Objectives
Better understand the relative roles of crack tip blunting and residual stress in WPS.
Assess the ability of Sylvain Bordets advanced local approach model to predict the WPS benefits and create a database of fracture test results for calibration of the parameters.
Model existing TWI and proposed French large scale tests to predict WPS benefits and compare with the experimental data and current procedures for accounting for WPS effect.
Project Outline
Fundamentals of WPS
A structured series of small scale uni-axial fracture tests will be carried out under various conditions. The measured fracture toughness will be interpreted with regard to micro-structural observations and finite element estimates of the residual stress at the crack tip prior to fracture. Finite element analyses will also be carried out in order to correlate the measured toughness with the effects of plasticity and constraint on WPS. The experimental programme is described below:
Reference fracture tests without WPS (20 tests)
Fracture tests after WPS using LUCF and LCF cycles (up to 20 tests each)
Tests with WPS using LUC and LC, no fracture, for examination (2 tests each)
Tests with WPS using LC, stress relief heat treatment, then fracture (up to 20 tests)
Tests with residual stress, WPS using LUC and LC, then fracture (up to 20 tests each)
The material to be tested is 18MND5 (French equivalent to A533-B) and is available from EDF as part of NESC-7. The number of tests will give a statistical database for the tasks described below.
Modelling WPS using Sylvain Bordets advanced local approach model
Sylvain Bordets advanced local approach model will be used in parallel with the experimental work described above in order to predict the initiation of cleavage micro-cracks during the WPS cycle. This model can predict separately the initiation of micro-cracks and their possible propagation by cleavage fracture. In this model, initiation depends on yielding and propagation depends mainly on the maximum principal stress. This model is considered adequate for the analysis of warm pre-stressing because it could predict the initiation of micro-cracks, which would blunt at high temperature and may not lead to cleavage fracture at low temperature unless the stress is increased. The local approach model may also account for the effect of residual stress and the main crack tip blunting.
Modelling of large scale fracture tests
Large scale uni-axial and bi-axial fracture tests were carried out in the 1980s and 1990s, partly at TWI, and biaxial fracture tests are currently being carried out by CEA in France for the NESC-7 project. The materials tested were A533B and other grades for offshore engineering or pressure vessels. Different conditions were applied in these tests: initial crack located in the parent metal or in the weld; fracture test carried out before or after WPS; various bi-axiality ratios. It is intended to select a number of these tests (about eight), and to calculate the crack driving force in a finite element model. The elastic-plastic analyses will account for the effect of loading history prior to fracture and for the constraint. Both isotropic and kinematic plasticity models will be considered.
Relevant Industry Sectors
Nuclear, Oil and Gas, Construction and Engineering
Technical and Economic Benefits
These are highlighted under Research Need. In summary the project has the potential to:
Provide a validated procedure for giving better prediction of the effect of WPS
Develop understanding of the fundamentals of cleavage fracture
Input into codes and standards