An equivalent uniaxial stress process for fatigue life estimation of mechanical components under multiaxial stress conditions

Fatigue life estimation or fatigue damage evaluation of mechanical components under a multiaxial state of stress time history has an important role in virtual design phases but this evaluation is a real problem to resolve because this goal is not reached by classical fatigue criterions. Moreover the virtual simulation of components and structures will only be efficient if it uses reliable and quick tools in each step of the analysis. Above all the analytical instruments used in fatigue damage evaluation must guarantee minimal computational effort starting from simulated stresses. In this context the authors chose the uniaxial equivalent stress as the approach useful for this purpose by combining it with a strength curve for the material (i.e. S–N curve) and a rule to evaluate damage (i.e. Miner’s rule). So the authors has developed a new equivalent stress that can be used as a uniaxial stress time history suitable for the application of damage evaluation methods. However the uniaxial equivalent process is not accepted by the majority of researchers because a definition in agreement with experimental results has not been found yet. Furthermore some theoretical problems appear when an equivalent stress defined in static conditions is extended to dynamic conditions. In this paper the authors introduce an original form of equivalent uniaxial process starting from an accurate theoretical analysis of octahedral planes. It will be demonstrated that this process can be defined both in the time domain and the frequency domain. Then the agreement between experimental tests found in literature and the expectations of the proposed method will be also shown by a comparison with the best multiaxial criteria.