Longitudinal Strain Distribution Research Articles

Weld Bead Distortion of Thin-Plate Using Weak Digital Image Correlation Method

Currently, the primary method used to investigate weld bead distortions is based on finite element prediction. However, in this paper, we proposed a three-dimensional optical method based on weak digital image correlation technology to track the dynamic distortion of the weld bead throughout the welding and cooling process. An experimental investigation demonstrated that the weld bead zone was distorted into a dish shape during the welding process and changed into a saddle shape after cooling down. In the weld bead, the out-of-plate distortion was significantly larger than the in-plate distortion, and the longitudinal plastic strain distribution was small and uniform, whereas the transverse strain was larger. This optical experimental method could be useful in the validation of numerical models in the welding process and important to research on welding distortions.

Experimental investigation of bond stress distribution and bond strength in unconfined UHPFRC lap splices under direct tension

Several studies on tension lap splices have shown the improvement of bond strength using Ultra-high performance Fibre Reinforced Concrete (UHPFRC). The bridging effect of fibres on cracks improves the bond splitting strength substantially in comparison to normal concrete. This paper investigates the influence of fibre content on the strength of tension lap splice of reinforcing bars in UHPFRC without additional transverse reinforcement. Different splice lengths and UHPFRC mixes were tested. Internal strain measurements were used to capture the force transfer mechanism and the evolution of longitudinal strain distribution and associated bond stresses. The bond performance is clearly related to the pre- and post-cracking tensile capacity of UHPFRC. At a distance exceeding 2 db from bar extremities, bond stress distribution at failure displayed a quasi-constant value regardless of the lap splice length up 10 db. This reveals for short lap splices that the bearing action of all ribs along the splice length contributes equally in resisting the applied force. This experimental program provides experimental results for understanding the local force transfer mechanism in UHPFRC lap splice and contribute for further developments on bond in UHPFRC.

EVALUATION ON ELONGATION OF REBAR BY USING IMAGE PROCESSING METHOD

In this study, longitudinal strain distributions of rebars in the uniaxial tension test are evaluated with an image processing method (IPM). The measurements of strain are conducted till rebar is ruptured. As a result of the test, the detailed longitudinal strain distributions of rebar in the tension test are clarified and the results show that the strain at necking region are almost 2-3 times of average strain in the gauge length of specimen. After the stress of rebar is reached at ultimate strength, the most of elongations are concentrated at necking region. In the other region except necking region, therefore, the strains are merely increased till the end of test. From these results, it is estimated that the necking region can make the over estimation of elongation of rebar, therefore, the estimation except the necking region can be suggested to evaluate the true elongation of rebar. Also the measured results with IPM are compared with the measured results by using conventional strain gauge. It is estimated that the IPM shows some noise in data, but the IPM has advantages to measure the strain distribution to the range of rupture. Therefore if the image acquisition of the better resolution and better quality are possible, it can be a good alternative measuring method for the strain measurement.

Theoretical and experimental study on notched steel beams strengthened with CFRP plate

CFRP external bonding is an alternative technique to repair or strengthen notch damaged steel beams. However, the premature debonding failure caused by the stress concentration at the notch location reduces the effectiveness of this reinforcement technique. This paper presents an integrated closed-form solution for obtaining the interfacial shear and normal stresses in steel beams strengthened with a CFRP plate. Simple expressions of the maximum interfacial stresses at the notch locations, the adhesive hollows and the plate ends are given. A parametric study indicates that the maximum stresses at the notch locations increased with the notch depth and the applied bending moment on notched cross-sections, but reduced with the thickness of the adhesive. An experimental study of model-scale CFRP bonded steel beams was conducted as well. The test results show the strength of the notched beam can be improved almost twice by the CFRP plate strengthening, while the brittle fracture caused by intermediate debonding initiated from the notch location limits the ductility enhancement of the retrofitted beams. The good agreement between the longitudinal strain distributions in the CFRP plate obtained from the tests and the corresponding analytical results demonstrates the validity of the theory.

