Date of Award

2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical, Industrial and Systems Engineering

First Advisor

Arun Shukla

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 increasing with increasing grain size. The compressive strengths of different grain sizes at a given temperature are related to the grain length by a Hall-Petch type relation.

Experimental studies were then conducted to investigation of mechanical response and fracture toughness of Ti3AlC2 under dynamic loading at different temperatures. SHPB apparatus in conjunction with an induction coil heater were used to dynamically load the specimen at various temperatures. The results of this study reveal that Ti3AlC2 exhibits brittle failure during dynamic loading, even for temperatures of 1100 °C. The peak compressive stress for failure decreased with increasing temperatures. The fracture toughness was also seen to decrease with increasing temperature. The SEM images indicate that under dynamic loading conditions the deformation behavior and fracture mechanisms do not change with temperature.

The constitutive behavior of Hastelloy X was evaluated over a wide range of strain rates. Shear compression specimen (SCS) geometry was utilized to achieve strain rates from 10-3-20000 s-1. A screw driven testing machine and a SHPB apparatus were used to load the specimens. An induction coil heating system was utilized in conjunction with SHPB to evaluate constitutive behavior at elevated temperatures under dynamic loading. Room temperature experiments were carried out at varying strain rates from 10-3-20000 s-1 exhibit the strain rate sensitivity of Hastelloy X. The rate sensitivity increases after rate exceeds 1000 s-1. The stress stain curves at various temperatures under dynamic loading showcase a yield strength anomaly. The stress peaks at the beginning of dynamic yielding at 850 °C and 1000 °C.

An experimental investigation was conducted to understand the effect of curvature on blast response of aluminum panels under extreme temperatures. Three aluminum 2024-T3 panels: flat panel and two curved panels, with radii of curvatures of 304.8 mm and 111.8 mm were used for the study. A shock tube apparatus was utilized to impart controlled shock loading. Propane flame torches were used to heat the specimens and experiments were conducted at 25 °C, 200 °C and 350 °C. High speed photography coupled with 3D Digital Image Correlation (DIC) technique was used to obtain full field deflection and velocity, as well as in-plane strain on the back face of the panels. The different modes of deformation and the mode transitions during the blast loading of the three panels have been identified. The effect of temperature on the deformation modes is investigated. The deflections of panel with 304.8 mm radius of curvature were found to be higher than the other two panels. With increase in temperature, the deflections increased and strain was seen to get localized.

Experiments were performed to study the shear banding phenomenon in Ti6Al4V. SCS of Ti6Al4V were used in this of study the formation and evolution of adiabatic shear bands. The gage section of specimen was speckled using high temperature paint and SHPB apparatus will be used to dynamically load the SCS of Ti6Al4V. High speed photography was used to capture the deformation of speckled specimen. The high speed images were analyzed using DIC software to extract the strain fields during shear banding. Before dynamic failure localization in strain field were noticed. These localizations indicate towards formation of shear bands.

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