MIL-HDBK-17-3F Volume 3,Chapter 3 Quality Control of Production Materials and Processes CHAPTER 3 QUALITY CONTROL OF PRODUCTION MATERIALS AND PROCESSES 3.1 INTRODUCTION Quality conformance tests are needed to assure the continued integrity of a previously characterized material system.The tests performed must be able to characterize each batch/lot of material so a proper assessment of critical properties of a material system can be made.These critical properties provide in- formation on the integrity of a material system with regard to material properties,fabrication capability, and usage.Additionally,the test matrix must be designed to economically and quickly evaluate a material system. Quality control in a production environment involves inspection and testing of composites in all stages of prepreg manufacture and part fabrication.Tests must be performed by the material supplier on the fi- ber and resin as separate materials,as well as on the composite prepreg material.The user of the pre- preg must perform receiving inspection and revalidation tests,in-process control tests,and nondestructive inspection tests on finished parts.These tests are described in the following sections and normal industry practice is discussed. 3.2 MATERIAL PROCUREMENT QUALITY ASSURANCE PROCEDURES 3.2.1 Specifications and documentation The specification for materials,fabrication processes,and material testing techniques must ensure compliance with the engineering requirements. Chapters 3,4,and 6 in Volume 1 of this handbook describe acceptance test methods for characteriz- ing fiber,matrix,and resin-impregnated fiber materials by their chemical,physical,and mechanical prop- erties.Sections 3.3 and 3.4 of this volume provide information on variable statistical sampling plans that are based on MIL-STD-414 (Reference 3.2.1(a)).These plans control the frequency and extent of mate- rial property verification testing to achieve targeted quality levels. The specifications for destructive and nondestructive test equipment and test methods should contain test and evaluation procedures.These procedures need to describe the means by which the equipment will be calibrated to maintain the required accuracy and repeatability;they should also establish the cali- bration frequency.Information on the standards to be used in the calibration of chemical analysis equip- ment will be found in preceding sections of this handbook which deal with the particular test technique. The standards for quality control documentation requirements are found in military and federal speci- fications such as the Federal Aviation Regulation Part 21 "Certification Procedures for Products and Parts"used by the Federal Aviation Administration production approval holders(Reference 3.2.1(b)). 3.2.2 Receiving inspection The composite material user typically prepares material specifications which define incoming material inspection procedures and supplier controls that ensure the materials used in composite construction will meet the engineering requirements.These specifications are based on material allowables generated by allowables development programs.The acceptance criteria for mechanical tests must be specified to as- sure that production parts will be fabricated with materials that have properties equivalent to the materials used to develop the allowables. The user material specifications typically require the suppliers to provide evidence that each produc- tion lot of material in each shipment meets the material specification requirements.This evidence will in- 3-1
MIL-HDBK-17-3F Volume 3, Chapter 3 Quality Control of Production Materials and Processes 3-1 CHAPTER 3 QUALITY CONTROL OF PRODUCTION MATERIALS AND PROCESSES 3.1 INTRODUCTION Quality conformance tests are needed to assure the continued integrity of a previously characterized material system. The tests performed must be able to characterize each batch/lot of material so a proper assessment of critical properties of a material system can be made. These critical properties provide information on the integrity of a material system with regard to material properties, fabrication capability, and usage. Additionally, the test matrix must be designed to economically and quickly evaluate a material system. Quality control in a production environment involves inspection and testing of composites in all stages of prepreg manufacture and part fabrication. Tests must be performed by the material supplier on the fiber and resin as separate materials, as well as on the composite prepreg material. The user of the prepreg must perform receiving inspection and revalidation tests, in-process control tests, and nondestructive inspection tests on finished parts. These tests are described in the following sections and normal industry practice is discussed. 3.2 MATERIAL PROCUREMENT QUALITY ASSURANCE PROCEDURES 3.2.1 Specifications and documentation The specification for materials, fabrication processes, and material testing techniques must ensure compliance with the engineering requirements. Chapters 3, 4, and 6 in Volume 1 of this handbook describe acceptance test methods for characterizing fiber, matrix, and resin-impregnated fiber materials by their chemical, physical, and mechanical properties. Sections 3.3 and 3.4 of this volume provide information on variable statistical sampling plans that are based on MIL-STD-414 (Reference 3.2.1(a)). These plans control the frequency and extent of material property verification testing to achieve targeted quality levels. The specifications for destructive and nondestructive test equipment and test methods should contain test and evaluation procedures. These procedures need to describe the means by which the equipment will be calibrated to maintain the required accuracy and repeatability; they should also establish the calibration frequency. Information on the standards to be used in the calibration of chemical analysis equipment will be found in preceding sections of this handbook which deal with the particular test technique. The standards for quality control documentation requirements are found in military and federal specifications such as the Federal Aviation Regulation Part 21 "Certification Procedures for Products and Parts" used by the Federal Aviation Administration production approval holders (Reference 3.2.1(b)). 3.2.2 Receiving inspection The composite material user typically prepares material specifications which define incoming material inspection procedures and supplier controls that ensure the materials used in composite construction will meet the engineering requirements. These specifications are based on material allowables generated by allowables development programs. The acceptance criteria for mechanical tests must be specified to assure that production parts will be fabricated with materials that have properties equivalent to the materials used to develop the allowables. The user material specifications typically require the suppliers to provide evidence that each production lot of material in each shipment meets the material specification requirements. This evidence will in-
