In order to investigate the effect of the interfacial property on stress transmissibility, the finite element analytical model in which fiber, matrix and interphase were divided independently into mesh elements has been applied. The interfacial region was assumed as orthotropy. The concept of orthotropy on the composite interphase is suitable for investigating the effects of stress bearing and stress transfer on mechanical behaviors. This paper deals with the study on the influence of the gradient Interphase on the stress transfer. Either the axial tensile modulus or the shear modulus was varied gradually in the interphase. From the analytical results, it is concluded that the difference in gradient distribution of the interfacial tensile modulus or the interfacial shear modulus affected both the interfacial stress transmissibility and stress bearing in the interphase. Therefore, it is obvious that this numerical analytical modelling on composite interface is suitable for the numerical approach of fiber reinforced composites, because it is possible to represent separately stress transfer and stress bearing as the role of the interface/interphase.
A computer program of three-dimensional intelligent finite element method by using shell elements has been developed in order to obtain the optimum fiber orientation and the optimum stacking sequence of laminate composites. The developed computer program can treat a balanced symmetric laminate composites. The design object to determine the fiber orientation and the stacking sequence is the strength of laminate. The strength is evaluated by the F-value of each lamina based on Hoffman's failure criterion. And the optimum fiber orientation and the optimum stacking sequence are determined by the minimum thickness with the F-value satisfying the allowable value. As the numerical examples for actual structures, the determination of fiber orientation in each layer for a cylindrical pipe and a T-type structural component under combined load has been carried out. In the case of cylindrical pipe under three loading conditions such as the combined internal pressure and tensile load, the only torsion load and the combined torsional and tensile load, the optimum laminates to withstand hoop stress, axial stress and shearing stress for each loading condition were obtained. Then, the optimum laminates of T-type structural component for each loading condition were estimated by using the initial fracture load. As the result, it is recognized that the proposed method is very useful for the structural design of composites.
The cured shape of anti-symmetric crossply composite laminates is theoretically calculated. The displacement functions are assumed to be polynomials of coordinates and the Green strain tensor is adopted so as to take into account the nonlinearity of deformation due to large curling of cured laminates which occurs because of discrepancy of thermal expansion coefficients in fiber and transverse directions in a lamina. Rayleigh-Litz method is adopted to obtain the cured shape of antisymmetric crossply laminates. The simultaneous equations for the curvature of curling can be analytically solved. The bifurcation points of solution are also theoretically obtained. An assessment of displacement assumptions is presented.
The purpose of the present work is to estimate the behavior of creep deformation of GFRP pipe under an internal hydraulic pressure where the both ends are free. The GFRP pipe is developed for renewal of an overage drain pipe or gas pipe. It is mainly made up of two layers where the glass fiber mat is used in the inside layer for corrosion resistance and the woven roving cloth is as the outside layer for pressure resistance. Before the internal hydraulic pressure creep test, the tensile creep tests are carried out with the dumbbell type specimen which is cut from the uncured laminated pipe and then is cured. The creep constitutive equation is obtained using the tensile creep data of short-term tests, by taking account of stress, temperature and tensile angle to reinforced glass fiber. Based on this creep constitutive equation, long-term creep behavior can be predicted. A reasonable comparison between the prediction and the tensile experiment is observed at time up to 106 second. The internal hydraulic pressure creep test on the pipe verified the creep constitutive equation.
In this paper, the residual life and the creep damage for glass fiber reinforced plastics are studied experimentally. For this purpose, emphasis is placed upon introducing a new damage parameter defined by acoustic emission (AE) intensity in order to evaluate the micro-damage accumulation generated before macro-crack initiation in a composite. Short-term creep experiments were carried out on three types of specimens, consisting of glass fiber fabric reinforced epoxy laminated composites with different weaves, counts, densities and thickness. The single-edge notched specimens were subjected to a series of stepwise loadings with increasing peak loads, in edgewise four-point bending so that tensile stress arose at the front end of the notch. AE activity was monitored with measuring mechanical displacement during a test using various AE intensities, which were analyzed in detail. The new damage parameter showed a good correlation with a conventional damage parameter defined by non-elastic deformation caused during cyclic load holdings. The correlation depended on test conditions and materials but was presumed to stand up under tensile deformation.
