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The Automated Fiber Placement (AFP) is the technology for automated layup of multiple narrow prepreg tapes in CFRP (Carbon Fiber Reinforced Plastics) manufacturing of structural components for aircraft. Because most AFP devices cut the material perpendicular to the feed direction, geometrically complex layup can result in localized gaps and overlaps that can affect the mechanical properties of the molded product. The bending modulus of prepreg tapes is the dominant factor for defect formation in the layup process. For the layup using CFRTP (Carbon Fiber Reinforced Thermoplastics) tapes, the material is heated during the tape layup and thus the temperature variation and distribution during forming have a large effect on the bending modulus of prepreg tapes [1]. In this study, we modeled the laminate with internal defects made by the AFP process, and we performed a coupled structural thermal analysis for the gap elimination when the out-of-plane pressure was applied, considering the temperature-dependent material properties of CFRTP. We showed a relationship between the bending stiffness of the layer above the gap and defect formation in CFRTP.

In this study, we investigated the improvement in carbon fiber (CF) bundle tensile tests. To effectively evaluate the mechanical properties of CF using the fiber bundle tensile test, measurement of system elongation correction considering nonlinear behavior and Weibull analysis using fitting of stress–strain curves was conducted. Shear-lag theory analysis revealed that the elongation in the nonlinear measurement system was largely governed by the adhesive layer deformation within the tabs. A nonlinear measurement elongation correction method was proposed that can be applied to specimens of different widths. Weibull parameters were extracted by fitting the stress–strain curves, while considering the effect of the kinetic frictional force. The obtained parameters from the fiber bundle tensile test corresponded to those obtained from the SFTT. Thus, the proposed fiber bundle test can be a simple method for evaluating fiber strength distribution.

A major modification has been made to the conventional mode-locked femtosecond laser autocollimator so that the apex angle of a small prism, which is a transparent object difficult to be measured by the conventional optical methods, can be measured. In the oblique-incidence optical setup, a collimated mode-locked femtosecond laser is at first made incident to a target prism, and the laser beam reflected from the prism surface is then projected onto a reflective-type grating for angle measurement based on the principle of the femtosecond laser autocollimator. With the robust signal processing technique in the femtosecond laser autocollimator where the spectrum of the first-order diffracted beams emanated from the grating is captured by the fiber detector, high-precision angle measurement can be realized even on a surface having a small area for measurement and/or on a surface having low reflectivity. An oblique-incidence femtosecond laser autocollimator with a fiber-based femtosecond laser source is developed, and its basic characteristics are then evaluated in experiments. An experimental setup composed of the oblique-incidence femtosecond laser autocollimator and a high-precision rotary table is also constructed for prism angle measurement. To avoid the influence of the internally reflected beam in a prism, a method employing the characteristic of Brewster's angle is newly proposed, and its feasibility is verified through experiments. The uncertainty analysis on the measurement of the prism apex angle by the oblique-incidence mode-locked femtosecond laser is also carried out based on GUM.

In this study, the mechanisms governing compressive strength in quasi-isotropic carbon-fiber reinforced-plastic laminates were clarified to gain an understanding of the results of non-hole compress...

In this study, impact and compressive after impact (CAI) tests using varying impact energies were conducted using ply-level hybrid laminates composed of thin and standard ply clusters. In addition, fiber metal laminates (FMLs), which consisted of thin-ply prepregs and stainless layers with a thickness of 0.04 mm, were made in order to suppress the fiber breakage that caused a decrease in CAI strength. Next, impact and CAI tests were conducted on FMLs in order to investigate the effect of the inserting location in the thickness direction and the number of metal layers in failure mode, as well as CAI strength. The results confirmed that CAI strength was improved by both ply-level hybridization and the insertion of metal layers. In ply-level hybrid laminates, the incidence of fiber breakage and delamination was reduced by changing the ratio of thin-ply prepregs and their through the thickness location. In addition, plastic deformation of metal layers in FMLs led to a larger dissipated energy; thus, the failure occurred in a limited location, such as fiber breakage at the non-impacted laminate face.

