In general, laminated cylindrical shells manufactured by filament-winding show the coupling effects and behave differently from homogeneous orthotropic ones. In the present study, such coupling effects on stress distributions in the laminated cylindrical shells were examined by use of Donnell's equation under the boundary conditions of clamped edges. The followings were obtained from the present analysis. (1) Circumferential displacements occur near the edges in the antisymmetric angle-ply laminates. (2) Twisting moments occur near the edges in the symmetric angle-ply laminates. (3) Bending moments occur in the central part of cylinder far away from both edges in the antisymmetric cross-ply laminates. These coupling effects decrease rapidly as the number of layers increases and the mechanical behaviors become close to those of homogeneous orthotropic laminates. In addition, the distributions of interlaminar shear stresses near the edges under high lateral shear force and the effects of stacking sequence and thickness ratio on them were analysed by using the finite element method because of low interlaminar shear strength in laminates. It was concluded that the interlaminar shear stresses decrease rapidly away from the clamped edges, but are considerably high near the edges especially in the case of large thickness.
This paper reports an investigation on the relationship between the stress propagation and the span ratio of a sandwich beam of a hybrid composite under impact load. The stress distribution in the impact bending process is very complicated when the hybrid construction has a different mechanical impedance. Experimentally, the impact test was carried out with a three points bending method by using a falling elastic bar. The transient strain in the loading elastic bar and the dynamic flexural strain of the specimen at the backface against the impact point were measured. The results obtained show that the initial strain of the bar depends on the material charcteristics of the beam member of the impact side, and is not affected by those of the backface. In the case of a large span ratio, the value of the initial stress does not depend on the ratio and is always maximum during initial process. On the contrary, the maximum value of stress wave does not coincide with the initial one in the case of a small span ratio. The value of maximum stress depends on the modules of elasticity of the surface member laminated at the impact side. Therefore, the value of impact stress depends largely on the characteristics of the loading surface member, and it increases with increasing modules of elasticity. The deformation mode of the sandwich beam after a long time period has no connection with high frequency stress waves at the impact loading point. The impact load acts as a distributed load against the lower lamina. As the span length becomes smaller and the characteristics of the face member laminated at the impact side becomes harder, the strain produced at the backface against the impact point becomes smaller. In an extreme case an inverse bending strain is produced.
It is well known that the fracture mode changes depending upon the span-to-depth ratio in three-point bending tests of unidirectional fiber reinforced composites. The characteristic change in fracture modes under three-point bending is clearly observed especially in unidirectional CFRP (Carbon Fiber Reinforced Plastics) because of its strong anisotropy in strength. In this study, the change in three-point bending behavior of unidirectional CFRP has been examined over a wide range of span-to-depth ratio. The changes in apparent maximum bending and shear stresses have been correlated to that in fracture modes. It has been shown that a unidirectional CFRP breaks into two pieces at the loading point in a certain range of span-to-depth ratio, and the regions of tension and compression side can be obviously distinguished in the broken fracture surface. The microscopic configurations of each region were examined by fractography using a scanning electron microscope and the results were discussed in conjunction with the fracture mechanism. It has also been shown that the broken fracture surface changes significantly with the strain rate or crosshead speed. A phenomenological correlation has been given between the strain rate dependency of apparent bending strength and the fracture surface based on the microfractographic examinations.
The purpose of this study is to investigate how the cyclic frequency of fatigue tests influences the fatigue behavior of fiber reinforced plastics (FRP) under natural convective conditions, especially in a higher cyclic frequency range than those of 1000 or 1800cpm used in conventional fatigue tests with constant stress. The reduction of rigidity and the raise of surface temperature were observed on the cantilever-type laminated FRP specimens excited sinusoidally by a shaker in a controlled high cyclic frequency of 6000, 8000 or 10000cpm. The time to failure was taken as the time period until the rigidity of a specimen became under 7/8 of its initial value according to the ASTM Standard, D671-63T. Two kinds of FRP were tested: the roving glass cloth reinforced unsaturated polyester resin (R) and the glass-fiber mat reinforced one (M). The observation of the surface temperature of specimens revealed two patterns in the process of surface temperature increase during fatigue and the cyclic frequency dependency for both FRP laminates. The raise in temperature of specimens M was more remarkable and sensitive for the stress level than that of specimens R. As for the cantilever bending fatigue strength evaluated on the basis of 1/8 rigidity reduction, a considerably large cyclic frequency effect was recognized. The fatigue strength under higher cyclic frequency loading was remarkably lower in comparison with that under the conventional cyclic frequency loading. It was found that fatigue strength σF is expressed in the form of λf-γ, where λ and γ are constants and f is the cyclic frequency.
