ADVANCED FRACTURE MECHANICS

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ADVANCED FRACTURE MECHANICS

Author: Dr Noel O’Dowd | Size: 3.7 MB | Format: PDF | Quality: Original preprint | Publisher: Imperial College London | Year: 2002 | pages: 107

Fracture mechanics concerns the design and analysis of structures which contain cracks or flaws. On some size-scale all materials contain flaws either microscopic, due to cracked inclusions, debonded fibres etc., or macroscopic, due to corrosion, fatigue, welding flaws etc. Thus fracture mechanics is involved in any detailed design or safety assessment of a structure. As cracks can grow during service due to e.g. fatigue, fracture mechanics assessments are required throughout the life of a structure or component, not just at start of life. Fracture mechanics answers the questions: What is the largest sized crack that a structure can contain or the largest load the structure can bear for failure to be avoided? How long before a crack which was safe becomes unsafe? What material should be used in a certain application to ensure safety?

Studies in the US in the 1970s by the US National Bureau of Standards estimated that “cost of fracture” due to accidents, overdesign of structures, inspection costs, repair and replacement was on the order of 120 billion dollars a year. While fracture cannot of course be avoided, they estimated that, if best fracture control technology at the time was applied, 35 billion dollars could be saved annually. This indicates the importance of fracture mechanics to modern industrialised society.

The topics of linear elastic fracture mechanics, elastic-plastic fracture mechanics and high temperature fracture mechanics (creep crack growth) are dealt with in this course. The energy release rate method of characterising fracture is introduced and the K and HRR fields which characterise the crack tip fields under elastic and plastic/creep fracture respectively are derived. The principal mechanisms of fracture which control failure in the different regimes are also discussed. In the later part of the course, the application of these fracture mechanics principles in the assessment of the safety of components or structures with flaws through the use of standardised procedures is discussed.

The approach taken in this course is somewhat different from that in Fundamentals of Fracture Mechanics (FFM) as here more emphasis is put on the mechanics involved and outlines of mathematical proofs of some of the fundamental fracture mechanics relationships are provided. There is some revision of the topics covered in FFM, particularly in the area of linear elastic fracture mechanics though the approach is a little different.

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