In this article, the interlaminar shear behavior of a [±45°]4s laminated carbon fiber reinforced plastic (CFRP) specimen is investigated, by utilizing microscale strain mapping in a wide field of view. A three-point bending device is developed under a laser scanning microscope, and the full-field strain distributions, including normal, shear and principal strains on the cross section of CFRP, in a three-point bending test, are measured using a developed sampling Moiré technique. The microscale shear strain concentrations at interfaces between each two adjacent layers were successfully detected and found to be positive-negative alternately distributed before damage occurrence. The 45° layers slipped to the right relative to the −45° layers, visualized from the revised Moiré phases, and shear strain distributions of the angle-ply CFRP under different loads. The absolute values of the shear strain at interfaces gradually rose with the increase of the bending load, and the sudden decrease of the shear strain peak value implied the occurrence of interlaminar damage. The evolution of the shear strain concentrations is useful in the quantitative evaluation of the potential interlaminar shear failure.
Mechanical properties and damage onset stress in 24 or 25-ply unidirectional CFRP laminates that contain different thicknesses and gap lengths of fiber discontinuity are investigated by tensile testing and analytical model. Same damage behavior, as with previously reported, that interlaminar delamination between fiber continuous and discontinuous plies follows after crack initiation at the edge of the discontinuous fibers is observed even if thinner and longer gap of the fiber discontinuities are introduced. It has been shown that the laminates with long gap fiber discontinuity show higher stress of crack onset than that with short gap, and the crack onset stress decreases with the number of the discontinuous plies. A similar trend can be seen for the onset stress of the interlaminar delamination though no delamination has been observed in the short and long gap 1-ply and long gap 2-ply discontinuous laminates. The crack onset stress is evaluated by representing the energy release rate with crack initiation by using stress change in shear-lag model. Due to the fact that the relation between the number of discontinuous plies and crack onset stress can be predicted for both short and long gaps of fiber discontinuity by assuming a certain value of critical energy release rate, it is shown that the crack onset behavior is not affected by thickness and gap length of the fiber discontinuity. Another analytical model with an assumed critical energy release rate has successfully predicted the delamination onset stress regardless of sizes of fiber discontinuities by applying the exact thicknesses of continuous and discontinuous plies. By comparing the predicted delamination onset stress and fracture stress of the laminates, it is concluded that two and more fiber discontinuous plies can affect the overall damage behavior of the laminates.
A buckling test of composite cylindrical shells with a radius–thickness ratio (r/t) = 893 under axial compression was conducted to investigate the effects of the radius–thickness ratio (r/t). It is known that the buckling load of cylinders shows large differences and scatter between theory and experiment. The ratio of the experimental buckling load and theoretical buckling load is called the knockdown factor (KDF). Many investigations have been conducted to find the cause of the degradation and scatter in the KDF, but as yet, no cause has been found. In 1968, NASA’s buckling design criterion, NASA SP-8007, gave an empirical KDF curve that decreased with the increasing r/t (up to 2000) for metal cylinders. The same curve has been applied to composite cylinders. Recently, Takano derived a flat lower-bound KDF in terms of A- and B-basis values (99% and 90% probability with a 95% confidence level) through a statistical analysis of experimental buckling loads. The result, however, based on experimental results up to r/t = 500 and, thus, the dependency on a large range of r/t, is not clear. Thus, the authors focused on a larger range of r/t. Cylindrical shells made from carbon fiber-reinforced plastic (CFRP) were tested. The nominal radius, thickness, and length were r = 100.118 mm, t = 0.118 mm, and L = 200 mm and, thus, the r/t = 848 and length-to-radius ratio (L/r) = 2.0. Shape imperfections were also measured by using in-house laser displacement equipment. The buckling load was slightly affected by the r/t, but the reduction in the KDF was insignificant.
This study presents a workflow to predict the upscaled absolute permeability of the rock core direct from CT images whose resolution is not sufficient to allow direct pore‐scale permeability computation. This workflow exploits the deep learning technique with the data of raw CT images of rocks and their corresponding permeability value obtained by performing flow simulation on high‐resolution CT images. The permeability map of a much larger region in the rock core is predicted by the trained neural network. Finally, the upscaled permeability of the entire rock core is calculated by the Darcy flow solver, and the results showed a good agreement with the experiment data. This proposed deep learning based upscaling method allows estimating the permeability of large‐scale core samples while preserving the effects of fine‐scale pore structure variations due to the local heterogeneity.
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