ARP1383

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AEROSPACE RECOMMENDED PRACTICE

ARP1383 REV.B

Issued 1978-03Revised 2003-02

Superseding ARP1383A

(R) Impulse Testing of Aerospace Hydraulic Actuators, Valves,Pressure Containers, and Similar Fluid System Components

NOTICE

Some test procedures are potentially dangerous. SAE Technical Reports do not purport to address all of the safety problems, if any, associated with their use. It is the responsibility of the user of an SAE Technical Report to establish and employ appropriate safety practices. Tests should only be conducted by individuals who have been properly trained in test procedure and who are aware of any hazards which may be present. Appropriate safety and health procedures must be employed when conducting any test.

1. SCOPE:

This SAE Aerospace Recommended Practice (ARP) establishes the minimum requirements and procedures for impulse testing of aerospace hydraulic actuators, valves, pressure containers, and similar fluid system components, for use in aerospace hydraulic systems; for exceptions, see paragraph 9.1. This ARP also refers to standard impulse test equipment, which may be used in conducting these impulse tests.

2. REFERENCES:

2.1 Applicable Documents:

The following publications form a part of this document to the extent specified herein. The latest issue of SAE publications should apply. The applicable issue of other publications shall be the issue in effect on the date of the purchase order. In the event of conflict between the text of this document and references cited herein, the text of this document takes precedence. Nothing in this document, however, supersedes applicable laws and regulations unless a specific exemption has been obtained.

SAE ARP1383 Revision B

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2.1.1 SAE Publications: Available from SAE, 400 Commonwealth Drive, Warrendale, PA 15096-0001.

ARP603 Impulse Testing of Hydraulic Hose Assemblies, Tubing, FittingsAIR1228 Standard Impulse Machine Equipment and OperationAS4265 Impulse Testing of Hydraulic Tubing and Fittings, S-N CurveAIR4298 Impulse Test Machine, Sine Wave, Equipment and Operation of

2.1.2 U.S. Government Publication: Available from DODSSP, Subscription Services Desk, Building 4D, 700 Robbins Avenue, Philadelphia, PA 19111-5094.

MIL-HDBK-5 Metallic Materials and Elements for Aerospace Vehicle Structures

2.1.3 NFPA Publication: Available from the National Fluid Power Association, 3333 N. Mayfair Road, Milwaukee, WI 53222-3218.

NFPA/T2 .6.1 Fluid power components - Method for verifying the fatigue and establishing the burst pressure ratings of pressure containing envelope of a metal fluid power component

2.2 Definitions:

2.2.1 Miners Rule: Palmgren-Miner cycle-ratio summation theory, also known as Miner's rule, may be expressed as:

A method that may be used to approximate the cumulative fatigue damage at a point in a metallic part from the repeated exposure to cycles of various stress levels. For given maximum and minimum stress levels, the fatigue life in terms of the number of cycles is found from a S/N curve. The damage fraction is the applied number of cycles divided by the fatigue life. The fractional damage amounts for each stress level are added together to calculate the total fatigue damage. Cumulative fatigue damage greater than or equal to 1 indicates that the part will fail.

2.2.2 Rate of Rise: The rate of rise is defined as the slope of the pressure-time curve in the straight portion of the initial pressure increase portion between 10% of the total rise above back pressure and 10% of the total rise below peak pressure.

2.2.3 Cycle: The time from minimum pressure to maximum pressure and back to minimum pressure (see Figures 1, 2 and 3).

2.2.4 Dwell time (Time at Pressure): The time for which the pressure exceeds the lower tolerance for P(max).

2.2.5 P(max): The maximum supply cavity test pressure.

2.2.6 P(min): The minimum return pressure experienced in normal operation.


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FIGURE 1 - Supply Cavity and Return Cavity Impulse Waveform


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FIGURE 2 - Alternate Impulse Trace - Damped Wave


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FIGURE 3 - Alternate Impulse Trace Sine Wave

2.2.7

2.2.8 P(oper): The nominal system design operating pressure.

2.2.9 PR: The nominal return line design pressure.

2.2.10 PR(max): The maximum return cavity test pressure.

P(Mean) P(Max) P(Min)2

-------------------------------------------=


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3. REQUIREMENTS:

The pressure-impulse test is basically an accelerated fatigue test. This document defines the impulse test procedures recommended for the hydraulic equipment listed in SCOPE.

