Jamie Kimberley

Visualization of the failure of quartz under quasi‐static and dynamic compression

[1] Quasi-static and dynamic compression experiments were performed on natural α quartz single crystal specimens at strain rates ranging from 10 ―3 to 10 3 s ―1 using a high-speed camera for visualization of failure. In one set of experiments, the specimens were compressed until catastrophic failure occurred, shattering the specimen into many small pieces. The results of the experiments show little strain rate dependence of the compressive strength of quartz for the range of strain rates applied in this study. In a second set of experiments, referred to here as interrupted compression, the specimens were compressed to a stress level of about half of the failure strength and then unloaded. For times up to when the peak load is achieved, images of the specimen recorded during the experiment show no crack initiation or propagation. However, in these experiments, the growth of large planar cracks was observed during (and only during) the unloading phase. The real-time visualization demonstrated that behavior of failure during unloading occurs in both the quasi-static and dynamic interrupted compression experiments. The crystallographic indices of the failure planes were identified to be of the {1101} and {1010} families, indicating cleavage failure on the positive and negative rhombohedral surfaces, respectively.

Characterization of light emission from mechanoluminescent composites subjected to high-rate compressive loading(Conference Presentation)

This study aims to devise multifunctional composites using fracto-mechanoluminescent (FML) materials and photoactive sensing thin films for autonomous and self-powered impact damage detection. In previous studies, multifunctional photoactive thin films were suggested as a strain sensor that does not require any external electrical source. Instead, the photoactive thin films generated direct current (DC) (or photocurrent) under ambient light, whose magnitude varied linearly with applied strain. In this study, multifunctional FML materials-photoactive thin film composites will be devised for autonomously sensing high-speed compressive strains without supplying any external photonic or electrical energy. FML materials exhibit transformative properties that emit light when its crystalline structures are fractured. The developed photoactive strain sensing thin film will be integrated with the FML materials. Thus, it is envisioned that the FML materials will emit light, which will be supplied to the photoactive sensing thin films when the high-speed compressive loadings break FML materials’ crystalline structures. First, synthesized europium tetrakit(dibenzoylmethide) triethylammonium (EuD4TEA) crystals will be embedded in the elastomeric and transparent polydimethylsiloxane (PDMS) matrix to prepare test specimens. Second, the FML properties of the EuD4TEA-PDMS composites will be characterized at various compressive strains, which will be applied by Kolsky bar testing setup. Light emission from the EuD4TEA-PDMS test specimens will be recorded using a high-speed camera. Intensity of the light emissions will be quantified via image processing techniques by taking into account pixel profiles of the high-speed camera captured images (e.g., pixel values, counts of pixels, and RGB values) at various levels of compressive strains. Lastly, the autonomous high-speed compressive sensor modules will be fabricated by integrating the EuD4TEA-PDMS composites with the photoactive thin film sensor. Self-powered sensing capability will be validated by measuring DC at various compressive strains.

Dynamic Strength and Fragmentation Experiments on Brittle Materials Using Theta-Specimens

Characterization of the strength and fragmentation response of brittle materials poses unique challenges related to specimen gripping and alignment. These challenges are often exacerbated when the characterization is to be conducted at elevated strain rates. Tensile strength of brittle samples are often characterized using the Brazilian disk testing geometry. While this ameliorates issues related to specimen alignment, the stress field in the specimen is not uniform, complicating the analysis of the test results. The theta specimen geometry was designed specifically to provide a uniform state of uniaxial stress in the specimen gage section when the exterior of the sample is subjected to compressive loading. Here we evaluate the use of the theta specimen geometry with a compressive Kolsky bar to measure the dynamic tensile strength and fragmentation response of a brittle polymer, Poly(Methyl methacrylate). Finite element simulations are used to investigate the effect of geometry and loading pulse shape on the ability to establish a state of uniaxial stress in the gage section. Particular attention is given the excitation of lateral vibrations in the gage section, which would perturb the desired uniaxial stress state.

A Miniature Tensile Kolsky Bar for Thin Film Testing

A miniature tension Kolsky (split-Hopkinson) bar has been developed to facilitate testing of metallic films with thicknesses on the order of 100 μm. The system consists of a cylindrical launch tube (which contains an internal striker), and incident and transmitted bars of rectangular cross section. The launch tube with internal striker facilitates pulse shaping by allowing for the use of traditional disk shaped pulse shapers. This ensures that tests are conducted under force equilibrium and at a nearly constant strain rate. The rectangular incident and transmitted bars facilitate specimen and strain gage mounting. The rectangular section also provides a reduced cross sectional bar area which increases the system sensitivity. Design considerations and analysis of different measurement techniques for bar strain/velocity will be discussed along with test results for Al foils.

Rate effects in the failure strength of extraterrestrial materials

—Most simulations of asteroid impacts use the properties of terrestrial analogs to approximate the behavior of the asteroid material because there is currently a lack of data for the mechanical properties of the investigate the mechanical response of a stony meteorite that is believed to be of asteroidal origin. We investigate the mechanical response of a stony meteorite that is believed to be of asteroidal origin. We have measured, at strain rates ranging from 10–3-10–3 S–1, the uniaxial (unconfined) compressive strength of specimens cut from an L5 ordinary chondrite, MacAlpine Hills 88118 (MAC88118). Quasistatic compression experiments were conducted using a servo-hydraulic load frame while dynamic compression experiments were conducted using a modified Kolsky bar. Images of the specimen were also recorded during the experiments were conducted using a modified Kolsky bar. Images of the specimen were also recorded during the experiments for visualization of the failure process. A large increase (3-4X) in compressive strength is observed when the strain rate is increased from 10–3-10–3 S–1.

A Scaled Model Describing the Rate-Dependent Compressive Failure of Brittle Materials

A universal relationship is developed that describes the rate-dependent compressive strength of brittle solids based on the micromechanics of the growth of brittle cracks from populations of initial flaws. Real-time observations of crack growth provide insight to the model which captures the dynamics of interacting and rapidly growing cracks. Fundamental time and length scales involved in the problem are used to develop expressions for a characteristic stress and a characteristic strain rate in terms of material and microstructural properties. Scaling simulation results by the characteristic stress and strain rate collapses the data to a single curve in failure stress-strain rate space. This curve represents the universal response, which captures both the relatively constant failure stress at low rates as well as the dramatic increase in strength observed in experiments as the applied strain rate increases above the transition rate. The resulting model for the universal response compares well with experimental data for ceramics and geologic materials, indicating that the model has adequately captured the physics of compressive failure for a wide range of materials. INTRODUCTION— The vast majority of compressive failures of brittle solids involve both significant amounts of crack nucleation (from pre-existing flaws or defects) and significant amounts of crack propagation and crack interactions. A new ansatz for high-rate compressive failure was recently developed by Paliwal and Ramesh (1) based on real-time ultra-high- speed visualization experiments (2) coupled with theoretical investigations of massive brittle failure. In brief, that work showed that the dynamic compressive failure process is controlled by the interactions of three terms: the initial defect distribution, crack growth dynamics and crack-crack interactions, and the coupling of these three terms with the superimposed rate of loading. That model describes the increase in the strength that is often observed in brittle materials subjected to uniaxial compression at high strain rates (3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13). This work develops a model that captures the behavior of brittle solids in an appropriately scaled form (14). RELEVANT SCALES—We begin by identifying the critical length scales and timescales associated with the physical problem of a brittle solid containing a pre-existing distribution of rectilinear flaws and subjected to dynamic loading. The pre-existing flaw distribution is described in terms of a flaw density