Model-based analysis of strain/stress behavior of polymer materials at constant strain rate regime

Tensile test at constant strain rate by Universal Testing Machine (UTM) is honored as a standard method for characterizing mechanical behavior of polymer materials, and is applied routinely in all polymer laboratories. Data collected from this test are never utilized to evaluate elastic, viscoelastic and viscous components of the deformation undergone by the material in the process, which is so important to evaluate its sustainability as constructional material. Assuming that a mechanical model consisting of elastic and viscous elements arranged in series, parallel or in combination would describe the deformational behavior of an object, mathematical expressions have been derived to describe stress development in five classical model objects (Kelvin, Maxwell, Hooke-Kelvin, Kelvin-Newton and Burger) subjected to constant strain rate. Finally, a procedure has been worked out to solve the reverse problem with UTM data (before neck formation in the sample), i.e. to identify the model representing the deformational behavior of the object and to evaluate the model parameters. The method has been applied to evaluate some tensile test data available in literature and appropriate models have been proposed. For the first time in literature, elastic, viscoelastic and viscous strain have been differentiated from a UTM test data. The methodology developed in this work can be used by researchers in developing their own models with different combinations of viscous and elastic elements and in differentiating properly the elastic, viscoelastic, and viscous strains of the material objects subjected to conventional UTM tests.

Graphical abstract