Amanda L. Forster

Testing and analyses of copolymer fibers based on 5-amino-2-(p-aminophenyl)-benzimidazole

Fibers containing 5-amino-2-(p-aminophenyl)benzimidazole are being considered for use as reinforcement in soft body armor applications. Past research in this laboratory has resulted in a suite of tests that have been used to detect degradation in other fibers and are now being applied to the fibers in question. Due to the architecture of the yarns in this study, two methods to extract single filaments for tensile testing were described and analyzed. A dry method resulted in fibers with surface damage, reflected in a high standard deviation in strength. A wet extraction method showed a reduction in surface damage and a lower standard deviation in strength. Fourier Transform Infrared analysis detected signs of hydrolysis in the fibers that were exposed to water. Although no noticeable loss in tensile strength was noted upon exposure, the possibility of hydrolysis in these fibers may indicate a need for further study.

Enhanced durability of carbon nanotube grafted hierarchical ceramic microfiber-reinforced epoxy composites

As carbon nanotube (CNT) infused hybrid composites are increasingly identified as next-generation aerospace materials, it is vital to evaluate their long-term structural performance under aging environments. In this work, the durability of hierarchical, aligned CNT grafted aluminoborosilicate microfiber-epoxy composites (CNT composites) are compared against baseline aluminoborosilicate composites (baseline composites), before and after immersion in water at 25 °C (hydro) and 60 °C (hydrothermal), for extended durations (90 d and 180 d). The addition of CNTs is found to reduce water diffusivities by approximately 1.5 times. The mechanical properties (bending strength and modulus) and the damage sensing capabilities (DC conductivity) of CNT composites remain intact regardless of exposure conditions. The baseline composites show significant loss of strength (44 %) after only 15 d of hydrothermal aging. This loss of mechanical strength is attributed to fiber-polymer interfacial debonding caused by accumulation of water at high temperatures. In situ acoustic and DC electrical measurements of hydrothermally aged CNT composites identify extensive stress-relieving micro-cracking and crack deflections that are absent in the aged baseline composites. These observations are supported by SEM images of the failed composite cross-sections that highlight secondary matrix toughening mechanisms in the form of CNT pullouts and fractures which enhance the service life of composites and maintain their properties under accelerated aging environments.

An Investigation of the Temperature and Strain-Rate Effects on Strain-to-Failure of UHMWPE Fibers

During a ballistic impact, Ultra High Molecular Weight Polyethylene (UHMWPE) fibers are subjected to high temperatures and high strain-rates. Their tensile strength increases with increasing strain-rate and decreases with increasing temperature. To understand the impact of both factors simultaneously, a single fiber heater has been fabricated to heat UHMWPE fibers up to the melting temperature (~148 °C) to measure the change in mechanical properties as a function of temperature and strain-rate. Custom grips have been fabricated for use with the single fiber heater and performed well across all strain rates and temperatures in this study. 251 tensile tests have been conducted on 10-mm gage length UHMWPE single fibers at temperature-strain-rate combinations spanning five strain-rates between 10−3and 550 s−1 and 11 temperatures from 20 to 148 °C. A non-failure boundary is created by temperature-strain-rate combinations where fibers can be strained to 25 % without mechanically failing. This occurs at 75 °C for 10−3 s−1, 100 °C for 10−2 s−1, 130 °C for 10−1 s−1, 148 °C for 100 s−1, and fail regardless of temperature at 550 s−1. It is estimated that for similar mechanical response, an increase in temperature of 25–30 °C is equivalent to lowering the strain-rate by one decade for strain-rates between 10−3 and 10−1 s−1. At 550 s−1 strain-rate, there was minor change in the strain-to-failure from 20 to 145 °C indicating strain-rate is the dominant factor.

Photofading in cotton fibers dyed using red, yellow, and blue direct dyes during examination with microspectrophotometry (MSP)

Microspectrophotometry (MSP) is a rapid, nondestructive technique for the analysis of color in textile fibers. This technique combines microscopy and ultraviolet (UV)/visible (Vis) spectroscopy, allowing for very small colored samples, like dyed textile fibers, to be analyzed directly and thereby eliminates the need for time consuming and destructive extractions. While MSP is generally accepted to be a nondestructive evaluation method, a loss of color during analysis, or photofading can occur. In this work, cotton fabric dyed with blue, yellow, and red direct dyes at different concentrations. Dye photofading during MSP examination was investigated by measuring the absorbance at a specific position on the fibers from these fabrics, periodically over the course of 30 minutes. Visible color loss and a reduction in absorbance was observed for all three colors, but was most pronounced for the fibers dyed red. A major goal of this study is to increase awareness of the photofading phenomenon when analyzing cotton fibers using MSP.

Disentangling High Strength Copolymer Aramid Fibers to Enable the Determination of Their Mechanical Properties

Traditionally, soft body armor has been made from poly(p-phenylene terephthalamide) (PPTA) and ultra-high molecular weight polyethylene. However, to diversify the fiber choices in the United States body armor market, copolymer fibers based on the combination of 5-amino-2-(p-aminophenyl) benzimidazole (PBIA) and the more conventional PPTA were introduced. Little is known regarding the long-term stability of these fibers, but as condensation polymers, they are expected to have potential sensitivity to moisture and humidity. Therefore, characterizing the strength of the materials and understanding their vulnerability to environmental conditions is important for evaluating their use lifetime in safety applications. Ballistic resistance and other critical structural properties of these fibers are predicated on their strength. To accurately determine the strength of the individual fibers, it is necessary to disentangle them from the yarn without introducing any damage. Three aramid-based copolymer fibers were selected for the study. The fibers were washed with acetone followed by methanol to remove an organic coating that held the individual fibers in each yarn bundle together. This coating makes it difficult to separate single fibers from the yarn bundle for mechanical testing without damaging the fibers and affecting their strength. After washing, fourier transform infrared (FTIR) spectroscopy was performed on both washed and unwashed samples and the results were compared. This experiment has shown that there are no significant variations in the spectra of poly(p-phenylene-benzimidazole-terephthalamide-co-p-phenylene terephthalamide) (PBIA-co-PPTA1) and PBIA-co-PPTA3 after washing, and only a small variation in intensity for PBIA. This indicates that the acetone and methanol rinses are not adversely affecting the fibers and causing chemical degradation. Additionally, single fiber tensile testing was performed on the washed fibers to characterize their initial tensile strength and strain to failure, and compare those to other reported values. Iterative procedural development was necessary to find a successful method for performing tensile testing on these fibers.

Specifying and testing idealized bust surrogates for testing of female stab-resistant body armor

Manufacturers of stab-resistant body armor continually strive to improve the comfort of the armor wearer and maximize protection. As female officers make up a larger portion of the law enforcement and corrections workplace, armor manufacturers have begun to offer designs that accommodate contoured body shapes, such as the female bust region, which results in non-planar armor. Conventional test methodologies for body armor were designed for armor to be tested flat and do not accommodate contoured armor. This work reviews the available literature concerning common bust sizes and discusses the research performed to specify bust surrogate size, shape, and materials for testing stab-resistant female body armor. Two standard cup sizes, B and D, modeled as a paraboloid with a modified base, are recommended to meet breast volume requirements. Recommendations for manufacture of these surrogates for incorporation into standardized test methods are provided. Instrumented stab testing of these surrogates shows that impact location on a structured armor and the type of backing material increase the maximum deceleration of a spike impact.