Characterization of the post-necking strain hardening behavior using the virtual fields method

Some comments on “The determination of best fit linear failure envelopes to Mohr circles” by J. V. Stafford, E. Audsley and J. R. Sharp

The paper is focused on the process of identifying stress fields from strain fields in the specimen with a hole. The experiment was realized on the specimen with a hole made from anisotropic material. The main attention is paid to the analysis of deformation in the areas of stress (near the hole). That geometry generates a heterogeneous strain field which has been measured during the test using a digital image correlation system. The advantage of using heterogeneous strain fields in the identification procedure is that a complex state of stress-strain can be analyzed at the same time. On the other hand the stress field cannot be directly computed from the test and a suitable identification procedure has to be developed. Here, the virtual fields method (VFM) adapted for plastic strain has been used to identify the hardening behaviour and the anisotropy of the material. The values obtained by the VFM have been compared with the results coming from a standard identification made with uniaxial tensile tests.

This paper deals with the direct identification of parameters governing anisotropic elastic constitutive equations. These parameters are identified from heterogeneous strain fields with the virtual fields method. This method is based on a relevant use of the principle of virtual work. Different numerical aspects of the implementation of the method are discussed in the paper, mainly in terms of stability of the identified parameters when noisy data are processed. It is shown that the sensitivity of the method to noisy data is compatible with a practical use during experiments. Copyright © 2004 John Wiley & Sons, Ltd.

Special virtual fields for the direct determination of material parameters with the virtual fields method. 2––Application to in-plane properties

The accuracy of a Finite Element Simulation for plastic deformation strongly depends on the chosen constitutive laws and the value of the material parameters within these laws. The identification of those mechanical parameters can be done based on homogeneous stress and strain fields such as those obtained in uni-axial tensile tests and simple shear tests performed in different plane material directions. Another way to identify plastic material parameters is by inverse modeling of an experiment exhibiting a heterogeneous stress and strain field. Material parameter identification methods, which integrate optimization techniques and numerical methods such as the finite element method (FEM), indeed offer an alternative tool. The most common approach is to determine the optimal estimates of the model parameters by minimizing a selected measure-of-fit between the responses of the system and the model, In the present study a method is proposed for the identification of the initial yield stress, the two parameters of a Swift isotropic hardening law and the four parameters of the Hil148 yield surface, based on the full-field surface measurements of a cruciform specimen subjected to biaxial tensile loading. Experimental forces and strains are in this case compared to the simulated values. A finite element model of the perforated specimen serves as numerical counterpart for the experimental set-up. The difference between the experimental and numerical strains (e x, e y and e xy) is minimized in a least squares sense by updating the values of the different parameters simultaneously. The sensitivities used to obtain the parameter updates are determined by finite differences, using small parameter perturbations. The optimization routine used, is based on a constrained Newton-type algorithm.

A numerical study of the constitutive characterization of thermoplastic materials submitted to finite strain

Full-Field Deformation Measurements of Aluminum Plates Under Free Air Blast Loading

A growing number of observations made using geodetic approaches have been able to detect large preparatory regions that experience accelerated deformation prior to and in close proximity to an earthquake’s hypocenter. An uptick in localized seismicity has also been observed in these regions and represents an opposite end-member of the spectra of deformation, in both space and time, from the opposite broad and slow process. If and how these preparatory observations are linked are not well understood. To study this, we conducted a triaxial experiment on a granitic rock sample instrumented with calibrated acoustic emission (AE) sensors and a distributed strain sensing (DSS) method using fibre optics. These two technologies were sensitive to seismic (100 kHz to 1 MHz) and aseismic (DC to 0.4 Hz) deformation at our sample scale and these were monitored as it was loaded and experienced brittle shear failure. DSS measurement allowed us to visualize the emergence of slow, heterogeneous strain fields that localized well before the failure of the sample. In the early stages of localized deformation, the regions exhibiting preferential damage were growing and doing so without producing seismicity. However, when approaching failure, these regions accommodating slow deformation began to accelerate and now produced clusters of seismicity. The cumulative seismic moment of the precursory seismicity was a fraction of the total anelastic deformation (< 0.1%) precluding the runaway dynamic failure. We also examined the clustering and frequency-magnitude distribution of the seismicity with respect to the localized strain field. In the later stages, moments prior to nucleation, the b-value begins to drop and becomes anti-correlated to the rapidly accelerating average volumetric strain rate measured using the DSS array. This observation better constrains the hypothesis that dilation of the relatively large preparation zone can host larger precursory earthquakes therein. These findings can help constrain models that better replicate the physics associated with the large spectrum of brittle deformation and will in turn help with our understanding of preparatory earthquake processes.

Isotropic points and lines in strain fields

Isotropic points and lines in strain fields

Analysis of the deformation paths and thermomechanical parameter identification of a shape memory alloy using digital image correlation over heterogeneous tests

The use of heterogeneous strain fields for the characterization of composite materials

Anisotropic plasticity model coupled with Lode angle dependent strain-induced transformation kinetics law

Abstract. The integration of Digital Image Correlation with Infrared Thermography (DIC-IRT) in heterogeneous tests provides information rich kinematic and thermal field data. Thanks to the commercial availability of optical devices, this technique is gaining attraction to characterize the thermomechanical behavior of metals. However, achieving accurate full-field results requires a deep understanding of all the uncertainties arising from the complex measurement and post-processing chains. To this end, this paper studies the previously unexplored effect of user-dependent DIC settings on temperature fields. The solution approach relies on a Digital Virtual Twin (DVT) simulating a post-necking tensile test on a thick S700MC steel assisted with DIC-IRT. The novelty here is that the DVT combines the state-of-the-art virtual image deformation with the metrological aspect of thermal measurement. The results show that, under heterogeneous temperature and strain fields, DIC settings introduce a systematic temperature error with local biases comparable to the measurement error. According to this finding, a spatially converged displacement field is required to measure an accurate thermal field in heterogeneous tests.

Processing heterogeneous strain fields with the virtual fields method