M.C. Shaw

ON DEFORMATION AT LARGE STRAINS

An improved method of determining the shear stress-shear strain characteristics of a metal with variable normal stress on the shear strain is presented and used to obtain a number of representative curves. These curves exhibit the strain hardening characteristics that metals normally exhibit at moderate strains. However, at higher strains, the strain hardening tendency disappears and becomes negative before gross fracture occurs. It is suggested that microcracks are responsible for this unusual behavior. As the metal deforms at large strain microcracks of small lateral extent are believed to appear and disappear. As strain increases, the equilibrium shifts in the direction of microcrack formation. Normal stress on the shear plane shifts the equilibrium in the direction of microcrack disappearance and postpones gross fracture. The deformation at very large strains appears to be associated with the motion of microcracks in a manner similar to that of the movement of structural holes in the case of a liquid.

Galling wear of materials at high speed sliding contact

Abstract Wear by galling is detrimental to the service life of bearing surfaces of machine elements in relative motion. This is especially so in the case of high speed rotating machinery when the lubricants fail to be effective. It is proposed, in this paper, that wear leading to galling can be influenced by changes (metallurgical, mechanical and chemical) that take place in situ during sliding. Experiments were conducted at high sliding speed (20,000 s.f.p.m.) using a newly developed microfeed mechanism on a grinding machine to simulate the conditions of rubbing. Various micromechanisms of wear leading to accelerated adhesive wear (galling) are identified and discussed. Using Scanning and Transmission Electron Microscopic and Auger Electron Spectroscopic techniques, the micromechanisms leading to galling were investigated.

Experiments with spherical tools

Abstract Spherical steel and carbide tools were scraped through the surface layers of steel, brass and aluminum specimens. Results of force measurements and records of groove shapes are presented and discussed. Force ratio ( R = ratio of tangential to radial components) was found to be related to the proportion of the spherical surface contacting the workpiece material, and to frictional resistance. Aluminum gave less side flow and higher values of R than steel or brass due to its inherently greater friction characteristics.

An investigation of the wear of abrasive grains by rubbing on ferrous and non-ferrous surfaces

Abstract The wear characteristics of abrasive grains have been extensively studied using stationary grains carrying a load of 1 or 2 lbs. bearing against a disk rotating at approximately 800 r.p.m. Mean wear volume was found to increase linearly with sliding distance. Carbon steel disks gave lower wear rates for the friable grains than the ductile grains while the reverse was found to be true for Waspaloy and stainless steel. The effect of sliding speed on wear rate and coefficient of friction is also discussed.

On friction and metal transfer of sliding surfaces undergoing subsurface plastic deformation

Abstract Results of sliding experiments under high normal pressure using a sapphire ball on a modified Brinell hardness testing machine and different work materials are reported. A rational approach to the mechanism of metal build-up on Al2O3 abrasive developed recently has been extended in the present sliding tests to verify the hypothesis, and good agreement was found. The results of this study show a wide range of behaviour relative to the equilibrium level of friction and the time required to reach equilibrium when different materials are rubbed against the sapphire ball.

Friction characteristics of coated tungsten carbide cutting tools

Abstract A series of experiments has been carried out to explain the lower frictional characteristics of carbide tools coated with TiN relative to those coated with TiC. The reason for the higher friction of TiC appears to lie in the tendency for C to diffuse from the TiC coating into the thin layer of steel that transfers to the tool surface, thus strengthening it. No such strengthening mechanism is evident when TiN is the coating.

Wear of silicon carbide in high speed sliding

Abstract Attritious wear of silicon carbide rubbing against a cobalt base superalloy at high speed was studied using a scanning electron microscope (SEM) and an Auger electron spectroscope (AES). The SEM study of the wear area on the silicon carbide grain showed it to be very smooth. The AES study of the groove-like marking generated by a silicon carbide grain showed a heavy concentration of carbon in areas where submicron wear debris was present. No indication of chemical reaction of the abrasive with the work material was evident. Instead, it appears that the surface atoms on the abrasive are removed preferentially, layer by layer, by oxidation under high temperature and a favorably directed shear stress.

On the attritious wear of abrasive grains

Abstract The overcut fly milling operation that closely simulates fine grinding has been used extensively to study the performance of several grain types at moderate and low wheel speeds. Results indicate that the wear of abrasive grains for a particular grain-work-piece combination is a function of chip thickness, chip length and wheel speed. For a constant chip length, the wear rate increases exponentially with increasing chip thickness. There is, however, an optimum value of chip length which gives minimum wear at any particular chip thickness.

An investigation of the wear of abrasive grains by rubbing on diamond disks

Abstract In earlier wear studies 1 single abrasive grains were rubbed against the surfaces of metal disks under light loads. These studies have now been extended to include an examination of wear behaviour when using diamond-impregnated surfaces at lower sliding speeds and when using more accurately controlled test conditions. These wear tests provided values of wear resistance in the absence of chemical effects ( i.e. at sufficiently low surface temperatures). The results are in excellent agreement with the conventional wear theory pertaining to lightly loaded sliders. The wear volumes are, however, two orders of magnitude greater than those obtained when rubbing tough grains against steel disks and three orders of magnitude greater than the results obtained when rubbing friable grains against similar metallic surfaces.

A mechanism for the accelerated wear of high speed sliding surfaces in the presence of spherical particles

Abstract A mechanism for the accelerated wear of bearing surfaces due to the cutting action of the spherical particles between bearing surfaces in sliding contact is proposed and verified experimentally.