MIL-HDBK-17-3F Volume 3,Chapter 3 Quality Control of Production Materials and Processes clude test data,certification,affidavits,etc.,depending upon the user quality assurance plan and pur- chase contract requirements for a particular material.The test reports contain data to verify the confor- mance of material properties to user specifications and acceptance standards. Acceptance test requirements may vary from user to user.However,the tests must be sufficient to assure the material will meet or exceed the engineering requirements.A typical example of acceptance tests required for carbon/epoxy unidirectional tape is shown in Table 3.2.2.Note that Table 3.2.2 is di- vided into two parts.The first part concerns uncured prepreg properties.The purpose of these tests is to assure that the resin and fibers materials are within acceptable limits.The second part involves tests on cured laminates or laminae.The mechanical property tests should be selected to reflect important design properties.They can be direct tests of a property or a basic test that correlates with critical design proper- ties.The 90/0 tension test evaluates the fiber strength and modulus.The 90/0 compression test evalu- ates the reinforced fiber/resin combination.The compression testing also includes hot dry tests since one resin-dependent mechanical property should include elevated temperature tests to ensure the material's temperature capability.A shear test should be run as a resin evaluation.The short beam shear test or the t45 tension test should be used depending on the end product's emphasis on interlaminar or in-plane properties. Receiving inspection test requirements should address test frequency and,in the event of initial fail- ure to satisfy these requirements,retest criteria.Test frequency is a function of the quantity of material (weight and rolls)in a batch.Typical testing may include specimens from first,last and random rolls.A retest criteria should be included for the cured lamina tests so that the material is not rejected because of testing anomalies.If a material fails a test,a new panel from the same suspect roll of material should be fabricated and used to rerun that specific test.If a batch has multiple rolls,that test should run on material from the roll before and after the suspect roll in order to isolate the potential problem.If the material fails the retest,the entire batch should be reviewed by material engineering.As use and confidence increase, the receiving inspection procedure can be modified.For example,the test frequency can be decreased or certain tests can be phased out. 3.3 PART FABRICATION VERIFICATION 3.3.1 Process verification The quality assurance department for the user generally has the responsibility for verifying that the fabrication processes are carried out according to engineering process specification requirements.The wide range of activities to control the fabrication process are described below. Material Control:The user process specifications must set the material control for the following items as a minimum. 1.Materials are properly identified by name and specification. 2.Materials are stored and packaged to preclude damage and contamination. 3. Perishable materials,prepregs and adhesives,are within the allowable storage life at the time of release from storage and the allowed work life at time of cure. 4. Prepackaged kits are properly identified and inspected. 5.Acceptance and reverification tests are identified. Materials Storage and Handling:The user material and process specifications set procedures and requirements for storage of prepregs,resin systems and adhesives to maintain acceptable material qual- ity.Storing these materials at low temperatures,usually 0F or below,retards the reaction of the resin materials and extends their useful life.Negotiations between the supplier and user result in an agree- ment on how long the supplier will guarantee the use of these perishable materials when stored under these conditions.This agreed to time is incorporated as one of the requirements in the user material specification. 3-2
MIL-HDBK-17-3F Volume 3, Chapter 3 Quality Control of Production Materials and Processes 3-2 clude test data, certification, affidavits, etc., depending upon the user quality assurance plan and purchase contract requirements for a particular material. The test reports contain data to verify the conformance of material properties to user specifications and acceptance standards. Acceptance test requirements may vary from user to user. However, the tests must be sufficient to assure the material will meet or exceed the engineering requirements. A typical example of acceptance tests required for carbon/epoxy unidirectional tape is shown in Table 3.2.2. Note that Table 3.2.2 is divided into two parts. The first part concerns uncured prepreg properties. The purpose of these tests is to assure that the resin and fibers materials are within acceptable limits. The second part involves tests on cured laminates or laminae. The mechanical property tests should be selected to reflect important design properties. They can be direct tests of a property or a basic test that correlates with critical design properties. The 90°/0° tension test evaluates the fiber strength and modulus. The 90°/0° compression test evaluates the reinforced fiber/resin combination. The compression testing also includes hot dry tests since one resin-dependent mechanical property should include elevated temperature tests to ensure the material's temperature capability . A shear test should be run as a resin evaluation. The short beam shear test or the ±45° tension test should be used depending on the end product's emphasis on interlaminar or in-plane properties. Receiving inspection test requirements should address test frequency and, in the event of initial failure to satisfy these requirements, retest criteria. Test frequency is a function of the quantity of material (weight and rolls) in a batch. Typical testing may include specimens from first, last and random rolls. A retest criteria should be included for the cured lamina tests so that the material is not rejected because of testing anomalies. If a material fails a test, a new panel from the same suspect roll of material should be fabricated and used to rerun that specific test. If a batch has multiple rolls, that test should run on material from the roll before and after the suspect roll in order to isolate the potential problem. If the material fails the retest, the entire batch should be reviewed by material engineering. As use and confidence increase, the receiving inspection procedure can be modified. For example, the test frequency can be decreased or certain tests can be phased out. 3.3 PART FABRICATION VERIFICATION 3.3.1 Process verification The quality assurance department for the user generally has the responsibility for verifying that the fabrication processes are carried out according to engineering process specification requirements. The wide range of activities to control the fabrication process are described below. Material Control: The user process specifications must set the material control for the following items as a minimum. 1. Materials are properly identified by name and specification. 2. Materials are stored and packaged to preclude damage and contamination. 3. Perishable materials, prepregs and adhesives, are within the allowable storage life at the time of release from storage and the allowed work life at time of cure. 4. Prepackaged kits are properly identified and inspected. 5. Acceptance and reverification tests are identified. Materials Storage and Handling: The user material and process specifications set procedures and requirements for storage of prepregs, resin systems and adhesives to maintain acceptable material quality. Storing these materials at low temperatures, usually 0°F or below, retards the reaction of the resin materials and extends their useful life. Negotiations between the supplier and user result in an agreement on how long the supplier will guarantee the use of these perishable materials when stored under these conditions. This agreed to time is incorporated as one of the requirements in the user material specification
MIL-HDBK-17-3F Volume 3,Chapter 3 Quality Control of Production Materials and Processes TABLE 3.2.2 Typical acceptance and revalidation tests required for suppliers and users. TESTING REQUIRED PROPERTY PRODUCTION PRODUCTION REVALIDATION SPECIMENS ACCEPTANCE ACCEPTANCE (USER)(3) REQUIRED (SUPPLIER)(3) (USER)(3) PER SAMPLE Prepreg Properties Visual Dimensional Volatile Content 0 Moisture Content X X 3 Gel Time X X X 3 Resin Flow X X 2 Tack Resin Content X X 0 Fiber Areal Weight X X 3 Infrared Analysis X 1 Liquid Chromatograph 2 Differential Scanning X X 2 Calorimetry Lamina Properties Density 3 Fiber Volume 0 Resin Volume Void Content X 3 Per Ply Thickness X X X 1 Glass Transition Temp X X X 3 SBS or t45°Tension X(2) X(2) X(2) 6 90/0°Compression X(1) X(1) X(2) 6 Strength 90/0°Tension X(2) X(2) X(2) 6 Strength Modulus (1)Tests should be conducted at RT/Ambient and Maximum Temperature/Ambient(See Volume I,Section 2.2.2). (2) Tests should be conducted RT/Ambient. (3)Supplier is defined as the prepreg supplier.User is defined as the composite part fabricator.Production ac- ceptance tests are defined as tests to be performed by the supplier or user for initial acceptance.Revalida- tion tests are tests performed by the user at the end of guaranteed storage life or room temperature out time to provide for additional use of the material after expiration of the normal storage or out time life. 3-3
