Application of digital-image-correlation techniques in analysing cracked cylindrical pipes

Cracks induced by external excitation on a material that has defects may generate the stress concentration phenomenon. The stress concentration behaviour causes local buckling, which will induce the damage of the members made of this material. Thus, developing techniques to monitor the strain variation of a cracked member is an important study. The traditional technique (such as strain gauge) can only measure the average strain of a region. The strain variation within this region cannot be determined. Therefore, it cannot sufficiently reflect the mechanical behaviour surrounding the crack. The Digital image correlation technique recently developed is an image identification technique to be applied for measuring the object deformation. This technique is capable of correlating the digital images of an object before and after deformation and further determining the displacement and strain field of an object based on the corresponding position on the image. In this work, this technique is applied to analyse the mechanics of a cylindrical pipe experiencing crack destruction. The fixing device is used to avoid shaking the specimen during the pressurizing process. The image capture instruments are fixed on the stable frame to measure the deformation of specimen accurately. Through the cylindrical pipe cracking test, the capacity of the digital image correlation technique for surveying the strain variation in a tiny region is validated. Then, the experimental results obtained using the digital image correlation analysis is used to demonstrate the crack development tendency in defect materials and the stress concentration zone.     

A numerical and experimental study of the yield surfaces in cross shaped aluminium plates

The behaviour of metals can be described through knowledge of its yield surfaces and constitutive relationships. Starting from this point, numerical and experimental analyses were developed about the yield surface evolution in aluminium cross shaped sheets subjected to incremental hardening. The results of using both the Szczepinski & Miastkowski experimental technique and an F.E.M. code for solving non-linear stress and strain problems, which uses a macro-programming language, were compared. The influence of the chosen offset and of the loading path on the shape and evolution of the yield surfaces were analysed. The determination of such surfaces and their evolution with the increase in plastic deformation is still of great importance.     

Translational motion of a polymer gel microrod via electrically induced wave propagation

The aim of the present study relies on the potential to control the motion of a polymer gel microrod by means of a suitable electric field. It is well known that a polyelectrolyte gel can change its conformation under the effects of an electric field. Inducing a local deformation in a gel rod, it is possible to propagate the deformation along the gel rod itself, shjfting it in the same direction as the electric field. This technique, which can be viewed as a sort of wave propagation method, can be used to perform a controlled translational motion of the whole gel body.A simple device has been designed to provide the desired application of the electric field and to study the gel microstructure response. Experimental measurements characterizing the main electro-kinetic properties of the polymer gel have been carried out, and the related results used in the simulation of the gel motion. Finally, real motion experiments have been performed.Both theoretical and experimental results show that this novel technique can induce and control the translational motion of a polymer gel microrod, but that a high degree of miniaturization of the system is still required to achieve reliability and performance necessary in technological and scientific applications.     

Adaptive strain estimation using retrospective processing [medical US elasticity imaging

Because errors in displacement and strain estimates depend on the magnitude of the induced strain, the strain signal-to-noise ratio (SNR) will be a function of the applied deformation. If deformation is applied at the body surface, it is difficult during data acquisition to select a single surface displacement providing the highest strain SNR throughout the image. By applying continuous deformation and capturing data in real-time, the surface displacement providing the highest strain SNR can be selected retrospectively. A method to adaptively optimize strain SNR over the image plane using retrospective processing is presented and demonstrated with experimental results.     

Viscoelastic Stress Analysis of a Strip Plate under Moving Contact with Dry Friction

Moving contact problems in a viscoelastic body with a rigidindentor are often seen in an industrial field. An evaluation of thetime-temperature-dependent stress and strain behavior around a contactregion is required in order to make clear the fundamental mechanism ofthe local fracture and wear on the contact surface of the viscoelasticbody under moving loads with dry friction. No analyses have yet beenpublished about the stress/strain of the viscoelastic moving contactproblem with the dry friction using both an experimental and a numericalmethod. The authors discuss an experimental and a numerical model forthe analysis of not only the viscoelastic stress and strain, but alsodeformations, taking into consideration the dry friction. Animage-processing-aided photoviscoelastic technique is applied foranalyzing the principal stress and strain behavior near the contactregion. Also, a two-dimensional plane stress model which consists of aviscoelastic strip and a rigid sliding cylinder is adopted in a finiteelement analysis of the same problem. The time-dependent stress andstrain and the coefficient of dry friction are successfully evaluated byexperimental and numerical methods.     

