Brendan O’Toole

Cell Morphology and Mechanical Properties of Rigid Polyurethane Foam

Polyurethane foam, used as a supporting or insulating material, is sometimes formed in complex molds with significant variations in geometry and size. This work investigates the relationships between cell morphology, density, and mechanical properties in a molded polyurethane material using relatively small cylindrical molds. Understanding these relationships will help mechanical designers to analyze and predict the responses of foam components accurately. Three mold sizes are used to study changes in cell morphology (cell area, cell diameter, aspect ratio, cell angle, cell edge length, cell face thickness, and cell edge thickness), density, and mechanical properties (Young’s modulus and collapse stress) with respect to position within the mold. The density is shown to increase from the top to the bottom of the reference mold but does not change significantly in the small and tall molds. However, the mechanical and cell morphology properties show more changes in the small and tall molds when compared to the reference mold even though there are only small changes in density.

Functional and dynamic response characteristics of a custom composite ankle foot orthosis for Charcot-Marie-Tooth patients

BACKGROUND Custom carbon-fiber composite ankle foot orthoses (AFOs) have been anecdotally reported to improve gait of Charcot-Marie-Tooth (CMT) patients. The purpose of the study was to characterize the spatio-temporal, joint kinetic and mechanical responses of a custom carbon fiber AFO during locomotion for persons diagnosed with CMT. METHODS Eight volunteers were fitted with custom AFOs. Three of the devices were instrumented with eight strain gauges to measure surface deformation of the shell during dynamic function. Following a minimum 10 weeks accommodation period, plantar- and dorsiflexor strength was measured bilaterally. Volunteers then walked unbraced and braced, at their preferred pace over a force platform and instrumented walkway while being tracked with a 12-camera motion capture system. Strength, spatio-temporal and lower extremity joint kinetic parameters were evaluated between conditions (single subject) using the model statistic procedure. Mechanical loads were presented descriptively. RESULTS All participants walked faster (89.4 ± 13.3 vs 115.6 ± 18.0 cm/s) in the braced condition with ankle strength negatively correlated to speed increase. As Δ velocity increased, maximum joint moments during loading response shifted from the hip joint to the ankle and knee joints. During propulsion, the hip joint moment dominated. Subjects exhibiting the greatest and least Δ velocity imposed an average load of 54.6% and 16.6% of body weight on the braces, respectively. Energy storage in the brace averaged 9.6 ± 6.6J/kg. CONCLUSION Subject-specific effects of a custom AFO on gait for CMT patients were documented. The force-deflection properties of carbon-fiber composite braces may be important considerations in their design.

Study of hypervelocity projectile impact on thick metal plates

Hypervelocity impacts generate extreme pressure and shock waves in impacted targets that undergo severe localized deformation within a few microseconds. These impact experiments pose unique challenges in terms of obtaining accurate measurements. Similarly, simulating these experiments is not straightforward. This study proposed an approach to experimentally measure the velocity of the back surface of an A36 steel plate impacted by a projectile. All experiments used a combination of a two-stage light-gas gun and the photonic Doppler velocimetry (PDV) technique. The experimental data were used to benchmark and verify computational studies. Two different finite-element methods were used to simulate the experiments: Lagrangian-based smooth particle hydrodynamics (SPH) and Eulerian-based hydrocode. Both codes used the Johnson-Cook material model and the Mie-Gruneisen equation of state. Experiments and simulations were compared based on the physical damage area and the back surface velocity. The results of this study showed that the proposed simulation approaches could be used to reduce the need for expensive experiments.

Optimization of a Vehicle Space Frame Under Ballistic Impact Loading

Shock from impact loading may risk the lives of the occupants of a military vehicle and damage the sensitive electronic components within it. A finite element model (FEM) for a space-frame based military vehicle is presented in this paper. An approach is developed to optimize the design of the joints within the space frame structure to reduce the mass of the vehicle while maintaining its structural integrity. The process starts by creating a parametric FEM of the vehicle. The optimization variables are the lengths of joint branches. The effect of joint location within the space frame is also explored. The problem is subject to geometry and stress constraints. Results show that a mass reduction can be achieved without adversely affecting integrity of the vehicle.Copyright

Finite Element Modeling of a Lightweight Composite Blast Containment Vessel

This paper presents various approaches for finite element modeling of a cylindrical lightweight composite vessel for blast containment purposes. The vessel has a steel liner that is internally reinforced with throttle and gusset steel plates and wrapped with a basalt fiber/epoxy composite. The vessel design is fairly complex, including many geometric details and several components with different material models. The objective of this work is to determine an accurate and efficient procedure for modeling this type of vessels. This model can be used within an iterative optimization process. Different modeling approaches using various combinations of element types, material models, and geometric details are explored. Results of these models are compared to available experimental data. Accuracy and computational time between all these models are also compared. A suitable modeling method is recommended based on these findings.