MIL-HDBK-17-3F Volume 3, Chapter 3 Quality Control of Production Materials and Processes 3-3 TABLE 3.2.2 Typical acceptance and revalidation tests required for suppliers and users. TESTING REQUIRED PROPERTY PRODUCTION ACCEPTANCE (SUPPLIER)(3) PRODUCTION ACCEPTANCE (USER)(3) REVALIDATION (USER)(3) SPECIMENS REQUIRED PER SAMPLE Prepreg Properties Visual & Dimensional X X - Volatile Content X X 3 Moisture Content X X X 3 Gel Time X X X 3 Resin Flow X X X 2 Tack X X X 1 Resin Content X X 3 Fiber Areal Weight X X 3 Infrared Analysis X 1 Liquid Chromatograph X X X 2 Differential Scanning Calorimetry X X X 2 Lamina Properties Density X 3 Fiber Volume X 3 Resin Volume X 3 Void Content X 3 Per Ply Thickness X X X 1 Glass Transition Temp X X X 3 SBS or ±45° Tension X(2) X(2) X(2) 6 90°/0° Compression Strength X(1) X(1) X(2) 6 90°/0° Tension Strength & Modulus X(2) X(2) X(2) 6 (1) Tests should be conducted at RT/Ambient and Maximum Temperature/Ambient (See Volume I, Section 2.2.2). (2) Tests should be conducted RT/Ambient. (3) Supplier is defined as the prepreg supplier. User is defined as the composite part fabricator. Production acceptance tests are defined as tests to be performed by the supplier or user for initial acceptance. Revalidation tests are tests performed by the user at the end of guaranteed storage life or room temperature out time to provide for additional use of the material after expiration of the normal storage or out time life
MIL-HDBK-17-3F Volume 3,Chapter 3 Quality Control of Production Materials and Processes Materials are generally stored in sealed plastic bags or containers to prevent moisture from condens- ing on the cold material and migrating into the polymer when it is removed from the freezer and allowed to warm up to ambient temperature.The time interval between material removal from the freezer and when the material bag or container may be opened is generally empirically determined.Physical characteristics such as material roll,stacking height thickness,or material type (e.g.,tape vs broadgoods)are consid- ered when determining this time interval.Therefore,the user should have procedures that prevent prema- ture removal of materials from storage bags or containers before material temperature stabilization oc- curs. Tooling:The tooling(molds)to be used for lay-up are subject to tool proofing/qualification proce- dures.This demonstrates that the tooling is capable of producing parts that conform to drawing and specification requirements,when used with the specified materials,lay-up and bagging methods,and cure profile.Also,cured material specimens made from the tool should be tested to ensure they meet specified mechanical and physical properties.Tool surfaces must be inspected before each use to en- sure the tool surface is clean and free of conditions which could contaminate or damage a part. Facilities and Equipment:The user will establish requirements to control the composite work area environment.These requirements are a part of the user's process specifications.The requirements should be commensurate with the susceptibility of materials to contamination by the shop environment. Inspection and calibration requirements for autoclaves and ovens must be defined. Contamination restrictions in environmentally-controlled areas typically prohibit the use of uncon- trolled sprays (e.g.,silicon contamination),exposure to dust,handling contamination,fumes,oily vapors, and the presence of other particulate or chemical matter which may affect the manufacturing process. Conditions under which operators may handle materials should also be defined.Lay-up and clean room air filtrations and pressurization systems should be capable of providing a slight positive overpressure. In-Process Control:During lay-up of composite parts,certain critical steps or operations must be closely controlled.Requirements and limits for these critical items are stated in the user process specifi- cations.Some of the steps and operations to be controlled are listed below: 1.Verification that the release agent has been applied and cured on a clean tool surface. 2.Verification that perishable materials incorporated into the part comply with the applicable mate- rial specifications. 3.Inspection of prepreg lay-ups to assure engineering drawing requirements for number of plies and orientation are met. 4.Inspection of honeycomb core installation,if applicable,and verification that positioning meets the engineering drawing requirements. 5.The user paperwork should contain the following information. a.Material supplier,date of manufacturer,batch number,roll number,and total accumulated hours of working life. b.Autoclave or oven pressure,part temperatures,and times. c.Autoclave or oven load number. d.Part and serial number. Part Cure:Requirements must be defined in user process specifications for the operating parame- ters for autoclaves and ovens used for curing parts.These include heat rise rates,times at temperature, 3-4
MIL-HDBK-17-3F Volume 3, Chapter 3 Quality Control of Production Materials and Processes 3-4 Materials are generally stored in sealed plastic bags or containers to prevent moisture from condensing on the cold material and migrating into the polymer when it is removed from the freezer and allowed to warm up to ambient temperature. The time interval between material removal from the freezer and when the material bag or container may be opened is generally empirically determined. Physical characteristics such as material roll, stacking height thickness, or material type (e.g., tape vs broadgoods) are considered when determining this time interval. Therefore, the user should have procedures that prevent premature removal of materials from storage bags or containers before material temperature stabilization occurs. Tooling: The tooling (molds) to be used for lay-up are subject to tool proofing/qualification procedures. This demonstrates that the tooling is capable of producing parts that conform to drawing and specification requirements, when used with the specified materials, lay-up and bagging methods, and cure profile. Also, cured material specimens made from the tool should be tested to ensure they meet specified mechanical and physical properties. Tool surfaces must be inspected before each use to ensure the tool surface is clean and free of conditions which could contaminate or damage a part. Facilities and Equipment: The user will establish requirements to control the composite work area environment. These requirements are a part of the user's process specifications. The requirements should be commensurate with the susceptibility of materials to contamination by the shop environment. Inspection and calibration requirements for autoclaves and ovens must be defined. Contamination restrictions in environmentally-controlled areas typically prohibit the use of uncontrolled sprays (e.g., silicon contamination), exposure to dust, handling contamination, fumes, oily vapors, and the presence of other particulate or chemical matter which may affect the manufacturing process. Conditions under which operators may handle materials should also be defined. Lay-up and clean room air filtrations and pressurization systems should be capable of providing a slight positive overpressure. In-Process Control: During lay-up of composite parts, certain critical steps or operations must be closely controlled. Requirements and limits for these critical items are stated in the user process specifications. Some of the steps and operations to be controlled are listed below: 1. Verification that the release agent has been applied and cured on a clean tool surface. 2. Verification that perishable materials incorporated into the part comply with the applicable material specifications. 3. Inspection of prepreg lay-ups to assure engineering drawing requirements for number of plies and orientation are met. 4. Inspection of honeycomb core installation, if applicable, and verification that positioning meets the engineering drawing requirements. 5. The user paperwork should contain the following information. a. Material supplier, date of manufacturer, batch number, roll number, and total accumulated hours of working life. b. Autoclave or oven pressure, part temperatures, and times. c. Autoclave or oven load number. d. Part and serial number. Part Cure: Requirements must be defined in user process specifications for the operating parameters for autoclaves and ovens used for curing parts. These include heat rise rates, times at temperature
