Appendix 1 knowledge based on flight history recorder data, design limitations established in compliance with § 29.309, and recommended operating conditions and limitations specified in the rotorcraft flight manual. The distribution of times at various forward flight speeds should reflect not only the relation of these speeds to VNE but also the recommended operating conditions in the rotorcraft flight manual that govern Vc or cruise speed. Where possible, it is desirable to conduct the flight strain-gage program by simulating the usage as determined above, with continuous recording of stresses and loads, thus obtaining directly the stress/load spectra for structural elements.
FATIGUE STRENGTH EVALUATION (INCLUDING ROTORCRAFT FLAW
TOLERANCE)
a. General. A means should be established using the conventional safe-life approach or another fatigue tolerant approach to control the airworthiness of principal structural elements identified under § 29.571(a)(1)(i). While the conventional safe-life approach is acceptable under certain circumstances as defined in § 29.571, the enhanced safe-life and fail-safe flaw growth approaches are to be used unless shown to be impractical as stated in § 29.571. A fatigue strength evaluation of structure considering tolerance to flaws is intended to ensure that even when flaws are present due to manufacturing of service operations, the structure will withstand service loads without failure until the flawed parts are replaced or until the flaws (including resulting fatigue cracks) are detected and appropriate action taken. Either of two types of fatigue strength evaluation may be used for flaw tolerance substantiation: flaw tolerant (enhanced) safe-life or fail-safe flaw growth methods. Flaw tolerant (enhanced) safe-life includes the testing and analyses currently associated with safe-life substantiation, plus consideration of flaws. Flaw growth methods include the testing and analyses currently associated with damage tolerance assessment (DTA). Tests are required to substantiate flaw propagation rates and residual strength. Either method or a combination can be used to meet the requirements of § 29.571 for tolerance to flaws. Flaw tolerance evaluation encompasses establishing the components to be designed as flaw tolerant, defining the loading conditions and extent of flaws for which the structure is to be designed, conducting structural tests and analyses to substantiate that the design objectives have been achieved, and establishing replacement times or establishing inspection programs as necessary to assure detection of fatigue damage. On components predominantly loaded by centrifugal force, care should be taken in selecting limit load to assure that it is the maximum expected in service. Design features that should be used in attaining a flaw tolerant structure are:
(1) Use of multi-path construction and the provision of crack stoppers to limit the growth of cracks and to provide adequate residual strength.
(2) Selection of materials and stress levels that preclude crack growth from flaws or that provide a controlled slow rate of crack propagation combined with high Page A1 - 8
Appendix 1 residual strength after initiation of cracks. Tests are required to substantiate crack propagation rates.
(3) Design to permit detection of cracks and other flaws, including the use of crack detection systems, in all critical structural elements before cracks can propagate and become dangerous or result in appreciable strength loss and to permit replacement or repair. Inspection means appropriate for flaw tolerant design follow:
(i) Routine Inspections. To support routine inspection programs, blind areas should be avoided, where practical. Access panels and openings should be considered early in design.
(ii) Special Inspections. These inspections will generally result from test results as well as the geometry of the design. Care should be given to special inspection techniques to be used in the field. Inspection techniques requiring facilities and resources beyond the capability of the small operator or not generally available in remote-area operations traditionally associated with rotorcraft operations should not be specified for field inspections. Conservative sizes for detectable cracks or other flaws should be used. Sufficient interval inspections should be provided to detect cracks before they grow from a detectable size to a size that reduces the remaining strength below design limit strength. If special inspection techniques not commonly available cannot be avoided, then use of enhanced safe-life substantiation techniques should be used.
(ili) Pressurized Chambers. This design feature may be used to detect cracks that cause a chamber to lose its pressure (either positive or negative). The loss of pressure can be indicated by gages, or dye may be used if it is shown to be a dependable indicator. Care should be taken in the design of pressurized chambers and their indicating systems to assure dependability. Undependable systems can cause an inordinate number of false indications and maintenance problems.
(iv) Vibration Generation. This characteristic should be considered both from the aspect of vibrations giving indications of a failure and from the aspect of the increased fatigue loading resulting from the vibrations.
(v) Noise Generation. If initial failure will result in a clear and unmistakable noise that is sufficiently continuous and loud, this characteristic can be used in achieving flaw tolerance without additional special inspections.
(vi) Crack Detection Wire, Foil, etc. Detection wire may be used in areas that are sufficiently well defined so that the wire can be properly located. This technique is appropriate in areas otherwise difficult to inspect. The potential for false readings and possible maintenance problems should be considered.
