《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 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