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Accueil du site > Divers > equipes de recherche > Matériaux Fonctionnels et de Structure (MFS) > Activités > Propriétés thermomécaniques des métaux et composites >

25 février 2011

 

This topic is concerning by the evaluation of the damage occurring in metallic alloys in the course of their thermomechanical history. Our works mainly focused on Nodular Cast Irons (NCI, Industrial support MPSA) and Dual Phase steels (DP, industrial support Faurecia).

Nodular Cast Irons (NCI) are widely used in the automotive industry because of the relative low cost fabrication of products with complicated shapes. These materials exhibit good mechanical properties, especially for the use in cyclically loaded components9. Some of automotive pieces, like manifolds, must be dimensionally stable at high temperature, typically around 800 °C. These structural components exhibit relatively low and constant elastic stresses, which can produce their creep straining, their damage, and occasionally their fracture. Ferritic SiMo based NCI are currently employed with various alloy elements for the conception of manifolds. These alloys give a good compromise between economical considerations, mechanical properties and high temperature corrosion resistance. Nevertheless, for the higher levels of exhaust gas temperature, austenitic Ni-rich NCI are often preferred in order to enhance the mechanical properties and prevent decarburation as well as phase transformation. The main goal of this work concerns the creep behaviour of NCI submitted to weak elastic stresses for temperatures ranging between 650 °C and 900 °C. The mechanisms of creep damage and fracture were analyzed using microstructural observations and quantitative measurements of the primary graphite dimensions (Fig.4). Damage evolution in NCI under creep solicitations was evaluated thanks to an adequate damage parameter linking the strain rate at a given time to the minimum strain rate (Hug, MSEA, 2009).

 

Figure 4 - Microstructure of ferritic NCI fractured by creep. (trupt = 995.6h, 800 °C, ecor = 0.34). Arrows indicate creep stress direction. (Hug, MSEA, 2009)

 

Similarly, another field of interest concerns the initiation and growth of damage in dual phase steel by means of microstructural quantification of voids generated at the interfaces between ferritic grains and martensitic islands. The influence of stress triaxiality and strain rate on the behaviour of these structural steel families is also taken account for predicting their limit of formability (Chottin et al Numiform 2010, Japon).

All these works give tools for the numerical modelling of various forming processes, and the predictability of damage and fracture of mechanical components. In particular, modelling of the blanking process remains a very difficult problem, taking account for large strains, friction, ductile fracture and damage. Numerical analysis are carried out in this area in collaboration with the Roberval laboratory (Université de Technolgie de Compiègne) and the Université Laval, Quebec.
 

 

 

 

 

 

 

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