PhD Thesis Defence: Mr. Sazid Khan (24/08/26)
Thesis title:
Normal And Shallow Oblique High-Velocity Impact of Ductile Metals: Experiments, Microstructure, And Modelling
Faculty advisor(s):
Prof. Karthikeyan S
When?
24th August, 2026 (Modday), 02:00 PM (India Standard Time)
Where
KPA Auditorium, Department of Materials Engineering
Abstract
High-velocity impact phenomena are important in the aerospace, defense, and manufacturing sectors, where material response under high-strain-rate loading conditions strongly influences structural integrity and performance. In particular, the impact of soft metallic spheres on hard surfaces is encountered in numerous engineering applications, including cold spray deposition, particle erosion, shot peening, and ballistic protection. This thesis investigates the high-velocity impact response of ductile copper and brass spheres on a hardened tool steel substrate in both normal and oblique configurations, addressing knowledge gaps in the understanding and modelling of such impacts. Copper and brass were selected because their annealed and as-received conditions provide a wide range of yield strengths and distinct strain-hardening characteristics, enabling a systematic investigation of the effects of yield strength and strain hardening on impact response. A hardened tool-steel substrate was used to maintain elastic target behavior. High-velocity impacts were studied through experiments and simulations, with velocities ranging from 1 to 300 m/s. The impact angles were 90° (normal impact) and 10°, 20°, and 30° (oblique impact). Experiments included ballistic impact of ductile projectiles accelerated by a single-stage gas gun, high-speed imaging, and microstructural characterization of the deformed samples via SEM-EBSD. Numerical simulations were conducted using LS-DYNA, employing constitutive models derived in this study for four materials: as-received and annealed copper and as-received and annealed brass.
The first part of the work presents the high-strain-rate mechanical characterization of copper and brass spheres in as-received and annealed conditions using the Split Hopkinson Pressure Bar (SHPB) apparatus at various strain rates and temperatures. Johnson-Cook constitutive parameters were derived from the stress-strain curves. Plastic strain levels were calibrated through GOS mapping and Vickers microhardness measurements. In some cases, EBSD analysis revealed dynamic recrystallization in Cu and brass due to adiabatic heating.
The second part investigates the normal impact behavior of copper and brass spheres against tool steel over velocities ranging from 1 to 300 m/s. Rebound behavior was quantified through rebound velocity and the coefficient of restitution (COR). Thermomechanical LS-DYNA simulations were validated against experiments and used to obtain additional quantities, such as contact force and duration, load-displacement response, strain/strain rate evolution, and adiabatic temperature rise, which are not accessible through experiments. Microstructural results corroborated the simulation-predicted deformation. A new unified contact model was proposed to capture the velocity dependence of COR over the entire velocity range. The model accounts for changes in the sphere’s geometry (and hence its stress state) during impact and inertial effects. The model not only predicts the COR but also models the force-displacement curves, shape change during impact, etc. While the baseline model was specific to elastic-perfectly plastic solids, elastoplastic materials with strain hardening could also be modelled using a representative strain-based yield strength.
The final part examines high-velocity oblique impact to determine the coefficient of friction (COF) under sliding speeds ranging from 1 to 260 m/s and extremely short contact durations (10-30 µs). The experimental results show that the coefficient of friction decreases with sliding speed, consistent with the literature. COF is significantly smaller (< 0.1) than typical steady state tribological studies, due to the very short contact duration and high contact pressure of the oblique impact. Therefore, energy loss and plasticity due to frictional forces are minimal and highly localized. The “teardrop” shape of the contact mark during oblique impact due to material transfer was observed. SEM and EDS analyses of the slid samples confirm the actions of both abrasive and adhesive wear mechanisms. Numerical simulations were used to investigate the effect of the normal velocity component in oblique impact, revealing that it causes both temporal and spatial variations in the sliding velocity at the contact surface.