Showing papers in "Strain in 2007"

Volumetric Digital Image Correlation Applied to X-ray Microtomography Images from Triaxial Compression Tests on Argillaceous Rock

Abstract: A set of triaxial compression tests on specimens of argillaceous rock were performed under tomographic monitoring at the European Synchrotron Radiation Facility in Grenoble, France, using an original experimental set-up developed at Laboratoire 3S, Grenoble Complete 3D images of the specimens were recorded throughout each test using X-ray microtomography Such images were subsequently analysed using a Volumetric Digital Image Correlation software developed at the Laboratoire de Me?canique des Solides in Palaiseau, France Full-field incremental strain measurements were obtained, which allow to detect the onset of shear strain localisation and to characterise its development in a 3D complex pattern Volumetric Digital Image Correlation revealed patterns which could not be directly observed from the original tomographic images, because the deformation process in the zones of localised deformation was essentially isochoric (ie without volumetric strain), hence not associated to density changes

Stress intensity factor gauging by digital image correlation: application in cyclic fatigue

Abstract: A fatigue crack in steel (CCT geometry) is studied via digital image correlation. The measurement of the stress intensity factor (SIF) change during one cycle is performed using a decomposition of the displacement field onto a tailored set of elastic fields. The same analysis is performed using two different routes, namely, the first one consists in computing the displacement field using a general correlation technique providing the displacement field projected onto finite element shape functions, and then analysing this displacement field in terms of the selected mechanically relevant fields. The second strategy, called integrated approach, directly estimates the amplitude of these elastic fields from the correlation of successive images. Both procedures give consistent results, and offer very good performances in the evaluation of the crack tip position (uncertainty of about 20 μm for a 14.5-mm crack), SIFs (uncertainty <1 MPa ) and opening properties.

Failure of Composite Materials

Abstract: The failure of composites has been investigated extensively from the micromechanical and macromechanical points of view. On the micromechanical scale, failure mechanisms and processes vary widely with type of loading and are intimately related to the properties of the constituent phases, i. e., matrix, reinforcement, and interface-interphase. Failure predictions based on micromechanics, even when they are accurate with regard to failure initiation at critical points, are only approximate with regard to global failure of a lamina and failure progression to ultimate failure of a multi-directional laminate. For these reasons a macromechanical approach to failure analysis is preferred.

Identification of the Orthotropic Elastic Stiffnesses of Composites with the Virtual Fields Method: Sensitivity Study and Experimental Validation

Abstract: This paper presents the use of a sensitivity study to find the best combinations of free length and fibre angle in an unnotched Iosipescu test processed with the virtual fields method. The sensitivity to noise coefficients arising from the special virtual fields procedure are used to build up a cost function. This function is aimed at balancing out the coefficients of the different orthotropic stiffnesses so that the same confidence level can be reached on all these parameters. Then, experimental validation was performed using a speckled interferometry (ESPI) system. Full-field strains were measured and stiffnesses identified and compared between the usual 0°, 30 mm configuration and an improved 25°, 40 mm configuration. The outcome of the optimisation was confirmed by testing the same specimen several times and comparing scatter between the two configurations. This is a first promising result on the way to the design of a new test for orthotropic stiffness identification on a single specimen from full-field measurements.

Thermographic Analysis of Fatigue Dissipation Properties of Steel Sheets

Abstract: This paper presents several properties of heat source fields accompanying the high-cycle fatigue of a dual-phase steel grade. Heat sources were derived from thermal data provided by an infrared focal plane array camera. An especially developed image processing estimates separately the thermoelastic coupling source amplitude and the mean dissipation per cycle. Our experiments underline that dissipation sources are heterogeneous and this forms the beginning of the fatigue test. They also point out a linear evolution of the mean dissipation per cycle as a function of the loading frequency for a given stress range and a given loading ratio.

3D Strain Field Measurement by Correlation of Volume Images Using Scattered Light: Recording of Images and Choice of Marks

Abstract: We have extended the Digital Image Correlation technique to the case in three dimensions. This new technique, allowing the full three-dimensional (3D) strain measurement in the bulk of a solid, needs volume images containing a 3D variation of the grey levels. Generally, volume images are obtained by X-ray computed tomography. In this paper, we present a procedure that is easier to implement and enables to generate volume image in transparent materials. The principle consists in the optical slicing of the specimen. To obtain a random distribution of grey levels within the volume image, we use the scattered light phenomenon induced by particles included in the specimen. The recording of 3D images by optical slicing is presented and the influence of different kinds of particles on the scattered light and on the accuracy of measurement is described. Through several tests involving rigid body displacements and a tensile test we show the performance of this technique and we evaluate the measurement error of displacement and strain components.