The procedure is divided into two categories:

CATEGORY I: GENERICALLY DERIVED TEST REQUIREMENTSCATEGORY II: SPECIFICALLY DERIVED TEST REQUIREMENTS

3.1 Category I: Generically Derived Test Requirements:

This category is applicable to generic aerospace hydraulic equipment impulse testing, with recommended pressure levels and test cycle requirements being provided. The pressure and return cavities of the component should be tested, as a minimum, to the number of pressure impulse cycles defined below for the applicable aircraft classification.

3.1.1 Fixed Wing Aircraft:

(1) Flight control actuators/valves/components, P(max) = 1.5 x P(oper), 200,000 cycles

(2) Utility actuators/valves/components, P(max) = 1.5 x P(oper), 100,000 cycles

(3) Return ports/passages and components subjected to return pressure only, PR(max) = PR x 1.5, 100,000 cycles, with PR taken as 0.5 x P(oper)

3.1.2 Rotary Wing Aircraft:

(1) Primary flight control actuators: 1,000,000 cycles, P(max) = 1.5 x P(oper)

(2) Secondary flight control actuators: 1,000,000 cycles, P(max) = 1.5 x P(oper)

(3) Utility actuators: 300,000 cycles, P(max) = 1.5 x P(oper)

(4) Flight control system hydraulic components: 1,000,000 cycles, P(max) = 1.5 x P(oper)

(5) Utility system hydraulic components: 300,000 cycles, P(max) = 1.5 x P(oper)

(6) Return ports/passages and components subjected to return pressure only: 300,000 cycles, PR(max) = 1.5 x PR with PR taken as 0.5 x P(oper)

(7) S-N curve generation. In addition to meeting the criteria specified in 1 through 6 above, a S-N curve should be generated for critical components in the rotary-wing aircraft hydraulic system. Unless otherwise specified, a minimum of six specimens should be utilized for testing. See Appendix A for additional data on S-N curve generation and usage.


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3.2 Category II: Specifically Derived Test Requirements:

Category II is applicable to all aircraft configurations (including helicopters), both military and commercial, where the test requirements are derived from specifically related system values.

The number of test cycles are to produce equivalent fatigue damage, when the component is tested at the design operating pressure, using the selected wave form (see Figures 1, 2 or 3), to the damage the component is expected to receive during the lifetime of the applicable aircraft. The determination of the number of test cycles must consider both mean and alternating stresses for in-service, ground, and test operations of the aircraft during its expected life. The cycles at various stress levels are combined by Miners Rule to determine the number of equivalent cycles at the design operating pressure. Appendix B provides details of the method to be used to determine test cycles by means of an example. Because in this example only one unit is tested during qualification, it is necessary for additional cycles to be performed to verify reliability. (Example: to demonstrate 95% reliability with 95% confidence, it would be necessary to test four lifetimes of one unit). For guidance on the testing of multiple samples, to obtain specific confidence levels, see NFPA/T2.6.1.

UNLESS OTHERWISE SPECIFIED, THE FOLLOWING GENERAL REQUIREMENTS ARE APPLICABLE TO TEST CATEGORIES I AND II.