For unidirectional thermoplastic composite materials, it is preferable to use tab-less specimens in tensile tests owing to the low adhesive performance between specimens and tabs, as well as considerable warpage in laminates due to compression molding. In this study, round-robin tests are performed for unidirectional laminates in the 0° and 90° directions by two types of thermoplastic composites – carbon/polyamide 6 and carbon/polypropylene. The purpose of the round-robin test is to examine the difference between tab-bonded and tab-less specimens. Statistical analyses determined the degree to which tab-less specimens influenced their evaluation of the mechanical performance. In addition, from the detailed experiments, precisely controlled gripping force, fine roughness of grip surfaces, and a few inserted abrasive papers had significant impact on the 0° tensile strength of tab-less specimens. Based on the results, 0° tab-less strength of the proposed gripping method was shown to be almost equal to that of tab-bonded specimens recommended by the present tensile test standard.

In this study, flexural tests using both thermoset and thermoplastic carbon fiber reinforced plastics (CFRP) laminates with varied ply thicknesses were conducted in order to evaluate the effect of ply thickness and matrix resin on the flexural properties of thin ply composites. Results confirmed that the flexural properties were improved by decreasing the ply thickness in both thermoset and thermoplastic laminates. Furthermore, in-situ observation of specimens during the test revealed that short delamination occurred in the carbon fiber carbon fiber reinforced thermoplastics (CFRTP) thin ply laminate, while long delamination was observed in case of carbon fiber reinforced thermosets (CFRTS). This indicates that damage progressions of the laminates were influenced by the toughness of the matrix resin. On comparing the in-situ strength in the first ply failure calculated from the classical laminate theory in the thin and thick ply laminates, it was confirmed that high damage resistance was caused by higher in-situ strength in each layer in the thin ply laminate.

This study investigates the influence of local hybridization of Thin and Thick-Ply CFRP laminates on the open-hole and bearing properties. The area weight of the CFRP unidirectional prepregs used is 40 gsm in the case of Thin-Ply layers and 160 gsm in the case of Thick-Ply layers. The steel used is a 1.4310 stainless-steel foil with the same layer thickness as the prepregs. In the hybrid area, 90 ∘ layers were locally replaced by stainless steel patches. The local metal foil content varies from 6.25%, 12.5% to 25.0%. For notched laminates, the open hole tensile strength is significantly decreased with thinner layer thicknesses. The failure behavior changes from complex delamination dominated failure to brittle failure. By using stainless steel foils in the regions of stress concentrations, energy can be dissipated by plastic deformation of the steel foil and stresses can be deflected to neighbouring areas. For Thin-Ply samples with a local steel content of 25% the open hole tensile strength increases by 64% in comparison to the reference Thin-Ply specimens and the sensitivity towards stress concentrations decreases. The bearing strength of the hybrid CFRP laminates is increased by up to 54.6% in comparison to the reference material, due to the confinement of the steel foil and the resulting higher compressive strength and the plastic deformation at high stresses. The stress–strain diagrams and micrographs of the fibre metal samples reveal that damage is initiated before the maximum bearing strength. However, the damage offset bearing strength increase concerning the specific density of the material significantly.

The present study investigated the damage analysis based on the peridynamics (PD) theory, which can deal with damage in composites with arbitrary fiber orientation, as a basis for enhancing the strength properties of the composites with local fiber orientation. The damage of the composites was reproduced by setting the appropriate bond properties (bond constant and critical stretch). The advantages and disadvantages of the PD models for the damage in composites with curvilinear fiber paths were also discussed. The PD model was further extended to address the multiple-type and multiple-site damage in multi-ply laminated composites with arbitrary fiber orientation. Finally, in order to apply the PD model to composites with the AP-PLY (Advanced Placed Ply) configuration allowed by current fiber placement technology, the efforts were made to establish an integrated framework of forming and mechanical analyses for future composite manufacturing and design.

