The effect of melting method on the creep rupture strength of 12%Cr heat resisting steels was investigated. The vacuum arc melted and air melted steels with almost the same chemical compositions were employed. At 550°C, the vacuum arc melted steel has higher creep rupture strength than that of the air melted steel. But above 600°C, the vacuum arc melted steel showed lower creep rupture strength than the air melted steel. This was attributed to the easiness of coarsening of carbide in vacuum arc melted steel.
In the present paper the dynamic yield of metallic materials under impact loading was analyzed by using the strain-rate dependent theory of Malvern for plastic wave propagation along a bar. Impact tests were also made on lead and copper specimens. The results obtained are summarized as follows: (1) In the case of step loading, a large increase of dynamic yield stress was predicted at the section near to the impact end of the bar by the rate dependent theory, but the increase was considerably small when the rise time of impact velocity was taken into consideration. (2) The dynamic yield stress near the impact end increased with increasing impact velocity and rising time but with decreasing the constant K in the constitutive equation. The yield delay time depended mainly on the magnitude of K. (3) The phenomena of the increase in dynamic yield stress and the yield delay at the impact end were clearly observed in the impact experiments on lead and copper specimens. If the rising time of impact velocity is taken into consideration the rate dependent theory can account for those phenomena quantitatively.
In this paper, an experimental method to detect an attenuation phenomenon at the leading wave front under torsional impact is presented. The relation between the attenuation phenomenon and the parameters of a constitutive equation for elastic/viscoplastic media was discussed theoretically. Incremental torsional impacts were applied to commercial pure aluminum tubes which had been given pre-stress and pre-strain, and the leading wave front was detected in the range of larger pre-strain. The parameters of the constitutive equation were obtained using the attenuation curve of the leading wave front. The experimental method proposed in this paper may be an effective method to investigate the strain rate dependency of the yield point of mild steel.
The process of quenching a long gear of carbon steel was analyzed by considering the coupling effect among temperature, stress and metallic structure. Emphasis was focused on the finite element scheme of so-called generalized plane strain condition; in-plane element divisions were made in the one-half of a tooth, and the axial strain of one layer of the elements in the plane was kept constant. The analytical results were verified by comparing them with the measured cooling rate, residual stress and deformation and Vicker's hardness obtained as the function of induced structure. Discussions were also made on the dependences of these quantities on cooling rate in order to achieve the optimum quenching condition.
For the improvement of wear-resistance of high manganese austenitic cast steel, explosion hardening treatment has been tried recently. In this study, in order to clarify the characteristics of explosion hardened Hadfield's manganese cast steel, the hardness distribution, grain size, microstructure, phase transformation, crack and residual stress in the explosion hardened layer were examined. The results obtained are as follows: (1) Explosion impact produced a deep hardened layer and high hardness. (2) Explosion treatment did not give much effect on grain size. (3) The strain markings in Hadfield's manganese steel were classified into five groups, namely simple, Widmannstäten-like, wide, distorted and complex ones. In explosion hardened steel, a good many Widmannstäten-like markings were observed. (4) γ→ε transformation occurred in the thin layer of explosion impact surface and deformation twins were observed in the explosion hardened layer. (5) Microcracks were observed in the surface layer of specimens given two and three explosion impacts but none was found in the specimens given one explosion impact. It was difficult to detect them by liquid-penetrant inspection. (6) High tensile residual stress generated in the surface hardened by explosion.
This study aims at making the relationship between the deterministic approach and the probabilistic approach in the structural earthquake response analysis clear by graphically depicting the constraint curves for responses. The present aseismic design of a structure is carried out by deterministic approach using the response spectrum and modal analysis. As most structures face random external forces, however, it is necessary to introduce the probabilistic concept into the aseismic design. In this study, the authors have formulated the structural earthquake response analysis by two approaches: one is the deterministic approach using the response spectrum and modal analysis, while the other is the probabilistic approach using the power spectrum density, random vibration theory and dynamic reliability analysis. Some structures of multi-degrees-of-freedom were analyzed by the two approaches, and the constraint curves for these responses were graphically illustrated. From these results, it is shown that the constraint curves determined by the deterministic approach are quite similar to those determined by the probabilistic approach. A quantitative relationship between the two responses has been obtained by evaluating these constraint curves.
