《纺织复合材料》课程参考文献（Composite Materials Handbook，Volume 3）Chapter 7 - Damage Resistance, Durability, and Damage Tolerance

MIL-HDBK-17-3F Volume 3,Chapter 7-Damage Resistance,Durability,and Damage Tolerance CHAPTER 7 DAMAGE RESISTANCE,DURABILITY,AND DAMAGE TOLERANCE 7.1 OVERVIEW AND GENERAL GUIDELINES 7.1.1 Principles Engineered structures must be capable of performing their function throughout a specified lifetime while meeting safety and economic objectives.These structures are exposed to a series of events that include loading,environment,and damage threats.These events,either individually or cumulatively,can cause structural degradation,which,in turn,can affect the ability of the structure to perform its function. In many instances,uncertainties associated with existing damage as well as economic considerations necessitate a reliance on inspection and repair programs to ensure the required structural capability is maintained.The location and/or severity of manufacturing flaws and in-service damage can be difficult to anticipate for a variety of reasons.Complex loading and/or structural configurations result in secondary load paths that are not accurately predicted during the design process.Some manufacturing flaws may not be detectable until the structure is exposed to the service environment.For example,joints with con- taminated surfaces during bonding may not be detectable until the weak bond further deteriorates in ser- vice.The numerous variables associated with damage threats(e.g.,severity,frequency,and geometry) are rarely well defined until service data is collected.Moreover,established engineering tools for predict- ing damage caused by well-defined damage events often do not exist.Economic issues can include both non-recurring and recurring cost components.The large number of external events,combined with the interdependence of structural state,structural response,and external event history,can result in prohibi- tive non-recurring engineering or test costs associated with explicitly validating structural capability under all anticipated conditions.Moreover,large weight-related recurring costs associated with many applica- tions rule out the use of overly conservative,but simpler approaches. The goal in developing an inspection plan is to detect,with an acceptable level of reliability,any dam- age before it can reduce structural capability below the required level.To accomplish this.inspection techniques and intervals for each location in the structure must be selected with a good understanding of damage threats,how quickly damage will grow,the likelihood of detection,and the damage sizes that will threaten structural safety.To avoid costs associated with excessive repairs,inspection methods should also quantify structural degradation to support accurate residual strength assessments. This concept of combining an inspection plan with knowledge of damage threats,damage growth rates and residual strength is referred to as "damage tolerance".Specifically,damage tolerance is the ability of a structure to sustain design loads in the presence of damage caused by fatigue,corrosion,envi- ronment,accidental events,and other sources until such damage is detected,through inspections or mal- functions,and repaired. Durability considerations are typically combined with damage tolerance to meet economic and func- tionality objectives.Specifically,durability is the ability of a structural application to retain adequate prop- erties (strength,stiffness,and environmental resistance)throughout its life to the extent that any deterio- ration can be controlled and repaired,if there is a need,by economically acceptable maintenance prac- tices.As implied by the two definitions,durability addresses largely economic issues,while damage tol- erance has a focus on safety concerns.For example,durability often addresses the onset of damage from the operational environment.Under the principles of damage tolerance design,the small damages associated with initiation may be difficult to detect,but do not threaten structural integrity. 7.1.2 Composite-related issues All structural applications should be designed to be damage tolerant and durable.In using composite materials,a typical design objective is to meet or exceed the design service and reliability objectives of the same structure made of other materials,without increasing the maintenance burden.The generally good fatigue resistance and corrosion suppression of composites,help meet such objectives.However, 7-1

MIL-HDBK-17-3F Volume 3, Chapter 7 - Damage Resistance, Durability, and Damage Tolerance 7-1 CHAPTER 7 DAMAGE RESISTANCE, DURABILITY, AND DAMAGE TOLERANCE 7.1 OVERVIEW AND GENERAL GUIDELINES 7.1.1 Principles Engineered structures must be capable of performing their function throughout a specified lifetime while meeting safety and economic objectives. These structures are exposed to a series of events that include loading, environment, and damage threats. These events, either individually or cumulatively, can cause structural degradation, which, in turn, can affect the ability of the structure to perform its function. In many instances, uncertainties associated with existing damage as well as economic considerations necessitate a reliance on inspection and repair programs to ensure the required structural capability is maintained. The location and/or severity of manufacturing flaws and in-service damage can be difficult to anticipate for a variety of reasons. Complex loading and/or structural configurations result in secondary load paths that are not accurately predicted during the design process. Some manufacturing flaws may not be detectable until the structure is exposed to the service environment. For example, joints with con￾taminated surfaces during bonding may not be detectable until the weak bond further deteriorates in ser￾vice. The numerous variables associated with damage threats (e.g., severity, frequency, and geometry) are rarely well defined until service data is collected. Moreover, established engineering tools for predict￾ing damage caused by well-defined damage events often do not exist. Economic issues can include both non-recurring and recurring cost components. The large number of external events, combined with the interdependence of structural state, structural response, and external event history, can result in prohibi￾tive non-recurring engineering or test costs associated with explicitly validating structural capability under all anticipated conditions. Moreover, large weight-related recurring costs associated with many applica￾tions rule out the use of overly conservative, but simpler approaches. The goal in developing an inspection plan is to detect, with an acceptable level of reliability, any dam￾age before it can reduce structural capability below the required level. To accomplish this, inspection techniques and intervals for each location in the structure must be selected with a good understanding of damage threats, how quickly damage will grow, the likelihood of detection, and the damage sizes that will threaten structural safety. To avoid costs associated with excessive repairs, inspection methods should also quantify structural degradation to support accurate residual strength assessments. This concept of combining an inspection plan with knowledge of damage threats, damage growth rates and residual strength is referred to as “damage tolerance”. Specifically, damage tolerance is the ability of a structure to sustain design loads in the presence of damage caused by fatigue, corrosion, envi￾ronment, accidental events, and other sources until such damage is detected, through inspections or mal￾functions, and repaired. Durability considerations are typically combined with damage tolerance to meet economic and func￾tionality objectives. Specifically, durability is the ability of a structural application to retain adequate prop￾erties (strength, stiffness, and environmental resistance) throughout its life to the extent that any deterio￾ration can be controlled and repaired, if there is a need, by economically acceptable maintenance prac￾tices. As implied by the two definitions, durability addresses largely economic issues, while damage tol￾erance has a focus on safety concerns. For example, durability often addresses the onset of damage from the operational environment. Under the principles of damage tolerance design, the small damages associated with initiation may be difficult to detect, but do not threaten structural integrity. 7.1.2 Composite-related issues All structural applications should be designed to be damage tolerant and durable. In using composite materials, a typical design objective is to meet or exceed the design service and reliability objectives of the same structure made of other materials, without increasing the maintenance burden. The generally good fatigue resistance and corrosion suppression of composites, help meet such objectives. However

