Development of a Split Hopkinson Tension Bar for Testing Stress-Strain Response of Particulate Composites under High Rates of Loading
Type of DegreeThesis
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Mechanical structures and their material constituents undergo an expansive range of loading conditions. Specifically, the rate at which the loading takes place can vary from being virtually static to being almost instantaneous. This presents an interesting and challenging problem of understanding the way a material will respond to various loading rates. In terms of mechanical behavior, it has been observed that the stress-strain response is elementally linked to this variation in loading rates. Due to this, it is imperative that the response for a particular material be known for the loading rates it will experience in service. Over the last half century, many investigators have studied this effect for different materials under compressive high strain rate loading conditions. In more recent decades, this research area has been extended to study the responses of materials in tension and in shear. One class of materials, particulate composites, is of specific interest in this research. Particulate composites have become progressively more popular in recent years, finding applications ranging from aerospace to electronics. Much research has been focused on studying their dynamic stress-strain behavior in compression. However, these materials are often employed under conditions that require an understanding of their dynamic tensile behavior. For instance, material fracture can often be driven by local maximum tensile stresses. The focus of this work is thus to design a split Hopkinson tension bar to be used for studying the dynamic tensile behavior of polymers and polymer-matrix particulate composites. This apparatus is designed using finite element analysis coupled with experimentation. A mechanism capable of producing a square loading pulse and transferring that loading pulse into the test specimens is developed. A data acquisition and post processing system is devised to capture and analyze the necessary data. A series of tests is then completed to demonstrate the validity and repeatability of the results. Next, the effect of filler particle diameter and filler volume fraction on dynamic tensile stress-strain response is investigated. The results from the filler volume fraction study are compared with those from predictions and various empirical models. Finally, a brief study of the effects of loading rate on a commercially available polymer biocement is undertaken. In general, the polymers and filled-polymer composites exhibit a stiffer response under the dynamic conditions. The dynamic material strength is typically higher than its quasi-static counterpart. Often, there is a reduction in strain at failure for the dynamic loads. All of these items are quantified in the present work.