Deformation Measurement System Research Articles

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

ABSTRACTAlthough 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.

Tensile Anisotropy of Cu/Al Clad Metals After Prestrain Using Digital Image Correlation

A dimensional deformation measurement system (XJTUDIC/VS) based on digital image correlation was employed to measure the deformation and strain field of Cu/Al clad metals made using the explosive welding technique. An extensometer with a gage length of 50 mm was used during the Q235 tensile test to verify the strain measurement accuracy of the system. The results demonstrate that the accuracy of the XJTUDIC/VS system is ≥0.5%, which is very close to that of the extensometer. The anisotropy of Cu/Al clad metals under uniaxial tension after 0, 1.5, 2.5, and 4.5% prestrain was investigated. With an increase in strain, the true stress-strain curve varied at different directions in the same prestrains. The strain hardening rate, r value (changes from 0.1 to 0.9), and n value (changes from 0.25 to 0.35) all strongly depended on loading directions and prestrains. The linear relations between longitudinal strain and transverse strain were also influenced by tensile directions and prestrains. Cu/Al clad metals generally presented strong anisotropy.

Meso-Mechanical Behavior of Sand under Shear Loading Test Research

A continuous deformation measurement system of two dimensional granular materials displacement based on digital image correlation method (DIC) has been build up. Direct shear tests have been performed on standard sand in a special laboratory apparatus with different vertical pressure. Using DIC a pairs of consecutive photographs taken during the tests has enabled fields of displacement velocity and strain to be determined. Assumed the proportional of stress and strain, the direct observation of displacements and principle strains has been possible for a series of force of grains under shear at various stages in the direct shear test.

A Method for Subsidence Monitoring of Similar Material Simulation Test in Coal Mining

To accurately monitoring the subsidence of similar material simulation test in coal mining, a static deformation measurement system is proposed based on the digital image processing and photogrammetry techniques. Firstly pastes reference points and observation points on model frame and working face respectively. Before exploitation, calculates the 3D coordinates of those artificial points, transfers the default coordinate system to user defined coordinate system using 3-2-1 transformation method, and defines this stage as basic stage. Then start mining, records enough images from different position and angle, and calculates the 3D coordinates of the deformation stage. Finally aligns the single deformation stage to the basic stage, by tracking and comparing the 3D coordinates of the homonymous points, the subsidence of observation points are obtained. One similar material simulation tests were conducted which prove good performance of the proposed measurement system.

Numerical and Experimental Study of Tube Hydroforming for Aerospace Applications

In the tube hydroforming (THF) process, a pressurized fluid is used to expand a thin walled tube inside a closed die in order to fill the die cavity. THF has many advantages that render this process interesting for different industries such as automotive and aerospace. In this work, to investigate the effect of different process parameters, such as the friction condition, tube thickness and end-feeding on the final product, THF experiments were performed on stainless steel 321 (SS 321) tubes using a round-to-square die. Experimental loading paths were obtained via the data acquisition system of the hydroforming press, which is fully instrumented. An automated deformation measurement system, Argus®, was used to measure the strains on the hydroformed tubes. The THF process was simulated using Ls-Dyna software. The variation in the strain and thickness measured from the experiments were compared to the simulation results at critical sections. Comparison of the results from the finite element (FE) simulations and experiments showed good agreement, indicating that the approach can be used for predicting the final shape and thickness variations of the hydroformed parts for more complex shapes in aerospace applications.

Development of Deformation Measurement System Consisting of High-Speed Camera and Digital Image Correlation, and Its Application to the Measurement of Large Inhomogeneous Deformations Around the Crack Tip

Development of Deformation Measurement System Consisting of High-Speed Camera and Digital Image Correlation, and Its Application to the Measurement of Large Inhomogeneous Deformations Around the Crack Tip

Laser–arc hybrid welding of Ti6Al4V titanium alloy: mechanical characterization of joints and gap tolerance

This paper reports the results of a comparison between the mechanical characteristics of butt joints in 3.0 mm thick Ti6Al4V titanium alloy sheets, made by laser welding with no filler material and by laser–arc hybrid welding. Vickers hardness tests have been performed on cross-sections of the beads obtained. In particular, the influence of the gap between the sheets in test welds performed using CO2 laser–MIG hybrid welding has been analysed. Cross-sections of the weld beads obtained with several different gap sizes have been analysed morphologically. The welded joints have been subjected to draw testing, and joint deformation behaviour has been analysed using an optical deformation measurement system based on stereoscopic image capture (ARAMIS 3D analysis system).

