Assessment of welding induced plastic strain using the thermoelastic stress analysis technique

Abstract

The work presented in the thesis is dedicated to the development and validation of a new technique to assess plastic strain based on thermoelastic stress analysis (TSA). Welding induced plasticity (WIP) and welding residual stresses can negatively affect the structural integrity of welded structures as they can exacerbate creep and stress corrosion cracking and limit the structure’s resistance to failure. Moreover, WIP has been shown to negatively affect weld integrity, since the associated accumulation of defects (dislocations) in the material will accelerate the nucleation of macro-scale defects that lead to component failure.

There has been considerable amount of work published on determining the magnitude and distribution of the residual stresses both experimentally and by using numerical techniques. WIP can be predicted using finite element analysis (FEA), however, there is currently no standardised experimental method to characterise plastic strain and hence, model predictions are not readily validated with confidence. Recently, two techniques, based on electron backscatter diffraction and indentation respectively, were developed to assess WIP. However, both techniques are destructive and would not be applicable on in-situ components.

TSA is a non-contact stress analysis method which is quick to apply and fully portable. TSA is based on the measurement of a small temperature change that occurs as a result of a change in the stress. The small temperature change is measured using an infrared detector. A method for plastic strain assessment (PSA) using TSA has been proposed based on the change in the thermoelastic response due to the plastic strain a material has experienced during a process, e.g. deformation or welding. TSA has the potential to be the first nondestructive, non-contact plastic strain assessment technique, termed as TSA-PSA.




The aim of the PhD is to investigate the potential of using the TSA-PSA approach for assessing WIP in austenitic (AISI 316L) and ferritic (SA508 Gr.3 Cl.1) steels. The influence of welding induced microstructural changes on the thermoelastic response is investigated to establish any changes in the thermoelastic response relating to plastic straining only. The study focuses on two typical nuclear grade steels; ferritic SA508 Gr.3 Cl.1 and austenitic stainless steel AISI 316L. The effect of plastic strain on the thermoelastic response of both steels is investigated through the design and assessment of a calibration specimen used to determine the thermoelastic constant variation with plastic strain alongside with microstructural changes. It was found that the plastic strain has a stronger influence on the thermoelastic constant in SA508 than in AISI 316L. For uniform microstructures the influence of plastic strain on the thermoelastic response can be defined and, a larger influence of plastic strain on thermoelastic response was reported for coarse grains of austenite in AISI 316L and coarse grains of ferrite in SA508.

The second part of the work concerns development finite element (FE) models of weld mock-ups to demonstrate application of TSA-PSA. The modelling enabled the plastic strain experienced during welding to be predicted and adjustments made to the design prior to the mock-up manufacture. Once satisfied that the mock-ups were suitable for TSA, they were manufactured at TWI Ltd. TSA experimental work was conducted on each mock-up and the outcome was compared with the outputs from the calibrated FE models. The capability of TSA to identify plastic strain in welded components is assessed through the use of the weld mock-ups. The thesis makes a novel contribution to the development of TSA as a portable non-destructive, non-contact technique to assess WIP in components with the investigation of the influence of microstructural changes similar to that found in welds on the technique, as well as the design, manufacture and plastic strain predictions in weldments dedicated to the technique. The results indicate a stronger influence of the plastic strain on the thermoelastic constant in coarse-grained microstructure in both grades of steel.

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