Deformation behavior and the microstructural evolution with strain were studied in the Pb-Sn eutectic alloys which were solidified at different cooling rates. The flow behavior of the as-cast specimens was explored in compression tests. Stress-strain curves showed a maximum stress at several percent strain, after which the flow stress decreased to a steady state. The flow stress level of the rapidly solidified specimens with finer microstructures was always lower than that of the slowly solidified specimens. The microstructures of specimens changed during deformation, which was introduced mainly by dynamic recrystallization. Equi-axed recrystallized grains formed during deformation increased with strain. As a result, the deformation became superplastic and the strain-rate sensitivity index m increased from 0.2 to 0.4. The as-cast materials, which showed low ductility in tensile tests, can show superplastic behavior in compression tests due to the microstructural evolution during deformation.
Longitudinal and torsional impact tests were individually performed on two types of CFRP (carbon fiber reinforced plastic) rods with a view to assessing the dynamic properties of the materials. The wave history resulting from the longitudinal impact tests on each CFRP rod was resolved into Fourier components in order to determine the complex tensile compliance. In a similar manner, the complex shear compliance was also obtained from the analysis of torsional impact tests. The viscoelastic models and parameters both for tension and for shear were subsequently determined based on the variations of the complex compliances with frequency. It was found for each material that the 4-element viscoelastic model should be applied to the dynamic behavior of the materials both in tension and in shear over a wide frequency range.
Directional variations of viscoelastic properties of CFRP (carbon fiber reinforced plastic) were examined in terms of the complex compliances and the viscoelastic models determined through impact tests on two types of CFRP rods, which were made from the same prepreg but in different fiber orientations. The coordinate transformation law of elastic constants in anisotropic solids was modified to treat the present viscoelastic case by substituting viscoelastic functions for corresponding elastic constants. According to this procedure, experimental data of viscoelastic properties obtained from impact tests were evaluated. The results revealed that the substitution procedure on the transformation law would provide a reasonable estimation of viscoelastic properties in arbitrary direction.
The temperature dependence of impact properties of polyurethane moldings made by Reaction Injection Molding (RIM), which are integral-skin foams with a sandwich structure consisting of high density skin and low density core layers, were investigated by instrumented Charpy impact tests. A remarkable difference in failure mode was observed depending on the layer composition and temperature. The failure modes obtained were classified as: (1) tensile fracture-type and (2) buckling fracture-type. As the density ratio of the skin layer to the core layer increased or the temperature rose, the failure mode changed from the tensile fracture to the buckling fracture. Then the Charpy impact value became over two times greater with the change of failure mode, because the strain after the maximum impact load point increased remarkably. In addition, it was shown that there existed the density ratio that maximized the Charpy impact value and the fracture stress of the moldings made by RIM when the overall density was kept constant. The results obtained here provide an important guide to determine the optimum layer composition concerning impact properties in the structural design of moldings made by RIM, when such moldings are applied to light-weight and thermal-insulating structural materials.
This investigation deals with the mechanism of ductile crack initiation from a notch under mode II loading on a weldable structural steel SM41A. Under mode II loading, a notch root becomes acute and ductile crack initiation in shear type occurs at the region of the acute notch tip by large shear deformation. The ductile crack initiation occurs in two stages. In the first stage, the crack is initiated ahead of the acute notch tip by shear decohesion, and in the second stage, the crack emanates from the tension side of the surface of the first stage crack under low stress triaxiality. It seems to be difficult to predict analytically the shear type crack initiation from a notch under mode II loading, because of the intense concentration of strain at the acute notch tip and the complexity of stress history of material elements around notch tip. From the mode I loading tests of the specimens subjected to mode II preloading, it is found that the ductile crack initiation under mode I loading is promoted by mode II preloading. This result suggests that, at the notch tip region under mode II loading, some microscopic damage other than voids is produced, though unobservable by an optical microscope.
