Mark Pankow

Three-dimensional digital image correlation technique using single high-speed camera for measuring large out-of-plane displacements at high framing rates

We are concerned with the development of a three-dimensional (3D) full-field high-speed digital image correlation (DIC) measurement system using a single camera, specifically aimed at measuring large out-of-plane displacements. A system has been devised to record images at ultrahigh speeds using a single camera and a series of mirrors. These mirrors effectively converted a single camera into two virtual cameras that view a specimen surface from different angles and capture two images simultaneously. This pair of images enables one to perform DIC measurements to obtain 3D displacement fields at high framing rates. Bench testing along with results obtained using a shock wave blast test facility are used to show the validity of the method.

Specimen size and shape effect in split Hopkinson pressure bar testing

The results from an experimental and computational study on the effects of specimen size and shape on split Hopkinson pressure bar (SHPB) testing are presented. The effects of the L/D ratio (length L divided by diameter D) along with specimen shape (cylindrical versus square prismatic) were investigated. Even though specimens having a circular cylindrical cross-section are traditionally used in SHPB testing, a square cross-section specimen has advantages associated with in situ imaging of the specimen side surfaces during stress wave propagation. The results presented here show a strong correlation in specimen response between the two types of specimens. Different ranges of L/D ratios are recommended for obtaining stiffness and strength data at near constant strain rates.

Point of impact: the effect of size and speed on puncture mechanics.

The use of high-speed puncture mechanics for prey capture has been documented across a wide range of organisms, including vertebrates, arthropods, molluscs and cnidarians. These examples span four phyla and seven orders of magnitude difference in size. The commonality of these puncture systems offers an opportunity to explore how organisms at different scales and with different materials, morphologies and kinematics perform the same basic function. However, there is currently no framework for combining kinematic performance with cutting mechanics in biological puncture systems. Our aim here is to establish this framework by examining the effects of size and velocity in a series of controlled ballistic puncture experiments. Arrows of identical shape but varying in mass and speed were shot into cubes of ballistic gelatine. Results from high-speed videography show that projectile velocity can alter how the target gel responds to cutting. Mixed models comparing kinematic variables and puncture patterns indicate that the kinetic energy of a projectile is a better predictor of penetration than either momentum or velocity. These results form a foundation for studying the effects of impact on biological puncture, opening the door for future work to explore the influence of morphology and material organization on high-speed cutting dynamics.

Split Hopkinson bar measurement using high-speed full-spectrum fiber Bragg grating interrogation.

The development and validation of a high-speed, full-spectrum measurement technique is described for fiber Bragg grating (FBG) sensors. A FBG is surface-mounted to a split-Hopkinson tensile bar specimen to induce high strain rates. The high strain gradients and large strains that indicate material failure are analyzed under high strain rates up to 500  s-1. The FBG is interrogated using a high-speed full-spectrum solid-state interrogator with a repetition rate of 100 kHz. The captured deformed spectra are analyzed for strain gradients using a default interior point algorithm in combination with the modified transfer matrix approach. This paper shows that by using high-speed full-spectrum interrogation of an FBG and the modified transfer matrix method, highly localized strain gradients and discontinuities can be measured without a direct line of sight.

Through-the-thickness response of hybrid 2D and 3D woven composites

3D woven materials are investigated for their through-the-thickness response. In this study, hybrid 3D woven composites, consisting of multiple fiber tows (carbon, glass and kevlar) have been investigated. Quasi-static tests were performed to determine the effective through-the-thickness response when subjected to confined compression. Confined compression was chosen because it shows similar failure modes to those seen under dynamic loading. The results presented in this paper show a correlation to changes in strength based on hybridization, however some of the observed results were not as expected.

A generalized analytical model for predicting the tensile behavior of 3D orthogonal woven composites using finite deformation approach

Abstract Over the past few decades, there have been an increasing interest in woven preforms as a reinforcement for composites. The invention of 3D Orthogonal Weaving (3DOW) technology introduced new and enhanced features to the conventional 2D woven preforms. Modeling the tensile behavior of 3DOW composites is very useful to the industry, it helps in characterizing the composite material with minimal need for coupon testing. In this study, a generalized analytical model was developed to predict the entire load–extension curve of the 3DOW preforms and composites including the non-linear region, using the finite-deformation approach. The model relies on the geometry of the structure and the tensile properties of the constituent yarn and resin components as input parameters. The model was generalized to predict the properties of any 3DOW structure, made with spun or filament yarn, jammed and non-jammed, which have any weave architecture, including hybrid composites. The model was verified experimentally for a broad range of experimental composites, including hybrid ones. The results indicated that there was a general good agreement between the experimental and theoretical curves.

