Thermomechanical design in the automotive industry

Abstract

A B S T R A C T In this paper thermomechanical fatigue assessment in the automotive industry is discussed.The design strategy is based upon a consistent approach of the thermomechanical loading, the mechanical constitutive law of the material, the damage parameters and the fatigue strength criteria. The good understanding of these different steps allows one to perform predictive calculations of automotive parts subjected to thermomechanical loading. The main hypotheses and modelling choices are presented and results are illustrated by a series of computations on real 3D structures. Cracked area and lifetime prediction are described in the case of aluminium alloy cylinder heads subjected to transient thermal loadings.Development in numerical simulation over the past 20 years have led to increased performance of numerical tools and the development of new algorithms. Today this enables ever-quicker calculations of loads and displacements in structures. Nevertheless, whilst academic work is quite active few studies have been performed on structures subjected to multiaxial thermomechanical loading. Only a small number of components in automotive technology experience thermomechanical loading cycles of a severity capable of producing low-cycle fatigue. Most of these parts are related to the engine: cylinderheads, exhaust manifolds, crankcases, . . . . It is important to note that the low-cycle fatigue problems in such engine components are related to the start-operate-stop cycles and not to the combustion cycles. Therefore they involve large temperature changes. A previous study was dedicated to cast-iron exhaust manifolds. 1 This work showed an original comprehensive approach, which resulted in a reliable prediction of damage location and of lifetime in spite of basic assumptions like the separation of the damage and the mechanical constitutive behaviour. The paper presents an overview of this approach and the specific details of the applications in the case of the cylinder head in aluminium. The results are illustrated by a series of computations on real 3D structures. The words comprehensive approach shall denote the following series of items:Correspondence: Jean-Jacques Thomas. r thermal analysis, r elasto-viscoplastic mechanical analysis and r fatigue analysis.These three aspects are linked together; the thermal loading and the material behaviour determine the mechanical response of the structure and from that the fatigue strength of the structure can be estimated through a damage indicator and a pertinent fatigue criterion.For a design strategy, one should not concentrate separately on these four aspects of the problem. It should be a comprehensive approach to obtain an important robustness and to allow its integration in a development process 2 by the ability to answer the asked question in the appropriate time. That is to say in our case: is the fatigue strength of the component guaranteed or not before the production of tooling for a prototype? The cylinder head is made of an aluminium alloy (in this case an A...

Analysis of fatigue cracks of cylinder heads in diesel engines

“…Low cycle fatigue is caused by repeated start-up and shout-down cycle of the engine (Mirsalim et al, 2009;Thomas et al, 2002Thomas et al, , 2004Ghasemi, 2012).…”

“…According to the introduction, due to lack of information on the behavior of hardening, softening and viscosity of materials, the analysis of cylinder heads is mostly based on simple models of material behavior like elastic-plastic, and the effects of viscosity and creep of cylinder heads are less taken into consideration. Aluminum alloy has creep behavior at about 300 • C and viscosity should also be taken into account (Su et al, 2002;Thomas et al, 2002Thomas et al, , 2004Koch et al, 1999). The main objective of this study is to simulate thermo-mechanical behavior of cylinder heads based on the two-layer viscoplasticity model.…”

“…The temperature difference, which is a result of turning the engine on and off, begets thermo-mechanical fatigue (TMF) loads on the cylinder heads (Azadi et Thomas et al, 2004) and, consequently, reduces their lifetime, especially in thinner regions (Remy and Petit, 2001). The crucial regions include the valves bridge and areas near spark plugs and injectors (Gocmez and Pishinger, 2011;Shojaefard et al, 2006;Ziehler et al, 2005).…”

“…TMF of cylinder heads was studied by Thomas et al (2002Thomas et al ( , 2004) using the energy model and elasto-viscoplastic law. Their research proved a good agreement between experimental and simulated results of the fatigue life of the cylinder heads and the location of crack initiation.…”

Modelling of Fatigue Damage in Aluminum Cylinder Heads

“…These cracks can lead to engine failure (Fig 1a and b). It is therefore easily understood that engineers and material scientists in the automotive industry pay very much attention to this mode of failure, see eg [2][3]5,8,12].…”