Performance Analysis of the Mesh‐Free Random Grid Method for Full‐Field Synthetic Strain Measurements

Abstract: In responding to the needs of the material characterization community, the recently developed mesh‐free random grid method (MFRGM) has been exhibiting very promising characteristics of accuracy, adaptability, implementation flexibility and efficiency. To address the design specification of the method according to an intended application, we are presenting a sensitivity analysis that aids into determining the effects of the experimental and computational parameters characterizing the MFRGM in terms of its performance. The performance characteristics of the MFRGM are mainly its accuracy, sensitivity, smoothing properties and efficiency. In this paper, we are presenting a classification of a set of parameters associated with the characteristics of the experimental set‐up and the random grid applied on the specimen under measurement. The applied sensitivity analysis is based on synthetic images produced from analytic solutions of specific isotropic and orthotropic elasticity boundary value problems. This analysis establishes the trends in the performance characteristics of the MFRGM that will enable the selection of the user controlled variables for a desired performance specification.     

Modelling Strategies for Property Identification Based on Full‐Field Surface Displacement Data

Abstract: The use of full‐field displacement measurements in mechanical testing provides detailed response information that can be used, in conjunction with modelling and optimisation, for precise material property identification. One limitation of this technique is that the collection of response data and the sectioning of a specimen to reveal the material microstructure are both destructive tests and mutually exclusive, as the displacement measurement occurs only on the exposed surface. Therefore, modelling of an experiment to interpret a full‐field experiment requires assumptions about the structure of the material below the visible surface. This study evaluates the effects of several possible modelling assumptions on the errors in model‐predicted response and on the resulting material property estimates. A 3‐D microstructural model, for which the subsurface grain geometry and orientations are known, provides the basis for comparison of several common modelling assumptions based on the grain geometry and orientations on the visible surface of a specimen.     

Addendum to ‘Characterising the Strain and Temperature Fields in a Surrogate Bone Material Subject to Power Ultrasonic Excitation’

Recently, a very interesting article was published in Strain where a rigid polyurethane foam specimen was submitted to longitudinal vibrational excitation in the ultrasonic range. The authors showed that it was possible to measure time‐resolved strain response maps by combining digital image correlation and ultra‐high‐speed imaging. The objective of this discussion is to propose further analysis of the data published in that article, showing that it is possible to extract meaningful values for Young's modulus by using the acceleration field in the specimen as a load cell. The aim here is not to provide a complete solution to this problem but to alert the readers on the possibilities offered by this kind of test. This method is an interesting alternative where the energy is input repeatedly instead of in one go as in impact‐based tests. Full‐field vibration measurements have already been used in the past to identify stiffnesses but only in bending and at much lower strain rates. This article shows that the method can be extended to cover a much wider strain rate range. Finally, only global stiffness values were identified then, whereas here, maps of stiffnesses can be derived.     

Influence of Wooden Board Strength Class on the Performance of Cross‐laminated Timber Plates Investigated by Means of Full‐field Deformation Measurements

Although cross‐laminated timber (CLT) plates are increasingly used in high‐performance building structures, a tailored composition of them or, at least, a performance‐based classification scheme is not available. Especially, the influence of the quality of the ‘raw’ material (wooden boards) on the load carrying capacity of CLT elements is hardly investigated yet. For this reason, within this work, bending tests on 24 CLT plates consisting of wooden boards from three different strength classes have been carried out. The global mechanical response as well as the formation of failure mechanisms were investigated, including a full‐field deformation measurement system, which allowed for a qualitatively as well as quantitatively identification of board failure modes. Interestingly, no influence of the board strength class on the elastic limit load of the CLT plates was observed, but the situation was different for the load displacement history beyond the elastic regime, where basically, two different global failure mechanisms could be distinguished. The obtained knowledge about the ‘post‐elastic’ behaviour of CLT plates may serve as a basis for the optimisation of CLT products and the development or improvement of design concepts, respectively. Moreover, the obtained large ‘post‐elastic’ capacity reserve of CLT consisting of high quality boards could lead to a better utilisation of the raw material.     

Performance Assessment of a Numerical Simulation Tool for Wooden Boards with Knots by Means of Full‐Field Deformation Measurements

In wooden boards, knots and the resulting fibre deviations in their vicinities are mainly responsible for qualitative downgrading of timber elements. Thus, the development of reliable numerical simulation tools for the determination of effective strength and stiffness properties of timber elements and, in a next step, for the development and evaluation of grading criteria is highly desirable. Due to the complexity of such tools, a comprehensive validation is required. Within this work, the suitability of full‐field deformation measurements for four‐point bending tests on wooden boards with knots is evaluated first. Next, the test series is used to validate a previously developed three‐dimensional numerical simulation tool, which combines a geometrical model for the grain course and a micromechanical model for a density and moisture dependent characterisation of the clear‐wood material. The digital image correlation technique proved to be capable to reproduce the strain fields in the vicinity of knots under bending load. Moreover, a very good correlation between numerical and experimental results was obtained.     

