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Abstract: Severe plastic deformation is an attractive method for manufacturing fully dense ultrafine-grained (UFG) materials in large geometric dimensions. This opens the door to extend research in fracture mechanics to the nanocrystalline (NC) grain size regime. In reality, structures and components can experience variable loading during service. The retardation of fatigue cracks by overload events has been well catalogued and various explanations mooted relating to a range of closure effects but many aspects remain unclear. In this thesis, overload fatigue experiments were conducted on Ni samples with different grain sizes (i.e. NC samples produced by electrodeposition, UFG samples cut so as to grow cracks along (AT) and perpendicular (TR) to the elongated grain direction and coarse grained (CG) samples) trying to understand various effects of microstructure, grain size and stress ratio (R ratio) on the fatigue overload behaviours. Synchrotron X-ray diffraction (SXRD) was used to map the elastic crack-tip stress fields at various stages through the loading cycle. A novel deconvolution method was proposed to accurately locate the crack-tip position of the stress fields, from which the variation in stress prior to, during and after overload was measured and the effective stress intensity factor (SIF) at the crack tip estimated. This was complemented by digital image correlation (DIC) which was used to map the displacement fields around the crack at the surface and thereby to measure the crack length and the total strain field as the applied load was varied and from which the fatigue crack growth (FCG) rate, crack opening displacement, SIF range and J-integral were determined as a function of crack length relative to the overload position. Post mortem EBSD characterisation, fractography analysis and roughness measurements were conducted to help better understand the retardation mechanisms. It was found that the FCG for CG Ni was most significantly retarded whereas the NC sample was least affected by the overload arising from the increasing yield stress and more planar crack surface morphology and thus less significant crack closure and residual stress effect. The anisotropic microstructure of the UFG samples was found to have a significant effect on the FCG rate. The rate was more significantly retarded by the overload for UFG Ni in the TR orientation than the AT due to the larger deflection angle and therefore more pronounced closure and residual stress effect. The R ratio effect was also investigated with the same overload being applied on the UFG samples in the TR orientation. It was found that the overload retardation distance was shorter for high R ratio (R = 0.5) due to the absence of crack closure. However the decrease in FCG rate immediately after overload was more remarkable for R = 0.5 than for R = 0.1 due to a larger reduction ratio in equivalent SIF which is defined by Walker's model considering both effects of crack closure and residual stress.