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Title: The R-Curve approach for the fracture assessment of an aluminium alloy bridge
Author: Cheung, C. M. S.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 2001
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A result of fatigue testing on a full scale aluminium alloy bridge, which is used by the military for temporary crossings, showed an unexpected increase in fracture resistance, compared with its plane strain KIC. This increase was due to a combination of low constraint and large stable crack extension in the bridge components. Previous work had attributed the increase to the loss of constraint alone, but the present work shows stable cracking is equally important. The effect of stable cracking in the bridge alloy was first examined experimentally in large 25mm thick 3PB specimens which were analysed numerically using the finite element method. The numerical 2-D results provided a concave JR-curve showing that dJR/da rises increasingly with crack extension. This increase is associated with the transition from flat to slant fracture in the experimental test pieces, due to the loss of plane strain constraint. Fracture assessment using the R-curve approach showed that long cracks, both in large fracture mechanics specimens and the bridge girder, are stable because, although G>JR, dG/daR/da. Short cracks in standard KIC test specimens are unstable because dG/da>dJR/da when G=JR. The JR-curve for low constraint geometries should be indexed by the T-stress. The fatigue crack growth rates for the bridge alloy from different sources were compared. The Paris law index was nearly 4 for large CT and tension specimens, but only about 2 from fatigue tests on a full scale bridge. For a large tension specimen, the corresponding KIC at the change in index was delayed from 35 (the plane strain KIC value) to 48MPam, because of the effect of low constraint. A simple model based on the JR-curve (at initiation) was developed for predicting crack extension under high cyclic load (Kmax>KIC), but with limited success.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available