MIL-HDBK-17-3F Volume 3,Chapter 7-Damage Resistance,Durability,and Damage Tolerance the unique characteristics of composite materials also provide some significant challenges in developing safe,durable structure. The brittle nature of some polymer resins causes concern about their ability to resist damage and,if damaged,their ability to carry the required loads until the damage is detected.While the primary con- cerns in metal structure relate to tension crack growth and corrosion,other damages,such as delamina- tion and fiber breakage resulting from impact events and environmental degradation are more of a con- cern in polymer matrix composites.In addition,composites have unique damage sensitivities for com- pression and shear loading,as well as tension. In composite structure,the damage caused by an impact event is typically more severe and can be less visible than in metals.As a result of the increased threat of an immediate degradation in properties, another property,damage resistance,has been used for composite structures and material evaluation. Damage resistance is a measure of the relationship between parameters which define an event,or enve- lope of events (e.g.,impacts using a specified impactor and range of impact energies or forces),and the resulting damage size and type. Damage resistance and damage tolerance differ in that the former quantifies the damage caused by a specific damage event,while the latter addresses the ability of the structure to tolerate a specific damage condition.Damage resistance,like durability,largely addresses economic issues (e.g.,how often a par- ticular component needs repair),while damage tolerance addresses safe operation of a component. Optimally balancing damage resistance and damage tolerance for a specific composite application involves considering a number of technical and economic issues early in the design process.Damage resistance often competes with damage tolerance during the design process,both at the material and structural level.In addition,material and fabrication costs,as well as operational costs associated with inspection,repair,and structural weight,are strongly influenced by the selected material and structural configuration.For example,toughened-resin material systems typically improve damage resistance rela- tive to untoughened systems,which results in reduced maintenance costs associated with damage from low-severity impact events.However,these cost savings compete with the higher per-pound material costs for the toughened systems.In addition,these materials can also result in lower tensile capability of the structure with large damages or notches,which might require the addition of material to satisfy struc- tural capability requirements at Limit Load.This extra material and associated weight results in higher material and fuel costs,respectively. 7.1.3 General guidelines There are a large number of factors that influence damage resistance,durability and damage toler- ance of composite structures.In addition.there are complex interactions between these factors which can lead to non-intuitive results,and often a change in a factor can improve one of the areas of damage resistance,durability,or damage tolerance,while degrading the other two.It is important for a developer of a composite structure to understand these factors and their interactions as appropriate to the struc- ture's application in order to produce a balanced design that economically meets all of the design criteria. For these reasons,this chapter contains detailed discussions of influencing factors and design guidelines in each of the areas of damage resistance,durability,and damage tolerance(Sections 7.5 through 7.8). The following paragraphs outline some of the areas where significant and important interactions occur. The intent is to highlight these items that involve areas of several of the following detailed information sec- tions. An important part of a structural development program is to determine the damages that the structure is capable of carrying at the various required load levels(ultimate,limit,etc.).This in- formation can be used to develop appropriate maintenance,inspection and real-time monitoring techniques to ensure safety.The focus of damage tolerance evaluations should be on ensuring safety in the event of "rogue"and "unanticipated"events,not solely on likely scenarios of dam- age 7-2

MIL-HDBK-17-3F Volume 3, Chapter 7 - Damage Resistance, Durability, and Damage Tolerance 7-2 the unique characteristics of composite materials also provide some significant challenges in developing safe, durable structure. The brittle nature of some polymer resins causes concern about their ability to resist damage and, if damaged, their ability to carry the required loads until the damage is detected. While the primary con￾cerns in metal structure relate to tension crack growth and corrosion, other damages, such as delamina￾tion and fiber breakage resulting from impact events and environmental degradation are more of a con￾cern in polymer matrix composites. In addition, composites have unique damage sensitivities for com￾pression and shear loading, as well as tension. In composite structure, the damage caused by an impact event is typically more severe and can be less visible than in metals. As a result of the increased threat of an immediate degradation in properties, another property, damage resistance, has been used for composite structures and material evaluation. Damage resistance is a measure of the relationship between parameters which define an event, or enve￾lope of events (e.g., impacts using a specified impactor and range of impact energies or forces), and the resulting damage size and type. Damage resistance and damage tolerance differ in that the former quantifies the damage caused by a specific damage event, while the latter addresses the ability of the structure to tolerate a specific damage condition. Damage resistance, like durability, largely addresses economic issues (e.g., how often a par￾ticular component needs repair), while damage tolerance addresses safe operation of a component. Optimally balancing damage resistance and damage tolerance for a specific composite application involves considering a number of technical and economic issues early in the design process. Damage resistance often competes with damage tolerance during the design process, both at the material and structural level. In addition, material and fabrication costs, as well as operational costs associated with inspection, repair, and structural weight, are strongly influenced by the selected material and structural configuration. For example, toughened-resin material systems typically improve damage resistance rela￾tive to untoughened systems, which results in reduced maintenance costs associated with damage from low-severity impact events. However, these cost savings compete with the higher per-pound material costs for the toughened systems. In addition, these materials can also result in lower tensile capability of the structure with large damages or notches, which might require the addition of material to satisfy struc￾tural capability requirements at Limit Load. This extra material and associated weight results in higher material and fuel costs, respectively. 7.1.3 General guidelines There are a large number of factors that influence damage resistance, durability and damage toler￾ance of composite structures. In addition, there are complex interactions between these factors which can lead to non-intuitive results, and often a change in a factor can improve one of the areas of damage resistance, durability, or damage tolerance, while degrading the other two. It is important for a developer of a composite structure to understand these factors and their interactions as appropriate to the struc￾ture's application in order to produce a balanced design that economically meets all of the design criteria. For these reasons, this chapter contains detailed discussions of influencing factors and design guidelines in each of the areas of damage resistance, durability, and damage tolerance (Sections 7.5 through 7.8). The following paragraphs outline some of the areas where significant and important interactions occur. The intent is to highlight these items that involve areas of several of the following detailed information sec￾tions. • An important part of a structural development program is to determine the damages that the structure is capable of carrying at the various required load levels (ultimate, limit, etc.). This in￾formation can be used to develop appropriate maintenance, inspection and real-time monitoring techniques to ensure safety. The focus of damage tolerance evaluations should be on ensuring safety in the event of "rogue" and "unanticipated" events, not solely on likely scenarios of dam￾age

MIL-HDBK-17-3F Volume 3,Chapter 7-Damage Resistance,Durability,and Damage Tolerance The damage tolerance approach involves the use of inspection procedures and structural design concepts to protect safety,rather than the traditional factors of safety used for Ultimate Loads. The overall damage tolerance database for a structure should include information on residual strength characteristics,sensitivities to damage growth and environmental degradation,mainte- nance practices,and in-service usage parameters and damage experiences. Fiber and matrix materials,material forms,and fabrication processes are constantly changing.This requires a strong understanding of the durability and damage tolerance principles,the multitude of parameter interactions,and an intelligent,creative adaptation of them to achieve durability and safety goals.Also,new materials and material forms may have significantly different responses than exhib- ited by previous materials and structures (i.e.,"surprises"will occur).Therefore,the information and guidelines based on previous developments should not be blindly followed. Focusing strictly on meeting regulatory requirements will not ensure economical maintenance practices are established.For example,the Ultimate Load requirements for barely visible impact damage,BVID,in critical locations (see FAR 23.573,AC 20-107A,etc.)result in insufficient data to define allowable damage limits(ADLs)in higher-margin areas.Similarly,demonstrating com- pliance for discrete source damage requirements typically involves showing adequate structural capability with large notches at critical locations.Neither of these requirements ensure safe maintenance inspection practices are established to find the least detectable,yet most severe de- fect(i.e.,those reducing structural capability to Limit Loads).As a result the supporting data- bases should not be limited to these conditions.An extensive residual strength database ad- dressing the full range of damage variables and structural locations is needed to provide insights on ADLs for use in Structural Repair Manuals.For example,clearly visible damage may be ac- ceptable(i.e.,below the ADLs)away from stiffening elements and in more lightly loaded portions of the structure.A more extensive characterization of the residual strength curves for each char- acteristic damage type (impact,holes.etc.)will also help define damage capable of reducing strength to Limit Load. Well-defined inspection procedures that (a)quantify damage sufficiently to assess compliance with Allowable Damage Limits (ADLs)and (b)reliably find damage at the Critical Damage Threshold (CDT),discussed in Section 7.2.1,will help provide maintenance practices which are as good or better than those used for metal structure.Clearly defined damage metrics facilitate quantitative inspection procedures,which can be used to define the structural response of the de- tected damage. Currently,most initial inspections of composite structure have involved visual methods.There- fore,dent depth has evolved as a common damage metric.Development efforts should define the dent depths that correspond to the threshold of detectability for both general visual(surveil- lance in Boeing terminology)and detailed visual levels.The influence of dent-depth decay,which can come from viscoelastic and other material or structural behaviors,must be considered for maintenance inspection procedures and the selection of damage that will be used to demonstrate compliance Another factor motivating a more complete characterization of damage and structural variables is that the internal damage state for a specific structural detail is not a unique function of the dent depth.It is a complex function of the impact variables (i.e.,impactor geometry,energy level,an- gle of incidence,etc.).A range of these variables should be evaluated to understand the rela- tionship between them and to determine the combinations that result in the largest residual strength degradation. Structure certified with an approach that allows for damage growth must have associated in- service inspection techniques,which are capable of adequately detecting damage before it be- comes critical.These inspection methods should be demonstrated to be economical before committing to such a certification approach.In addition,the damage growth must be predictable such that inspection intervals can be reliably defined. 7-3