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Appendix 1
(vii) Health Monitoring. Techniques such as vibration sensing and analysis or real time oil analysis can be used to provide information on establishing inspection time.
(4) Use of multiple element structures may be provided so that damage or failure occurring in one element of the member will be confined to that element and the remaining structure will still possess adequate load-carrying ability until the failed element is discovered by inspection.
(5) Provisions to limit the probability of concurrent multiple damage, particularly after long service, should be provided. These provisions should ensure adequate independence of each failure mode of multi-path constructions. The use of full-scale fatigue test articles are recommended in this evaluation. Examples of concurrent multiple damage to be avoided are:
(i) Simultaneous failure or partial failure of multiple path discrete elements working at similar stress levels.
(ii) Failures or partial failures, in adjacent areas, due to redistribution of loading following a failure of a single element.
b. Identification of Principal Structural Elements. Principal structural elements are those that contribute significantly to carrying flight and ground loads and whose failure could result in catastrophic failure of the rotorcraft. Typical examples of such elements are:
(1) Rotor blades and attachment fittings.
(2) Rotor heads, including hubs, hinges, and some main rotor dampers.
(3) Control system components subject to repeated loading, including control rods, servo structure, and swashplates.
(4) Rotor supporting structure (lift path from airframe to rotor head).
(5) Fuselage, including stabilizers and auxiliary lifting surfaces.
(6) Main fixed or retractable landing gear and fuselage attachment structure.
c. Identification of Locations Within Principal Structural Elements to be Evaluated.
The locations of damage to structure for damage tolerance evaluation can be determined by analysis or by fatigue test on complete structures or subcomponents.
However, tests will be necessary when the basis for analytical prediction is not reliable, Page A1- 10
Appendix 1 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.
(1) The following should be considered:
(i) Strain gage data on undamaged structure to establish points of high stress concentration as well as the magnitude of the concentration;
(ii) Locations where analysis shows high stress or low margins of safety;
(iii) Locations where permanent deformation occurred in static tests;
(iv) Locations of potential fatigue damage identified by fatigue analysis;
(v) Locations where the stresses in adjacent elements will be at a maximum with an element in the location failed;
(vi) Partial fracture locations in an element where high stress concentrations are present in the residual structure;
(vii) Locations where detection would be difficult;
(viii) Design details that service experience of similarly designed components indicates are prone to fatigue or other damage; and
(ix) Components fabricated from materials of potentially low fracture toughness or high flaw growth rate.
(2) In addition, the areas of probable damage from sources such as a severe corrosive and/or fretting environment, a wear and/or galling environment, or a high maintenance environment should be determined from a review of the design and past service experience.
d. Extent of Flaws. Each particular design should be assessed to establish appropriate damage criteria in relation to inspectability and flaw extension characteristics. In any flaw determination, it is possible to establish the extent of flaws in terms of detectability with the inspection techniques to be used, the associated single element failure or initially detectable flaw size, the residual strength capabilities of the structure, and the likely flaw extension rate (after either an element failure or a partial failure) considering the expected stress redistribution under the repeated loads expected in service and with the expected inspection frequency. Although multiple-element design should be used where practical, and obvious partial failure could be considered to be the extent of the flaw for residual strength assessment, provided a positive determination is made that manufacturing or service flaws will not initiate flaw growth within the life of the part or that the fatigue flaw growth will be Page A1-11
Appendix 1 detectable by the available inspection techniques at a sufficiently early stage of the flaw growth. In a swashplate or pin containing pressurized chamber, an obvious partial failure might be detectable through the inability of the chamber to maintain pressure after occurrence of the damage. Flaw tolerant (enhanced) safe-life evaluations from flaws should consider flaws to be expected during manufacturing (including handling) and during service. Special coatings against corrosion, flame plating or plasma plating against fretting corrosion, and energy absorption coating or shielding against damage associated with maintenance may be used in determining the type and extent of flaws to be considered in flaw tolerant (enhanced) safe-life evaluations. The following are typical examples of the type of partial failures that may be considered in the flaw growth fail-safe and enhanced safe-life evaluation. These may or may not be appropriate to the design being considered. These examples have been sources of service difficulty on prior/existing designs:
(1) Detectable skin cracks in the trailing edge sections of rotor blades.
(2) Detectable failures of individual straps in "strap packs."
