Considering the space environment, it is important to study the effects of thermal cycling on the damage progress behavior and mechanical properties of CFRP. In the present study, thermal cycling tests are performed on CFRP laminates first. After thermal cycling tests, tensile tests are conducted at room temperature. The purposes of the present study are to investigate damage progress behavior in CFRP laminates under thermal cycle loading experimentally and to discuss the effects of thermal cycles on the matrix cracking behavior under tension in CFRP
laminates. The specimens are (0/90/0), (0/90)s and (0/902)s for T 700 S/2500 and (0/90)s for T 800 H/3631. Thermal cycling tests are performed up to 1000 cycles. During the thermal cycling tests, the specimens are observed periodically by the replica technique and the X-ray to investigate matrix crack behavior. In all the laminates, matrix cracking is observed during thermal cycling. The matrix crack density is defined as the number of cracks per unit specimen length. It is found experimentally that the increasing rate of matrix crack density is higher in the laminates with thinner 90°ply. The crack growth rate is measured as a function of the matrix crack length. The thicker the thickness of 90°ply is, the larger the crack growth rate is. This matrix crack behavior is correlated to the energy release rate calculated by the finite element analysis. To discuss the effects of thermal cycles on matrix cracking under tension, tensile tests after thermal cycles are performed. In tensile tests, matrix crack density is measured as a function of the laminate strain. The effect of the number of thermal cycles on matrix cracking is investigated experimentally. It is found that the effect of thermal cycles on matrix crack behavior can be evaluated as a change in the critical values stress and energy release rate for matrix cracking.
A formulation was derived to evaluate the velocity of plate waves propagating in composite materials. The formulation was based on Mindlin plate theory including the effect of shear deformation and rotary inertia. The influence of thickness, stacking sequence and propagating direction on the wave velocity was investigated. The formulation was applied to identify delamination in composite materials. The plate waves propagating in GFRP with delamination on the mid-plane were detected with AE sensors. The time-frequency analysis of AE signals was carried out by the wavelet transform. The peaks of the magnitude of wavelet transform in the time-frequency domain were related to the arrival times of plate waves. The length of delamination was predicted from the arrival time of a certain frequency component of plate waves. The predicted length of delamination agreed well with the actual length.
Numerical simulation is conducted to investigate effect of relaxation moduli of constitutive laminae on creep curves for cross-ply FRP (Fiber Reinforced Plastic) laminates containing initial transverse cracking. The onedimensional
viscoelastic shear lag model is employed to predict the strain response to constant applied stress as a function of time. The effect of the time-dependent shear lag parameter on creep curves is also discussed. The strain
is calculated numerically with the aid of inverse Laplace transform technique. Two types of relaxation moduli, namely, an exponential type and a power law type, are applied to the model to predict the creep strain for various transverse crack density. It is found that the creep strain increment can be enhanced by relaxation of transverse and shear moduli especially for high transverse crack density.
Fatigue test have been conducted on impact damaged coupons of T 300 plane woven fabric / vinylester resin. Two damaged levels were used to represent BVD(barely visible damage)and NVD(non visible damage). Fatigue tests were conducted at a frequency of 5 Hz and a load ratio of R =-1. Three levels of maximum stress were prepared. Impact damage and damage growth behavior were observed using several nondestructive evaluations such as ultrasonic C-scan and thermo elastic stress analyzer(TESA). Ultrasonic C-scan was sensitive to delamination, on the other hand, TESA was sensitive to fiber breakage. For this reason, sum of principal stress at surface were evaluated using TESA image. Moreover, from TESA image, internal damage was simulated using FEM analysis.