The effect of stress ratio on the near-threshold growth of delamination fatigue cracks was investigated with unidirectional CFRP laminates made from toughened thermoplastic matrix resin. Tests were carried out under mode II shear loading by using end notched flexure specimens with a special loading device. The crack growth behavior under different stress ratios indicated that the fatigue crack growth rate was well correlated to the stress intensity range near the threshold region. The fatigue crack growth resistance of CF/PEEK laminates was slightly higher than that of conventional CF/epoxy laminates. However, the increase of the fatigue crack growth resistance by toughening the matrix resin was smaller than that under mode I loading. The reason for this poor toughening effect was discussed based on fractographic observation. Comparison of the change of stress-ratio dependency and the relative fatigue threshold was made from brittle CF/epoxy laminates to toughened CF/PEEK laminates under both mode I and mode II loadings. The stress-ratio dependency and the relative threshold were expressed by unique functions of the fracture toughness of the same laminates under the same loading modes.
This paper presents stress corrosion cracking (SCC) of woven GFRP laminates under an acid stress environment. The threshold stress intensity factor KISCC for the stress corrosion cracking was determined by cumulative stress decrement tests. Based on a fractographic observation, it is found that the progression of corrosion at the weft fiber strand and matrix crack leads to the crack propagation of the warp fiber strand and that the fracture surfaces at each crack propagation rate are different due to the combined effect of chemical reaction and stress state. The crack propagation rate is possible to be estimated from a microscopic fracture model of the warp fiber strand. The crack propagation rate calculated by using this model agrees with the experimental value. It is confirmed that the proposed model is appropriate for evaluating the crack propagation rate of GFRP in acid stress environment.
Experiments were carried out to study the solid particle erosion behavior of FRP degraded by hot water. The FRP used was unsaturated polyester resin reinforced by chopped glass fiber mat. The test specimens were soaked in hot water of 80°C and the degraded test specimens were used in the erosion tests. The Vickers hardness of a resin part in the FRP was measured and the interlaminar shear strength of the FRP was obtained from short beam three-point bending tests. According to these test data, the resin became ductile and the interface between resin and fibers was weakened due to the degradation by hot water. It is found from the erosion test data that the erosion behavior of the degraded FRP is affected both by increasing ductility of the resin which results in decreasing the erosion rate and by decreasing the interfacial strength between resin and fiber which results in increasing the erosion rate.
In this study, the yield behavior of wood under the compression-shear stress conditions was examined by compression-torsion combined loading tests. The materials used for the experiment were Sitka spruce (Picea sitchensis Carr.) and katsura (Cercidiphyllum japonicum Sieb. and Zucc.). The compression load was applied along the longitudinal direction of the specimen, whereas the torsional moment was applied around the longitudinal direction. By varying the combined stress conditions, the couples of compression and shear stresses at the occurrence of yielding were obtained, and the applicability of three yield criteria, Hill-type, Gol'denblat-Kopnov's, and Jenkin's criteria, was examined. Although the testing results were more close to Hill-type criterion than Jenkin's one for both species, it is still early to say that the yield behavior of wood under the compression-shear combined stress condition is described by Hill-type criterion precisely.
Dried boxed-heart square timbers of sugi with a split liner were tested for the dimensional stability and tangential stress in surface layer during environmental change. The tests were made under several cyclic conditions of temperature and humidity with reference to the actual day and night condition. The results obtained were as follows; (1) The change in moisture content with environment was less than 1% by the oven drying method and less than 4% by the high frequency moisture meter method. At the same time, the shrinkage chandes of width were less than 0.4% in the opposite directions for the fluted face and for the other three faces. (2) When the difference of equilibrium moisture content (ΔE.M.C.) between daytime and nighttime was over 4%, tension stress appeared clearly in the surface layer after changing to the daytime condition. (3) The above daily change in stress behavior depended on the moisture condition of the dried timber. For the insufficient dried timber with a large moisture gradient, the tension stress in surface layer during the daytime condition increased enough to develope into check.