Image Processing to Estimate the Heat Sources Related to Phase Transformations during Tensile Tests of NiTi Tubes

Abstract: Temperature and strain fields have been observed during superelastic tensile tests on NiTi tubes. They show strong localisations that take the shape of helical bands with characteristics that depend on global tensile strain and strain rate. To obtain quantitative energy information, allowing a better recognition of the deformation mechanisms involved inside and outside the helical bands, an estimation of the local heat sources based on image processing of the temperature fields is proposed in the present paper. This processing method is first calibrated and validated on numerically simulated temperature fields calculated using theoretical heat sources close to those associated with martensitic phase transformation. It is then applied to experimentally observed temperature fields during a typical superelastic tensile test of an NiTi tube.

Application of Response Surface Methodology in Describing the Residual Stress Distribution in CO2 Laser Welding of AISI304

Abstract: Laser welding is a low-distortion welding process. Despite this advantage some detrimental residual stresses are still introduced during the process, which reduce the service life of the welded component. Therefore, it is important to estimate the magnitude and distribution of the residual stress. This study aims to create mathematical models to determine the relationship between laser welding parameters and the magnitude of the residual stress at different locations by using response surface methodology. In this study, the process input parameters are laser power, travel speed and focal point position. Laser butt-welding of AISI304 plates joints were investigated using a 1.5 kW continuous wave CO2 Rofin laser as a welding source. Incremental hole-drilling method was employed to measure the magnitude and the distribution of the maximum residual stress. The experiment was developed using a three-factor, five-level central composite design with full replication. Analysis of variance and other adequacy measures were employed to check the adequacy of the developed models using the statistical package Design-Expert. Twenty-one models were developed to demonstrate the magnitude and distribution of the residual stress. The main effect of each factor and the interaction effect with other factors were determined quantitatively and presented graphically.

Modern Photoelasticity for Residual Stress Measurement in Glass

Abstract: Residual stress is one of the important indicators of the quality of any glass product. The paper gives a review of the methods, which are nowadays used in glass industry for measuring residual stress both in architectural and automotive glass panels and in hollow glassware.

Application of Stereovision to the Mechanical Characterisation of Ceramic Refractories Reinforced with Metallic Fibres

Abstract: Optical methods that yield displacement or strain fields are now emerging significantly in the mechanical sciences. At the Research Center on Tools, Materials and Forming Processes (CROMeP) at Ecole des Mines d'Albi, a binocular stereovision system has been developed that can be used to measure: (a) the three-dimensional (3D) shape of a static object, or (b) the surface strains of an object undergoing some 3D mechanical or thermal stress. In this paper, the application of the stereovision technique to investigate the behaviour of ceramic refractories reinforced with metallic fibres is presented: (i) after the rupture of the sample for assessing the 3D orientation of the fibres in order to correlate a micro-mechanical model of fibre pullout with the macro-mechanical results of tensile tests, and (ii) during a tensile test for measuring the 3D displacement/strain field around a notch.

Experimental and numerical investigation of mechanical and thermal residual strains in adhesively bonded joints

Abstract: This paper describes an investigation of residual and mechanical strains in aluminium/aluminium (Al/Al) and aluminium/carbon fibre-reinforced polymer (Al/CFRP) adhesively bonded double-lap joints. Residual strains were measured inside the adherends by means of neutron diffraction (ND) and modelled using finite element analysis (FEA). In the Al/Al joints the measured residual strains were negligible, showing good agreement with FE predicted results. However, considerable strains developed in the Al/CFRP joint because of differential thermal contraction of the two materials during joint manufacture. Although considerable scatter was seen in the ND results, the measured and predicted trends showed similar behaviour and were of comparable magnitude. The paper also reports measurements of internal strains in an Al/CFRP joint under tensile load using ND and of surface strains using moire interferometry (MI). In general, good agreement was observed between FE predictions, surface strains measured with MI and internal strains measured with ND for the loaded Al/CFRP joint.

A Review of Dynamic Fracture Studies in Functionally Graded Materials

Abstract: This article presents a review of dynamic fracture studies on functionally graded materials. A brief literature review on the fracture mechanics of graded materials is presented first. This is followed by a discussion on the higher-order asymptotic analysis of the transient elastic field surrounding the tip of a dynamically growing crack in a functionally graded material. A comprehensive experimental study of dynamic crack growth in model functionally graded material using the optical method of reflection photoelasticity and high-speed photography is then presented. The results are analysed to establish a generalised relationship between the crack velocity and the dynamic mode-I stress intensity factor (SIF). This relationship is found to be unique and is distinctly different from that previously established for the matrix material (polyester). Finally, an innovative experimental procedure is used to demonstrate the necessity of employing a fully transient stress-field representation in the analysis of optical data for an accurate prediction of the dynamic SIF history.

