Jeries Abou-Hanna

Finite element approach to modeling particulate bed fixtures

Abstract This study investigates the feasibility of utilizing the finite element method in modeling the static behavior of particulate bed fixtures. The bed fixture consists of a container filled with steel pellets of known properties that are modeled by the Drucker-Prager nonlinear plastic model that represents granular material behavior. The interfacing of the submerged workpiece with the bed material is modeled using three-dimensional gap elements. The resulting model was verified against the soil mechanics and earth pressure theories. The results of the static behavior agree in principle with those obtained previously by other researchers who used different types of modeling techniques. Experimental results of displacement due to lateral loading were obtained and compared with numerical model results. The finite element model seems to be a promising new tool for investigating the characteristics of the particulate bed fixture under static loads.

Dynamic behavior and creep characteristics of flexible particulate bed fixtures

Abstract Lateral step and impact responses and lateral, vertical, and torsional creep behavior of slender cylindrical workpieces placed in a fluidized particulate bed fixture (FPBF or PBF) were investigated. These experiments were designed, constructed, and performed to determine workpiece rigidity under dynamic loads encountered in manufacturing operations such as machining, milling, drilling, and routing. Results indicate that compaction pressure and rod depth have significant effects on lateral dynamic rigidity; lateral rigidity of the PBF can approach that of a conventional shop vise. Fixture rigidity is reliable and repeatable under lateral dynamic loading. In addition, the PBF sufficiently resists creep behavior due to external lateral, vertical, and torsional loads. Under the loading conditions above, workpieces displacement can be maintained within acceptable tolerances even for close-tolerance machining operations encountered in tight indexing applications such as machining and surface inspection. Results also contribute to a database for applying the PBF in a wider variety of manufacturing processes.

Mechanical properties of steel pellets in particulate fluidized bed fixtures

Abstract Experiments were designed, built, and used to measure the stress-strain profiles of steel pellets when confined under pressure. The principles of the mechanics of granular media were used to compute the pellets bulk mechanical properties. Spherical steel pellets were used in the tests. Shear tests of the granular material were performed for different compaction pressures. Shear tests produced the angle of internal friction that is characteristic of granules. Simple uniaxial compression tests produced information that led to the computation of Poissons ratio, Youngs modulus, and the modulus of rigidity of the compacted granular steel pellets. The results reflect the variation in these material properties with compaction loads and depth of the material. These results make up a database that is essential for the modeling, design, synthesis, and analysis of flexible fluidized bed fixtures in which steel pellets make up the fixturing medium.

Experimental study of static and dynamic rigidities of flexible particulate bed fixtures under external vertical and torque loads

Abstract Rigidity of particulate bed fixtures under vertical (extraction and downward) loads, both static and dynamic, was investigated experimentally using a slender rod to represent a workpiece. Rigidity under external static and dynamic torque loads was also studied experimentally. Results of rigidity are represented as a function of submergence depth, compaction pressure, and workpiece cross section. Dynamic loads were of two types—step and impact. Loads applied to the workpiece represented typical loads encountered in manufacturing operations, such as in machining, milling, drilling, routing, inspection, welding, and painting. Results show that compaction pressure and rod depth have signifacant effects on bed fixture performance. Fixture rigidity exhibits different trends with load types. Fixture rigidity is reliable and repeatable under static vertical loading; it is also somewhat linear. Under torque loads, rigidity is highly nonlinear. Fixture rigidity is highly nonlinear in the case of dynamic loads as well. Under the loading conditions outlined above, displacement of the workpiece can be maintained within acceptable tolerances even for close tolerance machining operations and tight indexing applications such as surface inspection. Results of rigidity are conservative because the type of workpiece used in the study represents the worst possible geometry for fixturing.

Sensitivity Analysis of Hydro-Rim Deep Drawing of Cylindrical Cups

Extensive nonlinear finite element analyses were conducted to help predict practical test conditions of intelligent hydro-rim deep forming of cylindrical cups under controlled cooled punch and heated blank temperatures, punch speed, chamber and rim pressures, and punch friction. The study focused on finding practical process conditions for maximizing the drawing ratio by variations in blank and punch temperatures, friction, rim pressure, chamber pressure, and punch speed. The study was based on an experimental cell that aimed at using real time control of the mentioned parameters to delay the necking process. The finite element material model considered the plastic behavior to be strain rate and temperature dependent. While conventional deep drawing is limited to a Limit Drawing Ratio (LDR) of about 2, the results show that a parameters listed above. Blank temperature, punch friction, rim pressure, and chamber pressure provide significant influence of various degrees on increasing the cup drawing ratio. Blank heating is very effective, but does not by itself guarantee higher LDR. The presence of punch friction coupled with chamber pressure tends to delay the necking and moves the latter up along the cup wall and away from the cup bottom corner. Rim pressure, while difficult to implement, results in significant improvement of the LDR, since it helps push the material into the die, and in doing so reduces the cup-wall tension that causes the material instability. High rim pressure, on the other hand, increases the blank thickness resulting in increased blank holder loads. Punch temperature does not play as critical a role as the blank temperature in maintaining a high LDR under the conditions investigated. The study revealed that punch speed had to be above a certain critical level for a LDR of 4. However, increased punch speed proved to cause higher variations in the thickness along cup wall. It is important to mention that the results of this study do not necessarily apply to all metals; copper material was used here. Metals with low ductility, for example would react differently, a subject of future studies.Copyright

