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Lamb wave propagation must be understood comprehensively for simply evaluating delamination during ultrasonic testing. However, the difference between wave propagation, visualized using laser Doppler vibrometer and pulsed-laser scanners, has not been sufficiently investigated, and knowledge of optimal conditions for evaluating delamination is limited. Thus, in this study, the mode conversion and multiple reflections of Lamb waves propagating in a delaminated cross-ply laminate were visualized using different laser scanners, delamination depths, and wave incident angles. Delamination was characterized using maximum-amplitude map postprocessing under specific conditions. Further numerical analysis revealed that owing to multiple reflections of the antisymmetric mode in incident and mode-converted waves, standing waves were generated in the delaminated sublaminate. Dispersion curve and flexural stiffness calculations confirmed the conditions required for high-amplitude standing waves, thereby providing guidelines for simply and reliably evaluating delamination during inspections.

In industrial processes, piercing and trimming are essential because composite structures are usually manufactured in a near-net shape to reduce machining operations. Punching and shear cutting using out-of-plane shear loading are expected to increase productivity. Nevertheless, little is known about the effects of such operations on polymer-matrix composites. This study presents on the characterization of piercing damage in typical carbon fiber reinforced plastic (CFRP) cross-ply laminates [0°2/90°2]s after punching using quasi-static (QS) and drop-weight impact (DWI) loadings. During QS punching, the upper and lower ply interfaces delaminate due to the high shear stress to cut fibers and gradual shear deformation in the middle ply; however, during DWI punching at a low impact velocity, delamination of the lower ply interface can be reduced due to the localization of shear deformation, as compared to that in QS punching. Finally, the damage accumulation process during DWI punching is discussed.

This paper presents a stiffness and permeability multi-objective optimization method for carbon-fiber-reinforced plastic mesostructures based on a homogenization method. To reduce the computational cost of dealing with multiple design variables for complicated fiber mesostructures, we generate and extract effective design variables from optimization results derived from a smaller number of design variables. We applied the proposed method to optimization of the in-plane and out-of-plane stiffness and permeability of non-crimp fabrics. The optimization results showed that the application of effective design variables enabled attainment of an improved elastic modulus and permeability. From investigations of the obtained optimized design, we clarified the trade-off relationship between the elastic modulus and permeability, and elucidated the effects of dimensions of non-crimp fabric mesostructures on the elastic modulus and permeability.

Understanding the shear behavior and resulting fiber failure of fiber-reinforced plastics is required for better prediction of their behavior during the machining process, but knowledge regarding the shear strength of fiber failure is limited. In this study, out-of-plane shear tests were conducted to observe the shear behavior of carbon fiber-reinforced plastic laminates subjected to high shear stress exceeding the shear strength of matrix failure. The longitudinal fibers in carbon fiber-reinforced plastic unidirectional laminates were cut by shear loading without severe internal damage and the maximum shear stress causing progressive fiber breaks was much higher than the shear strength of matrix failure. This result suggested the possibility of out-of-plane shearing as a machining method for fiber-reinforced plastics and shear tests were subsequently performed for carbon fiber-reinforced plastic cross-ply laminates. Delamination was generated by high shear stress to cut the reinforcing fibers, but the size of the remaining damage was small even in the thermoset carbon fiber-reinforced plastic laminates in which delamination likely occurs, without any optimization of the trimming conditions.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2017-08-27, 資料番号: PA1820040000

In mode II interlaminar fracture toughness tests of composite laminates, the fracture toughness evaluated based on linear elastic fracture mechanics deviates from the true toughness due to the material nonlinearity of the matrix. In this study, an elastoplastic finite-element analysis was performed on a new quasi-static mode II interlaminar fracture toughness test, i.e., the doubly end-notched tension (DENT) test, to investigate the difference between the perceived energy release rate and the J-integral. The effect of material nonlinearity was compared with other quasi-static mode II interlaminar fracture toughness tests: three- and four-point bend end-notched flexure tests. Analysis of the DENT showed that for a given J-integral loading, the size of the plastic zone at the delamination crack tip was the same regardless of the crack length. The difference between the perceived energy release rate and J-integral in the DENT test with respect to the plastic zone size was independent of the crack length, and was smaller compared with that of flexure-type tests. Furthermore, the compliance calibration method was applied to the elastoplastic analysis results of the DENT test to reduce the evaluation difference of the energy release rate.

