Date of Award

2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Martin H. Sadd

Abstract

To improve the performance and safety of future aerospace vehicles, worldwide efforts are being directed towards the development of novel aerospace 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 aerospace materials such as Functionally Graded Materials (FGMs), Ti2AlC, and Hastelloy X under extreme thermo-mechanical loadings.

Experimental studies were then conducted to evaluate two different materials, namely, Nanolayered Titanium Aluminum Carbide (a MAX phase material) and Hastelloy X under varying rates of loading and at different temperatures. The dynamic behavior of nanolaminated ternary carbide, Ti2AlC, was characterized under dynamic loading using Split Hopkinson Pressure Bar (SHPB) compression apparatus. The dynamic loading experiments were performed in the strain-rate range of 1500-4200s-1 and at temperatures ranging from room temperature (RT) to 1150°C. At room temperature, the failure stress and strain show little dependence on strain rate, whereas the failure stress drops considerably at temperatures above 900°C. At all strain rates and temperatures, Ti2AlC exhibits softening after failure initiation and a more graceful failure due to delamination and kink band (KB) formation. At temperatures higher than 900°C, grain boundary decohesion is suggested to contribute towards the deecrease in the failure stress.

The dynamic constitutive behavior of Hastelloy X (AMS 5754) was studied at room and elevated temperatures under varying rates of loading. A split Hopkinson pressure bar (SHPB) apparatus was used in conjunction with an induction coil heating system for applying dynamic loads at elevated temperatures. Experiments were carried out at different temperatures ranging from room temperature (25°C) to 1100°C at an average strain rate of 5000s-1. Room temperature experiments were carried out at varying strain rates from 1000s-1 to 4000s-1. The results show that as the strain rate increases from quasi-static to 4000s-1, the yield strength increases by approximately 50%. Also, under dynamic loading, the yield stress decreases with temperature up to 700°C, after which it (yield strength) shows a peak at 900°C before beginning to decrease again as the temperature is further increased. The Johnson-Cook model was used to predict the dynamic plastic response under varying rates of loading and at different temperatures.

A series of experiments were conducted to study the dynamic response of rectangular Hastelloy X plates at room and elevated temperatures when subjected to shock wave loading. The shock tube apparatus was modified and validated for testing Hastelloy X at elevated temperatures. Propane gas was used as the heating source and it was directed onto the sample via four nozzles. A cooling system was also implemented to prevent the shock tube from reaching high temperatures. High-speed photography coupled with the optical technique of Digital Image Correlation (DIC) technique was used to record the real-time deformation of the specimen under shock wave loading. The DIC technique was used in conjunction with band pass optical filters and a high intensity light source to record the full-field deformation images under shock loading at high temperatures up to 900°C. In addition, a high speed camera was utilized to record the side-view deformation images. The high temperature DIC system has been validated by comparing with the mid-point deflections obtained from the side-view camera. The dynamic response of Hastelloy X was evaluated as a function of temperature under shock wave loading.

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