A global collocation technique for the solution of opening mode crack problems

A review of various experimental and numerical techniques for determination of fracture mechanics calibration functions (i.e., the variation of K ₁ and CMOD with crack length) revealed that neither technique, employed independently, can determine K ₁, CMOD, and full field stresses in closed form over a wide range of crack lengths. To fill this void, a combined experimental/numerical collocation technique based on a series expansion of the modified Westergaard functions was developed. This technique uses both boundary conditions, known a priori, and interior stress field conditions, determined using a suitable experimental technique, for analysis of two dimensional, finite body, opening mode crack problems. This paper reports on an investigation of the accuracy of this technique and its sensitivity to errors in experimental data for a sample problem of practical interest.     

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.     

Displacement discontinuity method applied to nondestructive testing related stress problems

A numerical stress analysis technique using the displacement discontinuity method (DDM) is used to evaluate surface breaking and near surface flaws. DDM is described briefly, with the main body of the work devoted to exploring the use of DDM in a nondestructive evaluation (NDE) program. DDM is a subset of boundary element method technique, and it requires that only the boundaries of the problem be described. Two smooth subsurface voids are investigated. Results for two-dimensional circular and elliptical holes are presented. Comparisons with experimental and theoretical work are shown. The method of uneven element spacing is explored. The results are examined in the context of providing information for NDE inspection requirements and the definition of acceptable flaws. DDM is very well suited for examining the stress field around a surface crack. Surface cracks can be very dangerous and good NDE inspection techniques are required. A method of obtaining the stress intensity factors is developed and illustrated on angular surface breaking cracks. The results are compared with limited experimental and numerical results and interpreted in terms of NDE inspection requirements.     

Determination of surface heat-transfer coefficients of steel cylinder with phase transformation during gas quenching with high pressures

In numerical simulation of quenching process, the boundary conditions of temperature field and stress field are very important, in which the boundary conditions of temperature field are very complicated. In order to simulate the thermal strains, thermal stresses, residual stresses and microstructure of the steel during gas quenching by means of the numerical method, it is necessary to obtain an accurate boundary condition of temperature field. The surface heat-transfer coefficient is a key parameter. The explicit finite difference method, non-linear estimate method and the experimental relation between temperature and time during quenching have been used to solve the inverse problem of heat conduction. The relationships between surface temperature and surface heat-transfer coefficient of cylinder have been given. The non-linear surface heat-transfer coefficients include the coupled effects between phase transformation and temperature. In calculation, physical properties were treated as the function of temperature and volume fraction of constituent. The results obtained have been shown that this technique can determine effectual the surface heat-transfer coefficients during gas quenching.     

Investigations on different fatigue design concepts using the example of a welded crossbeam connection from the underframe of a steel railcar body

For thin-walled steel structures, welded joints often turn out to be the weak spots under cyclic loading. For the dimensioning of welded safety components in vehicle construction, strength concepts are necessary that are well-defined and well-reviewed, in terms of their predictive accuracy.Within the scope of the research results presented here, the applicabilities of the nominal, structural and notch stress approaches have been examined on the basis of different arc welded and cyclic loaded steel structures, taken from the railway sector. In detail, this is a crossbeam connection from the underframe of a railcar body. Different sheet thicknesses in the range 2.7 mm-4 mm in combination with a misalignment lead to an increase of load in the region of the weld seams. Several different components and specimens with critical regions of failure have been tested under cyclic loading with constant amplitude. With the help of strain gauges, the (technical) crack initiation has been determined. A short review of the possibility of obtaining information about the crack initiation by ultrasonic-burst-phase-thermography is also given.The specimens were the basis for the application and evaluation of the different concepts for the assessment of fatigue life. The numerical determination of the nominal, structural and notch stresses has been performed with finite-element models. For a local approach, according to the notch stress analysis, a submodelling technique has been used. The FE-models have been compared with the experimental data by means of optical 3D deformation analysis and strain measurements with strain gauges. Existing weld seam and specimen tolerances have been included through the use of parametric models. Finally the experimental and computational results have allowed the derivation of structural and notch S-N curves for the crack initiation and the rupture of the specimen.     

Motion estimation of elastic articulated objects from image contours

Abstract

This paper presents a new model of elastic articulated objects based on revolving conic surface. The model includes three-dimensional object skeleton and deformable surfaces that can represent the deformation of human body surfaces. In each limb, surface deformation is represented by adjusting only two deformation parameters. Then, the deformation parameters are determined by corresponding two-dimensional image contours from a sequence of stereo images. The three-dimensional motion parameters are estimated based on the skeleton points. The algorithm presented in this paper includes skeleton-based parameter estimation of motion and parameter determination of deformable surfaces.     

