Thirty one kinds of fiber reinforced cement boards (FRCB) were used to discuss the frost damage from the pore structural point of view. The complexity of a spacial distribution of pore size, expressed by the fractal dimension (D), was found to be correlated with the frost damage index (FD), in the same way as the samples of fired clay roofing tiles. The correlation analysis between FD and the specific pore volume in the respective fractionated pore size ranges has been carried out. As the result, it was found that the pores with larger radii than 50nm cause the frost deterioration of the FRCB materials under the present freezing-thawing condition.
It is widely known that elastic deformation is a reversible process and plastic deformation is an irreversible one in metal. In this paper, the temperature change of various aluminum specimens during the tension or compression tests up to the plastic region was measured by using copper-constantan thermcouples of 50μm diameter and the deformation process was examined by comparing the experimental results with the numerical ones in which the heat conduction and heat transfer are taken into consideration. The specimens were deformed up to about 16 percent strain under a range of strain rate from 4×10-4S-1 to 3×10-2S-1. The following results are obtained. In the compression test, the yield point was clearly observed in the temperature curve obtained at a low strain-rate level. The intermittent plastic deformation like the serration appearing in a stress-strain curve produced the serrated temperature change. Consequently, it is supposed that the temperature change of a specimen corresponds to the micro deformation mechanism. The loss due to heat transfer is weaker than that due to heat conduction, so that the specimen temperature is mainly determined heat generation due to the conversion of plastic work and heat loss to the tool by means of heat conduction.
This paper describes experimental press forming of glass-cloth reinforced thermo-plastic composite sheet. The sheet was formed with a set of deep drawing tools, i.e., punch, die and blank holder. Deformation of the sheet during the forming was measured by mean of the scribed grid method. Two types of punches -flat and hemispherical- were used to form the sheet. The main deformation occurs in the part of the sheet having a 45° with the fiber, while the rest of the sheet deforms only slightly. The formability of the sheet is limited by the wrinkling that occurred at right angle to the fiber direction. The amount of wrinkling was measured as being the compressive strain in the fiber direction. A newly designed blank holder that has two concentric circular edges on it improves the formability of the sheet. By giving a more radial tensile force on the sheet at 45°, this holder increases both the magnitude and area of the shear-deformed part of the sheet and reduces the compressive strain in the fiber direction, thus reducing the amount of wrinkling. Deeper cups can be produced by this new blank holder.
A personal computer program of three-dimensional thermal stress analysis for anisotropic materials like composites has been developed in this paper. As an example, the deformations of bi-metal and angle-ply laminates under thermal conditions have been analyzed. From the computational results, the torsional phenomenon, which is caused by the coupling and the effect of free edge on stress, has been recognized. When the residual stress is taken into consideration, the tensile strength of angle-ply laminates has been also analyzed. Furthermore, the effect of property of matrix on the tensile strength of angle-ply laminate has been investigated. As a result, it is shown that the fracture pattern can be changed by property of matrix. From these results, it can be recognized that the developed computer program is very useful for the analysis of mechanical behaviors of composites under thermal condition.
To clarify the effects of β-STA processing on fatigue behaviors of Ti-6Al-4V alloy, microcrack initiation and propagation processes were successively observed in a series of rotating bending fatigue tests of β-STA material and annealed Ti-6Al-4V alloy. The main results obtained are as follows: (1) When the stress higher than a fatigue limit is repetitively applied to the specimen, microcracks appear from the α phase for the annealed material, while they appear along the fine acicular microstructure for the β-STA material. When the stress corresponding to a fatigue limit is repetitively applied to the specimen, no microcrack appears in either of the materials. Accordingly, the fatigue limit is determined by the microcrack initiation limit. (2) The fatigue limit of the β-STA material becomes about 30% higher than that of the annealed material. (3) In both materials, the equation dl/dN=Cσ6l (C: constant, dl/dN: crack growth rate) holds, and constants C are nearly equal to each other. (4) The above conclusions (1), (2) and (3) mean that β-STA processing has little influence on resistance against the microcrack propagation but contributes to an increase in limiting stress for the microcrack initiation.
