This study investigates the validity and applicability of the correlation between scratch and tensile properties for extruded polymer strands. The mechanical properties could be predicted for extruded samples, which allows skipping the step of injection molding and therefore enables a faster material development. Extruded polymer strands and tensile test specimens out of PMMA, PS, POM, PP and PE have been investigated. A correlation of the Young’s modulus and the elastic deformation as well as a correlation of the yield stress and the plastic deformation during scratching is given for both flat molded and cylindrical extruded specimens. SEM images of the scratch grooves are used to analyze the scratch deformation mechanism. The deformation mechanism correlates well to the variation coefficient of the indentation depth. Polarized light microscopy of thin cross sections of both types of specimens provides information about skin layer thickness and morphology. However, the optical analysis could not provide an explanation for the different levels of the indentation depth in the two specimen types. Further investigations should include a study of differences in process induced morphology and the effect of two layers with different mechanical properties, i.e., skin and center, on the stress and strain fields underneath the scratch.
Microstructures on polymer surfaces are known to reduce friction and thevisibility of scratches. Due to the complex interaction of multiple surface areas in contactwith each other the prediction of coefficient of friction (COF) or wear is difficult and dependson an empirical solution. This article deals with possibilities of calculating the deformingpart of friction via an analytical solution and a FE-model. In a first step the modelling ofsingle contacts is demonstrated. The analytical calculation based on the Hertzian contactequations is extended regarding viscoelastic material parameters. The basic approach ofFE-modelling is explained including calibration of the material model using the softwareMCalibration®. The article introduces the different procedures of simulating and modellingCOF and wear taking into account the area of contact and resulting stress distribution.
The paper presents analytical approaches for calculating the effective contact area of the sphere-plane contact, which allow conclusions to be drawn about the coefficient of friction of thermoplastics with manageable effort. These approaches are verified experimentally utilizing friction and wear tests using the example of a steel sphere against PE-UHMW. The friction area of the sphere was varied using a self-constructed adjustment unit and a spherical wedge. With the help of parametric experiments regarding the angular position of the spherical wedge, a limiting contact angle was detected. This limiting angle allows the calculation of the deformative and adhesive friction. FE models are being developed for the simulative verification of the analytical approaches and further investigations of structures for friction reduction. These require specific material models as a basis for the representation of the stress-strain behavior in contact. The different FEM material models are calibrated and compared against each other based on experimental tests. The materials POM, PP, PMMA, and PE-UHMW were considered.
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