Prediction of Crack Initiation Based on Energy Storage Rate during Low-Cycle Fatigue of Austenitic Stainless Steel.

Wiera Oliferuk,Michał Doroszko,Jarosław Szusta,Wojciech Grodzki,Leszek Urbański

doi:10.3390/ma14195526

Abstract

The low-cycle deformation of 304L austenitic stainless steel was examined in terms of energy conversion. Specimens were subjected to cyclic loading at the frequency of 2 Hz. The loading process was carried out in a hybrid strain–stress manner. In each cycle, the increase in elongation of the gauge part of the specimen was constant. During experimental procedures, infrared and visible-range images of strain and temperature fields were recorded simultaneously using infrared thermography (IR) and digital image correlation (DIC) systems. On the basis of the obtained test results, the energy storage rate, defined as the ratio of the stored energy increment to the plastic work increment, was calculated and expressed in reference to selected sections of the specimen. It was shown that, before the specimen fracture in a specific area, the energy storage rate is equal to zero (the material loses the ability to store energy), and the energy stored during the deformation process is released and dissipated as heat. Negative and close-to-zero values of the energy storage rate can be used as a plastic instability criterion on the macroscale. Thus, the loss of energy storage ability by a deformed material can be treated as an indicator of fatigue crack initiation.

Highlights

Determining the fatigue properties of a material subjected to a cyclic strength test requires expensive research and time-consuming evaluation of obtained data
Determining the increments of plastic work for the selected surface sections requires taking into account such a stress state
Using infrared thermography (IR) thermography and a digital image correlation (DIC) system, the energy storage process in 304L austenitic stainless steel subjected to low-cycle fatigue was investigated

Summary

Introduction

Determining the fatigue properties of a material subjected to a cyclic strength test requires expensive research and time-consuming evaluation of obtained data. Further studies related to the determination and analysis of heat source dissipation in materials subjected to low-cycle fatigue [4,5,6,7] have led to the assumption that the dependency of dissipated energy in the form of heat with a cyclic response is not clear, whereas the stress–strain response during a fatigue test has a strict reference to the energy storage process [7]. This conclusion refers to the change in the material microstructure resulting in crack initiation and its impact on the energy storage process. The instability of plastic deformation during the fatigue process undoubtedly leads to the initiation of fatigue fracture

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