The effect of the through-thickness yarn component on the in- and out-of-plane properties of composites from 3D orthogonal woven preforms

Abstract Development of three-dimensional (3D) weaving technology introduced new and enhanced features to the 2D weaving technology. 3D Orthogonal Woven (3DOW) preforms have a through-thickness yarn component that significantly enhances the out-of-plane properties and delamination resistance. In this study, a range of 3DOW E-glass preforms were woven using 3D weaving technology and then converted into composites, using vacuum assisted resin transfer molding technology. The composite samples had varying Z to Y-yarn/ layer ratio, the objective is to study the effect of changing the Z to Y-yarn/ layer ratio on the in-plane and out-of-plane mechanical properties. The study concludes that changing the amount of Z-yarn in the structure has negligible effect on the tensile (in-plane), yet, it has a significant effect on the drop weight impact properties (out-of-plane). Moreover, it had a strong effect on the failure mechanisms, and as the amount of Z-yarn is reduced, delamination became more significant.

Dynamic Characterization of Textile Composites Part I: Uniaxial Tension

This two part series of papers will investigate the uniaxial and multiaxial response of textile composite materials subjected to high rates of loading. In this first paper, the uniaxial high rate tensile response of fiber tows commonly used in textile composites is studied. The rate dependent material response of IM7/SC-15 and S2 Glass/SC-15 are investigated in this work. The material is tested from quasi-static to dynamic rates using both a servo-hydraulic load frame and a split Hopkinson bar technique. Cross-ply laminates were investigated to determine the response due to the difficulty of preventing the splitting mode of failure for uni-composites. Stress–strain curves are generated and a discussion of the validity of results is framed by using DIC strain fields to examine the mechanics in the static and elevated rate regimes.

Predicting Body Armor Back Face Deformation (BFD)

Understanding how personal protection systems deform during an impact event is paramount to their proper use in mitigating injury to the user. Most Kevlar fabric characterization relies on testing fabric suspended in the air; however, in use, these materials may respond differently due to the resistance from the backing material. The current NIJ standard relies on a clay backing material, and measuring the deformation after impact. This does not provide any time of flight information about the deformation response of these materials. In order to characterize the response of the multiple layer Kevlar samples, experimental testing was performed using clear backing materials that simulate human flesh. A clear backing material made of ballistics gelatin was used, allowing images to be obtained of the back of the Kevlar sample during the impact event. From these images, the maximum depth, as well as width and shape profile of the deformation can be tracked as a function of time. In this work a spherical projectile was fired into the Kevlar samples using a gas gun to determine the deformation vs time history. The parameters of the impact velocity, number of layers of Kevlar, and areal density of Kevlar were examined to determine the response on the deformation vs time history. Two types of Kevlar fabrics are used in various multiple layer configurations tested at a range of projectile velocities. It was found that the multi-layer areal density of a sample and the number of layers affect the deformation response significantly and independently. In addition, it was found that shapes of the back face deformations become more rounded as more fabric layers are added.

A spectral profile multiplexed FBG sensor network with application to strain measurement in a Kevlar woven fabric

A spectral profile division multiplexed fiber Bragg grating (FBG) sensor network is described in this paper. The unique spectral profile of each sensor in the network is identified as a distinct feature to be interrogated. Spectrum overlap is allowed under working conditions. Thus, a specific wavelength window does not need to be allocated to each sensor as in a wavelength division multiplexed (WDM) network. When the sensors are serially connected in the network, the spectrum output is expressed through a truncated series. To track the wavelength shift of each sensor, the identification problem is transformed to a nonlinear optimization problem, which is then solved by a modified dynamic multi-swarm particle swarm optimizer (DMS-PSO). To demonstrate the application of the developed network, a network consisting of four FBGs was integrated into a Kevlar woven fabric, which was under a quasi-static load imposed by an impactor head. Due to the substantial radial strain in the fabric, the spectrums of different FBGs were found to overlap during the loading process. With the developed interrogating method, the overlapped spectrum would be distinguished thus the wavelength shift of each sensor can be monitored.