Fatigue monitoring of metals based on mechanical hysteresis, electromagnetic ultrasonic, electrical resistance and temperature measurements

Abstract

Alternatively to conventional stress-strain hysteresis measurements in this paper physically based materials data such as specimen temperature, electrical resistance, speed of sound or generator power are used to describe the cyclic deformation behavior of metals in the high cycle (HCF) and the very high cycle fatigue (VHCF) regime. The mentioned physical parameters depend in a characteristic manner on the load and cycle dependent microstructure of metals. The change in temperature is proportional to the dissipated energy due to cyclic plastic deformation, the electrical resistance and speed of sound in metals depend on the individual dislocation and defect structure. Amongst others the generator power of ultrasonic testing machines is influenced by the internal friction of the material and consequently from the microstructure of a fatigued material. In analogy to the plastic strain amplitude all these data can be used to describe the cyclic deformation behavior and to calculate S, N-curves. The materials investigated are taken from technical components such as power plants, high speed trains and automotive industry to demonstrate that the presented methods and physical parameters can be used to describe the cyclic deformation and fatigue behavior of different steels with different microstructures. Related to the regarded materials and fatigues states SEM and TEM investigations were used to identify microstructural details such as slip bands, micro cracks, crack initiation sites and typical dislocation structures as well as phase transformations.