As a part of our ongoing study to investigate the effect of adhesive defects on fatigue strength of adhesive-bonded joint, a series of fatigue tests were carried out by using adhesive-bonded butt joints with unbonded layer at adhesive/adherend interface under the push-pull fatigue load condition of stress ratio R=-1.0. The specimens used possessed two unbonded portions of nail shape along the adhesive/adherend interface, which were located at the opposite sides of the specimen. To characterize the defect size quantitatively, it was defined by the following two quantities; the unbonded width which was the same as the defect length exposed to the free surface of the specimen, 2r, and the unbonded depth from the free surface, l. The fatigue strength of the adhesive-bonded joint specimens with two kinds of unbonded width (2r=3, 5mm) was discussed in connection with the unbonded depth l. The main results obtained in this study are as follows: (1) For the specimen with 2r=3mm, the fatigue strength decreased with increasing l in the low stress cycle range. But, the effect of l on the fatigue strength diminished with increasing stress cycles. (2) For the specimen with 2r=5mm, the fatigue strength decreased with increasing l irrespective of the number of stress cycles.
Filament-wound composite has distinct interfaces differing from conventional metals, and the fracture at these interfaces (macroscopic fracture) causes a remarkable decrease in strength and stiffness. In this investigation, the behavior of macroscopic fracture (delamination) of filament-wound tubes was observed underr static lateral compression and the crack initiation at the end cross-sections of tubes was examined by the finite element method using the polar-anisotropy. The followings were revealed from the results. (1) The fracture behavior in filament-wound tubes under lateral compressive load was affected by winding-angle of reinforcement, and various fracture modes were observed for the specimens with variable winding-angles. (2) On the numerical analysis, the domains involving microscopic fracture were produced at different locations predictable from the anisotropic or isotropic properties. Therefore, for the design of filament-wound tubes the anisotropic effect should be taken into consideration.
The press moulding process is being applied extensively as the mass production technology for composite materials especially FRP. In this process, although ribs are installed to strengthen the products, they tend to reduce the strength of products by the formation of interface due to two symmetrical flows from the both sides of flat parts toward the rib part. This interface is called “Weld Line”. The aim of this paper was to clarify the behavior of weld line. At first, the initiation and growth of weld line were observed by flow visualization experiments. Then, three point bending tests were carried out to evaluate the press moulded products containing the weld line. Furthermore, fracture surface was observed microscopically by using SEM. From the above experiments, the following results were obtained. (1) The weld line initiated at the opposite surface of the rib part and grew into the rib. (2) The weld line reduced the bending strength to the value less than that of the specimen made of only resin. (3) No reinforcement existed across the weld line. Nextly, the numerical procedure to understand the growing of weld line was performed by applying the initial flow analysis presented previously. The results indicated that the weld line in the case of wide rib differed from that in the case of narrow rib width.
The phenomenon of unstability of ductile fracture in the upper shelf region of a forged steel for nuclear reactor pressure vessels A508 Cl. 3 was studied with a large compliance apparatus, whose spring constants were 100, 170 and 230kgf/mm, at the test temperatures of 100, 200 and 300°C and at the loading rates of 2, 20 and 200mm/min in the crosshead speed. The main results obtained are as follows: (1) The fracture modes of the specimens consisted of (a) stable fracture, (b) unstable fracture which leads to a complete fracture. rapidly and (c) quasiunstable fracture which does not lead to a complete fracture though a rapid extension of ductile crack takes place. (2) Side groove, high temperature or small spring constant made a ductile crack more unstable. (3) High temperature or large spring constant made the occurrence of quasiunstable fracture easier. (4) Quasiunstable ductile fracture took place before the maximum load, that is, at the J integral value of about 10kgf/mm. The initiation of a microscopic ductile crack, therefore, seems to lead to quasiunstable fracture. (5) The concept that unstable ductile fracture takes place when Tapp exceeds Tmat seems applicable only to the case in which unstable ductile fracture takes place after the maximum load has been exceeded.
In order to clarify the mechanism of the effect of specimen diameter on impact strength of FRP in the edge-wise direction obtained by the Hopkinson pressure bar method, the compressive creep tests were perfomed on FRP and polyester resin used as the matrix of FRP and the effects of time and temperature on the compressive properties of these materials were investigated. From the experiments, it was found that the principle of time-temperature superposition was applicable for both the creep compliance of polyester and the compressive creep strength of FRP.Then, a prediction method of the compressive creep strength of FRP was derived based on the law of composite where the visco-elastic properties of the matrix was taken into account. The mechanism of the diameter dependence of the observed impact strength of FRP was explained by the present prediction method.
