# Predicting Low-Cycle Fatigue Life Using New Energy-Based Fatigue Damage Measures

## TL;DR

This study introduces a new energy-based method to predict the fatigue life of S420M steel, showing that material anisotropy significantly affects durability.

## Contribution

The paper proposes an energy accumulation graph as a novel generalization of isodamage lines for fatigue life prediction.

## Key findings

- Fatigue life of S420M steel samples perpendicular to the sheet surface was significantly lower than parallel samples.
- The UMT thermodynamic approach offers a physically grounded framework for fatigue damage prediction.
- Energy accumulation graphs showed higher unit loop energy in parallel samples compared to perpendicular ones.

## Abstract

What are the main findings?
The research highlights the significant impact of S420M steel anisotropy resulting from rolling on the low-cycle fatigue life.The paper proposes and evaluates an energy accumulation graph as a generalization of isodamage lines for fatigue life prediction. It compares its effectiveness with the UMT thermodynamic approach.

The research highlights the significant impact of S420M steel anisotropy resulting from rolling on the low-cycle fatigue life.

The paper proposes and evaluates an energy accumulation graph as a generalization of isodamage lines for fatigue life prediction. It compares its effectiveness with the UMT thermodynamic approach.

What are the implications of the main findings?
Fatigue life was consistently lower for samples taken perpendicular to the sheet surface compared to samples cut parallel to the rolling direction. This reduction in fatigue life varied significantly, from 40% to almost 290%, depending on the strain amplitude level.The thermodynamic UMT approach provides a physically grounded framework for fatigue damage prediction, integrating energy, entropy, and state variables into a single constitutive formulation.

Fatigue life was consistently lower for samples taken perpendicular to the sheet surface compared to samples cut parallel to the rolling direction. This reduction in fatigue life varied significantly, from 40% to almost 290%, depending on the strain amplitude level.

The thermodynamic UMT approach provides a physically grounded framework for fatigue damage prediction, integrating energy, entropy, and state variables into a single constitutive formulation.

This work investigates methods for predicting low-cycle fatigue life by employing new energy-based fatigue damage measures. The primary goal of this research is to evaluate whether fatigue life can be predicted based on an energy accumulation graph, proposed as a generalization of the isodamage lines concept. The efficiency of fatigue life predictions using this approach, derived from the empirical linear Palmgren–Miner hypothesis, is compared against the physically grounded Unified Mechanics Theory thermodynamic approach, which allows for general understanding of material degradation, in contrast to empirical approaches. The study also accounts for the influence of anisotropy resulting from the sheet rolling process on the fatigue response of S420M steel. Samples were tested in orientations both parallel to the rolling direction and perpendicular to the sheet surface. Microstructural analysis revealed a visible banded structure in the perpendicular samples, which is a consequence of anisotropy. The fatigue life of samples taken perpendicular to the sheet surface was lower than that of parallel samples. Verification of the linear Palmgren–Miner damage summation hypothesis, using both the classical fatigue chart and the cumulative energy chart, resulted in calculated fatigue life consistently higher than the experimental fatigue life in all cases. The reduction in fatigue life ranged from 40% (for total strain amplitude equal to 1.0%) to almost 290% for a strain amplitude of 0.25%. A comparative analysis of the unit loop energy shows that at all tested levels of strain amplitude, the unit loop energy of parallel samples is higher than that of samples perpendicular to the surface.