This paper quantitatively clarifies the effect of the fiber cut angle on the shearing strength of carbon-fiber-reinforced thermoplastic laminates. First, the shearing strength of the unidirectional and cross-ply laminates was measured at various cut angles. Next, two mechanics models, a rule-of-mixture-based model and a failure criterion model, were established to predict the shearing strength of the unidirectional and cross-ply laminates. The rule-of-mixture model explains the effect of the cut angle on the shearing strength of the unidirectional laminate. The failure criterion model predicts the dependence of the cut angle on the shearing strength for both laminates. Finally, finite element analysis was performed to semi-quantitatively evaluate the assistance effect of bending stress. It was revealed that the effective bending stress is almost equivalent to the stress averaged over approximately the one-layer thickness from the surface.

This study evaluated fatigue-induced disbond areas in carbon fiber–reinforced polymer double-lap joints using embedded fiber Bragg grating sensors. When the disbond grew by cyclic loading, the embedded fiber Bragg grating sensors yielded reflection spectra having two peaks representing a step-like strain distribution generated by the disbond; the peak at the shorter wavelength corresponded to the unloaded disbond region. The ratio of the peak intensity at the shorter wavelength to that at the longer wavelength increased gradually with increasing disbond length. The relationship between the peak intensity ratio and the disbond length was analyzed by coupled structural–optical analysis and was validated by comparing analytical peak intensity ratio with the experiment results. The disbond length was then estimated from the measured spectra based on this analytical calibration relationship, but the estimated disbond area exceeded that observed using the ultrasonic C-scan technique. Additional experiments including destructive observation of the adhesive suggested that an embedded fiber Bragg grating sensor could detect a moving disbond tip earlier than conventional nondestructive techniques.

Particle simulation methods represent deformation of an object by motion of particles, and their Lagrangian and discrete nature is suitable for explicit modeling of the microstructure of composite materials. They also facilitate handling of large deformation, separation, contact and coalescence. Mesh-free particle methods will thus be appropriate for a part of issues throughout the lifecycle of composite materials despite their high calculation cost. This study focuses on three particle simulation methods, namely, smoothed particle hydrodynamics, moving particle semi-implicit method, and discrete element method, and reviews approaches for modeling composite materials through these methods. Applicability of each method as well as advantages and drawbacks will be discussed from the viewpoint of engineering of composite materials. This reviewing study suggests capability of particle simulation methods to handle multiphysics and to predict various complex phenomena that necessitate explicit modeling of the material’s microstructure consisting of reinforcements (inclusions), matrix, and voids.

A hollow cylinder specimen of Cr-Mo steel with 0.20 mass% C was used as a model which simplified a motor cycle transmission gear. The specimen was carburized in carrier gas and quenched in oil bath. After quenching, the internal residual strain distributions in the radial, axial and hoop directions were mapped nondestructively by neutron strain scanning, and were compared with results of elastoplastic finite element analysis. The carbon content and hardness gradients were also measured to determine the case depth. Residual strain was calculated from lattice spacing change. In this study, unstressed lattice spacing was determined experimentally by measuring diffraction angle of coupons that were cut from the interior of the same carburized cylinder specimen. As a result, the carbon content and hardness distributions were almost accorded with finite element analysis results. On the other hand, the measured strains in some measuring points discorded with the analyzed weighted average strains. The peak widths in the interior of specimen equaled to that of unstressed coupons. Internal residual strain distributions were complicated and found to be elastic strains that were balanced with equivalent plastic strains due to transformation plasticity of case layer.