The multiple strip-sandwich method is a specially devised method using quicklime and filter fabric for embankments with difficult cohesive soils. A quicklime layer of about 5cm thick, sandwiched between two sheets of filter fabric of 30cm wide, is placed in a triangular configuration with a horizontal pitch of about 2m at each stage of piling cohesive soils. The usefulness of this method has been proved by field tests and a practical large scale work, but some problems remain yet for the establishment of the design procedure of the method. To make the multiple strip-sandwich method more practical, this paper examines and analyzes the past data obtained in the laboratory tests, field tests and the practical work, and presents the diagrams to use for evaluating the improvement effect of this method.
Some experiments concerning the generation of a negative stress pulse have been performed for the purpose of investigating the response of material under negative shock stress. The electromagnetic induction method was used for the generation of a negative stress pulse. Strip plates made of photoelastic material (DAP) were prepared as the specimen. The stress pulse was observed by the dynamic photoelasticity method and confirmed to be negative by using a semiconductor strain gauge. The stress pulse was generated with a good reproducibility and its stress amplitude and pulse width were easily controllable. Moreover, a negative stress pulse was generated in the strip plate with a natural crack, and the growth phenomenon of the crack was observed while the pulse was passing the crack. The crack grew slowly till the peak of pulse reached the crack position and after that the crack grew rapidly. The growth speed of the crack was about 250m/sec. No growth phenomenon was observed for a positive stress pulse.
This paper aims to clarify the fracture phenomenon of a closed steel vessel generating a lot of random fragments by high internal pressure due to charge explosion. For this purpose, a fracture model based on the distribution of effective work required for individual fragment formation was established to estimate the fragment weight distribution, and the vessel wall strain rate at fracture was calculated. The interrelation between these two factors was examined by the fracture test of the vessel with such a high fracture strain rate, as an order of 104per second. It was confirmed from the test results that the fragment weight distribution is explained well by the fracture model proposed and the distribution characteristics q0, qm and β are the functions of fracture strain rate, namely the external force imposed to the vessel. Their empirical equations are presented, which will be analyzed theoretically in the next report.
In the previous Report No. 1, the empirical equations were obtained, showing that the characteristic indices, q0, qm and β of the fragment weight distribution function, F(q)=1-exp[-(q/qm)β] which is derived from the proposed fracture model, are the functions of fracture strain rate εf of a closed vessel wall. This paper gives the theoretical discussion on those equations, resulting in the following conclusions: (1) The dependence of the fragment mean weight upon εf is well explained by the nucleation process. (2) Generally, the total effective work to be required for fragmentation varies with the material even in the same loading condition. It is inferred that this comes from the existence of energy coefficient λ inherent to the material, which can be derived from the empirical equations. Theoretically, the λ-value, defined as the ratio of qmβ to q02/3, determines the interrelation between the indices, q0, qm and β, covering the range of 0<λ<0.872. (3) All the values of q0, qm and β decrease with increasing εf. The value of fracture exponent β is in the range of 0<β<2/3. In addition, two engineering cases are given as the examples of application of the fragmentation concept.
An attempt was made to combine the subcritical crack growth law with Weibull's bi-modal inert strength distribution function. The results obtained are as follows: (1) The inert strength distribution function after proof test was derived from Weibull's bi-modal distribution function involving both effects of internal and surface cracks of ceramics. (2) The static fatigue life distribution after proof test was formulated by assuming that only surface cracks may grow subcritically. (3) On the assumption that the strength distribution function after subcritical rack growth may be mapped into new inert strength one, the bi-modal distribution function for constant loading rate test was formulated. (4) The newly obtained bi-modal distribution function tended to coincide with the uni-modal one for internal cracks as the loading rate increased infinitely.
