The influence of temperature variation on the mechanical properties of rubber automotive components

Automotive component manufacturers are under pressure to give reliability over widening ranges of operating temperature. Japanese motor manufacturers require their vehicle components to function between temperatures of -45 degrees C and 120 degrees C. The previous limits were -40 degrees C and 100 degrees C. The new limits create difficulties for producers of numerous components, but none more so than those making rubber parts. It is far more difficult to characterise material behaviour for rubbers than for linear elastic materials. Though finite element analysis of hyperelastic materials has gained greater acceptance recently, sensible modelling of parts subjected to high, localised deformations is still uncertain. There is no universal model for the characterisation of the physical behaviour of rubber. Strain energy density functions based on stretch ratios are increasingly used in preference to those employing strain invariants. However, certain material models may give the best representation of particular rubbers whilst others are better characterised by one of a number of alternatives. It is broadly accepted that material constants should be determined by conducting as many different standards tests as possible. Yet many manufacturers do not do their own testing and most material parameters are calculated from uniaxial tensile test results. No agreement on conditioning procedures exists. What loading method is applied, with how much recovery time between cycles and to what strain? This paper describes the variation in material constants resulting from uniaxial tensile tests on polychloroprene over the new temperature range. The test-pieces were cut from sheets in two perpendicular planes, with and normal to the mould tooling marks. Conditioned and unconditioned samples were tested, but only the latter are considered here. Two-term Ogden models were determined for each situation at five temperatures and the tests modelled using MARC finite element software. As a result sensible material constants are recommended.