With a continuous mechanical grading-machine (MG) which can be used to measure Young's modulus (MOE) of lengthwise portion of a lumber, the average of MOE (MGEAVG) and the minimum of MOE were obtained on 6400 lumbers of Japanese larch. The average of the difference (=MGEAVG-MGEMIN) was 0.92GPa and MGEMIN was extremely lower than MGEAVG in some cases. Next, horizontally (H-type) and vertically (V-type) laminated glulams of homogeneous-grade composition were made to examine the bending strength of glulams. They were composed of lumbers classified according to MGEMIN, because it was supposed that MGEMIN should be useful for estimating the bending strength (MOR) of glulams. In V-type glulams, the laminating effect which lowers the variation of MOR distribution was evidently observed. On the contrary, MOR of H-type glulams was almost independent of the number of laminae, because it depends on the quality of lumber in the outermost layer. Particularly, this result suggests that lumbers for the outermost layer of H-type glulams should be classified according to MGEMIN instead of MGEAVG.
The applicability of colloidal silica for producing a wood-mineral composite by impregnation was investigated, and its properties such as dimensional stability and decay and fire resistances were evaluated. The colloidal silica solution was prepared by mixing silicic acid anhydride (30.4%), boric acid, and chitosan with a low molecular weight (0.3%). Instead of boric acid, metal ions of Cu, Ag, or Ti (1000ppm) were added to the solution. The solution was diffusion-penetrated into the water-saturated specimens of Sugi (Cryptomeria japonica D. Don), followed by drying to solidify the colloidal silica for a combination. The formation of silicic acid gel in the composites made by using the colloidal silica solution system was proved to be mostly insoluble. The composites showed an antiswelling efficiency with the greatest value of 23% due to the bulking effect despite being hygroscopic and a good reduction in water absorptivity. Also, the composites, particularly those by using the colloidal silica-boric acid system, were found to improve greatly the decay and fire resistances. For practical use, the composites were manufactured with boards of yellow poplar (Liriodendron tulipifera L.) applying the colloidal silica-boric acid system. They showed the enhancement of bending strength and modulus of elasticity to some extent of apparent weight percent gains. The absorbed energy in impact bending became greater than that of the untreated wood.
For the development of partial coloring technique of japanese beech (Fagus crenata), five logs of air -dried wood were penetrated with a dye solution by the capillary rise method and the vacuum impregnation method. The penetration rate into wood by capillary tension of liquid showed a wide variation among the five logs. In the permeable sample (No.5 log), the dye solution was selectively penetrated into earlywood, so that color contrast between earlywood and latewood was emphasized. A thin impenetrable zone existed in the close vicinity of the surface of sound wood as No.5 log, in which the vessels had been entirely blocked up by tyloses. Tylose development, after felling and cutting of tree trunk in summer, appears to be the natural defensive mechanism of wood against drying out. For the prevention of the tylose development, it may be necessary to immerse wood in a hot bath at a temperature above 50°C, immediately after felling and cutting the tree in winter. In the poor permeable sample, the bark seam and the encased knot were found in trunk. Penetration of dye solution in the growth-related defects and their neighboring tissue was frequently stopped by tylose. Typical tyloses were detected in the growth-related defects. It is considered that the blockage of vessels arise in the wood of living tree. Therefore, it is unsuitable to use the wood including these defects for partial coloring. In the case of impregnation by the vacuum evacuation and subsequent atmospheric pressure, the dye solution penetrated easily into the less permeable tissues, where penetration by capillary tension was difficult. However, in No.3 log which was the most impermeable wood, the unstainable spot remained.
