strain rate values were chosen to cover the range of 10−4–10−3 s−1 which is lower than maximum strain rate considered at 10−3 and intersects with the strain rate range realized in free bulging tests.
The appearance of specimen deformed at 4.0 × 10−4 s−1 is presented at Fig. 2. The stress-strain curves obtained by constant strain rate tests are presented in Fig. 3. Strain hardening can be observed at the beginning of deformation when the effective strain (e) is less than 0.4. In the strain range of 0.4–0.8 the stress-strain curves are almost horizontal and the maximum of each curve corresponds