MIL-HDBK-17-3F Volume 3, Chapter 7 - Damage Resistance, Durability, and Damage Tolerance 7-3 • The damage tolerance approach involves the use of inspection procedures and structural design concepts to protect safety, rather than the traditional factors of safety used for Ultimate Loads. The overall damage tolerance database for a structure should include information on residual strength characteristics, sensitivities to damage growth and environmental degradation, mainte￾nance practices, and in-service usage parameters and damage experiences. • Fiber and matrix materials, material forms, and fabrication processes are constantly changing. This requires a strong understanding of the durability and damage tolerance principles, the multitude of parameter interactions, and an intelligent, creative adaptation of them to achieve durability and safety goals. Also, new materials and material forms may have significantly different responses than exhib￾ited by previous materials and structures (i.e., "surprises" will occur). Therefore, the information and guidelines based on previous developments should not be blindly followed. • Focusing strictly on meeting regulatory requirements will not ensure economical maintenance practices are established. For example, the Ultimate Load requirements for barely visible impact damage, BVID, in critical locations (see FAR 23.573, AC 20-107A, etc.) result in insufficient data to define allowable damage limits (ADLs) in higher-margin areas. Similarly, demonstrating com￾pliance for discrete source damage requirements typically involves showing adequate structural capability with large notches at critical locations. Neither of these requirements ensure safe maintenance inspection practices are established to find the least detectable, yet most severe de￾fect (i.e., those reducing structural capability to Limit Loads). As a result the supporting data￾bases should not be limited to these conditions. An extensive residual strength database ad￾dressing the full range of damage variables and structural locations is needed to provide insights on ADLs for use in Structural Repair Manuals. For example, clearly visible damage may be ac￾ceptable (i.e., below the ADLs) away from stiffening elements and in more lightly loaded portions of the structure. A more extensive characterization of the residual strength curves for each char￾acteristic damage type (impact, holes, etc.) will also help define damage capable of reducing strength to Limit Load. • Well-defined inspection procedures that (a) quantify damage sufficiently to assess compliance with Allowable Damage Limits (ADLs) and (b) reliably find damage at the Critical Damage Threshold (CDT), discussed in Section 7.2.1, will help provide maintenance practices which are as good or better than those used for metal structure. Clearly defined damage metrics facilitate quantitative inspection procedures, which can be used to define the structural response of the de￾tected damage. • Currently, most initial inspections of composite structure have involved visual methods. There￾fore, dent depth has evolved as a common damage metric. Development efforts should define the dent depths that correspond to the threshold of detectability for both general visual (surveil￾lance in Boeing terminology) and detailed visual levels. The influence of dent-depth decay, which can come from viscoelastic and other material or structural behaviors, must be considered for maintenance inspection procedures and the selection of damage that will be used to demonstrate compliance. • Another factor motivating a more complete characterization of damage and structural variables is that the internal damage state for a specific structural detail is not a unique function of the dent depth. It is a complex function of the impact variables (i.e., impactor geometry, energy level, an￾gle of incidence, etc.). A range of these variables should be evaluated to understand the rela￾tionship between them and to determine the combinations that result in the largest residual strength degradation. • Structure certified with an approach that allows for damage growth must have associated in￾service inspection techniques, which are capable of adequately detecting damage before it be￾comes critical. These inspection methods should be demonstrated to be economical before committing to such a certification approach. In addition, the damage growth must be predictable such that inspection intervals can be reliably defined

MIL-HDBK-17-3F Volume 3,Chapter 7-Damage Resistance,Durability,and Damage Tolerance 7.1.4 Section organization This chapter of the handbook addresses the multitude of issues associated with the damage resis- tance,durability,and damage tolerance of composite materials.Discussions are heavily reliant on ex- perience gained in the aircraft industry,since it represents the area where composites and damage toler- ant philosophy have been most used.As the associated composite technologies continue to evolve,ad- ditional applications and service history should lead to future updates with a more complete understand- ing of:(1)potential damage threats,(2)methods to achieve the desired reliability in a composite design, and(3)improved design and maintenance practices for damage tolerance. Section 7.2 focuses on the requirements for military and civilian aviation applications,as well as methods of compliance.Discussion of the characteristics of various types of composite damage and a list of possible sources of the damage are given in Section 7.3.Composite damage inspection methods and their limitations are discussed in Section 7.4.Sections 7.1 through 7.4 are relatively mature in their content. Sections 7.5 through 7.8,which comprise the bulk of this section,address the major material and structural responses:damage resistance,durability.damage growth under cyclic loading,and residual strength,respectively.Each section includes detailed discussions of:(a)the major factors that affect re- sponse;(b)design-related issues and guidelines for meeting objectives and requirements;(c)testing methods and issues;and(d)analytical predictive methods,their use,and their success at predicting ob- served responses. At this point in time,not all parts of Sections 7.5 through 7.8 are complete.Section 7.5,Damage Re- sistance,currently contains information on influencing factors and guidelines;sections on test and analy- sis methods will be added in the future.Section 7.6,Durability,currently contains only limited information. Future updates will complete this section.Section 7.7,Damage Growth Under Cyclic Loading,contains some limited information on the growth of impact damages.Additional parts of this section will be added in the future.Section 7.8,Residual Strength,contains extensive information on influencing factors,guide- lines and analysis methods;the section on test methods will be added in the future. Section 7.9 includes several examples of successful damage-tolerant designs from a number of com- posite aircraft applications.These examples illustrate how different aspects of damage tolerance come to the forefront as a function of application. 7.2 AIRCRAFT DAMAGE TOLERANCE Damage tolerance provides a measure of the structure's ability to sustain design loads with a level of damage or defect and be able to perform its operating functions.Consequently,the concern with damage tolerance is ultimately with the damaged structure having adequate residual strength and stiffness to con- tinue in service safely until the damage can be detected by scheduled maintenance inspection (or mal- function)and be repaired or until the life limit is reached.The extent of damage and detectability deter- mines the required load level to be sustained.Thus,safety is the primary goal of damage tolerance. Damage tolerance methodologies are most mature in the military and civil aircraft industry.They were initially developed and used for metallic materials,but have more recently been extended and ap- plied to composite structure.The damage tolerance philosophy has been included in regulations since the 1970's.It evolved out of the "Safe Life"and "Fail Safe"approaches(Reference 7.2). The safe-life approach ensures adequate fatigue life of a structural member by limiting its allowed operational life.During its application to commercial aircraft in the 1950's,this approach was found to be uneconomical in achieving acceptable safety,since a combination of material scatter and inadequate fa- tigue analyses resulted in the premature retirement of healthy components.The approach is still used today in such structures as high-strength steel landing gear.Due to the damage sensitivities and rela- tively flat fatigue curves of composite materials,a safe-life approach is not considered appropriate. 7-4

