A Variable Amplitude Multiaxial Fatigue Life Prediction Method
by
Julie Ann Bannantine
Department of Materials Science and Engineering
Abstract
A method to estimate the fatigue life of a component subjected to variable amplitude multiaxial loading has been developed. It is based upon an extension of the strain-life approach which achieved success in correlating the fatigue lives of components subjected to uniaxial variable amplitude loading. In addition, it incorporates multiaxial damage models that relate fatigue damage to remote loading parameters. A computer model was developed to implement the proposed method.
Measured or estimated strain histories are used as input in the method. Corresponding stress histories are calculated using a nonproportional cyclic plasticity model. Damage is calculated using the stress and strain histories and the multiaxial damage models. The plane experiencing the maximum damage is identified as the critical plane and the fatigue life of the component is estimated from the damage calculations on this plane.
Experimental test results were used to verify and evaluate the proposed method. The measured stress-strain response of thin wall tubes loaded in combined tension and torsion was used to verify the nonproportional cyclic plasticity model. Good correlation between predicted and measured responses was observed. Results from tests conducted on SAE 1045 steel components, loaded in bending, proportional bending and torsion, and nonproportional bending and torsion, were used to evaluate the overall method. Again, good correlation between predicted and actual fatigue lives were achieved.
The computer model ws also used to gain a better understanding of the local stress and strain states developed under nonproportional cyclic multiaxial loading and the effects of this loading on fatigue life. Observations and examples leading to this increased understanding are presented.
Download (9 MB)