Experimental Study on the Evaluation of Necking and Fracture Strains in Sheet Metal Forming Processes

In this paper the formability of AA2024-T3 metal sheets is experimentally analyzed. For this purpose, a series of Stretch- Bending and Incremental Sheet Forming (ISF) tests are carried out. The former tests allow determine the formability limits through the evaluation of necking and fracture using the optical deformation measurement system ARAMIS® and measuring the thickness strains along the fracture line. The latter are performed with the aim of confirming the validity of these limits. In this case, the spifability, formability in Single Point Incremental Forming (SPIF), was studied in the light of circle grid analysis by means of the 3D deformation digital measurement system ARGUS®. Different punch diameters are used in both processes. The results exhibit the importance of the accuracy in the setting of the formability limits as well as the variability that these limits present depending on the forming process or some variables such as the tool radius.

A New Image-Based Soil Deformation Measurement System

A Soil Deformation Measurement System using OPENCV library and FFTW library in C++ was developed in this paper. The system applied camera calibration based on neural network and Fasst Fourier Transform (FFT) cross-correlation algorithm for Particle Image Velocimetry (PIV). It is used to obtain soil deformation data, such as displacements, velocity and strain, and visualize the deformation. Experiments show that this system could acquire deformation data from soil images accurately, efficiently and continuously, which provides a strong proof that image processing technology has practical significance and application value in the research field of geotechnical engineering.

Formability Evaluation of Non-Crimp Carbon Fabric by Non-Contact 3D Deformation Measurement System

Non-Crimp Carbon Fabric (NCF) consists of unidirectional plies which are kept together by stitching yarns arranged in a number of different orientations relative to the fabric production direction. It is reported that NCF possesses excellent drape performance compared to woven fabrics. However there is not a clear criterion of a drape evaluation on the drape characteristic of the NCF. In addition, it is not clarify that stitch pattern and stitch tension influence on the drape characteristic of the NCF. Moreover, in existing bias extension test, measurement of shear angle is based on the pin-jointed net (PJN) approximation. The PJN approximation doesnt takes into consideration the fiber sliding and the effect of the stitched parameters of the NCF. In this study, the bias extension test based on the measurement of shear angle by non-contact 3D deformation measurement system was conducted to evaluate the drape performance of the NCF. We made a proposal of the formability evaluation index based on the measurement results. Moreover, the 3D draping tests were conducted onto hemisphere geometry and regular tetrahedron, in order to verify availability of the formability evaluation index. The availability of the formability evaluation index was verified.

전체변형률 영역에 대한 K0압밀시 풍화토의 변형특성 평가

본 연구에서는 미소변형 영역에서 파괴에 이르는 전체변형률 영역에서 지반의 변형거동 평가가 가능한 미소변형측정 삼축시험장비에 횡방향 변위측정시스템을 추가하여 시료의 포아송비 측정이 가능하도록 하였다. 구축한 횡방향 변위 측정시스템을 폴리우레탄시편을 이용하여 검증하였으며, 일정 구속압 재하 후 횡변위를 모니터링하며 축하중을 조정함으로써 <TEX>$K_0$</TEX> 압밀을 구현하였다. 구축된 시험시스템을 이용하여 화강풍화토 시료에 대하여 <TEX>$K_0$</TEX>압밀 및 등방압밀 상태에서 구속응력조건을 달리하며 시험을 수행하였고, 전체변형률 영역에서의 변형 및 강도거동을 비교, 평가하였다. <TEX>$K_0$</TEX>압밀 조건은 등방압밀 조건과 비교하여 동일한 평균유효구속응력 조건에서 포아송비, 탄성계수, 비선형 변형거동특성에 있어서 상이한 특성을 나타내었다. The small-to-large strain traixal testing apparatus has been adopted to evaluate the deformation characteristics of soils at full strain ranges (<TEX>$10^{-4}%{\sim}10%$</TEX>). In this study, a lateral deformation measurement system was additionally installed to the apparatus in order to measure Poisson's ratio. It was verified using polyurethane specimens and then used to set up <TEX>$K_0$</TEX>-consolidated state of soil specimens by monitoring the lateral deformation variation and adjusting axial stresses during a consolidation step. Test results show that Poisson's ratio, elastic modulus, and nonlinear stress-strain relations under <TEX>$K_0$</TEX> condition are significantly different from those under isotropic condition.