MIL-HDBK-17-3F Volume 3, Chapter 7 - Damage Resistance, Durability, and Damage Tolerance 7-4 7.1.4 Section organization This chapter of the handbook addresses the multitude of issues associated with the damage resis￾tance, durability, and damage tolerance of composite materials. Discussions are heavily reliant on ex￾perience gained in the aircraft industry, since it represents the area where composites and damage toler￾ant philosophy have been most used. As the associated composite technologies continue to evolve, ad￾ditional applications and service history should lead to future updates with a more complete understand￾ing of: (1) potential damage threats, (2) methods to achieve the desired reliability in a composite design, and (3) improved design and maintenance practices for damage tolerance. Section 7.2 focuses on the requirements for military and civilian aviation applications, as well as methods of compliance. Discussion of the characteristics of various types of composite damage and a list of possible sources of the damage are given in Section 7.3. Composite damage inspection methods and their limitations are discussed in Section 7.4. Sections 7.1 through 7.4 are relatively mature in their content. Sections 7.5 through 7.8, which comprise the bulk of this section, address the major material and structural responses: damage resistance, durability, damage growth under cyclic loading, and residual strength, respectively. Each section includes detailed discussions of: (a) the major factors that affect re￾sponse; (b) design-related issues and guidelines for meeting objectives and requirements; (c) testing methods and issues; and (d) analytical predictive methods, their use, and their success at predicting ob￾served responses. At this point in time, not all parts of Sections 7.5 through 7.8 are complete. Section 7.5, Damage Re￾sistance, currently contains information on influencing factors and guidelines; sections on test and analy￾sis methods will be added in the future. Section 7.6, Durability, currently contains only limited information. Future updates will complete this section. Section 7.7, Damage Growth Under Cyclic Loading, contains some limited information on the growth of impact damages. Additional parts of this section will be added in the future. Section 7.8, Residual Strength, contains extensive information on influencing factors, guide￾lines and analysis methods; the section on test methods will be added in the future. Section 7.9 includes several examples of successful damage-tolerant designs from a number of com￾posite aircraft applications. These examples illustrate how different aspects of damage tolerance come to the forefront as a function of application. 7.2 AIRCRAFT DAMAGE TOLERANCE Damage tolerance provides a measure of the structure’s ability to sustain design loads with a level of damage or defect and be able to perform its operating functions. Consequently, the concern with damage tolerance is ultimately with the damaged structure having adequate residual strength and stiffness to con￾tinue in service safely until the damage can be detected by scheduled maintenance inspection (or mal￾function) and be repaired or until the life limit is reached. The extent of damage and detectability deter￾mines the required load level to be sustained. Thus, safety is the primary goal of damage tolerance. Damage tolerance methodologies are most mature in the military and civil aircraft industry. They were initially developed and used for metallic materials, but have more recently been extended and ap￾plied to composite structure. The damage tolerance philosophy has been included in regulations since the 1970’s. It evolved out of the “Safe Life” and “Fail Safe” approaches (Reference 7.2). The safe-life approach ensures adequate fatigue life of a structural member by limiting its allowed operational life. During its application to commercial aircraft in the 1950’s, this approach was found to be uneconomical in achieving acceptable safety, since a combination of material scatter and inadequate fa￾tigue analyses resulted in the premature retirement of healthy components. The approach is still used today in such structures as high-strength steel landing gear. Due to the damage sensitivities and rela￾tively flat fatigue curves of composite materials, a safe-life approach is not considered appropriate

MIL-HDBK-17-3F Volume 3,Chapter 7-Damage Resistance,Durability,and Damage Tolerance The fail-safe approach assumes members will fail,but forces the structure to contain multiple load paths by requiring specific load-carrying capability with assumed failures of one or more structural ele- ments.This approach achieved acceptable safety levels more economically,and,due to the relative se- verity of the assumed failures,was generally effective at providing sufficient opportunity for timely detec- tion of structural damage.Its redundant-load-path approach also effectively addressed accidental dam- age and corrosion.However,the method does not allow for explicit limits on the maximum risk of struc- tural failure,and it does not demonstrate that all partial failures with insufficient residual strength are obvi- ous.Moreover,structural redundancy is not always efficient in addressing fatigue damage,where similar elements under similar loading would be expected to have similar fatigue-induced damage. 7.2.1 Evolving military and civil aviation requirements The "duration of damage or defect"factor based on degree of detectability has been the basis for es- tablishing minimum Air Force damage tolerance residual strengths for composite structures in require- ments proposed for inclusion in AFGS-87221,"General Specification for Aircraft Structures".These strength requirements are identical to those for metal structure having critical defects or damage with a comparable degree of detectability.Requirements for cyclic loading prior to residual strength testing of test components are also identical.The non-detectable damage to be assumed includes a surface scratch,a delamination and impact damage.The impact damage includes both a definition of dent depth, i.e.,detectability,and a maximum energy cutoff.Specifically,the impact damage to be assumed is that "caused by the impact of a 1.0 inch(25 mm)diameter hemispherical impactor with a 100 ft-lb(136 N-m) of kinetic energy,or that kinetic energy required to cause a dent 0.10 inch(2.54 mm)deep,whichever is least."For relatively thin structure,the detectability,i.e.,the 0.1 inch(2.5 mm)depth,requirement pre- vails.For thicker structure,the maximum assumed impact energy becomes the critical requirement.This will be illustrated in Section 7.5.The associated load to be assumed is the maximum load expected to occur in an extrapolated 20 lifetimes.This is a one-time static load requirement.These requirements are coupled with assumptions that the damage occurs in the most critical location and that the assumed load is coincident with the worst probable environment. In developing the requirements,the probability of undetected or undetectable impact damage occur- ring above the 100 ft-Ib(136 N-m)energy level was considered sufficiently remote that when coupled with other requirements a high level of safety was provided.For the detectability requirement,it is assumed that having damage greater than 0.10 inch(2.5 mm)in depth will be detected and repaired.Conse- quently,the load requirement is consistent with those for metal structure with damage of equivalent levels of detectability.Provisions for multiple impact damage,analogous to the continuing damage considera- tions for metal structure,and for the lesser susceptibility of interior structure to damage are also included. In metal structure,a major damage tolerance concern is the growth of damage prior to the time of detection.Consequently,much development testing for metals has been focused on evaluating crack growth rates associated with defects and damage,and the time for the defect/damage size to reach re- sidual strength criticality.Typically,the critical loading mode has been in tension.Crack growth,even at comparatively low stress amplitudes,may be significant.In general,damage growth rates for metals are consistent and,after test data has been obtained,can be predicted satisfactorily for many different aircraft structural configurations.Thus,knowing the expected stress history for the aircraft,inspection intervals have been defined that confidently ensure crack detection before failure. By contrast,composites have unique damage sensitivities for both tension and compression loads. However,the fibers in composite laminates act to inhibit tensile crack growth,which only occurs at rela- tively high stress levels.Consequently,through the thickness damage growth,which progressively breaks the fibers in a composite,has generally not been a problem.In studying the effects of debonds, delaminations or impact damage,the concern becomes compression and shear loads where local insta- bilities may stimulate growth.Unlike cracks in metal,growth of delaminations or impact damage in com- posites may not be detected using economical maintenance inspection practices.In many cases,the degraded performance of composites with impact damage also cannot be predicted satisfactorily.Hence, there is a greater dependence on testing to evaluate composite residual strength and damage growth 7-5

