Advisory U.S. Depariment Circular of Transportation Federal Aviation Administration Subject: DAMAGE TOLERANCE AND Date: 4/29/98 AC No: 25.571-1C FATIGUE EVALUATION OF STRUCTURE Initiated By: ANM-112 Change:
PURPOSE. This advisory circular (AC) provides guidance for compliance with the provisions of 14 CFR Part 25 of the Federal Aviation Regulations (FAR) pertaining to the damage-tolerance and fatigue evaluation requirements for transport category aircraft structure.
It also provides rational guidelines for the evaluation of scatter factors for the determination of life for parts categorized as safe-life. Like all advisory circular material, this advisory circular is not, in itself, mandatory and does not constitute a regulation. It is issued to provide an acceptable means, but not the only means, of compliance with the rules. Terms used in this AC, such as "shall" and "must" are used only in the sense of ensuring applicability of this particular method of compliance when the acceptable method of compliance described herein is used.
While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in determining compliance with the pertinent FAR. This advisory circular does not change, create any additional, authorize changes in, or permit deviations from, regulatory requirements.
CANCELLATION. Advisory Circular 25.571-1B, dated February 18, 1997, is canceled.
3. DEFINITIONS OF TERMS USED IN THIS AC.
a. Damage tolerance is the attribute of the structure that permits it to retain its required residual strength for a period of use after the structure has sustained a given level of fatigue, corrosion, accidental or discrete source damage.
Fail-safe is the attribute of the structure that permits it to retain its required residual strength for a period of unrepaired use after the failure or partial failure of a principal structural element.
Safe-life of a structure is that number of events such as flights, landings, or flight hours, during which there is a low probability that the strength will degrade below its design ultimate value due to fatigue cracking.
d. Design service goal is the period of time (in flight cycles/hours) established at design and/or certification during which the principal structure will be reasonably free from significant cracking.
e. Principal structural element. PSE, is an element that contributes significantly to the carrying of flight, ground, or pressurization loads, and whose integrity is essential in maintaining the overall structural integrity of the airplane.
f. Single load path is where the applied loads are eventually distributed through a single member, the failure of which would result in the loss of the structural capability to cary the applied loads.
ailure of individual elements) the apied loads would be safety distributed to other load carrying members.
Widespread fatigue damage, WFD, in a structure is characterized by the simultaneous presence of cracks at multiple structural details that are of sufficient size and density whereby the structure will no longer meet its damage tolerance requirement (i.e., to maintain its required residual strength after partial structural failure).
(1) Multiple site damage, MSD, is a source of widespread fatigue damage characterized by the simultaneous presence of fatigue cracks in the same structural element (i.e.
fatigue cracks that may coalesce with or without other damage leading to a loss of required residual strength).
(2) Multiple element damage, MED, is a source of widespread fatigue damage characterized by the simultaneous presence of fatigue cracks in adjacent structural elements.
1. Scatter factor. A life reduction factor used in the interpretation of fatigue analysis and test results.
BACKGROUND.
Since the early 1970's, there have been significant state-of-the-art and industrypractice developments in the area of structural fatigue and fail-safe strength evaluation of transport category airplanes. Recognizing that these developments could warrant some revision of the existing fatigue requirements of §§ 25.571 and 25.573 of 14 CFR part 25, the Federal Aviation Administration, on November 18, 1976 (41 FR 50956), gave notice of the Transport Category Airplane Fatigue Regulatory Review Program and invited interested persons to submit proposals to amend those requirements. The proposals and related discussions formed the basis for the revision of the structural fatigue evaluation standards of §§ 25.571 and 25.573 and the 2
development of guidance material. To that end, § 25.571 was revised, § 25.573 was deleted (the scope of § 25.571 was expanded to cover the substance of the deleted section), and guidance material (AC 25.571-1) was provided which contained compliance provisions related to the proposed change.
b. Since issuance of AC 25.571-1 on 9/28/78, additional guidance material, including discrete source damage, was developed and incorporated in revision 1A on 3/5/86.