3.3 Test Conditions:

When this document is referenced in a design specification as part of the requirements, the following additional requirements must be specified:

a. Number of test cycles (for Category I testing, see 3.1; for Category II testing, see 3.2)

b. Test pressures for all cavities

c. Operating temperature(s) (see 4.4)

d. Fluid (see 4.5)

e. Wave shape (figure number if other than Figure 1) (see 4.1)

f. Component/valve position (see 6.3.1)

g. Minimum return line pressure and return line design pressure (for Category I testing see 3.1; for Category II testing see 3.2)

h. Number of actuator cycles extended and retracted and position, if applicable (see 6.3.2)

i. Cycling rate and tolerance (see 4.3)

j. Layering requirements (sequential test condition(s) groupings)


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3.3 (Continued):

k. Pressure transients/surges (pressure and return if known) (see 4.1)

l. When a S-N curve is to be developed and if less than 6 specimens are permitted (see Section 7)

m. Any post fatigue performance tests that must be satisfactorily completed (see 8.1)

4. GENERAL TEST REQUIREMENTS:

4.1 Shape of the Impulse Cycle:

The limits shown in Figure 1 define the pressure-time cycle for both pressure and return cavities when observed on an oscilloscope and instrumented per AIR1228. Alternative pressure-time cycles for the pressure and return cavities are shown in Figures 2 and 3.

The dynamic impulse trace, produced by the test machine, should be in conformance with Figures 1, 2 or 3 as applicable, and the actual pressure-time curve should be confined to the area shown.

4.2 Rate of Rise:

The rate of rise should be in accordance with Figures 1, 2 and 3, and as defined in 2.2.2.

4.3 Cycling Rate:

4.3.1 A cycling rate of 1 Hz is recommended. Consideration should be given to other cycling rates when pressures must penetrate between close fitting parts or restrictions and where hysteresis can affect stresses. Cycling rates up to 5 Hz may be allowed for small components, with actual cycling rate defined in the component specification.

4.3.2 See Figures 1 through 3 for suggested pressure waveforms. Verify that peak pressures are within their specified tolerances. Note restrictions when using the Damped Wave Impulse Trace (Figure 2) (see 4.3.4).

4.3.3 For test cycling rates exceeding 1 Hz the stress level of at least one test sample should be verified by analysis or by strain gauge.

4.3.4 The following additional verifications are recommended for impulse tests where the cyclic rate exceeds 1Hz, or when employing a Damped Wave Impulse Trace (Figure 2).

a. Verify by strain gauge or analysis that the ratio of induced stress to pressure at the test cycling rate is the same as during the static loading.

b. Verify that temperatures of the test component are within allowable limits.


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4.4 Temperature:

The fluid temperature during testing should be maintained at the nominal component operating temperature within +15 to 0 C (+27 to 0 F). The nominal component operating temperature should be specified in the detail specification. Components which operate at more than one stabilized temperature, such as during subsonic cruise and during supersonic flight, should be tested at each temperature for approximately the same percentage of time predicted for operation during service as defined in the detail specification.

4.5 Test Fluid:

The fluid used for the test should be the service fluid of the component undergoing test unless specified otherwise.

5. TEST EQUIPMENT:

The test setup should produce repeatable pressure pulses within the limits defined in 4.1.

5.1 Accuracy:

The test equipment and instrumentation should be set up and maintained so that all data is accurate within 4% of the maximum actual value unless otherwise specified. Temperature recording accuracy should be maintained at 3 C (5 F).

5.2 Oscilloscope:

Where oscilloscopes are utilized to record the cycle shape, the sweep rate on the oscilloscope should be adjusted so that the slope of the pressure rise takes advantage of the full size of the screen. The trace and photos of the impulse cycle should be an accurate record of the impulse cycle and show a grid or other means to permit accurate checking.

6. TEST PROCEDURE:

The method of testing is intended to determine the ability of hydraulic actuators, valves, and pressure containers (for exceptions see 9.1) to withstand the hydraulic pressure-impulse cycling for qualification testing.


