Date of Award

2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Carl-Ernst Rousseau

Abstract

The response of materials to shock loading has been investigated through use of a plate impact experimental technique. A single stage gas gun was utilized to drive projectiles to velocities between 50-500 m/s, facilitating investigations into low to moderate shock loading conditions. Temporal records of the dynamic events were captured with the use of commercial manganin stress gauges that were embedded within layers of test material.

Within this thesis, there is a bimodal theme. The first portion of this thesis investigated the spall fracture of cast irons with varying microstructure. Although the study of the spall fracture of materials is a common theme in literature, there exists a noteworthy scarcity of research specific to cast iron. Given that cast iron is one of the most widely utilized materials in engineering structures, this research was pursued in an effort to characterize its strength and identify the fracture mechanisms and kinetics associated with its failure process. The second portion of this thesis involved the development of a new technique that could be utilized to generate multiple Hugoniot states in a single experiment. Generation of a material’s Hugoniot is a fundamental theme in shock wave studies because it allows researchers to determine all mechanical and thermodynamic properties associated with dynamic loading conditions. Traditionally, the locus of points necessary to construct a material’s Hugoniot is obtained through a rigorous series of experiments, where each test produces a single data set. By considering the shock wave processes associated with layered plates, a new method was developed that will significantly reduce the process of obtaining material Hugoniots.

Within the study of the spall fracture of cast iron, experiments were designed to induce an extreme tensile state within test samples from the interaction of decompression waves. The dynamic fracture strength, known as spall strength, was determined from temporal records of the stress evolution inside the samples. In order to encompass a vast majority of castings typical to industry, five separate cast irons were tested. Four of these castings consisted of gray cast iron with graphite in flake form, where three were classified as Type VII A2 and the other contained a bimodal distribution of Type VII A4 and VII D8. The fifth casting consisted of ductile cast iron with graphite in nodular form, classified as Type I with an average of 200 nodules per square millimeter of size class 5. The spall strength for the Type VII A2 gray cast irons was found to vary 40-370 MPa, and the additional gray cast iron was found to vary between 410-490 MPa. The spall strength of the ductile cast iron was found to fall within the range of 0.94-1.2 GPa. It was shown that the spall strength is linked to the damage level at the spall plane, where an increased amount of tensile stress is required to generate higher levels of damage. Post mortem analysis was performed on recovered samples in order to establish a relationship between microstructure and the fracture mechanisms of the failure process. This study has identified the graphite phase as the primary factor governing the spall fracture of cast irons, where crack nucleation is directly correlated to the debonding of graphite from the metal matrix. It has been noted that the average length of graphite found within a casting is linked to the material’s strength, where strength has been shown to increase as a function of decreasing length. The morphology, and mean free path of graphite precipitates, has been shown to further govern the subsequent coalescence of initiated cracks to form a complete fracture plane. In cases where graphite spacing is large, an increased amount of energy is required to complete the fracture process. A secondary factor governing the spall fracture of cast irons has been linked to the microstructure of the metal matrix. It has been noted that pearlite will yield higher spall strengths in cast irons than free ferrite.

Within the second portion of this thesis, an experimental approach was developed to induce shock reflections in a low impedance inner-layer embedded within a high impedance bulk structure. By capturing temporal records of the stress evolution at each side of the inner-layer, step-like loading profiles were obtained that allowed for the capture of multiple Hugoniot states. The mathematical framework employed in this technique utilized the classical Rankine-Hugoniot equations in the method of impedance matching, where either the bulk material (case 1) or inner-layer (case 2) was required to have a known Hugoniot. Validation of the new technique was performed by testing well classified materials in order to facilitate comparison of the Hugoniots generated from the method with published data found in literature. For the first case, where the Hugoniot of the bulk material is known, the Hugoniot Ring-Up Method (HRUM) was shown to accurately generate states along the Hugoniot of the inner-layer, where the number of states acquired is directly linked to the experimental design. Factors including the wave velocities in the materials, input pulse duration (controlled by the thickness and wave velocity of the impactor), thickness of the innerlayer, and diameter of the test samples (arrival of the radial release) affect the number

of states that can be generated from a single experiment. Experiments employing 6061 aluminum and polycarbonate, respectively, as the bulk material and inner-layer, accurately generated six Hugoniot states for polycarbonate. Additionally, experiments employing A572 grade 50 structural steel as the bulk material were able to accurately generate ten Hugoniot states of the polycarbonate inner-layer. In these experiments, the method was extended to generate a Hugoniot equation defining the material response of the inner-layer within the domain encompassed by the specific test. Through comparison of these experimentally determined equations to the real Hugoniot of polycarbonate, it has been shown that a single HRUM experiment can yield an accurate Hugoniot for the inner-layer. For the second case, when the Hugoniot of the inner-layer is known, the HRUM failed to accurately generate states along the Hugoniot of the bulk material. Thus, the HRUM requires significant improvements before it can be used in this application. In light of these shortcomings, a procedure utilizing over-deterministic methodology has been proposed, that may allow future researchers to extend application of the HRUM to the case of determining the Hugoniot of the bulk material.

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