University: University of Bristol
Main academic supervisor: Dr Alexander Velichko
Project description
Non-destructive measurement of material microstructural properties remains an important problem with a wide range of applications in many areas. For example, the knowledge of material microstructural parameters is crucial for accurately estimating the lifetime of safety critical components in aerospace (jet engines and landing gears) and energy sectors (nuclear power plants), or for characterising material performance and designing new materials with specific properties.
Detailed microstructural examination of metallic components can be performed using several established characterisation methods, such as optical microscopy, X-ray, Electron Back-Scattered diffraction (EBSD), as well as the emerging Spatially Resolved Acoustic Spectroscopy (SRAS). However, these methods are laboratory-based and usually require destructive sample extraction. Microscopy methods (optical, EBSD, SRAS) require complex surface preparation and are limited to relatively small inspection regions representing 2D slices through 3D structures. X-ray methods are mostly restricted to small samples or hard to access national facilities. In many industrially important cases the measurements to identify microstructural anomalies must be performed in-situ during manufacturing or in-service, so current techniques are not sufficient. Examples include detection and characterisation of high-density inclusions and microtextured regions in titanium alloys, in-service characterisation of cast austenitic stainless-steel components in the nuclear power industry, and the production of complex metal and alloy components using additive manufacturing techniques. Therefore, there is a clear need for alternative methods capable of quantifying material microstructure in large sample volumes while in service. It is also critical that such techniques significantly reduce both the time and surface quality requirements compared to conventional methods such as optical microscopy, EBSD and SRAS.
Typically, the mechanical behaviour of materials analysed by continuum fracture mechanics assumes that a damage (crack or void) already exists within the material. Consequently, most of traditional non-destructive evaluation (NDE) methods aim to detect, localise and characterise macro- or micro- defects in a material component. However, ultrasound also represents an attractive solution to detect and characterise microstructural “weakest link” locations on the meso-scale, allowing the screening of material volumes of hundreds of cubic centimetres with a large penetration depth and sub-millimetre resolution. The major limitation of traditional ultrasonic methods, used in medical diagnostics or NDE, is that they are mostly restricted to qualitative imaging. Quantitative ultrasound approaches are designed to predict only averaged material properties (for example, mean grain size or crystallographic texture) of the entire volume and are unable to detect and quantify local microstructural variations.
The aim of this project is twofold. The first work-package will complement the new core RCNDE research project (started in July 2025 and leaded by Dr. Velichko) with the goal to develop 3D ultrasonic array imaging method for characterisation of material microstructure. This work will be based on extracting and analysing the set of scattering coefficients (scattering matrix) from each local region in an inspected component. The significant advantage of this data representation is that the scattering matrix does not depend on the measurement system and represents a “fingerprint” of a local material region, providing a new dimension for microstructure characterisation and local material information beyond that directly available in ultrasonic images.
In the second work-package, the developed microstructural imaging tools will be used to relate microstructural features to damage initiation sites. The identification and description of influencing microstructural features on damage initiation and growth remains an open problem and is a current major research area. Irrespective of the material or the loading conditions, previous studies have highlighted two important factors. Firstly, microstructural “hot spots” are characterised by significant local heterogeneities (the misorientation between neighbouring grains), which results in deformation incompatibility and, therefore, leads to large strain and stress concentrations. Secondly, the damage nucleation process can only be fully described by considering multiple length scales from nano- (sub-grain) to meso-scale (grain clusters). For example, morphology and crystallographic orientation of grain clusters are both key factors in the magnitude of localised stresses.
Moreover, in many cases critical microstructural features occur infrequently in materials, which leads to a sampling problem (size effect). It has been widely recognized that fatigue strength is strongly affected by the component size. However, fatigue tests are time-consuming, and the location of critical grain combinations is difficult to identify before the testing. One of the significant benefits of a large-scale 3D ultrasonic microstructural imaging is the possibility to address this problem by screening and pre-selecting material samples before conducting complex mechanical tests.
This part of the project will consist of several case studies, which will aim to identify general, as well as application specific characteristics of the damage sensitive microstructural locations. For example, one important industrial case is related to the long-standing problem of crack nucleation and growth in titanium alloys under cyclic and dwell fatigue loading. Numerous previous studies have demonstrated particular importance of micro-textured regions (MTRs, clusters of grains with similar crystallographic orientations) present in the material as a life-limiting microstructural feature. However, the relationship between parameters of MTRs and fracture is still not completely understood. Although the focus will be on titanium alloys, similar fatigue behaviour was observed in other alloy systems, so other case studies can also be considered.
The experimental work will be conducted in close collaboration with the Solid Mechanics research group at University of Bristol, which has extensive expertise in the material science and structural integrity fields, as well as across a wide range of materials characterisation techniques.
Enquiries to a.velichko@bristol.ac.uk
Applications to Future Innovation in Non-Destructive Evaluation at the University of Bristol