Search

Solid panicle erosion of metal surfaces is a major problem in several fluid handling industries due to unpredicted equipment failure and production loss. The prediction of erosion is difficult even in a single-phase flow. The complexity of the problem increases significantly in a multiphase flow due to the existence of different flow patterns where the spatial distribution of the phases changes with the change of phase flow rates. Earlier predictive means of erosion in single and multiphase flows were primarily based on empirical data and were limited to the flow conditions of the experiments. A mechanistic model has been developed for predicting rosion in single-phase and multiphase flows considering the effects of solid particle impact velocities that cause erosion. Local fluid velocities and simplified equations are used to calculate erosion rates assuming a uniform distribution of solid particles in the liquid phase in the multiphase flow. Another assumption was that the solid particle velocities are similar to the velocity of the fluids surrounding the particles. As the model is based on the physics of multiphase flow and erosion phenomenon, it is more general than the previous models. The predicted erosion rates obtained by the mechanistic model are compared to experimental data available in the literature showing a reasonably good agreement. [DOI: 10.1115/1.2767336]

Hemming is a manufacturing process of folding a panel onto itself or another sheet. Quality of hemming is characterized by geometry and formability. This paper presents a response surface study of three-dimensional (3D) curved-surface-curved-edge hemming of an aluminum alloy, AA6111-T4, using finite-element ( FE) analysis. Solid elements and explicit FE solver are used for simulations of flanging, pre- and final hemming, and shell elements with implicit solver are deployed for springback prediction. A novel procedure called 'solid-to-shell mapping' is developed to bridge the solid elements with the shell elements. Verified to be accurate and efficient, the model is utilized in a central composite design to quantitatively explore the relationships between certain key process variables and the hem dimensional quality and formability. The most significant variables are identified as: (i) prehemming angle on roll-in/roll-out; (ii) nominal surface curvature on sheet springback; and (iii) initial sheet strain and flanging die radius on the maximum hemline surface strain of the produced hem. These results provide insights for process parameter selections in designing and optimizing 3D hems under material formability constraints. [DOI: 10.1115/1.2515430] Keywords: aluminum hemming, dimensional quality, formability, solid-to-shell mapping, central composite design

In the present paper the applicability of state and rate dependent friction laws in describing the phenomena of high speed slip at metal-on-metal interfaces is investigated. For the purpose of model validation, results of plate-impact pressure-shear friction experiments were conducted by Irfan in 1998 and Irfan and Prakash in 2000 using a Ti6AI4V and Carpenter Hampden tool-steel tribo pair are employed. In these experiments high normal pressures (1-3 GPa) and slip speeds of approximately 50 m/s were attained during the high-speed slip event. Moreover, these experiments were designed to investigate the evolution of friction stress in response to step changes in normal pressure and also in the applied shear stress during the high-speed slip event. A step drop in normal pressure is observed to result in an exponential decay of the friction stress to a new steady-state characteristic of the current normal pressure and the current slip velocity. A step drop in applied shear stress is observed to lead to an initial drop in friction stress, which later increases toward a new steady-state friction stress level. In response to the step drop in applied shear stress the slip velocity initially increases and then decreases to a new steady-state level consistent with the new friction stress level. A modified rate and state dependent friction model that employs both velocity and normal stress dependent state variables is used to simulate the experimental results. A good correlation is found between the experimental results and the predictions of the proposed state and rate dependent friction model. [DOI: 10.1115/1.2401217] Keywords: mathematical modeling, high speed frictional slip, plate impact friction experiments, state and rate dependent friction

Scuffing or scoring is an important form of damage leading to component replacements in lubricated mechanical systems such as power drives, gears, bearings, cams and followers, piston rings etc. Since scuffing necessarily involves localized welding of asperities, suitable surface modifications or coating can impart a good resistance to scuffing. A new class of low temperature salt bath nitriding process provides good resistance to scuffing. The current work is on evaluation of the scuffing resistance of AISI 4340 steels imparted with two such treatments. Experimental investigations were conducted on Sursulf and Arcor treated AISI 4340 steel specimen using a pin on ring test system under boundary lubrication condition. Continuous monitoring of friction records and (near surface) bulk temperature were done under step loading. Limiting loads and load velocity relations were evaluated and using the data generated and a thermo-mechanical wear model, performance is indexed. Off line studies on surface finish, hardness variations and surface transformations were also accomplished. Some salient aspects of the investigation and data generated are presented here. Onset of scuffing was observed to be far delayed with pin specimen imparted with these treatments compared to hardening. [DOI: 10.1115/1.1504089]

