Jahan Rasty

On the measurement of friction coefficient utilizing the ring compression test

Abstract The main objective of this research was to investigate whether generalized friction calibration curves, as recommended in the literature for use with ring compression tests, are applicable to all types of materials and test conditions. Specifically, the effects of material properties, strain-rate sensitivity, and “barreling” on the behavior of friction calibration curves were investigated. To this end, a series of ring compression tests were conducted in order to determine the magnitude of the friction coefficient, μ , as well as the corresponding calibration curves for two types of modeling materials, white and black Plasticine. The experiments were first conducted using the Physical Modeling Technique (PMT) and then simulated via an elastic–plastic finite element code (ABAQUS). In contrast to the results available in the literature, where the same friction calibration curves are recommended for all types of materials and test conditions, the results of this investigation showed that friction calibration curves are indeed affected by the material properties and test conditions and every material possesses its own distinctive friction calibration curve.

Flow behavior of Plasticine used in physical modeling of metal forming processes

Abstract In this study, compression tests were conducted to determine the flow behavior of various types of Plasticine. True stress–true strain relations are presented in the form of a well-known power law equation. It is shown that the strength coefficient and the strain hardening exponent vary significantly among the Plasticine with different colors commercially available. Furthermore, since three dimensional simulations require arranging slabs of alternating colors to form a grid pattern, two different colors, exhibiting similar flow characteristics, are identified. The flow behavior of the composite model is usually not the same as that of the individual colors. The results presented in this paper allow selection of the type of Plasticine which most closely resembles the strain hardening characteristic of the actual metal, thereby satisfying the “similarity condition” of the Physical Modeling Technique.

Protective effect of dietary long-chain n -3 polyunsaturated fatty acids on bone loss in gonad-intact middle-aged male rats

This study evaluated the effect of a fat blend containing long-chain (LC) n-3 PUFA on bone mineral density (BMD) and bone metabolism in gonad-intact middle-aged male rats (12 months old, n 28). Seven rats were killed on day 0 of dietary intervention to determine the baseline BMD. The remaining rats (seven per group) were fed a diet with one of the following dietary lipid treatments (g/kg diet): 167 g safflower oil + 33 g menhaden oil (N6 + N3 diet, control), 200 g safflower oil (N6 diet, almost devoid of LC n-3 PUFA), or 190 g menhaden oil + 10 g corn oil (N3 diet, rich in LC n-3 PUFA) for 20 weeks. After 20 weeks, all dietary treatment groups had a lower BMD compared with the baseline reference. However, rats fed the N3 diet had the highest bone mineral content and cortical + subcortical BMD compared with those fed the N6 and control N6 + N3 diet. Compared with the control (N6 + N3) group, rats fed the N3 diet had higher values for serum insulin-like growth factor-I, parathyroid hormone, 1,25-(OH)2 vitamin D3 and bone-specific alkaline phosphatase activity, but lower bone NO production and urinary Ca, whereas rats fed the N6 diet had higher bone prostaglandin E2 production and serum pyridinoline. These findings indicate a protective action of LC n-3 PUFA on ageing-induced bone loss in gonad-intact middle-aged male rats through a modulation of local factors and systemic calcitrophic hormones.

Determination of Friction Coefficient by Employing the Ring Compression Test

The main objective of this research was to investigate the effect of material properties. strain-rate sensitivity, and barreling on the behavior of friction calibration curves. The compression tests were conducted to obtain the necessary material properties for the finite element analysis. A series of ring compression tests were then conducted in order to determine the magnitude of the friction coefficient, μ. The experiments were first conducted for the modeling materials, namely, white and black plasticine and later on, for aluminum, copper, bronze, and brass. The experiments were then simulated via an elastic-plastic finite element code (ABAQUS). Contrary to the results available in the literature, where the same friction calibration curves are recommended for all types of materials and test conditions, the results of this investigation showed that friction calibration curves are indeed affected by the material properties and test conditions.

Improvement of Bone Quality in Gonad-Intact Middle-Aged Male Rats by Long-Chain n-3 Polyunsaturated Fatty Acid

The effect of long-chain n-3 polyunsaturated fatty acid (PUFA) on bone measurements was evaluated in gonad-intact middle-aged male rats. Seven rats were killed on day 0 of dietary intervention to determine bone parameters at baseline. Experimental rats (7/group) were fed one of the following lipid treatments (g/kg diet): 167 g safflower oil + 33 g menhaden oil (N6+N3 diet, control), 200 g safflower oil (N6 diet), or 190 menhaden oil + 10 g corn oil (N3 diet). After 20 weeks of dietary treatment, all groups had lower values for peak load and ultimate stiffness in femurs compared to baseline values. Rats fed the N3 diet had the highest values for peak load, ultimate stiffness, and Young’s modulus compared with those fed the N6 and control diets. Compared to baseline, all dietary treatment groups had significantly lower values for trabecular thickness and number in proximal tibia but higher values for trabecular separation and formation rate in proximal tibia and endocortical bone formation rate in tibial shaft. Compared with the control group, rats fed the N3 diet had lower values for formation rate, osteoclast number, and eroded surface in proximal tibia but higher values for periosteal mineral apposition and formation rates in tibia shaft. These findings indicate that a diet rich in long-chain n-3 PUFA mitigate aging-induced loss of bone integrity in intact middle-aged male rats through reducing bone turnover rate by suppressing both bone formation and resorption as a result of a larger net bone volume and modulating endocortical and cancellous bone compartments.