Stiffness and Reinforcement Effect of Electrical Resistance Strain Gauges

Abstract: The increasing use of low-modulus materials, on which the reinforcement effect of the electrical resistance strain gauge is not negligible, has re-opened the research interest into this issue. This study deals with the evaluation of stiffness, and of the strain gauge Young's modulus involved in the estimation of both the global and the local reinforcement effect; the relationship between the strain gauge stiffness and the local reinforcement effect is also analysed. In particular, the experimental technique used to determine the stiffness of some commercial strain gauges is described. The results show that the strain gauge stiffness alone does not permit an accurate evaluation of the local reinforcement effect.

Damage detection using stress waves and multivariate statistics, an experimental case study of an aircraft component

Abstract: The work will focus on generating statistical models of the normal condition based on the ultrasonic-guided wave response of the selected specimens and testing for novelty in subsequently acquired signals. Novelty refers to any computed Mahalanobis squared distance that exceed the appropriate threshold value diagnosed as having the damaged state of the structure. While the study proved extremely successful, with cuts of 1 mm diagnosed unambiguously, it was mainly of interest from the point of algorithm development as the specimen concerned bore only a vague resemblance to the sort of components which would comprise a real structure.

A Challenge for High-Performance Full-Field Strain Measurement Systems

Abstract: The advent of digital imaging technology and huge increases in data-processing power have led to enormous advances in optical techniques for evaluating strain fields. A standardised test material designed for evaluating the capabilities of the most sophisticated optical systems using these techniques is described. The standardised test material presents a significant challenge to optical techniques for full-field strain evaluation by employing complicated and reproducible strain fields for which analytical solutions are available. The design philosophy and process employed in developing the standardised test material are described and sample results are presented. The relationship between a standardised test material and reference material for calibration is discussed.

Mechanical Property Measurement at the Micro/Nano Scale

Abstract: There are three main challenges in measuring the mechanical properties of specimens with minimum dimensions on the order of microns or less — preparation and handling, force application and measurement, and strain measurement. This is a brief overview of the various test methods with emphasis on tensile testing because it produces a uniform stress and strain field, which enables unambiguous property measurements.

An Experimental Study on Wrinkling Behaviours and Characteristics of Gossamer Space Structures

Abstract: Ultra-large and ultra-lightweight gossamer space structures have received wide attention because of their small packaging volume and low cost. Wrinkle is the common defor- mation outcome and the main failure mode of such structures. The experimental test is essential for the wrinkling analysis, as contact test cannot be used to accurately determine wrinkling behaviour because of lightweight and flexibility. A novel non-contact dot-printing photogrammetry (DPP) based on the technology of the printed-dot targets is presented to measure wrinkling characteristics. The characteristics of wrinkle development of gossamer space structures are obtained by incor- porating the DPP method and a tension wrinkling (TW) test. Results reveal that wrinkle formation is the predominant form of bifurcation when the membrane is undergoing in-plane displacement. Over-contraction deformation is the special characteristic of the wrinkles, which reflects the degree of wrinkling and region in the wrinkled membrane. The development of wrinkling can be divided into five characteristic phases, and the occurrence of the wrinkles is followed by an abrupt jump vari- ation. DPP measurement and TW test are the effective ways to obtain accurately the wrinkling behaviours and characteristics in small-scale gossamer structures. Finite element results based on nonlinear post-wrinkling analysis are used to validate the experimental test. The numerical predic- tions agree well with the experimental results.

Burst Pressures for Torispheres and Shallow Spherical Caps

Abstract: Burst pressures are computed for shallow spherical caps and torispheres loaded by uniform static pressure. They are defined as maximum pressures at which proposed plastic strain criteria are met. The criteria are taken from true stress–true strain curves of uniaxial tensile tests. Experimental tests, carried out on pressure vessel components made from aluminium alloy 6061 and mild steel, provide experimental results against which the proposed criteria for burst pressures are benchmarked.

Maximum Stresses in a Taper‐shanked Round‐ended Lug Loaded by an Oblique Concentrated Force*

Abstract: In this study, the stress state in a round-ended, taper-shanked lug loaded by an oblique point force is investigated. Numerical, analytical and, partially, photoelastic studies were carried out. In the analytical model, the lug is modelled as a plane problem, and it is partially likened to a ring examined with a Michell-type series solution; the differences in shape between lug and ring are overcome by endowing the series solution with degrees of freedom suitable for adapting the ring solution to the lug actual geometry. Three prompt access design charts are presented, which report the normalised stress concentration for three lug shank taper angles, for selection of five aspect ratios between bore radius and radius of the rounded extremity, and for load inclinations ranging from longitudinal to transverse directions.