The growth behaviour of small fatigue cracks was investigated on an aluminum alloy 7075-T6 at the stress ratios R of 0, -1 and -2. The effect of stress ratio was discussed on the basis of the detailed observation with special interest in the stage I region of small crack growth. Cracks initiated at R=-1 and -Z, grew to a certain depth by the stage I growth mechanism, and the stage II crack growth followed. The stage I to stage II transition occurred under a constant maximum stress intensity factor which was approximately consistent with the effective threshold stress intensity range, ΔKeff, th, for large cracks. At R=0, on the other hand, the stage I crack growth was not observed because of crack initiation at inclusions. At all the stress ratios, small cracks grew more rapidly than large cracks in the same nominal stress intensity range, and grew below the threshold stress intensity range, ΔKth, for large cracks. Particularly, the stage I cracks showed much higher growth rate and grew below ΔKeff, th for large cracks. It is suggested that the stage II crack growth rate can be characterized in terms of the effective stress intensity range, while a micromechanics approach is successful to evaluate the stage I crack growth rate.
The characteristic of fatigue crack growth was investigated in pure titanium specimens with two different grain sizes of 27μm (fine grain) and 70μm (coarse grain). Two types of fatigue crack growth tests, i.e. the stess ratio R-constant and maximum stress intensity factor Kmax-constant tests, were carried out in laboratory air at room temperature. Based on the crack closure measurements, it was found that the crack growth rate was an unique function of effective stress intensity range ΔKeff independent of grain size and stress ratio. The transition in crack growth occurred at approximately constant values of ΔKeff (10MPa√m and 5MPa√m) in both fine-and coarse-grained materials. Fracture surface examination revealed that the transition behaviour was attributable to the change of crack growth mechanism which is related with crack growth resistance, fracture surface roughness and crack branching. The crack growth rate obtained in the Kmax-constant tests was compared with those of the other structural materials such as steels, a nodular cast iron and an aluminum alloy.
The behavior of fatigue cracks propagating along the weld interface of friction welded butt joint plate specimens with a single side edge notch and/or with a center notch composed of a free cutting stainless steel JIS. SUS303 was observed under a repeated tension load condition with a stress ratio of R=0 and with a frequency of 30Hz. Reference data were also obtained by using diffusion welded butt joint plate specimens with a single side edge notch, where the base metal was the same as the above. The experimental results indicated that the crack growth rate in the friction welded joints was much lower than that in the base metal for the cracks propagating the side edge of the specimen, despite that the crack growth rate from the center of the joints was much higher in comparison with that of the base metal. This suggests that the residual stress distributed along the crack growth path, together with the microstructure, gave a great influence on the crack growth behavior of the friction welded joint. To verify this points, the crack growth rate was evaluated against the modified stess intensity factor K' which was calculated numerically in considering both of the applied stress and the residual stress. The da/dN-ΔK' relation showed the similar tendency to that of the base metal in the case of the cracks propagating from the side edge. Furthermore, the diffusion welded joint plate specimen with a single side edge notch showed much higher crack growth rate than the base metal. This is attributed to the difference in the metallurgical feature at the welded interface between the friction welded and diffusion welded joints.
The effect of a single peak overload on the fatigue crack propagation was investigated in polyvinyl chloride (PVC), polymethyl methacrylate (PMMA) and polyamide (PA). The results obtained are summarized as follows: (1) A single peak overload was found to cause the acceleration of crack propagation during overloading and the retardation after overloading in all materials used. (2) The acceleration factor of crack propagation due to the overload was higher than that reported in metals. Especially PMMA which is a brittle material showed a remarkable acceleration. (3) The maximum retardation occurred immediately after the overload and the so-called delayed retardation was not observed in all the cases. The maximum retardation rate was fairly high in ductile PVC and PA, but extremely low in brittle PMMA. (4) The application of a single peak overload showed a tendency to prolong the fatigue life in PVC and PA since the retardation effect was larger than the acceleration effect, but in PMMA the retardation effect was small and the life was shortened under a certain condition. (5) From the examinations of fracture surface and crack closure, it was inferred that the craze formed at the crack tip and the blunting or orientation hardening at the crack tip played an important role for the acceleration and retardation, respectively.
The fatigue crack growth rates of material A (iron base alloy A286), material M (austenitic stainless steel 12Cr-12Ni-10Mn-5Mo) and material S (austenitic stainless steel SUS304L) were investigated experimentally. The fatigue tests were conducted with compact specimens at cryogenic temperatures (4 and 77K) and room temperature. The results obtained were as follows. (1) For materials A, M and S, the crack growth rates at 4 and 77K were lower than that at room temperature. However, the temperature dependence of the crack growth rate was different among the three materials and no clear tendency was observed. (2) At 4K, the crack growth rate of material A was lower than that of material M or S. (3) The crack growth rates at 4K of materials A, M, S, titanium alloy and aluminum alloy were related to the strain intensity factor range ΔK/E. (4) For materials A and M, a good correspondence between the striation spacing and the crack growth rate was observed in the crack growth rate range of 10-4-10-3mm/cycle, irrespective of test temperatures.
