Stress concentration, fatigue life prediction

Fatigue calculations for large bolted joints, such as those found on cylinders with threaded plugs or hydraulic accumulators, for example, may need to take account of thread-bottom plasticity during the stress cycle.

The aim of this work was therefore to develop a fatigue calculation methodology for screwed assemblies that incorporates cyclic plastic deformation at the bottom of the thread, and to consider the possibility of applying it in a fatigue post-processing module, after finite element analysis.

Various approaches have been considered, the principle being to use fatigue test results obtained in previous studies on an industrial case to test current calculation methods that take cyclic plasticity into account:

• calculation of the stabilized stress at the bottom of the notch using the "stationary" method developed at the solid mechanics laboratory (LMS) at École polytechnique ;

• local elastoplastic approach (cyclic strain-hardening curve, Manson-Coffin curve), taking into account possible preloads or average stresses using Morrow or Smith-Watson-Topper corrections ;

• fatigue post-treatments available in calculation codes (e.g. Abaqus).

It was also planned to validate the method on a simple case (single notch). This validation is presented below.