MIL-HDBK-17-3F Volume 3,Chapter 3 Quality Control of Production Materials and Processes cool-down rates,temperature and pressure tolerances,and temperature uniformity surveys in the auto- clave or ovens. Process Control Specimens:Many manufacturers require special test panels to be laid up and cured along with production parts.After cure,these panels are tested for physical and mechanical prop- erties to verify the parts they represent meet the engineering properties. The requirements for physical and mechanical testing are frequently defined by drawing notes which designates a type or class for each part.Non-critical or secondary structure may require no test speci- mens and no testing.Critical or safety-of-flight parts may require complete physical and mechanical test- ing. During early composite material production,most users required tests for 0 flexure strength and modulus and short beam shear strength.However,in recent years these tests have been changed by many manufacturers to require glass transition temperature,per ply thickness,fiber volume,void content, and ply count on samples taken from designated areas on the production part. 3.3.2 Nondestructive inspection Having assured in-process control,the detail composite parts must also be inspected for confor- mance to dimensional and workmanship requirements and nondestructively inspected for processing- induced defects and damage. Assembly Inspection:Laminates are prone to particular types of defects unless they are machined and drilled properly.Workmanship standards,required by manufacturer's process specifications,are needed to control the quality of trimmed edges and drilled holes.These standards establish visual accep- tance/rejection limits for the following typical defects:splintering,delamination,loose surface fibers, overheating,surface finish,off-axis holes,and surface cratering.Typical defects in the drilling operations are delaminations and broken fibers which start at the hole boundary.Since these defects are internal in nature,an evaluation of the seriousness of the flaws is not possible by visual inspection alone.It should be backed up by nondestructive inspection techniques.Internal defect acceptance and rejection limits must be established for nondestructive inspection. The extent of nondestructive(NDI)inspection on composite parts is dependent on whether the parts are primary structure,safety-of-flight or secondary structure,non-safety-of-flight.The type or class of part is usually defined on the engineering drawing.The engineering drawing also references a process speci- fication which defines the NDI tests and the accept/reject criteria.The NDI tests are used to find flaws and damage such as voids,delaminations,inclusions,and micro-cracks in the matrix. NDI techniques commonly used in production include visual,ultrasonic and X-ray inspection.Other methods,such as infrared,holographic,and acoustic inspection are being developed and may be used in production applications in the future. Visual inspection is an NDI technique involving checks to assure the parts meet drawing requirements and to evaluate the surface and appearance of the part.The inspection includes examination for blisters, depressions,foreign material inclusions,ply distortions and folds,surface roughness,surface porosity, and wrinkles.Accept/reject criteria for such defects are given in the manufacturer's process specifica- tions. The most widely used nondestructive inspection technique for composites production is ultrasonic thru-transmission C-scan inspection,followed by ultrasonic pulse echo A-scan inspection.Since the sub- ject is so broad,the engineering requirements and criteria are usually contained in a document that is ref- erenced in the user's process specification.The principal defects evaluated by ultrasonics are internal voids,delaminations,and porosity.These inspections require fabrication of standards with built-in known defects.The output is in the form of charts which shows the sound attenuation variations over the entire part.The charts are compared to the part to show the locations of the sound attenuation variations.If defects are found outside the limits allowed by the specification,the parts are rejected and dispositioned 3-5
MIL-HDBK-17-3F Volume 3, Chapter 3 Quality Control of Production Materials and Processes 3-5 cool-down rates, temperature and pressure tolerances, and temperature uniformity surveys in the autoclave or ovens. Process Control Specimens: Many manufacturers require special test panels to be laid up and cured along with production parts. After cure, these panels are tested for physical and mechanical properties to verify the parts they represent meet the engineering properties. The requirements for physical and mechanical testing are frequently defined by drawing notes which designates a type or class for each part. Non-critical or secondary structure may require no test specimens and no testing. Critical or safety-of-flight parts may require complete physical and mechanical testing. During early composite material production, most users required tests for 0° flexure strength and modulus and short beam shear strength. However, in recent years these tests have been changed by many manufacturers to require glass transition temperature, per ply thickness, fiber volume, void content, and ply count on samples taken from designated areas on the production part. 3.3.2 Nondestructive inspection Having assured in-process control, the detail composite parts must also be inspected for conformance to dimensional and workmanship requirements and nondestructively inspected for processinginduced defects and damage. Assembly Inspection: Laminates are prone to particular types of defects unless they are machined and drilled properly. Workmanship standards, required by manufacturer's process specifications, are needed to control the quality of trimmed edges and drilled holes. These standards establish visual acceptance/rejection limits for the following typical defects: splintering, delamination, loose surface fibers, overheating, surface finish, off-axis holes, and surface cratering. Typical defects in the drilling operations are delaminations and broken fibers which start at the hole boundary. Since these defects are internal in nature, an evaluation of the seriousness of the flaws is not possible by visual inspection alone. It should be backed up by nondestructive inspection techniques. Internal defect acceptance and rejection limits must be established for nondestructive inspection. The extent of nondestructive (NDI) inspection on composite parts is dependent on whether the parts are primary structure, safety-of-flight or secondary structure, non-safety-of-flight. The type or class of part is usually defined on the engineering drawing. The engineering drawing also references a process specification which defines the NDI tests and the accept/reject criteria. The NDI tests are used to find flaws and damage such as voids, delaminations, inclusions, and micro-cracks in the matrix. NDI techniques commonly used in production include visual, ultrasonic and X-ray inspection. Other methods, such as infrared, holographic, and acoustic inspection are being developed and may be used in production applications in the future. Visual inspection is an NDI technique involving checks to assure the parts meet drawing requirements and to evaluate the surface and appearance of the part. The inspection includes examination for blisters, depressions, foreign material inclusions, ply distortions and folds, surface roughness, surface porosity, and wrinkles. Accept/reject criteria for such defects are given in the manufacturer's process specifications. The most widely used nondestructive inspection technique for composites production is ultrasonic thru-transmission C-scan inspection, followed by ultrasonic pulse echo A-scan inspection. Since the subject is so broad, the engineering requirements and criteria are usually contained in a document that is referenced in the user's process specification. The principal defects evaluated by ultrasonics are internal voids, delaminations, and porosity. These inspections require fabrication of standards with built-in known defects. The output is in the form of charts which shows the sound attenuation variations over the entire part. The charts are compared to the part to show the locations of the sound attenuation variations. If defects are found outside the limits allowed by the specification, the parts are rejected and dispositioned
