MIL-HDBK-17-3F Volume 3,Chapter 8 Supportability ume 1 include material and structural testing,material types and properties,and joint types;in Volume 3 include materials and processes,quality,design,joints,reliability,and lessons learned needed to supple- ment those decisions that influence supportability. 8.2 DESIGN FOR SUPPORTABILITY 8.2.1 In-service experience The first step toward designing reliable and cost-effective design details is to understand the history of composite structure.Composite materials,as we know them today,were introduced into the commercial aircraft industry during the early 1960's and used mostly glass fiber.Development of more advanced fi- bers such as boron,aramid,and carbon offered the possibility of increased strength,reduced weight,im- proved corrosion resistance,and greater fatigue resistance than aluminum.These new material systems, commonly referred to as advanced composites,were introduced to the industry very gradually and cau- tiously to ensure their capabilities. The early success of the first simple components,such as wing spoilers and fairings,led to the use of advanced composites in more complex components such as ailerons,flaps,nacelles,and rudders.The increased specific stiffness and strengths of composites over aluminum,coupled with weight-driven re- quirements caused by fuel shortages,led to the application of thin-skin sandwich structures.Long-term durability requirements of the original aluminum parts were not fully accounted for when these composite parts were originally designed.To compound the problem further,damage phenomena such as delamination and microcracking were new and complex in comparison to traditional aluminum structure. The original composite parts,particularly thin-gage sandwich panels,experienced durability problems that could be grouped into three categories:low resistance to impact,liquid ingression,and erosion. These parts were either control panels or secondary structure,such as fixed trailing edge panels,and given the emphasis placed on weight and performance,the face sheets of honeycomb sandwich parts were often only three plies or less with a TedlarTM film.This approach was adequate for stiffness and strength,but never considered the service environment where parts are crawled over,tools dropped,and where service personnel are often unaware of the fragility of thin-skinned sandwich parts.Damages to these components,such as core crush,impact damages and disbonds,are quite often easily detected with a visual inspection due to their thin face sheets.However,sometimes they are overlooked,or dam- aged by service personnel,who do not want to delay aircraft departure or bring attention to their acci- dents,which might reflect poorly on their performance record.Therefore,damages are sometimes al- lowed to go unchecked,often resulting in growth of the damage due to liquid ingression into the core. Non-durable design details (e.g.,improper core edge close-outs)also led to liquid ingression. The repair of parts due to liquid ingression can vary depending upon the liquid,of which water and Skydrol(hydraulic fluid)are the two most common.Water tends to create additional damage in repaired parts when cured unless all moisture is removed from the part.Most repair material systems cure at temperatures above the boiling point of water,which can cause a disbond at the skin-to-core interface wherever trapped water resides.For this reason,core drying cycles are typically included prior to per- forming any repair.Some operators will take the extra step of placing a damaged but unrepaired part in the autoclave to dry so as to preclude any additional damage from occurring during the cure of the repair. This is done to assure they will only need to repair the part once.Skydrol presents a different problem. Once the core of a sandwich part is saturated,complete removal of Skydrol is almost impossible.The part continues to weep the liquid even in cure such that bondlines can become contaminated and full bonding does not occur.Removal of contaminated core and adhesive as part of the repair is highly rec- ommended. Erosion capabilities of composite materials have been known to be less than that of aluminum and, as a result,their application in leading edge surfaces has been generally avoided.However,composites have been used in areas of highly complex geometry,but generally with an erosion coating.The durabil- ity and maintainability of some erosion coatings are less than ideal.Another problem,not as obvious as 8-2MIL-HDBK-17-3F Volume 3, Chapter 8 Supportability 8-2 ume 1 include material and structural testing, material types and properties, and joint types; in Volume 3 include materials and processes, quality, design, joints, reliability, and lessons learned needed to supple￾ment those decisions that influence supportability. 8.2 DESIGN FOR SUPPORTABILITY 8.2.1 In-service experience The first step toward designing reliable and cost-effective design details is to understand the history of composite structure. Composite materials, as we know them today, were introduced into the commercial aircraft industry during the early 1960's and used mostly glass fiber. Development of more advanced fi￾bers such as boron, aramid, and carbon offered the possibility of increased strength, reduced weight, im￾proved corrosion resistance, and greater fatigue resistance than aluminum. These new material systems, commonly referred to as advanced composites, were introduced to the industry very gradually and cau￾tiously to ensure their capabilities. The early success of the first simple components, such as wing spoilers and fairings, led to the use of advanced composites in more complex components such as ailerons, flaps, nacelles, and rudders. The increased specific stiffness and strengths of composites over aluminum, coupled with weight-driven re￾quirements caused by fuel shortages, led to the application of thin-skin sandwich structures. Long-term durability requirements of the original aluminum parts were not fully accounted for when these composite parts were originally designed. To compound the problem further, damage phenomena such as delamination and microcracking were new and complex in comparison to traditional aluminum structure. The original composite parts, particularly thin-gage sandwich panels, experienced durability problems that could be grouped into three categories: low resistance to impact, liquid ingression, and erosion. These parts were either control panels or secondary structure, such as fixed trailing edge panels, and given the emphasis placed on weight and performance, the face sheets of honeycomb sandwich parts were often only three plies or less with a Tedlar™ film. This approach was adequate for stiffness and strength, but never considered the service environment where parts are crawled over, tools dropped, and where service personnel are often unaware of the fragility of thin-skinned sandwich parts. Damages to these components, such as core crush, impact damages and disbonds, are quite often easily detected with a visual inspection due to their thin face sheets. However, sometimes they are overlooked, or dam￾aged by service personnel, who do not want to delay aircraft departure or bring attention to their acci￾dents, which might reflect poorly on their performance record. Therefore, damages are sometimes al￾lowed to go unchecked, often resulting in growth of the damage due to liquid ingression into the core. Non-durable design details (e.g., improper core edge close-outs) also led to liquid ingression. The repair of parts due to liquid ingression can vary depending upon the liquid, of which water and Skydrol (hydraulic fluid) are the two most common. Water tends to create additional damage in repaired parts when cured unless all moisture is removed from the part. Most repair material systems cure at temperatures above the boiling point of water, which can cause a disbond at the skin-to-core interface wherever trapped water resides. For this reason, core drying cycles are typically included prior to per￾forming any repair. Some operators will take the extra step of placing a damaged but unrepaired part in the autoclave to dry so as to preclude any additional damage from occurring during the cure of the repair. This is done to assure they will only need to repair the part once. Skydrol presents a different problem. Once the core of a sandwich part is saturated, complete removal of Skydrol is almost impossible. The part continues to weep the liquid even in cure such that bondlines can become contaminated and full bonding does not occur. Removal of contaminated core and adhesive as part of the repair is highly rec￾ommended. Erosion capabilities of composite materials have been known to be less than that of aluminum and, as a result, their application in leading edge surfaces has been generally avoided. However, composites have been used in areas of highly complex geometry, but generally with an erosion coating. The durabil￾ity and maintainability of some erosion coatings are less than ideal. Another problem, not as obvious as