Failure criterions for polymer composites subjected to time-dependent loadings
Danuta Witemberg-Perzyk Politechnika Warszawska, Instytut Mechaniki i Konstrukcji, ul. Narbutta 85, 02-524 Warszawa, Poland
Quarterly No. 1, 2008 pages 22-25
DOI:
keywords: polymer composites, failure stress, strain history, strain energy failure criterion
abstract The mechanical properties of polymer materials, including composites, can be significantly time-dependent. In particular, an influence of the course of loading or deformation, i.e. history effect, on failure stress has been observed and described in several previous works. An acknowledged approach to the formulation of the failure criterion of inelastic materials is that utilizing the specific strain energy concept. In the present work tensile strength of a woven fiber reinforced thermoplastic composite was investigated. The tensile tests were carried out in the warp direction, using the MTS model 810 machine, which allowed arbitrary load or cross-head displacement programming in time. Two basic types of loading programs were examined: monotonic with two different machine cross-head speeds 0.05 mm/min and 5 mm/min as well as creep at several constant load levels, preceded by fast monotonic loading. The stored and dissipated energy components were calculated from a rheological material model consisting of the following elements connected in series: Hook’s term, time-independent plastic term responsible for deviation of the stress-strain curve due to proceeding structure damages as well as Kelvin’s and Newton’s terms, responsible for time-dependent energy dissipation and conservation. The model’s parameters were calculated by fitting the theoretical curves to stress-strain-time experimental results using the simulated annealing optimization method. The results presented indicate that none of the main energy components is constant, but all of them seem to be linear functions of the failure stress and, consequently, they cannot be directly used as a failure criterion. The failure criterion requires further elaboration and validation for various types of time-load time-dependencies. A good starting point may be the stored strain energy, modified by including its dependency on stress.