In the design of high-temperature equipments, the inelastic analysis comes to be frequently used in predicting a creep-fatigue life of the equipments. This paper describes the accuracy of the inelastic analysis and the life prediction. The material used in the investigation was normalized and tempered 2-1/4Cr-1Mo steel. The constitutive equation in which the total strain is equated as the sum of elastic, plastic and creep strain was used in the inelastic analysis. The accuracy of the inelastic analysis was pre-checked by applying the analysis to the experiments studying the strain rate effect. Uniaxial strain-controlled creep-fatigue tests were performed at 600°C in air, and the actual creep-fatigue lives were compared with the predicted lives using the inelastic analysis and two kinds of life prediction methods; the life fraction rule of ASME B & PV Code N-47 (LFR) and the strain range partitioning method (SRP). The comparison showed that LFR was influential to the accuracy of a calculated value of the stress, and that the predicted lives were far shorter because of higher calculated stresses in the prediction. In order to improve the accuracy of the life prediction by LFR, plasticity-creep interaction should be considered in the inelastic analysis. On the other hand, SRP was not so influential to the accuracy of a calculated value of the stress, and a good agreement was observed between the predicted and actual lives.
To investigate the time and temperature dependences of the longitudinal flexural strength of unidirectional CFRP, three-point bending tests were conducted at various temperatures and strain rates. The longitudinal flexural strength of CFRP showed a remarkable dependence upon time and temperature. The master curve for this strength was constructed based on the thermo-rheologically simple properties. The time and temperature shift factors were quantitatively in good agreement both for the longitudinal flexural strength of CFRP and the tensile strength of matrix resin, and also coincided well with those for the creep compliances of these materials. The fracture of CFRP under three-point bending was classfied into the three modes; a) breaking on both the tension and compression sides at the loading point of specimen in the region of low temperature, b) breaking on only the compression side in the region of middle temperature and c) microbuckling of fibers on the compression side in the region of high temperature. It was found that the time and temperature dependences of bending fracture were dominated by those of compressive fracture rather than tensile fracture.
Evidently, there is the lack of test data for composite materials, particularly for fiber reinforced plastics most likely to be used in the coupling areas. The knowledge of load-carrying capability for different joint geometries and associated fracture modes are needed for bolted-joint design. An experimental program to determine the bolted-joint strength and fracture modes for Mat-FRP at 120°C, 80°C and room temperature has been conducted in order to obtain such preliminary design data. In this paper, the effects of joint geometry and temperature on joint strength and fracture mode are presented. It is concluded that both fracture load and mode can be predicted from joint geometry and temperature. By taking fracture mode into consideration, the following experimental equation is proposed to estimate the fracture load. Pmax=k·P0 k=exp[α(w-d/d)+β(w-d/d)+γ(T-20)2]
The mathematical model of crack growth given by the following equation is proposed, d(Δa)/dt=Kn/A(Δa) where Δa is the crack extention and K is the stress intensity factor. The proposed equation has been applied to the three-point-bending tests on beam specimens under the condition of a constant rate of load-point-displacement, u. The results calculated showed a linear increase in Kmax with u1/(n+1). The experiments have been carried out by changing u from 10-6 to 10-1cm/sec. The experimental results also showed a linear increase in Kmax with u1/(37+1). Subsequently, the experiment has been carried out in order to determine the function A(Δa). The experimental procedure starts at loading a specimen up to K under a constant u, and then the specimen is unloaded measuring the compliance (COD)/(LOAD) to calculate the crack length. By changing the level of K, a total of 28 tests have been carried out. The K-Δa curve obtained indicated a rapid increase in K at small Δa, followed by a gradual decrease in its increasing rate, and finally levelling off of K above Δa=1cm. From this K-Δa curve, the A(Δa) was obtained and found to increase rapidly at small Δa and level off above Δa=1cm. Through various simulations by a computer, the proposed model was found to explain the behaviour of Sanjome Andesite specimen with a crack subjected to three-point-bending under the condition of a constant u. It is remained for future research to verify the equation in the cases of creep and relaxation.