Three-Dimensional Displacement Analysis by Windowed Phase-Shifting Digital Holographic Interferometry

Abstract: Digital holography is useful to reconstruct three-dimensional shapes of objects. The reconstruction process of the objects is fast and accurate because the reconstruction is performed by computer calculation without developing photographic plate. Phase-shifting digital holographic interferometry is possible to measure shape and displacement of objects quantitatively. The authors developed windowed phase-shifting digital holographic interferometry (WPSDHI) which provides very accurate results by decreasing the effect of speckle noise.

Large-Deformation Constitutive Theories for Structural Composites: Rate Dependent Concepts and Effect of Microstructure

Abstract: Continuous-fiber reinforced structural composite materials are typically classified as “brittle,” with strains to break in the principal fiber directions of only a percent or so. However, fiber architectures and material choices provide“ ductility” that is highly dependent on microstructure. Woven glass-reinforced polymer systems, for example, with strains to break on the order of 20 percent for uniaxial loading in off-axis directions are now in common use in a variety of applications, including Naval structures. An example of such behaviour for three different strain rates is shown in Fig. 1.

New Initiatives on Comparison of Whole-field Experimental and Numerical Results

Abstract: A simple approach to plot photoelastic fringes in grey scale and also in colour from finite element (FE) results is presented for better recognition and comparison with experiments. This requires proper identification of the plotting variable from FE results. For comparison with transmission photoelasticity, post-processing of principal stress difference is needed and for reflection photoelasticity the principal strain difference is to be used. The importance of the use of appropriate correction factors for comparison with reflection photoelastic results is emphasised. A newer approach to evaluate Rf for complicated geometries is indicated. Plotting of experimental fringes from finite elements is useful not only for validating the numerical model based on experiments but also for validating the experiments. To illustrate this, the problem of an interfacial crack in a bi-material Brazilian disc is discussed.

Micro/Nano Speckle Method With Applications to Material, Tissue Engineering and Heart Mechanics

Abstract: Using a random speckle pattern as a quantitative transducer for displacement and deformation measurement constitutes a major milestone in the advancement of experimental mechanics techniques. Heretofore, displacement or strain transducers or gages utilize regular patterns as the measurement base. It is the dimensional changes of these bases that are converted into displacement/strain information. The speckle method, on the other hand, uses a random array of particles (either physical or virtual) as displacement transducers. The displacement information of a cluster of random particles is obtained either through correlation calculations with its neighbors or a Fourier transform processing scheme. It is the latter approach that will be discussed in this paper.

Progress in Microelectromechanical Systems

Abstract: Recent advances in microelectromechanical systems (MEMS) technology have led to the development of a multitude of new devices heretofore impossible. However, applications of these devices are still hampered by challenges posed by their integration and packaging. The current trend in micro/nanosystems is to produce ever smaller, lighter and more capable devices at a lower cost than ever before. In addition, the finished products have to operate at very low power and in very adverse conditions while ensuring durable and reliable performance. Some of the new devices have been developed to function at high operational speeds, and others to make accurate measurements of operating conditions of specific processes. Regardless of their applications, the devices have to be packaged to facilitate their use. MEMS packaging, however, is application-specific and, usually, has to be developed on a case-by-case basis. To facilitate advances of MEMS, educational programmes have been introduced addressing all aspects in their development. This paper addresses progress in MEMS by presenting pertinent aspects in a development of MEMS including, but not limited to, design, analysis, fabrication, characterisation, packaging, and testing. This presentation is illustrated with selected examples.

Experimental Mechanics at the Nanometric Level

Abstract: Recent advances in the science and technology have originated the need of developing new tools to make observations at the molecular level. Nano-sciences and technology provide a great challenge to experimental mechanicians and at the same time extend the horizons of the discipline to new grounds and novel problems. Of the different disciplines that are required to meet the challenges presented by nano-sciences and technology, optics is a tool that seems particularly adapted for this task. However, classical optics puts a limit to the possibility of making measurements beyond half of the wavelength of light: that is, a couple of hundreds of nanometers at best. However, this quantity is not good enough to perform studies in the nanometric range.

Photomechanics in Dynamic Fracture and Friction Studies

Abstract: Studies of fracture and friction under dynamic loading conditions have benefited enormously from the use of optical methods of diagnosfis. Specifically, photoelasticity and high-speed photography have provided methods of detailed characterisation of these phenomena. In this paper, we present the results from experiments aimed at investigating fracture and frictional sliding under shear loading conditions.