Christopher J. Freitas

Dendritic processes of osteocytes are mechanotransducers that induce the opening of hemichannels

Osteocytes with long dendritic processes are known to sense mechanical loading, which is essential for bone remodeling. There has been a long-standing debate with regard to which part(s) of osteocyte, the cell body versus the dendritic process, acts as a mechanical sensor. To address this question experimentally, we used a transwell filter system that differentiates the cell body from the dendritic processes. Mechanical loading was applied to either the cell body or the dendrites, and the osteocyte’s response was observed through connexin 43 hemichannel opening. The hemichannels located on the cell body were induced to open when mechanical loading was applied to either the dendritic processes or the cell body. However, no significant hemichannel activity in the dendrites was detected when either part of the cell was mechanically stimulated. Disruption of the glycocalyx by hyaluronidase on the dendrite side alone is sufficient to diminish a dendrite’s ability to induce the opening of hemichannels on the cell body, while hyaluronidase has no such effect when applied to the cell body. Importantly, hyaluronidase treatment to the dendrite side resulted in formation of poor integrin attachments with the reduced ability of the dendrites to form integrin attachments on the underside of the transwell filter. Together, our study suggests that the glycocalyx of the osteocyte dendritic process is required for forming strong integrin attachments. These integrin attachments probably serve as the mechanotransducers that transmit the mechanical signals to the cell body leading to the opening of hemichannels, which permits rapid exchange of factors important for bone remodeling.

The issue of numerical uncertainty

Abstract Computational fluid dynamics (CFD) computer codes have become an integral part of the analysis and scientific investigation of complex, engineering flow systems. Unfortunately, inherent in the solutions from simulations performed with these computer codes is error or uncertainty in the results. The issue of numerical uncertainty addresses the development of methods to define the magnitude of error or to bound the error in a given simulation. This paper reviews the status of methods for evaluation of numerical uncertainty, and provides a direction for the effective use of some techniques in estimating uncertainty in a simulation.

Modeling of Fabric Impact With High Speed Imaging and Nickel-Chromium Wires Validation

Ballistic tests were performed on single-yarn, single-layer and ten-layer targets of Kevlar® KM2 (600 and 850 denier), Dyneema® SK-65 and PBO® (500 denier). The objective was to develop data for validation of numerical models so, multiple diagnostic techniques were used: (1) ultra-high speed photography, (2) high-speed video and (3) nickel-chromium wire technique. These techniques allowed thorough validation of the numerical models through five different paths. The first validation set was at the yarn level, where the transverse wave propagation obtained with analytical and numerical simulations was compared to that obtained in the experiments. The second validation path was at the single-layer level: the propagation of the pyramidal wave observed with the high speed camera was compared to the numerical simulations. The third validation consisted of comparing, for the targets with ten layers, the pyramid apex and diagonal positions from tests and simulations. The fourth validation, which is probably the most relevant, consisted of comparing the numerical and experimental ballistic limits. Finally for the fifth validation set, nickel-chromium wires were used to record electronically the waves propagating in the fabrics. It is shown that for the three materials the waves recorded during the tests match well the waves predicted by the numerical model.

Dynamic response due to behind helmet blunt trauma measured with a human head surrogate

A Human Head Surrogate has been developed for use in behind helmet blunt trauma experiments. This human head surrogate fills the void between Post-Mortem Human Subject testing (with biofidelity but handling restrictions) and commercial ballistic head forms (with no biofidelity but ease of use). This unique human head surrogate is based on refreshed human craniums and surrogate materials representing human head soft tissues such as the skin, dura, and brain. A methodology for refreshing the craniums is developed and verified through material testing. A test methodology utilizing these unique human head surrogates is also developed and then demonstrated in a series of experiments in which non-perforating ballistic impact of combat helmets is performed with and without supplemental ceramic appliques for protecting against larger caliber threats. Sensors embedded in the human head surrogates allow for direct measurement of intracranial pressure, cranial strain, and head and helmet acceleration. Over seventy (70) fully instrumented experiments have been executed using this unique surrogate. Examples of the data collected are presented. Based on these series of tests, the Southwest Research Institute (SwRI) Human Head Surrogate has demonstrated great potential for providing insights in to injury mechanics resulting from non-perforating ballistic impact on combat helmets, and directly supports behind helmet blunt trauma studies.

Simulation and analysis of a 23-mm HEI projectile hydrodynamic ram experiment

Abstract An Eulerian wavecode was used to simulate the impact, penetration, and detonation of a 23-mm high-explosive projectile into a water-filled tank. The pressure–time response is compared with results from an experiment conducted by Lundstrom and Andersen (Symp. on Shock and Wave Propagation, Fluid–Structure Interaction, and Structural Responses, 1989). Computed peak pressures and impulses compare well with the experimental values.

