High strain-rate tensile testing and viscoplastic parameter identification using microscopic high-speed photography

Sutton

et al. 2008

Mechanics of Materials

120

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.

“…(7) that give the yield stress in function of the plastic strain rate. Perzyna's model [2] fulfils this requirement for instance. Perzyna's model is deduced from Eq.…”

Section: Standard Tensile Characterization Of Elasto-visco-plasticitymentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

Avril

Sutton

et al. 2008

Mechanics of Materials

120

“…Such measurements have also been used to have better insights into strain localization in uniaxial tests [17,18]. More rarely, the full-field data have been used to identify a model using a uniform stress approach [17] or finite-element (FE) model updating [19][20][21]. It is to be noted that large strains (metals in plastic deformation or polymers) are usually measured, which is in the favourable end of the DIC usage range.…”

Section: Introductionmentioning

confidence: 99%

Beyond Hopkinson's bar

Phil. Trans. R. Soc. A.

Zhu

Siviour

2014

122

In order to perform experimental identification of high strain rate material models, engineers have only a very limited toolbox based on test procedures developed decades ago. The best example is the so-called split Hopkinson pressure bar based on the bar concept introduced 100 years ago by Bertram Hopkinson to measure blast pulses. The recent advent of full-field deformation measurements using imaging techniques has allowed novel approaches to be developed and exciting new testing procedures to be imagined for the first time. One can use this full-field information in conjunction with efficient numerical inverse identification tools such as the virtual fields method (VFM) to identify material parameters at high rates. The underpinning novelty is to exploit the inertial effects developed in high strain rate loading. This paper presents results from a new inertial impact test to obtain stress–strain curves at high strain rates (here, up to 3000 s −1 ). A quasi-isotropic composite specimen is equipped with a grid and images are recorded with the new HPV-X camera from Shimadzu at 5 Mfps and the SIMX16 camera from Specialised Imaging at 1 Mfps. Deformation, strain and acceleration fields are then input into the VFM to identify the stiffness parameters with unprecedented quality.

“…It is beyond the scope of the present paper to recall the details of these methods, information can be found in the following review paper [14]. FEMU has been applied in the past to identify the constitutive parameters of materials from full-field measurements at high strain rates [15]. However, in that work, the cost function used to identify the model parameters only included the measured and simulated impact forces, also meaning that the assumptions necessary to obtain the force from the strain gauge readings on the input bar still had to hold.…”

Section: Introductionmentioning

confidence: 99%

Exploration of Saint-Venant’s Principle in Inertial High Strain Rate Testing of Materials

Zhu

2015

Exp Mech

Current high strain rate testing procedures of materials are limited by poor instrumentation which leads to the requirement for stringent assumptions to enable data processing and constitutive model identification. This is the case for instance for the well known Split Hopkinson Pressure Bar (SHPB) apparatus which relies on strain gauge measurements away from the deforming sample. This paper is a step forward in the exploration of novel tests based on time and space resolved kinematic measurements obtained through ultra-high speed imaging. The underpinning idea is to use acceleration fields obtained from temporal differentiation of the full-field deformation maps measured through techniques like Digital Image Correlation (DIC) or the grid method. This information is then used for inverse identification with the Virtual fields Method. The feasibility of this new methodology has been verified in the recent past on a few examples. The present paper is a new contribution towards the advancement of this idea. Here, inertial impact tests are considered. They consist of firing a small steel ball impactor at rectangular free standing quasi-isotropic composite specimens. One of the main contributions of the work is to investigate the issue of through-thickness heterogeneity of the kinematic fields through both numerical simulations (3D finite element model) and actual tests. The results show that the parasitic effects arising from non-uniform through-the-thickness loading can successfully be mitigated by the use of longer specimens, making use of Saint-Venant's principle in dynamics.