Gale A. Holmes

The influence of silane coupling agent composition on the surface characterization of fiber and on fiber-matrix interfacial shear strength

It is well known that the fiber-matrix interface in many composites has a profound influence on composite performance. The objective of this study is to understand the influence of composition and concentration of coupling agent on interface strength by coating E-glass fibers with solutions containing a mixture of hydrolyzed propyl trimethoxysilane (PTMS) and n -aminopropyl trimethoxysilane (APS). The failure behavior and strength of the fiber-matrix interface were assessed by the single-fiber fragmentation test (SFFT), while the structure of silane coupling agent was studied in terms of its thickness by ellipsometry, its morphology by atomic force microscopy, its chemical composition by diffuse reflectance infrared Fourier transform (DRIFT), and its wettability by contact angle measurement. Deposition of 4.5 ‐ 10 m 3 mol/L solution of coupling agent in water resulted in a heterogeneous surface with irregular morphology. The SFFT results suggest that the amount of adhesion between the glass fiber and epoxy is dependent not only on the type of coupling agent but also on the composition of the coupling agent mixture. As the concentration of APS in the mixture increased, the extent of interfacial bonding between the fiber and matrix increased and the mode of failure changed. For the APS coated glass epoxy system, matrix cracks were formed perpendicular to the fiber axis in addition to a sheath of debonded interface region along the fiber axis.

A General Method for Quantitative Measurement of Molecular Mass Distribution by Mass Spectrometry

A method is presented to test whether the conversion of the mass spectrum of a polydisperse analyte to its molecular mass distribution is quantitative. Mixtures of samples with different average molecular masses, coupled with a Taylor’s expansion mathematical formalism, were used to ascertain the reliability of molecular mass distributions derived from mass spectra. Additionally, the method describes how the molecular mass distributions may be corrected if the degree of mass bias is within certain defined limits. This method was demonstrated on polydisperse samples of C60 fullerenes functionalized with ethylpyrrolidine groups measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry; however, it is applicable to any polydisperse analyte and mass spectrometric method as long as spectrum resolution allows individual oligomers to be identified. Mass spectra of the derivatized fullerenes taken in positive ion mode were shown to give an accurate measurement of the molecular mass distribution while those taken in negative ion mode were not. Differences in the mechanisms for ion formation are used to explain the discrepancy.Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States of America.

The Effect of Nonlinear Viscoelasticity on Interfacial Shear Strength Measurements

Experimental evidence demonstrates that diglycidyl ether of bisphenol-A (DGEBA)/meta phenylenediamine (m-PDA) epoxy resin matrix used in the single fiber fragmentation tests exhibits nonlinear stress strain behavior in the region where E-glass fiber fracture occurs. In addition, strain hardening after the onset of yield is observed. Therefore, linear elastic shear-lag models and the Kelly-Tyson model are inappropriate for the determination of the interfacial shear strength for this epoxy resin system. Using a strain-dependent secant modulus in the Cox model, the calculated interfacial shear strength is shown to be relatively lower by at least 15% than the value determined using a linear elastic modulus. This decrease is consistent with numerical simulations which show the linear elastic approximation over predicts the number of fragments in the fragmentation test. In addition, the value obtained by the strain-dependent secant modulus is approximately 300% relatively higher than the value predicted by the Kelly-Tyson model.

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.

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.

Effects of fiber gripping methods on single fiber tensile test using Kolsky bar

Preliminary data for testing fibers at high strain rates using the Kolsky bar test by Ming Cheng et al. [1] indicate minimal effect of strain rate on the tensile stress-strain behavior of poly (p-phenylene terephathalamide) fibers. However, technical issues associated with specimen preparation appear to limit the number of samples that can be tested in a reasonable time. In addition, under the Kolsky bar testing condition fiber fracture may occur at the interface between the fiber and adhesive rather than in the gage section. In this study, the authors investigate the effects of different gripping methods in order to establish a reliable, reproducible, and accurate Kolsky bar test methodology for single fiber tensile testing. As many single fiber tests have been carried out associated with ballistic research, we compare the Kolsky bar test results with the quasi-static test results to determine the tensile behavior over a wide range of strain rates.

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.

A methodology for detecting interfacial debonding in clay/epoxy nanocomposites

A novel methodology is presented for detecting the onset of debonding in clay-based nanocomposites. The procedure is based on constant illumination of the test specimen as it is subjected to tensile deformation by sequential strain-steps. After each strain-step, an image of the specimen is taken along its gauge length using a digital camera and the image is stored in a computer for later analysis using image analysis software. Test results from a nanocomposite containing a weak interface between the clay and the matrix indicate that interface debonding begins to occur above 1% strain, as evidenced by a reduction in the transmitted light through the specimen with increasing strain. Based on related research, the darkening of the specimen was interpreted as clay/matrix debonding. In contrast to the approximately 11% failure strain of the base epoxy resin, the nanocomposite specimen with the weak interface failed at 3.6% strain.

Measuring Interface Strength in Single Fiber Composites: The Effect of Stress Concentrations

Fiber-matrix interface strength is known to be a critical factor in controlling the long-term performance of structural composites. This parameter is often obtained by using the average fragment length data generated from the single-fiber fragmentation test (SFFT). The interfacial shear strength is then obtained by using this data in a micro-mechanics model that describes the shear-stress transfer process between the matrix and the fiber. Recently, a non-linear viscoelastic micro-mechanics model was developed to more accurately account for the matrix material properties. This new model indicates that the interface strength is dependent on the testing rate. Experimentally, it has been shown that the final fragment length distribution in some systems is dependent on the testing rate. However, data analysis using the new model indicates that the distribution change with testing rate is promoted by the presence of high stress concentrations at the end of the fiber fragments. From the model, these stress concentrations were found to exist at very low strain values. Experimentally, the fragment distributions obtained from specimens tested by different testing rates were found to be significantly different at strain values well below the strain values required to complete the test. These results are consistent with the research of Jahankhani and Galiotis and finite element calculations performed by Carrara and McGarry. These authors concluded that stress concentrations can promote failure of the fiber-matrix interface on the molecular level. Our results support this conclusion. In addition, our research results suggest that altering the SFFT testing rate can lower the magnitude of these stress concentrations and minimize failure of the fiber-matrix interface.