A two parameter characterisation of defects (with reference to pipe work)

[摘要] The work described in this thesis comprises an investigation into the effect of the non-singular elastic stresses ( T and S) on elastic-plastic crack tip deformation. A necessary preliminary of the present work involved calculating the T stress for a wide range of single edge cracked geometries loaded in tension and bending. Direct stress and displacement methods have been used to calculate the T stress. The effect of the T stress on J dominance for these geometries has been investigated and the stress fields ahead of the crack tip were compared with modified boundary layer formulations involving K and T, following Larsson and Carlsson (1973) and Betegon and Hancock (1990). Part of the work presented in this thesis comprises of an experimental investigation on the effect of the crack depth on fracture toughness and a (J-T) and (5-T) locus for three point bend specimens has been produced. Experimental tests were carried out on a carbon- manganese structural steel grade 500 under a range of temperatures from room temperature (+23C) to liquid nitrogen temperatures (-196C). Previous investigations involved two dimensional plane strain geometries in which only the effect of the T stress was considered. However for three dimensional geometries such as those involving semi-elliptical cracks and circumferential cracks, it is important to understand the effect of the out of plane (S) stress on the crack tip stress field. The effect of the out of plane stress (S stress) has been investigated using both modified boundary layer formulations and full field solutions. For modified boundary layer formulations the displacement field associated with the elastic singular term K and the two non-singular terms T and S were imposed as a boundary condition at a distance remote from the crack tip. Modified boundary layer formulations were compared with full field solutions involving two different types of axisymmetric geometries, namely the circumferential crack in a round bar and central crack in a round bar. These geometries were chosen to represented different values of the S and T stress. The present work also includes an estimation of the non-singular terms S and T for circumferential crack in pipes. Two analyses were considered, a continuum model and line spring model. The results of the two models were in a good agreement with each other. As a first attempt to provide a completely workable fracture design approach based on the J-T locus curve, the T stress for a semi-elliptical crack has been derived using a line spring analysis. The geometry chosen for this calculation was an internal circumferential and longitudinal semi-elliptical cracks allocated in an internally pressurised cylinder.

[发布机构] University:University of Glasgow

[关键词] Mechanical engineering