Estimation of fatigue strength of TiN coatings using cyclic micro-impact testing

This study delves into the behaviour of a thin TiN coating on a tool steel substrate material under dynamic and cyclic impacts through a comprehensive approach combining experimental testing and computational modelling. In dynamic impact tests, a pendulumbased setup investigates material responses under varying acceleration loads, revealing a distinctive "ringing effect" as the indenter bounces off the specimen's surface, with all plastic deformation concentrated during the initial impact. The study also quantifies dynamic hardness values, highlighting load-dependent behaviour and assessing the coating system's energy dissipation capabilities. In cyclic impact tests, materials experience permanent plastic deformation with each cycle, ultimately leading to coating failure. Chemical analysis identifies an interlayer between the coating and substrate, while cross-sectional analysis reveals the extent of coating damage due to cycling and load. A three-dimensional map is constructed, connecting acceleration load, sensed depth, and cycles to coating failure, and an empirical equation characterizes the relationship between depth and cycles before failure. The computational model scrutinizes traction component distribution during loading and unloading, with a focus on normal and shear tractions. The findings suggest the potential significance of normal traction in interface fatigue failure. Overall, offering implications for understanding and mitigating fatigue-related failures across various applications

An iterative method for reference pattern selection in high-resolution electron backscatter diffraction (HR-EBSD)

For high (angular) resolution electron backscatter diffraction (HR-EBSD), the selection of a reference diffraction pattern (EBSP0) significantly affects the precision of the calculated strain and rotation maps. This effect was demonstrated in plastically deformed body-centred cubic and face-centred cubic ductile metals (ferrite and austenite grains in duplex stainless steel) and brittle single-crystal silicon, which showed that the effect is not only limited to measurement magnitude but also spatial distribution. An empirical relationship was then identified between the cross-correlation parameter and angular error, which was used in an iterative algorithm to identify the optimal reference pattern that maximises the precision of HR-EBSD

HR-EBSD analysis of in situ stable crack growth at the micron scale

Understanding the local fracture resistance of microstructural features. such as brittle inclusions, coatings, and interfaces at the microscale under complex loading conditions is critical for microstructure-informed design of materials. In this study, a novel approach has been formulated to decompose the J-integral evaluation of the elastic energy release rate to the three-dimensional stress intensity factors directly from experimental measurements of the elastic deformation gradient tensors of the crack field by in situ high (angular) resolution electron backscatter diffraction (HR-EBSD). An exemplar study is presented of a quasi-static crack, inclined to the observed surface, propagating on low index {hkl} planes in a (001) single crystal silicon wafer

In situ full-field characterisation of strain concentrations (deformation twins, slip bands and cracks)

This thesis has developed novel methods to characterise the deformation field, in and ex situ, of strain concentrations (i.e., deformation twins, slip bands and cracks) using diffraction methods to map the local elastic deformation field, and calculate elastic strain energy release rate (J-integral) and stress intensity factors (SIFs) to parametrise the field under conditions of small-scale yielding. To calculate the J-integral from an elastic strain field, the field was integrated to equivalent displacement to use the finite element formulation for high accuracy. The method was validated against two and three dimensional synthetic crack fields and then applied to strain fields measured using synchrotron energy-dispersive X-ray diffraction (EDXD) for a fatigue crack propagating in the heat affected zone (HAZ) of a welded bainitic steel. The integrated displacement field was then used to calculate the J-integral and mode I, showing a good agreement with the standard analytical solution and results obtained using displacement fields of the same crack measured using Digital Image Correlation (DIC). The parametrisation via the J-integral and SIFs was extended to use the finite element solver’s equivalent domain integration (EDI), and anisotropic elastic and elasto-plastic material properties. The high-resolution electron backscatter diffraction technique (HR-EBSD) was employed to map the elastic strain field. The effect of the unknown deformation conditions at the electron backscatter reference pattern (EBSP0) were investigated to select an optimum EBSP0. The developed analytical techniques were then applied to study deformation twinning, slip band and crack local fields, in situ. First, an age-hardened duplex stainless steel (DSS) sample was deformed in tension, promoting plastic deformation by deformation twinning in the ferrite phase. The local in-plane strain fields ahead of a loaded deformation twin were measured, in and ex situ, and decomposed to the opening mode I and in-plane shear mode II SIFs. The analysis showed that the increase in twin lateral thickness correlates with mode I, while the elastic recovery when the load was removed was mainly in mode II. By estimating the EBSP depth resolution using Monte Carlo (MC) simulation, the analysis was extended to the third dimension and applied to study, in situ, intragranular slip bands in the ferrite phase of an age-hardened DSS. The integrated displacement fields gave information about the in- and out-of- plane movement of the surface, which was decomposed to the three dimensional stress intensity factors, KI,II,III. This showed that constraint of the topological changes due to out-of-plane shear induces additional tensile stresses, and the ratio of mode II to mode III depends on the direction of the slip band Burgers vector relative to the observed surface. Finally, a simplified method that did not rely on FE solvers to calculate the J-integral and SIFs was derived and used to investigate mixed-mode cleavage crack propagation in (001) single silicon crystal. The mixed-mode crack field was consistent with a constant maximum potential energy release rate (MPERR) criterion for crack propagation and the expected cleavage toughness of silicon

In situ characterisation of the strain fields of intragranular slip bands in ferrite by high-resolution electron backscatter diffraction

High angular resolution electron backscatter diffraction has been used to quantify the local elastic field at the tip of mechanically loaded intragranular slip bands observed in situ in the ferrite grains of an age-hardened duplex stainless steel (Zeron 100). The surface elastic strain field was integrated to calculate in-plane and out-of-plane surface displacements. This allowed the elastic fields to be parameterised in a finite element analysis, which used the displacement field as the boundary conditions, to obtain the potential strain energy release rate (J-integral) and three-dimensional stress intensity factors (,,). This new analysis method is demonstrated by examining the elastic fields around the tip of an incipient slip band, an array of slip bands and the loading of a slip band. Direct measurement of the stress tensor in the grain identified the active slip systems with the highest Schmid factor. The stress intensity factors ahead of the slip band, measured under load, were directly affected by the magnitude of loading and the inclination angle of the slip band to the observed surface