MIL-HDBK-17-3F Volume 3,Chapter 3 Quality Control of Production Materials and Processes by Engineering.Parts may be dispositioned 1)acceptable as is,2)subjected to further rework or repair to make the part acceptable or 3).scrapped. X-ray inspection is frequently used in NDI testing to evaluate bonding of inserts in laminate panels and honeycomb core to facesheet bonds in sandwich panels.The extent of testing required is designated on the engineering drawing by type or class of inspection.The type or class is usually defined in a sepa- rate document that is referenced in the manufacturer's process specification.As with ultrasonic inspec- tion,standards with built-in defects are usually required to evaluate the radiographic film properly. 3.3.3 Destructive tests 3.3.3.1 Background Destructive tests are often used to ensure the structural integrity of a component whenever assurance cannot be gained by nondestructive techniques alone.These tests include periodic dissection of the part to examine the interior of complex structures and mechanical testing of specimens cut from excess parts of the component(Figure 3.3.3.1). Quality Assurance of Composite Parts Adequate NDI Details not adequately inspected by NDI Low Experience Higher Experience Net Trim Trim Areas No Destructive Similar Parts Testing Required First Article Full Part Trim Sections Dissection Increased Experience Critical Parts Micrographs Micrographs Mechanical Ply Verification Ply Verification Tests (Process Control) FIGURE 3.3.3.1 Use of destructive tests. 3.3.3.2 Usage Destructive tests are often used to ensure the structural integrity of a component whenever assurance cannot be gained by nondestructive techniques alone.These tests include periodic dissection of the part to examine the interior of complex structures and mechanical testing of coupons cut from excess parts of the component. 3-6
MIL-HDBK-17-3F Volume 3, Chapter 3 Quality Control of Production Materials and Processes 3-6 by Engineering. Parts may be dispositioned 1) acceptable as is, 2) subjected to further rework or repair to make the part acceptable or 3), scrapped. X-ray inspection is frequently used in NDI testing to evaluate bonding of inserts in laminate panels and honeycomb core to facesheet bonds in sandwich panels. The extent of testing required is designated on the engineering drawing by type or class of inspection. The type or class is usually defined in a separate document that is referenced in the manufacturer's process specification. As with ultrasonic inspection, standards with built-in defects are usually required to evaluate the radiographic film properly. 3.3.3 Destructive tests 3.3.3.1 Background Destructive tests are often used to ensure the structural integrity of a component whenever assurance cannot be gained by nondestructive techniques alone. These tests include periodic dissection of the part to examine the interior of complex structures and mechanical testing of specimens cut from excess parts of the component (Figure 3.3.3.1). FIGURE 3.3.3.1 Use of destructive tests. 3.3.3.2 Usage Destructive tests are often used to ensure the structural integrity of a component whenever assurance cannot be gained by nondestructive techniques alone. These tests include periodic dissection of the part to examine the interior of complex structures and mechanical testing of coupons cut from excess parts of the component
MIL-HDBK-17-3F Volume 3.Chapter 3 Quality Control of Production Materials and Processes 3.3.3.3 Destructive test approaches There are two primary categories of destructive tests:dissection of the full part or examination of trim sections of the part.Full dissection,generally done for the first part from a new tool,gives a complete ex- amination of the part,but is expensive to perform.Examination of excess trim sections is the preferable approach whenever possible.The part is not destroyed,structural details can still be examined and me- chanical test specimens can be obtained. Full Part Dissection:Full part dissection is the approach often envisioned when the term "destructive testing"is mentioned.Since it prevents future use of the part,full part dissection should be reserved for parts that meet the following criteria: Areas cannot be adequately inspected by NDI Part is complex and there is a low experience level for working with the structural configuration or fabrication process Part is net trim;detail areas of interest cannot be examined using excess trim areas or part ex- tensions. Trim Sections:Examination and testing of trim sections offers a balance of quality assurance and cost. Trim sections can be part extensions that are intentionally designed to go beyond the trim line or can be taken from cutout areas inside the part.Section cuts from detail areas can be examined for discrepancies Test coupons can be machined from the sections and mechanically tested to ensure the structural capa- bility of the part and verify the quality of the fabrication process.Using coupons in this way can satisfy destructive testing requirements and process control requirements(Ref.Section 3.2.2). 3.3.3.4 Implementation guidelines The frequency of destructive tests are dependent on part type and experience.If the producer has significant fabrication experience,complex parts may not require periodic destructive testing,but only a first article dissection.For low experience with complex parts,periodic inspection with increasing intervals may be preferable.Critical(safety of flight)parts warrant consideration for destructive testing. Examination and testing of trim sections can be carried out on a more frequent basis and at less cost than full part dissection.Quality assurance can be enhanced by using more frequent and less elaborate trim section examinations. Destructive tests should be conducted before the part leaves the factory.Periodic destructive tests monitor the manufacturing processes to assure the quality of parts.If a problem does occur,the periodic inspections bracket the number of suspect parts.Not every part series needs to be examined.If many parts reflect the same type of configurations and complexity,they can be pooled together for sampling purposes.Parts made on tools fabricated from one master splash can also be grouped together. Sampling:A typical sampling plan might include first article full part dissection followed by periodic in- spections employing dissection of trim sections.The periodic inspection intervals can vary depending on success rate.After a few successful destructive tests,the interval can be increased.If nonconforming ar- eas are found in destructive tests,the inspection interval can be tightened up.If problems are found in service,additional components from the same production series can be dissected to assure that the prob- lem was isolated. For the trim section approach,periodic destructive tests can be conducted at smaller intervals since the cost is much less.Small intervals may be especially desirable in the case of critical parts. For first article inspection,one of the first few articles may be chosen to represent first article.Some of the reasons for not stipulating the very first structure built are:(1)it may not be as representative of the production run because of lessons learned and special handling;and(2)another part with processing problems or discrepancies may reveal far more information. 3-7
