Fracture Mechanics Research in the Department of
Mechanical Engineering
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Contents
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The traditional approach attempts to ensure that cracks or defects do not extend by measuring the lowest toughness which the material may be expected to have. Material data is typically obtained from deeply cracked bend specimens which develop highly three dimensional stress fields at the crack tip. These highly constrained fields provide a lower bound estimate of toughness, and ensures a conservative approach when applied to structural defects. |
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Fracture Mechanics
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Research activities
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Although this approach is safe, it is often
too conservative, and may lead to the imposition of prohibitive repair
and inspection policies when applied to short cracks which develop
unconstrained flow fields. Economic pressure demands that plant
down-time be minimised by avoiding unnecessary repairs and shutdowns. To
achieve this goal it is essential that structural integrity
methodologies minimise defects being prematurely sentenced as critical.
Unnecessary levels of conservativism must therefore be avoided as they
entail expensive and unnecessary inspections, outages and repairs. An
over conservative approach also penalises life extension procedures on
existing plant.
Developments, led by work at Glasgow University, have established a framework for quantifying the effect of constraint on fracture toughness through the application of two parameter fracture mechanics. The first parameter quantifies the strength of the crack tip singularity, while the second term is a measure of constraint. International collaboration with MIT and Idaho National Engineering laboratories has provided the experimental work which has verified the theory ( Hancock, Reuter and Parks 1992). This has provided the basis for advances in the defect assessment codes used by the Nuclear Industry both in the US and in the UK. Within the UK the work of MacLennan and Hancock ( Proc. Roy . Soc 1995 and Int J. Press Vessels and Piping 1995) has provided the foundation to the codification of Appendix 14 to the Nuclear Electric defect Assessment Code R6 which is related to BS 7910. |
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Publications
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Contact me
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work has examined the defect re-characterisation procedures recommended
by BS7910 :1999 code and their applicability to fracture under ductile
and brittle conditions, by invoking statistical size effects and
constraint arguments (Bezensek and Hancock 2004). Fundamental research
on the near tip constraint effects have enabled procedures to unify
mixed-mode toughness data with mode I data for cleavage (Bezensek and
Hancock 2004). Practical dimension on the work has also been given
through a development of a new model to quantify the benefits of
constraint expressed as a temperature shift of toughness data in the
ductile-brittle transition range (Bezensek and Hancock 2003, 2005) which
allows enhanced margins on the transitional temperature to be
established. Fundamentals understandings of the crack tip fields that develop in
non-homogeneous materials containing cracks parallel and perpendicular
to sharp and smooth interfaces have been examined analytically (Sham, Li and
Hancock 1999, Li and Hancock 1997, Banerjee and Hancock 2002, 2004) and
verified experimentally with tests of mismatched laser welded joints. Guidelines
have been given on recommended best practice of testing of laser welded
joints to account for fracture path deviation phenomena of such
welds. These developments have been supported by grants from the EPSRC and from
the Marine Technology Directorate. The national and industrial relevance
of the work is confirmed by funding from the DRA, Rolls-Royce and
British Energy Generation Limited, while the European dimension has been
supplied by funding from Shell in Amsterdam.
The work led by Dr. A. Banerjee and Dr. B. Bezensek developed a stochastic local approach model to account for gradients in strength and local toughness that govern fracture in welded joints. Procedure was verified with experiments where temperature gradient was established across a cracked steel bar and with tests on laser welded specimens. These confirm competition between the strength governed sampling volume and intrinsic toughness of a welded joint that establish the failure path and subsequently the integrity of the joint. Current work is motivated towards application of fracture mechanics methodologies to functionally graded materials, development of model material schemes and numerical tools for assessment of defect interaction of embedded (submerged) defects, and towards fundamental aspects of three dimensional near tip fields. National and international recognition of this work has Professor Hancock is a member of the EPSRC Committee for peer revue in Mechanical Engineering, a member of the editorial board of Materials Science and Technology, and a member of The Advisory Group on Structural Integrity which revues structural integrity in the Nuclear Industry. Notable in this context is the co-authored peer review of the Atomic energy Authority code R5: An Assessment Procedure for the High Temperature Response of Structures.
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| Web editor:
Bostjan Last update: November 2004 |
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