PhD Thesis Colloquium: Mr. Sazid Khan (11/12/25)
Thesis title:
Direct and oblique high-velocity impact of ductile metals: experiments, microstructure and modelling
Faculty advisor(s):
Prof.Karthikeyan S
When?
11th December, 2025 (Thursday), 2:30 PM (India Standard Time)
Where
KPA Auditorium, Department of Materials Engineering
Abstract:
High-velocity impact phenomena are important in the aerospace, defence, and manufacturing sectors, where the material response under high-strain-rate loading conditions plays a vital role in determining structural integrity and performance. This thesis investigates the high-velocity impact response of ductile copper and brass spheres on a hardened tool steel substrate in direct and oblique configurations addressing knowledge gaps that exist in the understanding and modelling of such impacts. High velocity impacts were studied via experiments and simulations—velocities ranged from 1-300 m/s and the impact angles were 900 (normal impact) and 100, 200, 300 (oblique impact). Experiments included ballistic impact of ductile projectiles accelerated by single-stage gas gun, high-speed imaging, and microstructural characterisation of the deformed samples via SEM-EBSD. Numerical simulations were carried out using LS-DYNA which utilized constitutive models which were also derived in this study for four different materials: as-received and annealed copper, as-received and annealed brass.
The first part of the work presents the high-strain-rate mechanical characterisation 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 recrystallisation in Cu and brass due to adiabatic heating.
The second part investigates the direct impact behaviour of copper and brass spheres against tool steel over velocities ranging from 1 to 300 m/s. Rebound behaviour 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, parameters that are not accessible to 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 the change in geometry (and hence the stress-state) of the sphere during the 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 of 1-260 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 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 contact model previously developed for normal impact was suitably modified to predict the “teardrop” shape of the contact scar during oblique impact. SEM and EDS analyses of the slid samples confirm the actions of both abrasive and adhesive wear mechanisms.