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6.1 Selection of Specimen:

The test specimen should meet the following criteria:

a. The unit should be a production unit or representative of production. Conservative substitutions, adequately substantiated by the equipment manufacturer are acceptable. Material Review Board (Group that provides dispositions/corrective actions for discrepancies occurring in the manufacturing process) actions should be documented and reviewed for effect on test validity.

b. The unit should not have been previously subjected to loads exceeding the limit load or pressures exceeding proof pressure, unless such application of load or pressure is a normal step in the production process. Any yielding, however microscopic, in stress concentration areas may result in unrealistic fatigue life.

c. The unit should not have been previously subjected to vibration tests or other fatigue type tests. The life capability used up in such tests may result in unrealistic low pressure-impulse fatigue life.

6.2 Preparation of Specimen:

Prior to pressure-impulse testing, the unit should be disassembled and thoroughly examined for cracks or structural failure or flaws in accordance with production requirements, such as, visually and by use of fluorescent-penetrant, ultrasonic or magnetic-particle inspection. The inspection of test specimen for flaws prior to testing shall not include any inspections not required for production units. It should then be reassembled with new seals, proof pressure tested, and given a baseline functional test including measurement of internal and external leakage prior to pressure-impulse testing. This requirement may be waived if the unit is inspected prior to assembly and impulse testing is the first qualification type test imposed on the test specimen.

The production acceptance test may be used to satisfy requirements for initial baseline test requirements pertinent to impulse testing and may eliminate non-pertinent tests such as frequency response, insulation resistance, no-load velocity, etc.

NOTE: A component should not be subjected to a proof pressure test prior to the pressure impulse test if it does not have a proof test as part of its ATP (Acceptance Test Procedure).

6.3 Test Setup:

All entrapped air should be bled as well as possible from the test component and test circuit. All drains and low-pressure ports that are not part of the area under test should be allowed to drain freely and kept at atmospheric pressure. Metal shot or loosely fitting metal pieces may be placed in the test unit if desired to minimize fluid volume. Measurement of the pressure cycle shall be made as close to the test specimen as is practical.


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6.3.1 Valve Positions: Variable-position valves, such as directional-control valves and servovalves, should be tested at their various positions so that each port and chamber is appropriately stressed. The total number of pressure-impulse cycles should be apportioned and applied at each position in approximately the same percentage expected in service. Simplifying tests should not result in undertesting any area. However, no chamber should see less than the number of cycles appropriate to the procedure selected.

6.3.2 Actuator Piston Positions: Actuating cylinders should be tested so that the pressure application should reflect conditions of operation. For example, if both sides of a piston are pressurized simultaneously during operation, then the test conditions should reflect this condition. In addition, actuating cylinders should be tested so that all elements exposed to fluid pressure are appropriately stressed. Unless otherwise specified in the detail procurement specification, actuators should be tested with the piston rod restrained in mid-position and the maximum number of cycles should then be applied in the retract pressurized direction. The required number of impulse cycles should then be repeated with pressure applied to the extend direction. Pressure should not be applied simultaneously to extend and retract directions. Servoactuators should see the full impulse range P(max) to PR during impulse testing.

6.3.3 Inlet Check Valves: For impulse testing of pressure vessels containing inlet check valves, the inlet check valve should be removed to obtain full pressure excursions for all valve components during testing.

6.3.4 Dual Pressure Systems: Components which operate at more than one pressure level, such as in dual pressure level systems, should be tested at each pressure level. The percentage of the total impulse test cycles, which are run at the higher pressure, should be equivalent to twice the percentage of the operating time at that (higher) pressure, predicted in service. For Category 1 testing, the apportioning (percentage) of the cycles to be run at the higher pressure should be defined in the detail specification.

6.4 Impulse Test:

The impulse test should be conducted as required under Section 3 and the detail specification. Where applicable (i.e., Category II), the impulse test should be conducted in cyclical layers with each layer including no more than the total number of pressure-impulse cycles expected during the components service life. If testing is done at more than one fluid temperature or test position, those variables should be interspersed within each layer in the same proportions expected in service.