The tensile test is conducted on the FRPA with the crack initiated by the fatigue test, including fiber content n weight of 50% made by direct molding method. The fractal dimensions mAE by the AE amplitude distributions detected during the tensile test are compared with the fractal dimensions mS by the section shapes of fracture surface after the tensile test. The normalized fractal dimensions mAE/mAE (N) and mS/mS (N) are affected by the fatigue damage, and there is a correlation on the relation between mS/mS (N) and mAE/mAE (N), where the subscript (N) shows the nonfatigue specimen. The accuracy can be explained with the slope of straight line showing the relation between the normalized fractal dimension m/m(N) and the normalized strain energy w/w(N)．
The interphase between fiber and matrix has a very important role, which includes the load transmission between fiber and matrix. In this study, the effect of interphase characteristics on the various mechanical properties of plain woven composites were most concerned. Basic mechanical properties of plain woven fabric composites with flexible interphase were evaluated experimentally. Nonlinear stress-strain behavior in off-axis tests was characterized using 3 parameter plasticity model. Cyclic tensile tests were conducted to evaluate the effect of interphase thickness on the durability.
An attempt was made to investigate the relation between the interfacial affinity and the electrical conductivity of carbon black (CB) filled polymer composites. High density polyethylene (HDPE) and polypropylene (PP) were examined as the matrix polymer. The interfacial affinity of CB particles with HDPE is higher than that with PP. The affinity of the matrix polymer can be improved by introducing the maleic anhydride (MAH) group. HDPE or PP was modified by the addition of the MAH grafted PE (MAH-PE 1 or MAH-PE 2) or the MAH grafted PP (MAH-PP 1 or MAH-PP 2), respectively. The graft ratios of MAH groups in MAH-PE 1 (0.5 wt%) and MAHPP 1 (0.6 wt%) are lower than those in MAH-PE 2 (3.0 wt%) and MAH-PP 2 (9.6 wt%). By the improvement of the interfacial affinity, it is expected that CB particles disperse finely and homogeneously in the matrix and the matrix resin may form the insulating skin around the CB particle. Experimental results obtained in this paper suggest the following conclusions on the relation between the dispersion state of CB particles and the electrical conductivity of the composite. (a) Owing to the finer dispersion of CB particles, the conductive network can be formed by the smaller content of CB. With the increase of CB content, the density of the conductive network increases and the electrical resistivity of the composite decreases. (b) By the homogeneous dispersion of CB particles, values of the measured resistivity become stable. (c) By the formation of insulating skin around the CB particle, the electrical resistivity shows the inclination to increase. When the temperature increases, the distance between CB particles extends widely with the thermal expansion of matrix polymer. Therefore, both of the electrical resistivity at the peak temperature and the PTC intensity (resistivity at peak temperature / resistivity at room temperature) increase. By the addition of 8 wt% of MAH-PE 2, the CB/HDPE composite exhibits the PTC intensity of 10^4.4 at the peak temperature 135℃.
The generation mechanism of residual stress in thermosetting resin by the curing shrinkage as well as the thermal shrinkage during curing process was studied in this paper. First,4-element thermoviscoelastic model, which can express the thermoviscoelastic behavior in the curing process of thermosetting resin was proposed. Second, the three-layer type laminate is assumed and the residual stress generated when this laminate is casted was analyzed. As results, it was turned out that the residual stress by curing shrinkage dose not generate when curing temperature is higher than the glass transition temperature of casting resin, but it generates when curing temperature is lower than the glass transition temperature of casting resin. And it is possible to make residual stress zero by controlling curing temperature, which becomes higher than the glass transition temperature by curing.
In this study, noticing foaming temperature as the factor, which controls thermodynamic instability for creating cells of microcellular plastics(MCP),the effect of foaming temperature on cell size and cell density of MCP was investigated. Concretely, two foaming methods were employed ; one is the elevated temperature method that grows up cells by raising temperature after decompression in the foaming process, the other is the constant temperature method in which the temperature is already kept at high temperature before the decompression. As results, although cell density of foamed plastics becomes large and average cell size becomes small, the maximum cell size becomes large by the elevated temperature method. On the other hand, although the maximum cell size becomes small, average cell size becomes large by the constant temperature method.
For the 10m diameter reflector the inflatable structure was constructed and was inflated in the gymnasium of Kanazawa Institute of Technology in Feb 25, 2002. The designing, making and inflation of the inflatable structure with the reflector are reported. The purpose of the experiment is to investigate the feasibility of the method of making and inflating, and to define technical problems and solutions, which are also reported.