In this study, void distributions in unidirectional carbon fiber-reinforced plastic (CFRP) laminates were measured using X-ray computed tomography (CT), and the stress concentrating effects of the voids on the reduction of the CFRP matrix-dominated strength were modeled. We first investigated the statistical features of the void sizes, geometrical shapes, and locations by analysis of the X-ray CT three-dimensional images. The natural logarithms of the void sizes showed a three-dimensional normal distribution; consequently, the void shapes and aspect ratios were correlated. Next, using the measured results regarding the voids, the stress concentration effect on the reduction of transverse tensile strength was modeled based on the Weibull model considering the volume dependence of strength. From the results, we attributed the decrease in the matrix-dominated strength with increases in void volume fraction to the increase in stress concentration volumes accompanying the increased number of voids.

In this study, the viscosity of unidirectional CFRTP laminates near the melting point was evaluated to discuss the relationship between the thermal fusion conditions and the flow of fiber and matrix. The laminates were regarded as a fluid flowing in a direction perpendicular to the fiber. The microscopic flow of the fiber and matrix near the melting point was observed during a parallel plate compression test. Based on this observation, viscosity and flow properties were evaluated by the finite element method. The results of the parallel plate compression test showed that the test conditions and the decrease in the thickness of the laminates are related to the apparent viscosity of the unidirectional plate. In addition, cracks in the direction of the maximum shear stress were detected via in situ observation near the melting point of the matrix. The occurrence of this phenomenon depended on the apparent viscosity of the unidirectional laminates and it was found that no cracks occurred when the apparent maximum viscosity of the unidirectional laminates was lower than a certain threshold.

The present study investigated nonlinear elastic-plastic stress-strain relationships of discontinuous carbon fiber-reinforced thermoplastics (CFRTPs) using experiments and numerical simulations. In the experiments, we conducted uniaxial tensile tests and three-point bending tests for two types of CF/PA6 (carbon fiber/polyamide 6) specimen: injection-molded specimens with short fiber length and aligned fiber orientation, and compression-molded specimens with long fiber length and random fiber orientation. Comparison of the experiment results indicated that the injection-molded specimens exhibited a nonlinear stress-strain response, while the compression-molded specimens exhibited an almost linear response. These results implied that long discontinuous fibers effectively increased the yielding point of composites, even if the composites had random fiber orientation, which eliminated orientation dependence on mechanical properties of the composites. Furthermore, we attempted to simulate elastic-plastic stress-strain relationships of discontinuous CFRTPs in an effort to understand the effect of the microstructure, including fiber length. For this purpose, we employed fiber-based simulations to deal with the microstructure of fibers and matrix and the constitutive law of the matrix. The simulated results indicated that fiber length influences the nonlinearity of the stress-strain relationships of discontinuous CFRTP composites.

There are two types of flow mechanism in prepreg processing: shear flow, in which resin and fibers flow together, and percolation flow, in which resin flows out of the gaps between fibers. In this study, we experimentally evaluated how these flow mechanisms change during cure including the gelation stage. Results of compression tests at various initial degree of cure at the start of compression showed that the maximum amount of shear flow is almost constant before matrix gelation. In the gelation region of matrix resin, shear flow still occurs at the gel point whilst percolation flow has completely stopped. Furthermore, it was suggested that the limit of the shear flow in the gelation stage can be described by the combined effects of the shear modulus of the fiber-bed and the shear storage modulus of matrix resin.

