Wedge Mechanism Research Articles

Numerical modeling of accretionary wedge mechanics: Application to the Barbados subduction problem

The mechanisms for the development of accretionary prisms in subduction complexes is numerically simulated using finite element plasticity analysis and the Barbados problem as an example. The accreted and subducting sediments are modeled as elastoplastic continua separated by rigid‐plastic contact elements to simulate the thrusting mechanism. Results of the analyses demonstrate the importance of both lateral compression and topographic slope in the development and propagation of accretionary prisms in the Barbados subduction complex, affirming the analytical theory on the mechanics of thin‐skinned fold‐and‐thrust belts presented by Chapple [1978]. Numerical analyses also reveal that the shape of the accretionary prism requires a critical balance in material characteristics such as the frictional strength of the basal layer, the modulus of the décollement, and the flow laws governing the yielding of the elastoplastic accretionary prism itself. Finally, it is shown that, unless the high normal stress at the base of the overriding plate is counteracted by a high fluid pressure localized in the décollement region, the frictional component of the basal traction will make the horizontal propagation of accretionary prisms difficult.

On the two modes of operation of monolithic Ag-C brushes

The previously observed transition from low-temperature (mode I) to high-temperature (mode II) behavior in silver-graphite brushes can be explained either by changes of constriction resistance through the temperature dependence of electrical resistivity and hardness at negligible film resistivity and one to three contact spots, or by loss of graphite lubrication within the interfacial film. These two interpretations were tested by controlling the contact spot temperature: (1) by heating in an oven and (2) by local heat input through friction. Correlated studies of contact resistance, friction, wear rate, wear chip size, wear chip microstructure, and calculations of contact spot temperatures favor the second hypothesis. Micrographic evidence suggests that this occurs through desorption of water vapor from the graphite. It is found that primary wear particles form due to the wedge mechanism and by cutting by graphite fragments steeply inclined to the interface. Silver fragments may consolidate into secondary wear particles.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Laboratory Study of Hydraulic Fracturing

Laboratory tests on Teton Dam soil were performed by increasing the water pressure in model bore holes and in simulated rock joints. The water pressure required to cause hydraulic fracturing was found to depend on soil density, water content, confining stress and test duration. On the basis of the test results it was hypothesized that: (1)Hydraulic fracturing is a weak link phenomenon, in that fracturing will occur in the least resistant soil subjected to increased water pressure; and (2)hydraulic fracturing can probably occur only in the presence of a discontinuity, within which the water pressure can act to create a wedging mechanism, thus creating tensile stresses in the soil.

Base Oil Effects on Fatigue Life for Additive Blends as Measured on a 4-Ball Fatigue Machine

As part of a study to determine the lubricant factors affecting fatigue of rolling contact bearings, the effects of base oil composition and viscosity for additive blends on the fatigue life of CVD 52100 steel balls were evaluated with a 4-ball fatigue machine. Using pure hydrocarbons as base stocks, aromatic hydrocarbons were found to not only give longer lives than paraffins or naphthenes but also exhibit extremes in additive response. Using different distillation cuts to cover the viscosity range from 2.3 to 24 mm2/s, life tended to increase with increasing viscosity for each of four series of mineral oils whereas the addition of antiwear and EP additives generally reduced life. In contrast, increasing the viscosity by reducing the test temperature increased life for most additive blends. The hydraulic wedge mechanism for fatigue, modified to include surface coating formation effects, seems to account for the observed results. Presented as an American Society of Lubrication Engineers paper at the ASLE/...