Distributed cracking mechanisms in textile-reinforced concrete under high speed tensile tests

Distributed cracking mechanisms in textile reinforced concrete (TRC) subjected to high speed loads and temperature variations were studied using digital image correlation (DIC) analysis and finite difference modelling. Three different TRC composites made of laminated AR glass, warp-knitted AR glass and polypropylene were used. Both plain textile and TRC specimens were tested at a nominal strain rate of 100 s−1 at the temperature range of −30–80 °C. The non-uniform distribution of longitudinal strain in TRC systems was divided into three distinct zones of localization, shear lag, and uniform strain and the strain distribution in each zone was quantitatively measured. Tensile strength and postcrack stiffness decreased in various TRCs as the temperature increased with the highest tensile strength of 38.1 MPa, work-to-fracture of 46.6 J, and postcrack stiffness of 459.7 MPa recorded for the warp-knitted AR glass TRC specimens at −30 °C. A finite difference model was used to simulate the experimental crack spacing and stress–strain behaviors as well as the degradation in postcrack stiffness as a function of interfacial bond strength. The experimentally observed crack patterns and failure modes in TRC systems agreed with the numerical simulations and the measurements of slip zone size using DIC.

가변롤성형 공정을 이용한 단면이 가변하는 프로파일의 형상변수 분석에 관한 연구

Flexible roll forming allows profiles to have variable cross-sections. However, the profile may have some shape errors, such as, warping which is a major defect. The shape error is induced by geometrical deviations in both the concave zone and the convex zone. In the current study, flexible roll forming was modeled with FE simulations to analyze the shape error and the longitudinal strain distribution along the flange section over the profile. A distribution of analytically calculated longitudinal strains was used to develop relationships between the shape error and the longitudinal strain distribution as a function of the defined shape parameters for the profile. The FE simulations showed that the shape error is primarily affected by the deviations between the distribution of analytically calculated longitudinal strain and the longitudinal strain distribution of the profile. The results show that the shape error can be controlled by designing the shape parameters to control the geometrical deviations at the flange section in the transition zones.

The Impact of Hemodialysis on Segmental and Global Longitudinal Myocardial Strain

BackgroundStrain analysis derived from the analysis of speckle tracked imaging echocardiography can be used to examine ventricular contractile functions. In this study, we examined the relationship of hemodialysis (HD)-induced circulatory stress with overall ventricular function assessed according to global longitudinal strain (GLS) and segmental distribution of strain. MethodsThis prospective observational study included 104 conventional HD patients at Royal Derby Hospital. Averaged values of segmental and GLS were determined from the echocardiography of these patients before and at peak dialysis. These values were compared with the reference values of healthy individuals, correlated with their demographic characteristics, and the effect on survival was assessed. ResultsThe global strain value was −11.5% ± 4.42, and the segmental strain values were significantly greater in HD patients than in healthy individuals by 2.7%-9.8% (P < 0.001). The strain values were not significantly different before dialysis and at peak dialysis (P > 0.05), except within the basal lateral segment (P = 0.01). The adjusted hazard ratio for mortality was 4.3 (95% confidence interval, 1.2-14.9; P = 0.021) when > 80% of the segments exhibited more than the mean of segmental strain values. For the 46 patients who died, there were statistically significant negative correlations between survival time and GLS (r = −0.30; P = 0.04). ConclusionsGlobal and segmental strain measured using speckle tracked imaging provides information relating to the effects of HD-induced cardiac injury. The segmental strain abnormalities in the watershed area of the left ventricle suggest a higher degree of vulnerability to HD-induced demand ischemia.

