Akinori Yoshimura

X-ray computed tomography used to measure fiber orientation in CFRP laminates

This paper explains a new method to measure the fiber orientation in carbon fiber reinforced plastics (CFRP) laminates from X-ray CT images. In the method, the fiber orientation is analyzed by the application of digital image correlation (DIC) method to the acquired tomographic images. Using DIC, the brightness pattern, which results from the radiodensity difference between fiber and resin, is compared between two different planes in the thickness direction. Then, the three-dimensional displacement of the brightness pattern, which indicates the fiber orientation, can be measured. This study applied the proposed method to a quasi-isotropic CFRP laminate. After X-ray CT imaging, the sample was sectioned and polished. The fiber orientation was then measured experimentally using microscopy. The fiber orientation calculated using the proposed method agrees very well with the experimentally measured one. After demonstrating the validity of the proposed method, we applied it to a plain woven CFRP laminate. Results revealed that an invalid fiber orientation might be calculated for fibers parallel to the plane of the CT image, or for the fiber orientation of the pattern around the outer edge of CT images.

Interlaminar Fracture Toughness of 5 Harness Satin Woven Fabric Carbon Fiber/Epoxy Composites

Abstract This paper presents experimentally obtained results of mode-I and mode-II interlaminar fracture toughness (G IC and G IIC) of unidirectional and 5 harness satin (5HS) woven fabric carbon fiber/epoxy composites (CFRPs). The mode-I delamination resistance of 5HS specimens, which was evaluated using a double cantilever beam (DCB) method, depends on the weave pattern and the ply stacking sequence at the delamination growth plane. Higher toughness was observed for crack propagation between surfaces with more transverse bundles (L–T and T–T plane) than those with more longitudinal bundles (L–L plane) because of transverse tow delamination pinning the crack and causing it to arrest. The intrinsic mode-I fracture toughness values of the 0°/0°, 0°/90° and 90°/90° fiber combinations were estimated from the G IC values obtained from three kinds of 5HS specimens which have different mid-plane stacking patterns (L–L, L–T and T–T). The G IC of 0°/0° combination corresponded to that of unidirectional specimen. The G IC of 0°/90° combination was almost identical to that of 90°/90° combination. Results suggest that the interlaminar fracture toughness of woven fabric composites can be estimated from the G IC of 0°/0° and 0°/90° (or 90°/90°) combinations.

Titanium alloy foil-inserted carbon fiber/epoxy composites for cryogenic propellant tank application

This paper presents the mechanical and gas barrier properties of titanium alloy foil-inserted carbon fiber/epoxy composites (CFRPs) undertaken to improve gas barrier properties for cryogenic propellant tank applications. A newly developed β-titanium alloy (GumMetal, GM) was applied. A sheet of titanium foil (α-Ti or GM, 0.05 or 0.1 mm) was inserted between cross-plied composite laminates ([0°2/90°2]S) without adhesive. Epoxy resin in the prepreg contributes to bonding between the Ti-foil and CFRP. The Ti-alloy foil insert did not strongly affect the tensile or compressive strength. Transverse cracks in the 90° layer never penetrated into the Ti-alloy foil layer before the final failure under tensile loading. Nonlinear stress–strain behaviors attributable to the pseudo-elastic behaviors of both unidirectional CFRP and β-Ti alloy (GM) were estimated using high-order stiffness and classical lamina theory. The numerical results agreed with the experimentally obtained results. Helium gas leakage under tensile stress was not observed before the final failure of the composites (1.4% of tensile strain). The excellent mechanical and gas barrier properties were successfully demonstrated.

Improvement of Out-of-Plane Impact Damage Resistance of CFRP Due to Through-the-Thickness Stitching

The present study investigated, both experimentally and numerically, the improvement of low-velocity impact damage resistance of carbon fiber reinforced plastic (CFRP) laminates due to through-the-thickness stitching. First, we conducted drop-weight impact tests for stitched and unstitched laminates. The results of damage inspection confirmed that stitching did improve the impact damage resistance, and revealed that the improvement effect became greater as the impact energy increased. Moreover, the stitching affected the through-the-thickness damage distribution. Next, we performed FEM analysis and calculated the energy release rate of the delamination crack using the virtual crack closure technique (VCCT). The numerical results revealed that the stitching affected the through-the-thickness damage distribution because the stitch threads had a marked effect on decreasing both the modes I and II energy release rate around the bottom of the laminate. Comparison of the results for models that contained delaminations of various sizes revealed that the energy release rate became lower as delamination size increased; therefore the stitching improved the impact resistance more effectively when the impact energy was higher.

Local out-of-plane deformation of CFRP ablator subjected to rapid heating

This study investigates local out-of-plane deformation of a carbon fiber reinforced phenolic polymer ablator subjected to very rapid heating. Local out-of-plane deformation was measured using a three-dimensional digital image correlation technique at high temperatures. This was achieved by attaching high temperature resistant random patterns on the specimen surface using a ceramic bond. Additionally, blue filters intended for cutting strong infrared radiation from the specimen were also used. This study then discusses the mechanisms of the local out-of-plane deformation under rapid heating conditions in terms of carbonization, pyrolysis gas occurrence, gas pressure storage, and interlaminar debonding due to gas pressure.

