An Analysis of a Programmed Load Fatigue Failure.
ROYAL AIRCRAFT ESTABLISHMENT FARNBOROUGH (ENGLAND)
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The assessment of the airworthiness of an aircraft structure is based normally upon the results of full-scale laboratory fatigue testing of components of the aircraft. Ideally, these full-scale fatigue tests should duplicate the working environmental of the aircraft. A correct simulation of the natural loadings and environmental conditions is, however, virtually impossible and usually the full-scale tests are simplified to the repeated application of a programme of loading cycles applied under laboratory conditions. The interpretation of the results of such tests is still complicated by the nature of the loading sequence, the complex geometry of the full-scale components and materials variables. This Report illustrates, in detail, a fractographic technique for the analysis of a typical full-scale test of an undercarriage fitting. The technique enables the effects of the programme of variable amplitude loads to be separated from those of defects in the forging and the introduction of the concept of an effective load, representing the entire sequence of variable amplitude loads, allows a fatigue life to be predicted for a defect-free forging. The fractographic technique relies upon the measurement, in the scanning electron microscope, of the spacings of fatigue crack striations that are formed on the fracture surface.
- Metallurgy and Metallography