One of the failings of the pressure die casting process is that variable surface quality is a common result. This paper provides evidence that die vibrant can be used to polish castings as they form during solidification. Evidence is obtained from experiments performed on a purpose built rig that is designed to be representative of the pressure die casting process. The rig's design is such that part of the casting face can be subjected to tangential vibration at prespecified frequencies and amplitudes. The castings made are viewed under a microscope and micrographs produced. These combined with Talysurf readings show that die vibration can be used to generate casting surfaces with a quality significantly superior to that of the die. Supporting evidence is also provided by thermocouple readings and the solution of the inverse parabolic heat equation. The expected increase in the interfacial heat transfer coefficient is obtained with improved casting surface quality. Established in the paper is the effect of vibration frequency and amplitude on the Ra value of the casting surface. It is shown that threshold values for frequency and amplitude exist, below which surface improvement is diminished. [DOI: 10.115/1459085]

The Mode I stress intensity factor of a sector crack in a three-dimensional Voronoi polycrystal is computed by the body force technique. Microstructural stresses arising from the elastic anisotropy of grains (cubic and hexagonal) and the random grain orientations are estimated using the Eshelby procedure and incorporated in the stress intensity factor calculations. For metallic polycrystals, it is shown that the stress intensity factor depends significantly on the elastic anisotropy ratio, the grain orientations, the remote stress state, and the microstructural stresses. [S0021-8936(00)03401-2]

A theoretical model of the gap nucleation process during pure metal solidification on a deformable mold of finite thickness is presented. Both surfaces of the mold follow a sinusoidal lay for which the ratio of the amplitude to the wavelength, or aspect ratio, is much less than one. This makes the aspect ratio a convenient perturbation parameter for the thermal and mechanical problems since it is indicative of the spatial variation in the surfaces. The thermal and mechanical fields are coupled along the upper surface of the mold through a pressure-dependent thermal contact resistance. The main goal of the model is to develop a means for examining the contact pressure along the mold-shell interface and how variation of the mold surface wavelength affects the time and location of gap nucleation. Gaps, which result from irregular distortion of the shell due to the modest variation of the mold surface geometry, are assumed to nucleate when the contact pressure locally falls to zero. The model leads to two coupled differential equations for the shell thickness and contact pressure perturbations which are solved with a numerical scheme. Using a series solution methodology, it is shown that the contact pressure perturbation predicted by the present model reduces to that for a rigid, perfectly conducting mold (which was considered in another work) in the limit of zero mold thickness. In the companion paper, we specifically examine various combinations of pure materials acting either as the shell or the mold material. The concept of a critical wavelength, which separates those wavelengths that lead to gap nucleation at the crests, from those that lead to gap nucleation at the troughs, is introduced. The potential for development of design criteria for mold surface topographies using the present theoretical model as a limiting solution for finite element models of more complex casting processes is discussed. [S0021-8936(00)03201-3]

Flattening of random rough surfaces on a workpiece undergoing bulk deformation has been analyzed using a model of the surface consisting of just two wavelength components. Asperities are flattened at a rate which depends on the ratio of the initial r.m.s, amplitudes of the long and short wavelength components. The flattening behavior of the long wavelength asperities only becomes important when the amplitude of the long wavelength asperities is much greater than that of the shorter wavelength asperities. The surface modification was investigated experimentally by cold rolling of aluminum strips. The power spectral density of the roughness was used to extract appropriate amplitudes for the short and long wavelength components of roughness. The change in roughness amplitudes showed excellent agreement with theory.