Moiré fringe multiplication method using the interference of diffracted laser beams was used to evaluate the strain distribution at the fatigue crack tip which was crossing the interface of two-layered composite plates composed of low carbon steel and ferritic or austenitic stainless steel. All the tests were carried out under pulsating tension with the stress ratio R≅0. The area Ω surrounded by equi-strain loci of εy, the strain component in loading direction, was obtained from the measurements, and its variation was discussed with relation to the complicated crack propagation behavior near the interface. It was concluded that the variation of Ω could be understood by considering both of the distribution of residual stress in the constituent materials and the difference in yield strength between two materials, and that a modified stress intensity factor, which was obtained by correcting the ordinary stress intensity factor with consideration of the variation of Ω, was a much better parameter controlling the crack growth rate near the interface.
The influence of microstructure on fatigue strength and resistance to fatigue crack propagation has been investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel, Then, the impact fatigue tests were carried out by using a rotating disk type impact fatigue testing machine, These results were compared with those for non-impact fatigue tests, The results obtained are summarized as follows. (1) For the non-impact fatigue, the fatigue strength and the resistance to fatigue crack propagation were markedly influenced by the microstructure. In other words, the 600°C tempered material had the most superior property of crack propagation resistance, while this material showed the most inferior resistance to crack initiation among the materials tested. The 300°C tempered material has a considerably inferior resistance to both the initiation and the propagation of fatigue crack because of low temperature temper brittleness. It was concluded from the present work that the temper treatment at 450°C was one method to improve the resistance to both the initiation and the propagation of fatigue crack. (2) For the impact fatigue, the resistance to crack propagation increased with decreasing tempering temperature. This result was not in agreement with the one described above for the non-impact fatigue. If the impact fatigue fracture behavior of component is estimated from the results of non-impact fatigue, it may involve a considerable risk. (3) The crack growth rate for impact fatigue was larger than that for non-impact fatigue. This difference increased with increasing tempering temperature. This behavior could be explained by the authors' concept of the equivalent stress ratio, in which the effect of impact loading is replaced with the stress ratio in non-impact fatigue.
In order to investigate the effect of cyclic stress history on corrosion fatigue strength, two-level fatigue tests were performed on the 0.45C carbon steel smooth specimens in rotary bending in laboratory air and in salt water (3.0%NaCl). It was clarified that the values of the cumulative damage changed complexly under various conditions depending upon the magnitude of stress, stress sequence and environment. The tendency of the cumulative damage mentioned above, excepting the case of the tests in which the micro-crack growth rate under the secondary stress was influenced by the first stress, were explained theoretically by regarding the maximum crack length obtained from the statistical properties of distributed cracks on the smooth specimen surface as fatigue damage.
The effects of loading history and corrosive environment on the retardation in crack growth were investigated in rotating bending. The material used was 200°C tempered SNCM439. Tests were made at two-step varying loading, K1→K2, in room air and in 1%NaCl solution. The stress intensity was estimated by K=σmax√πa/2 for convenience sake, where σmax is the stress amplitude and a is the crack length measured circumferentially on the surface. The results obtained are as follows: (1) The number of retardation cycles was influenced by the environment, K2, and the intermediate treatment (ex. hydrogen charge) during the change in K from K1 to K2. (2) The retardation crack length was determined by K1, irrespective of environment. (3) The relationship between the number of retardation cycles and a parameter of varying load, (K1-K2)/K2, was derived for each environment. The number of retardation cycles increased linearly with (K1-K2)/K2. (4) The crack propagation rate in retardation period decreased with an increase in K1 and increased with an increase in K2.
Corrosion fatigue crack growth tests in low ΔK region in 3%NaCl solution and in distilled water have been carried out on 13Cr stainless steels at various stress ratios (R=-1.0, 0.1, 0.4, 0.8). The effect of frequency on corrosion fatigue crack growth in 3%NaCl solution was also investigated. The results are summarized as follows. (1) The corrosive environment studied here had little effect on crack growth rate in ΔK≥ΔKth (ΔKth: threshold value in air) for R=-1.0 and 0.1, but it slightly increased the crack growth rate for R≥0.4. (2) As critical crack growth rate dropped from (2∼5)×10-10m/cycle in air to (2∼5)×10-11m/cycle in 3%NaCl, ΔKthCF(ΔKthCF: threshold value in corrosive environment) decreased 0.4 times (R=-1.0) to 0.6 times (R=0.1) of ΔKth. ΔKthCF in distilled water was (0.65∼0.70) time (R=-1.0 and R=0.1) of ΔKth. (3) Corrosion pits often occurred at the corrosion fatigue crack tip in the region below da/dN=(2∼8.7)×10-11m/cycle. Therefore, the ΔK value required to reinitiate crack growth from those pits was about 1.5 times higher than that of ΔKthCF. (4) The crack growth law derived previously by considering the effects of waveform and frequency was also found valid as the first approximation for the materials tested.