The AC was further revised on 2/18/97, revision 1B, to add guidance on the elements to be considered in developing scatter factors for certification. This current revision adds guidance.
material whose objective is to preclude widespread fatigue damage (MSD or MED) and to aid in the determination of thresholds for fatigue inspection and/or other special fleet actions.
INTRODUCTION.
The contents of this advisory circular are considered by the FAA in determining compliance with the damage-tolerance and fatigue requirements of § 25.571.
(1) Although a uniform approach to the evaluation required by § 25.571 is desirable, it is recognized that in such a complex field new design features and methods of fabrication, new approaches to the evaluation, and new configurations could necessitate variations and deviations from the procedures described in this advisory circular. Close adherence to the procedures in this advisory circular is encouraged.
(2) Damage tolerance design is required, unless it entails such complications that an effective damage-tolerant structure cannot be achieved within the limitations of geometry, inspectability, or good design practice. Under these circumstances, a design that complies with the fatigue evaluation (safe-life) requirements is used. A typical example of structure that might not be conducive to damage-tolerance design is the landing gear and its attachments.
(3) Experience with the application of methods of fatigue evaluation indicates that a test background should exist in order to achieve the design objective. Even under the damage tolerance method discussed in paragraph 6 of this AC, it is the general practice within industry to conduct damage tolerance tests for design information and guidance purposes.
Damage location, growth, and detection data should also be considered in establishing a recommended inspection program.
Typical loading spectrum expected in service. The loading spectrum should be based on measured statistical data of the type derived from government and industry load history studies and, where insufficient data are available, on a conservative estimate of the anticipated use of the airplane. The principal loads that should be considered in establishing a loading spectrum are flight loads (gust and maneuver), ground loads (taxiing, landing impact, turning, engine runup, braking, thrust reversing, and towing), and pressurization loads. The development of the loading spectrum includes the definition of the expected flight plan which involves climb, cruise, descent, flight times, operational speeds and altitudes, and the approximate time to be 3
spent in each of the operating regimes. Operations for crew training and other pertinent factors, such as the dynamic stress characteristics of any flexible structure excited by turbulence or buffeting, should also be considered. For pressurized cabins, the loading spectrum should include the repeated application of the normal operating differential pressure, and the superimposed effects of flight loads and external aerodynamic pressures.
c. Components to be evaluated. In assessing the possibility of serious fatigue failures, the design should be examined to determine probable points of failure in service. In this examination, consideration should be given, as necessary, to the results of stress analyses, static tests, fatigue tests, strain gage surveys, tests of similar structural configurations, and service experience. Service experience has shown that special attention should be focused on the design details of important discontinuities, main attach fittings, tension joints, splices, and cutouts such as windows, doors, and other openings. Locations prone to accidental damage (such as that due to impact with ground servicing equipment near airplane doors) or to corrosion should also be considered.
d. Analyses and tests. Unless it is determined from the foregoing examination that the normal operating stresses in specific regions of the structure are of such a low order that serious damage growth is extremely improbable, repeated load analyses or tests should be conducted on structures representative of components or subcomponents of the wing, control surfaces, empennage, fuselage, landing gear, and their related primary attachments. Test specimens should include structure representative of attachment fittings, major joints, changes in section, cutouts, and discontinuities. Any method used in the analyses should be supported, as necessary, by test or service experience. Typical (average) values of material properties that account for the quantifiable effects of environment and other parameters may be used in residual strength, crack growth, and damage detection analyses for damage tolerance evaluations per paragraph 6, and discrete source damage per paragraph 8.
