TWI Industrial Member Report Summary 276/1985
By T L Anderson
Fracture analyses based on crack tip opening displacement (CTOD) were examined. The applied conditions and critical conditions were viewed as separate components of the analysis of a structure. It was concluded that there is an upper limit to the accuracy that can be achieved in a fracture assessment because of uncertainties in material properties. A new design equation, which utilises a material's stress-strain curve, was used to predict the applied CTOD in seven wide plate specimens made from 110mm thick A533B steel and 51mm thick 316 stainless steel plates. In most cases the predictions agreed very well with experiment, although slight variations in flow properties caused significant errors in some instances. This new method was compared with the CTOD design curve and strip yield equations. For the A533B steel, the strip yield equations were reasonably accurate below 85% of yield while the CTOD design curve was conservative in this range. At higher stresses, both equations tended to be unsafe. Critical CTOD values in 52mm thick BS 4360 50D steel plate at -65°C were compared for full thickness bend specimens and wide plate specimens. The toughness values in the bend specimens were significantly lower than those in the wide plates, implying that bend specimens have higher constraint. A 'yield before break' criterion is proposed as a simple method for specifying material toughness. Also, a new ductile instability analysis was developed.