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Karine Bavard
Abstract
High temperature behaviour and low cycle fatigue of spheroidal graphite cast iron
Karine Bavard - 10 june 2004
Spheroidal graphite cast iron are usually subject to severe and complex
thermo-mechanical cycles in temperatures. Such loadings are at the
origin of appearance of cracks and thus have a direct impact on the
reliability of the part. The aim of this study was to get a better
understanding monotone and cyclic mechanical behaviour of a classical
spheroidal graphite cast iron used at LCF (exhaust manifold, glass
bottle forming moulds).
Ambient tensile tests in-situ (MEB) showed a damage occurring by
decohesion of the interface nodule/matrice and growth of vacuums. A
heterogeneity of deformation on the scale of the clusters of nodules was
shown by a bimodal extensometry (correlation of images). A mechanism of
oxidation was proposed following observations of oxidation in-situ with
the environmental SEM.
A sensitivity to dynamic strain ageing was highlighted 150°C at 400°C by
instabilities of deformation in traction and opposite sensitivity strain
rate in fatigue. In this temperature range, the material hardens. From
500°C, softening or cyclic stability was noticed, just as a
quasi-viscous behaviour.
Several approaches (Manson-Coffin, Skelton and cumulated energy) were
used in order to better exploit the results of lifetime. The existence
of three fields was announced and corresponds to those already found at
the time of the study of the monotonic behaviour (25°C-500°C,
600°C-700°C and 300°C). The fractographic analysis made it possible to
note a dynamic brittleness with 300°C correspondent with a transition
from propagation transgranulaire/intergranulaire.
Keywords:
Spheroidal graphite cast iron, low cycle fatigue, monotonic and cyclic
behaviour, high temperature, lifetime, fractography, tensile test
in-situ (MEB), bimodal extensometry, oxidation
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