DAMAGE-TOLERANCE EVALUATION.
a. General. The damage tolerance evaluation of structure is intended to ensure that should serious fatigue, corrosion, or accidental damage occur within the design service goal of the airplane, the remaining structure can withstand reasonable loads without failure or excessive structural deformation until the damage is detected. Included are the considerations historically associated with fail-safe design. The evaluation should encompass establishing the components that are to be designed as damage-tolerant, defining the loading conditions and extent of damage, conducting structural tests or analyses, or both, to substantiate that the design objective has been achieved, and establishing data for inspection programs to ensure detection of damage. Although this evaluation applies to either single or multiple load path structure, the use of multiple load path structure should be given high priority in achieving damage-tolerant design. Design features that should be considered in attaining a damage-tolerant structure include the following:
(1) Multiple load path construction and the use of crack stoppers to control the rate of crack growth, and to provide adequate residual static strength;
(2) Materials and stress levels that, after initiation of cracks, provide a controlled slow rate of crack propagation combined with high residual strength;
(3) Arrangement of design details to ensure a sufficiently high probability that a failure in any critical structural element will be detected before the strength has been reducer below the level necessary to withstand the loading conditions specified in § 25.571(b), so as to allow replacement or repair of the failed elements; and
(4) Provisions to preclude the possibility of widespread fatigue damage (MSD or MED) prior to reaching the design service goal and to prevent or control the effects of such damage beyond that time. Examples of concern are:
(i) A number of small cracks that might coalesce to form a critical single long crack;
(ii) Loss of residual strength for a lead crack (or accidental damage) due to very small MSD along its growth trajectory;
(iii) Failures, or partial failures, in adjacent areas due to the redistribution of loading following a failure of a single element; and
(iv) Simultaneous failure, or partial failure, of multiple load path discrete elements, working at similar stress levels.
Freedom from Widespread Fatigue Damage. Paragraph (b) of § 25.571 requires the effectiveness of the provisions to preclude the possibility of widespread fatigue damage occurring within the design service goal of the airplane to be demonstrated with sufficient fullscale fatigue test evidence. Although it is not generally practical to produce enough certification test evidence to absolutely preclude the possibility that WFD will develop on any particular airplane within the design service goal of its parent model, it is possible to give some guidelines for test evidence that give a high degree of confidence that it will not. The determination of what constitutes "sufficient full-scale test evidence" requires a considerable amount of engineering judgment and is a matter that should be discussed and agreed to between an applicant and the FAA early in the planning stage for a certification project. In general, sufficient full-scale test evidence consists of fatigue testing to two or more times the design service goal, followed by specific inspections and analyses to determine if widespread fatigue damage (MSD or MED) has not occurred. While the data is not needed at the time of certification, a plan to produce it must be approved prior to certification. In addition, the following language (or appropriate equivalent) should be included in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness until such time as the testing is completed:
¿Until completion of certification fatigue testing, the airplane shall not be operated beyond a number of cycles (i.e., landings) equal to the number of cycles accumulated on the fatigue test article divided by 2."
The type certificating FAA office will monitor the progress of testing conducted after issuance of a type certificate and notify FAA operational maintenance inspectors and other regulatory authorities of the status of fatigue testing if airplanes in service are expected to have a problem meeting this condition because of the pace of the testing or some other circumstance. Further failure to complete testing in accordance with the plan may result in issuance of an Airworthiness Directive (AD) which limits airplane usage until the required testing is completed. The following factors should be considered in determining the sufficiency of the evidence:
Factor 1: The comparability of the load spectrum between the test and the projected usage of the airplane.
Factor 2: The comparability of the airframe materials and design between the test article and the certificated airplane.
Factor 3: The extent of post-test teardown inspection and analysis for determining if widespread fatigue cracking has occurred.
Factor 4: The duration of the fatigue testing.
Factor 5: The size and complexity of a design change. This factor applies to design changes made to a model that has already been certificated and for which full-scale fatigue test evidence for the original structure should have already been determined to be sufficient. Small, simple design changes, comparable to the original structure, could be analytically determined to be equivalent to the original structure in their propensity for WFD. In such cases, additional full-scale fatigue test evidence should not be necessary.
Factor 6. The age of an airplane being modified. This factor applies to airplanes that have already accumulated a portion of their design service goal prior to being modified.
