Adhesive joint and adhesive/rivet or adhesive/spot weld combined joints are getting special attention recently because of their advantage of joining thin steel plate from the viewpoint of high joint efficiency. In this study, the strength characteristics of adhesive/rivet combind joints were investigated by static tensile and fatigue tests. In addition, the stress distributions of the joints were analyzed by the finite element method. The maximum stress in the adhesive layer was found to correlate with fatigue strength of the joints.
Advanced fiber-reinforced laminated composite materials have been used for structural members in various fields, because of their high specific strength and stiffness. In general, laminated composite cylindrical shells behave differently from homogeneous orthotropic cylindrical shells due to their anisotropy and unsymmetric lamination. In the present paper, the buckling problems of carbon fiber/epoxy (CFRP) cross-ply and angle-ply laminated cylindrical shells under hydrostatic pressure are considered. That is, the effects of stacking sequence, number of layers, lamination angle, buckling modes and dimension of cylinders, on the buckling pressure are analysed by assuming the buckling patterns which satisfy the equilibrium equation based on the Donnell-type expressions.
Threshold strength of delayed failure, KISCC, under cyclic mixed mode I and II loading conditions was investigated for a Ni-Cr-Mo steel (SNCM439), that was quenched and tempered at 673K. When the mode II static stress intensity factor KII or the range of mode II dynamic stress intensity factor ΔKII was superposed on the static stress intensity factor KI, the threshold strength KISCC decreased with an increase in KII or ΔKII. The amount of decrease of KISCC due to the superposition of ΔKII was larger than that of KII. When the ratio of stress intensity factor KR was large, the crack propagation direction under superposed KII or ΔKII on KI at near the KISCC was normal to the maximum tensile stress. Therefore, when the ratio of stress intensity factor KR is large, the threshold condition on the delayed failure crack growth initiation can be described by the maximum tensile stress criterion, which is given by σθmax√2πr=constant.
Corrosion fatigue behaviour of plant materials such as 21/4;Cr-1Mo, SUH309 and S20C was investigated in a molten salt environment of (NaNO240%, NaNO37%, KNO353%) at 723K. The rotating bending fatigue strength at 4×107 cycles of 21/4;Cr-1Mo steel in the molten salt environment was 8 percent lower than the fatigue limit in air. The fatigue strength at 4×107 cycles of SUH309 in the molten salt environment was deteriorated by 27 percent. Corrosion fatigue cracks were initiated from corrosion pits and propagated accompanying striation predominantly in the molten salt environment. The morphology of the corrosion products on the specimen surface was examined by X ray-diffractions. The major corrosion product of these steels was Fe3O4. It may be concluded that the formation of corrosion pits at the initiation of fracture surface is the major cause of fatigue strength decrease of these steels in a molten salt environment containing Cl-. The corrosion fatigue strength of S20C steel was higher than the fatigue limit in air. This may be caused by the retardation of crack initiation from the hardened surface layer due to nitriding in a molten salt environment. Neither the initiation nor the propagation of stress corrosion cracking of these steels was observed in the molten salt environment during the initiation test up to 2880hrs and the propagation test up to 2160hrs in the stress intensity factor range KIO of 248 and 372MPa√m, respectively.
Fatigue crack growth and crack closure at moderately elevated temperatures were investigated in low carbon steel (SM41A), Cr-Mo-V steel and type 304 stainless steel (SUS304). A critical experiment was made to examine the cause of increase in fatigue threshold at elevated temperatures in air environment. Samples of low carbon steel and type 304 stainless steel were tested in argon gas at 300°C. The fatigue threshold and crack closure were compared with those in air at the same temperature. It was concluded that the oxidation at elevated temperatures would contribute to the increase in fatigue threshold through the double effects of the enhanced crack closure and the increased fatigue crack growth resistance.
In order to obtain the fundamental information on the remaining life evaluation of high temperature components, the high cycle fatigue strength of austenitic stainless steel used in service for a long time in fossil fuel power plant was investigated at 873K, and it was compared with that of the virgin material. The fatigue strength of the postservice material was significantly lower than that of the virgin material, and the carbide precipitation in the postservice material played an important role in fatigue crack initiation. The small fatigue crack growth behavior in both materials was also studied. Small cracks in both materials propagated even in the region of lower J-integral range than the long crack threshold ΔJth of the virgin material. In addition, the small crack growth rate of the postservice material was several times faster than that of the virgin material. It was concluded that the information on crack growth in the virgin material is not always justifiable to the safety assessment of high temperature long term service components.
The a.c. electric potential method was adopted for the crack length measurement in high temperature fatigue tests. The method was found easy to get the same relationship between the electric potential and the crack length as that in the d.c. potential method, when a constant a.c. of the relatively low frequency of 93Hz was supplied to the plate specimens of SUS304 stainless steel and titanium alloy, Ti-17, with a center crack or double edge cracks. Fatigue crack propagation tests were conducted on SUS304 at 600°C and on Ti-17 at 482°C under the trapezoidal waveforms with tension hold time in the crack center opening displacement (CCOD) control as well as the load control conditions. A creep-dominated, time-dependent relationship between the crack propagation rate, dl/dN, and the creep J-integral range, ΔJc, in the CCOD control condition was in good agreement with that in the load control condition, although the CCOD control test was a kind of ΔJc-decreasing tests. The titanium alloy, Ti-17, showed a lower bound of the available dl/dN-ΔJc data band of some heat resisting steels.