Shape and deformation measurement system by combining fringe projection and digital image correlation

An optical measurement system, which is capable of simultaneously measuring surface profile and three-dimensional (3-D) deformation of small objects, has been developed by the combination of fringe projection and a two-dimensional digital image correlation (DIC) technique. In the proposed approach, it requires only one camera and one projector. The surface profile and out-of-plane deformation are determined by fringe projection with the phase-shifting method. The in-plane deformation is determined by comparing the surface texture of specimen before and after deformation with the DIC technique. A simple procedure is described which enables determination of texture image from phase-shifting images. As an application, the developed system was used to investigate the mechanical behavior of molded pulp material in micro-zone during tension testing.

An Experimental Study of the Microformability of Very Thin Materials

This article extends the concept of forming limit curves to very thin (0.1 mm) metallic materials. The forming limit curves for these materials are referred in this paper as microforming limit curves. To study the microformability of such materials, a microdrawing press based on the Marciniak principle, coupled with an image acquisition system, was designed and built in the laboratory. Images of the sample surface were obtained during the microdrawing process and processed using a deformation measurement system based on image correlation. The microformability of aluminium 1050A (99.5 %) and aluminium alloy (Al-3 % Mg) were characterised in tests at three different geometries and then analysed. Besides, a new technique for estimating the strain at the onset of necking was developed and tested against a proven method. This technique is more easily automated because it does not rely on subjective criteria. Thus, it is possible to have the microforming limit curve for very thin sheet by a rapid and objective experimental method.

Study on Mechanical Properties of the Composite Materials Film by Speckle Projection Method

An improved accurate speckle projection method is used for study the mechanical properties of the composite material film in the paper. A system for deformation measurement is developed with the telecentric lenses, in which such conventional lens’ disadvantages such as lens distortion and perspective error will be diminished. Experiments are performed to validate the availability and reliability of the calibration method. The system can also be used to measure the dynamic deformation and then results are also given.

Calibration error for dual-camera digital image correlation at microscale

Digital image correlation (DIC) has been widely conducted in many engineering applications. This paper describes a dual-camera system which is mounted on a stereo light microscope to achieve 3D displacement measurement at microscale. A glass plate etched with precision grids was used as the calibration plate and a translation calibration procedure was introduced to obtain the intrinsic and extrinsic parameters of the cameras as well as the aberration of the imaging system. Two main error sources, including grid positioning and stage translation, were discussed. It was found that the subpixel positioning errors had limited influences on displacement measurement, while the incorrect grid positioning can be avoided by analyzing the standard deviation between the grid spacing. The systematic translation error of the stage must be eliminated to achieve accurate displacement measurement. Based on the above analysis, a precisely controlled motorized calibration stage was developed to fulfill fully automatic calibration for the microscopic dual-camera system. An application for measuring the surface texture of the human incisor has been presented. It is concluded that the microscopic dual-camera system is an economic, precise system for 3D profilometry and deformation measurement.

