1,721,195 research outputs found
A Fourier-series-based virtual fields method for the identification of three-dimensional stiffness distributions and its application to incompressible materials
Full text linkThis is the peer reviewed version of the following article: Nguyen, TT and Huntley, JM and Ashcroft, IA and Ruiz, PD and Pierron, F (2017) A Fourier-series-based virtual fields method for the identification of three-dimensional stiffness distributions and its application to incompressible materials. Strain, 53 (5). e12229-e12229 which has been published in final form at 10.1111/str.12229 This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving."
We present an inverse method to identify the spatially varying stiffness distributions in 3 dimensions. The method is an extension of the classical Virtual Fields Method—a numerical technique that exploits information from full-field deformation measurements to deduce unknown material properties—in the spatial frequency domain, which we name the Fourier-series-based virtual fields method (F-VFM). Three-dimensional stiffness distributions, parameterised by a Fourier series expansion, are recovered after a single matrix inversion. A numerically efficient version of the technique is developed, based on the Fast Fourier Transform. The proposed F-VFM is also adapted to deal with the challenging situation of limited or even non-existent knowledge of boundary conditions. The three-dimensional F-VFM is validated with both numerical and experimental data. The latter came from a phase contrast magnetic resonance imaging experiment containing material with Poisson's ratio close to 0.5; such a case requires a slightly different interpretation of the F-VFM equations, to enable the application of the technique to incompressible materials
Discussion of the article: Biaxial Testing of Unidirectional Carbon-Epoxy Composite Using Biaxial Iosipescu Test Fixture
No full textSaint-Venant effects in the Iosipescu specimen
No full textThe main purpose of this paper is to explain the shear strain differences observed between the two faces of certain Iosipescu specimens. The different parasitic moments that can act on the specimen are investigated and their effects assessed. Different Iosipescu fixtures from the literature are checked for these parasitic moments and it is shown that modification of the Wyoming fixture can lead to the elimination of its parasitic in-plane and out-of-plane movements. Finally, it is shown that the strain differences that remain are caused by Saint-Venant effects due to the proximity of the loading points to the notch line but that averaging the strains between the two faces eliminates this effect. This is an important feature since for certain material configurations (thick and transversely stiff specimens), preventing twisting of the fixture is not sufficient to enable the use of only one strain gauge. <br/
Exploration of Saint-Venant’s Principle in inertial high strain rate testing of materials
No full textCurrent 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
Heat dissipation measurements in low stress cyclic loading of metallic materials: from internal friction to micro-plasticity
No full textThe present paper deals with a procedure to measure the very small quantities of heat generated during the very first cycles of mechanical loading on metallic specimens. A dedicated procedure is presented to reach the required sensitivity in terms of heat sources with some original features with respect to the present state of the art, assuming that the heat sources are uniformly distributed within the specimen as such low strain levels. Then steel (cold rolled and annealed) and aluminium specimens were tested at two different aspect ratios (R?=0.1 and R?=-1). It was shown that for small stress levels, the heat generated was more or less constant with cycle number. This energy was associated to visco-elastic behaviour (internal friction) and it was shown that the same energy could be measured with the two stress ratios when plotted against strain rate (stress amplitude at constant frequency, here 15 Hz). Then, after a certain stress level, an initial outburst of energy was evidenced over about 10–15 cycles before a constant level was reached again. This was associated to micro-plastic adaptation. This procedure provides an advanced tool to tackle the problem of very early fatigue damage detection and is aimed at providing some physical justification to the procedures of rapid fatigue limit detection by self-heating tests.<br/
Identification of the plastic behaviour in the post-necking regime using a three dimensional reconstruction technique
No full textTowards the design of a new standard for composite stiffness identification
No full textThis paper presents a step towards the design of a novel test for simultaneous identification of all the stiffness components of orthotropic composite materials. A simulator was adopted to numerically simulate the whole identification process. Synthetic images were generated and then processed by Digital Image Correlation (DIC) to calculate the strain fields. The Virtual Fields Method (VFM) was used to identify the material stiffness parameters and error functions were finally defined to evaluate the identification error. Two steps of optimization were applied to obtain the best design variables of different specimens and the optimal DIC processing parameters. Four types of test configuration were simulated including short off-axis tensile test, short off-axis open-hole tensile test, off-axis Brazilian disc and off-axis unnotched Iosipescu test and the most promising configuration was identified
Identification of the through-thickness moduli of thick composites from whole-field measurements using the Iosipescu fixture: theory and simulations
No full textThis paper presents a method to measure the four through-thickness moduli of a thick coupon subjected to a shear-bending load obtained with the Iosipescu fixture. The identification method relies on the global equilibrium of the specimen written with the principle of virtual work with different virtual fields. It requires the measurements of the in-plane strains over the whole surface of the specimen. The choice of a set of virtual fields is presented and validated using finite element simulated measurements. The stability of the method is checked by adding simulated experimental noise. The stability is found to be compatible with experimental implementation except for Poisson's ratio.<br/
- …
