Cyclic lifetime, fatigue testing

In the case of a pressure vessel subjected to cyclic loading (as here) cracks can grow by fatigue and a vessel initially passed as safe may subsequently become unsafe due to this crack growth. The probable extent of crack growth can be determined by making fatigue tests on pre-cracked pieces of steel of the same type as that used in the pressure vessel, and the safe vessel lifetime can be estimated by the method illustrated in Case Studv 3. 

Numerous studies have shown that the cyclic lifetime of a test bar or component varies inversely with the magnitude of the applied cyclic stress, strain, or deflection. As a consequence, various test procedures have been developed to obtain mechanical property information useful in fatigue design analysis. Long... 

Fig. 7.18 Comparison of cyclic fatigue lifetime under various kinds of loading wave forms. (Arrow indicates that failure of specimen did not occur when the test was terminated) [14]. With kind permission of John Wiley and Sons...

Another basic major advantage is that the cyclic-fatigue fracture-mechanics data may be gathered in a relatively short time-period, but may be applied to other designs of bonded joints and components, whose lifetime may then be predicted over a far longer time-span. Obviously, the fracture-mechanics tests need to be conducted under similar test conditions and environments as the joints, or components, whose service-life is to be predicted. This is important since the fracture-mechanics test specimens do need to exhibit a similar mechanism and locus of failure (e.g. cohesively through the adhesive layer, or interfacially between the adhesive and substrate, or through the oxide layer on the metallic substrate, etc.) as observed in the joints, or components, whose lifetime is to be ranked and predicted. 

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