800x600 The short and long Japanese swords “WAKIZASHI” (back up sword of the main sword, “KATANA”) were made by one sword craftsman. The short and long swords “WAKIZASHI” were machined by rough grinding and final hand polishing, respectively. The microstructure, carbon content and hardness of short sword were measured experimentally on the cross section of sword blade. The 2θ-sin2ψ diagrams from Fe-211 plane using Cr-Kα radiation on the ground and polished blade were measured. An influence of final hand polishing process on the surface residual stress field of sword blade was examined. As a result, biaxial principal compressive residual stresses were generated and had constant stress gradients in depth on the ground and polished surfaces because the ψ-splitting was not observed and the measured 2θ vs. sin2ψ relations could be approximated as a parabolic curve. Large compressive residual stresses more than −1.0 GPa were distributed on the ground surface from “HASAKI” to “HAMON” of short sword. The surface compressive residual stress and its gradient were diminished gradually from “HAMON” to “MUNE” (the ridge of sword). On the other hand, compressive residual stresses more than −650MPa were distributed on the pol­ished surface from “HASAKI” to “HAMON” of long sword. The surface compressive residual stress and its gradient were also diminished gradually from “HAMON” to “MUNE”, and the residual stress gradient in the transverse direction were greatly degraded in comparison to the short sword. Additional compressive residual stress field induced by rough grinding was superimposed on the residual stress field after tempering process. The residual stress field near the blade surface after rough grinding was released partly by final hand polishing. Normal 0 21 false false false DE X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Century","serif";}

This study investigates characteristics of foreign-object damage in plain-woven SiC/SiC composites after thermal loading. High-speed impact tests were conducted on virgin specimens, thermally exposed specimens, and thermally shocked specimens, in which the maximum temperature during thermal loading was 600 °C or 1000 °C. An oxide layer was generated on the specimen surface by thermal loading at 1000 °C. Damaged areas on the front and back surfaces induced by particle impact were independent of thermal loading. However, in specimens thermally loaded at 1000 °C, brittle failure, i.e. cone cracking without fiber pull-out, occurred due to oxidation of the fiber/matrix interfaces, and the ballistic limit velocity significantly decreased. Finally, the ballistic limit is predicted using static strength properties, and the effect of thermal loading on impact resistance is discussed.

This paper aims at characterizing the high-velocity impact damage behavior in carbon fiber reinforced plastic (CFRP) unidirectional (UD) and cross-ply (CP) laminates. First, the surface and internal damages of CFRP plates impacted at a velocity of 200 and 430m/s were observed by using optical microscopy together with radiography. Next, dynamic finite element analysis was performed to simulate the damage process. Cohesive elements were introduced to express the delamination and splitting cracks while the maximum stress fracture criteria were employed to express the intralaminar failure. Finally, the simulations were compared with the experiment results to verify the reasonability of the analysis. The damage process in both laminates was as follows : 1. A crater accompanied by splitting cracks is generated at the impact point on the front surface while splitting cracks due to bending are observed on the back surface. 2. Cone-shaped matrix cracking is observed in the UD laminate while few matrix cracking is generated in the CP laminate. 3. The shape of the delamination in the UD and CP laminates is a galaxy and a circle, respectively. Delamination size is larger in the CP laminate than in the UD laminate, which results in higher energy absorption capability in the CP laminate. Simulations are in qualitatively good agreement with the experiment results.

This study experimentally and numerically investigate fatigue damage in CFRP cross-ply laminates with an open hole under some test conditions, and verified the numerical simulation for predicting the fatigue damage progress proposed by the authors. Splits in the 0° plies, transverse cracks in the 90° plies and delaminations at 0°/90° ply interfaces were experimentally observed due to the cyclic loading, even if the maximum stress was much lower than the stress to generate the initial damage under the static loading condition. All the cracks gradually extended with increasing number of cycles under the low stress levels. After a damage analysis with cohesive elements was confirmed to represent the damage under static loading condition, the fatigue damage observed near the hole was analyzed by the damage analysis with a fatigue damage law that was applied to the stiffness degradation process of cohesive elements. The predicted fatigue damage progress agreed well with the observations, and this approach accurately represented the difference of the damage state due to the change in the critical energy release rate or in the maximum stress in cyclic loading. These experiments and simulations demonstrated the validity of the numerical approach for predicting the fatigue damage progress in composite laminates, and the parameters in the fatigue law for cohesive elements were discussed.

