The impact loading of steel is critical for structural applications, particularly in naval structures, where high strain rate loading induces significant changes in both microstructural and mechanical properties. This has driven extensive research into the behaviour of steel under ballistic and blast events, which have become more frequent due to civil wars, insurgencies, and terrorist attacks. High Strength Low Alloy (HSLA) steels are highly valued in the shipbuilding industry for their exceptional mechanical properties, making them ideal for naval vessels exposed to such extreme conditions. However, HSLA steels are prone to adiabatic shear band (ASB) formation under high strain loading, which can lead to material failure. The formation and behaviour of ASBs remain a key area of study, with no universally accepted model yet established.

This thesis focuses on the ballistic impact on thermomechanically processed (TM) and normalized (N) EH36 HSLA steels, commonly used in the hulls of ships. Past research efforts highlighted that EH36TM steel exhibits anisotropic tensile behaviour after ballistic perforation—a phenomenon not observed in EH36N steel. Understanding the anisotropic behaviour of EH36TM steel is the primary research goal of this thesis. This research aims to experimentally investigate the microstructural mechanisms responsible for the anisotropy resulting from ballistic impact. The objective is to enhance our understanding of the material's behaviour under extreme conditions and to potentially improve the performance and safety of naval structures.

The central hole tensile (CHT) tests confirmed the anisotropic behaviour of perforated EH36TM steel. However, tensile tests on as-received EH36TM and N steels demonstrated that both steels exhibit isotropic tensile behaviour along the rolling direction (RD) and transverse direction (TD), indicating that the anisotropy is attributed to the impact. To understand this behaviour, an initial analysis of the base microstructure of both TM and N steels was conducted to establish the starting point of the material's characteristics. Both steels' microstructural changes were studied after ballistic impact to identify the alterations that may contribute to the observed anisotropy of the EH36TM steel. The study of as-received steels revealed similar behaviour between EH36TM and EH36N samples, with no inherent anisotropy detected in EH36TM along the TD. The finer microstructure of TM steel was the primary difference, suggesting a slightly higher susceptibility to ASB formation, which could be a possible link for the occurrence of anisotropy after steel's perforation. Microstructural analysis showed that both perforated steels formed comparable regions, after the severe plastic deformation. However, Scanning Electron Microscopy (SEM) analysis revealed a higher susceptibility to micro-cracks and micro-voids formation in EH36TM steel, which could lead to diminished tensile behaviour and hence anisotropy under specific conditions. For both steels, adjacent to hole, ASBs were identified in the entire perimeter. Therefore, they were not correlated directly with EH36TM steel's anisotropy. Pronounced cracks parallel to the rolling direction were detected in TM steel, possibly contributing to its decreased tensile performance, in perpendicular direction, while these cracks were absent in perforated EH36N steel. Further SEM analysis across the hole's thickness revealed that ASB initiated within the thickness of the TM steel and were oriented almost parallel to the rolling direction. This could potentially weaken its tensile properties across the transverse direction. EBSD analysis indicated a high probability of (110)<111> and (112)<111> slip systems alignment across RD in TM steel, correlating with higher ductility in RD compared to TD. This behaviour of slip systems was not observed in N steel. Fractographic analysis via SEM confirmed that TM steel could sustain greater plastic deformation when loaded along rolling, as evident from an area of clear ductile fracture, while the transverse-loaded sample showed brittle and mixed modes. In EH36N samples, brittle fracture was observed adjacent the perforated hole, with mixed fracture behaviour close to it. These observations were consistent with the isotropic tensile behaviour of the perforated EH36N steel.

This thesis significantly advances the understanding of why EH36TM steel exhibits anisotropic tensile behaviour along TD after perforation. By thoroughly investigating the material before and after perforation, this research sheds light on the effects of ballistic impact on EH36TM and EH36N materials' microstructure and mechanical behaviour. The findings not only deepen our insights into the post-impact behaviour of EH36TM steel but also have broader implications for improving the design and safety of naval structures that uses this material.

In recent decades, high strength steels in thick sections have been increasingly used in offshore structures where they are subjected to harsh service conditions such as freezing temperatures and high static/dynamic loading. At these conditions they are susceptible to a transition from ductile failure to a dangerous brittle (cleavage) predominant type of failure, which occurs well before yielding. One of the main challenges in employing thick sections is the through-thickness heterogeneous variance of microstructures as a result of the processing route owing to a gradient of cooling rates from the surface to the bulk. As the material’s mechanical and fracture behaviour strongly depend on the microstructure, the through-thickness microstructural heterogeneity leads to a significant scatter in mechanical and fracture properties, which makes it difficult to predict and control cleavage fracture.

From the body of literature establishing microstructural dependence on cleavage failure, several features contributing to this type failure can be identified. Phases, grain size, grain boundary misorientations and the presence of secondary phase constituents can play a major role in failure through cleavage. Additionally, cleavage failure is also sensitive to the crack depth to width ratio (a/W). This study investigates the microstructural features contributing to cleavage failure in a 100 mm thick S690QT high strength steel plate by performing mechanical and fracture toughness tests. In order to improve mechanical properties and cleavage fracture toughness, an isoparametric study employing rapid cyclic heating with the objective of grain refinement was performed.

The steel plate has coarser prior austenite grain (PAG) sizes, and a larger area and number fraction of inclusions in the middle section. Additionally, segregation bands as a result of solute segregation during the solidification process were observed to be dispersed throughout the middle section. The middle section was also characterized by lower hardness compared to the top section. The detrimental effects of the middle section were evidenced by inferior low temperature tensile properties and cleavage fracture toughness. This was attributed to the larger PAG sizes, larger area and number fractions of inclusions, and segregation bands in the middle section. The specimen orientation with respect to the rolling direction was found to have no effects on the tensile properties. Additionally, different a/W geometries and notch orientations with respect to the rolling direction were used to investigate the role of constraint effect and rolling orientation, respectively, in the fracture behaviour. Shallow-notched specimens representative of the defects found in offshore structures demonstrated a higher fracture toughness than the deep-notched specimens. This was attributed to lower hydrostatic stresses at the crack tip, which reduces stress triaxiality. The isoparametric study resulted in average grain size reduction by 41% and proved to improve micro-hardness, low temperature tensile properties and cleavage fracture toughness by 5%, 13% and 41% respectively. Fractographic analysis on the fracture toughness specimens revealed the presence of O, C-rich regions which are known to promote brittle behaviour.