Characterization of Novel Materials Under Extreme Dynamic Loading Conditions

Abstract

To improve the performance, efficiency and safety of future equipment’s in commercial, aerospace, nuclear and defense structures, worldwide efforts are being directed towards the development of novel materials which exhibit superior structural and multifunctional capabilities in extreme environments. Fundamental investigation into the thermo-mechanical response and dynamic failure of the materials is paramount before they can be incorporated into the design of future space access vehicles that can operate reliably in combined, extreme environments. For this purpose, a comprehensive study was conducted to evaluate the performance of variety of materials such as Ti2AlC, Ti3AlC2, Hastelloy X and 2024 Aluminum under extreme thermo-mechanical loadings. An experimental investigation was conducted to evaluate the compressive constitutive behavior and fracture initiation toughness of fine grained (~4.2 μm) Ti2AlC in dynamic and quasi-static loading at different temperatures. A Split Hopkinson Pressure Bar (SHPB) apparatus was used in conjunction with an induction coil heating system for dynamic experiments at elevated temperatures. A series of experiments were conducted at different temperatures from 25°C to 1200 °C and strain rates of 10-4 s-1 and 500 s-1. A single edge notched specimen was used to determine the fracture initiation toughness in dynamic and quasi-static loading from ambient temperature to 900 °C. The SHPB apparatus was modified for this purpose and high speed photography was incorporated to calculate the dynamic fracture toughness. The results from these experiments reveal that the peak compressive failure stress in dynamic conditions decreases with increases in temperature, from 1600 MPa at room temperature to 850 MPa at 1200 °C. In the dynamic testing condition, the failure remains predominately brittle even at temperatures as high as 1200 °C. However, brittle-to-plastic transition was observed at around 900 °C under quasi static loadings. The fracture experiments reveal that the dynamic fracture toughness is higher than the quasi-static value by approximately 35%. The effect of grain size on the constitutive behavior of Ti2AlC from room temperature to 1100°C under dynamic and quasi-static loading was also investigated using high density Ti2AlC samples of three different grain sizes. The results show that under quasi-static loading the specimens experience a brittle failure for temperatures below Brittle to Plastic Transition Temperature (BPTT) of 900-1000 °C and pseudo-plastic behavior at temperatures above the BPTT. During dynamic experiments, the specimens exhibited brittle failure, with the failure transitioning from catastrophic failure at lower temperatures to graceful failure at higher temperatures, and with the propensity for graceful failure