Abstract
Within the scope of this contribution, a method for the determination of a strain limit for designing components made of elastomeric polyurethane systems is presented. The knowledge of a material-specific strain limit is essential for the structural-mechanical calculation of plastic components in the context of component design. Compared to a commonly used component design, based on a simplified dimensioning approach taking only linear viscoelastic deformations into account, the strain limit determined in this study allows an improved utilisation of lightweight construction potential in the dimensioning of technical components made of polyurethanes through the consideration of permissible nonlinear viscoelastic deformations. The test method comprises a sequence of quasi-static loading and unloading cycles, with a subsequent load-free recovery phase, allowing the relaxation of the viscoelastic forces. Standardised tensile and simple shear test specimens and a dynamic mechanical thermal analyser (DMTA) are used within the tests. The strain limit is determined by means of the so-called residual energy ratio, which is a characteristic quantity for the evaluation of hystereses of load–unload cycles. These hystereses are increasingly formed by deformations outside the range of linear viscoelastic deformations. The residual energy ratio relates the proportion of deformation energy recovered during unloading to the deformation work that is applied. In this contribution, the residual energy ratio is successfully used to detect a significant evolution of loss energy under increasing load and to correlate this transition to a characteristic strain. The latter is used as a dimensioning parameter for the design of components made of elastomeric polyurethane materials for quasi-static load cases. The determination of this strain limit is performed under consideration of the criterion of reversibility of deformation.
Highlights
Depending on their chemical composition, the mechanical properties of polyurethanes (PUR) include the entire range of material behaviour of engineering plastics, from rubberelastic to hard-elastic brittle deformation behaviour
The characterisation and modelling of the material behaviour and the determination of mechanical parameters of elastomeric polyurethanes are of high importance for engineering applications
Only a few data sets with basic material properties are available in the literature, such as those used for thermoplastics in the form of the database CAMPUS, for example
Summary
Depending on their chemical composition, the mechanical properties of polyurethanes (PUR) include the entire range of material behaviour of engineering plastics, from rubberelastic to hard-elastic brittle deformation behaviour. Only a few data sets with basic material properties are available in the literature, such as those used for thermoplastics in the form of the database CAMPUS, for example. This results in a deficit regarding appropriate design parameters for polyurethane materials for the performance of strength assessments as they are known for thermoplastics or composite materials. The design methods for plastic components with corresponding design parameters and their limit values are divided into stress-, strain-, and energy-based approaches. The advantages of a strain-based design using a so-called critical strain or limit strain are presented, for example, in investigations of Menges [1,2] and Kunz [3,4]
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