Three points bending tests were conducted on specimens of plain woven glass cloth/epoxy laminates (fiber volume fraction≈0.4) with notch and R curves were obtained. The effects of the thickness and the size of the specimens on the fracture toughness were investigated. The crack propagation was experimentally observed with the ink method. The electric potential method and the acoustic emission method were carried out to detect the initiation of the crack. In this study, the validity of these nondestructive inspection method was examined corresponding to the load-displacement curves. The following results were obtained: (1) the effect of the thickness on the fracture toughness was less than that of the specimen size, (2) the outputs in the electric potential method were correlatable with the initiation of the damage and the propagation of macroscopic crack, and classified into three distinct patterns and (3) AE counts gave so much information for the fracture behavior in such a way that the initiation of the damage and the proportional limit corresponded to the initiation and the rapid increase of the AE event counts, respectively.
To study the effects of hydrogen charging direction on the hydrogen-induced phenomena and susceptibility to hydrogen embrittlement of SCM435, static bending tests, and observations of hydrogen blistering and hydrogen-induced cracking were carried out, and the plastic deformation on the specimen surface due to hydrogen introduction was investigated by X-ray diffraction technique. The main results obtained are summarized as follows; (1) The hydrogen blistering was observed in the case of hydrogen charging perpendicular to the rolling direction not only on low strength steels but also on high strength level steels having tensile strength up to 1200MPa. On the other hand, in the case of hydrogen charging parallel to the rolling direction, no hydrogen blistering appeared in all specimens. (2) It was found that the susceptibility to hydrogen embrittlement depended on the strength level of material and was affected by the hydrogen charging direction in the case of materials sensitive to hydrogen such as those tempered at 300°C and 400°C. (3) From the measurements of half-value breadth and residual stress by X-ray diffraction, it was estimated that the values of plastic deformation due to hydrogen dissolution for the specimens tempered at 500°C, 600°C and the annealed specimen were about 0.8%, 1.2% and 2.2%, respectively, while those of the specimens tempered at 300°C and 400°C were within 0.5%. (4) Hydrogen-induced cracking initiated around the nonmetallic inclusions (MnS) or micro-segregation bands and grew parallel to the rolling direction. Their fracture morphology was quasicleavage fracture such as river like pattern.
A method to estimate the fatigue crack initiation life of notched specimens from the S-N curve of smooth specimens had been proposed in the previous papers by the present authors. In the method, the local stress and strain were calculated through the relationships of cyclic stress-strain and Kσ-Kε, where Kσ and Kε are the stress and strain concentration factors respectively, based on the assumption that the damage in local material by local stress was just the same as the damage in material of smooth specimen by nominal stress, provided that the local stress of the notched specimen was equal to the nominal stress of the smooth one. From the test results, the applicability of the method was shown to be good for carbon steels (S35C, S15C) in reversed stress. In order to confirm its applicability to high strength steel, fatigue tests were executed on the notched specimens of an alloy steel (SCM435). The influence of mean stress on fatigue crack initiation life of notched specimens was also investigated on both SCM435 and S15C steels. The results showed that the fatigue life prediction method proposed by the authors was valid to SCM435 and that the influence of mean stress was insignificant.
The effect of stress frequency on fatigue crack initiation in 99.5 percent pure titanium, which has remarkable strain rate dependence in plastic region, was studied. Fatigue crack initiation tests were carried out under three frequencies (20Hz, 1Hz, 0.2Hz). An elasto/visco-plastic analysis was performed by the finite element method (FEM), and the comparison between the cycles to crack initiation and the viscoplastic strain range at notch root obtained by analysis was made. The results obtained in this study are summarized as follows: (1) It was found from the experiments that the cycles to crack initiation depended on stress frequency, and were approximately in proportion to fn(n<0), where f is the stress frequency. (2) The effect of stress frequency on the fatigue crack initiation may be explained by the variation of viscoplastic strain range at notch root resulting from the strain rate dependence of the material. (3) A parameter related closely to the fatigue crack intiation is the visco-plastic strain range at the notch root part of stress concentration.
