Title:
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The influence of warm prestressing and proof loading on the cleavage fracture toughness of ferritic steels
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This thesis presents a combination of numerical and experimental
studies performed to assess the influence of the warm prestress effect
on the cleavage fracture toughness of two ferritic pressure vessel
steels. The aims of the research are to gain a detailed knowledge of
the materials low temperature response under uniaxial and fracture
conditions; to examine, using the finite element method, crack tip
stress fields during warm prestress LUCF load cycles; and provide a
clear and consistent method of classifying the warm prestress effect.
An experimental programme investigated the room temperature and
low temperature response of two candidate steels, A533B and
BS1501. These steels were tested uniaxially under monotonic and
cyclic conditions, and in the cracked condition in the as-received and
warm prestressed conditions. Application of a three parameter
statistical model to the experimental data showed that the distribution
of data in the as received and warm prestressed conditions can be
described accurately. The shift in the cleavage toughness distribution
following warm prestressing was predicted by combining the statistical
model with a validated analytical model of the warm prestress effect.
Repeated proof loading was shown to increase cleavage toughness in
A533B steel, providing the loading was load controlled. There were
negligible effects of repeated proof loading on BS1501 steel. Some
further enhancement of cleavage fracture toughness was observed
when sub critical crack extension was introduced following warm
prestressing, although the results were highly scattered.
The finite element method was employed to simulate experimental
fracture events. It was found from these simulations that fracture
occurs following warm prestressing, when the reloaded crack tip
stress distribution matches the as-received fracture crack tip stress
distribution. The stress matching was observed to occur well into the
elastic stress field ahead of the crack tip. This fracture criterion was
employed to provide predictions of cleavage toughness following
varying applied preload levels. The results were compared to
experimental data sets and various analytical models. The Chell
model of the warm prestress effect was observed to provide the best
agreement with the finite element predictions. Crack tip blunting
during the preload steps was found to have no influence on the
predictions of cleavage fracture toughness. Differences in hardening
response of the material was also shown to have little influence of the
predictions of cleavage toughness. Simulations incorporating sub
critical crack extension prior to reloading to fracture demonstrated that
cleavage 'toughness can be enhanced further by limited crack
extension. Large increments of crack growth were shown to reduce
the warm prestress effect. The finite element predictions were
validated against the appropriate analytical solution proposed by Chell
and experimental results.
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