Jack C. Roberts

Characterization of a peek composite segmental bone replacement implant

A composite material of polyetheretherketone and short, chopped E-glass fibers was used to produce a segmental bone replacement implant. Problems with current metallic implants include stress-shielding of the surrounding bone and subsequent loosening of the implant. A better match between the bulk material properties of the implant and the bone it replaces can decrease the occurrence of these problems. Composite materials were chosen because their properties can be tailored to match the requirements. Material selection was accomplished with the aid of modeling software, which predicted the composite properties based on its composition and fiber directional parameters. Prototype parts were completed through a series of in-house molding and machining processes. Sections complete with an embedded metallic porous surface were tested to measure the strength of the attachment of the surface. The molded parts were characterized both destructively and nondestructively. The results of tensile tests performed on molded parts were comparable to those using commercially supplied samples. The fiber orientation was measured to verify the random positioning of fibers throughout the part, as assumed in the initial material selection. Ultrasonic C-scanned images confirmed that the molded parts had a very low density of air pockets or voids.

Human Surrogate Head Response to Dynamic Overpressure Loading in Protected and Unprotected Conditions

The ballistic performance of helmets has contributed to increased soldier survivability through the prevention of penetrating injuries. However, the efficacy of helmets in mitigating primary blast–induced traumatic brain injury (bTBI) is unclear. The objective of this effort was to utilize the Human Surrogate Head Model (HSHM) to investigate brain response to shock tube overpressure loading conditions, both with and without personal protective equipment (PPE). The HSHM is a physical surrogate which includes a brain, skull, facial structure and skin, all fabricated using biosimulant materials. The system was mounted to a Hybrid III Anthropomorphic Test Device neck to allow head motion during overpressure exposure. Pressure sensors were embedded along the sagittal plane in the anterior and posterior regions of the biosimulant brain. A series of shock tube tests using driver pressures at four levels (ranging from 420 to 1150 kPa) were conducted to simulate blast loading conditions. Internal pressure response was highly correlated to driver pressure, thus demonstrating the surrogate models sensitivity to load conditions. Characteristic features observed in the archetypical pressure waveform were used to evaluate differences between test parameters, including the effects of a helmet system on response. Results suggest that the helmet does not alter the initial peak pressure response. However, certain subsequent pressure peaks were found to undergo statistically significant reductions when a helmet was placed on the HSHM. The results of this test series demonstrate the use of a surrogate head system in characterizing the brain response to overpressure loading. Future studies will further evaluate the efficacy of PPE and contribute to the understanding of blast-induced injury mechanisms.

Development of a Human Head Physical Surrogate Model for Investigating Blast Injury

The objective of this effort was to develop a Human Surrogate Head Model (HSHM) and measure its response to pressure loading conditions representative of a blast environment. The HSHM consists of skin, face, skull, and brain fabricated using biosimulant materials and mounted to the neck of a Hybrid III Anthropomorphic Test Device to allow head motion during loading. The HSHM instrumentation includes pressure and displacement sensors embedded in the anterior and posterior areas of the brain along the saggital plane. The displacement sensors are a custom solution developed for this particular application. A series of shock tube tests at three varying load levels were conducted with the HSHM to simulate blast loading conditions. As pressure loading levels increased, the intracranial pressures and brain displacements increased as well. However, the spatial response of the displacement sensors varied with location in the brain. The results of this test series provide the first instance of intracranial pressure and directly measured brain displacements recorded from an anatomically correct head surrogate exposed to conditions representative of blast loading.Copyright

Design, analysis and fabrication of a graphite/epoxy electronics enclosure flanged aperture, with supporting electromagnetic interference test data

The structural design of a rectangular metal electronics enclosure for airborne or space applications is straightforward and contained in a number of texts. However, the design of a composite electronics enclosure, optimized for weight and structural and thermal load paths, has not been addressed. In particular, the design of flanges that attach the walls to one another or the enclosure to its cover is an important topic because of the need for EMI shielding and flange structural stability in a random vibration environment. Several carbon fabric/epoxy flanged aperture (two overlapping flanges with gap) designs were considered for integration into a composite electronics enclosure. The design evaluation was based on structural integrity, EMI shielding effectiveness, ease of fabrication, development time, number of parts, total cost, and total weight. The most promising design consisted of two overlapping L flanges with a double EMI gasket in the aperture. This design was analyzed for instability of the flanged aperture, local buckling of the individual flanges between fasteners, and bearing and shearout of the flanged aperture. Laminate configurations used in the structural analysis consisted of [(0°)]3, [(0°), (45°)] s , and [(0°), (45°), 0.076 mm thick syntactic foam core, (45°), (0°)]. The preliminary analysis indicated that the [(0°)] 3 laminate configuration would fail by local buckling between flanges. The [(0°), (45°), 0.076 mm thick syntactic foam core, (45°), (0°)] laminate had the highest margins of safety in overall flanged aperture instability, local buckling, bearing, and shearout. This configuration was thought to be the best lay-up because large margins of safety would be needed in random vibration to compensate for the unknown torsional and bending loads in the flanges. EMI shielding tests on a Ni plated carbon fabric flanged aperture from 200 MHz to 40 GHz showed that the geometry of the flange alone provided significant shielding over that of an open hole. The shielding effectiveness of the Ni plated carbon fabric/epoxy flanged aperture with EMI gasket was equivalent to that of a continuous solid Ni plated carbon fabric/epoxy panel. The [(00), (450)], and [(00), (450), 0.076 mm thick syntactic foam core, (45°), (00)] laminate configurations are currently being incorporated into a finite element model of the electronics enclosure.

Evaluation of an Instrumented Human Surrogate Torso Model in Open Field Blast Loading

A 50th percentile Human Surrogate Torso Model (HSTM50) was constructed using biosimulant materials to represent the thoracic skeletal structure, internal organs, and soft tissues. The model was instrumented with pressure sensors embedded in each organ, accelerometers rigidly mounted to the sternum, and a load cell aligned with the vertebral column. The HSTM was exposed to a series of open-field blast tests. Sensor data clearly conveyed an initial rise in organ pressure due to the arrival of the incident shock wave followed by a delayed secondary peak of lesser magnitude due to the arrival of the ground-reflected incident shock wave. For repeat test conditions, the HSTM provided sensor response deviation within the inherent variability of field pressure data recorded for various tests of equal weight charges. This test series demonstrated the HSTM50 sensitivity to blast threat conditions including variations in charge weight and type. The HSTM50 proved to be a repeatable, durable, non-homogeneous test device complete with skeletal structure and soft tissue. The system allowed for the dynamic measurement of internal pressures, acceleration, and spinal load as a result of various blast conditions.Copyright