Derivation of endurance data C Component testing

(1) Whenever force spectra or stress history data are known component testing can be done to verify the design of critical parts of the structure. The component to be tested should be manufactured to exactly the same dimensions and procedures as are intended to be used in the final design. All these aspects should be fully documented before manufacture of the test component is carried out. In addition any method of non-destructive testing and the acceptance criteria should be documented, together with the inspector's report on the quality of the joints to be tested.

(2) The test specimens should be loaded in a similar manner to that described in C. 1.1. Strain gauges should be used to verify that the stress fluctuations are as required. The siting of strain gauges should be such that they are recording the correct stress parameter (see 7.5). If the nominal stress is being recorded the gauge should be at least 10mm from any weld toe. Where the stress gradient is steep three gauges should be used to enable interpolation to be carried out.

(3) In order to obtain a Aa-N curve for design purposes a minimum of eight identical specimens should be tested to give endurance in the range 104 to 107 cycles. Testing should be carried out with reference to the appropriate standard procedures. A mean curve should be calculated and a design curve obtained which is parallel to the mean curve in a double logarithmic diagram but not less than 2 standard deviations away nor greater than 80% of the strength value, whichever is the lower. This allows for wider variations in production than is normally expected in a single set of fatigue specimens.

(4) For damage tolerance designs a record of fatigue crack growth with cycles should be obtained.

(5) Alternatively, if the design stress history is known and variable amplitude facility is available the specimen may be tested under the unfactored stress history.

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