A procedure to estimate transverse strength of unidirectional composite is proposed in this study. Most of the micro structure of fiber reinforced plastic is unidirectional composite. The estimation of the transverse strength of unidirectional composite is very important because damages in FRP originate from the transverse crack. As the method to estimate the strength of unidirectional composite, theoretical methods and experimental methods have been applied. However, the transverse strength is influenced by several factors such as the volume fraction of fiber, interfacial strength, and residual thermal stresses. In order to estimate the transverse strength considering the factors, we propose an FE method in this study. Firstly, FE model of unidirectional composite considering the random arrangement of filaments are generated. Interfacial elements are installed between filaments and resin to simulate the interfacial behavior. The damage conditions such as damage criterion and strength of resin and interface are determined by comparing the static transverse tensile test of single-filament embedded specimen and FEM. The transverse strength is estimated by FE analysis based on damage mechanics considering the effects of residual thermal stresses. Several FE models with volume fraction of fiber of 40%, 50% and 60% have been generated respectively and the strengths have been estimated. As the results, the strength decreased when the volume fraction of fiber increases without considering residual thermal stresses. On the other hand, it is clear that the strength is dependent on the random arrangement of filament strongly with considering the residual stress.
In this study, we have extended the concept of fiber hybrid and proposed fiber structural hybrid composite as the new concept. Fiber structural hybrid is defined as reinforcements consist from same kinds of fiber materials with different structure such as the fineness, the twisting state, and etc. In this study, the concept of the fiber structural hybrid was applied to the glass woven fabric composite and fiber structural hybrid composite materials with different twisting count and twisted structure were fabricated. By static tensile test and Finite Element Method (FEM) considering of the internal structure, the effects of the difference in twisting state of weft fiber on the initial fracture of fiber structural hybrid composites were investigated. In tensile test, it was clarified that tensile modulus and initial fracture stress were decreased by the increase in the twisted count and the number of doubling. In FEM, apparent interfacial strength inside of weft fiber bundle was decreased by the increase in twisting count and apparent interfacial strength was increased by the increase in number of doubling. From these results, initial fracture can be designed by applying the fiber structural hybrid concept to the composite materials, and the safer structural material can be realized.
Specific elastic modulus of bamboo fiber paper was evaluated when the shellac was added into the paper during the paper making process. For the fabrication of the paper, bamboo fibers were treated with the alkali solution after extraction. The bamboo paper was fabricated with a heat pressing technique using the wet bamboo fibers sheet obtained by the filtration. Particle type of the shellac was used as the binder material of the paper. To compare the test results, lacquer type of the shellac was also applied in which ethanol was contained in the process. When the particle type of shellac was added into the paper, the specific elastic modulus of the paper was improved. It was shown that effective weight content of the shellac was about 20% to obtain the improvement and steady properties of visco-elastic behaviors. The improvement of the specific elastic modulus was explained with the increase of contact points between fibers in the bamboo fiber paper, when particles of shellac were added into the paper.
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.
Strain monitoring of resin during cure process is an effective method to manufacture high-quality FRP products because it is known that residual stress generated during molding decreases strength of final products. Although both cure and thermal shrinkage occur during cure process, it is difficult to measure cure shrinkage of FRP products due to dramatic changes in mechanical properties of resin by cure reaction. Among cure monitoring methods, it is proven that fiber optic embedded strain sensors can measure cure shrinkage of resin. However, quantitative evaluation of cure shrinkage strain measured by optical fiber strain sensors has not been conducted. In the present paper, we monitored curing shrinkage of epoxy resin by an embedded EFPI sensor. Mechanical and thermochemical properties of the resin during cure process were experimentally obtained by several methods. Volume shrinkage of resin during cure process was also measured by a dilatometer. In this paper, a viscoelastic model during cure process was driven and the model was used for FEM analysis of cure shrinkage strain generated in the embedded EFPI sensor. From the results, it appeared that the calculated cure shrinkage strain of the EFPI sensors agreed well with the measured strain especially when the degree of cure was high. Therefore, it was concluded that an embedded EFPI sensor can measure cure shrinkage strain quantitatively.
