Determination of Fatigue Failure Parameters from the Depth of Plastic Zones Beneath the Fracture Surface

Fatigue failure is the most common type of failure in aircrafts, motor vehicles, medical devices, and other engineering systems, and its study is crucial for predicting the service life of such systems. Of particular interest is to investigate the fatigue failure of new bulk ultrafine-grained nanostructured metal materials produced by severe plastic deformation. The major fatigue failure parameters such as the maximum cycle stress sigma(max), which characterizes the applied failure load, and the cycle asymmetry coefficient R = sigma(min)/sigma(max), which characterizes the loading conditions, can be determined with resort to fracture mechanics, in particular, to the relation between the sizes of plastic zones at the tip of a growing crack and the fatigue fracture parameters. Here we demonstrate the possibility of determining sigma(max) and R from the depth of plastic zones beneath the surface of fatigue fractures on the example of coarse- and ultrafine-grained materials with different lattice types: carbon steels (steels 20 and 45), austenitic steel (07Cr13Ni4NMn20), and aluminum (D16) and magnesium (Mg6Al) alloys exposed to three-point bending. The depth of plastic zones beneath the surface of fatigue fractures was measured by the X-ray method. The maximum cycle stress sigma(max) was estimated from the relation between the depth of a monotonic plastic zone h(y) at a given crack length l and the factor K-max.

Automated summary

Fatigue failure is the most common type of failure in various engineering systems, and its study is crucial for predicting system's service life. The investigation of fatigue failure in new ultrafine-grained nanostructured metal materials is particularly interesting. This study demonstrates the possibility of determining the maximum cycle stress and cycle asymmetry coefficient from the depth of plastic zones beneath the surface of fatigue fractures.