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

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

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

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

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

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

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

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

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

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

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