River channel erosion by plucking is poorly understood even though it is a dominant mechanism for bedrock river profile evolution. In an experimental flume with fractured slabs of plaster model bedrock, plucking from a bed lacking protrusion is produced by nonuniform flow, particularly in rapidly varied flow with hydraulic jumps and free-surface undulations. Model bedrock slides upstream toward water-surface depressions in regions lacking recirculation, groups of blocks bulge up when a trough of free-surface waves moves above, and bubbles and debris particles move in the bed crack network. The likelihood and completeness of plucking increases with average flow strength but relies on local flow properties for initiation of motion. Particle image velocimetry (PIV) analysis of flow during a plucking event suggests that flow structures smaller than the average size of the blocks may be important in the plucking process by generating velocity or pressure variations around the blocks. Because plucking typically occurred near free-surface undulations and we have observations consistent with crack network flow, we propose that the mechanism driving block lift starts in the static pressure gradients developed in the sub-bed crack network, which are locally and temporally enhanced by turbulent pressure fluctuations. Positive feedback occurs when plucked blocks alter flow character and allow other blocks to slide around the bed, promoting additional plucking. Negative feedback occurs where the deposition of plucked blocks downstream of nonuniform-flow reaches limits transport capacity by changing or damping the nonuniform flow upstream. Our experimental results are consistent with previous engineering studies of slab uplift under plunging jets and high-Froude-number hydraulic jumps in energy-dissipating spillways. Our results also point toward the ability of nonuniform flow in bedrock rivers with a low Froude number to generate lift of fractured bedrock below steps and constrictions, and suggest a need for further study of mechanisms that initiate block plucking in experimental and field settings.

The friction of tread compounds on ice is affected to a great extent by the methods and conditions of measurement. As velocity of sliding is increased from zero to about two centimeters per second, there is an increase in friction, after which there is a gradual decrease with further increase of velocity. Static friction depends on the length of time the sample is in stationary contact with the ice, and is of little practical significance. Of the test variables studied, temperature has the greatest influence. At the lowest temperatures used, −30 to −40° C, the coefficient was approximately double that usually observed slightly below the freezing point. Friction was also found to vary with pressure. An empirical equation of the form,k=C1−C2 log P, was found to fit results in many cases. Minor differences were noted, resulting from changes in sample shape and surface treatment. When testing conditions were held constant, it was found that several variables in the samples themselves affected the friction. The type of polymer used had a definite effect. Of about equal importance was hardness. In similar compounds the softer samples had higher coefficients of friction. The unusual frictional characteristics of rubber sliding on ice may be explained by taking into account the water layer formed between the sliding surfaces. The melting of ice to form this water layer takes place because of the heat generated by the work done against friction. The friction itself is believed to be caused by alternate welding and breaking of junctions between surface asperities. Although the process is initiated in part by pressure melting, that phenomenon has only minor importance after the motion is in progress.

