Brendan O'Toole

Bone strength is maintained after 8 months of inactivity in hibernating golden-mantled ground squirrels, Spermophilus lateralis.

SUMMARY Prolonged inactivity leads to disuse atrophy, a loss of muscle and bone mass. Hibernating mammals are inactive for 6–9 months per year but must return to full activity immediately after completing hibernation. This necessity for immediate recovery presents an intriguing conundrum, as many mammals require two to three times the period of inactivity to recover full bone strength. Therefore, if hibernators experience typical levels of bone disuse atrophy during hibernation, there would be inadequate time available to recover during the summer active season. We examined whether there were mechanical consequences as a result of the extended inactivity of hibernation. We dissected femur and tibia bones from squirrels in various stages of the annual hibernation cycle and measured the amount of force required to fracture these bones. Three groups were investigated; summer active animals were captured during the summer and immediately killed, animals in the 1 month detraining group were captured in the summer and killed following a 1-month period of restricted mobility, hibernating animals were killed after 8 months of inactivity. A three-point bend test was employed to measure the force required to break the bones. Apparent flexural strength and apparent flexural modulus (material stiffness) were calculated for femurs. There were no differences between groups for femur fracture force, tibia fracture force, or femur flexural strength. Femur flexural modulus was significantly less for the 1 month detraining group than for the hibernation and summer active groups. Thus, hibernators seem resistant to the deleterious effects of prolonged inactivity during the winter. However, they may be susceptible to immobilization-induced bone loss during the summer.

Photoelastic investigation of crack-inclusion interaction

An experimental procedure is presented for determining the mode I stress-intensity factor of an edge crack with a nearby rigid elliptical inclusion in a finite plate loaded in uniform tension. The rigid inclusion was modeled by bonding two identical steel inclusions on to the faces of a thin plate of polycarbonate. Models were constructed with edge cracks and various inclusion geometries so that the effect of parameters such as inclusion shape, orientation, and cracktip position on the stress-intensity factors of the crack could be determined. Photoelasticity experiments were used for this investigation and the results were compared to the results of a similar theoretical analysis of the interaction between a crack and an inclusion in an infinite plate. A good correlation was found between the experimental and theoretical models indicating that the results may help provide a better understanding of the toughening mechanisms in materials such as short-fiber-reinforced composites and ceramics. This experimental method is relatively easy to use making it an attractive candidate to be applied to similar problems involving cracks and inhomogeneities.

Temperature and mold size effects on physical and mechanical properties of a polyurethane foam

Rigid polyurethane foams are used as thermal or vibration insulators and energy absorption material, and are often molded directly in place, where a smooth, thin skin forms between the mold and the cellular structure. Density gradients and the presence of a skin are known to affect the mechanical properties of the foam. We investigate the effect of processing temperature (25, 40, 65, and 85 C) and mold size (aluminum cylinders with diameters of 29, 41, and 51mm) on the average density and density gradients (radial and vertical) of a free-rise, water blown, rigid polyurethane foam system, and measure the effects on compressive modulus of elasticity and collapse stress. In general, both average density and radial density gradients decrease with increasing processing temperature and larger mold sizes. A reduction in average foam density corresponds with decreases in the elastic modulus and compressive strength. These mechanical properties are compared to reference samples extracted from very large batches of foam with a uniform density of 0.10 g/cc, where normalization of the compressive data shows the elastic modulus to exhibit the strongest dependence on processing temperature and mold size.

Tensile Properties of Martensitic Stainless Steels at Elevated Temperatures

Tensile properties of quenched and tempered martensitic alloys EP-823, HT-9, and 422 were evaluated at temperatures ranging from ambient to 600 °C as a function of three different tempering times. The results indicated that the yield strength, ultimate tensile strength, and the failure strength were gradually reduced with increasing temperature. The ductility parameters were enhanced at elevated temperatures due to increased plastic flow. However, the tempering time did not significantly influence these properties. The evaluation of the fracture surfaces by scanning electron microscopy revealed reduced cracking and dimpled microstructures, indicating enhanced ductility at higher testing temperatures.

Experimental and Finite Element Analysis of Preloaded Bolted Joints Under Impact Loading

*† ‡ One of the primary parameters in analyzing bolted joints is preload in the bolt. We have considered several possible preload modeling techniques to analyze the effect of preload on the dynamic response of the bolted joints. Five different methods of applying preload in the nonlinear finite element analysis are evaluated. These methods are “force on bolt and nut”, “force on bolt shank”, “interference fit”, “thermal gradient” and “initial stress method”. Explicit and implicit analyses are used for transient response and preload generation in bolt respectively. Time history and shock response spectrum are used to compare experimental and simulation results. Simulation results compared fairly well with the experimental results.