(3) Detectable skin cracks emanating from the edge of structural openings or cutouts;
(4) Detectable circumferential or longitudinal skin crack in the basic fuselage or tail boom structure;
(5) Complete severance of interior frame elements or stiffeners in addition to a detectable crack in the adjacent skin;
(6) Presence of a detectable fatigue failure in at least the tension portion of the spar web or similar element;
(7) Detectable failure of a primary attachment, including blade attachment fittings and control surface hinge and fittings; and
(8) Fretting, corrosion, and galling conditions expected in service.
e. Provisions for Inspection. The designer should strive to ensure adequate inspectability of all structural parts to qualify them under the fail-safe flaw growth provisions. In those cases where blind areas or surfaces exist, suitable design features should be provided to allow inspection techniques (either visual or nondestructive testing, as necessary) to assure adequate residual strength is achieved unless shown to be impractical due to limitations of geometry and good design practice. In addition, the alternate safe-life approach to fatigue tolerance should be implemented if the inspection techniques are shown to be too complicated and impractical.
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Appendix 1 †. Lesting of Principal Structural Elements. The nature and extent of 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. For flaw tolerant safe-life testing, simulated flaws should be as representative as possible of actual gouges, scratches, pitting, or fretting to be expected in manufacture and service. For fail-safe testing considering crack propagation, simulated cracks should be as representative as possible of actual fatigue damage. Where it is not practical to produce actual fatigue cracks, flaws can be simulated by cuts made with a fine saw, sharp blade, guillotine, or other suitable means. The validity of saw cuts, etc., should be verified by comparison to coupon tests of a cracked specimen of the same material. In those cases where bolt failure, or its equivalent, is to be simulated as part of a possible flaw configuration in joints or fittings, bolts can be removed to provide that part of the simulation.
g. Flaw Tolerance Demonstration (Flaw Tolerant (Enhanced) Safe-Life or Fail-Safe Flaw Growth).
(1) It should be determined by analysis, supported by test evidence, that the structure with the extent of damage established for residual strength evaluation can withstand the specified design limit loads (considered as ultimate loads). Flaw tolerant safe-life substantiation provides a safe period of operation of structure with flaws with only routine inspections necessary. Safe crack growth (fracture mechanics) substantiation for fail-safe designs, on the other hand, provides for limited operation after crack initiation from flaws until the cracks can be safely detected. Since flaw tolerant safe life does not provide for detailed crack growth beyond the threshold of detectability, it will tend to apply to life-limited parts, particularly those hard to inspect.
Safe crack growth substantiation of fail-safe designs is applicable to readily inspectable parts and may provide for replacement "on condition" rather than at a specified life.
(2) The enhanced safe-life of metallic components will use analyses and testing similar to that of basic safe-life except that "flawed" specimens will be tested rather than "pristine" specimens.
(i) In order to determine the mean fatigue strength and the variability in fatigue strength considering flaws, it is necessary to test a number of specimens in establishing stress versus number of cycles (S-N) curves. Both full-scale and coupon specimens may be used to account for the variability in fatigue strength. A reduction factor should be applied to the mean curve in arriving at a working S-N curve. This factor should include consideration of the number of specimens tested, the variability of the fatigue results and, where available, previous test data on the same materials or similar components, as well as service experience.
(ii) Where new materials or designs are being evaluated, it is recommended that a larger reduction factor be used until additional test data justifying a change are available. The mean and reduced S-N data acceptable to the Administrator on Page A1-13
Appendix 1 specimens with stress concentration factor, as applicable. A reduced S-N curve and the loading spectrum of Paragraph 6b should be used in determining replacement times.
(iii) Figure 1 represents the method of constructing a typical S-N curve from the fatigue test data. Four to six full-scale specimens have commonly been used in past sate-life programs to determine mean S-N curves. The applicant may propose a specific number of specimens that may be evaluated with respect to the proposed methodology in determining the acceptability of the structural substantiation program.
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Appendix 1 * simple safe life ** enhanced safe life *** X is commonly 3 sigma for pristine specimens but may be
- MEAN CURVE IHROUCH TEST DATA less for flawed specimen testing STRESS (S) OR LOAD (P) WEAN CURVE THROUGH TEST DATA (1 lowed specimens) ** **** XX% varies from 50% to 80% based on expected material and usage variablity
• Denotes bottom of scatter REDUCED CURVE (MEAN MINUS "X" SIGMA) *** - (pristine specimens)*_ - REDUCED CURVE (XXx OF MEAN) ****
NUMBER OF CYCLES FIGURE APX1-1 S-N OR P-N CURVE USAGE Page A1-15
Appendix 1
(iv) Additional coupon tests have been used, when necessary, to identify more completely test scatter due to material properties variability. When the agreed-upon number of specimens is tested at various acceptable load levels, a reduction by three mean standard deviations, sigma (o), necessary to account for material properties, should be made to establish a reduced S-N curve for determination of retirement times. This reduction may take into account the fact that flawed specimens were tested to preclude a dual penalty situation. Reduction by two mean standard deviations rather than three, may be used if justified by appropriate design features such as multiple elements or unmistakable flaw indications or by material properties that provide benign types of failure modes.
