Time-Dependent Characterization of Viscoelastic Materials

Daniel Tscharnuter

Research output: ThesisDoctoral Thesis

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The mechanical behavior of polymers depends on time, stress, strain and other influences such as temperature or moisture. Nonlinear behavior can be observed at strains well below 1%, but nevertheless linear viscoelastic behaviour is frequently assumed in the description of time-dependent mechanical properties. However, the nonlinear behavior is typically such that increased stress leads to an increased compliance, unless the deformation is large enough to introduce strong orientation. In some cases, in addition to the viscoelastic deformation, a load history can cause viscoplastic deformation. For comprehensive modeling of the mechanical behavior of a polymer and the simulation of polymer components, a thorough understanding of the viscoelasticity and viscoplasticity is required. Despite its limitations, the theory of linear viscoelasticity is useful for the characterization of time-dependent behavior at low strains, e.g. to characterize the effect of temperature on the relaxation processes. A new method for the determination of linear model parameters based on cubic splines was introduced. It provides an accurate model for the stress or strain and allows for an analytic solution of the constitutive equation in combination with a Prony series model. The Prony series is directly determined by fitting the entire set of data to the solution of the constitutive equation, thus avoiding a calculation of point values of the viscoelastic material function. This approach proved especially useful in the determination of the Prony series for the time-dependent Poisson's ratio, for which a direct calculation assuming instantaneous loading is prone to large scatter. The applicability of linear viscoelasticity is limited to very low strains and stresses. Nonlinear viscoelastic effects were observed e.g. already at strains around 0.6% in stress relaxation. This shows the necessity of a nonlinear viscoelastic model but before a suitable model can be determined, the limits of reversible loading must be known. Hence, three experimental techniques were applied to gain insight into the viscoplastic deformation and the limit of reversible loading. First, a thermomechanical analysis was performed. An experimental setup for very precise and accurate temperature measurements during mechanical testing was developed. The description of heat release or absorption due to elastic behavior follows from the thermodynamic theory. Heat released due to viscoelastic and viscoplastic effects leads to a deviation from the theoretical elastic behavior. The determination of the deformation heat can thus provide information on these processes. Second, a quantitative analysis of the stress whitening during uniaxial tensile testing was conducted. The experiment was designed to allow for the simultaneous measurement of true strain and stress whitening by means of digital image correlation. The stress whitening relates to cavitational processes in the polymer. Hence, the onset of cavitation could be determined by evaluating the gray levels during tensile testing. Third, strain recovery from uniaxial testing was measured to determine the viscoplastic strain component for various load levels. The viscoplastic strain is the irrecoverable strain that is observed after the completion of the recovery. By the latter two methods it was shown that irreversible deformation occurs below 4\% strain in the uniaxial tensile tests. The strain recovery data provides a good basis for modeling. The strain range covers strains close to yield point and thus nonlinear behavior is contained in the data. The stress-controlled recovery phase provides further information on the time-dependence. These data are used to define a uniaxial Schapery-type nonlinear viscoelastic viscoplastic model. The constitutive equation of this model is solved using an iterative scheme and the parameters are determined by fitting the sol
Translated title of the contributionZeitabhängige Charakterisierung viskoelastischer Werkstoffe
Original languageEnglish
  • Pinter, Gerald, Assessor A (internal)
  • Rink, Marta, Assessor B (external), External person
Publication statusPublished - 2010

Bibliographical note

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  • Viscoelasticity
  • Time-dependence
  • Viscoplasticity
  • Polypropylene

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