Y. Nadot

High cycle fatigue of 1045 steel under complex loading: Mechanisms map and damage modelling

A model of life prediction is proposed for a polycrystalline metal subjected to multiaxial complex loading in high cycle fatigue. In order to understand the mechanisms of plasticity and damage to be modelled, a campaign of experiments conducted on 1045 steel is carried out in the first part of the study. The modelling options are chosen according to this precise observation of mechanisms. The modelling focuses on the prediction of finite life regime (105–107 cycles). In order to overcome a purely phenomenological description, a two-scale damage model (macro–meso) integrates a multiaxial fatigue criterion and is formulated in the framework of thermodynamics of irreversible processes allowing to capture as closely as possible degradation mechanisms at mesoscopic scale as well as phase shift effect and nonlinear fatigue damage accumulation. The incremental formulation of the proposed model is an asset to deal with variable amplitude loadings in future works.

Gradient approach for the evaluation of the fatigue limit of welded structures under complex loading

Welded ‘T-junctions’ are tested at different load ratio for constant and variable amplitude loading. Fatigue results are analyzed through the type of fatigue mechanisms depending on the loading type. A gradient approach (WSG: Welded Stress Gradient) is used to evaluate the fatigue limit and the comparison with experimental results shows a relative good agreement. Nonlinear cumulative damage theory is used to take into account the variable amplitude loading.

Improvement of Fatigue Properties of Cast Aluminum Alloy A356 by Warm Deformation

Aluminum foundry alloys such as A356 are used extensively in applications where high cycle fatigue (HCF) resilience is a key design consideration. Fully reversed, multiaxial HCF studies on this alloy in the T6 condition have shown that endurance limits are governed by maximum principal stress and driven by crack propagation as opposed to initiation. In light of these fatigue characteristics, it has been found that warm deformation imparted via flow forming prior to heat treatment improves the fatigue resilience by upwards of 30% depending on the degree of deformation. The fatigue performance improvement has been attributed to eutectic particle size and morphology changes and potential recrystallisation of the primary phases. This hypothesis is supported by extensive particle characterisation and preliminary EBSD results.