Stress analysis fatigue testing

Stress-controlled fatigue tests and the properties generated by these tests are useful for HCF analysis where elastic strains are dominant. One... 

The thin-wall bellows element should be designed for membrane stresses to conform to code-allowable stresses. The sum of membrane and secondary bending stresses should not exceed 1.5 times the yield stress in order to prevent the collapse of the corrugations caused by pressure. Bellows subjected to external pressure can be analyzed in a manner similar to a cylinder, utilizing an equivalent moment of inertia. The fatigue life can be estimated based on the sum of deflections and pressure stresses as compared to S/N curves based on bellows test data or using the curves in B31.3 Appendix X, Metal Bellows Expansion Joints. Formulas for the stress analysis of bellows are available in the Expansion Joints Manufacturing Association (EJMA) Standards (37). 

The lap shear fatigue test for the joints of the front cab module of the Maglev Transrapid TR08 has been analyzed by a fine detail analysis with 3D solid elements using the real cross-section geometry. The adherend materials are aluminum on one side and GRP on the other see also Section 32.5.1. The PU adhesive Sikaflex -254 Booster is modeled with the Ogden strain energy equation (Eq. (1), with N=2). The computed stiffness correlates well with the test results. The local stress distribution is visualized in Fig. 32.11. 

Another fatigue testing related consideration is that in an ideal test dK/da = 0, in other words a constant amplitude loading is maintained. A real structure, however, experiences a spectrum of stresses in service, which can result in overloads. A detailed analysis of the subject of variable amplitude loading and the associated concepts are covered by Anderson [16]. 

V. M. Sglavo and D. J. Green, The interrupted static fatigue test for evaluating threshold stress intensity factor for ceramic materials a numerical analysis, J. Eur. Ceram. Soc, 15(1995)777-86. 

The need is to develop a test to produce adhesive fatigue properties which are useful to the designer. We consider that the skin-doubler specimen has this potential. It combines the capability of stress analysis with a failure mode faithful to that of the actual structure (see Ref. 4). The results will reflect the strength (durability) of the attachment in terms of stress (load) in the skin. 

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