Dynamic strain distribution measurement and crack detection of an adhesive-bonded single-lap joint under cyclic loading using embedded FBG

In this study, the dynamic strain distribution measurement of an adhesive-bonded single-lap joint was carried out in a cyclic load test using a fiber Bragg grating (FBG) sensor embedded into the adhesive/adherend interface along the overlap length direction. Unidirectional carbon fiber reinforced plastic (CFRP) substrates were bonded by epoxy resin to form the joint, and the FBG sensor was embedded into the surface of one substrate during its curing. The measurement was carried out with a sampling rate of 5 Hz by the sensing system, based on the optical frequency domain reflectometry (OFDR) throughout the test. A finite element analysis (FEA) was performed for the measurement evaluation using a three-dimensional model, which included the embedded FBG sensor. The crack detection method, based on the longitudinal strain distribution measurement, was introduced and performed to estimate the cracks that occurred at the adhesive/adherend interface in the test.

Investigation of a new incremental counter forming in flexible roll forming to manufacture accurate profiles with variable cross-sections

A roll-formed profile with variable cross-sections fabricated by flexible roll forming has a shape error, such as warping, because of geometrical deviations in transitional zones of the profile between the initial metal strip and the roll-formed profile. To reduce the shape error, a new process called incremental counter forming (ICF) is proposed. Our investigation of the ICF process shows that the longitudinal strain distribution at the flange of the roll-formed profile can be controlled by combinations of forming parameters of the ICF process. As the forming parameters increase the longitudinal strain distribution in the concave zone, the shape error decreases. However, when the longitudinal strain distribution in the straight zone reaches a critical limit, the additional longitudinal strain works as an excessive longitudinal strain to worsen the shape error. An analytical model, which describes the longitudinal strain at the flange during roll forming, is adopted to reveal that the increase of the longitudinal strain is induced by increasing derivatives of a bending angle, which is controlled by the forming parameters of the ICF process. Finally, the FE simulation has been carried out to compare with the experimental results, which show that the ICF process is effective for reducing the shape error of the profile with variable cross-sections in flexible roll forming.

Development of a Combined Method between MPS and FEM for Simulating Friction Stir Welding

In order to simulate the large deformation surrounding the rotational tool of friction stir welding (FSW) precisely, the moving particle semi-implicit (MPS) method was employed and the temperature distributions near the tool were obtained. Also, the temperature distributions in the whole model except for the area computed by MPS were calculated by the finite element method (FEM) and then the elastic-plastic analysis was conducted using the temperature distributions obtained by MPS and FEM. The inhomogeneous temperature distributions through the thickness near the joint line could be simulated and the maximum temperature distributions computed had a good agreement with the experiments. In addition, the longitudinal plastic strain distributions indicates that this plastic strain near the tool is not governed by only the temperature distributions and the influence of plastic flow should be taken into account.

Analysis of SBS Gain Shaping and Threshold Increase by Arbitrary Strain Distributions

Fibers with a longitudinal strain distribution can be used to shape the stimulated Brillouin scattering (SBS) gain spectrum or to increase the SBS threshold power. Theoretical analysis and experimental results on the SBS gain spectrum of coiled fibers with a continuous and arbitrary distribution of longitudinal strain are presented. The concept of a critical gain factor for determining the SBS threshold power with a simple formula is reviewed for fibers with different parameters and spectral shapes. A fiber coiling machine for high coiling forces is described for realizing permanent strain distributions with a maximum strain of more than 3%. SBS spectra were broadened to a spectral width of 1.7 GHz. Measurements verify a significant SBS gain suppression by a factor of 40, which is in excellent agreement with the theoretical analysis. Finally, synthetic triangular and broadened Lorentzian SBS spectral shapes with a spectral width of 550 MHz are demonstrated. The results can be used to suppress SBS in high-power fiber lasers and amplifiers or to tailor almost any arbitrary SBS spectral shapes, which could be useful for slow-light or active optical filter applications.

