NSIRC PhD student Ali Same has been accepted for Neutron Diffraction Beam-Time at the Paul Sherrer Institute (PSI), Switzerland, after successfully completing his submission with help from his academic supervisor, Professor Xiang Zhang of Coventry University, and industrial supervisor, Tyler London at TWI Ltd.
In the last two decades, X-ray and neutron diffraction have been primary tools in the natural sciences. Neutron scattering and synchrotron sciences represent one of the premier examples of interdisciplinary research.
Residual stresses, or their development under applied external force, can have a strong influence on the basic mechanical properties of materials. Indeed, it is very rare to encounter engineered components that do not contain residual stresses, caused either by fabrication, joining, assembly, heat or surface treatment, service use or, more probably, by a complex combination of these. They displace atoms from their original positions in a crystalline material and this can be non-destructively quantified by diffraction-based techniques (in special neutron diffraction) or through electromagnetic radiation using the Bragg Diffraction Angle Analysis. The other advantage of neutron scattering is the large penetration depth (of the order of several cm) as compared to X-rays, which make it an increasingly important technique in assisting engineering design and the advancement of engineering materials. Because of these ubiquitous scientific endeavours, the instrument becomes an attractive technique to be developed around medium flux research reactors.
In view of this, a proposal for Neutron Diffraction beam-time was submitted to the Paul Sherrer Institute (PSI), which is the largest research institute for natural and engineering sciences in Switzerland conducting cutting-edge research in three main fields; matter and materials, energy and environment and human health. The title of the proposal was ‘Investigation of Mechanically Induced Residual Stress Field on Conventional Fracture Specimen using Non-destructive Techniques.’
The results from this testing will contribute to Ali’s PhD thesis (titled: ‘Development of Advanced Crack Modelling Techniques for Fracture Mechanics Assessments’) and will be disseminated by the regular academic routes, through journal publications and conferences, with direct contributions to the structural integrity work of TWI and BS7910.
Having started his PhD with NSIRC in September 2016, Ali’s project focuses on the development of new numerical modelling techniques for fatigue crack growth and, in particular, non-uniform crack front evolution and crack growth through complex, through-wall residual stress profiles. Ultimately, the aim of the combined numerical-experimental research programme is to improve guidance in existing codes and standards (e.g. R6 and BS 7910) to reduce the potential over-conservatism of fracture mechanics and fitness-for-service assessments. By simulating the residual stress field and measuring the residual stresses using multiple experimental methods, the project will have a high degree of confidence in the state of residual stress through which the crack will grow.
The experimental programme will be used to validate and fine-tune the proposed numerical models for crack propagation. Throughout the project, a focus will be on the development of automation software for modelling crack propagation. This will allow for efficient computations and simulation run-times, enabling a high-level of functionality for the advanced crack propagation models. The results of this project are expected to have a significant impact on existing knowledge as well as integrity assessment methods, where crack propagation and ductile tearing play important roles in the life cycle of complex welded structures.
Useful link: Paul Scherrer Institute (PSI)