A Digital Image-Based Deformation Measurement System for Triaxial Tests

Abstract This paper describes the development of a new digital image-based deformation measurement system for triaxial tests on soils. Three digital cameras placed on radii at intervals of 120° viewed on plan outside a transparent triaxial cell were used to capture images of a deforming cylindrical soil specimen at various instants. A digital image correlation program was written to analyze the captured images, using ray tracing to take account of image distortion due to refraction at the interfaces between the cell fluid, the cell wall, and the atmosphere. The technique is validated with reference to a sand specimen tested in drained triaxial compression. Typical implementations of the system to study surface deformation characteristics (e.g., barreling, onset of localization, and shear band evolution) of deforming soil specimens are illustrated.

Influence of Different Pre-Stretching Modes on the Forming Limit Diagram of AA6014

In order to evaluate the formability of sheet materials forming limit diagrams (FLD) are recorded which represent the values of major and minor strain when necking occurs. FLDs are recorded based on the assumption that exclusively linear strain paths occur. In real forming parts, however, particularly in those with complex shapes, predominantly non-linear strain paths occur which reduce the accuracy of the failure prediction according to a conventional FLD. For this reason forming limits after loading with non-linear strain paths have to be investigated. In this contribution a systematic analysis of the forming limits of a conventional AA6014 alloy after loading with non-linear strain paths is presented. This material is pre-stretched in uniaxial, plane strain and biaxial direction up to several levels before performing Nakajima experiments in order to determine FLDs. During the pre-stretching process as well as during the Nakajima experiment the strain distribution can be measured online very precisely with the optical deformation measurement systems GOM Aramis or VIALUX. The gained curves are compared to the FLD of the as-received material. The results prove a significant influence of the pre-stretching condition on the forming limits of the used aluminum alloy. For a low pre-stretching in uniaxial as well as in biaxial direction the FLDs show a slightly reduced formability while after higher pre-stretching levels the forming limit can be improved such as for biaxial loading after uniaxial pre-stretching. The formability after pre-stretching in plane strain direction was changed. Also, a shift of the FLD depending on the direction of pre-stretching can be observed.

Linear friction welding of a near-β titanium alloy

The linear friction welding (LFW) behaviour of near-β titanium alloy Ti–5Al–5V–5Mo–3Cr (Ti-5553) was investigated by varying the processing conditions of frequency and axial pressure. The examined mechanical properties of the welded material included microhardness and tensile properties. The maximum strains experienced by the material during LFW for each set of welding parameters were estimated based on the process parameters and then evaluated using Aramis, a three-dimensional optical deformation measurement system. The LFWed Ti-5553 was examined with electron backscatter diffraction techniques to relate the texture and phase changes to the thermomechanical conditions. Characterisation of the welds included analysis of the microstructural features of the weld region and the thermomechanically affected zone in relation to the parent material.

Determination of Global and Local Tensile Behaviours of Laser Welded Ti-6Al-4V Alloy

In this study, the global and local tensile behaviours of laser welded 5.1-mm thick Ti-6Al-4V alloy were obtained at various welding speeds and defocusing distances using a digital image correlation (DIC) technique with a full field three-dimensional deformation measurement system. The local tensile properties including elastic modulus, yield stress, and maximum plastic strain were determined at various locations by assuming an iso-stress condition. It was found that the elastic modulus and yield stress were maximum in the fusion zone (FZ) and minimum in the heat-affected zone (HAZ). Porosity and underfill defects were the main reasons for the failures in the FZ and/or the HAZ. Maximum plastic strain at fracture was observed at the failure location in most cases.

Enhanced electromechanical performance of cellular polypropylene electrets charged at a high temperature

In order to enhance the electromechanical performance of polypropylene (PP) electrets, cellular PP electrets were heated during corona poling at a high temperature in order to accelerate its chemical structure change and increase the quantity of formed dipole within the electrets. We investigated the behaviour of surface potential decay after corona poling, and the results showed that corona poling at a high temperature can improve the charge retention of commercially used cellular PP. The results of thermally stimulated depolarization current indicated the different localized states of the electric charges in cellular PP electrets after corona poling. Then, enhanced electromechanical ability was measured by a deformation measurement system and large strain (about 5%) under a moderate applied electric field intensity (30 MV m−1) was obtained.