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Marie-Cécile Robin
Abstract
Mechanical behaviour and fatigue lifetime of tubular parts, welded or not, made of ferritic stainless steel (1.4509): car manifold lifetime assessment
Marie-Cécile Robin-Boudaoud - 14 December 2010
Faurecia uses stainless ferritic steels to produce car
manifolds, which are made of several tubular parts welded
together.
In order to reduce design costs, Faurecia needs to develop a
robust numerical design tool. In the frame of this project, this
thesis is a contribution to fatigue life assessment of 1.4509
stainless steel tubular parts, welded or not, submitted to
cyclic thermomechanical loadings.
In a first part, the cyclic mechanical behaviours of the base
metal (1.4509 steel) and the melted metal are experimentally
studied under various loads. They are then described thanks to
an elastoviscoplastic model. The specific mechanical behaviour
of the heat affected zone is also considered. The ability of the
proposed models to describe the mechanical response of a welded
zone is assessed thanks to a traction compression test including
local strain measurements.
In a second part, fatigue life of 1.4509 steel is addressed. A
large experimental database is built up. A non isothermal
continuum damage model (J. Lemaître model) is
identified. The same test conditions are applied to welded
specimens. Then, the results of those tests are analysed thanks
to an uncoupled approach of mechanical behaviour and damage. It
is so considered that the strain distribution at the stabilized
cycle, governs the fatigue life of the welded specimen. Thanks
to this method, most the tests are well predicted (within a
scattering band of factor 3 maximum).
Finally, last part of this thesis concerns the integration of
the identified models in a robust numerical tool in order to
predict fatigue life of industrial welded parts. A sub model
methodology is used in order to precisely predict the stabilized
mechanical behaviour of welded zone. This technique is
successfully applied to predict the fatigue life of two welded
parts, included a car manifold.
Keywords: mechanical behaviour, fatigue lifetime, weld, stainless steel, lifetime assessment
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