This study proposes a numerical approach for predicting the injection molding process of short-fiber-reinforced plastics using the moving particle semi-implicit (MPS) method, which is a particle-simulation method. Unlike conventional methods using orientation tensors, this approach represents all fibers and resin as an assembly of particles, and automatically analyzes the interaction between fiber and resin and between fibers. In addition, this method can follow the motion of a specific fiber, which is a significant advantage over orientation tensors. This study simulated the injection molding of short-fiber-reinforced plastics; the thermoplastic resin was considered as an incompressible viscous fluid and the fibers were modeled as rigid bodies. The numerical result illustrated that the molding material was unidirectionally reinforced by short fibers since the fibers rotated and were aligned parallel to the flow direction due to the velocity gradient near the wall boundary. Moreover, the stagnation of resin at a corner was predicted. The results agreed well with previous studies, and the present approach was confirmed. Beyond this, we predicted the accumulation of fibers near the wall due to the velocity gradient, which could not be represented by conventional simulations based on orientation tensors.

This study investigated nondestructive testing of welded steel plates by imaging ultrasonic wave propagation based on laser generation. The specimens were scanned by a pulsed laser, and the signals were received by a fixed piezoelectric transducer. A moving diagram of wave propagation from the fixed point was obtained directly from the collected signals. Wave scattering due to a weld defect (e.g., toe crack or root crack) was successfully imaged on weld specimens, and was easily recognized by visual observation in the measured moving diagrams. These experiments demonstrated the ability of the imaging technique to inspect a large area in a short time and to reliably detect a defect. Furthermore, the position-time-amplitude maps (B-scope images) were reconstructed from the moving diagram along some lines perpendicular to the defect, and the location and the size of a root crack were evaluated based on the B-scope images.

This paper presents a fracture criterion for predicting the strength of a CFRP unidirectional laminate with a hole. We formulated the point stress criterion (PSC) for notched composites with the characteristic distance at which the Tsai–Hill criterion for orthotropic materials was satisfied. The strengths of CFRP off-axis unidirectional laminates with a hole predicted by the PSC agreed well with the measured strengths, and the validity of the characteristic distance determined by the Tsai–Hill criterion was confirmed. Moreover, we observed the fractured surfaces of the holed specimens with small off-axis angles and found that the fracture mode within the characteristic distance (fiber breakage mode) was different from that away from the hole (shear fracture mode). These observations of the fractured surfaces suggested that the characteristic distance represented the size of the initially damaged area at the notch tip.

This study presents a numerical simulation to predict damage progress in notched composite laminates under cyclic loading by using a cohesive zone model. A damage-mechanics concept was introduced directly into the fracture process in the cohesive elements in order to express crack growth by cyclic loading. This approach then conformed to the established damage mechanics and facilitated understanding the procedure and reducing computation costs. We numerically investigated the damage progress in holed CFRP cross-ply laminates under tensile cyclic loading and compared the predicted damage patterns with experiment results. The predicted damage patterns agreed with the experiment results that exhibited the extension of multiple types of damage (splits, transverse cracks, and delamination) near the hole. A numerical study indicated that the change in the distribution of in-plane shear stress due to delamination induced the extension of splits and transverse cracks near the hole.

This study proposes a novel technique to visualize ultrasonic waves propagated in a general solid medium. A specimen was scanned by a pulsed laser for generation of ultrasound, and the propagated waves were received by a fixed transducer. This technique then provided a snapshot of traveling waves based on the reversibility for the wave propagation, which is confirmed in this paper. We experimentally and numerically investigated and visualized the wave propagation in a steel plate with a slit. Furthermore, we demonstrated inspection of an elbow pipe with an artificial defect, as an application of the proposed technique to a structural component. Non-contact laser scanning for ultrasound excitation enabled us to inspect a wide area of an arbitrarily shaped object. The distinguishing advantages of this technique were determined to be well-suited to the quick and easy wide-range inspection of real structures.

