The survivability of armour steel under multi-threat loading is influenced by microstructural damage initiated by fragment perforation. This damage affects the way the material deforms and fractures under subsequent blast-induced loading. This thesis investigates the failure micro-mechanisms in Armox 440T after perforation by 20 mm Fragment Simulating Projectiles (FSPs) at 900 m/s and 1600 m/s, followed by Drop-Weight Impact (DWI) loading that simulates blast-induced blast wave loading. A multiscale approach using high-speed digital image correlation (DIC), fractography using Optical Microscopy (OM) and Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD) with Kernel Average Misorientation (KAM) mapping, and microhardness profiling reveals the microstructural response to plastic deformation and failure by the formation of adiabatic shear bands (ASBs) and the ductile crack growth phenomena.

Fragment perforation produces complex networks of deformed (dASB) and transformed (tASB) adiabatic shear bands concentrated at the hole surface. With increasing FSP velocity, (I) plug and hole size increases, (II) local hardness peaks intensify reaching ≈ 650 HV within tASBs, and (III) The number of ASBs, branching and junction frequency increases. Microcracks are commonly found near dense tASB clusters at the hole surface and internally in tASB junctions indicating a reduced dynamic fracture tolerance.

Under DWI, cracks initiate at the hole surface and initially propagate perpendicular to the length of the plate, then change direction to ≈ 45◦ shear accompanied by pronounced necking. All fracture surfaces show ductile microvoid coalescence behaviour. The crack centre fails under tensile tearing, which transition into shear failure towards the edges. OM and SEM reveal that microcracks from perforation form under microvoid coalescence and void sheeting. EBSD/KAM mapping on adiabatic shear bands shows ultrafine equiaxed grains and low dislocation density within tASBs, confirming severe strain localisation and dynamic recrystallisation.

Collectively, the results show that tASB networks constitute preferential crack paths that lower the energy absorption capacity in blast impact loading. These findings offer new insights into ASB-enabled damage evolution and provide critical microstructural constraints for modelling and design of armour systems capable of withstanding multi-threat environments.