In‐Field Measurement of Forces and Deformations at the Rear End of a Motorcycle and Structural Optimisation: Experimental–Numerical Approach Aimed at Structural Optimisation

Abstract: The present study concerns the analysis of the asymmetric displacement behaviour of the rear part of a motorcycle. Stylistic reasons led to the design of a vehicle with only one suspension located on the left‐hand side. Experimental tests performed on a circuit with seven obstacles along a straight line confirmed that the bending displacement is higher on the right‐hand side than on the left. This study aimed to perform a structural optimisation of the components involved at the rear end of the motorcycle, to find a solution to the problem. A hybrid approach is applied: the force acting on the suspension and bending displacements at the rear end were simultaneously measured in working conditions; a finite‐element method model was then set up, validated and applied for design optimisation purposes. Both methodological aspects and applicative results are presented and discussed. Finally, a solution in accordance with design specifics is proposed.     

A Review of the Challenges and Limitations of Full‐Field Measurements Applied to Large Heterogeneous Deformations of Rubbers

Abstract: This paper presents an overview of the use of full‐field measurement techniques, more precisely digital image correlation (DIC) and coupled DIC and infrared thermography, for the material and structure characterisation of rubber reported in the literature. Even though such techniques have increasingly been applied for approximately 30 years for moderate deformations in metal and composite materials, they are still under‐employed in the measurement of full kinematic and thermal fields in the case of large deformations undergone by rubber materials. To date, the applications addressed are crack propagation at both macroscopic and microscopic scales, model validation and constitutive parameter identification.     

The Grid Method for In‐plane Displacement and Strain Measurement: A Review and Analysis

The grid method is a technique suitable for the measurement of in‐plane displacement and strain components on specimens undergoing a small deformation. It relies on a regular marking of the surfaces under investigation. Various techniques are proposed in the literature to retrieve these sought quantities from images of regular markings, but recent advances show that techniques developed initially to process fringe patterns lead to the best results. The grid method features a good compromise between measurement resolution and spatial resolution, thus making it an efficient tool to characterise strain gradients. Another advantage of this technique is the ability to establish closed‐form expressions between its main metrological characteristics, thus enabling to predict them within certain limits. In this context, the objective of this paper is to give the state of the art in the grid method, the information being currently spread out in the literature. We propose first to recall various techniques that were used in the past to process grid images, to focus progressively on the one that is the most used in recent examples: the windowed Fourier transform. From a practical point of view, surfaces under investigation must be marked with grids, so the techniques available to mark specimens with grids are presented. Then we gather the information available in the recent literature to synthesise the connection between three important characteristics of full‐field measurement techniques: the spatial resolution, the measurement resolution and the measurement bias. Some practical information is then offered to help the readers who discover this technique to start using it. In particular, programmes used here to process the grid images are offered to the readers on a dedicated website. We finally present some recent examples available in the literature to highlight the effectiveness of the grid method for in‐plane displacement and strain measurement in real situations.     

Identification of Johnson–Cook's Viscoplastic Model Parameters Using the Virtual Fields Method: Application to Titanium Alloy Ti6Al4V

Abstract: The identification of viscoplastic material parameters is addressed using a new powerful method: the virtual fields method (VFM). Contrary to classical procedures that are statically determined, the VFM is applied to heterogeneous mechanical fields. Without any hypotheses of homogeneity required, the exploitation of tests with the VFM is not limited to small levels of strains anymore and it can be taken advantage of the large amount of information available thanks to full‐field measurements. In the case of viscoplastic models, the characterisation of strain‐rate sensitivity with the VFM is attempted in this paper using only one test under high‐speed loading conditions, whereas several tests performed at different constant strain‐rates are required for the classical procedures. This article focuses on the development of the VFM for the characterisation of Johnson–Cook's (JC) viscoplastic model. To his aim a return‐mapping algorithm was developed according to the JC's model with an implicit Euler scheme implemented to integrate the constitutive relations. The whole viscoplastic behaviour of a Titanium alloy (Ti6Al4V) is successfully characterised by the VFM using only two tensile tests on notched flat specimens, with full‐field strain measurements by digital image correlation.     

Digital Image Correlation and Noise‐filtering Approach for the Cracking Assessment of Massive Reinforced Concrete Structures

This paper describes the use of Digital Image Correlation (DIC) techniques for the cracking assessment of reinforced concrete (RC) massive beams and walls. DIC is known to provide accurate and detailed information on displacement and strain fields. Non‐contact measurements can be used to evaluate concrete cracking of destructive tests carried out on a wide range of specimen scales. When applied to large RC structures tested outdoors or in difficultly controllable conditions, DIC‐based methods may lead to erroneous results. In this study a post‐processing procedure is presented to cope with noisy full‐field measurements. The proposed cracking assessment approach is validated on a large experimental campaign. Four points bending tests are carried out on RC beams: firstly on full‐scale rectangular beams and then on mock‐ups scaled down by 1/3. In addition, fours RC walls are tested under in‐plane cyclic shear up to failure. Digital images taken throughout the tests are processed by DIC techniques to provide in‐plane displacement and strain fields. Full‐field measurements are post‐processed by the noise‐filtering technique and the cracks patterns are identified. Crack widths are measured and compared with measurements obtained from conventional point‐based sensors (linear variable differential transformer LVDT and fibre‐optic FO transducers). The proposed DIC‐based post‐processing provides accurate estimation of cracks width for most of the tests. The analyses carried out on the two groups of RC beams show a scale‐effect on the cracks width.