MIL-HDBK-17-3F Volume 3, Chapter 7 - Damage Resistance, Durability, and Damage Tolerance 7-5 The fail-safe approach assumes members will fail, but forces the structure to contain multiple load paths by requiring specific load-carrying capability with assumed failures of one or more structural ele￾ments. This approach achieved acceptable safety levels more economically, and, due to the relative se￾verity of the assumed failures, was generally effective at providing sufficient opportunity for timely detec￾tion of structural damage. Its redundant-load-path approach also effectively addressed accidental dam￾age and corrosion. However, the method does not allow for explicit limits on the maximum risk of struc￾tural failure, and it does not demonstrate that all partial failures with insufficient residual strength are obvi￾ous. Moreover, structural redundancy is not always efficient in addressing fatigue damage, where similar elements under similar loading would be expected to have similar fatigue-induced damage. 7.2.1 Evolving military and civil aviation requirements The “duration of damage or defect” factor based on degree of detectability has been the basis for es￾tablishing minimum Air Force damage tolerance residual strengths for composite structures in require￾ments proposed for inclusion in AFGS-87221, “General Specification for Aircraft Structures”. These strength requirements are identical to those for metal structure having critical defects or damage with a comparable degree of detectability. Requirements for cyclic loading prior to residual strength testing of test components are also identical. The non-detectable damage to be assumed includes a surface scratch, a delamination and impact damage. The impact damage includes both a definition of dent depth, i.e., detectability, and a maximum energy cutoff. Specifically, the impact damage to be assumed is that “caused by the impact of a 1.0 inch (25 mm) diameter hemispherical impactor with a 100 ft-lb (136 N-m) of kinetic energy, or that kinetic energy required to cause a dent 0.10 inch (2.54 mm) deep, whichever is least.” For relatively thin structure, the detectability, i.e., the 0.1 inch (2.5 mm) depth, requirement pre￾vails. For thicker structure, the maximum assumed impact energy becomes the critical requirement. This will be illustrated in Section 7.5. The associated load to be assumed is the maximum load expected to occur in an extrapolated 20 lifetimes. This is a one-time static load requirement. These requirements are coupled with assumptions that the damage occurs in the most critical location and that the assumed load is coincident with the worst probable environment. In developing the requirements, the probability of undetected or undetectable impact damage occur￾ring above the 100 ft-lb (136 N-m) energy level was considered sufficiently remote that when coupled with other requirements a high level of safety was provided. For the detectability requirement, it is assumed that having damage greater than 0.10 inch (2.5 mm) in depth will be detected and repaired. Conse￾quently, the load requirement is consistent with those for metal structure with damage of equivalent levels of detectability. Provisions for multiple impact damage, analogous to the continuing damage considera￾tions for metal structure, and for the lesser susceptibility of interior structure to damage are also included. In metal structure, a major damage tolerance concern is the growth of damage prior to the time of detection. Consequently, much development testing for metals has been focused on evaluating crack growth rates associated with defects and damage, and the time for the defect/damage size to reach re￾sidual strength criticality. Typically, the critical loading mode has been in tension. Crack growth, even at comparatively low stress amplitudes, may be significant. In general, damage growth rates for metals are consistent and, after test data has been obtained, can be predicted satisfactorily for many different aircraft structural configurations. Thus, knowing the expected stress history for the aircraft, inspection intervals have been defined that confidently ensure crack detection before failure. By contrast, composites have unique damage sensitivities for both tension and compression loads. However, the fibers in composite laminates act to inhibit tensile crack growth, which only occurs at rela￾tively high stress levels. Consequently, through the thickness damage growth, which progressively breaks the fibers in a composite, has generally not been a problem. In studying the effects of debonds, delaminations or impact damage, the concern becomes compression and shear loads where local insta￾bilities may stimulate growth. Unlike cracks in metal, growth of delaminations or impact damage in com￾posites may not be detected using economical maintenance inspection practices. In many cases, the degraded performance of composites with impact damage also cannot be predicted satisfactorily. Hence, there is a greater dependence on testing to evaluate composite residual strength and damage growth

MIL-HDBK-17-3F Volume 3,Chapter 7-Damage Resistance,Durability,and Damage Tolerance under cyclic loads.In the absence of predictive tools for growth,design values are typically established with sufficient margins to ensure that damage growth due to repeated loads will not occur.This method for avoiding the potential growth of damage in design and certification is known as the"no-growth"ap- proach.It has been practical for most composite designs,which have proved to be fatigue insensitive at typical design stress levels. The damage tolerance design procedures for civil/commercial aircraft are expressed more generally but with equal effectiveness.Civil aviation requirements are addressed in Federal Aviation Regulations (FAR)23.573,25.571,27.571,29.571 and Joint Airworthiness Requirements(JAR)25.571.Advisory Cir- cular 20-107A and ACJ 25.603 provide means of compliance with the regulations concerning composite material structure.Advisory Circular AC25.571-1(rev.B was issued 2/18/97)provides means of compli- ance with provision of FAR Part 25 dealing with damage tolerance and fatigue life (25.571).Unlike mili- tary requirements,civil/commercial ones do not recommend any energy level or detectability thresholds. In fact,they do not assume the inspections will be visual.Relative to impact damage,it is stated in the FAA guidelines in AC20-107A,Paragraph 6.g."It should be shown that impact damage that can be realis- tically expected from manufacturing and service,but not more than the established threshold of detect- ability for the selected inspection procedure,will not reduce the structural strength below Ultimate Load capability.This can be shown by analysis supported by test evidence,or by tests at the coupon,element, or subcomponent level.This guidance is to ensure that structure with barely detectable impact damage will still meet ultimate strength requirements.A similar wording to the above has been added to FAR 23.573.In practice.visual inspections are most often used for initial detection.It is important to consider lighting conditions when determining visibility.Dent depth thresholds are typically used to quantify visibil- ity,with typical values being 0.01 to 0.02 inches(0.25 to 0.50 mm)for tool-side impacts and 0.05 inches (1.3 mm)for bag-side impacts. It is also stated in 7.a(2)of AC 20-107A"The extent of initially detectable damage should be estab- lished and be consistent with the inspection techniques employed during manufacture and in service. Flaw/damage growth data should be obtained by repeated load cycling of intrinsic flaws or mechanically introduced damage."And,in 7.a.(3)of AC 20-107A,it is stated "The evaluation should demonstrate that the residual strength of the structure is equal to or greater than the strength required for the specified de- sign loads(considered as ultimate)."This guidance is to ensure that visible impact damage (VID)will be detected in a timely manner and will be repaired before strength is reduced below Limit Load capability Damage such as runway debris,which may not be immediately obvious,would likely be considered as VID.The difference in the Air Force specification and the FAA guideline is primarily in the residual strength value.Also,while the Air Force specification assumes visual inspection,the FAA guideline leaves the inspection method to be selected.Consequently.since specifications and guidelines differ with the type of aircraft,the manufacturer must be aware of the differences and apply those guidelines and specifications appropriate to the situation. The FAA guidelines for discrete source damage are stated in 8.b of AC 25.571-1A.They state that "The maximum extent of immediately obvious damage from discrete sources(S 25.571(e))should be de- termined and the remaining structure shown,with an acceptable level of confidence,to have static strength for the maximum load(considered as Ultimate Load)expected during completion of the flight."It is stated in 8.c.(2)of AC 25.571-1A"(2)Following the incident:Seventy percent(70%)limit flight maneu- ver loads and,separately,40 percent of the limit gust velocity(vertical or lateral)at the specified speeds, each combined with the maximum appropriate cabin differential pressure (including the expected external aerodynamic pressure)."The discrete sources listed in 25.571(e)are as follows:(1)Impact with a 4- pound bird;(2)Uncontained fan blade impact;(3)Uncontained engine failure;or(4)Uncontained high energy rotating machinery failure.These high-energy sources are likely to penetrate structures.Damage from a discrete source that is not immediately obvious must be considered as VID with Limit Load.MIL- A-83444 has similar requirements for "in-flight"and "ground evident damage".The design loads for these two conditions are the maximum loads expected in 100 flights. The following summarize current aeronautical requirements for composite aircraft structures with damage: 7-6