(v) "Run-outs" (specimens that do not fail) may be treated in one of three ways: deleted from data (conservative), considered as a failure at maximum test cycles (conservative), or considered in a rational manner to develop a more realistic "o." To further account for variability in fatigue strength not totally accounted for by this reduction in S-N data, the lesser of this reduction, 80 percent of mean, or bottom of scatter, may be used as a limit, where justified, depending on material properties.
(3) The procedures in Paragraph 7a of AC 20-107A provide criteria for substantiating safe flaw growth for composite structure.
(4) Safe crack growth (fracture mechanics) substantiation should show that the damage growth rate under the repeated loads expected in service (between the time at which the damage becomes initially detectable and the time at which the extent of damage reaches the value for residual strength evaluation) provides a practical basis for development of the inspection programs and procedures described in Paragraph 7h of this Appendix. For multiengine load paths, a minimum of three inspection intervals is recommended between the initially detectable damage time and the time when residual strength is reduced to design limit load by crack growth. For single element structures, a minimum of four inspection intervals is recommended. The repeated loads should be 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 it is significant.
(5) For flaw tolerance to achieve an improvement in safety over safe-life for composite structures, the procedures of Paragraph 7a of AC 20-107A are recommended. For flaw tolerance to achieve and improvement in safety over simple safe-life for metallic structures, the following testing criteria are recommended to supplement flaw tolerant safe-life analysis using flawed specimen S-N curves or crack growth analyses using appropriate stress intensity factors and da/dn data from coupon tests:
(i) Flaw Tolerant (Enhanced) Safe-Life Testing Criteria. Test full-scale specimens with flaws or a mix of full-scale and coupon specimens with flaws to obtain Page A1- 16
Appendix 1 S-N data. Plot the data as shown in Figure APX1-1. Utilize the S-N data and loading spectrum of Paragraph 6b in substantiating a crack-free life or in arriving at a replacement time by cumulative damage analysis means. Where practical (as allowed by the number of damaging cycles in the loading spectrum), spectrum testing may be used for fatigue substantiation in lieu of S-N testing followed by analysis. The replacement time established should be included in the airworthiness limitations section of the document established under § 29.1529.
(ii) Fail-Safe Crack Growth (Fracture Mechanics) Substantiation. Test two or more specimens to obtain crack propagation data using either a realistic load spectrum or an accelerated load (spectrum or single) associated with the use of propagation theory and data after cracks have been initiated. Unless a more rational method with an equivalent level-of-satety is applied for, the tollowing methods of setting inspection intervals should be applied. In all cases, the inspection methods and intervals should adequately consider variables such as inspectability, type of inspection, crack growth behavior, and other scheduled maintenance considerations.
(A) For single element (load path) structure, plot the data and set the inspection as shown in Figure APX1-2.
(1) Set the initial inspection at L1/3.
(2) Set the repetitive inspection intervals at L2/4.
Page A1-17
Appendix 1
FLAW SIZE CRITICAL CRACK LENGTH FOR LIMIT LOAD
FIELD DETECTABLE FLAW SIZE
FLAW SIZE CONTROLLED I
BY MANUFACTURING QUALITY LIFE FIGURE APX1-2 CRACK GROWTH FOR SINGLE ELEMENT STRUCTURE Page A1- 18
Appendix 1 (B) For multi-element load path structure:
(1) Test all paths simultaneously with a 0.05-inch thick crack (or size detectable by manufacturing quality control procedures) in the critical element at the start of tests.
(2) Note when field detectable cracking occurs.
(3) Note when one element fails.
(4) Note when the residual strength of the remaining elements decreases to limit load due to crack growth.
(5) From Figure APX1-3, set initial inspection at L, /3.
(6) Set repetitive inspection intervals at (L2 + Lp)/3.
NOTE: If partial failure of the critical element is not detectable, then L2 becomes zero.
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