This paper discusses the fracture criteria for predicting the strength of unidirectional discontinuous glass fiber reinforced composites with side-notches. The point stress criterion (PSC) for notched composites was employed so that the Tsai-Hill criterion for orthotropic materials is satisfied. The fracture criterion proposed on the basis of linear notch mechanics (LNM) proposed by Hyakutake et al. was also discussed in association with the above two criteria. Strength of intact and notched unidirectional composites was experimentally obtained for various kinds of off-axis angle and notch depth. The stress concentration factor in the vicinity of a notch was calculated with the use of finite element analysis. It was found that the notched to intact strength ratio is almost independent of off-axis angle though it slightly depends on notch depth. The characteristic length for the PSC estimated through the above experimental and analytical results increases with increasing off-axis angle and notch depth. The critical maximum stress of the LNM-based criterion was shown to be a function of fiber orientation angle, but almost independent of notch depth. The LNM criterion was proved to be equivalent to the PSC that satisfies the Tsai-Hill criterion.

This study experimentally and numerically investigated the tensile damage progress in stitched laminates. In particular, it focused on the effects of stitching on the damage progress. First, we experimentally confirmed that ply cracks and delamination appeared under load regardless of stitching. We then performed damage-extension simulation for stitched laminates using a layer-wise finite element model with stitch threads as beam elements, in which the damage (ply cracks and delamination) was represented by cohesive elements. A detailed comparison between observation and the simulated results confirmed that stitching had little effect on the onset and accumulation of ply cracks. Furthermore, we demonstrated that the stitch threads significantly suppressed the extension of the delamination.

This study proposes two new approaches for identifying damage patterns in a holed CFRP cross-ply laminate using an embedded fiber Bragg grating (FBG) sensor. It was experimentally confirmed that the reflection spectrum from the embedded FBG sensor was significantly deformed as the damage near the hole (i.e. splits, transverse cracks and delamination) extended. The damage patterns were predicted using forward analysis (a damage analysis and an optical analysis) with strain estimation and the proposed damage-identification method as well as the forward analysis only. Forward analysis with strain estimation provided the most accurate damage-pattern estimation and the highest computational efficiency. Furthermore, the proposed damage identification significantly reduced computation time with the equivalent accuracy compared to the conventional identification procedure, by using damage analysis as the initial estimation.

This study proposes a new approach to monitoring the damage process in holed CFRP laminates using an embedded chirped fiber Bragg grating (FBG) sensor. To this end, we experimentally and numerically investigated the damage process and the damage-induced changes in the spectrum shape. It was experimentally confirmed that multiple types of damage (e.g., splits, transverse cracks and delamination) appeared near a hole, and that the spectrum shape of the embedded chirped FBG sensor changed as the damage extended. Our proposed simulation for the reflection spectrum considering the damage agreed with the experiments. Furthermore, this study investigated the effect of each damage pattern on the changes in the spectrum shape. Finally, based on these discussions, we present simple damage identifications with the embedded chirped FBG for the holed CFRP laminates under completely unloaded conditions.

This paper proposes a new approach to estimate the damage pattern of a notched cross-ply laminate using an embedded Fiber Bragg Grating (FBG) sensor. The damage pattern near the notch was investigated by finite element analysis with cohesive elements for the onset and the extension of various cracks. Then, the reflection spectrum of the FBG sensor was analyzed from the strain distribution obtained in the damage analysis. Based on the above analysis, the damage pattern is optimized as an inverse problem in the damage analysis, while the spectrum shape is adopted as the objective function. In the estimation scheme, the strain distribution of the FBG sensor is calculated by the damage analysis using the damage pattern expressed by the residual strength distribution of the cohesive elements. We estimated the damage patterns of the notched cross-ply laminate from the reflection spectra obtained in the analysis and those measured in the experiment. The estimated damage patterns agreed well with those corresponding to the input spectra.

This study applied a newly developed technique for visualizing ultrasonic waves to damage detection in holed CFRP laminates. This technique provided a moving diagram of propagating waves with the use of a pulsed laser for generation scanning a specimen and a fixed transducer for reception. Multiple types of damage (i.e. splits, transverse cracks and delamination), as well as delamination during processing, were observed near the hole in the loaded specimens by soft X-ray radiography. The specimens were then inspected by visualizing the propagation of ultrasonic waves. The S0 and A0 Lamb modes were visualized on the CFRP cross-ply laminate. Furthermore, we successfully detected delamination near the hole through the appearance of scattered waves. These inspection results agreed well with that by soft X-ray radiography, and consequently confirmed the usefulness of the technique for visualizing ultrasonic waves in inspection for composite laminates.