MIL-HDBK-17-3F Volume 3, Chapter 7 - Damage Resistance, Durability, and Damage Tolerance 7-6 under cyclic loads. In the absence of predictive tools for growth, design values are typically established with sufficient margins to ensure that damage growth due to repeated loads will not occur. This method for avoiding the potential growth of damage in design and certification is known as the "no-growth" ap￾proach. It has been practical for most composite designs, which have proved to be fatigue insensitive at typical design stress levels. The damage tolerance design procedures for civil/commercial aircraft are expressed more generally but with equal effectiveness. Civil aviation requirements are addressed in Federal Aviation Regulations (FAR) 23.573, 25.571, 27.571, 29.571 and Joint Airworthiness Requirements (JAR) 25.571. Advisory Cir￾cular 20-107A and ACJ 25.603 provide means of compliance with the regulations concerning composite material structure. Advisory Circular AC25.571-1 (rev. B was issued 2/18/97) provides means of compli￾ance with provision of FAR Part 25 dealing with damage tolerance and fatigue life (25.571). Unlike mili￾tary requirements, civil/commercial ones do not recommend any energy level or detectability thresholds. In fact, they do not assume the inspections will be visual. Relative to impact damage, it is stated in the FAA guidelines in AC20-107A, Paragraph 6.g. “It should be shown that impact damage that can be realis￾tically expected from manufacturing and service, but not more than the established threshold of detect￾ability for the selected inspection procedure, will not reduce the structural strength below Ultimate Load capability. This can be shown by analysis supported by test evidence, or by tests at the coupon, element, or subcomponent level.” This guidance is to ensure that structure with barely detectable impact damage will still meet ultimate strength requirements. A similar wording to the above has been added to FAR 23.573. In practice, visual inspections are most often used for initial detection. It is important to consider lighting conditions when determining visibility. Dent depth thresholds are typically used to quantify visibil￾ity, with typical values being 0.01 to 0.02 inches (0.25 to 0.50 mm) for tool-side impacts and 0.05 inches (1.3 mm) for bag-side impacts. It is also stated in 7.a(2) of AC 20-107A “The extent of initially detectable damage should be estab￾lished and be consistent with the inspection techniques employed during manufacture and in service. Flaw/damage growth data should be obtained by repeated load cycling of intrinsic flaws or mechanically introduced damage.” And, in 7.a.(3) of AC 20-107A, it is stated “The evaluation should demonstrate that the residual strength of the structure is equal to or greater than the strength required for the specified de￾sign loads (considered as ultimate).” This guidance is to ensure that visible impact damage (VID) will be detected in a timely manner and will be repaired before strength is reduced below Limit Load capability. Damage such as runway debris, which may not be immediately obvious, would likely be considered as VID. The difference in the Air Force specification and the FAA guideline is primarily in the residual strength value. Also, while the Air Force specification assumes visual inspection, the FAA guideline leaves the inspection method to be selected. Consequently, since specifications and guidelines differ with the type of aircraft, the manufacturer must be aware of the differences and apply those guidelines and specifications appropriate to the situation. The FAA guidelines for discrete source damage are stated in 8.b of AC 25.571-1A. They state that “The maximum extent of immediately obvious damage from discrete sources (§ 25.571(e)) should be de￾termined and the remaining structure shown, with an acceptable level of confidence, to have static strength for the maximum load (considered as Ultimate Load) expected during completion of the flight.” It is stated in 8.c.(2) of AC 25.571-1A “(2) Following the incident: Seventy percent (70%) limit flight maneu￾ver loads and, separately, 40 percent of the limit gust velocity (vertical or lateral) at the specified speeds, each combined with the maximum appropriate cabin differential pressure (including the expected external aerodynamic pressure).” The discrete sources listed in 25.571(e) are as follows: (1) Impact with a 4- pound bird; (2) Uncontained fan blade impact; (3) Uncontained engine failure; or (4) Uncontained high energy rotating machinery failure. These high-energy sources are likely to penetrate structures. Damage from a discrete source that is not immediately obvious must be considered as VID with Limit Load. MIL￾A-83444 has similar requirements for “in-flight” and “ground evident damage”. The design loads for these two conditions are the maximum loads expected in 100 flights. The following summarize current aeronautical requirements for composite aircraft structures with damage:

MIL-HDBK-17-3F Volume 3,Chapter 7-Damage Resistance,Durability,and Damage Tolerance 1.Structure containing likely damage or defects that are not detectable during manufacturing in- spections and service inspections must withstand Ultimate Load and not impair operation of the aircraft for its lifetime(with appropriate factor). 2.Structure containing damage that is detectable during maintenance inspections must withstand a once per lifetime load,which is applied following repeated service loads occurring during an in- spection interval(with appropriate factor). 3.All damage that lowers strength below Ultimate Load must be repaired when found. 4. Structure damaged from an in-flight,discrete source that is evident to the crew must withstand loads that are consistent with continued safe flight. 5. Any damage that is repaired must withstand Ultimate Load. Static and fatigue tests are usually conducted during design development and validation to show that composite structures satisfy certification requirements(Reference 7.2.1(a)). The [inverse]relationship between design load levels and damage severity is shown in Figure 7.2.1(a).As is the case with metal commercial aircraft components,ultimate strength and damage toler- ance design philosophies are used to help maintain the reliable and safe operation of composite struc- ture.The load and damage requirements are balanced such that there is an extremely low probability of failure.Residual strength design requirements for relatively small damage,which are likely to occur in service,are matched with very high(unlikely)load scenarios(ultimate).The design requirement for more severe damage states,such as those caused by impact events that have a very low probability of occur- rence,are evaluated for the upper end of realistic load conditions(limit).The most severe damage states considered in design are those occurring in flight (e.g.,engine burst).The flight crew generally has knowledge of such events and they limit maneuvers for continued safe flight.Depending on the specific structure and an associated load case,continued safe flight load requirements may be as high as limit (e.g.,pressure loads for fuselage). Maintenance technology for composite aircraft structure benefits from a complete assessment of ser- vice damage threats on structural performance.Unfortunately,the necessary links between composite design practices and maintenance technology has not received the attention required to gain acceptance by commercial airlines and other customers.In the past,damages selected to size structure for the de- sign load conditions shown in Figure 7.2.1(a)have not met all the needs of maintenance.A more com- plete database is needed to determine the effects of a full range of composite damages on residual strength.A complete characterization of the residual strength curve (i.e..residual strength versus a measurable damage metric)can help establish the Allowable Damage Limits (ADL)and Critical Damage Threshold (CDT)as a function of structural location.Well-defined ADLs can help airlines accurately de- termine the need for repair.Generous ADLs in areas prone to damage may help minimize maintenance costs by allowing cosmetic repairs instead of structural repairs that require more equipment and time. The amount of damage that reduces the residual strength to the regulatory requirements of FAR 25.571 are referred to as the Critical Damage Threshold(CDT).It is desirable to design structure such that service damage falling between the ADL and CDT limits can be found and characterized using practi- cal inspection procedures.This goal provides aircraft safety and maintenance benefits.By definition,all damage of this extent must be repaired when found.Damage approaching the CDT must be found with extremely high probability using the selected inspection scheme(i.e.,it should be reliably detectable with the specified inspection scheme).A complete description of the critical damage characteristics,as re- lated to the inspection scheme,is valuable information for maintenance planning activities.As with met- als,damage tolerant design to relatively large CDTs provides the confidence for safe aircraft operations with economical inspection intervals and procedures. The ADL and CDT definitions in Figure 7.2.1(a)both imply zero margins of safety for respective load cases.These parameters will vary over the surface of the structure as a function of the loads and other factors driving the design.As such,they have meaning to maintenance and should not be thought of as the design requirement for ultimate and Limit Loads.Design requirements and objectives are established for a given application,within general guidelines set by industry experience and the FAA.The design cri- 7-7