After completion of each test layer, the external surfaces of the component should be visually examined for cracks, leaks, or other evidence of structural failure. Then, before proceeding with the next test layer, the baseline performance should be rerun and the results, including internal and external leakage, recorded. Elastomeric, plastic, or other nonstructural wear sensitive components may be replaced prior to proceeding to the next test layer.


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6.4 (Continued):

If seals or gaskets are replaced during testing, it is recommended that the torque required to remove any fasteners be recorded and that on reinstallation, these fasteners be torqued to the same value as determined during removal. Thus if the prestress has been reduced during the cycles previously conducted, the test is resumed at conditions similar to those conditions before seal replacement was necessary.

7. S-N CURVE:

If specified (see 3.3[l]), a S-N curve should be generated for critical components. Unless otherwise specified, a minimum of six specimens should be utilized for testing. See Appendix A for additional data on S-N curve generation and usage.

8. POST TEST REQUIREMENTS:

8.1 Performance After Test:

The component should conform to the post impulse test performance requirements specified after completion of the impulse cycling (see 3.3[m]). Depending upon the test procedure used, performance testing may or may not be required.

8.2 Component Verification:

Upon completion of post test performance tests specified, the unit should be disassembled and inspected for cracks or structural failure. The presence of detectable cracks constitutes failure unless otherwise determined by the procuring activity. If a post test performance test is required and any performance characteristic fails to meet the requirements specified in the detail specification, the unit should be reassembled with new seals, proof tested, and re-subjected to the post test performance requirements (see 8.1) to determine the reason for deteriorated performance.

8.3 Approval:

Designs are considered acceptable if no cracks or other structural damage occur during or as a result of the impulse test. Alternatively, in less critical circumstances where some degree of performance degradation is considered acceptable, the detail specification should specify the associated allowable conditions, such as, maximum acceptable crack size, inspection methods and procedures to be employed (example: Visual; with degree of magnification, Penetrant, and/or Magnetic Particle Inspections), and the resulting minimum acceptable performance requirements. If unacceptable performance degradation is found following the first layer of a multilayer test, the test should be interrupted until corrective redesign or part replacement is implemented.


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9. NOTES:

This section does not contain mandatory requirements and is for information only.

9.1 Intended Use:

This document, as indicated by the Scope (Section 1) is intended for use as a standard for impulse test requirements and procedures for qualification and evaluation testing of the hydraulic system components indicated. This document does not apply to the impulse testing of the following hydraulic components; where known the applicable impulse test document is noted.

9.2 Key Words:

Test, cycling rate, wave form, fatigue test

9.3 The change bar ( l ) located in the left margin is for the convenience of the user in locating areas where technical revisions, not editorial changes, have been made to the previous issue of this document. An (R) symbol to the left of the document title indicates a complete revision of the document.

PREPARED UNDER THE JURISDICTION OFSAE COMMITTEE A-6, AEROSPACE FLUID POWER, ACTUATION,

AND CONTROL TECHNOLOGIES

TABLE 1

Components Refer toHydraulic AccumulatorsHydraulic Pump and Motor Rotating Elements AS7997 & AS19692Hydraulic Hose Assemblies, Tubing, and Fittings ARP603 & AS4265


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APPENDIX AS-N CURVE GENERATION

A.1 SCOPE:

This appendix specifies the procedure to be used in preparation of S-N curves as required by 3.1.2 (7) Category I, and where required in detail specifications (see Section 7).

A.2 APPLICABLE DOCUMENTS:

This section in not applicable to this appendix.

A.3 REQUIREMENTS:

A.3.1 Upon completion of test, plot the test point(s) as an S-N plot. Designate point(s) as either a run-out (no test failure for the number of cycles tested) or failure point (see legend, Figure A1).

A.3.2 Draw the appropriate curve through the point(s) using the equivalent data point routine. Mean endurance limit is acceptable for a mix of fractures and run-outs. For run-outs the highest run-out may be used to establish the mean S-N curve (Figure A1). The low cycle reduction allows for increased scatter factor at low number of cycles.