Three-dimensional steady temperature field and the resulting thermal stress field are analyzed in a functionally graded hollow hemisphere (FG hollow hemisphere) subjected to unaxisymmetric heating at the inner and outer hemispherical surfaces. The FG hollow hemisphere whose thermal and mechanical properties vary arbitrarily in the radial direction is approximated as a multi-layered hollow hemisphere with distinct thermal and mechanical constants in each layer. An analytical solution to the three-dimensional heat conduction problem is obtained by introducing Fourier's cosine transform and Legendre transform. The associated three-dimensional thermal stress problem is estimated by the use of thermoelastic displacement potential and Papkovich-Neuber's displacement functions. Numerical calculations of the temperature and thermal stresses are carried out for SiC/Al-Alloy FG hollow hemisphere with three types of graded material composition expressed in the form of a power function. The effects of the graded composition, inner/outer radius ratio, and number of layers on relaxation characteristics of thermal stress are quantitatively discussed.
Nylon 6 clay hybrid (NCH) composite consists of nano-sized Montmorillonite and nylon 6 matrix. Tensile tests at near glass transition temperature (35°C, 50°C) were conducted at a strain rate of 10-2s-1. Fatigue tests were performed with a stress ratio of 0.1 and frequency of 0.1Hz. Although the tensile strength and fatigue strength of 2 mass percent clay reinforced composite (NCH-2) were the highest at room temperature, the tensile strength and fatigue strength of 5 mass percent clay reinforced composite (NCH-5) became the highest at 35°C. Fatigue fracture surfaces showed that fatigue fracture of NCH-2 wais brittle at room temperature, but ductile at 35°C, while fatigue fracture of NCH-5 was brittle at both room temperature and 35°C. A parameter based on elliptical zone area was proposed to express fracture toughness of the composites, which was not dependent on number of cycles to failure.
In this paper, the method of determination the orthotropy ratio and the principal material direction of orthotropic brittle material in diametrical compression test is shown. For this method, strains and angles between principal strain and the direction of load are calculated at the center of cross section under several orthotropy ratios and several angles between direction of load and principal material direction by theoretical solution. The relation between ratio of these strains and principal material direction, and the relation between the direction of principal strain and that of principal material direction are shown by graphical representations in this paper. When strains and directions of principal strain obtained by rosette gages are applied to these graphical representations, the orthotropy ratio and the principal material direction of this specimen are able to be determined.
In this study, the thermal calculation algorism was developed and introduced to the original distinct element code, and the coupled thermo-mechanical behavior of rock mass around the HLW disposal tunnel was simulated. The simulation results were compared with the experimental data obtained from the pillar stability experiments with mechanical loading and heating at Aspo HRL (hard rock underground laboratory), Sweden. For the simulation, input microscopic parameters were calibrated to match the macroscopic properties of the Aspo Diorite. As a result, the crack propagation process, temperature change and displacement at measuring point during thermal evolution were successfully simulated and calculated results agreed qualitatively well with the measuring and observation results at the site. After the confining pressures were applied, microcracks were generated around the borehole. All microcracks were generated by tensile failure, and the direction of the crack propagation was parallel to the borehole surface. These microcracks were gradually propagated with increasing temperature during heating process. However, the number of microcracks was a few and exfoliation of rock surfaces observed in the in-situ experiment was not formed in the simulation even though the calculated maximum temperature was higher than the experimental data. Therefore, for further improvement of quantitative accuracy, different bonding parameters as a function of thermal aspect will be required. Moreover, calibration of laboratory experiments considering heat should be conducted to investigate the effect of heat on the microscopic parameters to develop more realistic DEM models with bonded particles.
This paper presents a theoretical approach for estimating the fracture strength of brittle materials under variable loads. First, a model was established on the basis of the slow crack growth concept in conjunction with two-parameter Weibull distribution for a brittle material subjected to variable loads. Assuming fracture of a brittle material under constant load, a fracture criterion obtained from the model was related with the empirical life law. Next, four-point bending test was carried out for soda glass under two step crosshead speeds. The experiment result was compared with the predictions to verify the validity of the model.
We examined the internal friction (IF) behavior of Y2O3-stabilized ZrO2 with a large amount of oxygen vacancy by using forced torsion method in the temperature range of 25∼420°C. In the polycrystalline Zr1-xYxO2-x/2 (x = 0.3 and 0.5), a single and broad IF peak was observed at around 150∼350°C. The analysis of IF peak suggested that the internal friction was induced by the relaxation of oxygen vacancy in the local structure. Also, the formation of locally immobile coordination of vacancy was suggested by the IF amplitude for heavily defective cubic zirconia.