An applicant should only be required to demonstrate freedom from WFD up to the design service goal for the original airplane.
The following examples offer some guidance on the types of data sets that might constitute "sufficient evidence" for some kinds of certification projects.
(1) New Type Certificates. Normally this type of project would necessitate its own full-scale fatigue test to represent the new structure and its loading environment.
Nevertheless, prior full-scale fatigue test evidence from earlier tests performed by the applicant, or others, may also be used and could supplement, or in rare instances eliminate the need for,
additional tests on the new model. Ultimately, the evidence needs to be sufficient to conclude with confidence that, within the design service goal of the airframe, widespread fatigue damage will not occur. Factors 1 through 4 should be considered in determining the sufficiency of the evidence.
A test duration of a minimum of twice the design service goal for the airplane model would normally be necessary if the loading spectrum is realistic, the design for the test article principal structure is the same as for the certificated airplane, and the post-test teardown is exhaustive. If the conformance to Factors 1 through 3 is less than ideal, a significantly longer test duration would be needed to conclude with confidence that WFD will not occur within the design service goal. Moreover, no amount of fatigue testing will suffice if the conformance to Factors 1 through 3 above is not reasonable.
(2) Derivative Models. It may be possible to reliably extrapolate the occurrence of widespread fatigue damage for part or all of the derivative model from the data that the applicant generated or assembled during the original certification project. Nevertheless, the evidence needs to be sufficient to allow confidence in the calculations that show that widespread fatigue damage will not occur within the design service goal of the airplane. Factors 1 through 5 should be considered in determining the sufficiency of the evidence for derivative models. For example, a change in structural design concept, a change in aerodynamic contour, or a modification of structure that has a complex internal load distribution, might well make analytical extrapolation from the existing full-scale fatigue test evidence very uncertain. Such changes might well necessitate full-scale fatigue testing of the actual derivative principal structure. On the other hand, a typical derivative often involves extending the fuselage by inserting "fuselage plugs" that consist of a copy of the typical semimonocoque construction for that model with slightly modified material gauges. Normally this type of project would not necessitate its own full-scale fatigue test.
(3) Type Design Changes--Service Bulletins. Normally this type of project would not necessitate its own full-scale fatigue test because the applicant would have generated, or assembled, sufficient full-scale fatigue test evidence during the original certification project that could be applied to the change. Nevertheless, as cited in the previous example, the evidence needs to be sufficient to allow confidence in the calculations that show that widespread fatigue damage will not occur within the design service goal of the airplane. In addition, Factor 5, The Size and Complexity of a Design Change, should be considered.
(4) Supplemental Type Certificates (STC).
(1) Sufficient full-scale fatigue test evidence for structure certified under an STC may necessitate additional full-scale fatigue testing, although the extent of the design change may be small enough to use Factor 5, to infer the sufficiency of the existing fullscale fatigue test evidence. In addition, although the applicant for an STC may not have access to the original equipment manufacturer's full-scale fatigue test data, the applicant may assume the basic structure was shown to comply with the regulation, unless the FAA has taken, or intends to
take, Airworthiness Directive (AD) action to alleviate a WFD condition. This assumption implies that sufficient full-scale fatigue test evidence exists, demonstrating that multiple-site damage will not occur within the design service goal of the airplane. For the purpose of the STC applicant's demonstration, it may be assumed that model types certified under § 25.571, as amended by Amendment 25-96, and which are not subject to AD action to alleviate aWFD condition, have received 2 full design service goals of fatigue testing, under realistic loads, and have received a thorough post-test inspection that did not detect any widespread fatigue damage.
With this assumption, and Factors 1 through 5, the STC applicant may be able to demonstrate that WFD will not occur on its modification (or the underlying original structure) within the design service goal. If, however, the modification significantly affects the distribution of stress in the underlying structure, or significantly alters loads in other parts of the airplane, or significantly alters the intended mission for the airplane, or if the modification is significantly different in structural concept from the certificated airplane being modified, additional full-scale fatigue test evidence would likely be necessary.