The correlation between tensile strength and low cycle fatigue life at elevated temperatures was discussed on a cast Ni base supper alloy (16Cr-3Al-3Ti-9Co-Ni) and a Co base super alloy (11Ni-29Cr-7W-Co). Also, an equation for predicting low cycle fatigue life was developed. In the low cycle fatigue tests, the Co base super alloy showed similar fracture behavior in a wide range of temperature. However, the Ni base super alloy showed brittle fracture behavior at low temperatures (ex. 500°C) because of poor ductility, but showed ductile fracture behavior at high temperatures (ex. 900°C) because of high ductility. In the Ni base super alloy, the failure life was defined as the number of cycles when the nominal stress value of a specimen began to decrease from a steady state stress. Tensile and low cycle fatigue data obtained at 500-900°C for the Ni base super alloy and 600-900°C for the Co base super alloy were analyzed by the stepwise multiple regression procedure, and the effect of tensile strength characteristics on the low cycle fatigue strength was discussed. The results of this analysis showed that the relationship between the plastic strain range (Δεp) and cycles to failure (Nf) was expressed by a function of εf, while the relationships between the elastic strain range (Δεe) and cycles to failure was given by a function of (σ0.2/E) for both super alloys in a wide range of high temperatures, unlike the results of the Universal Slope method. The life prediction equation obtained was as follows: Δε=Δεe+Δεp=A(σ0.2/E)lNkf+BεnfNmf where A, B, k, l, m and n are material constants that are independent of temperature.
The effect of grain size on the strainrange partitioning life relation, that is, Δεij-Nij relation, was quantitatively determined for 347H stainless steel at 750°C by conducting creep-fatigue tests on five kinds of test materials with different grain size at 750°C both in air and in an imperfect vacuum. It was assumed that the ductility normalized strainrange partitioning (DN-SRP) life relation in a perfect vacuum was exactly the same as that reported in the author's previous paper and was not influenced by grain size explicitly. As the results, it was found that the grain size dependence of tensile and creep ductilities, Dp and Dc, can be given by the equations, Dp=0.160+0.140d-1/2 and Dc=0.540+0.140d-1/2 (d: grain size in mm), and the Δεij-Nij relation in air can be expressed by Dp, Dc and d as follows: Δεpp=(0.640-0.0438d-1/2)DpN-0.6pp Δεpc=1.26D0.787pN-0.787pc Δεcp=0.261D0.722cN-0.722cp Δεcc=1.31D0.943cN-0.943cc
It is well known that slow-fast or tension-hold strain waveforms, which are so-called c-p type waveforms, yield the shortest life of high-temperature low-cycle fatigue for 304 stainless steel. This may be attributed to an irreversible creep deformation such as an accumulation of grain boundary sliding in the tension going direction which results from the difference in strain rate between in tension and in compression in every cycle. This characteristic seems to be a fundamental nature of creep-fatigue being typical to many kinds of heat resistant steels. From this point of view, a monotonic tensile test was conducted under a constant strain rate being equal to the tension going strain rate of slow-fast waveform for a fatigue test using a 304 stainless steel smooth specimen, and the difference between monotonic tension and fatigue in the behavior of small crack initiation and early growth along grain boundaries on the surface of the specimen was examined. The crack initiation life was shorter and the crack density was higher in monotonic tension than in fatigue. All the small cracks were arrested with the blunting of the tips in monotonic tension, although some of the cracks propagated discretely to become large cracks in fatigue. It can be concluded that the easy crack initiation and the following crack coalescence due to local necking of the specimen in monotonic tension causes the shorter failure life as compared with the fatigue life.
