Page 902 - CP2009

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Influence of discrete residual stress fields on fracture
D. T. Asquith
, Y. H. Tai
and J. R. Yates
Department of Mechanical Engineering, University of Sheffield, UK,
The rate and direction in which cracks grow during ductile fracture is
influenced by both stress state and material properties, particularly plasticity and
anisotropy. Previous work has shown that compressive surface residual stress fields
such as those caused by shot-peening or burnishing can be used to modify the
behaviour of a propagating crack. To exploit the controlling behaviour of residual
stresses it is necessary to understand the interaction between secondary applied
residual stresses and the crack tip stress field. Controlled plasticity burnishing has
been used to create near 1-dimensional compressive residual stress fields in the surface
of AA2024 in attempts to modify cracking behaviour. A direct optical method (Digital
Image Correlation) has been used to characterise the crack tip displacement fields and
subsequently calculate values of CTOA. A series of tests on specimens with continuous
and intermittent compressive stress zones was used to explore the relationship between
the crack tip and the applied residual stress.
The ductile tearing behaviour of aluminium alloys is of particular interest in the
aerospace industry for life prediction and mitigation of catastrophic failures. Improved
understanding and the development of techniques for improving and controlling the
tearing behaviour would allow for more economical approaches to aircraft design and
operation. It is well established that the presence of residual stresses alters structural
integrity and this provided the inspiration to undertake this work.
For this investigation, a series of compact tension (CT) tearing tests were carried out
g thin s
heet 2024-T3 aluminium. This material was chosen because of its
widespread use in the aerospace industry and it is known to have stable tearing
properties. Near one dimensional compressive residual stresses were imparted onto the
surface using the controlled plasticity burnishing technigue developed at the University
of Sheffield [1]. Burnishing was done in different orientations to determine its effects
on crack stability and fracture toughness. The CTOA fracture parameter was used
because it has been shown to be well suited for characterising ductile tearing fracture