Evaluation of Spaceborne SiC Mirror Materials Using Samples Cut from the Periphery of a Mirror Body

The Japan Aerospace Exploration Agency has studied large-scale, lightweight mirrors constructed of SiC-based materials as a key technology for future earth observations and astronomical missions. One of the most important technical issues for large-scale ceramic components is their quality stability (viz., differences in material properties depending on the part and the processing), which might influence the structural and/or thermal reliability through unforeseen deformation and breakage. In this study, the authors used a simple, low-cost method for evaluating the properties of SiC mirror materials. Using mechanical testing, thermodilatometry, and microstructural analysis on samples cut from the periphery of a prototype 800-mm-diameter mirror body, the overall quality of the mirror body material was determined.

A new approach for evaluating crack growth resistance curve of mode II delamination by doubly end-notched tension tests

Quantitative determination of interlaminar fracture toughness that governs onset and growth of delamination is essential for engineering of composite materials and structures. This study proposes a new approach to evaluate both the initial fracture toughness and the crack growth resistance property of pure mode II delamination by tensile tests of specimens having two initial cracks, which were conceived from double-lap joints. The proposed test method achieves stable growth of mode II delamination using a fundamental testing system. This study presents the specimen configuration, the theory to evaluate the energy release rate, and experiment results. The mode II initial fracture toughness measured by the present approach agreed well with the results of conventional end-notched flexure tests. Furthermore, the crack growth resistance curves were evaluated by unloading-reloading tests of the proposed doubly end-notched tension specimens.

Development of a three-dimensional finite element model for a unidirectional carbon fiber reinforced plastic based on X-ray computed tomography images and the numerical simulation on compression

A unidirectional carbon fiber reinforced plastic (CFRP) was scanned by an X-ray computed tomography (CT) system. Based on the X-ray CT images, a three-dimensional model with random fiber waviness was developed. Each fiber location was identified in a sectional CT image. Subsequently, the relative displacement of fibers between adjacent sectional CT images was obtained with a digital image correlation method. This procedure was repeated to obtain fiber waviness along the axial direction. The constructed three-dimensional fiber model showed random waviness of each fiber in the unidirectional CFRP. Finite element analysis was performed using the three-dimensional model. Simulation results showed bending and twisting deformations coupled with axial contractions during axial compression, which developed due to fiber waviness. A reduction of the fiber directional Young’s modulus due to fiber random waviness was quantitatively evaluated.

Measurement of Thermal Deformation of CFRP Under Rapid Heating

CFRP which consists of carbon fiber/phenol resin is widely used for a thermal protection system in Aerospace field. Rapid-heating-induced thermal deformation of CFRP is measured using digital image correlation (DIC) technique in the present study. Specimen temperature is above 1200 K so that long wavelength radiation is significant and the DIC technique is not easy to apply. A heat resistance random pattern on the specimen surface, which is made by ceramic materials, and low pass filter allow to apply the DIC method. A complicated deformation including rapid-heat transfer and simultaneously occurring CFRP ablation is measured transitionally.

Damage Simulation of Stitched CFRP Laminates Under High-Velocity Projectile Impact

In this study, numerical analytical model which simulates the damage behavior of the through-the-thickness stitched CFRP laminates under high velocity impact (HVI) is developed. The simulation results are compared with experimental results for validation of the developed model. Through-the-thickness stitched CFRP laminates have good mechanical properties such as higher delamination toughness and better impact resistance than that of conventional laminates. In this study, rigid body impact tests were conducted for the stitched CFRP laminates. Test results revealed that through-the-thickness stitching effectively suppressed the propagation of the delamination, and it increased the performance against the perforation. Among them, moderate stitched CFRP had the highest performance. In order to analyze the HVI damage behavior of the stitched CFRP laminate, special finite element model was developed. In the model, the in-plane tensile and compressive fiber damages were considered by using stress criterion. Transverse cracks were modeled by continuum damage mechanics (CDM). Delamination was modeled by cohesive zone model (CZM). The effect of the stitch threads were introduced by multiscale approach. Out-of-plane tensile tests were performed for small specimens those include single stitching. Small finite element model which contains a non-linear spring element was prepared, and the load-displacement relationship of the non-linear spring was decided so that the result of the model agreed with the tests. Then, the spring elements were introduced to the HVI simulation model. The simulation results revealed that stitching bridges the delamination and delamination area was reduced. On the other hand, the simulation results revealed that delamination reduces the tensile stress in the bottom surface of the laminate. Because delamination easily propagate in the moderate stitched laminates, the performance against the perforation of the moderate stitched laminates is the highest among the stitched laminates.Copyright