Fracture mechanics 2nd Edition PDF

Introduction

Conventional engineering design is based on avoidance of failure by general plastic collapse. The material property specified in design codes is the flow stress: usually the yield stress or 0.2% proof stress, but occasionally, in older codes, the tensile strength. The DESIGN STRESS is then the applied stress calculated to cause collapse, divided by a SAFETY FACTOR. Typical safety factors are: 1.5 for wrought steel in applications such as pressure vessels or boilers; perhaps, 4 for steel castings in similar applications; and some 5-10 for wire ropes, supporting crane hooks or lift cages. The prime aim of the safety factor is to take account of any extra stresses imposed during erection, fabrication, or service, which may raise the applied stress to the value required to cause plastic collapse and failure.

As defined above, the safety factor does not recognise the possibility of failure by an alternative mode such as 'brittle' or 'fast' fracture. It was generally believed that the safety factor could 'safeguard' against this type of low stress fracture by the use of higher figures applied to the tensile strength. However experience has shown this not to be the case, there being a number of instances where total failure of a component or structure has occurred in the presence of a material defect or crack at stresses well below the design stress. Moreover, the higher safety factors applied for castings, as compared with wrought material, stem from fears that the castings might contain more inherent defects, which could lead to fast crack propagation at or below the applied design stress. This, in the engineering sense, is a 'brittle' failure and it is clearly necessary that a STRESS CONCENTRATOR must be present to obtain a brittle failure because the plastic strain required to operate the fracture mechanism has to be able to develop in a local region, without causing overall general collapse.

In service, the stress concentrators of importance are CRACK-LIKE DEFECTS, particularly if these are situated in regions of high background stress, such as those around fillets, keyways, nozzle openings or hatchways. Typical examples of crack-like defects include:-

Solidification cracking in welds of-castings

Hydrogen cracking in heat-affected zones

Lamellar tears around inclusions in rolled plate

Cracks which have grown in a 'sub-critical' manner by fatigue or

stress-corrosion mechanisms.

It is usually possible to detect such defects, using ultrasonic inspection or some other NDT technique and to determine the maximum size of defect in the region of interest.

Edited by: John Knott, Paul Withey