A Novel Method to Obtain Manson-Coffin Law of Metallic Materials for Low-cycle Fatigue by Small Punch Test

Key words:  SPT  Manson-Coffin law  LCF  theoretical model  stress-strain relation

Abstract:

The assessment of materials low-cycle fatigue (LCF) properties using micro-samples extracted from in-service structures is crucial for structural integrity evaluation. Existing small punch tests for low cycle fatigue (SPTLCF) methodologies predominantly rely on load-controlled approaches and remain constrained by dependencies on conventional standard round bar specimen data. To address this limitation, we propose a displacement-controlled (strain-controlled) SPTLCF framework. Firstly, a disc with center hole (Discch) specimen with dual mechanically fixed boundaries (inner and outer circular boundaries) is developed to enable symmetrical displacement-controlled cyclic loading. Then, a comprehensive theoretical framework is presented, including displacement-load model, equivalent stress-strain model, stress-strain field model, and maximum stress-strain model. Additionally, a new approach is proposed to determine material stress-strain relationships under cyclic steady states, coupled with a novel SPTLCF methodology for LCF performance assessment. Finite element analysis (FEA) of the specified materials demonstrates less than 4% deviation from model predictions. Experiment results of 316L stainless steel and P91 steel validate that the cyclic steady-state stress-strain relationships and the Manson-Coffin law derived from Discch specimens align closely with those from standard round bar. This study provides a reliable and material-efficient approach to evaluate LCF properties in metallic materials using miniaturized specimens, particularly for in-service structural applications.