Identification of elasto-visco-plastic parameters and characterization of Lüders behavior using digital image correlation and the virtual fields method

Abstract

Abstract. In this study, tensile loading experiments are performed on notched steel bars at an average applied strain rate of 1s -1 . Displacement fields are measured across the specimen by coupling digital image correlation (DIC) with imaging using high speed CCD cameras (4796 fps). Results from the experiments indicate the presence of local strain rates ranging from 0.1 to 10s -1 in the notched specimens. The heterogeneity of the strain rate fields provides suitable conditions for determining simultaneously all the elasto-visco-plastic constitutive parameters governing the material behavior. For that, the whole stress fields are reconstructed in the specimen using the fullfield deformation measurements. This reconstruction is repeated with different constitutive parameters until the average stress in the specimen matches the one measured with the load cell response. Perzyna's model is firstly considered for the reconstruction of stresses but it is shown to be unsuited for providing the drop in the average stress that is systematically detected at the onset of plasticity by the load cell. This drop is attributed to the sudden occurrence of plasticity in the material due to Lüders effect. A modified model for elasto-visco-plasticity taking account of Lüders behavior in the material is considered afterwards. It yields a better agreement between the reconstructed stresses and the load cell response, and a more accurate identification of the parameters driving the visco-plastic model. Eventually, it is shown how to use DIC measurements for replacing the load cell measurements when the transient effects in the test reach the resonance frequency of the load cell.

Full-Field Strain Measurement and Identification of Composites Moduli at High Strain Rate with the Virtual Fields Method

“…-Identification of a strain rate dependance law over a range of strain rates with a single specimen making use of the presence of an heterogeneous strain rate map, as in [6]. -Explore the identification of more complex elastoplastic models (including cyclic loads) by using heterogeneous tests as in [52,53].…”

“…Unfortunately, their results, obtained with an interframe time of 8 μs, present a very small field of view (166 × 192 pixels) which did not allow to completely describe the localization process due to lack of spatial resolution. The second paper by Avril et al deals with the identification of elasto-visco-plastic constitutive parameters at more moderate strain rates (around 1 s −1 ) [6]. The authors used digital image correlation on dogbone steel specimens submitted to a tensile test on a universal hydraulic test machine.…”

“…13). Taking advantage of this heterogeneity, a strain rate sensitivity of the mechanical properties could be identified using the VFM as it is done in [6]. But, unfortunately here, the strain-rate resolution is not good enough to do it.…”

Identification of the Dynamic Properties of Al 5456 FSW Welds Using the Virtual Fields Method

“…Since then, it has been successfully applied to the identication of constitutive parameters for homogeneous materials in elasticity [24,25], elasto-plasticity [26,27], and visco-plasticity [28]. The method has also been used for heterogeneous materials (welds) in quasi-static loading and elasto-plastic material response [29].…”

Identification of plastic constitutive parameters at large deformations from three dimensional displacement fields

“…A review of the VFM is given by Grédiac et al [34] and its application to elasto-plasticity is described in Grédiac and Pierron [35] and Avril et al [36], moreover Pierron et al [37] extended the method to cyclic loads and kinematic hardening. In dynamic, examples of application of the VFM are given in Pierron et al [38] and Avril et al [39].…”