Polymer materials exhibit a high ductility. Determination of the yield strain as well as the break (ultimate) strain is usually done on the basis of tensile tests performed on standard samples and evaluated for normalizhed measured length, supposing the homogeneous material deformation along the sample axis. This experimental approach does not take into account the plastic strain concentration ii small neck area during the final deformation phase before the sample rupture, what is typical for the plastics behavior. Application of so defined material characteristic in the case of Finite element metod (FEM) analyses of real constructions made of TSCP plastics (typical semi-crystal polymer) led to significantly conservative (smaller) values of ultimate loads compared to the measured ones. A special experimental method making use of high-speed camera has been developed to determine the strain in defined small area of local strain concentration being also in correlation with the FEM element size. Application of the more realistic (higher local) break strain value in the case of FEM analyses of real TSCP constructions led to much better agreement between the calculated and measured stiffness and ultimate load values.