Solomon Boakye-Yiadom

Deformation Mapping and the Role of Carbides on the Microstructure and Properties of Evolved Adiabatic Shear Bands

Impacting hardenable steel such as 4340, results in the formation of adiabatic shear bands (ASBs). Previous studies have shown that the presence of carbides/second-phase particles in the pre-deformation microstructures of 4340 steel increases their susceptibility to the formation of ASBs. The current study examines the role of carbides on the microstructure and properties within evolved ASBs in 4340 steel after impact. Geometric phase analysis was used to map local deformation fields within the evolved ASBs. It was observed that carbide fragmentation due to plastic deformation of carbides produces both residual carbides and residual carbide particles in regions away from the shear bands. Extensive carbide fragmentation produces fine residual carbide particles which are redistributed within the ASBs. This is attributed to strain localization within the ASBs which result in higher local strain and strain rates within the shear bands than in regions outside the bands. In addition, it is observed that the residual carbide particles trap and pin dislocations within the shear bands and contribute to an increase in local hardening. A more homogenous distribution of narrower and shorter rotational and shear-strain fields were revealed by the local deformation maps within the evolved ASBs. Lattice deformation mapping revealed that the ferrite matrix, prior to impact, had broader and longer rotational and shear-strain fields perpendicular to the direction of impact. This is attributed to lattice-invariant deformation and shape deformation processes that occur on specific crystallographic planes during martensitic transformation. It is concluded that strain localization during high strain rate deformations does not occur on specific crystallographic planes. This results in a more regular distribution of internal lattice rotational and strain fields within the evolved ASBs.

Microscopical study of the formation of adiabatic shear bands in 4340 steel during dynamic loading

In this study, optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and electron probe microanalyser were used to analyse the changes in microstructure of AISI 4340 steel specimens caused by impact at high strain rates and large strains. The structures of the steel prior to dynamic deformation and after dynamic deformation were examined to understand on a microscale level, the mechanism of formation of adiabatic shear bands (ASBs). The study also includes the structural changes that occur during post-deformation annealing processes which may relate to understanding of the mechanism of formation of ASBs. Prior to deformation, the tempered steel specimens consisted of lenticular laths of α-ferrite with precipitated platelet and spherical M3C carbides. After impact, the structure inside the shear band was characterized by refined and recrystallized grains immersed in dense dislocation structures. In addition, residual carbide particles were observed inside the shear bands due to deformation induced carbide dissolution. Regions away from the shear bands developed ‘knitted’ dislocation walls, evolving gradually into sub-boundaries and highly misoriented grain boundaries at increasing strains, leading to grain refinement of the ferrite. After impact, annealing the shear bands at 350 °C resulted in an increase in hardness regardless of the heat treatment before impact, amount of deformation and the time of annealing. This is because of the occurrence of extensive reprecipitation of dissolved carbides that existed in the steel structure prior to deformation. It is concluded that dynamic recovery/recrystallization, development of dislocation structures and carbide dissolution all contribute simultaneously to the formation of ASBs in quench-hardened steels.

On the Importance of Retaining Stresses and Strains in Repositioning Computational Biomechanical Models of the Cervical Spine

Human body models are created in a specific posture and often repositioned and analyzed without retaining stresses that result from repositioning. For example, repositioning a human neck model within the physiological range of motion to a head-turned posture prior to an impact results in initial stresses within the tissues distracted from their neutral position. The aim of this study was to investigate the effect of repositioning on the subsequent kinetics, kinematics, and failure modes, of a lower cervical spine motion segment, to support future research at the full neck level. Repositioning was investigated for 3 modes (tension, flexion, and extension) and 3 load cases. The model was repositioned and loaded to failure in one continuous load history (case 1), or repositioned then restarted with retained stresses and loaded to failure (case 2). In case 3, the model was repositioned and then restarted in a stress-free state, representing current repositioning methods. Not retaining the repositioning stresses and strains resulted in different kinetics, kinematics, or failure modes, depending on the mode of loading. For the motion segment model, the differences were associated with the intervertebral disc fiber reorientation and load distribution, because the disc underwent the largest deformation during repositioning. This study demonstrated that repositioning led to altered response and tissue failure, which is critical for computational models intended to predict injury at the tissue level. It is recommended that stresses and strains be included and retained for subsequent analysis when repositioning a human computational neck model.

Study of the formation of adiabatic shear bands in steel during impact

Abstract Both metals and other materials may exhibit the formation of narrow bands of extreme strains when impacted at high strain rates and large strains. These are known as Adiabatic Shear Bands (ASBs). They are observed during material processing such as forging and machining as well as wear and in armor plates during impact by projectiles. The prevailing theory for their formation is that they form in narrow bands because of two competing mechanisms occurring sequentially: strain hardening followed by thermal softening from the retained heat due to the impact. However, recent studies suggest that the formation of ASBs may be a simultaneous occurrence of different mechanisms which starts with the emergence of dislocations depending on the imposed local strain and strain rate. This study uses different methodologies to explore the microstructure of ASBs in a hardened low alloy steel. The study includes the effect of the initial microstructure on the formation of ASBs. The Focused Ion Beam technique was used to prepare transmission electron microscopy samples from regions within the shear bands to eliminate the induced further deformation which could be produced by conventional approaches of electropolishing. The present study reveals that each of these methodologies complements each other. Also more complex series of mechanisms including dislocation cell formation, texture development, dynamic recrystallization and carbide dissolution accommodate the excessive strains that occur during the evolution of the shear bands.

Microstructural Evolution from Shaped Charge through Steel Plates

A set of 18 armour steel plates were stacked on top of each other and subjected to shape charges that went through the plates and created a hole that decreased in diameter as it went through consecutive plates. Afterwards, the plates were examined and the hardness near the hole and away from the hole was taken to determine the effect of the passing of the shaped charge through the plates. Also, specimens from the walls of the holes were taken to determine changes in the microstructure due to the shock wave and the resulting excessive heating from the shape charge. It was observed that the shock wave produced significant changes in the microstructure resulting in the appearance adiabatic shear bands (ASBs). These ASBs persisted in holes in plates placed further down the stack (up to 8th in the stack). More complex microstructural mechanisms are thought to take place as opposed to erosion from the flow of the molten metal through the holes in the plates.