A.3.3 Construct a working curve based on the number of samples tested. (The smaller the number of test specimens the greater the reduction in allowables in the mean S-N curve). The working curve using the three sigma scatter factor is appropriate (Figures A1, A2, and A3).

A.3.4 When one specimen is tested, whether a significant run-out or fracture, a 50% reduction in the mean endurance (one-half of the mean curve pressure values at any given number of cycles) shall be used unless otherwise specified by the procuring agency (Figure A4). This procedure may be used to establish interim life of the component.

A.3.5 The working S-N curve shall be used to calculate the component life or replacement time.


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FIGURE A1 - S-N Curves



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APPENDIX BDETERMINATION OF TEST CYCLE REQUIREMENTS USING CATEGORY II

B.1 SCOPE:

This appendix presents an example to show how the number of test cycles applicable to 3.2 Category II can be determined for an application.

B.1.1 To use Category II of this document it is necessary to define the unique characteristics of the fatigue cycles to be encountered during the life of each individual component to be evaluated and determine the number of equivalent cycles at test pressure limits. The following example uses an aileron actuator for a commercial airplane to illustrate the method recommended. This component, used as an example, has two details that may be sensitive to fatigue and the analysis includes many of the aspects typifying duty cycle definition. The actuator manifold and cylinder barrel are pressure vessels for which the duty cycle is defined by pressure limits between zero and operating pressure. The piston rod and rod end experience reversing loads during actual service. The piston rod assembly may be tested concurrently with the cylinder by applying impulse pressure alternately to opposing ends of the piston. Table B1 shows a summary of the determination of the test cycle requirements for this actuator. The following text describes the procedure for defining test cycles and applies it to the example.

B.2 APPLICABLE DOCUMENTS:

B.2.1 U.S. Government Publications:

Available from DODSSP, Subscription Services Desk, Building 4D, 700 Robbins Avenue, Philadelphia, PA 19111-5094.

MIL-HDBK-5 Metallic Materials and Elements for Aerospace Vehicle Structures


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TABL

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ifold

Det

ail

min

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ail

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min

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105


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B.3 REQUIREMENTS:

B.3.1 Step 1, Determine Manufacturers Design Requirements:

Obtain from the aircraft manufacturer a definition of the load cycles (in terms of the fluctuations of hydraulic pressure) that the component experiences during normal operation, the number of times each load cycle is repeated during one flight, and the number of flights in the aircraft lifetime. For components that experience reversing loads, this requires definition of pressures on either side of the piston.

B.3.2 Step 2, Determine the Ground-Air-Ground Pressure Cycle:

Determine the maximum peak-to-peak pressure variation that occurs during one flight using the data from Step 1.

TABLE B2 - Aircraft Manufacturers Data for the Example Chosen

Duty Conditions Manifold/Cylinder Detail Rod/Rod End DetailOne life = 62,000 flights 7075-T6 Aluminum AISI 4340, H.T. to 200 ksi3000 psig design pressure Kt = 3 in control pressure Kt = 3 in threadsFlight segments pressure fluctuations pressure fluctuations- controls check approx 2/flt 750/2250 psi -1500/1500 psi- cruise1 approx 435/flt 1980/2050 psi 960/1100 psi- roll1 approx 40/flt 2160/2595 psi 320/2190 psi- on/off approx 1/flt 0/1500 psi -

NOTE: Large load offset from neutral during flight cycles.

TABLE B3 - G-A-G Cycle for the Example Chosen

Pressure Range Component Element0 to 2595 psig Manifold-1500 to 2190 psig Rod End


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B.3.3 Step 3, Determine the Stress Cycle:

Select a stress/pressure ratio consistent with the materials and type of construction used in the component. Refer to the notes at the conclusion of this appendix for suggestions related to making this selection.

Apply the ratio K to the pressures from Steps 1 and 2 and to define the stress cycle for each flight segment.

