In this work, electron backscatter diffraction (EBSD) and digital image correlation were employed to study tensile behavior of friction-stir welded magnesium alloy AZ31. The character of measured ...strain patterns indicated the prevalence of basal slip but also suggested significant activity of prism slip. A substantial strain gradient was found lengthways in the stir zone. The failure was shown to be initiated at the weld root and to originate from double {101¯1}−{101¯2} twinning. Crack propagation was influenced by the “onion-ring structure” of the stir zone.
A method for evaluating mode I, mode II and mixed-mode stress intensity factors from in-plane displacement fields using the method of nonlinear least-squares is proposed in this paper. Along with ...stress intensity factors, crack tip location and rigid body displacement components are determined simultaneously from both displacement components obtained using full-field optical methods or numerical methods. The effectiveness is validated by applying the proposed method to mixed-mode displacement fields obtained through digital image correlation, displacement fields obtained by analysis using elasto-plastic finite element method, and displacement fields around a fatigue crack obtained by electronic speckle pattern interferometry. Results show that the proposed method can extract stress intensity factors from the displacement fields both accurately and easily. Furthermore, they can be determined even if the material at a crack tip exhibits small-scale yielding. It is expected that the proposed method is applicable to various fracture problems during experimental and numerical evaluation of structural components.
In this work, electron backscatter diffraction and digital image correlation techniques were employed to study the microstructure and microstructure-property relationship in friction-stir welded TRIP ...steel. It was found that the thermal effect of the welding process led to material softening in the heat-affected zone and promoted martensite transformation in the stir zone. These microstructural changes provided rapid strain localization during subsequent transverse tensile tests and thus resulted in premature failure of the welds.
Material softening in the heat-affected zone was deduced to be a combined result of dissolution and spheroidisation of retained austenite as well as recovery in bainitic ferrite. The stir zone martensite was characterized by significant orientation spread and therefore its orientation relationship with austenite essentially deviated from the ideal Kurdjumov-Sachs relation. Moreover, the martensite transformation was found to be influenced by variant selection.
ABSTRACT
Path integral, domain integral and least squares methods for evaluating J‐integral from measured displacement fields for a power‐law hardening material are described in this paper. The ...values of the J‐integral are evaluated by applying the path and domain integral methods to the displacement fields obtained by elastoplastic finite element analysis and the displacement fields obtained through the measurement using digital image correlation. Results show that the values obtained by the domain integral method are slightly better than those by the path integral method, because the domain integral method efficiently uses the full‐field measurement data. The values of the J‐integral are also evaluated by the least squares method with the Hutchinson, Rice, and, Rosengren displacement fields. Results show that the J‐integral can be obtained by the least squares method simply and easily without any calculation of the integration. The J‐integral values obtained by the least squares method agree well with the values obtained using other methods. Because J‐integral can be evaluated easily by any method described in this paper, it is expected that these methods are applicable to various fracture problems during experimental evaluation of structural components.
•Stress fields around a curving crack in a thin glass plate is measured using instantaneous phase-stepping photoelasticity.•An experimental–numerical hybrid method is applied to the analysis of ...stress field at the crack tip.•Mode II stress intensity factor exhibits nonzero value although the crack growth smoothly.•Compressive stresses are distributed surrounding the tensile stresses at the crack tip.•The compressive stress field leads to both higher stress intensity factor and crack oscillation.
The stress fields around an oscillating crack tip in a thin plate are studied using an experimental–numerical hybrid method. Instantaneous phase-stepping photoelasticity using a CCD camera equipped with a pixelated microretarder array is used for measuring the stress fields around a propagating crack tip. Not only the principal direction but also the principal stress difference around a growing crack is obtained. Then, the stress distributions around a crack are evaluated by the experimental–numerical hybrid method. Results show that the proposed hybrid method is effective for the study of crack growth behavior in the glass plate. The results obtained in this study show that the compressive stresses exist around the tensile stress region at the crack tip. It can be considered that the existence of the compressive stress distribution surrounding the tensile stress field at the crack tip leads to both the high value of the stress intensity factor and the crack oscillation.