Rotating bending fatigue tests were conducted on S55C steel under two-step spectrum loading, where nH cycles of the overstress σH were followed by nL cycles of the understress σL in subsequent blocks. Crack initiation and growth lives in spectrum loading tests were much shorter than the estimated values based on the linear cumulative damage rule when the damage given by the understress was neglected. The stress amplitude at 60% of the endurance limit of the steel was judged effective to make both of crack initiation and growth lives shorter when the number of cycles of the understress repeated in a block was large enough. A larger value of nL gave shorter crack initiation and growth lives until nL reached about 5×105, and beyond this a larger value of nL did not give shorter lives, when nH was fixed. A smaller value of nH gave shorter crack initiation and growth lives. The crack growth amount at the understress was large, because the number of cycles at the understress was larger compared to that at the overstress, though the crack growth rate at the former was low from about one fifth to one tenth of the growth rate at the latter.
The influence of blowholes on the fatigue strength of members with longitudinal welded joints was investigated by probabilistic approach. Random variables and parameters used were blowhole size, member length, probability of fatigue crack occurrence from blowholes, distribution of stress range and parameter C in Paris's law. Probabilistic functions of these variables were given from the results of previous fatigue tests or field measurements. By parametrical analysis, the influences of the above random valiables on the fatigue strength of longitudinal welded joints were evaluated quantitatively.
Relationship between inelastic mechanical response and the evolution of matrix cracking and delamination under axial cyclic loading is elucidated. Tension-compression loadings at several levels of maximum stress and stress ratio were applied to [±45°]4 Graphite/Epoxy laminate tubular specimens. Significant inelastic deformation was observed under every cyclic loading condition, and the effect of stress reversal on the enhancement of plastic deformation was first discussed. Then, the relation between inelastic behavior and fatigue damage process was discussed by examining the internal state of damage. It was observed that delamination has more crucial effect on the material deterioration than the matrix cracking. The matrix cracking was observed to result from the development of interfacial debonding between fibers and matrix. The significant plastic deformation and salient hysteresis behavior were attributed mainly to this delamination. The changes in Young's modulus and total strain range in the process of fatigue damage were also elucidated.
Unidirectional CFRP has superior in-plane mechanical properties. Its low delamination resistance, however, is a week point. Thus the delamination strength has been evaluated based on the fracture mechanics as delamination resistance or delamination fracture toughness in these days. In the previous paper, the authors have shown the effects of loading rate and load holding on the delamination resistance and the occurrence of stick-slip phenomena. In this paper, the effect of load holding was examined in detail by using a scanning acoustic microscope and the effect of stacking sequence of 0° laminae on the delamination resistance was evaluated by conducting mode I delamination tests on two types of stacking sequence. Based on these results, the stick-slip phenomena during the load holding was explained by heterogeneous crack growth with visco-elasticity in the resin rich region and the micromechanism of delamination between 0° laminae of the multidirectional laminates was proved to be the same as the unidirectional reinforced lamina except for the effect of residual stress caused by curing.
In order to predict the long term creep property, the creep deflection during 3 point bending creep tests were measured at various elevated temperatures below the glass transition temperature on CFRP samples at off-axis angles of 0°, 15°, 30° and 45°, three kinds of CF/GF sandwich laminates and GFRP used as the cores of sandwich. The master curves of creep property were obtained by applying the reciprocation law between time and temperature of laminates. The results obtained were compared with long term creep properties measured under a constant testing condition (T0=50°C). A good agreement between the master curves of creep modulus and long term creep results was found on CFRP and its off-axis samples, but not on the sandwich laminates.
The strain rate dependence of the Mode II interlaminar fracture toughness of unidirectional CF/epoxy composite laminates, Q-C133 (Besfight IM-600/Epoxy #133) and T300/2500 (Torayca T300/Epoxy #2500), was studied by the dynamic End Notch Flexure (ENF) test using a Split Hopkinson Pressure Bar (SHPB) system. The experimental technique using the ramped incident wave to avoid the high frequency vibration and the formula estimating energy release rate derived with aid of FEM analysis, both of which were obtained previously by the authors11), were used to carry out the high strain rate experiments. The results obtained are as follows: In the range of shear strain rate γ of 10-5-102sec-1, the Mode II critical energy release rate GIIC tends to decrease with increasing γ. The value of GIIC at γ=102sec-1 is approximately 20% lower than the value at γ=10-5sec-1 on each specimen. This tendency is similar to the results reported in 10) and is considered to be caused by the fractographic difference between high and low strain rate loadings. That is, hackle-looking fracture surface due to the fracture of the matrix resin is observed at a low strain rate, but at a high strain rate, smooth fracture surface due to the debonding between the matrix resin and the reiforce fibers is observed.