A fatigue crack initiated in welded structure propagates through the welded residual stress field. It is a very important problem to clarify the effect of welded residual stress on fatigue crack growth rate. For the butt welded joint of 600MPa grade steel, the effects of tensile and compressive residual stresses were investigated in association with the crack closure phenomenon. The results obtained are summarized as follows. (1) For the welded joint in which the fatigue crack propagated through the tensile residual stress field, the fatigue crack growth rate was accelerated by the tensile residual stress. But, the acceleration of fatigue crack growth rate was independent of the stress ratio and the intensity of residual stress. (2) For the welded joint in which the fatigue crack propagated through the compressive residual stress field, the fatigue crack growth rate decreased with increasing stress intensity range, and the crack arrested as the stress intensity range approached the threshold for crack growth, ΔK0. At the stress intensity range above ΔK0, the fatigue crack growth rate increased with increasing stress intensity range. The increase of stress ratio has given rise to increase in fatigue crack growth rate and decrease in ΔK0 value. Such a stress ratio effect was the same as the results of base metal. (3) It is considered that the welded residual stress has an effect on the fatigue crack growth rate to change the mean stress at the crack tip. This effect may be explained by the equivalent stress ratio, Req, defined by the following equation. Req=R-2Kres/Kmax
The fatigue limit prediction of butt welded joints with incomplete penetration was investigated by paying attention to the case when the loading direction caused the closure of incomplete penetration. The experimental results indicated the depth of incomplete penetration affected not only the fatigue crack initiation life but also the location of fatigue crack initiation, and there was a trend toward lower fatigue crack initiation life with increasing the depth of incomplete penetration. From an engineering point of view, the fatigue crack initiation limit on the opposite side of incomplete penetration can be estimated by a local strain approach. The fatigue crack initiation limit at the tip of incomplete penetration can be estimated conveniently by the linear fracture mechanics analysis based on the experimental assumption of contact between the face of incomplete penetration.
It is well known that the welded residual stress has marked influence on fatigue crack growth rate in welding structure. In particular, because the tensile residual stress may accelerate the fatigue crack growth rate, it is important to develope the method of residual stress-relief. We have succeeded in relief of residual stress up to about 80% by using a method of mechanical stress relief which was the slow preloading of welded joint. The effects of residual stress relieved, stress ratio and preloadingg on fatigue crack growth rate in welded joints were investigated in associated with crack closure phenomenon. The results obtained are summarized as follows. (1) For the tensile residual stress fields after preloading, the fatigue crack growth rate was accelerated by the tensile residual stress relieved even if the stress ratio was low. Then, the fatigue crack growth rate accelerated was not changed regardless of the strength of residual stress fields and stress ratio. (2) For the compressive residual stress fields after preloading, the fatigue crack growth rate was considerably lower than that of base metal. After the crack arrested as the stress intensity range approached the threshold for crack growth, the fatigue crack growth rate increased with increasing stress intensity range and stress ratio. (3) The preloading scarecely resulted in the change of crack growth rate in the tensile residual stress field but markedly accelerated the rate in the compressive residual stress field. (4) It became clear that the effects of welding residual stress, stress ratio and preloading on fatigue crack growth rate in welding residual stress fields can be explained by the effective stress intensity range defined from both the welding residual stress- and plastic- induced crack closure.
The effect of service loading on the fatigue behavior of steam turbine rotor steel (1Cr-1Mo-1/4V steel) was studied at 550°C. The service loading was simulated by a multiple two-step straining. Under the programed fatigue tests the life was reduced remarkably (1/10-1/100), as compared to the estimated values based on the conventional methods using a linear cumulative law in terms of fatigue and creep damages. This decrease in fatigue life was largely attributed to the accumulation of increased plastic strain range corresponding to the small strain cycles after large strains. For the programed fatigue tests, the fatigue life was estimated under the assumption of linear cumulative fatigue damage law in terms of the measured plastic strain range. The results of this estimation were in agreement with those of the constant strain range tests. The plastic strain range which increased after a large strain cycle could be expressed as a relatively simple function of strain con-ditions. This was indicative of a possible system for monitoring fatigue life of turbine rotors by using the above-mentioned linear cumulative damage law. The fatigue curve could be modified with the data above so as to estimate fatigue life under variable strains for use in designing. It was shown, however, that the curves were affected largely by the number of cycles of small strains (n2) and tended to increase in life with n2. The significance of further examinations on the effects of service loadings was indicated.