Nonlinear vibration characteristics of tread compounds, as evidenced by a dependence of modulus and internal friction on amplitude, were studied in order to reach a n understanding of these unexplained phenomena in terms of plausible structural alterations which may occur in the tread compounds because of vibration. This information is interesting both for a more exact description of the deformation processes in tread compounds, and because of the necessity of dealing with the effects in any dynamic testing procedure. Nonlinear vibration characteristics are readily observed for tread stocks of both Hevea and synthetic rubbers in the mechanical range of frequencies.Experiments were undertaken t o study the extent to which these effects are dependent upon temperature, compounding variables, and type of vibration. I t may be shown that the nonlinearity is not due to temperature rise from vibration, although precise measurements are complicated by the temperature rise. The effects occur for stocks reinforced with fine silica pigment, Pliolite resin, and higher loadings of pigments generally considered to be nonreinforcing, as well as for those compounded with reinforcing blacks. The pigment loading necessary for the same degree of nonlinearity increases with the pigment size. The effects are observed in unvulcanized tread stocks and occur for vibrations in both shear, compression, and tension. Measurements of the nonlinearity for tread stocks over a range of temperatures suggest that the vibration elicits structural changes in the rubber analogous to those observed in the rheology of rubber solutions and raw rubber. The nature of these changes can be interpreted by the rheological criteria of structure developed for non-Newtonian systems. Such recognized mechanisms for structural viscosity as orientation, deformation, and breaking and reforming of bonds of the flow units are useful in explaining the phenomena observed. The effects appear to be of such magnitude that they should be taken into account in any exact study of the effect of modulus on tread wear and performance. S E of the most distinctive features of the vibration of tread 0 stocks is a dependence of the dynamic modulus and internal friction upon the amplitude. This has been consistently observed over a wide range of experimental conditions (4-6, 8, 12, 13, 16, 18-20). For gum stocks these effects are so small that they have usually escaped detection. However, analogous effects with gum stocks of a magnitude of a few per cent have been ieported and it is very probable that they exist. In contrast, nonlinear vibration characteristics for tread stocks are so pronounced that they must be takeh into account in any dynamic testing procedure for evaluating tread compounds, and also in any precise effort to estimate the effective hardness of tire treads on the road.These phenomena with tread stocks are of particular interest because of their puzzling nature, the difficulty in formulating any exact explanation for them, and the possibility that a better understa...

Nonlinear vibration characteristics of tread compounds, as evidenced by a dependence of modulus and internal friction on amplitude, were studied in order to reach an understanding of these unexplained phenomena in terms of plausible structural alterations which may occur in the tread compounds because of vibration. This information is interesting both for a more exact description of the deformation processes in tread compounds, and because of the necessity of dealing with the effects in any dynamic testing procedure. Nonlinear vibration characteristics are readily observed for tread stocks of both Hevea and synthetic rubbers in the mechanical range of frequencies. Experiments were undertaken to study the extent to which these effects depend on temperature, compounding variables, and type of vibration. It may be shown that the nonlinearity is not due to temperature rise from vibration, although precise measurements are complicated by the temperature rise. The effects occur for stocks reinforced with fine silica pigment, Pliolite resin, and higher loadings of pigments generally considered to be nonreinforcing, as well as for those compounded with reinforcing blacks. The pigment loading necessary for the same degree of nonlinearity increases with the pigment size. The effects are observed in unvulcanized tread stocks and occur for vibrations in both shear, compression, and tension. Measurements of the nonlinearity for tread stocks over a range of temperatures suggest that the vibration elicits structural changes in the rubber analogous to those observed in the rheology of rubber solutions and raw rubber. The nature of these changes can be interpreted by the rheological criteria of structure developed for non-Newtonian systems. Such recognized mechanisms for structural viscosity as orientation, deformation, and breaking and reforming of bonds of the flow units are useful in explaining the phenomena observed. The effects appear to be of such magnitude that they should be taken into account in any exact study of the effect of modulus on tread wear and performance.

A technique was developed for vulcanizing cylinders of rubber 0.5 inch in diameter and up to 1 inch long at pressures as high as 150,000 pounds per square inch, using a small laboratory press (8-inch square platens). The specially designed mold was made of alloy steel according to general principles from the high pressure work of Bridgman. It was prestressed at 200,000 pounds per square inch. The temperature in the mold cavity was carefully calibrated with thermocouples to determine the equivalent length of cure of the test-specimens. Cylinders of GR-S tread stock were vulcanized for a range of cures at 1000, 20,000, 50,000, and 100,000 pounds per square inch. GR-S gum stock and Hevea gum and tread stocks were vulcanized for a range of cures at 100,000 pounds per square inch. Control specimens were cured in a regular type of mold, for which the pressure was not determined. All the volume compression during vulcanization is recovered when the pressure is released. This was determined by precise density measurements of the test-specimens. The electrical resistivity, dynamic modulus, internal friction, and resilience of the test-specimens were determined and the results are discussed.