This paper describes tool wear in cutting of unidirectional glass fiber reinforced plastic materials with high-speed steel bits. It was clarified that the changes in both the characteristic of matrix and the direction of fiber influenced the tool wear. It was found that the roughness of the cut surface of specimen was strongly affected by as well. The following results were obtained: (1) The wear width of flank face was made longer with an increase in Young's modulus of matrix. (2) The hard matrix produced unusual wear of rake face like a step at both directions of fiber, π/2 and 2π/3. (3) The lower Young's modulus of matrix resulted in the larger roughness of surface. (4) The wear width of flank face was strongly affected by the direction of fiber, while that of rake face was not so much.
Experiments were carried out to study the effects of matrix material, reinforcement fibers, impact angle and particle velocity on the solid particle erosion behavior of thermoplastic resins reinforced by short fibers. Special attention was focussed on the incubation period of erosion. Two types of FRPs were used for the erosion experiments. One was the New Thermoplastic Poly-Imide (New-TPI) resin reinforced by short glass or carbon fibers. The other was the Poly Ether Ether Ketone (PEEK) resin reinforced by short glass or carbon fibers. The FRPs with different fiber contents were utilized as the test materials to examine the effect of fiber content. The initial and eroded surfaces of test specimens were observed with a scanning electron microscope and an optical microscope. The results of the experiments show that the incubation period is strongly dependent on the initial surface roughness.
Thermoplastic resin is being applied as the matrix of the continuous fiber reinforced composites. But it is difficult to impregnate reinforcement strands with the melted resin because of its high viscosity, and the adhesive condition at the fiber/matrix micro interface becomes poor. Therefore, it is necessary to estimate the adhesive condition at the interface. The purpose of this investigation is to estimate the adhesive condition at the fiber/matrix micro interface of E-glass/PEI laminates, which are made by co-woven and film stacking method. To promote the degradation of the adhesive condition at the fiber/matrix micro interface of laminates with no matrix dissolution loss, hot-wet and thermal spiking tests were carried out. The absorption properties and flexural properties of each laminates were compared. Consequently the following results were obtained. (1) Under a hot-wet condition, the degradation of flexural strength of E-glass/PEI laminates is mostly caused by micro interface deterioration, and it is possible to estimate the adhesive condition at the micro interfaces of the laminates by hot-wet test and thermal spiking test. (2) The adheive condition at the fiber/matrix micro interface of co-woven laminates is better than that of film stacked laminates because of their good impregnation ability.
In order to investigate the behaviour of Rayleigh surface wave passing through a slit notch, a surface wave generated by collision of a steel ball and traveled on the edge of a steel plate was observed. If h/λ, which is the ratio of the wave length of Rayleigh wave to the length of the notches, is smaller than 0.2, the amplitude of Rayleigh wave after the notch was more than 60% of that before the notch. And if h/λ>0.6, the amplitude of the wave after the notch was less than 20%. A numerical analysis due to a finite differential method (FDM) was also performed. The results of the analysis agree well with the experimental results. It was also found from the FDM results that Rayleigh wave does not propagate along the surface of the notch maintaining it's original shape, but passes through the notch inducing new waves at the tip of the notch and/or at a point of the plate edge after the notch.
Nondestructive evaluation has been playing a critical role in durability and reliability assessment of advanced composite structures. In the present paper, vibration pattern imaging technique and finite element method were applied to characterize the dynamic response of the rectangular composite laminates with through-the-width delamination. A simple delamination model for finite element analysis was proposed and applied to the analysis of the eigenfrequency and model pattern of the delaminated specimen. A Laser Doppler vibrometer was applied as a non-contacting transducer for the measurement of vibration of a specimen. The agreement between the numerical and experimental results was excellent and the results indicated that the size of the delamination has an obvious effect upon the eigenfrequency and the model patterns.
Fiber-reinforced plastic (FRP) composite formed by filament-winding (FW) technique shows a distinct advantage in its high strength. However, the mechanical property data for FW composite, particularly under impact loading are not sufficient for structure design. In this report, Charpy impact property of unidirectional FRP formed by FW technique was investigated. Charpy impact tests were carried out on several types of GFRP and CFRP specimens having widely-spread levels of strength, and the correlation between Charpy impact value and strength was investigated. The experimental results showed that the charpy impact values linearly inceased with increasing tensile strength. The case of unidirectional GFRP especially showed high impact values.