B.3.4 Step 4, Determine the Life Cycle Expectancy:

Identify a curve in MIL-HDBK-5C that most closely relates to the material and condition being analyzed. See Figures B1 and B2 related to example chosen. Determine from the intersection of lines representing minimum stress and maximum stress, using the results for each segment in Step 3, the life cycle expectancy (N) in cycles for each segment.

TABLE B4 - Stress/Pressure Ratio for the Example Chosen

Component Element Stress Pressure K FactorManifold 50,000 psi 3000 psi 16.7Rod End 80,000 psi 3000 psi 26.7

TABLE B5 - Stress Cycle for the Example Chosen

FlightSegment

ManifoldPressure

(psi)min

ManifoldPressure

(psi)max

ManifoldK

ManifoldStress(ksi)min

ManifoldStress(ksi)max

Rod EndPressure

(psi)min

Rod EndPressure

(psi)max

Rod EndK

Rod EndStress(ksi)min

Rod EndStress(ksi)max

Controls Chk. 750 2250 16.7 12.5 37.5 -1500 1500 26.7 -40.0 40.0

NOTE: See Table B1 for data on remainder of flight segments.


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FIGURE B1 - Example - Typical Constant Life Diagram, 7075T6 Aluminum AlloyData from MIL-HDBK-5C Figure 3.7.3.1.8(D) Detail Omitted for

Clarification of Example

TABLE B6 - Life Cycle Expectancy for the Example Chosen

Flight Segment

ManifoldStress(ksi)min

ManifoldStress(ksi)max

ManifoldMIL-HDBK-5C

ManifoldN

Rod EndStress(ksi)min

Rod EndStress(ksi)max

Rod EndMIL-HDBK-5C

Rod EndN

Controls Chk. 12.5 37.5 Figure B1 5 x 104 -40.0 40.0 Figure B2 4 x 104


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FIGURE B2 - Example - Typical Constant Life Diagram, AISI 4340 Notched Bar Datafrom MIL-HDBK-5C Figure 2.3.1.1.8(C) Detail Omitted for

Clarification of Example

B.3.5 Step 5, Calculate n/N and n/N:

Use the data from Steps 1 and 4, where n = cycles in one operational life and N = stress life cycle expectancy for each flight segment. Calculate the arithmetic sum of the segment results.

TABLE B7 - n/N, n/N for Example Chosen

Flight Segment Control Check Cruise Roll On-Off G-A-G n 1.25 x 105 2.7 x 105 2.5 x 106 6.2 x 104 6.2 x 104 N/AManifold N 5 x 104 5 x 108 3 x 105 1 x 104 N/AManifold n/N 2.5 0 0.005 0.21 6.2 8.915Rod End N 4 x 104 -- 2 x 104 N/ARod End n/N 3.125 0 0 -- 3.1 6.225


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B.3.6 Step 6, Calculate n/N for Test Pressure:

The pressure fluctuations used in conducting the test per Category II should be the best representation of the service expected in the component using only one set of minimum/maximum pressures. For most components this is the design operating pressure (0 to 3000 psi for a 3000 psi system). However, if the operational cycle has reversing loads, the test should be conducted with reversing loads.

B.3.6.1 n/N for Test of the Example Chosen: Inspection of Table B1 shows the rod end is subjected to reversing cycles. The controls check segment was selected as the test condition although the G-A-G segment could serve the same purpose. Using the controls check for the rod end N = 4 x 104. The pressure application during testing is to alternate 1500 psi impulses to the head end and rod end of the cylinder with the actuator mounts restrained to hold the piston at near mid position.

B.3.7 Step 7, Calculate n for Testing:

Perform the following calculation:

(Eq. B1)

where:

ref = the data form the segment used in Step 6 to define the test conditions.

B.3.7.1 n for Test of the Example Chosen:

(Eq. B2)

B.4 NOTES:

The method of analysis for calculation of equivalent cycles has several differences from conventional fatigue analysis.