The susceptibility to stress corrosion cracking (SCC) and its potential dependence of Zircaloy-2 were investigated. The SCC tests were conducted with CH3OH solutions containing HCl under given potentials at room temperature. The critical potential for SCC in 0.4% HCl solution moved to the negative direction, and the susceptibility to SCC increased. The potential range for SCC corresponded with the active region in the potentiostatic anodic polarization diagram, and the test specimen was attacked by uneven general corrosion when the potential was moved to the positive direction. The fracture mode of SCC differed from that of general corrosion, i.e., the transition of intergranular fracture to cleavage-like fracture was observed in the SCC potential region, whereas only intergranular corrosion was found in the active region. It is, therefore, considered that cleavage-like fracture is caused by mechanical stresses associated with the absorption of hydrogen generated by an intergranular corrosion reaction. A model for cracking was proposed based on these experimental results.
In order to examine the computational accuracy of the boundary element method for predicting galvanic corrosion and cathodic protection in the actual complicated field, the galvanic field with a screen plate was analyzed by using single and multiple region methods. It was found from the analysis of two-dimensional problems that the difference between the computational results by the two methods increased with increasing screen height. However, the results by the two methods agreed well with each other by reducing the mesh size. Three-dimensional analyses were also performed on a cylindrical vessel of cast iron and type 304 stainless steel with a screen plate. A good agreement was obtained between the computational and experimental results.
Various kinds of carbon were used as the surface materials in the manufacture of fire resistive composite boards. The fire resistive boards were tested by an oxygen index method in accordance with the Japanese Industrial Standards (JIS) K 7201, and by a burn-through method. Flammability of the carbon-based boards decreased with an increase in board density. Fire endurance of the carbon-based boards was found to increase in the order of natural amorphous graphite, synthetic graphite, charcoals from bark, sawdust and rice hulls, and natural crystalline graphite. Electric resistivity of these boards increased in the order of natural crystalline graphite, synthetic graphite, natural amorphous graphite, rice hull charcoal, bark charcoal, and sawdust charcoal. Bending strength of the carbon-based boards increased with an increase in density of boards. Reinforcing with glass whiskers did not improve the bending strength of natural crystalline graphite-surfaced composite board.
Recently, the amount of waste materials such as muds obtained from construction and/or hedoro obtained from dredging, has been increasing according to increasing demands of construction as a results of industrial development, economic growth and need for constructing better environments. It is, therefore, very important to use them as construction materials, from the viewpoint of natural resource preservation and environmental impact mitigation. The main purpose of this paper is to present a method for improving properties of hedoro soils by using cement group hardening materials added with CAS materials and gypsum, and to clarify the utilizing feasibility of the hedoro soils for construction materials. As a result, it is found that the strength of the treated hedoro is affected by water content, grain-size characteristics and the clay minerals in hedoro, and reduces due to the existance of humic acid. However, the hardening effect is magnified due to cement group hardening materials added with CAS reagents and gypsum. Especially, the strength appearance in an early age is remarkable. It is concluded that the treated hedoro can be applied as subgrade, embankment and so on. Furthermore, it is clear that Ettringite, which is considered as the dominant reaction product, contributes to the development of strength of hedoro.
Phenol was esterified with boric anhydride and the reaction products were polycondensed with paraformaldehyde. The higher was the boric anhydride content to phenol, the higher was the esterified content of the product. As the boron content of phenyl borate increased, the adequate amount of paraformaldehyde decreased. Flexual strength of the cured resin strongly depended on the boron content. The strength gradually decreased with increasing heat treatment temperature. However, flexual modulus was hardly affected by boron content or by heat treatment temperature. From the results of differential scanning calorimetry, both high temperature and long time were necessary to cure perfectly the phenyl borate resins. In order to explain these curing behaviors, frontier electron density calculations were carried out with triphenylborate (as a model for phenyl borates) and phenol. It was found that phenol showed high reactivity at both ortho and para positions, while triphenylborate had high reactivity only at para position.
Dynamic response of circular rings with various diameter ratios to inplane concentrated impact was examined. The electromagnetic induction method was employed for the generation of impulse because of its good reproducibility and controllability of amplitude. The dynamic photoelastic method and semiconductor strain gauges were used for the measurement of the stress wave propagation through the circular ring. It was confirmed that compressive stress waves propagated into the circular ring transformed to tensile stress waves at the position of conflux stress waves. The stress concentration ratio in the circular ring under the present condition was 12-13% higher than that of the case subjected to diametrically opposite external concentrated static load. Moreover, when excessive stress waves were generated in the circular ring, the appearance of a group of cracks was observed.