Simulation and experimental verification of flexible roll forming of steel sheets

Roll forming is a sheet metal forming process that has been used for decades. Usually roll-formed sections have a constant cross section. Flexible roll forming is a brand new forming process that produces parts with variable cross sections, in which the rollers translate back and forth in a direction that is perpendicular to the sheet feeding direction. Theoretical analysis gives an explanation of the plane strain state, compressive stresses, tensile stresses, and shear stresses in flexible roll forming. In order to analyze the mechanics and the deformation characteristics of flexible roll forming, the finite element method (FEM) model of a 17-step flexible roll forming process is established. The yield criterion used in the FEM simulation is Hill 48, and the parameters of which are solved with the yield stresses under different loading conditions and are firstly verified with a plane strain tensile test. The complicated roller paths are realized with data extracted from the computer-aided design (CAD) files with VC++ programs developed by the authors. We developed the first flexible roll forming prototype machine in China, with which the roll forming experiment of a side door beam is performed. Final shapes of the experimental and numerical results are compared. It is shown that the numerical results based on Hill 48 yield criterion that is solved with yield stresses agree well with the experimental results, which indicates that the simulation model can well reflect the real forming process. Detailed analysis of the distribution and history of plastic strain, longitudinal strain, shear strain, and thickness of both the constant cross section and the variable cross section is performed, which is of great help to understand this forming process.

Experimental investigation of CFRP anchorage systems used for strengthening RC joints

Fibre-reinforced polymer (FRP) composite materials are widely used for strengthening existing reinforced concrete structures because of their light weight, easy implementation and high efficiency. However, recent seismic studies have revealed that FRP-strengthened specimens underperform due to the debonding of the FRP strips in the vicinity of L-shaped structural joints, resulting from the lack of an anchoring system to transfer loads from the FRP strips to the supporting structural element. To address this issue, four new anchorage techniques are presented in this paper, and experimental results for each are analysed in the form of failure modes, load displacement curves, longitudinal strain distribution curves, hysteresis loops and the ultimate capacity. This research paper concludes that these Carbon Fiber (CF) anchor systems enhance the overall seismic performance of FRP-strengthened specimens.

Mechanical characterization of flat specimens in tensile test and numerical simulation

Tensile test experiments with flat specimens for SPCC and JAC340H are presented and simulated in this paper. Digital image correlation (DIC) technology is used to measure the deformation during tension procedure, and the accuracy of the home-developed DIC equipment is discussed. The 3-D finite element models of tensile specimens are developed to analyze the influences of different yield functions upon evaluating the engineering stress-engineering strain relationship, load-true strain curve, transversal-longitudinal true strain relationship and the distribution of longitudinal strain after necking. The comparisons between the simulations and the experimental results show that the Barlat 1991 yield function offers better results than the von Mises yield function and Hill 1948 yield function do.

Longitudinal strain monitoring of rail using a distributed fiber sensor based on Brillouin optical correlation domain analysis

A fiber-optic distributed sensor system is configured to measure the longitudinal strain distribution of a rail in real time. The system is based on the Brillouin correlation domain analysis (BOCDA), in which the variation of local Brillouin frequency ( ν B ) is measured that linearly depends on the strain applied to the optical fiber. In the test measurement, the longitudinal strain distribution along a 2.8 m rail is measured under different loading conditions with a spatial resolution of 3.8 cm and an accuracy of ±15 με.

Analysis of Metal Flow and Deformation Features during Continuous Tube Rolling Process with Mandrel Mill

With the aid of commercial FE code MSC.SuperForm, the structural steel tube continuous rolling process of a typical hollow tube specification 152.5×12.5mm is simulated based on Bao Steel 152.5 main pass sequence of 140mm 8-stand mandrel mill, and the distribution characteristics of stress/strain, outline lateral spread, temperature changes of workpiece in continuous rolling process and distribution of stress/strain, friction of workpiece in deformation zone are analyzed. Analysis results indicate that deformation of workpiece along the width of the groove, especially at the top and the bottom of the groove is highly inhomogeneous due to the unequal draught and the longitudinal stress of special position (the top and the bottom of the groove) of workpiece is always an alternate state, in a tensile-compressive-tensile manner, and has a distinct rule. In the first stand, outline dimension of workpiece gradually increases during reducing process and early stage of wall thickness reduction, but it gradually decreases during middle-late stage of wall thickness reduction and tends towards stability at last. It is inhomogeneous that distribution of unit compressive stress and longitudinal strain of workpiece in deformation zone, and contact stress and total equivalent plastic strain are maximal in inner surface of workpiece contacting with mandrel. Temperature difference between the outer and the inner surfaces of workpiece is obvious.

