Influence of residual stresses on fracture
This thesis presents numerical and experimental research concerned with developing laboratory
test specimens containing well-characterised residual stress fields. These specimens were then
used to examine how residual stresses influenced fracture conditions. Three different materials
were used in this work; an A508 ferritic steel, and two aluminium alloys, 2650 and 2024.
Residual stresses were generated using a technique called local compression on both uncracked
plates and cracked compact tension, C(T), specimens. Residual stresses introduced by single
punching tools on the uncracked specimens were examined theoretically and numerically to
benchmark further developments. Also residual stresses were measured using three techniques,
deep-hole drilling (DHD), centre-hole drilling (ICHD) and synchrotron diffraction (HEXRD)
and excellent agreement between measurement methods was obtained. A parametric study was
carried out to determine the features of the residual stress field generated in cracked specimens.
The position of single and double pairs of punching tools relative to the crack tip as well as the
size of the punches were examined systematically. The numerical analyses revealed that
positioning a single punching tool tangentially to the crack tip resulted in the generation of a
tensile residual stress field ahead of a crack. Furthermore, double pairs of punching tools were
shown to generate either tensile or compressive residual stresses normal to the crack plane
depending on the relative position of the tools to the crack tip. The numerical findings were
confirmed experimentally through HEXRD measurements and fracture tests.
Local compression and prior overloading were applied to C(T) specimens to generate a residual
stress field, either independently or in combination. It was found that tensile residual stresses
reduced the apparent fracture toughness and that compressive residual stresses resulted in
increased the fracture toughness. The shift in the apparent fracture toughness depended on the
magnitude of the residual stresses and material, with the aluminium alloys being more
susceptible to the presence of tensile residual stresses.
A local approach based on the Beremin model was used to predict failure in the presence of
residual stress fields in terms of fracture toughness for cleavage fracture in steel specimens.
The overall trends from predictions were similar to the experiments, but there remain
limitations in the model. For aluminium specimens, a method based on the William's series
was employed to predict the stress intensity corresponding to a residual stress field (Kres). The
measured changes in initiation toughness matched the predicted values of K1es.