Composite materials dynamic back face deflection characteristics during ballistic impact

The current rationale for development of composite combat helmets is to either maintain performance at reduced weight or maintain weight with a significantly higher level of ballistic performance. Typically, weight reduction with maintained performance is the design approach used. In order to reduce weight with the same materials requires a reduction of material thickness. Thinner structural materials then introduce the complicating and often limiting factor of greater back face deflection. To further understand the tradeoffs of ballistic performance and efficiency, weight and back face deflection, a research project was undertaken. In this research project, a set of 17 composite materials were investigated. The digital image correlation method was used to directly measure the characteristics of the dynamic back face deflection of targets engaged by a set of threats. The analysis of this data, which includes dynamic deflection time histories, back face velocity time histories, strain time histories and spatial distributions of these quantities, allowed for assessment of candidate material performance and characterization of back face deflection. The details of this experimental program and key data results are presented in this paper.

Rapid impactor sample return (RISR) mission scenario

Abstract Due to the long lead time and great expense of traditional sample return mission plans to Mars or other astronomical bodies, there is a need for a new and innovative way to return materials, potentially at a lower cost. The Rapid Impactor Sample Return (RISR) mission is one such proposal. The general mission scenario involves a single pass of Mars, a Martian moon or an asteroid at high speeds (7 km/s), with the sample return vehicle skimming just 1 or 2 m above a high point (such as a top ridge on Olympus Mons on Mars) and releasing an impactor. The impactor strikes the ground, throwing up debris. The debris with roughly the same forward velocity will be captured by the sample return vehicle and returned to Earth. There is no delay or orbit in the vicinity of Mars or the asteroid: RISR is a one-pass mission. This paper discusses some of the details of the proposal. Calculations are presented that address the question of how much material can be recovered with this technique. There are concerns about the effect of Mars tenuous atmosphere. However, it will be noted that such issues do not occur for RISR style missions to Phobos, Deimos, or asteroids and Near Earth Objects (NEOs). Recent test results in the missile defense community (IFTs 6–8 in 2001, 2002) have scored direct hits at better than 1 m accuracy with closing velocities of 7.6 km/s, giving the belief that accuracy and sensing issues are developed to a point that the RISR mission scenario is feasible.

The characterization of a wide area network computation

Distributed parallel computing using message-passing techniques on Networks of Workstations (NOW) has achieved widespread use in the context of Local Area Networks. Recently, the concept of Grid-based computing using Wide Area Networks (WAN) has been proposed as a general solution to distributed high performance computing. The use of computers and resources at different geographic locations connected by a Wide Area Network and executing a real application introduces additional variables that potentially complicate the efficient use of these resources. Presented here are the results of a study that begins to characterize the performance issues of a WAN-based NOW, connecting resources that span an international border.

High-Speed DIC on Inside Perma-Gel During Ballistic Peneration

Perma-Gel® is a melt-castable, reusable synthetic ballistic gelatin used as a soft tissue simulant in research, forensics, and shooting sports. It can also be used to study the temporary wound cavity that forms during a penetrating injury from a ballistic projectile such as a small arms bullet or a blast fragment. This cavity is typically imaged with high-speed video cameras and its measured manually in each video frame. The dynamic cavity size alone, however, does not directly provide data on the strain that material nearby, but outside of, the wound cavity is subject to. In a recent project, SwRI® determined that it would be feasible to embed a dot pattern inside of the Perma-Gel block and use DIC to directly measure the strain field surrounding the wound cavity. Successfully and reliably embedding a dot pattern on a flat plane in the middle of the gel block proved to be challenging, in part because casting on top of a set layer with the pattern tended to melt the bottom layer and distort the pattern. After some modifications, a successful technique was developed. The method developed and example applications of it and the resulting DIC strain fields measured from a variety of projectiles and impact speeds are presented. The data demonstrate that DIC is an effective technique for measuring dynamic response of ballistic penetration in Perma-Gel, and can provide new insights that may be difficult or impossible to achieve with other measurement techniques.

High-Speed DIC on Flat Panels Subjected to Ballistic Impacts

Over the past several years, Southwest Research Institute® (SwRI®) has developed and applied DIC to measure back-face deflection and strain response on hundreds of flat panel test articles subjected to ballistic impacts from both small arms fire and bird-strike. The data collected has been used to develop new composite panel layups and validate material models to a level that no previous measurement technique was able to achieve. Getting consistent, high-quality DIC data during a ballistic impact test, however, presents some unique challenges not present in lower-rate testing methods. Proper pattern application techniques to avoid spalling during impact and protection of the expensive stereo high-speed camera pair must be considered and are addressed here. For small arms tests, the required image exposure time to avoid motion blur is on the order of 1 μs, requiring appropriate illumination methods. Bird strike presents a separate set of challenges including an outdoor environment, larger measurement area, and biohazard. The methods used to address these will also be presented. Materials tested include metals, transparent plastics, and composites including carbon, aramid, and polyethylene. Target sizes range from 6 × 6 to 36 × 36 in. and data rates from 10,000 to 50,000 Hz. Comparison of the DIC data with traditional measurement techniques such as strain gauges and calipers was also made. Agreement proved to be good, prompting the most recent series of tests to be run with DIC as the exclusive data collection method.