Towards a temperature dependent and probabilistic lifetime concept for nodular ductile cast iron materials undergoing isothermal and thermo-mechanical fatigue

Elena Garcia Trelles,Christoph Schweizer,Stefan Eckmann,G Hénaff

doi:10.1051/matecconf/201816519006

Elena Garcia Trelles, Christoph Schweizer + Show 2 more

Open Access

https://doi.org/10.1051/matecconf/201816519006

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Abstract

In this investigation, the fatigue behaviour of a ductile cast iron with high content of silicon and molybdenum, was experimentally characterized by performing isothermal low cycle fatigue (LCF) tests as well as out-of-phase thermomechanical fatigue (OPTMF) tests within the temperature range RT – 500 °C. The studied material shows an embrittlement at temperatures nearby 400 °C. A possible explanation for the observed lifetime reduction is intergranular embrittlement (IE). A mechanism based lifetime model is proposed for assessing the lifetime. The model is based on the assumption that the crack advance per cycle is correlated with the cyclic crack tip opening displacement (ΔCTOD) attributed to the crack tip blunting caused by accumulation of plastic and creep deformations ahead of the crack tip. Intergranular embrittlement is accounted for by introducing a temperature and strain rate dependent prefactor in the crack growth law, which only acts in a certain temperature range. The model is calibrated for a GJS material and successfully applied to predict the lifetime of this material when undergoing isothermal and non-isothermal mechanical loadings. A probabilistic interpretation of the scatter of the investigated material is presented in conjunction with the random nature of the initial defect size distribution.

Highlights

Nodular ductile cast iron materials (DCI) provide a good performance at high temperatures, especially DCI materials with high content of silicon and molybdenum
The presence of intergranular embrittlement (IE) damage is related to a shorter fatigue lifetime and the modelling of DCI materials suffering from IE damage demands a new formulation
The first aim of this study is to propose a lifetime model including the damaging effect of IE to enable reliable lifetime assessment of DCIs

Summary

Introduction

Nodular ductile cast iron materials (DCI) provide a good performance at high temperatures, especially DCI materials with high content of silicon and molybdenum Due to their good mechanical properties, castability and cost efficient production processes, the use of these materials for fabricating components undergoing thermomechanical fatigue (i.e. cylinder heads, turbochargers, motor housings) is widely spread. For non-isothermal conditions, the time and temperature dependent damage parameter DTMF [1] was successfully applied for describing the fatigue damage of numerous high temperature resistant alloys This parameter accounts for the temperature dependent material properties, the cyclic stresses and plastic strains, fatigue crack closure as well as for creep accelerated fatigue crack growth. The lifetime assessment is performed by assuming that the crack growth per cycle is proportional to the crack tip opening displacement This model formulation does not account for IE damage. The presence of IE damage is related to a shorter fatigue lifetime and the modelling of DCI materials suffering from IE damage demands a new formulation

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