This paper presents a method to simultaneously estimate the interfacial stiffnesses between the adhesive and the adherends of an adhesive joint, and the adhesive thickness by combining the experimental measurement based on the ultrasonic reflection method and theoretical analysis. The interfacial stiffnesses and the adhesive thickness were evaluated for an aluminum adhesive joint to study the effects of contamination of the release agent. The optically observed thickness of the adhesive is in good agreement with the theoretical results, indicating that the estimation is appropriate. The interfacial stiffness of the interface contaminated by the release agent was estimated to be approximately 24% of the properly bonded interface. In addition, double-cantilever beam tests were performed on different specimens to evaluate the mode I fracture toughness. The results showed that the mode I fracture toughness decreased to 10% of that of the properly bonded interface owing to the contamination. The results suggest that the reduction in the mode I fracture toughness induced by adherend contamination can be detected by identifying the decrease in the interfacial stiffness based on the proposed method.

In this study, fiber metal laminates (FMLs), which consisted of thin-ply prepreg and stainless steel layers with a ply thickness of 0.04 mm, were fabricated with the aim of suppressing the fiber breakage that decreases the compression-after-impact (CAI) strength. Then, static flexural tests of FMLs were conducted to investigate the effects of the number of metal layers on the failure mode and mechanical properties during static out-of-plane loading, and the results were compared with those of our previous impact and CAI tests. The results confirmed that the plastic deformation of metal layers improved the energy dissipation and thus failure occurred in a limited range of locations. Furthermore, the investigation of the in situ stress during flexural tests by finite element method (FEM) analysis revealed that a hybrid material that both suppresses ply failure and has a high modulus of the metal can be realized owing to the constraining effect of thin-ply laminates.

The crack initiation and propagation under the application of opening load were analyzed in situ using nondestructive synchrotron radiation X-ray computed tomography (nanoscopic SR X-CT) with a spatial resolution of ∼50 nm. The results show that the voids and cracks initiation are not only the result of local stresses but also are due to two competing nanoscale mechanisms, that is, fiber/plastic interface debonding and in-resin crack initiation. In the “thin” epoxy region in which the resin thickness between the adjacent carbon fibers is small, cracks propagate mainly by debonding along the carbon fiber/epoxy interface. In the “thick” epoxy region in which the resin thickness between the carbon fibers is large, cracks propagate through the extensive plastic deformation of the epoxy resin, and the propagation mechanism largely depends on the resin thickness, which affects the plastic deformation behavior of the epoxy around the crack tip. Voids (sub-μm) often form in front of the crack tip and merge with the propagating cracks. Nanoscopic SR X-CT provides indispensable information on these nanoscale mechanisms, which cannot be obtained with traditional methods, including macroscopic observations and mechanical theory. These nanoscale mechanisms are essential for the mechanical modeling and analysis at multi-scales.

Crack initiation and propagation in carbon fiber-reinforced plastic (CFRP) was observed in situ under the application of an opening load using nanoscopic synchrotron radiation X-ray computed tomography (nanoscopic SR X-CT) at a high spatial resolution of ∼50 nm. Two datasets of reconstructed and segmented images were produced in typical regions, namely in the thin and thick epoxy regions where the resin thickness between the adjacent carbon fibers was small and large, respectively. This novel study presents the first non-destructive three-dimensional (3D) visualization of resin deformation behavior around crack tips, and provides a valuable and unique insight for the future design of CFRPs.

The present study investigated the effect of reinforcement on the viscoelasticity of shape-memory polymer composites (SMPCs) from two different scale views utilizing two different techniques. From a nano-scale view, we conducted dynamic viscoelastic simulations using molecular mechanics simulation models proposed by our research group [1]. These nano-scale simulations could reproduce a clear glass transition from glassy state to rubbery state in the frequency domain; therefore, they are suitable for studying the filler effect including interface properties. From a macro-scale view, we conducted experiment dynamic mechanical analysis using woven fabric based SMPCs. Through comparison of multi-scale analyses, we investigated the importance of nano-scale analysis for the composites’ design of viscoelasticity.