This investigation was concerned with the effect of internal structure factor of particleboard on the characteristics of acoustic emission (AE) during internal bond testing. The tensile tests perpendicular to the plane of board were conducted with the aid of AE monitoring system for handmade particleboards having various internal structure factors, such as particle size, board density, board thickness and resin content. Especially, the effects of particle size on the internal bond strength and AE generation behavior were studied. The tensile strength (IB) of particleboard was affected by particle size, board density, board thickness, resin content and schedule of hot-pressing. Generally, the higher the value of IB was, the more the cumulative AE event counts (TAE) were monitored; therefore, it was considered that IB was closely related to TAE. The ratio (Ra) of the stress level at the initiation of AE generation (σG) to that at failure (IB) was negatively correlative to either of IB or TAE. Furthermore, from the results of AE measurement with several threshold levels, it was confirmed that the particleboard made of larger particles generated higher amplitude and more AEs than that made of smaller particles. And it was also confirmed that σG was higher in the board made of smaller particles, because of reduction of amplitude of AE to be detected with AE sensor.
This paper deals with interaction problems of interface cracks with unequal length. The problems are analyzed by using the singular integral equations on the basis of the body force method. In the numerical analysis, the unknown function of body force density is approximated by the product of the fundamental density function and power series. The stress intensity factor of interface cracks is systematically calculated for various crack dimensions, spacing and elastic constants. The interaction effect of these variables is discussed by comparing the present results with the results of ordinary crack problems in a homogeneous material. The calculation shows that the effect of elastic constants of dissimilar materials on the dimensionless stress intensity factor F1 for interface cracks is small and F1-value of interface cracks is almost the same as that of ordinary cracks with the same geometrical condition.
The effects of interaction and coalescence of cracks on the fatigue behavior of high brittle materials such as glass and ceramics are examined. For glass ceramics specimens with two different center distance collinear cracks introduced by Vickers microhardness indentation, the cyclic fatigue tests were carried out under four point bending load. It was found that the time to failure for a certain applied stress significantly decreased with increasing the number of indentation cracks and the center distance of indentation cracks. The prediction of time to failure with crack interaction and crack coalescence was also carried out. The results were in good agreement with the experimental results. The validity of the proposed method of fatigue life prediction was confirmed.
The effect of fluctuating stress on the fatigue strength of type 304 stainless steel in air at 288°C was examined. In any of the 2 step increasing stress, 2 step decreasing stress, repeated 2 step stress (high to low), repeated 2 step stress (low to high) and gradual increasing stress, the cumulative usage factor calculated based on the linear damage law exceeds 1. In the 2 step increasing or decreasing stress, the usage factor for the decreasing stress type is larger than the increasing stress type. On the other hand, the cummulative usage factor for the repeated 2 step stress (low to high) is larger than the repeated 2 step stress (high to law), and the usage factor increases with increasing number of cycles at the first stress. The smaller the difference between the first stress and the second stress, the larger the usage factor. In the gradual increasing stress, the usage factor increases with increasing number of cycles at each stress.
In order to study the accuracy of creep residual life prediction by the Omega method which is based on creep deformation, a series of creep tests on modified 9Cr-1Mo steel were conducted at 500°C, 550°C and 600°C, and the Omega method was applied to the residual life estimation. The main results obtained are as follows: (1) There was a obvious linear portion, which corresponded to the tertiary creep, in the relationship between logarithm of strain and srain. So it was easy to define the Omaga value as a gradient of linear portion. (2) It was proved that the Omega value depended on stress and temperature in such a way as it was the larger, the lower the stress and the lower the temperature. (3) By using the Omega value and strain rate which were determined experimentally, the residual life could be predicted within a factor of 1.5 at the stage of 50% and 80% of actual life. It was confirmed that the accuracy of this method was higher than that of the former method based on rupure time. (4) To apply this method to the residual life evaluation of operating plant materials, the Omega value has to be determined in the lower stress condition. So it is important to develop the extrapolation method of the Omega value based on the laboratory acceralation test to the longer service life.