Effects of Processing Temperature on ReCrete Polyurethane Foam

Research is conducted to determine the effect of processing temperature on some of the physical and mechanical properties of a polyurethane foam called ReCrete. The polyurethane foaming process is manipulated to change the foams density, chemistry, and mechanical properties. There is a 30-min period after ReCrete components are mixed when the materials are still undergoing significant chemical reaction. Researchers manipulate these chemical reactions by changing the environmental temperature during this process. This study investigates the effect of processing temperature on the chemistry and the resulting mechanical properties for a polyurethane foam system molded in aluminum cylinders and boxes. Processing temperature is varied from 25°C to 85°C. Researchers show that the processing temperature has a significant effect on ReCrete chemistry and density. The average density decreases by 19% over this temperature range. The chemistry, in turn, affects the static and dynamic mechanical properties. The axial compressive modulus and strength decrease by 24 and 16%, respectively. The chemistry changes that results from higher processing temperatures produce foam that is less rigid in compression, but tougher and more flexible. The dynamic flexural failure strength increases by 38% when the processing temperature is increased from 25°C to 85°C. Foam processed at 85°C has significantly greater resistance to brittle failure under impact.

Optimization of Finite Element Modeling Methodology for Projectile Models

Gun-fired projectiles are subjected to severe loads over extremely short duration. There is a need to better understand the effects of these loads on components within a projectile. While experimental data can be helpful in understanding projectile launch phenomena, collecting such data is usually difficult. There are also limitations on the reliability of sensors under these circumstances. Finite element modeling (FEM) can be used to model the projectile launch event. Currently, engineers usually use large number of elements to accurately model the projectile launch event, which results in an extremely long computational time. FEM results in these cases are always subject to questions regarding accuracy of the results and proof of mesh stability This paper presents an expert system that can reduce computational time needed to perform FEM of gun-fired projectiles. The proposed approach can result in reducing computational time while ensuring that accuracy of results is not affected. Recommendations of the expert system are reached through two stages. In the first stage, an equivalent projectile with simple geometry is created to reduce the complexity of the model. In the second stage, parameters controlling mesh density of the equivalent projectile are used as variables in an optimization scheme with the objective of reducing computational time. Accuracy of the acceleration results from an optimized model with respect to a model with an extremely fine mesh is used as an inequality constraint within the optimization search. A projectile model meshed with aspect ratios obtained from the optimization search produces good agreement with the finite element results of the original densely-meshed projectile model while significantly reducing computational time. It is anticipated that this approach can make it easier to conduct parametric analysis or optimization studies for projectile design.Copyright

Shock reduction for electronic components within a projectile

Electronic components within a projectile are subjected to severe loads over extremely short duration. Failure of these components is likely to have negative implications to the projectile or mission. While experimental data can be helpful in understanding the failure phenomena, collecting such data is usually difficult. There are also limitations on the reliability of sensors under these circumstances. Finite element modeling (FEM) can offer a means to better understand the behavior of these components. It can also be used to design better techniques to mitigate the shocks these components are subjected to. A model of a typical projectile and the gun barrel is presented. The projectile is modified to include a payload of a one-pound mass that represents a typical electronic package, which is supported by a plate. The model, which is subjected to a realistic launch pressure-time history, includes the effects of friction between the gun barrel inner surface and the projectile. The effect of the flexibility of the gun barrel on the vibrations of the electronic package is also considered. This paper proposes using a composite plate, with carbon fibers embedded in an epoxy matrix, to reduce the shocks transmitted to the payload. A parametric study of the effects of varying the thickness of the supporting plate and the fiber volume fraction on accelerations and stresses is included.Copyright

The effect of lay-up, core material, and cross-sectional geometry on the structural performance of pultruded fiberglass utility poles

The structural performance of several pultruded fiberglass utility poles is investigated and compared with wood pole standards. The fiberglass poles are thin-walled tubes with an approximate hexagonal configuration. The outside of the poles have longitudinal grooves which create a dovetail configuration. The dovetails provide a convenient mechanism for attaching crossarms and other hardware to the tubular structure and contribute additional stiffness and strength at these attachment points. Manually operated clamping devices which can run up and down the grooves provide a safe environment for utility linemen. Structural evaluation includes a comparison of axial stiffness, flexural stiffness, maximum load, maximum bending moment, and load-deflection curves for several different pole cross-sectional geometries. Material elastic properties and fiber orientations are varied to determine the effect on structural performance. Several different fiber preforms, including unidirectional rovings and multidirectional fabrics, are evaluated. Results of the analyses are compared with the manufacturers (Composite Power Corporation) preference in terms of manufacturabilit,. The effect of fiberglass sleeve inserts and solid foam cores is also investigated. Some of the analyses are compared with full scale test data and both the experimental and theoretical results are compared with utility standards for pole design. Failure mechanisms for different loading conditions and future analysis is discussed. Preliminary results show that the composite poles can achieve the standards required for the highest quality wood poles.

Flexural failure of notched curved composite beams

An analytical technique was developed to predict the tensile strength of notched curved composite beams loaded in bending. Based upon the Damage Zone Model (DZM), the current analysis was applied to notched curved beams with J cross sections. Similar sections composed of a long, discontinuous fiber laminate were tested to failure in four point bend. Predicted and experimental values were compared and results discussed. Discrepancies may be due to out-of-plane deformations observed during the experiments which are not accounted for in the current analysis.