Steven R. Schmid

Elastohydrodynamic Film Thickness and Tractions for Oil-in-Water Emulsions

Emulsions, consisting of a small volume of oil dispersed in water in the form of small particles, are popular lubricants for metal rolling and some machine design applications. A number of mechanisms have been suggested for the lubricating behavior of emulsions, among which plate-out, starvation, and dynamic concentration are of particular interest here. At low speeds, the emulsion provides essentially the same lubricating ability as neat oil for a point contact, consistent with plate-out. At some critical speed, the emulsion behavior departs from the neat oil, associated with starvation of the inlet zone. At a second critical speed, dynamic concentration becomes the important mechanism. This article measures the film thickness and traction coefficients of oil-in-water emulsions in the different regimes of behavior and compares the results to existing theoretical understanding. The effect of droplet size is isolated as a causative element in fluid film formation.

Simulation of asperity plowing in an atomic force microscope Part 1: Experimental and theoretical methods

Abstract The plowing of a soft workpiece by a hard tool asperity is recognized as a fundamental tribological problem and key contributor to friction and wear in bulk metal forming processes such as rolling and extrusion. However, experiments on an asperity scale have been difficult to perform to date since measurements of plow track geometry, the forces required to create the plow track, and the plowing speed must be made over a very small length scale. It is therefore the purpose of this paper to describe a new experimental method that stimulates single asperity plowing of a soft workpiece material in an atomic force microscope. Using a sharp diamond indentor as a tool asperity, plow tracks were formed in 99.99% pure aluminu which was chosen to demonstrate that the method gave repeatable results over a range of forces and speeeds. A mathematical model of plowing was modified to take the plow track geometry, the measured normal and plowing forces, and the plowing speed as input variables and provide a measure of the friction factor (and hence shear stress) along the indentor-workpiece interface, as well as the workpiece flow strength. The companion paper presents the results of asperity plowing experiments (based upon the methods presented in the present article) on specific aluminum alloys of differing hardnesses.

Effects of pre-cooling and pre-heating procedures on cement polymerization and thermal osteonecrosis in cemented hip replacements

Numerical studies were performed to investigate bone cement polymerization, temperature history and thermal osteonecrosis in cemented hip replacements with finite element methods. In this paper, the effects of pre-cooling and pre-heating of the prosthesis and/or the cement prior to implantation were simulated. It was found that the cement polymerization initiated near the bone-cement interface and progressed toward the prosthesis when both the cement and prosthesis were initially at room temperature. When the prosthesis and/or cement were pre-cooled, a reduction of the peak temperature at the bone-cement interface resulted, and this may reduce thermal osteonecrosis. However, this also slowed the polymerization process, and may result in a weaker bone cement. If the prosthesis was significantly initially heated, bone cement polymerization reversed reaction direction, started from the cement-prosthesis interface and proceeded toward the bone. Such polymerization direction may reduce or eliminate the formation of voids at the cement-prosthesis interface. Numerical results also showed that pre-heating seemed unlikely to produce significant thermal damage to the bone. The method of pre-heating the prosthesis prior to implantation may decrease the likelihood of cement-prosthesis loosening and increase the life of total hip arthroplasty.

Simulation of asperity plowing in an atomic force microscope. Part II: Plowing of aluminum alloys

Abstract In this second part, we present results of plowing tests on three commercial aluminum alloys using the atomic force microscope technique presented in Part I. Pure (99.99%) aluminum, 5182-O, and 7150-T6 were chosen as substrates because of their different material properties. These properties create different tribological conditions in processes such as rolling, forging, and extrusion where a metal workpiece flows plastically due to contact with a hard tool surface having many asperities. In the present technique, a single plow track was generated with a sharp diamond indentor which simulated a single tool asperity. A series of plow tracks was thus generated on each substrate surface over a range of plowing speeds and normal forces. The resulting plow track geometries and plowing forces were measured for input to the theoretical model presented in Part I. The model was used to predict material flow stress as a function of normal force as well as friction factors at the different indentor faces. The flow stress predictions from the model w ere compared with flow stress measurements from Vickers microhardness tests of each material. This helped to reveal possible scale effects and highlight tribological phenomena that are peculiar to plowing. The friction factor was found to be strongly dependent on the ratio of the measured ridge height to the depth of cut. This ratio is known to depend upon material properties such as strain hardening, elastic recovery, densification beneath the indentor, etc., that are not captured within the theoretical framework. Results from some plowing tests with a blunt indentor are reported and comparisons are made with those with the sharp indentor.

Improved mechanical properties of acrylic bone cement with short titanium fiber reinforcement.

Acrylic bone cements are widely used in total joint arthroplasties to grout the prosthesis to bone. The changes in the tensile properties and fracture toughness of polymethylmethacrylate (PMMA) bone cements obtained by the addition of control and heat treated short titanium fibers are studied. Heat treatment of titanium fibers is conducted to precipitate titania particles on the fiber surface to improve the biocompatibility of the metal. Control and heat treated short titanium fibers (250 μ long and 20 μ diameter) were used as reinforcements at 3 volume %. X-ray diffraction indicated the presence of a rutile form of titania due to the heat treatments. The tensile and fracture properties were improved by the addition of fibers. Bone cements reinforced with titanium fibers heated at 550∘C for 1 h followed by 800∘C for 30 minutes show the largest increase in fracture toughness along with the smallest changes in elastic modulus and needs to be further investigated.

