SHEAR-STRENGTH RESULTING FROM STATIC FRICTION OF SOME THERMOPLASTICS

The effects of normal load and roughness on the static shear strength of three thermoplastics have been investigated for the case of contact between a plastic sample and a very smooth metallic plate. An apparatus which provides a gradual increase in the tangential load was used to measure the minimum force required to shear the adhesion between the two surfaces when subjected to a normal load, This force was set equal to the adhesion component of friction F(f). The real area of contact A(r) was also measured using an optical device designed to handle samples and experimental conditions similar to the friction tests. This permitted the evaluation of static shear strength tau as the ratio F(f)/A(r). The experimental results for ultra- high molecular weight polyethylene (UHMWPE) show that the relationship between tau and the real contact pressure P can be represented by a regression function of order three. For this material, there is a decrease of tau when the asperities undergo elastic deformations. However, an increase in tau is found when plastic deformation of the asperities occurs. This transition was noted at the region where P is equal to the yield strength S(y) of the material. The acetal Delrin also showed a decrease of tau when P < S(y) and, although it could not be verified experimentally, it is assumed that it would behave in the same manner as the previous material for higher P. A strong dependence of tau on the initial roughness of the contacting surface was observed for both materials. The results obtained with nylon (PA 66) show an increase of tau with P, even when P is lower than S(y). This difference in behaviour compared to the previous materials is related to the brittleness of the asperities which is a result of its relatively low glass transition temperature. No effect of the roughness on the shear strength was found for this material. The relationship between tau and P can be very well approximated by linear functions whose parameters depend on the type of deformation that occurs at the interface. These parameters are derived and reported.