Showing papers by "Noel P. O’Dowd published in 2007"

Comparison of Crack Driving Force Estimation Schemes For Weld Defects In Reeled Pipelines

Abstract: In this work, analytical estimation procedures for the evaluation of the crack driving force under strain control for surface cracked pipelines have been examined. Pipes with outer diameter in the range from 10.75” to 18” (approx. 0.25 m to 0.5 m) and corresponding minimum wall thickness have been considered. For an assumed level of critical toughness and a range of crack lengths from 25 mm to 200 mm, the maximum acceptable crack depths have been determined for each pipe size. The crack driving forces from different estimation schemes have been compared with the results of a finite-element analysis.

The Effect of Residual Stress and Microstructure on Distortion in Thin Welded Steel Plates

Abstract: The current trend in ship construction is to reduce the thickness of the ship panels, in order to minimize weight and maximize vessel speed. The ship panels of interest consist of 4 mm thick butt welded plates. This reduction in panel thickness may lead to excessive plate distortion during welding, resulting in significant additional costs during assembly. A ferritic-pearlitic DH-36 steel is used, in which phase transformations during welding may affect the distortion and stress states observed. Two large plates, representative of ship panels, have been butt welded using a metal inert gas (MIG) process. The temperature histories have been recorded during welding and the resulting distortion profile has been obtained using digital photography. Neutron diffraction measurements have been performed to determine the residual stress state in the plates before welding, due to e.g. processing and laser cutting, and after butt welding of the plates. Reference matchsticks from the weld, heat affected zone (HAZ) and parent plate have been taken from similar locations in nominally identical plates and measured to obtain the strain/stress free lattice parameter, α0 . A Rietveld analysis has been performed on the diffraction data. Post welding, hardness surveys have indicated the microstructural variation in the weld, parent plate and HAZ. Results from these on-going studies are presented which identify the key factors responsible for thin plate distortion.Copyright © 2007 by ASME

Computational modelling of crack initiation in a single crystal superalloy under fatigue-oxidation conditions

Abstract: Crack initiation in a single crystal nickel base superalloy was studied under fatigue–oxidation conditions, using a crystallographic constitutive theory in conjunction with a mass diffusion model. Finite element (FE) analyses were carried out on a notched compact tension (CT) specimen with a void close to the notch surface. The number of cycles to crack initiation was predicted using a strain ratchetting based failure criterion. Microcracks were predicted to initiate from the void due to the more severe ratchetting at the void surface. The applied load level and ratio, as well as the void location, strongly affect the number of cycles to crack initiation from the void. A high temperature oxidising environment is predicted to reduce the number of cycles to crack initiation by enhancing the ratchetting in the vicinity of the void, a consequence of microstructural degradation of the material near the notch due to oxidation linked diffusion processes at the notch root.

Evaluation of Two-Parameter Approaches to Describe Crack Tip Fields in Engineering Structures

Abstract: The application of two parameter approaches to describe crack tip stress fields has generally focused on power law materials which limits the range of applicability of such approaches. In this work we consider the applicability of a J-Q or J-A2 approach (the latter is designated here as the J-A approach) to describe the stress fields for power law materials and for materials whose tensile behaviour cannot be described by a simple power law. The predictions of the two parameter approaches are compared with full field finite-element predictions. Results are presented for shallow-cracked and deep-cracked tension and bend geometries, as these are expected to provide the expected range of constraint conditions in practice.Copyright © 2007 by ASME

A Comparison of Measurement and Modelling of Plastically Induced Residual Stresses in a 316H and a Weld 347 Stainless Steel

Abstract: Neutron diffraction measurements on two types of stainless steel have been carried out on Compact Tension (CT) specimens containing plastically induced residual stresses at the blunt notch root. The materials were a type 316H stainless steel parent material and a type 347 stainless steel weld material. The former exhibited a high creep ductility of ∼25% and the latter exhibited brittle behaviour under operating conditions with less than 10% creep ductility. The work is based in part on an ongoing collaborative effort by the Versailles Agreement on Materials and Standards, Technical Working Area, VAMAS TWA 31 Committee working on ‘Crack Growth of Components Containing Residual Stresses’. The objective of this paper is to examine how residual stresses and/or prior straining and subsequent relaxation at high temperature (550 °C for 316H and 650 °C for 347 weld) contribute to creep crack initiation and growth in the two steels. Elastic/plastic/creep finite-element results and neutron diffraction measurements are presented for the CT specimens before and after elevated temperature exposure. The results suggest that the mechanical induced normalised stresses and strains profiles ahead of the crack tip are insensitive to material, however the relaxation response of the materials appear to be dependent on the creep behaviour and ductility. Localised cracking in the plastically deformed material has been observed in both materials due to the redistribution of the residual stress field and associated creep deformation at elevated temperature.Copyright © 2007 by ASME