Oxygen effects on the corrosion of niobium and tantalum by liquid lithium

The effect of oxygen on the corrosion of niobium and tantalum by liquid lithium at 600°C was studied with static capsules. An increase in the oxygen concentration of the lithium from 100 to 2000 ppm had no measurable effect on the dissolution of either refractory metal. Exposure to lithium reduced the oxygen content of niobium and tantalum to less than or equal to 20 ppm regardless of the oxygen concentration of the lithium, results that agree with thermodynamic calculations. When the initial oxygen concentration of niobium and tantalum exceeded a threshold level, lithium (with no oxygen added) penetrated the refractory metal. Penetration resulted from the formation of a ternary oxide on grain boundaries or preferred crystallographic planes and proceeded by a wedging mechanism, which is caused by stresses generated by the corrosion product. Penetration was along grain boundaries at low oxygen concentrations, the attack depth and number of affected boundaries increasing with increasing oxygen concentration. At higher oxygen concentrations, transgranular attack also occurred. The threshold oxygen concentrations for attack were 400 and 100 ppm for niobium and tantalum, respectively.

The effect of oxygen on tantalum-sodium compatibility

The effect of oxygen in both phases on the compatibility of tantalum with sodium was investigated at 600°C by exposing tantalum specimens to liquid sodium in static capsules. Increasing the initial oxygen concentration of the sodium from 50 to 12,000 ppm led to increased dissolution of the tantalum in the sodium; oxygen migrated from the sodium to the tantalum. As the initial oxygen concentration of the sodium increased, the accompanying tantalum specimens first gained and then lost weight. We concluded that the addition of oxygen to sodium results in the formation of a loosely adhering ternary oxide scale on the tantalum surface. The increased dissolution of tantalum and the weight change behavior of the tantalum specimens when the initial oxygen concentration of the sodium is increased indicates that the nonadherent ternary oxide flakes off in progressively greater amounts as its thickness increases. When the concentration of oxygen in tantalum before test exceeded 200 ppm, sodium penetrated the tantalum. Penetration, as in other refractory metal-oxygen-alkali metal systems previously investigated, proceeds with the formation of a ternary oxide by a wedging mechanism. At low oxygen concentrations attack occurs along grain boundaries, and as the oxygen concentration of the tantalum is increased, the number of grain boundaries attacked and the depth of attack increase. Still higher oxygen concentrations induce transgranular attack.

Paper 1: The Hydrodynamics of Flat Contacts: Friction Studies in the Pin and Disc Machine

The friction of run-in flat steel specimens lubricated with plain mineral oils has been measured in a pin and disc machine over a wide range of operating conditions. The hydrodynamic region was identified with the aid of electrical contact measurements and the corresponding friction data were considered in terms of the various theories which have been proposed to explain hydrodynamic action in nominally flat sliding contacts. It was concluded that the Lewicki inflow, the surface roughness, and the viscosity-density wedge mechanisms could not explain the observed friction; but that it could be explained if the surfaces formed a wedge whose angle was constant over the whole range of operating conditions. It is suggested that the wedge may be formed during the running-in process by mechanical effects such as flexural distortion. Particularly striking is the strength of the hydrodynamics which can occur in such ‘flat’ contacts: in terms of the wedge analysis the contact was operating close to the maximum theoretical load-carrying capacity. The implications with respect to the use of the pin and disc machine as a test rig are discussed and it is suggested that there might be a practical application in the design of ‘self-adjusting‘ slider bearings.

The Wear of Copper, Aluminum, Mild Steel, and Zinc, and Their Wear Particle Shape Factors

Unlubricated, highly loaded sliding experiments were conducted for systems of mild steel, copper, aluminum, and zinc. Load is critical in determining the wear mode. A Cocks wedge mechanism is the primary method of wear for copper, and for aluminum until a critical temperature is reached. The wear of steel is characterized by the formation of small un-joined fragments. The surfaces of wear particles were of two types; copper showed one type, and aluminum, mild steel, and zinc the other. Wear particle shape can be characterized by the ratios of dimensions (L/W) and (L/H). These factors are easily determined to good accuracy, and have distributions which are both far different in form and mean values for the various metals. The mean values of the shape factors are independent of particle size, and of each other. Surface roughness is not related to wear particle size for highly loaded sliding.