A fractographic study on mode II and mode III static- and cyclic-SCC crack growth in 3.5% NaCl solution has been carried out for a high-strength Al-Zn-Mg alloy. Static- and cyclic-SCC crack growth was introduced under mode II and mode III loads. Unlike the mode I SCC, the fracture surface morphologies of mode II and mode III SCC were characterized by transgranular stress-assisted dissolution, piled-up trench-like dissolution along slip planes, slip planes and slip traces. Transgranular stress-assisted dissolution was dominant in static SCC, whilst piled-up trench-like dissolution along slip planes, slip planes and slip traces were dominant in cyclic SCC due to repeated stress. Mode II and mode III SCC crack grew at much smaller K values than the thresh-hold stress intensity factor for mode I SCC crack growth, KISCC.
Carbide tools of P10 and K10, a CBN tool and two kind of sintered diamond powder tool (hereinafter referred to as DC tool) were used to dry-cut copper silumin (AC4B-T6) and two kinds of hypersilumin (Al-17%Si-T6 and Al-23%Si-F), and their wear was observed. Furthermore the detail of wear mechanism of DC tool, which showed the highest wear resistance among the tools used, was examined by EPMA. The following results were obtained. The wear of DC tool was small in comparison with carbide tools and CBN tool in any cutting condition. At low cutting speed, the wear of DC tool was extremely small because it is caused only by the adhesive wear process of silicon grains. At high cutting speed, however, it is caused by the following two processes: One is the falling-off of diamond grains which is attributable to the decrease in its binding force, resulting from the bonding of the work material with cobalt binder during cutting. The other is the same as that discribed above for the case of low cutting speed. DC tool which has the small diamond grain size and the narrow distribution of grain size showed greater wear resistance.
The procedure to predict the skin density of moldings made by RIM (Reaction Injection Molding) was investigated as follows: (1) By regarding the skin formation process as the shrinkage process of bubbles in viscous liquid, a series of equations to predict skin density were derived as the function of viscosity and gel time of RIM material, vapor pressure of blowing agent, and mold pressure. (2) Nine kinds of materials having different foaming properties and reactivities were used as the experimental moldings. Then, the measured skin density was compared with the predicted value. It was found that both agreed well within an error of ±15%. This result suggests that the proposed bubble shrinkage model for skin formation can be useful in determination of material's formulation and molding conditions for RIM.
Two kinds of coal ashes were investigated about their reactivities. One was fly ash discharged from a pulverized coal combustion boiler and the other was FCB ash discharged from a fluidized-bed combustion boiler. Both of these ashes have similar chemical compositions because their origins are inorganic substances such as clay minerals, quartz and feldspars contained in coal. However, these ashes have considerably different structures and characteristics depending on the combustion temperature of the boilers. The results of the investigations for practical utilizaton of these ashes are as follows. (1) The formation of C2AS from FCB ash and limestone, as well as the decomposition of C2AS to C2S and C3A occured rapidly up to 1250°C. But the production rate of C3S from both of the ashes at higher temperatures didn't differ from that from clay generally used in cement production. (2) Higher strength was obtained when FCB ash was used as a pozzolan in steam curing. But fly ash was superior in strength to FCB ash as a pozzolan in autoclave curing. (3) The rate determining step of the pozzolanic reaction of fly ash was the ionic diffusion through the inner hydrate layer. (4) FCB ash showed higher reactivity in low temperature synthesis of molecular shieve, Na-A type zeolite. Fly ash gave only a small amount of Na-P type zeolite rather than Na-A type zeolite under the same conditon.