Evaluations of thermal shock resistance, including the onset of crack propagation and progressive crack growth, and examinations to the deterioration property for tetragonal zirconia polycrystalline (TZP), silicon nitride and silicon carbide were conducted under various repeated thermal shock conditions. Vickers indented specimens with various surface flaws sizes were employed for these experiments. The results obtained were summarized as follows: (1) The minimum thermal shock condition required for the onset of crack propagation was proportional to the inverse square root of surface crack length under repeated thermal shock conditions. (2) The actual stress intensity factor KI of crack near the specimen surface was obtained by using a fracture mechanics formula for semielliptical flaws and making some corrections to the thermal stress value by taking the Biot modulus difference into considerations. As a result, KI values of TZP, silicon nitride and silicon carbide were found to be about 17-33%, 17% and 6.9% compared with those calculated without corrections from the surface half crack length by employing usual fracture mechanics formula. (3) TZP, which has the largest KIc value and bending strength, showed superior character, when the resistance of material under repeated thermal shock conditions was evaluated by the amount of crack growth per each cycle. Silicon carbide had the smallest resistance under the repeated thermal shock condition.
Tension test data and blast erosion test data are presented for the epoxy composite systems in which the volume fractions of dispersion were changed. The dispersed materials were SUS304, Al and Al2O3 powders and their average particle sizes were 44-68μm. The results showed that the tensile strength of epoxy composites could be slightly increased by choosing a proper volume fraction of SUS304 or Al powders. However, the tensile strength decreased with increasing the volume fraction of sintered Al2O3 powder. Also a profound effect was found on the resistance to blast erosion of the composites, depending on the volume fraction of the dispersion used. These experimental results indicated a good correlation between the resistance to blast erosion of the composites and the tensile strength.
In order to investigate the change of strength of ceramics due to the size effect, the strength distribution was simulated by the Monte-Carlo method. The tensile strength, 3- and 4-point bending strength of four kinds of specimens of different sizes were calculated, by using the model of specimens with a number of penny-shaped inner cracks of which diameters followed the prescribed distribution. The strength obtained was plotted on the Weibull probability papers and represented by the 2-parameter Weibull distribution. As some experimental data had indicated, the Weibull modulus had a tendency to increase as the effective size decreased. It became clear that this tendency was caused by the existence of the upper limit of crack strength. The strength distribution function for this model was also expressed and compared with the simulation results. Although the Weibull plot of that function was slightly concave, it showed good agreement with the simulation results. Furthermore, the relation between the mean strength and the effective volume was discussed, using the Weibull statistics. As a result, it was indicated that the Weibull statistics could not explain the change of strength due to the size effect satisfactorily.
A method of estimating the reliability of double reinforced concrete members under an axial load at a given eccentricity was proposed by using the probabilistic method, considering the variations in strength of both reinforcing bars and concrete and improving the method which was proposed previously to obtain the reduction factor of the balanced steel ratio, the minimum steel ratio, and the ultimate design strength of the single reinforced flexural member with a specified reliability. The validity of the method was examined by comparing the results with the values provided in the ACI-Code which were decided in the safety side, based on a large number of the test data.
This paper is concerned with evaluation of the upper bound of probability of failure when the distribution forms of the strength R and the stress S are unknown and only their means μR and μS, and variances σ2R and σ2S are known. In the previous paper, assuming (a) the distribution of Z≡R-S is continuous and unimodal and (b) the mode of Z is equal to the mean of Z, the present author derived the formula PfU=(4/9)(f2cη2R+η2S)/(fc-1)2 using Camp-Meidell inequality, where fc≡μR/μS is the central factor of safety, and ηR≡σR/μR and ηS≡σS/μS are the coefficients of variation of R and S, respectively. However, the assumption (b) can be satisfied only in limited cases. Thus, it was attempted in the present paper to evaluate PfU assuming only (a) without assuming (b). The problem was formulated as an optimization problem and solved using linear programming. It was found that the above formula can still be used under the assumption (a) alone. That is, the formula gives an almost exact value of PfU under the assumption (a) alone when ηZ, the coefficient of variation of Z, is small, and gives a somewhat higher value when ηZ is large. Hence, practical applicability was given to the above formula in the present study. The upper bound of the central factor of safety can also be obtained from this formula only by assuming the condition (a). Thus, by merely assuming that the distribution of Z≡R-S is continuous and unimodal, considerably lower upper bounds can be obtained for the probability of failure and for the factor of safety as compared with the case when no assumption is made.
The possibility of thermal spraying over epoxy composites was investigated by paying attention to their temperature increase, and a suitable flame spraying process was selected for this experiment. The mechanism of adhesion for the flame sprayed 18Cr-8Ni steel coating on the epoxy composite system was also investigated by changing the volume fraction of dispersion. The dispersed materials were SUS304, Al and Al2O3 powders and their average particle sizes were 44-68μm. The experimental results showed that the adhesion strength of the sprayed coatings increased with increasing the volume fraction of SUS304 and Al powders. However, the adhesion strength markedly decreased with increasing the volume fraction of Al2O3 powder. From the observation of the interface of thermal sprayed coatings by EDX, it is clear that the primary contributor to increasing adhesion strength is the bonding between the sprayed coating and the dispersed SUS304 or Al.
With ABS resin as matrix material and stainless steel fibers as conductive filler, the continuous kneading of composite conductive plastics was performed by changing operation conditions, such as the filling ratio of fillers, the feeding rate of raw materials, the revolution speed of paddles and the kneading temperature. The electric conductivity and shielding effectiveness of the material kneaded and the dispersion state of fillers were investigated. It was found that the composite conductive plastics being useful as an EMI shield material for electronic instruments were obtained with the filling ratio above 10 wt%, and the electric conductivity and shielding effectiveness could be improved by kneading under a proper operation condition, i.e. an optimum feeding rate, lower revolution speed and higher kneading temperature. Also, it was clarified that under such an operation condition the dispersion state of fillers in the matrix resin were dispersed effectively, forming a network of conductive paths and reducing breakage of fillers during kneading.