In this paper, the equivalent stress range was proposed as a new mechanical parameter controlling the biaxial low-cycle fatigue life of a heat resistant steel at elevated temperatures. Push-pull and torsional tests were conducted on an austenitic stainless steel of SUS304 at 296K, 723K, 823K and 923K with a frequency of 0.1Hz in air. The reason for selecting these four temperatures was to vary the constitutive relation of the material tested. The conclusions obtained are as follows. (1) The maximum principal stress was inadequate to compare the fatigue lives in push-pull tests with those in torsional tests at all the temperature levels from 296K to 923K. (2) Both the maximum principal stress and the temperature level did not influence the accerelation of crack propagation rate caused by the minimum principal stress parallel to the crack. (3) The equivalent stress range, Δσ*, including the effect of principal stress parallel to the crack is a good parameter for arranging low-cycle fatigue data obtained in biaxial tests at temperatures ranging from 296K to 923K.
In our country, many banks are made of allophenic-volcanic ash soils which are notorious problem soils. Kuroboku soil which has high organic matter content and Akaboku are typical allophnic-volcanic ash soils. These soils in the banks are subjected to traffic loads, dead loads of itself and other structures under the nearly undrained condition during construction and for a short period after construction because of their low permeability and high water content. The hardening effect of undrained preloading with a repeated stretch load or a sustained stretch load on the highly saturated impact compacted Kuroboku soil had been studied seldomly in the past.In this research, the effect of preloading under undrained condition with repeated or sustained stage loads was investigated experimentally on impact compacted specimens made of two kinds of the allophenic-volcanic ash soils (Kuroboku and Akaboku). These stage loadings were more practical than stretch loadings. The following results were obtained. (1) The hardening effect of preloading with repeated load was larger on Akaboku than on Kuroboku. (2) The smaller the increasing ratio of load or the longer the loading period of one load stage, the larger the hardning effect of preloading was observed. (3) Organic matter in Kuroboku showed a buffer effect against the impact of repeated loading. (4) The banks of allophenic-volcanic ash soils may increase in density by preloading because their degree of saturation was lower than that of the specimens used in these experiments. From the above results, it is found that preloading with some stage loads under nearly undrained condition is practically effective for stabilizing subgrade and filled-up ground made of allophenic-volcanic ash soils.
The effect of firing operation on the thermal properties of heat insulators in the vermiculite-phosphate system was investigated by measuring their compressive strength and thermal conductivity.The aggregates used were the raw vermiculite from Fukushima Prefecture, Japan and Palaboroa area, N.E. Transvaal, S. Africa, and the bonding material of aggregates was sodium metaphosphate. Magnesium dihydrogen phosphate and aluminum dihydrogen phosphate were tried as the additives. The compressive strength was increased when a proper quantity of magnesium dihydrogen phosphate was added to the vermiculite-sodium metaphosphate system. This was attributed to the formation of bridge bonds due to the reactions of magnesium oxide component in vermiculite with sodium metaphosphate and magnesium dihydrogen phosphate. A considerable decrease in strength was observed when aluminum dihydrogen phosphate was added. This was explained by the X-ray diffraction evidence that alminum dihydrogen phosphate did not react with vermiculite. The thermal conductivity of the vermiculite-phosphate system was considerably smaller than that of the expanded vermiculite-bond clay-phosphate system or the expanded vermiculite-portland cement system. This corresponded to the porosity increase caused by the dehydration reaction of magnesium dihydrogen phosphate with raw vermiculite on firing.
Shock response of curved beams with various curvatures and open angles were examined by using stress pulses. The dynamic photoelasic method and the semicondutor strain gauge were used for the measurement of stress pulses. It was observed that the phase and mode transitions occurred during the process of stress pulse propagation through the beam. On the incidence of positive stress pulses into the curved beam, the transition of the pulse phase to negative occurred at the position of stress concentration, while on the incidence of negative pulses the phase was converted into positive. It was also ascertained that the transition of the pulse mode occurred also at the position of stress concentration. In addition, the position of the stress concentration and the dynamic stress concentration factor were discussed in relation to the curvature and open angle of curved beams.