MIL-HDBK-17-3F Volume 3, Chapter 7 - Damage Resistance, Durability, and Damage Tolerance 7-7 1. Structure containing likely damage or defects that are not detectable during manufacturing in￾spections and service inspections must withstand Ultimate Load and not impair operation of the aircraft for its lifetime (with appropriate factor). 2. Structure containing damage that is detectable during maintenance inspections must withstand a once per lifetime load, which is applied following repeated service loads occurring during an in￾spection interval (with appropriate factor). 3. All damage that lowers strength below Ultimate Load must be repaired when found. 4. Structure damaged from an in-flight, discrete source that is evident to the crew must withstand loads that are consistent with continued safe flight. 5. Any damage that is repaired must withstand Ultimate Load. Static and fatigue tests are usually conducted during design development and validation to show that composite structures satisfy certification requirements (Reference 7.2.1(a)). The [inverse] relationship between design load levels and damage severity is shown in Figure 7.2.1(a). As is the case with metal commercial aircraft components, ultimate strength and damage toler￾ance design philosophies are used to help maintain the reliable and safe operation of composite struc￾ture. The load and damage requirements are balanced such that there is an extremely low probability of failure. Residual strength design requirements for relatively small damage, which are likely to occur in service, are matched with very high (unlikely) load scenarios (ultimate). The design requirement for more severe damage states, such as those caused by impact events that have a very low probability of occur￾rence, are evaluated for the upper end of realistic load conditions (limit). The most severe damage states considered in design are those occurring in flight (e.g., engine burst). The flight crew generally has knowledge of such events and they limit maneuvers for continued safe flight. Depending on the specific structure and an associated load case, continued safe flight load requirements may be as high as limit (e.g., pressure loads for fuselage). Maintenance technology for composite aircraft structure benefits from a complete assessment of ser￾vice damage threats on structural performance. Unfortunately, the necessary links between composite design practices and maintenance technology has not received the attention required to gain acceptance by commercial airlines and other customers. In the past, damages selected to size structure for the de￾sign load conditions shown in Figure 7.2.1(a) have not met all the needs of maintenance. A more com￾plete database is needed to determine the effects of a full range of composite damages on residual strength. A complete characterization of the residual strength curve (i.e., residual strength versus a measurable damage metric) can help establish the Allowable Damage Limits (ADL) and Critical Damage Threshold (CDT) as a function of structural location. Well-defined ADLs can help airlines accurately de￾termine the need for repair. Generous ADLs in areas prone to damage may help minimize maintenance costs by allowing cosmetic repairs instead of structural repairs that require more equipment and time. The amount of damage that reduces the residual strength to the regulatory requirements of FAR 25.571 are referred to as the Critical Damage Threshold (CDT). It is desirable to design structure such that service damage falling between the ADL and CDT limits can be found and characterized using practi￾cal inspection procedures. This goal provides aircraft safety and maintenance benefits. By definition, all damage of this extent must be repaired when found. Damage approaching the CDT must be found with extremely high probability using the selected inspection scheme (i.e., it should be reliably detectable with the specified inspection scheme). A complete description of the critical damage characteristics, as re￾lated to the inspection scheme, is valuable information for maintenance planning activities. As with met￾als, damage tolerant design to relatively large CDTs provides the confidence for safe aircraft operations with economical inspection intervals and procedures. The ADL and CDT definitions in Figure 7.2.1(a) both imply zero margins of safety for respective load cases. These parameters will vary over the surface of the structure as a function of the loads and other factors driving the design. As such, they have meaning to maintenance and should not be thought of as the design requirement for ultimate and Limit Loads. Design requirements and objectives are established for a given application, within general guidelines set by industry experience and the FAA. The design cri-

MIL-HDBK-17-3F Volume 3,Chapter 7-Damage Resistance,Durability,and Damage Tolerance teria used to meet these requirements become even more program-specific,depending on available da- tabases for the selected structural concept. Structural durability affects the frequency and cost of inspection,replacement, repair,or other maintenance Structural damage tolerance ensures Design damage will be found by maintenance Ultimate Load practices before becoming a safety threat 1.5 Factor Level of Safety Discrete source events (e.g., Limit engine burst,birdstrike) can cause severe damage Maximum load but it is known to pilot per fleet lifetime Continued safe flight Allowable Critical Damage Damage Limit Threshold (ADL) (CDT) Increasing Damage Severity FIGURE 7.2.1(a)Design load and damage considerations for durability damage tolerance. Figure 7.2.1(b)helps illustrate the requirements for damage subjected to time in service(i.e..re- peated loads and environmental cycling).For relatively small damages,which likely exist in the structure and may be undetected by either quality control at the time of manufacturing or service inspection,the structure should retain static strength for Ultimate Loads over the aircraft's life.When detailed visual in- spection techniques are used for service,barely visible impact damage(BVID)is usually classified as a threshold for undetectable damage.If damage is of a size and characteristic that can be detected by se- lected service inspections (e.g.,visible impact damage,VID),then the load requirement drops to Limit Load.Structure with such damage is only expected to sustain the service environment for a period of time related to the inspection interval.In the cases of both undetectable and detectable damages,factors are typically applied in fatigue testing,damage tolerant design and maintenance to account for the vari- ability in material behavior under repeated loading and the reliability of inspection techniques.In certifica- tion practice for composite materials,a load enhancement factor is often used to reduce the additional test cycles needed to account for material variability (References 7.2.1(b)to 7.2.1(d)). 7-8

MIL-HDBK-17-3F Volume 3, Chapter 7 - Damage Resistance, Durability, and Damage Tolerance 7-8 teria used to meet these requirements become even more program-specific, depending on available da￾tabases for the selected structural concept. Allowable Damage Limit (ADL) Increasing Damage Severity Ultimate ~ Maximum load per fleet lifetime Design Load Level Continued safe flight Limit Critical Damage Threshold (CDT) 1.5 Factor of Safety Structural durability affects the frequency and cost of inspection, replacement, repair, or other maintenance Structural damage tolerance ensures damage will be found by maintenance practices before becoming a safety threat Discrete source events (e.g., engine burst, birdstrike) can cause severe damage but it is known to pilot FIGURE 7.2.1(a) Design load and damage considerations for durability & damage tolerance. Figure 7.2.1(b) helps illustrate the requirements for damage subjected to time in service (i.e., re￾peated loads and environmental cycling). For relatively small damages, which likely exist in the structure and may be undetected by either quality control at the time of manufacturing or service inspection, the structure should retain static strength for Ultimate Loads over the aircraft’s life. When detailed visual in￾spection techniques are used for service, barely visible impact damage (BVID) is usually classified as a threshold for undetectable damage. If damage is of a size and characteristic that can be detected by se￾lected service inspections (e.g., visible impact damage, VID), then the load requirement drops to Limit Load. Structure with such damage is only expected to sustain the service environment for a period of time related to the inspection interval. In the cases of both undetectable and detectable damages, factors are typically applied in fatigue testing, damage tolerant design and maintenance to account for the vari￾ability in material behavior under repeated loading and the reliability of inspection techniques. In certifica￾tion practice for composite materials, a load enhancement factor is often used to reduce the additional test cycles needed to account for material variability (References 7.2.1(b) to 7.2.1(d))

MIL-HDBK-17-3F Volume 3,Chapter 7-Damage Resistance,Durability,and Damage Tolerance Design Load Selected manufacturing or service flaws which may Requirement go undetected by selected OC or service inspection (e.g.,BVID,small delaminations,porosity) Ultimate Selected rogue manufacturing or service flaws which likely will be detected by selected service inspection (e.g.,VID,missing fasteners,small penetrations, delaminations) Damage Size (Severity) Maintenance Inspection Interval x Factors (accounting for design factors of safety,reliability of inspection methods,material variability) Designed Service Life x Factors(accounting for factors of safety,material variability) Time(repeated loads,environment) FIGURE 7.2.1(b)Repeated load and residual strength requirements for damaged composites. Figure 7.2.1(c)illustrates another important aspect of damage tolerance,which is related to rare acci- dental damage and discrete source impact events that yield relatively large damages.Such damages are typically treated as obvious or assumed to exist when a discrete source event occurs in service that is known to the crew.In both cases,there is no repeated load requirement.The requirements for discrete source damage are defined in aeronautical regulations.There is generally no specific damage size re- quirements for obvious damage,but to be classified as such,it must be detectable without directed in- spection(e.g.,large penetrations or part malfunction).Service databases have shown that such damage does occur and may go undiscovered for a short period of time.As a result,it is good fail-safe design practice to ensure structure is capable of sustaining Limit Load with obvious damage.The analyses and test databases used to meet discrete source damage requirements typically characterize the residual strength curve,which can also be used to meet design criteria for obvious damage.For bonded struc- ture,there are other requirements to ensure fail safety in the case of large debonds(e.g.,FAR 23.573). Such requirements relate to the unreliability of secondary bonding. The range of damages shown in Figures 7.2.1(b)and 7.2.1(c)have traditionally provided a basis for durability and damage tolerance assessments of composite structure.However,complex design details and secondary load paths can also result in damage initiation and significant growth in composites struc- tures.Since these details and load paths are difficult to analyze,the resulting damage initiation and growth are often not identified until large-scale tests of configured structure are conducted.Alternatively, damage growth must either be arrested by design features or be predictable and stable(e.g.,analogous to metal crack growth).In this case,safety is achieved through damage tolerant design and maintenance practices similar to those for metal structures. 7-9