B.4.1 In Step 3, somewhat unrealistic high stress levels may be assumed without compromising the analysis. Since the "error" is contained in all elements of the calculation and the total damage, (n/N), is divided by the reference cycle damage (n/N ref), the "error" tends to cancel out.

B.4.2 Damage values (n/N) exceeding 1.0 are acceptable for the reasons given in B.4.1 above. Experience has shown that inaccuracy due to assumptions is minimal when (n/N) is in the range of 0.2 to 5.0. Any value of (n/N) greater than 10.0 should not be used.

ntest(n/N)n/Nref----------------- nref=

ntest(n/N)n/Nref----------------- nref

(6.225)3.125

------------------- 1.25 105 2.49 105= = =


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B.4.3 The ratio of pressure to stress in Step 3 (K) should be assumed as high as is permitted by B.4.2 above. This improves the ability to read S-N curves and, hence, to distinguish among load cases. Even with high assumed stress levels, some fatigue load cases will still have very high allowable lives and so, will by analysis, do no fatigue damage. The cruise flight segment for the example illustrates this condition.

B.4.4 Once a pressure to stress relationship (K) is assumed, it MUST be held constant throughout the analysis. If a problem occurs which requires changing the assumption, the entire analysis must be repeated.

B.4.5 Although comparative analyses can be performed using any reputable S-N curve, the basis of the S-N curve must be examined carefully before performing a fatigue analysis and predicting the absolute life of parts. Some considerations are:

a. The S-N curve selected should be based on a given stress concentration factor. The curve used shall represent the closest stress concentration factor (either higher or lower value) in published data to the condition being examined.

Material processing effects should also be considered e.g., fatigue life degradation due to:

1. Surface coatings-chrome plate, anodize, etc.,2. Electro-discharge machining recast layers,3. Electro-chemical or similar surface material removal processes and4. Metal working/machining or heat treatment induced tensile residual stresses.

Where appropriate, because of high loading in a pressure vessel situation, steps should be taken to mitigate these effects, i.e., by mechanically removing recast layers, shot peening before application of certain coatings and by proper sequencing of manufacturing machining, heat treating and processing operations.

Conversely, beneficial effects due to compressive residual surface stresses induced into components via such processes as shot peening, thread rolling, cold expansion, coining, roller burnishing, autofrettage, etc. or resulting from carburizing, nitriding, etc. should be acknowledged as favorable (life enhancing) in the fatigue analysis.

b. The test specimens used to generate the S-N curves are often fabricated with extremely smooth surface finishes in the test section. These finishes are much smoother than is reasonable for many production parts.

Efforts should be made in production to control or eliminate any machine tool marks, notches, nicks, dings, grind tears, etc. which will adversely impact fatigue life. In addition, care should be taken to eliminate all unforeseen stress risers by proper deburring. Where applicable, contact wear (i.e. fretting) fatigue should be taken into account. The analysis can compensate for some of these factors by increasing the calculated stress concentration factor.


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B.4.5 (Continued):

Components should be stress relieved (relaxed) during heat treatment and properly baked for embrittlement relief after chromium, cadmium, or any other plating which introduces hydrogen into the part.

c. The material properties of the test specimens are often significantly higher than the minimums that can exist in production.

Material section thickness/size effects and temperature reduction factors should be applied when fatigue calculations are made.

d. Nearly all S-N curves represent typical test lives. They have not been constructed to statistically reflect higher reliability than typical. Probability determinations for reliability must consider the confidence factor in the probability. For example: if it is the objective to evaluate a component to 95% reliability with 95% confidence by testing one unit, the number of cycles required in testing is four lifetimes.

B.4.6 Sources such as Joseph E. Shigley and Larry D. Mitchell, MECHANICAL ENGINEERING DESIGN, Fourth Edition, McGraw-Hill Inc., 1983 discuss ways of accounting for the above in performing detail life prediction analysis, otherwise known as fatigue analysis.

ARP1383

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