Composite materials appear to offer substantial advantages over metals for application to structures subjected to various types of loading. Their characteristic response to time-dependent phenomena, however, is substantially different from that of metals. In the present study, the fracture machanism related to short time-dependent phenomena at room temperature of roving glass cloth FRP has been examined. The results obtained are as follows. A close correlation was observed between the modulus of elasticity and the strength which varied depending on the dispersion of the material characteristics. Therefore the stress and the strain at the fracture point did not scatter independently but distributed along a line in the stress-strain diagram, leading to a fracture criterion called as the fracture envelope. Creep fracture was also able to be determined by the fracture envelope similar to that obtained from the statical behavior. In both cases, the gradient of the fracture envelope increased with the ply number of laminate. The phenomenon that the specimens sometimes fracture during the stress relaxation tests can also be explained by the concept of the fracture envelope.
An investigation was made on the fatigue properties under random loadings of satin woven glass cloth FRP. Constant displacement fatigue tests were conducted on FRP test specimens with a random loading fatigue test apparatus. The effects of the input signal type of loading wave and the frequency of input signal of loading wave on th fatigue life of FRP were investigated. Moreover, 8-step stress amplitude fatigue test and randomly programmed ones were conducted on the FRP test specimens and their fatigue lives under these conditions were evaluated. In order to investigate the effect of filler in the resin on the fatigue life, the test was conducted on FRP test specimens containing the filler (CaCo3) with a rotating bending fatigue test machine. According to the results of repeated bending fatigue test, a longer fatigue life was obtained at the long life region in the case where sinusoidal wave was used for the loading signal compared with other types of loading waves like pulse and triangle waves. As for the fatigue life, there was no difference between the FRP test specimens with and without filler. However, the specimens with filler showed a larger scatter of fatigue life with increasing filler content.
Fiber reinforced plastics (FRP) are widely used in many fields because of their characteristics achived by the combination of fibers of high strength and elasticity and excellent matrix. In this paper, a fundamental investigation was made to develop light weight composite materials with excellent spring properties from hybrids of glass/epoxy and carbon/epoxy laminates. As a result, the CRP/GRP hybrid structure was found to be superior in fatigue properties to any other GRP laminates. In this structure, fracture took place stepwise in slow motion. Furthermore, the relation between the flexural rigidity of the hybrid laminates and the spring constant was calculated using the number of plies as the structural factor of laminates. The result agreed with the experimental value. Therefore, it was confirmed that the CRP/GRP hybrid structure shows the excellent spring properties and can be used for a long time in a wide elastic region.
This paper describes a systematic approach for analyzing and processing the load data containing high frequency components caused by oscillation of a hammer in the instrumented Charpy tests of FRP. Special analytical and measuring techniques were used to separate such components and to smooth the jagged load line. First, the digitized data of raw load time-history obtained from the Charpy tests on plain woven glass cloth FRP and chopped strand glass mat FRP were analyzed by means of the energy spectral density function to determine the cut-off frequency of a filter. Then the load data were passed through the recursive low pass digital filter and smoothed. Finally, the smoothed load line was fitted to a polynomial function of time by the least squares approximation with due consideration for precision in maximum load and ductility index. The results of the analyses of experimental data and numerical values show that the high frequency components in load data are due to the interaction between the specimen and testing machine.
Punching shear properties of selected glass-fiber reinforced polyester under both static and impact loadings were investigated. A modified Split Hopkinson Pressure Bar Method was applied to the impact test. The test results were as follows. (1) The impact shear strength is about twice as high as the static shear strength of the same material. (2) Under impact loading, the law of mixture holds approximately. (3) The impact shear strength is influenced by the textile structure of fiber reinforcements. (4) The residual deformation of glass-fiber reinforced polyester at fracture is larger than that of the matrix itself, and it is influenced by both the textile structure of fiber reinforcements and the fiber content.
This paper deals with the trial production of a chain-loading fiber tensometer and the statistical properties of the tensile strength of glass fibers with the nominal diameter of 13μm. For the tensometer, the tensile load was increased by increasing the suspended length of the chain and a constant-stress-rate condition was realized. A newly introduced adhesive grip of the tensometer eliminated the breakage of fibers due to the gripping and also contributed the reduction in testing time. High accuracy in measuring fracture stress was achieved by using an appropriate loading chain. The statistical properties of the tensile strength of glass fibers was clarified by a method used in reliability analysis. The probability of failure was calculated including censored data so as to achieve more accurate analysis. The strength distribution was well approximated by a normal or Weibull distribution function. Consequently, a statistical analysis can be done with a normal distribution function and a stochastic process theory can be discussed with a Weibull distribution function. The mean strength of glass fibers with the nominal diameter of 13μm was 1.78GPa (gage length=100mm, stress rate=0.0153GPa/s). The coefficient of variation was 28.9%, but it should be considered that this value contains the variation of the sectional area of glass fibers.
