Applied Mechanics and Materials Vol. 782

Abstract: In order to make the specimen deformed under a constant strain rate and the stress in the specimen kept homogeneous, the wave shaper technology was adopted to modify the incidence waves of the normal Split Hopkinson Press Bar. A method of changing the shape of the bullet was suggested to be applied on the SHPB. Bullets with different length and different curvature have been researched in this paper. And the effection of the button head bullet about incidence pulse was simulated with Lagrange method by ANSYS/LS-DYNA. It is shown in the results that changing the curvature of the bullet impact the rising edge of incidence waves, and the peak stress increase with the speed of the bullet increase, the peak stress and length of incidence waves increased with the length of the button head bullet, when the peak stress reached a certain strength, increasing the bullet length could make the stress peak value lasted longer. Due to the reason that the button head bullet was based on the elastic wave theory, the wave length and the max stress of the shaped wave would be controlled conveniently and avoid the shortcoming that the analogue specimens could not be recycled in the normal pulse shaper technology.

Abstract: Digital speckle pattern interferometry (DSPI) is an advanced technique for both in-plane and out-of-plane deformation measurements of diffuse surfaces in nanoscale. It has been widely used in aerospace engineering and other high-tech industries due to the advantages of non-contact, high-accuracy and full-field measurement. Traditionally, DSPI uses temporal phase shifting method to achieve precise deformation measurement, but it is only suitable for quasi-static deformation. Spatial-carrier method is another effective phase retrieval method used in DSPI and its validity has been verified in some DSPI setups. DSPI with spatial-carrier method enjoys the advantages of simple optical arrangement, easy operation, and above all, high-speed measurement of deformation. This paper introduces a dual-beam spatial-carrier digital speckle pattern interferometry system, with which in-plane and out-of-plane deformations can be measured simultaneously as well as quickly. In the optical setup, two lasers are employed to illuminate the measured object with different illumination angles, and two single-mode fibers server as carriers to transmit the reference beams. In-plane and out-of-plane deformations can be obtained by combining the phase maps of both channels. Theoretical discussion and experimental analysis are both presented.

Abstract: VIsual Background Extractor (VIBe) is a simple and effective background subtraction model, it is important to choose the threshold according to different environments. A new method based on the VIBe is presented which uses the adaptive threshold. We give a new description of the similarity degree without threshold, and then use the energy minimization model to combine the spatial information and show that dynamically choosing threshold in our kernel estimates at each individual pixel can significantly improve results. To compare with the traditional VIBe algorithms, a synthetic test SABS and a real world test I2R are used. Experiment shows that the presented method is on average 10% better than the traditional VIBe in accuracy.

Abstract: Residual stress is one of the important factors in thermally sprayed deposits which affects both processing and performances. In the present study, the hole-drilling method and moire interferometry were used to determine the residual stress of thermally sprayed coatings to overcome certain limitations of the strain rosette version of hole drilling. The methodology was established based on the elastic theory and finite element method. The residual displacement was measured by moire interferometry and the residual stress of the thermally sprayed coating was calculated. The results have demonstrated that moire interferometry for surface displacement caused by hole-drilling is high sensitive, reliable and stable.

Abstract: Digital Volumetric Speckle Photography (DVSP), which is the extension of 2D digital speckle photography, is applied to measure the interior deformation and strain of the object by 2-step 3D FFT. For using FFT, DVSP has some advantages in computational efficiency. With the special loading setup for Micro-CT, the coal specimen under different uniaxial loadings is scanned, and the volumetric images are acquired. By using DVSP, the interior 3D displacements of the specimen are obtained, and then the equivalent strains and volumetric strains are estimated. Based on the interior displacement and strain mappings, the process of deformation or strain localization in the coal specimen is studied.

Abstract: Developing a true 3D experimental stress analysis technique has been the holy grail of solid mechanics researchers since the genesis of the field. Combined the high-resolution x-ray computer tomography (Micro CT) with the 2D speckle photography technique developed by one of us (Chiang), we recently proposed a new 3D strain analysis technique called Digital Volumetric Speckle Photography (DVSP). The elements of DVSP technique are as follows. A reference volumetric image of the specimen is recorded by a Micro-CT scan and stored as a reference. Under load the deformed CT image of the specimen is also recorded. Both volume images are divided into subimages of certain voxel arrays. Each corresponding pair of the subimages are ‘compared’ via a two-step 3D Fourier Transform analysis. The result is a 3D map of displacement vectors representing the collective displacement experienced by all the speckles within the subset of voxels. Strain distribution of the entire specimen can then be calculated using appropriate displacement strain relations. Application of this technique to strain mapping of red sandstone and composite are presented.

Abstract: In recent years, the research on the mesostructure of the concrete by using numerical simulation has been a hot topic. Mesoscopic research would contribute to reveal the concrete cracking failure mechanism. In this paper, the aggregate, the mortar , the interface and the pore are segmented based on CT images . With the aid of Mimics software, the numerical model for ANSYS is built. Besides this, the concrete specimen under uniaxial compression is scanned by using micro-CT in situ. The volumetric images under different loadings are acquired. By using digital volumetric speckle photography (DVSP) method, interior 3D deformation is obtained. Combined with the experiment results, the numerical model is optimized. Based on the optimized model, mesoscopic mechanics of concrete is analyzed by using numerical simulation.