Hyperelastic Behaviour Identification by a Forward Problem Resolution: Application to a Tear Test of a Silicone-Rubber

Abstract:  The mechanical behaviour of synthetic rubbers shows very high deformability, compressibility, time-dependent effect and strain softening. The present study is devoted to the analysis of local mechanical behaviour of silica-filled silicone rubber. New testing and identification are proposed in this paper by using standardised tear test, kinematic field measurements and a numerical inverse problem resolution to investigate localisation strain phenomena. The experimental procedure described hereafter, is based on strain field measurements using digital image processing. In-plane kinematic measurements by the digital image correlation are suitable to analyse non-homogeneous mechanical tests performed especially on thin sheets: indeed, rubber-like materials are characterised by a very high deformability and a non-linear behaviour leading to important gradients of deformation. The identification procedure is conducted in two steps. First, parameters of the viscosity and stress softening (Mullins effect) are evaluated analytically by using axial and biaxial tensile tests. Then, hyperelastic parameters are identified by an inverse resolution based on standardised tear tests. The mechanical model is implemented into the finite element code Zebulon (Transvalor/ENSMP). The numerical model is built up by using informations on geometry and boundary conditions extracted from image sequence that were acquired during the test. Usage of different functions evaluating the distance between computed and experimental quantities (cost functions) in a minimisation process is discussed.     

An approximated computational method for fast stress reconstruction in large strain plasticity

This article describes a numerical method to reconstruct the stress field starting from strain data in elastoplasticity. Usually, this reconstruction is performed using the radial return algorithm, commonly implemented also in finite element codes. However, that method requires iterations to converge and can bring to errors if applied to experimental strain data affected by noise. A different solution is proposed here, where an approximated numerical method is used to derive the stress from the strain data with no iterations. The method is general and can be applied to any plasticity model with a convex surface of the yield locus in nonproportional loading. The theoretical basis of the method is described and then it is implemented on two constitutive models of anisotropic plasticity, namely, Hill48 and Yld2000-2D. The accuracy of the proposed method and the advantage in terms of computational time with respect to the classical radial-return algorithm are discussed. The possibility of using such method to reconstruct the stress field in case of few temporal data and noisy strain fields is also investigated.     

Boundary element analysis of wave-current interaction around a large structure

A numerical approach for wave-current interaction around a large structure is investigated, based on potential flow theory, linear waves and small current velocity approximation. The velocity potential in a wave-current coexisting field is separated into a steady current potential and an unsteady wave potential. The boundary element method was then employed to compute the unsteady wave potential with effects of both a uniform current and a large body taken into consideration. It is demonstrated that the steady current potential can be expressed as the sum of a uniform current and a steady disturbance due to the presence of the object. The variation of current velocity in the vicinity of the object is then calculated by using a surface vorticity boundary integral meethod. Boundary element analysis is also used for the numerical solutions of the surface vorticity method. Substituting both unsteady wave potential and current velocity into the first-order dynamic surface boundary condition, the water surface elevation around a large structure in a wave-current coexisting field can then be obtained. Comparisons of numerical predictions with experimental results ar also made; qualitative good agreements are obtained.     

Local strain in a 5-harness satin weave composite under static tension: Part I - Experimental analysis

This paper presents an experimental method for determining the local strain distribution in the plies of a thermoplastic 5-harness satin weave composite under uni-axial static tensile load. In contrast to uni-directional composites, the yarn interlacing pattern in textile composites causes heterogeneous strain fields with large strain gradients around the yarn crimp regions. In addition, depending on the local constraints that are imposed by the surrounding plies, the deformation behavior of the laminate inner layers may vary from that of the surface layers, which are relatively more free to deform, compared to the inner layers. In order to validate the above hypothesis, the local strains on the composite surface were measured using digital image correlation technique (LIMESS). Internal strains in the composite laminate were measured using embedded fibre optic sensors (FOS).Based on the DIC results, the strain profiles at various locations on the composite surface were estimated. Using the FOS results, the maximum and minimum strain values in the laminate inner layers were evaluated. Comparison of the local strain values at different laminate positions provides an estimate of the influence of the adjacent layers on the local longitudinal strain behavior of a satin weave composite. Part II of this paper elucidates the local strain variation computed using the meso-FE simulations. In addition to the comparison of numerical and experimental strain profiles, Part II presents the maximum and minimum strain envelopes for the carbon-PolyPhenelyne Sulphide (PPS) thermoplastic 5-harness satin weave composite.     