Most of structural FRP components are now compression-molded using SMC (Sheet Molding Compound). SMC are, however, characterized by considerable variations of the mechanical properties of molded parts. The advanced utilization of SMC demands to clearify the main factors controlling the variation of SMC properties. This paper deals with three kinds of SMC laminates of which charge ratios were 30, 60 and 90 percents. The main results obtained in this work are summarized as follows; (1) SMC laminates showed the anisotropy of tensile strength which is directly related to the direction and also to the extent of SMC flow in molding. The relation between the anisotropy of laminate and material flow was complicated due to a localization phenomenon in SMC flow. (2) The average and the coefficient of variation of the tensile strength of SMC laminates were calculated by use of the mixture laws of strength and its variation in FRP and were found to agree with the experimental data. The sensitivities of each variable on the tensile strength of SMC were computed. The simplified mixture laws of strength and its variation of SMC were proposed, which consist of the variables having high sensitivity.
In the field of composite materials, such as glass fiber reinforced plastics, the molding process using a matched die is attracting wide attention as a mass production technology. BMC has been developed for the process, but it seems to have some troubles. One of them is a surface appearance with thin wall. Another one is brittle weakness at the bottom of a fin portion caused by a stress concentration in the resin rich fillet part. In order to prevent these troubles, it is necessary to investigate the flow state of BMC during the molding process. In BMC glass fibers are randomly oriented at the initial state. However, in order to prepare a specimen with fibers oriented initially in the flow direction, a block of BMC is prepressed from 50mm to 6mm in its height. So, the flow of BMC shows an anisotropic nature, such as different fluid resistances in normal and parallel directions to the flow, as reported in the previous paper. In this paper, the dependence of flow state of BMC on the closing speed and the influence of temperature were considered. At first, the experiments were carried out at room temperature so as to avoid the effect of polymerization and obtain only the effect of closing speed. Then the experiments were made at 110°C, which is close to the actual product condition. The results indicate that the flow state of BMC shows more anisotropic nature with increasing closing speed. With decreasing closing speed, the troubles described above disappear at room temperature but at 110°C only a limited closing speed prevents the troubles. And the“surge phenomenon” appears remarkably with increasing speed.
Carbon fiber or glass fiber unidirectionally reinforced epoxies (CFRP or GFRP) were immersed in water, and 4, 9 and 23% salt water at 20°C at various pressures in the range from 50 to 1000kg/cm2 for one month, and the measurements of the amount of water absorbed, flexural strength and interlaminar shear strength were carried out on them before and after immersion. The amount of water absorbed was more in CFRP than in GFRP, and it decreased with increasing pressure and increasing concentration of salt in both of CFRP and GFRP. The flexural strength and the interlaminar shear strength of CFRP were decreased by 5.0 and 7.5% on the average, respectively, by the immersion in water and salt water under pressure, and those of GFRP by 1.2 and 1.9%, respectively. In this case, the flexural strength as well as the amount of water absorbed in CFRP decreased as the immersion pressure increased. The flexural and interlaminar shear strengths of CFRP and GFRP after immersion were not lower in salt water than in water. From these results the effect of hydraulic pressure was considered to decrease the flexural strength of CFRP.
Carbon fiber or glass fiber unidirectionally reinforced epoxies (CFRP and GFRP) were immersed in water and 9% salt water at 20°C at a pressure of 800kg/cm2 for various periods in the range from 1 to 12 months, and the measurements of the amount of water absorbed, flexural strength and interlaminar shear strength were carried out on them before and after immersion. The amount of water absorbed in CFRP in water or salt water was twice that absorbed in GFRP for the same period of immersion. The flexural strengths of CFRP and GFRP were decreased by 4-13 and 3-8%, respectively, by the immersion in water and salt water for twelve months, and their interlaminar shear strength 13-14 and 1-9%, respectively. The amount of water absorbed in salt water was less than that in water, and the decrease in flexural strength of CFRP and GFRP in salt water also was less than that in water. The flexural and interlaminar shear strengths of CFRP and GFRP decreased with increasing amount of water absorbed, and the decrease in flexural strength of CFRP was practically equal to that of GFRP at the same amount of water absorbed, but the decrease in interlaminar shear strength of CFRP was slightly greater than that of GFRP.