In addition, Factor 6, The Age of the Airplane Being Modified, comes into play for modifications made to older airplanes. The STC applicant should demonstrate freedom from WFD up to the design service goal of the airplane being modified. For example, an applicant for an STC to an airplane that has reached an age equivalent to 75 percent of its design service goal should demonstrate that the modified airplane will be free from WFD for at least the remaining 25 percent of the design service goal. Although an applicant could attempt to demonstrate freedom from WFD for a longer period, this may not be possible unless the original equipment manufacturer cooperates by providing data for the basic structure. A short design service goal for the modification could simplify the demonstration of freedom from WFD for the STC applicant. Nevertheless, the applicant should also be aware that the design service goal of the airplane is not a fixed life; it may be extended as a result of a structural reevaluation and service action plan, such as has been developed for certain models under the FAA's aging airplane program. Unless the modifier also reevaluates its STC modification, the shorter design service goal for the modification could impede extending the design service goal of the modified airplanes. Because of these unique considerations for STC's, it is especially important that the applicant address this issue with the FAA at an early stage of the STC project.
(5) Repairs. New repairs that differ from repairs contained in the FAA- approved original equipment manufacturer's structural repair manual, but that are comparable in design to such repairs, and that meet the FAR in other respects, would not necessitate full-scale fatigue testing.
c. Damage Tolerance Assessment Methodologies. Normally, the damage tolerance assessment consists of a deterministic evaluation of the design features described in paragraphs 6a(1), (2), (3), and (4). Paragraphs d. through k. below provide guidelines for this approach. In certain specific instances, however, damage-tolerant design might be more realistically assessed by a probabilistic evaluation employing methods such as risk analysis. Risk analyses are
routinely employed in fail-safe evaluations of airplane systems and have occasionally been used where structure and systems are interrelated. These methods can be of particular value for structure consisting of discrete isolated elements where damage tolerance depends on the ability of the structure to sustain redistributed loads after failures of discrete elements resulting from fatigue, corrosion, or accidental damage. Where considered appropriate on multiple load path structure, a probabilistic analysis may be used if it can be shown that loss of the airplane is extremely improbable, and the statistical data employed in the analysis is based on tests o.
operational experience, or both, of similar structure.
d. Identification of principal structural elements (PSE). A PSE is an element of structure that contributes significantly to the carrying of flight, ground, or pressurization loads, and whose integrity is essential in maintaining the overall structural integrity of the airplane.
Typical examples of such elements are as follows:
(1) Wing and empennage.
(1) Control surfaces, slats, flaps, and their mechanical systems and attachments (hinges, tracks, and fittings);
(ii) Integrally stiffened plates;
(iii) Primary fittings;
(iv) Principal splices;
(v) Skin or reinforcement around cutouts or discontinuities;
(vi) Skin-stringer combinations;
(vii) Spar caps; and
(vill) Spar webs.
(2) Fuselage.
(i) Circumferential frames and adjacent skin;
(ii) Door frames;
(ı1) Pilot window posts;
(iv) Pressure bulkheads;
(v) Skin and any single frame or stiffener element around a cutout;
(vi) Skin or skin splices, or both, under circumferential loads;
(vii) Skin or skin splices, or both, under fore and aft loads;
(viii) Skin around a cutout;
(ix) Skin and stiffener combinations under fore and aft loads;
(x) Door skins, frames, and latches; and
(xi) Window frames.
(3) Landing gear and their attachments.