Lateral Stability of Towed Flexible Vehicles

SUMMARY The highway transport of mobile homes is a matter of concern for the increasingly safety-minded driving public. The low speeds of towed vehicles necessary to maintain stability, together with the requirements for excessive lane widths due to clearance for the lateral motion, result in increased likelihood of traffic accidents, impeded traffic flow, and reduced highway capacity. A safe increase in the stable cruising speed, coupled with a decreased amplitude in the pendular motion helps alleviate all three of the aforementioned problems. Energy input at hitch point and lateral forces between the road and tires permit lateral vehicular motions, which occur above a critical speed, to increase in amplitude until possibly a limit cycle or instability is reached. One would expect that structural dynamics could have a pronounced influence on the lateral response of towed vehicles with large and relatively flexible chassis, such as mobile homes. The objective of this research is to determine the influence...

Finite Element Based Automated Analysis for Determining Ratchet, Shakedown and Elastic Limits

In order to determine the ratchet and shakedown limit curves for even a simple component, such as a tube under a constant pressure load and cyclic thermal load, can be a daunting task when using conventional analysis methods (elasto-plastic cyclic finite element analysis) that require repeated iterative simulations to determine the state of the structure, elastic, shakedown, plastic or ratchet. In some cases, the process is further complicated by the difficulty in interpreting results of the cyclic loading to determine in which regime the structure is. Earlier work by Abou-Hanna and McGreevy was able to demonstrate limit load analysis of a structure whose yield strength is modified based on cyclic load, provided the ratchet limit [1]. The method, called Anisotropic Load Dependent Yield Modification (LDYM), was implemented by using a user subroutine with ABAQUS, a general commercial finite element code. The approach adopted provided ratchet limits for only one individual cyclic load value. The work presented here describes a process for analyzing the structure and determining the elastic, shakedown and ratchet boundaries all in one finite element simulation using only one analysis step. The approach manipulates the structure material behavior that enables the resetting of the material characteristics to their original values in order to be able to analyze the structure for different sets of cyclic and primary load combinations. The process was verified using problems available in the literature, such as the Bree tube and Ponter’s Holed Plate. Additionally, a tubular T-joint was used as an example of the effectiveness of the process for a three dimensional complex geometry. The tubular T-joint results are verified against baseline data from the iterative elastic-plastic simulations used to determine the elastic, shakedown, and ratchet limits. The work presented highlights the advantages and limitations of this numerical approach which requires little interaction with the analyst.Copyright

Impact of Surface Roughness on Formation of Micro Cracks and Fatigue Performance

Tungsten-doped diamond-like carbon (DLC) coatings have been magnetron sputtered with chromium and chromium/tungsten carbide dual interlayers onto 410 stainless steel rods. The surface finish (Ra) of the substrate before deposition was 0.10–0.25 μm for a set of rough rods and 0.05 to 0.10 μm for a set of smooth rods. SEM analyses revealed different kinds of flaws in the as-deposited films (virgin coating). Two samples, one from each set, were fatigue tested under uni-axial tension to determine the effect of the substrate surface roughness on the performance of the coating. Surface analysis revealed a different response between films deposited on rough and smooth substrates. Severe failure modes such as spalling and wide cracks developed from initial film flaws in the rough substrate sample. Cracks and film spalling were also observed in the smooth substrate sample but the severity, in terms of crack dimensions and density was considerably lower than the rough substrate sample.Copyright

Applicability of Simplified Methods to Alloy 617 in Excess of 649°C

The Alloy 617 draft Code case does not permit the use of simplified methods to assess ratcheting when temperatures exceed 649°C. This restriction was placed due to an apparent difficulty in distinguishing between creep and plastic deformation at various strain rates for this material. A numerical evaluation of the B-1 and B-2 Tests for a tube under constant pressure and cyclic thermal gradients (linear and nonlinear) was made. Analysis results indicate that simplified methods currently in Appendix T of ASME-NH are applicable for Alloy 617 in excess of 649°C.Copyright

Simplified Inelastic Time Independent (SITI) Method for Predicting Creep

In an effort to address inelastic creep behavior for very high temperature (VHT) applications, a unified state variable material model was used in a time dependent finite element analysis to generate isochronous curves. The resulting isochronous curves were then used in an efficient time-independent plastic analysis to predict the creep behavior of components. This simplified inelastic time-independent (SITI) method can significantly reduce the geometric and load uncertainties, and the over-conservatism in predicting inelastic strain levels. SITI is an effective and computationally efficient approach for predicting inelastic strains of components operating at high and very high temperatures such as the case in the Next Generation Nuclear Plant. This work compares the SITI inelastic strains to those obtained using fully inelastic time-dependent elastic-plastic-creep analysis, and illustrates the effectiveness of the approach in obtaining creep strain predictions without elaborate full inelastic time-dependent simulation.© 2007 ASME