MIL-HDBK-17-3F Volume 3, Chapter 3 Quality Control of Production Materials and Processes 3-7 3.3.3.3 Destructive test approaches There are two primary categories of destructive tests: dissection of the full part or examination of trim sections of the part. Full dissection, generally done for the first part from a new tool, gives a complete examination of the part, but is expensive to perform. Examination of excess trim sections is the preferable approach whenever possible. The part is not destroyed, structural details can still be examined and mechanical test specimens can be obtained. Full Part Dissection: Full part dissection is the approach often envisioned when the term "destructive testing" is mentioned. Since it prevents future use of the part, full part dissection should be reserved for parts that meet the following criteria: • Areas cannot be adequately inspected by NDI • Part is complex and there is a low experience level for working with the structural configuration or fabrication process • Part is net trim; detail areas of interest cannot be examined using excess trim areas or part extensions. Trim Sections: Examination and testing of trim sections offers a balance of quality assurance and cost. Trim sections can be part extensions that are intentionally designed to go beyond the trim line or can be taken from cutout areas inside the part. Section cuts from detail areas can be examined for discrepancies. Test coupons can be machined from the sections and mechanically tested to ensure the structural capability of the part and verify the quality of the fabrication process. Using coupons in this way can satisfy destructive testing requirements and process control requirements (Ref. Section 3.2.2). 3.3.3.4 Implementation guidelines The frequency of destructive tests are dependent on part type and experience. If the producer has significant fabrication experience, complex parts may not require periodic destructive testing, but only a first article dissection. For low experience with complex parts, periodic inspection with increasing intervals may be preferable. Critical (safety of flight) parts warrant consideration for destructive testing. Examination and testing of trim sections can be carried out on a more frequent basis and at less cost than full part dissection. Quality assurance can be enhanced by using more frequent and less elaborate trim section examinations. Destructive tests should be conducted before the part leaves the factory. Periodic destructive tests monitor the manufacturing processes to assure the quality of parts. If a problem does occur, the periodic inspections bracket the number of suspect parts. Not every part series needs to be examined. If many parts reflect the same type of configurations and complexity, they can be pooled together for sampling purposes. Parts made on tools fabricated from one master splash can also be grouped together. Sampling: A typical sampling plan might include first article full part dissection followed by periodic inspections employing dissection of trim sections. The periodic inspection intervals can vary depending on success rate. After a few successful destructive tests, the interval can be increased. If nonconforming areas are found in destructive tests, the inspection interval can be tightened up. If problems are found in service, additional components from the same production series can be dissected to assure that the problem was isolated. For the trim section approach, periodic destructive tests can be conducted at smaller intervals since the cost is much less. Small intervals may be especially desirable in the case of critical parts. For first article inspection, one of the first few articles may be chosen to represent first article. Some of the reasons for not stipulating the very first structure built are: (1) it may not be as representative of the production run because of lessons learned and special handling; and (2) another part with processing problems or discrepancies may reveal far more information
MIL-HDBK-17-3F Volume 3,Chapter 3 Quality Control of Production Materials and Processes Potential areas:Potential areas and items to examine include: Primary load paths within the part, Areas that showed indications from non-destructive inspection, Tool markoff near cocured details, Ply drop offs at a taper, Ply wrinkles, Resin starved and resin rich areas, Corner radii and cocured details, Core to face sheet fillets, Tapered core areas. 3.3.3.5 Test types Both full part dissection and trim sections involve examination of detail areas.After machining the de- tail areas,photomicrographs can be obtained to examine the microstructure.Another type of destructive testing is ply verification.Only a small section is need to perform a deply or grind down to verify that the plies are laid up in the correct stacking sequence and orientation.For machine lay-up,this procedure should not be necessary after initial validation.To investigate items such as ply lay-up,potential ply wrin- kles and porosity,initial core plugs can be taken at fastener hole locations and photomicrographs can be developed. When mechanically testing specimens that were machined from trim sections,the coupons should be tested for the critical failure mode for that part or that area of the part.Tests addressing typical failure modes are unnotched compression,open hole compression and interlaminar tension and shear. 3.4 STATISTICAL PROCESS CONTROL 3.4.1 Introduction Since composites exhibit a strong capacity for variability,the tools used to identify,assess,and hope- fully control variability become critical.Statistical process control is a term used to tie together several different aspects of statistical and other quality methods. 3.4.2 Quality tools There are several methods which form the bulk of SPC efforts.They range from fairly simple method- ologies for gathering and evaluating data,to sophisticated statistical techniques for answering very spe- cific questions.What is described in the following sections should not be construed as a comprehensive evaluation.There are many other techniques,or variants on the techniques discussed,which can be re- viewed in the literature. 3.4.3 Gathering and plotting data One of the first concepts in evaluating data is to collect them in a rigorous manner.Once the data have been gathered,the data should almost always be plotted in some fashion.It can be very difficult to discern even moderate trends in tabular data.This can be true with even just a handful of data points.In many cases,the same data can and should be plotted in several different manners,looking for patterns and relationships between factors.or trends over time. 3.4.4 Control charts One of the specific ways that data can be plotted is as a part of a control chart.With control charts, variability in a process output is measured.The sources of variation are partitioned into chance or com- 3-8
MIL-HDBK-17-3F Volume 3, Chapter 3 Quality Control of Production Materials and Processes 3-8 Potential areas: Potential areas and items to examine include: Primary load paths within the part, Areas that showed indications from non-destructive inspection, Tool markoff near cocured details, Ply drop offs at a taper, Ply wrinkles, Resin starved and resin rich areas, Corner radii and cocured details, Core to face sheet fillets, Tapered core areas. 3.3.3.5 Test types Both full part dissection and trim sections involve examination of detail areas. After machining the detail areas, photomicrographs can be obtained to examine the microstructure. Another type of destructive testing is ply verification. Only a small section is need to perform a deply or grind down to verify that the plies are laid up in the correct stacking sequence and orientation. For machine lay-up, this procedure should not be necessary after initial validation. To investigate items such as ply lay-up, potential ply wrinkles and porosity, initial core plugs can be taken at fastener hole locations and photomicrographs can be developed. When mechanically testing specimens that were machined from trim sections, the coupons should be tested for the critical failure mode for that part or that area of the part. Tests addressing typical failure modes are unnotched compression, open hole compression and interlaminar tension and shear. 3.4 STATISTICAL PROCESS CONTROL 3.4.1 Introduction Since composites exhibit a strong capacity for variability, the tools used to identify, assess, and hopefully control variability become critical. Statistical process control is a term used to tie together several different aspects of statistical and other quality methods. 