ROUTINE FE DETERMINATION OF STRESS INTENSITY FACTORS USING A MÜLLER-BRESLAU INFLUENCE FUNCTION TECHNIQUE

Abstract A remarkably simple and accurate one-step application of the finite element (FE) method is suggested as a means for the designer's routine determination of stress intensity factors in linear fracture mechanics for complicated non-symmetric geometries. The vector-valued influence functions (Green functions) introduced here can be seen as a special kind of weight functions. Each of them is numerically found as the displacement field resulting from a certain unit deformation singularity being implanted at the crack tip through a prescribed set of mutual nodal displacements between the crack surfaces. Mode separation is inherent to the procedure. Plane model, mode II and mixed mode I and II numerical examples demonstrate the ease and accuracy of the method. Detailed guidance to the design of the FE mesh at the crack tip is given and is related to accuracy. Any standard FE code can be used. The literature in the field of computational fracture mechanics is surveyed, and some suggestions for further work are made. The present method draws on a classical technique for the calculation of influence lines in structural mechanics. The method is believed to have an added value in that it promotes an overview and understanding of how different load combinations on a given cracked body contribute to a stress intensity factor. Field plots of a calculated influence function are given in one of the examples.     

The SEĖ method for determination of behaviour laws for strain rate dependent material: Application to polymer material

The need to model fracture in crashworthiness by means of finite element codes is a real challenge for research. Before implementing fracture criteria, an excellent knowledge of the stress and strain states in the material just before the crack appearance is the first condition necessary to ensure the model development. At present, most of the material behaviour laws, for example for steel, are only defined until the maximum force when necking occurs. For polymers, the early occurrence of the diffuse necking leads to an experimental technique in which the speed loading is controlled in real time to maintain a constant strain rate during the test. This technique is not however used, due to technical limitations, for high strain rate behaviour laws. In this paper, the authors propose to use the heterogeneity of the displacement field on the surface of the tensile specimen as an initial condition to identify behaviour laws. The method developed uses the information in all the surface zone of the specimen by using digital image correlation. Stresses, strains and strain rates are then obtained to build a surface behaviour called the SE? surface. By cutting it, the experimental behaviour laws for a range of large strains and strain rates are then defined for model identification.     

Application of the Voronoï tessellation to study transport and segregation of grains inside 2D and 3D packings of spheres

This article provides a simple method to simulate the transport of a small bead through a static granular medium as a random walk on a network. This kind of displacement is strongly related to the geometry of the porous structure. A way to map the interparticle space is to calculate the Voronoï tessellation of the packing whose edges describe the network of pores of the medium. Then, the calculation of the probabilities to use each bond and a Monte-Carlo method for the choice of them, can simulate the displacement of the sphere. We introduce this technique for the inter-particle percolation of a fine particle through a packing of monosize spheres. We compare our numerical simulation with experiments performed in our laboratory. Then this technique is extended to the surface segregation. For the two kinds of segregation, we study the transverse diffusion and obtain a good agreement with experimental results. That our numerical simulation based on a rigorous geometric analysis of the medium is in agreement with inter-particle percolation and surface segregation experiments shows the importance of the study of the geometry of granular materials.     

Experimental investigation of the deformation of Hopkinson bar specimens

Split Hopkinson bar (SHB) experiments are often used to study the strain rate dependent mechanical properties of materials. During a SHB experiment a small sample of the material under study is subjected to a high strain rate, uni-axial, tensile, compressive or torsion load. From the classical measurements the time history of the mean stress, strain rate and strain in the specimen can be derived. For some applications, more detailed information concerning the variation of the deformation in the specimen is necessary. In this contribution a technique is presented which makes it possible to obtain the deformation along the length of the specimen. The deformation of a line grid attached to the specimen is recorded during an experiment by means of a streak camera. An advanced and innovative numerical technique, based on a combination of geometric moiré and phase shifting, is developed to extract the time history of the deformation along the axis of the specimen from the picture of the deforming grid automatically. Large specimen deformations are allowed, and the technique proved to give highly accurate results. In this contribution results are presented of a SHB experiment on a steel sheet specimen. Some remarks are formulated concerning the generally assumed homogeneity of the deformation in the specimen, and the deformation obtained with the classical measurement techniques.     

Numerical–experimental assessment of roughness-induced metal–polymer interface failure

A numerical–experimental method is presented to study the initiation and growth of interface damage in polymer–steel interfaces subjected to deformation-induced steel surface roughening. The experimentally determined displacement field of an evolving steel surface is applied to a numerical model consisting of a polymer coating and interface layer. The measured displacement field is obtained with a Finite Element based Digital Image Correlation method.The resulting simulations provide novel insights into the mechanical behaviour of the polymer–steel interface during interface roughening. The appearance of local hills and valleys on the evolving steel surface results in local bands of intensified stress in the polymer layer. These localized deformation bands trigger interface damage, which grows as the surface deformation increases. Polymer ageing initially delays the initiation of interface damage. However, for increased polymer ages the average interface damage increases. Likewise, the critical strain, at which the interface integrity is locally compromised, decreases.The presented method allows for a detailed study of the interface integrity during deformation-induced steel surface roughening. With properly identified material parameters, it becomes possible to tailor the polymer–steel material properties to minimize interface damage during production and storage of cans or canisters, e.g. for food and beverages.