Boundary Additive Effect on Abrasive Wear During Single Asperity Plowing of a 3004 Aluminum Alloy

Aluminum forming processes such as rolling, extrusion, and ironing involve the transfer of large loads through a tooling/workpiece interface to plastically deform the workpiece to a desired shape. Sharp tool surface asperities can plow the workpiece and lead to elevated friction and temperatures in the interface with a subsequent increase in abrasive wear debris which in turn degrades the surface aesthetics of the final product. To minimize associated friction and wear levels in aluminum forming processes, a base oil with one or more boundary additives is used as a lubricant. At the present time, however, little is known about the mechanisms by which a given additive influences abrasive wear in an aluminum metal forming interface. In the present work, a series of single asperity plowing experiments on a 3004-O aluminum alloy with selected lubricant components was conducted. Three additives were separately investigated, viz., stearic acid, butyl stearate, and lauryl alcohol. The plowing motion of a pyramidal diamond indentor with a cutting edge oriented in the plowing direction (i.e., a sharp indentor) was controlled with the piezo-electric transducers of an atomic force microscope. The experiments help to provide insight about the interplay between additive reaction with the surface and plowing mechanics. Further insight into this interplay and abrasive wear debris generation was sought, albeit qualitatively, through additional experiments involving a diamond indentorfor which no one cutting edge was oriented in the plowing direction (i.e., a blunt indentor). The tests allowed evaluation of the boundary lubricant mechanism and propensity for generating wear debris.

Survivability of Laminated Polymer Lubricant Films in Ironing

Polymer-coated steels are of interest in the production of food and beverage containers because of their potential utility as solid lubricants and because of the potential elimination of a post-forming polymer spray operation which is time consuming and produces environmentally hazardous emissions. Unfortunately, little work has been done regarding formability of polymer-laminated steels in metal-forming operations, and this is especially true of the ironing process. This paper presents an experimental and theoretical investigation into the ironing formability of polymer coated steels. A strip ironing simulator is used to perform ironing tests at a large number of tooling geometries and thickness reductions. From the experiments, it is found that polymer coated steels will either iron successfully or will be shaved off of the substrate, and that the active mechanism depends most strongly on the die angle. An upper bound model is developed which predicts the circumstances under which successful ironing and...

Reinforcement of bone cement using zirconia fibers with and without acrylic coating

Acrylic (polymethylmethacrylate or PMMA) bone cement was modified by the addition of high-strength zirconia fibers with average lengths of 200 microm and diameters of 15 microm or 30 microm. A novel emulsion polymerization process was developed to encapsulate individual fibers in PMMA. Improvements in tensile and compressive properties as well as in fracture toughness were investigated upon incorporation of uncoated and acrylic coated zirconia fibers. Bone cements were reinforced with 2% by volume of the 15 microm diameter and 5% by volume of the 30 microm fibers. Results indicate that elastic modulus and ultimate strength of bone cements reinforced with zirconia fibers were higher than controls, being the largest for cements reinforced with 30 microm diameter fibers. The fracture toughness of the cement increased by 23% and 41% by the addition of 15 microm and 30 microm fibers, respectively. Coating of individual zirconia fibers did not result in improved material properties of bone cements. The use of uncoated or acrylic coated 30 microm fibers is recommended based on the significant increases in ultimate strength and fracture toughness of the cements.

Formability of porous tantalum sheet-metal

Over the past ten years, a novel cellular solid, Trabecular Metal?, has been developed for use in the orthopaedics industry as an ingrowth scaffold. Manufactured using chemical vapour deposition (CVD) on top of a graphite foam substrate, this material has a regular matrix of interconnecting pores, high strength, and high porosity. Manufacturing difficulties encourage the application of bending, stamping and forming technologies to increase CVD reactor throughput and reduce material wastes. In this study, the bending and forming behaviour of Trabecular Metal? was evaluated using a novel camera-based system for measuring surface strains, since the conventional approach of printing or etching gridded patterns was not feasible. A forming limit diagram was obtained using specially fabricated 1.65 mm thick sheets. A springback coefficient was measured and modeled using effective hexagonal cell arrangements.

Thermal Effects on Polymer Laminated Steel Formability in Ironing

In the current manufacturing processes for can making, a time consuming and therefore expensive process involves spraying a food-contact safe polymer coating onto the can interior before filling. This process can be eliminated by using a prelaminated metal workpiece as long as the polymer will survive the manufacturing operations involved in can making. The most demanding operation in can making is ironing because of the high pressures involved as well as the necessary generation of new surface Previous research [5] has demonstrated the feasibility of using a polymer coated steel sheet stock for can making. However, ironing is commonly performed with elevated tooling temperatures which result from friction and plastic deformation in the workpiece. As such, it is possible that the polymer could significantly soften or melt during the ironing process when tooling/workpiece contact is most intimate. In this paper, the thermal effects of hot tooling on polymer coated steel formability are explored through both experiments and mathematical models.