Modeling cycle and time dependent creep/relaxation effects on fatigue lives of notched members

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Virginia Polytechnic Institute and State University


Mechanical cyclic variations in mean stress and strain amplitude is a well-known occurrence for metals even at room temperature. Many fatigue analysis procedures ignore these variations. Fatigue analysis which included both time and cycle dependent mechanical material behavior for metals at room temperature had not been previously studied except for the case of creep. An investigation studying transient mechanical effects on Ti-6Al-4V titanium and 7475-T651 Al alloys was done to determine how great an effect transients at room temperature would have on fatigue life under cyclic conditions. The mechanical material response was modeled using viscoplasticity constitutive laws and Neuber’s rule eliminating the need for finite element modeling of uniaxially loaded notched members. However, the Nenber’s modeling may be used with any material constitutive law. The procedures for fatigue damage used cycle counting to compute strain amplitude and mean stress. Since a large amount of fatigue data is reported as strain-life curves, the fatigue analysis was developed using this fatigue data although it did not include transients. If favorable results are obtained, development of modeling and testing to include transients in strain-life fatigue data could be avoided, and the existing fatigue data base utilized.

Experimental work was undertaken and nonlinear optimization techniques used to compute model constants for the two alloys. However, small amounts of rate dependence was found for cyclic strain control testing. The viscoplasticity models became stiff when rate dependence was low causing numerical problems, and model constants for the viscoplastic constitutive law could not be determined since convergence was not achieved. Also, only small amounts of transient static stress relaxation was observed for extended hold periods.

Finally, experimental verification was done for the local surface stresses in a notched member under load using advanced x-ray stress equipment. Measurements during brief pauses were made over a cycle. From the x-ray results, an anomalous surface behavior was observed. The surface yielded before the bulk material with the lower surface yielding seeming to be time dependent in nature. Since rolled plates of the alloys were used, texture was measured and studied in the form of pole figures, and extreme texture was found for both alloys. However. successful x-ray measurements were made for the alloys studied even though assuming linear d-spacing versus sin²ψ. Finally. x-ray measurements for a cycled notched member, exhibited relaxation of mean stress and not relaxation of residual stress.