Hany A. Sherif

Relationship between normal and tangential contact stiffness of nominally flat surfaces

Abstract A study of the static frictionless contact between two elastic bodies having nominally flat surfaces is made. The normal and tangential contact stiffnesses between these two elastic bodies are determined by measuring the systems natural frequencies under the effect of different values of normal load. The theoretical analysis has been carried out to compute the contact stiffness between flat surfaces from profile measurements using the theories of Greenwood and Williamson and Onions and Archard. Comparison of the calculated values, based on these theories, with the experimental data shows good agreement. It is found that the ratio between the normal and tangential contact stiffness is constant and is merely a function of the Poissons ratio of the contact surfaces.

Effect of contact stiffness on the establishment of self-excited vibrations

Abstract The onset of friction-vibration instability is sometimes attributed to the inverse variation in the friction force with the relative velocity of sliding whereby the friction-velocity gradient becomes high compared with the system damping coefficient. However, when account is taken of the effective contact stiffness of sliding surfaces, it is found that the instability of frictional vibrations is mainly dependent on the value of the total tangential contact stiffness of sliding surfaces as well as the system stiffness. The values of both the friction-velocity gradient (the slope of the friction-velocity curve) and the mass of the slider are found to have negligible effects on the stability of motion.

Parameters affecting contact stiffness of nominally flat surfaces

Abstract The problem of the elastic contact of nominally flat surfaces is reviewed towards the mathematical derivation of the resultant contact stiffness in terms of the surface topographies. The present study puts forth two approaches based on the elementary theories of elastic contact. When considering the theory of Greenwood and Williamson (GW model), it is found that the contact stiffness is dependent upon the standard deviation of height distribution of asperities, the effective radius of curvature of asperities and the density of asperities per unit area. When considering the theory of Onions and Archard (OA model), the contact stiffness is found to be dependent upon the correlation distance, the apparent area of contact and the standard deviation of height distribution of asperities. However, it is found that the standard deviation in both models strongly affects the value of contact stiffness. Moreover, the variation of contact stiffness with normal load is similar to that of a stiffening spring. It is also shown that the GW model underestimates the contact stiffness when it is compared with the OA model.

Ambient vibrations piezoelectric harvester array with discrete multiple low frequencies

This work presents an approach to enhance kinetic energy harvesting from low multi-frequency discrete ambient vibrations. It is based on an array of three piezoelectric harvesters each with a certain resonant frequency that is triggered at a specific frequency in the vibration environment. The harvesters used in this work are of commercially available cantilever beam type. Their resonant frequencies are experimentally estimated. They are compared with the theoretical values from the model and good agreements are found. During operation, each harvester is modeled as a voltage source with a resistance and a capacitance load (i.e. RC circuit). The proposed approach discretely broadens the frequency band of power harvesting process and makes it more efficient and more suitable for practical implementations.