(4) Engine mounts.
e. Extent of damage. Each particular design should be assessed to establish appropriate damage criteria in relation to inspectability and damage extension characteristics. In any damage determination, including those involving multiple cracks, it is possible to establish the extent of damage in terms of detectability with the inspection techniques to be used, the associated initially detectable crack size, the residual strength capabilities of the structure, and the likely damage-extension rate, considering the expected stress redistribution under the repeated loads expected in service and with the expected inspection frequency. Thus, an obvious partial failure could be considered to be the extent of the damage for residual strength assessment, provided a positive determination is made that the fatigue cracks will be detectable by the available inspection techniques at a sufficiently early stage of the crack development. In a pressurized fuselage, an obvious partial failure might be detectable through the inability of the cabin to maintain operating pressure or controlled decompression after occurrence of the damage.
The following are typical examples of partial failures that should be considered in the evaluation:
() Detectable skin cracks emanating from the edge of structural openings or cutouts;
(2) A detectable circumferential or longitudinal skin crack in the basic fuselage structure;
(3) Complete severance of interior frame elements or stiffeners in addition to a detectable crack in the adjacent skin;
(4) A detectable failure of one element of components in which dual construction is used, such as spar caps, window posts, window or door frames, and skin structure;
(5) The presence of a detectable fatigue failure in at least the tension portion of the spar web or similar element; and
(6) The detectable failure of a primary attachment, including a control surface hinge and fitting.
f. Inaccessible areas. Every reasonable effort should be made to ensure inspectability of all structural parts, and to qualify them under the damage tolerance provisions (reference 14 CFR § 25.611).
g- Testing of principal structural elements. The nature and extent of residual strength tests on complete structures or on portions of the primary structure will depend upon applicable previous design, construction, tests, and service experience, in connection with similar structures. Simulated cracks should be as representative as possible of actual fatigue damage.
Where it is not practical to produce actual fatigue cracks, damage can be simulated by cuts made with a fine saw, sharp blade, guillotine, or other suitable means. If sawcuts in primary structure are used to simulate sharp fatigue cracks, sufficient evidence should be available from element tests to indicate equivalent residual strength. In those cases where bolt failure, or its equivalent, is to be simulated as part of a possible damage configuration in joints or fittings, bolts can be removed to provide that part of the simulation.
M.. Identification of locations to be evaluated. The locations of damage to structure for damage tolerance evaluation should be identified as follows:
() Determination of general damage locations. The location and modes of damage can be determined by analysis or by fatigue tests on complete structures or subcomponents. However, tests may be necessary when the basis for analytical prediction is not reliable, such as for complex components. If less than the complete structure is tested, care should be taken to ensure that the internal loads and boundary conditions are valid.
(i) If a determination is made by analysis, factors such as the following should be taken into account:
(A) Strain data on undamaged structure to establish points of high stress concentration, as well as the magnitude of the concentration;
(B) Locations where permanent deformation occurred in static tests;
(C) Locations of potential fatigue damage identified by fatigue analysis; and (D) Design details that service experience of similarly designed components indicates are prone to fatigue or other damage.
(ii) In addition, the areas of probable damage from other sources, such as severe corrosion and accidental damage, should be determined from a review of the design and past service experience.
(2) Selection of critical damage areas. The process of actually locating where damage should be simulated in principal structural elements (identified in paragraph 6d of this AC) should take into account factors such as the following:
(1) Review analysis to locate areas of maximum stress and low margin of safety;
(i) Select locations in an element where the stresses in adjacent elements would be the maximum with the damage present;
(iii) Select partial fracture locations in an element where high stress concentrations are present in the residual structure; and
(iv) Select locations where detection would be difficult.
1. Damage tolerance analysis and tests.
(1) It should be determined by analysis, supported by test evidence, that:
(i) The structure, with the extent of damage established for residual strength evaluation, can withstand the specified design limit loads (considered as ultimate loads);
and The damage growth rate under the repeated loads expected in service (between the time the damage becomes initially detectable and the time the extent of damage reaches the value for residual strength evaluation) provides a practical basis for development of the inspection program and procedures described in paragraph 6j of this AC.
(2) The repeated loads should be as defined in the loading, temperature, and humidity spectra. The loading conditions should take into account the effects of structural flexibility and rate of loading where they are significant.