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The R-curve approach for the fracture assessment of an aluminium alloy bridge. / C.M.S Cheung

Swansea University Author: C.M.S Cheung

Abstract

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 K[IC]. This increase was due to a combination of low constraint and large stable crack exten...

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Published: 2001
Institution: Swansea University
Degree level: Doctoral
Degree name: Ph.D
URI: https://cronfa.swan.ac.uk/Record/cronfa42424
Abstract: 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 K[IC]. 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 J[R]-curve showing that dJ[R]/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 for a given load, G=J[R], dG/dadJ[R]/da when G=J[R]. The J[R]-curve for low constraint geometries should be indexed by the T-stress. The fatigue crack growth rate for the bridge alloy, taken from different sources, was 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 K[IC] at the change in index was delayed from 35 (the plane strain K[IC] value) to 48MPa√m, because of the effect of low constraint. A simple model based on the J[R]-curve (at initiation) was developed for predicting crack extension under high cyclic load (K[max]>K[IC]), but with limited success.
Keywords: Civil engineering.
College: Faculty of Science and Engineering

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