The purpose of the present study is to model shape fixity and time-dependent deployment in shape-memory polymer composites (SMPCs) and to evaluate the effect of textiles’ tensile and bending moduli on these properties. We constructed an SMPC model by combining SMP layers and a reinforcing layer. We also considered the thermo-viscoelasticity of SMP and the difference in values between the tensile and bending moduli of the reinforcing layer. Employing this model, we simulated deployment under pure bending conditions. Comparison with experimental results confirmed that our proposed model is able to simulate shape fixity and time-dependent deployment in SMPCs. We also confirmed that the bending modulus is an important factor for shape fixity and time-dependent deployment, whereas the tensile modulus has nothing to do with these properties.

Intralaminar and interlaminar fatigue crack growth behaviours under mode I loading were investigated with conventional and interlayer toughened unidirectional CFRP laminates. For intralaminar crack growth tests, initial defects were introduced using “intralaminar film insertion method”, in which a release film is inserted inside a single lamina prepreg. A fatigue test under a constant maximum energy release rate, Gmax, was carried out using DCB specimens. It was found that the intralaminar fatigue crack growth property of the interlayer toughened CFRP laminates was the same as that of the conventional CFRP laminates. For the interlayer toughened CFRP laminates, the Gmax with a given crack growth rate, da/dN, was much lower for intralaminar crack growth than for interlaminar crack growth. The da/dN-Gmax curve at zero crack extension, Δa = 0, which was estimated by extrapolating the da/dN-Δa relationship, was not affected by bridging fibres, and most conservative for the interlayer toughened CFRP laminates.

The present study presented a numerical model for the tensile damage in fiberreinforced composites with an open hole, using a peridynamics theory. Especially, peridynamics theory can deal with the damage in the composites having arbitrary fiber orientations by setting the bond properties (bond constant and critical stretch) appropriately. We compared two approaches for the peridynamics analysis of fiberreinforced composite materials, proposed by Oterkus and Madenci [1] and Ghajari et al. [2] and discussed the advantages and disadvantages of the peridynamics modeling for the damage in composites with curvilinear fiber paths. Finally, we reproduced the multiple-type and multiple-site damage in multi-ply laminated composites with arbitrary fiber orientations, using peridynamics simulations.

A novel initial crack insertion method, “intralaminar film insertion method”, was proposed to investigate the fracture toughness of unidirectional carbon fiber reinforced plastic (CFRP) laminates when the crack propagates inside the ply and not in the interlayer resin-rich area. Here, a release film was inserted inside a single lamina during the resin impregnation process of prepreg manufacturing. Mode I intralaminar fracture toughness tests were carried out for conventional CFRP laminates and interlayer toughened CFRP laminates. For comparison, two conventional methods were used to introduce initial cracks. One is the “interlaminar film method”, where a release film is inserted between two prepreg plies during the lay-up process. The other is the “machined slit method”, where a slit notch is machined in parallel to the layer of CFRP laminates. It was demonstrated that the proposed “intralaminar film method” can correctly evaluate the intralaminar fracture toughness of both conventional CFRP laminate and interlayer toughened CFRP laminate from the initial value to the propagation value. For this range, it was also found that the intralaminar fracture toughness of interlayer toughened CFRP laminate was the same as that of conventional CFRP laminate. Thus, the intralaminar fracture toughness was not influenced by interlayer toughening.

In the present study the effect of carbon nanofiber interlayers on the fatigue crack propagation of woven fabric CFRP laminates made by VaRTM is investigated. Double cantilever beam (DCB) tests are carried out to determine the mode I static fracture toughness and mode I fatigue crack propagation curve. The experimental results clearly show that the interlaminar fracture toughness and the fatigue crack growth resistance can be substantially improved by the addition of CNF interlayers to the CFRP laminates. In more details, as far as the mode I fatigue crack tests are concerned, thanks to the addition of MWNT-7 interlayers the number of cycles to failure becomes 1.5 or more times greater than that of base laminates. Furthermore, it is found that the fatigue fracture toughness G Imax (the upper limit of the energy release rate in a fatigue test) is increased by about 300%.