Kirk D. Rice

Effect of fiber gripping method on the single fiber tensile test: II. Comparison of fiber gripping materials and loading rates

Single poly(p-phenylene terephthalamide) (PPTA) fiber tensile tests were carried out under quasi-static and high strain rate loading conditions using poly(methyl methacrylate) and rubber grips to investigate effects of grip materials and loading rates on fiber tensile properties. Differences in ultimate tensile strengths, failure strains, and moduli of PPTA fibers obtained by two different grip materials were insignificant. On the other hand, the fiber tensile properties showed significantly rate-dependent behaviors, which were graphically confirmed by kernel density plots as a non-parametric statistical analysis. Strength models considering three aspects (stochastic, fracture mechanics, and polymer chain domain behaviors) were also shown to link the loading rate effect in relation to fracture mechanisms.

Characterization of clay composite ballistic witness materials

Mechanical and thermal properties of Roma Plastilina Clay #1 (RP1) were studied through small-amplitude oscillatory shear (SAOS), large-amplitude oscillatory shear (LAOS), and differential scanning calorimetry (DSC), supplemented with thermogravimetric analysis, X-ray diffraction, and X-ray florescence. Rheological characterizations of RP1 through SAOS indicate that the clay composite softens as it is worked and slowly stiffens as it rests. Upon heating, the clay composite softens, prior work history is erased, and the composite undergoes a melting transition, although melted clay is significantly stiffer when returned to the usage temperature. Continuing mechanical characterizations into the LAOS or nonlinear region, RP1 transitions from a transient network to a viscous shear-thinning material as the temperature is increased. Using the MITlaos framework, RP1 exhibits intra-cycle strain stiffening and intra-cycle shear thinning at all temperatures.

Single Fiber Tensile Properties Measured by the Kolsky Bar Using a Direct Fiber Clamping Method

The Kolsky bar test has been widely used in measuring material behavior under high strain rate conditions. In particular, polymers used in ballistic applications have been characterized by this method to investigate high strain rate behavior during ballistic impact. Research conducted by Cheng et al. (J Eng Mater Technol-Trans ASME 127(2):197–203, 2005) and Lim et al. (J Mater Sci 45(3):652–661, 2010) measured high strain rate properties of single PPTA [Poly (p-phenylene terephthalamide)] fibers and aramid co-polymer fibers by gluing the fiber directly to the Kolsky bar, which is time consuming work and can be affected by wicking of the glue into the fiber gauge length area. Kim et al. (J Mater Sci. doi: 10.1007/s10853-013-7142-y, 2013) investigated clamping effects of the glue-tab and direct gripping methods on the single PPTA fiber tensile properties under the quasi-static loading condition and applied the direct grip method for the Kolsky bar test to measure the tensile strengths at a high strain rate (Kim et al., Compos Sci Technol. doi: 10.1016/j.compscitech.2012.03.021, 2012). This study extends the measurement capability for the tensile strength, failure strain and modulus that are important parameters that influence performance of soft body armor.

Statistical Characterizations for Tensile Properties of Co-polymer Aramid Fibers: Loading Rate Effects

High strength polymer fibers such as poly(p-phenylene terephthalamide) (PPTA), and poly(p-phenylene benzobisoxazole), (PBO) have been used for ballistic body armors. In addition to these para-oriented fibers, copolymer aramid fibers are being considered. Although mechanical properties of these fibers measured under quasi-static loading conditions are reported to be excellent, fiber tensile properties measured at comparable loading conditions for ballistic impact are rarely reported. In this study, we measure single fiber tensile properties at high rate loading conditions by clamping a fiber to the grips of a mini Kolsky bar, and investigate their statistical distributions as well as fiber morphologies.

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.

Statistical Characterization of Single PPTA Fiber Tensile Properties from High Strain Rate Tests

Single [poly (p-phenylene terephalamide)] PPTA fiber tensile strengths were measured under quasi-static and high strain rate loading conditions, and poly (methyl methacrylate) (PMMA) and rubber as gripping materials were used to investigate gripping effects for the tests. To incorporate the strength distributions of single PPTA fibers into a rate dependent stochastic strength model, it is important to estimate uncertainties of the model parameters as well as the best-fitting-distribution for the parameter estimation. We demonstrated the appropriateness of a Weibull model for the tensile strengths obtained by the quasi-static test and preliminary results for the corresponding Weibull shape parameters with approximately ±20 % parameter confidence intervals. These results will be used to characterize of the strengths obtained by the high strain rate test using the Weibull model.

Tensile Behavior of Single PPTA Fibers Measured by the Kolsky Bar Using the Direct Fiber Clamping Method

The Kolsky bar test has been widely used in measuring material behavior under high strain rate conditions. In particular, this methodology has been used to characterize the high strain rate behavior of polymer and polymer composites during ballistic impact. Chen et al. measured the tensile properties of single poly(p-phenylene terephthalamide) (PPTA) fibers at high strain rates by gluing the fiber directly to the Kolsky bar. However, the application of this technique is somewhat limited because of the time-consuming nature of the gluing procedure [1]. Due to the highly stochastic nature of fiber tensile strength, large test-sample sizes are required to analyze distributions of fiber properties such as tensile strength, modulus, and strain-to-failure. The authors have investigated the direct gripping method to facilitate high throughput single fiber tensile testing at high strain rates. As an extension of these efforts, the polymethyl methacrylate (PMMA) platens used for directly gripping the fibers are replaced here by rubber platens to assess the impact of the platen material properties on the stress transfer from the Kolsky bar to the fiber at high strain rates. In this study, the system compliance for the PMMA grips is measured under the quasi-static condition to assess the stress transfer from the grip and to correct the modulus and failure strain of the fiber. In preliminary tests using a PMMA grip, the system compliance was obtained by the 5, 10 and 60 mm gauge lengths. These data will be compared with similar data using the rubber grips to investigate the influence of the gripping material properties.

Statistical analysis of fiber gripping effects on Kolsky bar test

Preliminary data for testing fibers at high strain rates using the Kolsky bar test by Ming Cheng et al. [1] indicated minimal effect of strain rate on the tensile stress-strain behavior of PPTA [poly (p-phenylene terephathalamide)] fibers. In a different study, Lim et al. [2] reported that the tensile strengths of the copolymer aramid fibers are strain rate dependent. Usually with single fiber tests, a large sample size is needed to get statistically significant results. To date with high strain rate tests, technical issues associated with specimen preparation appear to limit the number of samples that can be tested within a reasonable time. In this study, the authors investigate the feasibility of a gripping design to establish a reliable, reproducible, and accurate Kolsky bar test methodology for single fiber tensile testing with high throughput. Preliminary results measured under a quasi-static strain rate with 5 mm and 60 mm gauge lengths showed that the PPTA fibers are gauge length dependent. This size effect must be considered in determining the true strain rate effect on fiber strength in conjunction with the gripping effect.