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Title: Constraint based fracture assessment of through-thickness cracks in a bridge girder structure
Author: Henry, B. S.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1995
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The Christchurch bridge is a lightweight and readily transportable utility which can be rapidly erected to cover a range of emergency bridging requirements. The bridge is manufactured from a weldable high tensile aluminium alloy. During large scale laboratory fatigue testing, cracks were detected in a tension cord of an I-section bridge girder where damage is most critical. The bridge girder sustained much longer critical crack lengths before failure than estimated assuming linear elastic fracture mechanics conditions and using fracture toughness data obtained from highly constrained geometries. To solve this discrepancy, the current study provides a comprehensive elastic-plastic fracture mechanics assessment of the bridge girder containing through-thickness cracks in the tension chord. Three-dimensional finite element modes of the I-section girders are analysed and a wide range of fracture parameters are evaluated. The bridge girder containing cracks is shown to be a low constraint geometry, where constraint is parameterised by the T-stress and the Q-value. The J-T theory enables the characterisation of the opening stress fields at a distance of 2Jys ahead of the crack fronts to within 10% of the actual fields evaluated from the finite element analyses. Numerical analyses of a wide range of experimentally tested high and low constraint specimens are used to quantify the relationship between constraint and fracture toughness of the bridge material. Low constraint geometries show up to eight-fold increase in toughness in comparison with high constraint geometries. The J-integral values of the cracked I-section bridge girder models, calculated at the maximum strain levels obtained in the experiments, are matched with those of the specimens through the fracture toughness loci. The I-section results are within the upper boundaries of the loci. The J-T and J-Q theories provide the means of explaining, in terms of loss of constraint, why the bridge girder is able to sustain much longer cracks than first predicted. The loss of constraint enhances the fracture resistance of the bridge girder.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available