A method to estimate the fatigue life of a component subjected to variable amplitude multiaxial loading has been developed. The strain-life approach, which achieved success in correlating the fatigue lives of components subjected to uniaxial variable amplitude loading, is used as the foundation of this approach. The method incorporates critical plane 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. Corresponding stress histories are calculated using a nonproportional cyclic plasticity model. Damage is calculated using the stress and strain histories and 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 evaluate and verify the proposed method. The measured stress-strain response of thin wall tubes loaded in combined tension and torsion were 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. Good correlation between predicted and actual fatigue lives was achieved.