Characterization of Electronic Board Material Properties Under Impact Loading

On-board electronics in advanced military apparatus are often subjected to severe ballistic shocks and vibrations. Safeguarding on-board electronic sensors from such transient shocks due to ballistic impact is of concern. While several studies document material characteristics of electronic boards under quasi-static and low impacts, few researchers addressed the behavior of these boards under severe impact loading. This paper presents the results of testing electronic boards under different strain rates to assess the effects of strain rates on modulus of elasticity of the boards. The results are used to suggest material models that can be used in finite element codes to accurately describe the behavior of these boards under impact loading.Copyright

Comparison of Failure Mechanisms Due to Shock Propagation in Forged, Layered, and Additive Manufactured Titanium Alloy

The objective of this paper is to propose experimental techniques for studying the behavior of titanium alloy, Ti-6Al-4 V (Grade 5), under shock loading. Single-layer and multi-layered stacks of forged titanium, and additive manufactured (AM) titanium plates were considered. In these experiments, target materials were subjected to ballistic impact using a two-stage light gas gun. A Photonic Doppler Velocimetry (PDV) diagnostics system was used to measure free-surface velocity on the back of each target. The experimental measurements were used to describe the behavior of these materials under shock loading. In addition to velocity measurements, physical damage and spall crack formation were monitored.

Simulation of Shock Response in a Lab-Scale Space Frame Structure Using Finite Element Analysis

Space frames are usually used to enhance the structural strength of a vehicle while reducing its overall weight. Impact loading is a critical factor when assessing the functionality of these frames. In order to properly design the space frame structure, it is important to predict the shocks moving through the members of the space frame. While performance of space frame structures under static loads in well-understood, research on space frame structures subjected to impact loading is minimal. In this research, a lab-scale space frame structure, comprising of hollow square members that are connected together through bolted joints which allow for quick assembly/disassembly of a particular section, is manufactured. Non-destructive impact tests are carried out on this space frame structure and the resulting acceleration signals at various locations are recorded. A finite element (FE) model of the lab-scale structure is created and simulated for the experimental impact loads. Acceleration signals from the FE model are compared with the experimental data. The natural frequencies of the structure are also compared with the results of the FE model. The results show a good match between the model and the experimental setup.Copyright

Structural Response Optimization of a Light-Weight Composite Blast Containment Vessel

This paper proposes an optimization technique for increasing the structural integrity of a light-weight composite blast containment vessel. The vessel is cylindrical with two hemispherical ends. It has a steel liner that is internally reinforced with throttles and gusset plates and wrapped with a basalt-plastic composite. A computationally-efficient finite element model of the blast containment vessel was proposed and verified in an earlier work. The parameters of the vessel are incorporated within an iterative optimization procedure to decrease the peak strains within the vessel, which are caused by internal blast loading due to an explosive charge placed at the center of the vessel. The results of the proposed procedure are validated for different initial guesses of the design variables.

Experimental Investigation of Shock Mitigation of Electronic Boards Within Projectiles

Electronic components inside a projectile are subjected to high acceleration during launch. Failure of these components, affects the performance of the projectile. The objective of the paper is to better understand how shocks are transmitted to electronic boards and to investigate ways to mitigate these shocks. A projectile model is created to mimic an actual projectile. As projectiles are usually composed of threaded components, the effect of the tightening preload torque on accelerations and frequencies of components on boards is discussed. Experimental results show that increasing the tightening preload torque above a specific level can ensure that projectile behaves as a single body, which makes acceleration of a component on the board independent of tightening torque. Effect of mounting the board on polyurethane rubber is also considered. The paper also presents suggestions to simulate experimental test using finite element methods.Copyright