engleski

Prediction of thin-rim gears bending fatigue crack initiation life

Numerical analyses of rim thickness effect on bending fatigue crack initiation life of thin-rim gears manufactured from case carburized 14NiCrMo13-4 alloy steel taking into account actual gear operation are performed. Thin-rim gears are often used in aircraft applications in order to reduce weight. Rim deflections have significant effect on stress amplitudes in the tooth root which may lead to bending fatigue problems. Generally, fatigue process can be divided into two main stages: crack initiation and crack propagation. Since crack initiation stage usually accounts for most of the load cycles to gear failure, especially in high-cycle fatigue, a simplification that failure has occurred when a crack has initiated can be adopted. Service life of gear, according to ISO, can be estimated from tooth bending life factor. For surface-hardened steels, this factor is given with respect to the experimentally obtained number of load cycles required to initiate bending fatigue crack in gear tooth root from pulsator testing and therefore does not take into account fact that in actual thin-rim gear operation a significant reversed stress occurs at the root of the tooth adjacent to the loaded tooth. A proposed numerical model is based on the continuum mechanics approach and on the assumption that fatigue cracks are initiated on the component’s surface. Material is assumed to be homogenous, isotropic and linear elastic. Commercial finite element package ABAQUS/Standard is used to determine stresses in gear. Two-dimensional finite element model is created, assuming plane stress conditions and uniformly distributed load on the tooth flank. Elastic strains are calculated from obtained stresses and corrected using Neuber’s rule to account plasticity effects. The number of load cycles required to initiate fatigue crack is calculated using strain-life approach based on principal strain criterion. Strain-controlled fatigue properties of the material are approximated from hardness. The mean stress as well as residual stress effects are included through Morrow’s mean stress correction. Analysis is performed for all the nodes in the root fillet region of the loaded tooth side. Node with the smallest number of load cycles required to initiate fatigue crack is regarded as location of potential crack initiation. Numerical model is validated by comparison with available experimental data from literature and used for parametric studies of thin-rim gears. Predicted numbers of load cycles required to initiate potential bending fatigue crack as well as its location in tooth root fillet region are presented with respect to the evaluated rim thickness.

gear; rim thickness; bendig fatigue; crack initiation; strain-life approach

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