Hysteresis Strain Energy Behavior of Al6061-T6 With Multi-Fatigue Load Levels as Applied to an Energy-Based Fatigue Life Prediction Method

Abstract

Fatigue testing is a time and resource-consuming task. Historically, SN testing was conducted at many stress levels on simple representative specimen in order to determine an SN curve, which could then be used to design a component from the same type of material. Recently, an energy-based fatigue life prediction method has been in development. The goal of this method is to quickly determine a material’s fatigue characteristics using simple test procedures. The main theory behind the energy-based fatigue life prediction method is that the strain energy in a monotonic tensile test is equal to the cumulative hysteresis energy of a cyclic test. This theory has always been tested using a single stress level on each specimen. The hysteresis loop information was then used to make fatigue life predictions at other stress levels. Further testing has been done to learn more about the hysteresis energy behavior throughout the lifetime of a specimen, but only for a single stress value. In this study, several stress levels were tested on a single specimen. This new information will help make fatigue life predictions by completely removing the difficult and inconsistent process of determining experimental curve fit coefficients traditionally used in the energy-based fatigue life prediction method.

On the relationship between mechanical energy rate and heat dissipated rate during fatigue for a C45 steel depending on stress ratio

“…The energy during a cyclic process, _ W , can be defined by considering stress and strain over the time 10,11,13,16,17 :…”

“…The energy during a cyclic process, , can be defined by considering stress and strain over the time 10,11,13,16,17 : where σ m , σ a , ε m , and ε a are mean and amplitude of the stress, mean and amplitude of the strain, φ the phase shift between stress and strain, and f the loading frequency. As it is possible to observe ε m and ε a values are clearly dependent on stress ratio ( R ) and stress amplitude.…”

“…The dissipated energy in the material (cyclic plastic energy) because of mechanical loading is related to the number of cycles for failure and can be used as a damage parameter for the material fatigue life estimation 4–6 . Such an energy dissipation is of critical importance for setting up energy‐based fatigue life prediction approaches 10–14 based on the assessment of mechanical energy input 10–19 or heat converted energy by measuring the material surface temperature 20–30 …”

“…[20][21][22][23][24][25][26][27][28][29][30] Even if many efforts have been made to better understand the relationships between surface thermal measurements and heat converted energy, and in turn between heat converted energy and intrinsic dissipations, how they vary depending on loading conditions is still unclear. [30][31][32][33][34][35][36][37][38][39][40][41] Referring to mechanical energy input, the aim is to assess the area under the hysteresis loop that represents the dissipated energy, by investigating the constitutive stress-strain law [9][10][11]13,16,17 or by modeling the strain hardening behavior. 7,11 Both approaches require the definition of a model and the experimental assessment of coefficients (i.e., hardening coefficients) that depend on imposed loading, fatigue regime, material.…”

“…Referring to mechanical energy input, the aim is to assess the area under the hysteresis loop that represents the dissipated energy, by investigating the constitutive stress–strain law 9–11,13,16,17 or by modeling the strain hardening behavior 7,11 . Both approaches require the definition of a model and the experimental assessment of coefficients (i.e., hardening coefficients) that depend on imposed loading, fatigue regime, material.…”