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Title: Influence of residual stresses on fracture
Author: Mahmoudi, Amir Hossein
ISNI:       0000 0001 3616 7759
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2005
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available