3.4.2 Quality tools There are several methods which form the bulk of SPC efforts. They range from fairly simple methodologies for gathering and evaluating data, to sophisticated statistical techniques for answering very specific questions. What is described in the following sections should not be construed as a comprehensive evaluation. There are many other techniques, or variants on the techniques discussed, which can be reviewed in the literature. 3.4.3 Gathering and plotting data One of the first concepts in evaluating data is to collect them in a rigorous manner. Once the data have been gathered, the data should almost always be plotted in some fashion. It can be very difficult to discern even moderate trends in tabular data. This can be true with even just a handful of data points. In many cases, the same data can and should be plotted in several different manners, looking for patterns and relationships between factors, or trends over time. 3.4.4 Control charts One of the specific ways that data can be plotted is as a part of a control chart. With control charts, variability in a process output is measured. The sources of variation are partitioned into chance or com-
MIL-HDBK-17-3F Volume 3,Chapter 3 Quality Control of Production Materials and Processes mon cause,and assignable variation.Data are plotted as it is generated by a process,and a simple set of rules can be used to determine if an assignable cause should be pursued.With proper application, issues can be identified and addressed prior to reaching rejectable levels. 3.4.5 Process capability A fundamental question for a manufacturing process is given the variability present,what percentage of product would meet specification requirements.Numbers representing this concept are termed meas- ures of process capability.The process variability,represented by the standard deviation,is used to es- tablish tolerance limits which describe where almost all of the product should fall.For one measure of process capability the range between these limits is compared to the specification range. The lower the quantity of product produced outside the specification limits,the more capable the process.Various ratios can be used to assess process capability.An important issue is whether the proc- ess mean is centered between the specification limits,and the implications if it is not. 3.4.6 Troubleshooting and improvement Many times a new process requires characterization and development,or improvements become necessary for an established process.A process that was once in control may not be any longer for rea- sons which are not well understood.In situations such as these,tools for troubleshooting established processes,and making improvements to new or established process become valuable.Three common methods are described below. 3.4.6.1 Process feedback adjustment Introduction Process control is achieved through both process monitoring and adjustment.Process monitoring is accomplished through Statistical Process Control(SPC).including tools such as process control(or She- whart)charts and cumulative sum(Cusum)charts.These are used to interrogate the process or system to determine its stability.Process adjustment is used to bring a process back from drifting and is usually termed Engineering Process Control(EPC).SPC and EPC do not compete but can work together. They can be adapted for environments where an appreciable cost is associated with making a change to the system or taking the measurement.These look at minimizing the overall cost of controlling the system using also the cost of being off the process target.EPC can also implement bounded adjust- ment charts that will dictate both the necessity and magnitude for an adjustment to the process.Finally, the monitoring of a process that is undergoing feedback control is covered. The stable,stationary state,which is the environment under which traditional Statistical Process Con- trol(SPC)is supposed to take place,is actually very difficult to attain and maintain.While the more famil- iar technique of process monitoring through the use of control charts can help achieve this control,fre- quently processes require adjustment of parameters to attain the desired output.While some of the tools and procedures are similar to those for process monitoring.the intent and approach is actually quite dif- ferent. Process monitoring is defined as the use of control charts that are used to continuously interrogate the stability of the process being investigated.When unusual behavior is detected,assignable causes as the source of the behavior are searched for,and if possible,eliminated.This technique has been widely used in the standard parts industry as SPC. Process adjustment utilizes feedback control of some variable related to the desired output in order to keep the process as close as possible to a desired target.The origins of this procedure are in the process industry,which is termed Engineering Process Control(EPC). 3-9
MIL-HDBK-17-3F Volume 3, Chapter 3 Quality Control of Production Materials and Processes 3-9 mon cause, and assignable variation. Data are plotted as it is generated by a process, and a simple set of rules can be used to determine if an assignable cause should be pursued. With proper application, issues can be identified and addressed prior to reaching rejectable levels. 3.4.5 Process capability A fundamental question for a manufacturing process is given the variability present, what percentage of product would meet specification requirements. Numbers representing this concept are termed measures of process capability. The process variability, represented by the standard deviation, is used to establish tolerance limits which describe where almost all of the product should fall. For one measure of process capability the range between these limits is compared to the specification range. The lower the quantity of product produced outside the specification limits, the more capable the process. Various ratios can be used to assess process capability. An important issue is whether the process mean is centered between the specification limits, and the implications if it is not. 3.4.6 Troubleshooting and improvement Many times a new process requires characterization and development, or improvements become necessary for an established process. A process that was once in control may not be any longer for reasons which are not well understood. In situations such as these, tools for troubleshooting established processes, and making improvements to new or established process become valuable. Three common methods are described below. 3.4.6.1 Process feedback adjustment Introduction Process control is achieved through both process monitoring and adjustment. Process monitoring is accomplished through Statistical Process Control (SPC), including tools such as process control (or Shewhart) charts and cumulative sum (Cusum) charts. These are used to interrogate the process or system to determine its stability. Process adjustment is used to bring a process back from drifting and is usually termed Engineering Process Control (EPC). SPC and EPC do not compete but can work together. They can be adapted for environments where an appreciable cost is associated with making a change to the system or taking the measurement. These look at minimizing the overall cost of controlling the system using also the cost of being off the process target. EPC can also implement bounded adjustment charts that will dictate both the necessity and magnitude for an adjustment to the process. Finally, the monitoring of a process that is undergoing feedback control is covered. The stable, stationary state, which is the environment under which traditional Statistical Process Control (SPC) is supposed to take place, is actually very difficult to attain and maintain. While the more familiar technique of process monitoring through the use of control charts can help achieve this control, frequently processes require adjustment of parameters to attain the desired output. While some of the tools and procedures are similar to those for process monitoring, the intent and approach is actually quite different. Process monitoring is defined as the use of control charts that are used to continuously interrogate the stability of the process being investigated. When unusual behavior is detected, assignable causes as the source of the behavior are searched for, and if possible, eliminated. This technique has been widely used in the standard parts industry as SPC. Process adjustment utilizes feedback control of some variable related to the desired output in order to keep the process as close as possible to a desired target. The origins of this procedure are in the process industry, which is termed Engineering Process Control (EPC)
