Brett Sanborn

Pre-strain Effect on Frequency-Based Impact Energy Dissipation through a Silicone Foam Pad for Shock Mitigation

Silicone foams have been used in a variety of applications from gaskets to cushioning pads over a wide range of environments. Particularly, silicone foams are used as a shock mitigation material for shock and vibration applications. Understanding the shock mitigation response, particularly in the frequency domain, is critical for optimal designs to protect internal devices and components more effectively and efficiently. The silicone foams may be subjected to pre-strains during the assembly process which may consequently influence the frequency response with respect to shock mitigation performance. A Kolsky compression bar was modified with pre-compression capabilities to characterize the shock mitigation response of silicone foam in the frequency domain to determine the effect of pre-strain. A silicone sample was also intentionally subjected to repeated pre-strain and dynamic loadings to explore the effect of repeated loading on the frequency response of shock mitigation.

Orientation Dependent Compressive Response of Human Femoral Cortical Bone as a Function of Strain Rate

Abstract Under extreme environments, such as a blast or impact event, the human body is subjected to high-rate loading, which can result in damage such as torn tissues and broken bones. The ability to numerically simulate these events would help improve the design of protective gear by iterating different configurations of protective equipment to reduce injuries. Computer codes capable of simulating these events require accurate rate-dependent material models representing the material deformation and failure (or injury) to properly predict the response of human body during simulation. Therefore, the high-rate material response must be measured to allow for simulation of high-rate events. This study seeks to quantify the high-rate mechanical response of human femoral cortical bone for use in high fidelity human anatomical models. Cortical bone compression specimens were extracted from the longitudinal and transverse directions relative to the long axis of the femur from three male donors, ages 36, 43, and 50. The compressive behavior of the cortical bone was studied at quasi-static (0.001/s), intermediate (1/s), and dynamic (~1000/s) strain rates using a split-Hopkinson pressure bar to determine the strain rate dependency and anisotropic effect on the strength of bone. The results indicate that cortical bone is anisotropic and stronger in the longitudinal direction compared to the transverse direction. The human cortical bone compressive response was also rate dependent in both directions, demonstrating significant increase in strength with increase in strain rate. Additionally, as the strain rate increased from intermediate to dynamic, a decrease in the elongation at transverse orientation was observed, which would indicate the bone becomes more brittle.

Effect of Pre-strain, Processing Conditions, and Impact Velocity on Energy Dissipation in Silicone Foams and Rubber

Silicone foams and rubber are used in a variety of applications to protect internal components from external shock impact. Understanding how these materials mitigate impact energy is a crucial step in designing more effective shock isolation systems for components. In this study, a Kolsky bar with pre-compression and passive radial confinement capabilities was used to investigate the response of silicone foams and rubber subjected to impact loading at different speeds. Using the preload capability, silicone foam samples were subjected to increasing levels of pre-strain. Frequency-based analyses were carried out on results from silicone foams and rubber to study the effect of both pre-strain and material processing conditions on the mechanism of energy dissipation in the frequency domain. Additionally, effects of impact speed on energy dissipation through silicone foams and rubber were investigated.

Poisson‘s Ratio Induced Radial Inertia Confinement During Dynamic Compression of Hyperelastic Foams

Hyperelastic foams have excellent impact energy absorption capability and can experience full recovery following impact loading. Consequently, hyperelastic foams are selected for different applications as shock isolators. Obtaining accurate intrinsic dynamic compressive properties of the hyperelastic foams has become a crucial step in shock isolation design and evaluation. Radial inertia is a key issue in dynamic characterization of soft materials. Radial inertia induced stress in the sample is generally caused by axial acceleration and large deformation applied to a soft specimen. In this study, Poisson’s ratio of a typical hyperelastic foam – silicone foam – was experimentally characterized under high strain rate loading and was observed to drastically change across the densification process. A transition in the Poisson’s ratio of the silicone foam specimen during dynamic compression generated radial inertia which consequently resulted in additional axial stress in the silicone foam sample. A new analytical method was developed to address the Poisson’s ratio-induced radial inertia effects for hyperelastic foams during high rate compression.

Dynamic Fracture Response of a Synthetic Cortical Bone Simulant

This work characterizes the fracture response of a composite material designed to mimic the response of human cortical bone. We have identified additive manufacturing, more generally known as 3-D printing, as a means of reproducing the curvature, variation in thickness, and gradient in porosity characteristic of the human bone between the cortical and trabecular regions. As the base material for developing bone surrogates via additive manufacturing, we evaluate a photocurable polymer with a high loading of ceramic particulate reinforcement that is compatible with stereolithographic additive (SLA) manufacturing. Specimens were printed in two orientations to measure fracture response perpendicular and parallel to the direction of deposition of the layer-by-layer manufacturing process. Mode I fracture behavior of the material was measured in four point bending configuration at high rate via modified split Hopkinson pressure bar for both orientations. In this paper, the fracture behavior of the bone simulant are presented and are compared to the mode I fracture behavior of human cortical bone perpendicular to the long axis of the human femur characterized under the same conditions.

EXPERIMENTAL EVALUATION OF LOW-PASS SHOCK ISOLATION PERFORMANCE OF ELASTOMERS USING FREQUENCY-BASED KOLSKY BAR ANALYSES

ELASTOMERIC MATERIALS ARE USED AS SHOCK ISOLATION MATERIALS IN A VARIETY OF ENVIRONMENTS TO DAMPEN VIBRATIONS AND/OR ABSORB ENERGY FROM EXTERNAL IMPACT FOR MINIMIZING ENERGY TRANSFER BETWEEN TWO OBJECTS OR BODIES. SOME APPLICATIONS REQUIRE THE SHOCK ISOLATION MATERIALS TO BEHAVE AS A LOW-PASS MECHANICAL FILTER TO MITIGATE THE SHOCK/IMPACT AT HIGH FREQUENCIES BUT TRANSMIT THE ENERGY AT LOW FREQUENCIES WITH MINIMAL ATTENUATION. IN ORDER TO FULFILL THIS REQUIREMENT, A SHOCK ISOLATION MATERIAL NEEDS TO BE CAREFULLY EVALUATED AND SELECTED WITH PROPER EXPERIMENTAL DESIGN, PROCEDURES, AND ANALYSES. IN THIS STUDY, A KOLSKY BAR WAS MODIFIED WITH PRE-COMPRESSION (UP TO 15.5 KN) AND CONFINEMENT CAPABILITIES TO EVALUATE LOW-PASS SHOCK ISOLATION PERFORMANCE IN TERMS OF ACCELERATION ATTENUATION THROUGH A VARIETY OF ELASTOMERS. THE EFFECTS OF PRELOAD AND SPECIMEN GEOMETRY ON THE LOW-PASS SHOCK ISOLATION RESPONSE WERE ALSO INVESTIGATED.