High-pressure x-ray diffraction study of the giant dielectric constant material CaCu3Ti4O12: Evidence of stiff grain surface

We measured the high-pressure x-ray diffraction of the giant dielectric constant material CaCu3Ti4O12 (CCTO) under both hydrostatic and uniaxial compressions. We found that the cubic structure of CCTO is stable up to 57GPa. Nevertheless we observed that CCTO has unusual compression behaviors under hydrostatic pressure. Specifically, the volume reduction is less than that under uniaxial compression below 25GPa, above it the volume reduction starts to approach and finally reaches the same value as that under the uniaxial compression at about 30GPa. We explained these remarkable phenomena by using the model that the samples are composed of grains that have shells stiffer than the cores.

Prediction of Elastic-Plastic Boundary Around Cold-Expanded Holes Using Elastic Strain Measurement

The cold-expansion process is used routinely for improving the fatigue life of holes in a variety of components. The expansion process involves drawing a slightly oversized tapered mandrel through the hole. Upon expansion, the material near the hole deforms plastically while material away from the hole undergoes elastic deformation. Upon removal of the mandrel, the ensuing elastic recovery of the surrounding material results in the development of a compressive residual stress field around the hole. Since the magnitude of plastic deformation sustained by the material near the hole depends on the severity of the expansion, the elastic-plastic boundary radius (EPBR) during the expansion process can be used to characterize the extent of cold expansion. The elastic-plastic boundary is an important parameter in characterizing the residual stress distribution around cold-expanded holes, as well as in determining required spacing between successively expanded holes. This paper presents a new method for determining the EPBR using strain measurements within the elastic region. Analytical equations are developed relating elastic strains measured away from the hole to EPBR. A methodology is presented for employing strain data (obtained via miniature resistance strain gauges located away from the hole and within the elastic region) to be used as an input variable into the developed equations for determining EPBR. Using the method described in this paper, an average normalized EPBR of 2.38 (normalized with respect to the initial hole radius) was calculated utilizing elastic strain measurements during 4.0% cold expansion of a set of 4.826 mm thick 7075-T6 aluminum specimens containing a 6.0 mm diameter hole. The results showed excellent agreement with numerical simulations using a nonlinear elastic-plastic finite element code ( A BAQUS). The deviation between the average EPBR determined by the analytical-experimental method and the finite element analysis was about 4.0%. The proposed method for using elastic strain measurements away from the hole provides improvement over earlier methods that rely on fringe observations or strain measurements within the relatively narrow plastic zone that has an uneven surface near the hole.

Conductivity measurements of explosively shocked aluminum and OFHC copper used for armature material in a magnetic flux compression generator

Modeling and characterization of a magnetic flux compression generator (MFCG) requires detailed knowledge of the changes in conductivity of the MFCG materials during the shock-loading phase. In the studies reported here, a thin metallic strip is shocked with an explosively generated shock wave produced from a charge of composition C-4. The shock wave is intended to simulate the shock wave and pressures produced in MFCG research currently being conducted at Texas Tech University. These pressures are estimated to be between 1 and 3 GPa. The experimental setup is arranged so that the shapes of the metallic strip and shock front are the same, as confirmed using optical fibers. This was to ensure that the test sample was shocked uniformly. The metallic test strip is pulsed with a 70 A current pulse during application of the shock wave. The current and voltage across the test sample are measured directly to determine the change in conductivity. Pressure measurements are conducted in separate tests under similar conditions using strain gauges. The results are then compared to results determined previously using a split Hopkinson pressure bar apparatus (SHPB).

Probabilistic Analysis of Steel Roof Damage From Hail Strike

In this paper, two probabilistic analyses are performed to try to gain a better understanding of the damage inflicted on galvanized roofs from hail strikes. The first analysis addresses the probability that a hailstone will have the energy required to cause yielding in steel sheets. The second probability study lays the theoretical groundwork for determining the probability of a loss in structural integrity of the steel roof. Data and equations are combined from various sources to create a separate limit state function for each analysis. The probabilistic analyses are performed in NESSUS. For determining probability of yielding, an energy related method for determining stress in an impacted plate is used in conjunction with LS-DYNA finite element software to discover that all perpendicular hail strikes will induce some degree of plastic deformation on simply supported steel sheets in the gauge range of galvanized steel roofs. More data is still required for successful determination of probability of a loss in structural integrity, but the foundation is established, which would allow probabilistic determination of the most economic roof gauges in a given region. There appears to be a logarithmic relationship between the analytically determined initial impact force of a hailstone and the indentation diameter that remains after striking in which the thickness of the sheet plays an important role, but more data points need to be collected to verify this.Copyright

Experimental and numerical investigation of the armature/stator contact in magnetic flux compression generators

The efficiency of a magnetic flux compression generators (MFCG) is highly dependent on the expanding characteristics of the exploding armature and the nature of contact between the armature and the surrounding stator coil. A hydrodynamic finite element (FE) model was developed to simulate the expansion characteristics of the armature and its ensuing impact with the stator. The effectiveness of the FE model to simulate the explosive behavior of the armature was qualified by comparing the numerical results with experimentally measured parameters. Specifically, the radial displacement of the armature as well as the axial velocity of the armature/stator contact point were measured experimentally and compared with numerical results showing excellent agreement between the two. The results indicated that the radial and axial velocity with which the armature impacted the stator did not change through the length of the armature. However, the results showed that the velocity with which the contact point between the armature and the stator traveled along the length of the armature decreased as the explosion process went on. As expected, the axial propagation velocity of the contact point was found to be at its highest value (2.25 X detonation velocity) at the region close to the detonation end while approaching the detonation velocity at points away from the detonation end.