The influence of harmonic normal loads on sliding friction is investigated through analysis of contacts consisting of conical and spherical sliders of hard materials on softer metal surfaces. Friction for such contacts is assumed to result from a plowing component and a shearing component. Calculations and experiments show that the coefficient of friction is essentially independent of normal load for contacts with conical sliders. However, for spherical sliders the relation between the coefficient of friction and normal load is highly nonlinear. In the presence of harmonic variations in normal load, this non-linearity causes a shift in the average coefficient of friction. For ideal lubricated contacts, the shearing component of friction is very small and for this case, it is shown that the maximum average reduction in the coefficient of friction is ten percent. When the shearing component is more significant, as with dry contacts, the shift is less. For example, when the shear strength is one-sixth the hardness of the softer material, the maximum average reduction in the coefficient of friction is five percent.

Research is conducted to examine the applicability of an existing model created for coated metallic surfaces for use over the range of parameters utilized in an engine block. Experimental data is compared to a thermal contact conductance model created for plastic asperity deformation of uncoated surfaces. Results indicate that material composition, anisotropic surface roughness and interface porosity will cause the measured thermal contact conductance values to deviate from the values predicted by the two models.

The velocity, normal pressure, and slider size dependence of the coefficient of dry friction of metals in the range of high sliding velocities (V [greater than or equal to] 1 m/s) is investigated theoretically. Failure of the adhesive junctions by adiabatic shear banding is considered as the underlying process. The concept of asperity shearing by the adiabatic shear banding mechanism represents a new approach to unlubricated high velocity friction. Analytical solutions of a coupled thermomechanical problem are given for two constitutive relations. Numerical solutions for steel-on-steel friction showing a decrease of the coefficient of friction with the sliding velocity for different normal pressures are presented. The model is considered to be adequate in the velocity range of 1-10 m/s where friction enhanced oxidation or surface melting are believed not to interfere with the asperity shearing process.

Fretting is the tribological phenomenon observed in nominally-clamped components which experience vibratory loads or oscillations. Associated with fretting contacts are regions of small-amplitude relative motion or microslip that occurs at the edges of contact. A newly-available infrared technology capable of resolving temperatures fields finely, both spatially and temporally, is used to characterize the near-surface conditions associated with fretting contact between an aluminum alloy cylinder and fiat. Both frictional heating due to interfacial slip and the coupled-thermoelastic effect arising from strains in the material induce these temperatures. The experimental results provide insight into not only the magnitude and distribution of near-surface temperatures, but also the nature of the contact stress field and the mechanics of partial slip fretting contacts. Comparisons of the measured temperature fields are made with those predicted by considering both conduction of the frictional heat flux and coupled-thermoelastic theory.

A technique for measuring temperature close to the primary cutting edge in turning has been developed. The cutting temperatures of a 0.16 percent carbon bright drawn mild steel, have been measured for a range of cutting speeds and feedrates at a constant depth of cut. Tool nose radius was also varied. The correlations for the workpiece temperature of cutting speed and feedrate have been developed. The results show that the temperature correlates well with cutting speed and feedrate but the nose radius has little effect. Cutting forces were measured by a dynamometer and these were used to find the non zero forces at zero feedrate. These forces have been related to the deformation of the work material near the cutting edge of the tool and a method for calculating the cutting temperatures from these has been proposed.

In many instances machined parts experience sliding motion while in contact with a counter surface. The performance of the contacting parts depends on their friction and wear characteristics. In this work the effects of the machining condition on the tribological performance of steel, brass, and duralumin are investigated. For steel, it is shown that the friction coefficient value remains about the same whereas the rate of wear varies as the cutting condition is altered. The friction coefficients for brass and duralumin were lower than that of steel and their tribological properties seem to be less sensitive to the machining condition. In conclusion, it is emphasized that the machining condition directly influences the surface integrity which in turn affects the tribological performance of the machined parts, and therefore, the machining condition for parts that experience contact sliding motion should be optimized for best tribological performance in addition to dimensional accuracy and surface roughness.

A finite element method simulates the flame-cutting process of two 50 millimeter thick steel plates to estimate the risk of crack occurrence in the plate due to flame-cutting under different conditions. The parameters considered in the simulation include temperature history effects. The samples are cut in air after preheating, which reduces residual stresses, and by inference, which reduces crack occurrence.