MIL-HDBK-17-3F Volume 3.Chapter 3 Quality Control of Production Materials and Processes Control Charts Control charts that are used to observe frequencies and proportions are covered in the Section 3.4.4. Different types of charts are used for monitoring of measurement data.These look at a sample aver- age and range and are known as X bar and R charts.Some of the useful simplifications used for fre- quency and proportion data are not applicable for measurement type data.The same general terminol- ogy is used except as noted. Several rules,some of which are industry or even company specific,can be applied to these control charts.The most widely known set of rules is the Western Electric rules that are applied to the control chart to determine if a deviation warrants the search for an assignable cause to be eliminated. There are assumptions made for the application of these control charts.While some mild violation of these assumptions is not usually catastrophic,an unstable system can result in inappropriate warning and action limits Process Adjustment In the process regulation the object is not to test hypotheses about the likelihood of a set of data indi- cating special cause,but rather statistical estimation of a disturbance to the system which is then com- pensated for in various manners. As an indication of the differences between the objectives between process monitoring and process adjustment,waiting to implement a process adjustment until the process monitoring indicates a statisti- cally significant deviation as a control strategy would usually lead to excessive process output variation. For many processes acceptable control may not be achievable without process adjustment at some interval.These adjustments must not be made in an arbitrary fashion for consistent results.Important concepts in implementing process adjustment are the processes resistance to change,termed inertia, and the use of models to predict the future output of the process. A unit change in the adjustment variable will not most likely result in a unit change of the process out- put.The relationship between these factors is termed the system gain.In attempting to predict the output of the process,it is useful the split the response in the categories of the white noise,and the system drift. It is important to note that many processes,if left alone,will continuously drift away from a target value.Because of this drift,a low value is more likely to be followed by another low value,termed auto- correlation.These changes may be in the form of step changes,spikes,or changes in slope.With proc- ess adjustment,it is attempted to estimate the direction of the process,and then adjust the process, compensating to keep the process directed toward the target value. The types of disturbances that induce this drift can be environmental changes such as temperature and humidity,or changes in the composition of the input materials.Whether or not these variable have been identified,some sort of feedback control may be necessary to compensate for their effect,allowing the process output to return to the target value.These feedback adjustment procedures have a direct re- lation to the types of automatic control methods used in the process industry. While some sort of alteration of the process based on SPC input is frequently employed,a consistent methodology is seldom implemented.By using some sort of feedback adjustment scheme,the variation in the process output can be reduced several-fold in many applications. What has allowed feedback adjustment the opportunity for more widespread application outside the traditional chemical process industries is the determination that substantial errors in modeling the system, and even significant errors in applying the feedback adjustment result in minimal effects on the process output variability. 3-10
MIL-HDBK-17-3F Volume 3, Chapter 3 Quality Control of Production Materials and Processes 3-10 Control Charts Control charts that are used to observe frequencies and proportions are covered in the Section 3.4.4. Different types of charts are used for monitoring of measurement data. These look at a sample average and range and are known as X bar and R charts. Some of the useful simplifications used for frequency and proportion data are not applicable for measurement type data. The same general terminology is used except as noted. Several rules, some of which are industry or even company specific, can be applied to these control charts. The most widely known set of rules is the Western Electric rules that are applied to the control chart to determine if a deviation warrants the search for an assignable cause to be eliminated. There are assumptions made for the application of these control charts. While some mild violation of these assumptions is not usually catastrophic, an unstable system can result in inappropriate warning and action limits. Process Adjustment In the process regulation the object is not to test hypotheses about the likelihood of a set of data indicating special cause, but rather statistical estimation of a disturbance to the system which is then compensated for in various manners. As an indication of the differences between the objectives between process monitoring and process adjustment, waiting to implement a process adjustment until the process monitoring indicates a statistically significant deviation as a control strategy would usually lead to excessive process output variation. For many processes acceptable control may not be achievable without process adjustment at some interval. These adjustments must not be made in an arbitrary fashion for consistent results. Important concepts in implementing process adjustment are the processes resistance to change, termed inertia, and the use of models to predict the future output of the process. A unit change in the adjustment variable will not most likely result in a unit change of the process output. The relationship between these factors is termed the system gain. In attempting to predict the output of the process, it is useful the split the response in the categories of the white noise, and the system drift. It is important to note that many processes, if left alone, will continuously drift away from a target value. Because of this drift, a low value is more likely to be followed by another low value, termed autocorrelation. These changes may be in the form of step changes, spikes, or changes in slope. With process adjustment, it is attempted to estimate the direction of the process, and then adjust the process, compensating to keep the process directed toward the target value. The types of disturbances that induce this drift can be environmental changes such as temperature and humidity, or changes in the composition of the input materials. Whether or not these variable have been identified, some sort of feedback control may be necessary to compensate for their effect, allowing the process output to return to the target value. These feedback adjustment procedures have a direct relation to the types of automatic control methods used in the process industry. While some sort of alteration of the process based on SPC input is frequently employed, a consistent methodology is seldom implemented. By using some sort of feedback adjustment scheme, the variation in the process output can be reduced several-fold in many applications. What has allowed feedback adjustment the opportunity for more widespread application outside the traditional chemical process industries is the determination that substantial errors in modeling the system, and even significant errors in applying the feedback adjustment result in minimal effects on the process output variability