Predicting the Lifetime of Steel

Summary

Even today, lifetime predictions of construction parts are still based on the Wöhler method, which is almost 150 years old. To construct a reliable Wöhler diagram, it is necessary to perform alternating load fatigue experiments on a huge number of equivalent samples for up to 10⁸ or 10⁹ load cycles. The lifetime under a specific applied load is then deduced from this diagram using statistical techniques.

Physically, the reason for fatigue and finally fracture is the accumulation of lattice defects like dislocations, vacancies and vacancy clusters, which are produced even when the load is significantly below the material’s yield strength. The progress of fatigue can be observed from its earliest stages — after only a few load cycles — up to the final state of fracture by employing positrons as extremely sensitive lattice defect probes. In situ experiments can be performed to study test samples or real construction parts under realistic conditions. In steels a critical defect density is reached just before fatigue failure occurs. The point of failure can therefore be extrapolated from the early stages of fatigue by monitoring the defect density.

Spatially resolved experiments performed on a simple carbon steel and employing the Bonn Positron Microprobe indicate significant variations in defect densities over the region under stress even after just a few load cycles. These inhomogenieties grow from a typical starting size of less than a millimeter to encompass the entire volume after further fatigue. With more experimental experience and a better theoretical understanding of this process, this new prediction method should lead to much simpler and more reliable predictions of the lifetimes of metallic materials in the near future.