Strain monitoring of composite steel girder bridge using distributed optical fibre sensor system

An I type steel girder was assembled with a concrete upper plateand then placed on two roller supports. Four vertical load actuators were laid on the upper plateto add the vertical load. A single mode optical fiberwas attached on the surface of the lower flange of a composite steel girder bridge with a lengthof 39 m. Longitudinal strain distribution of a composite steel girder bridge is monitoreddirectly using optical fibre sensor system based on Brillouin frequency shift dependency of the strain applied on the optical fiber.Brillouin scattering signal is generated through the Brillouin optical correlation domain analysisat a selected location. Strain data wereacquired at 195 points with the interval of 20cm over the full range.From the strain data over the entire length of the composite steel girder bridge, the strain distribution shape was shownwhen the vertical load was added onthe bridge.

Experimental Testing of Fiber Reinforced Polymer-Concrete Composite Beam

A new form of fiber reinforced polymer (FRP)-concrete composite beam is proposed in this study. The proposed composite beam consists of a GFRP box beam combined with a thin layer of concrete in the compression zone. The interaction between the GFRP beam and the concrete was obtained by bonding coarse-sand on the top flange of the GFRP beam. One GFRP box beam and one GFRP-concrete composite beam were investigated in four-point bending test. Load-deflection response, mid-span longitudinal strain distributions and interface slip between GFRP beam and the concrete for the proposed composite beam were studied. Following conclusions are drawn from this study: (1) the stiffness and strength of the composite beam has been significantly increased, and the cost-to-stiffness ratio of the composite beam has been drastically reduced comparing with GFRP-only box beam; (2) a good composite action has been achieved between the GFRP beam and the concrete; (3) crushing of concrete in compression defines flexural collapse of the proposed composite beam..

Measurement of the 3D arterial wall strain tensor using intravascular B-mode ultrasound images: a feasibility study

Intravascular ultrasound (IVUS) elastography is a promising tool for studying atherosclerotic plaque composition and assessing plaque vulnerability. Current IVUS elastography techniques can measure the 1D or 2D strain of the vessel wall using various motion tracking algorithms. Since biological soft tissue tends to deform non-uniformly in 3D, measurement of the complete 3D strain tensor is desirable for more rigorous analysis of arterial wall mechanics. In this paper, we extend our previously developed method of 2D arterial wall strain measurement based on non-rigid image registration into 3D strain measurement. The new technique registers two image volumes acquired from the same vessel segment under different levels of luminal pressure and longitudinal stress. The 3D displacement field obtained from the image registration is used to calculate the local 3D strain tensor. From the 3D strain tensor, radial, circumferential and longitudinal strain distributions can be obtained and displayed. This strain tensor measurement method is validated and evaluated using IVUS images of healthy porcine carotid arteries subjected to a luminal pressure increase and longitudinal stretch. The ability of the algorithm to overcome systematic noise was tested, as well as the consistency of the results under different longitudinal frame resolutions.

Knots in trees: strain distribution in a naturally optimised structure

Electronic speckle pattern interferometry was applied to directly measure the distribution of longitudinal, tangential, and shear strains in small boards of Norway spruce (Picea abies (L.) Karst.) exposed to tensile load in longitudinal direction. A sample with a central intergrown knot and one with an equivalent loose knot were compared with reference samples made of clear wood with an artificial central circular or square hole, respectively. The observed measurements were compared with a finite element (FE) simulation. The FE model was based on a geometric model to quantify the local fibre orientation and a micromechanical model to estimate elastic constants of clear wood and knot tissue. Both the measurements and simulation clearly illustrate a rather homogenous strain distribution around the intergrown knot. In comparison, the natural optimisation of dispersing strain peaks is less efficient in the case of loose knots. The artificial circular and square holes in samples with parallel fibre orientation lead to high gradients in the strain field and peak values in vicinity of the disturbance.