: A method for determining mixed‐mode stress intensity factors from displacement fields obtained by digital image correlation is studied. To perform automatic evaluation, not only stress intensity ...factors but crack‐tip location, higher‐order terms in the series expansion of displacement fields and rigid‐body displacement components are also determined simultaneously using nonlinear least squares based on the Newton–Raphson method. Experimental results show that the mixed‐mode stress intensity factors are evaluated accurately from either radial or circumferential displacement components. As mixed‐mode stress intensity factors can be evaluated easily, simply and automatically by the technique with digital image correlation and nonlinear least squares, it is expected that the proposed method can be applied to solve various fracture problems.
For the structural evaluation of bridges, various tests are usually performed to know the structural properties such as natural frequencies, dynamic responses, deflections, and strains.1,2 In ...addition, it is important to monitor the variation of these properties as well as damages to evaluate the structural integrity of bridges.3,4 One of these properties is vertical deflection of bridge girders. Thus, a lot of applications of this method to various problems can be found, such as the studies of shape memory alloys18 and time-dependent materials.19 In addition to the simplicity of measurement, the recent progress of high resolution and handy digital cameras with zoom lenses has led the method to various new applications. The effect of random pattern on an object surface is also investigated by measuring the bridge surfaces with and without random pattern because the paint of random pattern that is found in solid mechanics applications is not possible for real bridges.
The aim of the present study is to investigate the nucleation and growth behavior of twin region around yield point of polycrystalline pure Ti under deformation. Firstly, we prepare commercial ...polycrystalline pure Ti plate, and investigate the microstructure and pole figures using an Electron Backscatter Diffraction Patterns device. Secondly, tensile specimens are cut out from 0°, 30°, 45° and 90° relative to plate rolling direction. Then, we measure the macroscopic stress–strain curve, local strain distribution and nucleation and growth of twin region arising in specimens under uniaxial tensile loading. Results show the anisotropic characteristics in those behaviors. Those could be related to
c
axis in hcp lattice. However, detailed anisotropic mechanism may have something to do with several interactions between slips and twins arising in its body. It is also understood that the avalanche behavior of twin region nucleation occurs as a result of larger twin region formation, with inhomogeneous small twin region nucleation in transient process. Finally, we could suppose the bridge mechanism of deformation behaviors from macroscale to microscale for polycrystalline pure Ti under deformation.
Melittin, a bee venom, is a basic amphiphilic peptide, which mainly acts on the lipid matrix of membranes, lysing various cells. To elucidate the molecular mechanism, we investigated its interactions ...with phospholipid vesicles. The peptide formed a pore with a short lifetime in the membrane, as revealed by the release of an anionic fluorescent dye, calcein, from the liposomes. Our new double-labeling method clarified that the pore size increased with the peptide-to-lipid ratio. Upon the disintegration of the pore, a fraction of the peptides translocated across the bilayer. The pore formation was coupled with the translocation, which was proved by three fluorescence experiments recently developed by our laboratory. A novel model for the melittin pore formation was discussed in comparison with other pore-forming peptides.
•The full Stokes parameters can be obtained from a single image instantaneously using a camera equipped with microreaders.•The distributions of the isochromatic and isoclinic parameters as well as ...their time variations can be measure by the proposed instantaneous phase-stepping photoelasticity.•The crack tip stress fields are obtained by analyzing the stress intensity factors and T-stresses from the measured principal stress difference.•Time-variations of the stress intensity factors and the T-stress explain the complicated oscillating crack growth behavior.
A method for analyzing the phase of photoelastic fringes by polarization image analysis using the camera equipped with a pixelated microretarder array is described. Unlike the conventional phase-stepping method, this method does not require multiple images to be acquired and can analyze problems that vary from moment to moment. Therefore, this method can be applied to viscoelastic problems, crack propagation problems, and other problems involving variation with time.
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