MIL-HDBK-17-3F Volume 3, Chapter 7 - Damage Resistance, Durability, and Damage Tolerance 7-9 Damage Size (Severity) Design Load Requirement Selected manufacturing or service flaws which may go undetected by selected QC or service inspection (e.g., BVID, small delaminations, porosity) Time (repeated loads, environment) Ultimate Limit Selected rogue manufacturing or service flaws which likely will be detected by selected service inspection (e.g., VID, missing fasteners, small penetrations, delaminations) Designed Service Life x Factors (accounting for factors of safety, material variability) Maintenance Inspection Interval x Factors (accounting for design factors of safety, reliability of inspection methods, material variability) FIGURE 7.2.1(b) Repeated load and residual strength requirements for damaged composites. Figure 7.2.1(c) illustrates another important aspect of damage tolerance, which is related to rare acci￾dental damage and discrete source impact events that yield relatively large damages. Such damages are typically treated as obvious or assumed to exist when a discrete source event occurs in service that is known to the crew. In both cases, there is no repeated load requirement. The requirements for discrete source damage are defined in aeronautical regulations. There is generally no specific damage size re￾quirements for obvious damage, but to be classified as such, it must be detectable without directed in￾spection (e.g., large penetrations or part malfunction). Service databases have shown that such damage does occur and may go undiscovered for a short period of time. As a result, it is good fail-safe design practice to ensure structure is capable of sustaining Limit Load with obvious damage. The analyses and test databases used to meet discrete source damage requirements typically characterize the residual strength curve, which can also be used to meet design criteria for obvious damage. For bonded struc￾ture, there are other requirements to ensure fail safety in the case of large debonds (e.g., FAR 23.573). Such requirements relate to the unreliability of secondary bonding. The range of damages shown in Figures 7.2.1(b) and 7.2.1(c) have traditionally provided a basis for durability and damage tolerance assessments of composite structure. However, complex design details and secondary load paths can also result in damage initiation and significant growth in composites struc￾tures. Since these details and load paths are difficult to analyze, the resulting damage initiation and growth are often not identified until large-scale tests of configured structure are conducted. Alternatively, damage growth must either be arrested by design features or be predictable and stable (e.g., analogous to metal crack growth). In this case, safety is achieved through damage tolerant design and maintenance practices similar to those for metal structures

MIL-HDBK-17-3F Volume 3,Chapter 7-Damage Resistance,Durability,and Damage Tolerance Design Load Requirement Large accidental damages and failsafe design considerations treated as obvious damages; Ultimate hence,not requiring repeated loads (e.g..large debonds penetrations) Limit 009 Discrete source damage defined by specified criteria (e.g.,engine rotor burst.birdstrike) for pressure loads 7 Allowable Critical Damage No Repeated Load Damage Limit Threshold (ADL) (CDT) Requirement (residual strength only) Increasing Damage Size(Severity) FIGURE 7.2.1(c)Residual strength requirements for large damage in composite structure. 7.2.2 Methods of compliance to aviation regulations There is a notable difference between military and civil aviation methods of compliance.For military aircraft,the government defines the requirements (Military Specifications)and works with the manufac- turer to establish the method of compliance.The government is also the customer in this instance.In civil aviation,the government defines the requirements through regulations(FAR's,JAR's)and accepted means of compliance through guidance material(Advisory Circulars).Compliance must be demonstrated to the agency(FAA,JAA).In this instance the government is a neutral,third party. This difference in ultimate ownership also influences the attitude the different agencies adopt regard- ing durability.To the extent that durability is an economic issue,it is not generally of concern to civil avia- tion authorities.It is a concern to military agencies because maintainability expenses affect their cost of ownership. The reason why visual inspection methods,rather than a special one(requiring some special tech- niques like ultrasonic pulse echo for instance),is preferred by the aircraft manufacturers and operators for impact damage detection is purely economic.Unlike fatigue cracks in metallic structure that can only be initiated at restricted and easily identifiable areas (where stress raisers and/or corrosion exist)impact damage may occur anywhere on large exposed surfaces,raising the cost of an inspection plan covering the entire surface of the structure. The use of visual methods for initial damage detection results in a more conservative(i.e.,heavier) design than would the use of more stringent inspection methods,since the damage level required for visi- bility is more severe.However,the visual approach results in improved damage tolerance capability, since the structural strength is typically less sensitive to changes in damage severity as damage severity increases.A majority of the compression strength reduction occurs for energy levels below the detectabil- ity threshold that will govern static strength requirements.Then,limited extra strength reductions should be expected for higher energies to be considered for damage tolerance evaluation. 7-10

MIL-HDBK-17-3F Volume 3, Chapter 7 - Damage Resistance, Durability, and Damage Tolerance 7-10 Allowable Damage Limit (ADL) Increasing Damage Size (Severity) Ultimate Design Load Requirement Limit Critical Damage Threshold (CDT) Large accidental damages and failsafe design considerations treated as obvious damages; hence, not requiring repeated loads (e.g., large debonds & penetrations) Discrete source damage defined by specified criteria (e.g., engine rotor burst, birdstrike) * for pressure loads No Repeated Load Requirement (residual strength only) * residual strength to limit load and below) is sparse FIGURE 7.2.1(c) Residual strength requirements for large damage in composite structure. 7.2.2 Methods of compliance to aviation regulations There is a notable difference between military and civil aviation methods of compliance. For military aircraft, the government defines the requirements (Military Specifications) and works with the manufac￾turer to establish the method of compliance. The government is also the customer in this instance. In civil aviation, the government defines the requirements through regulations (FAR’s, JAR’s) and accepted means of compliance through guidance material (Advisory Circulars). Compliance must be demonstrated to the agency (FAA, JAA). In this instance the government is a neutral, third party. This difference in ultimate ownership also influences the attitude the different agencies adopt regard￾ing durability. To the extent that durability is an economic issue, it is not generally of concern to civil avia￾tion authorities. It is a concern to military agencies because maintainability expenses affect their cost of ownership. The reason why visual inspection methods, rather than a special one (requiring some special tech￾niques like ultrasonic pulse echo for instance), is preferred by the aircraft manufacturers and operators for impact damage detection is purely economic. Unlike fatigue cracks in metallic structure that can only be initiated at restricted and easily identifiable areas (where stress raisers and/or corrosion exist) impact damage may occur anywhere on large exposed surfaces, raising the cost of an inspection plan covering the entire surface of the structure. The use of visual methods for initial damage detection results in a more conservative (i.e., heavier) design than would the use of more stringent inspection methods, since the damage level required for visi￾bility is more severe. However, the visual approach results in improved damage tolerance capability, since the structural strength is typically less sensitive to changes in damage severity as damage severity increases. A majority of the compression strength reduction occurs for energy levels below the detectabil￾ity threshold that will govern static strength requirements. Then, limited extra strength reductions should be expected for higher energies to be considered for damage tolerance evaluation