Mixed Mode I and II Fully Plastic Crack Growth. Summary Report.
MASSACHUSETTS INST OF TECH CAMBRIDGE DEPT OF MECHANICAL ENGINEERING
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If a structure cracks, it is desirable that any crack growth be fully plastic to provide large deflections, both for stability by load-shedding to other parts of the structure, and for facilitating crack detection before failure of the entire structure. Unstable fractures are not only serious in themselves, but they may lead to a transition to brittle cleavage fracture that can propagate through parts of the structure that are much more lightly loaded. Materials tests traditionally involve symmetric specimens, in which the crack advances between two slip bands into undamaged material. In practice, asymmetries often occur for cracks near welds, fillets, or shoulders. Then the deformation is focused into a single band, along which the crack advances into pre-damaged material. A measure of fully plastic crack toughness is the crack growth ductility, defined as the extension required as a fraction of the original ligament per unit load drop as a fraction of maximum load. The crack growth ductility thus provides a measure of the stiffness of the surrounding structure that is required for stability of the entire structure. The main result of this work is to show that in typical structural alloys with low strain hardening, asymmetries reduce the crack growth ductility by a factor of two to three. Asymmetries have little effect on the displacement for initiation, or on the crack growth ductility of higher hardening alloys, such as annealed, hot rolled, or normalized steels.
- Metallurgy and Metallography
- Structural Engineering and Building Technology