Olaf Hertel

Notch stress and strain approximation procedures for application with multiaxial nonproportional loading

Abstract An extension of several algorithms is presented for estimating elastic-plastic notch stresses and strains for metallic materials under cyclic multiaxial nonproportional loading. The extension allows for using advanced models of incremental plasticity. The accuracy of the algorithms, i.e. the structural yield surface approach according to Köttgen et al. and the incremental Neuber approach according to Glinka et al., is shown by comparisons with finite element calculations and notch strain measurements. The accuracy of both approaches is comparable. The differences in predicted stress-strain paths are in the same order as those to measured and FE-calculated paths. These differences lead to a factor of up to two in calculated fatigue lives. Glinkas approach can be applied much more easily; Köttgens approach offers the opportunity for further improvements in accuracy.

A Unified Fatigue Life Calculation Model for Notched Components Based on Elastic-Plastic Fracture Mechanics

Based on Dankert’s et al. [1] initial model for the elastic-plastic evaluation of fatigue crack growth in sheets providing elliptical notches, a generalized procedure enabling an improved evaluation of the effective ranges of the crack driving force (i.e. the J-Integral) as well as the application to arbitrary notched components has been developed [2]. The present paper presents the basic topics of the calculation model as well as its verification using experimental results from notched specimens with various notch shapes subjected to cyclic loading with various load ratios.

Fatigue crack growth at notches considering plasticity induced closure

Classical fatigue analyses discriminate between technical crack initiation (crack length of about 1mm) and crack propagation stages. The stage of crack initiation, however, is itself dominated by the growth of short fatigue cracks. Based on the assumption that a fatigue life to initiate a microstructurally short crack of dimensions in the order of 10µm may be neglected Dankert et al. [1] have proposed a so-called unified elastic-plastic model for fatigue crack growth evaluation which describes the whole fatigue life (technical crack initiation and stable crack growth) of notched and unnotched components by integrating an appropriate crack growth law. In the meantime this model has entered a guideline for the proof of the strength of components [2].

Short Fatigue Cracks in Notched and Unnotched Specimens under Non-Proportional Loading

A dominant part of fatigue life is spent in short crack growth. Modelling this process using fracture mechanics based methods has led to both increased insight into fatigue and improved prediction accuracy. A short crack growth model for the prediction of constant amplitude fatigue life to technical crack initiation under multiaxial nonproportional loading has been proposed by Doring et. al. [1]. Here, results of further work are presented which are intended to improve and extend the mentioned short crack model.