Ultra fine MgO powders with or without a small amount of SiC whisker were capsulated in a metallic mold with dimensions of O.D. 22φ-I.D. 10φ×30lmm. The mold was preheated at a temperature of 900°-1600°C for 8-10 minutes, and was press-forged in few seconds under an uniaxial compressive load. The deformation ratio of the mold was about 30% of the original height, and the maximum pressure was about 5000kgf/cm2. The polycrystalline MgO without SiC whisker prepared by the hot-forge processing at 1300°C was composed of submicron grains, and its Vickers hardness was almost the same as that of the theoretically dense polycrystalline MgO prepared by hot-pressing. The hardness of the MgO-SiC whisker composite ceramics prepared by this processing, on the otherhand, decreased with increasing whisker content, and the whiskers in the composites were preferentially orientated perpendicular to the press-forging axis. The fracture toughness of the MgO-SiC whisker composite ceramics was about 1.5 times as large as that of the polycrystal without SiC whisker.
The effects of porosity on mechanical properties and deformation behavior of four isotropic polycrystalline graphites were studied. The pore size distributions of the graphites were measured using a conventional mercury penetration technique. The average pore radius of ISO-88 graphite was about one-tenth of that of ISEM-1, IG-11 or IG-15 graphites. Young's modulus of the graphites decreased with increasing porosity. The stress-strain curve of each graphite was measured in its lateral and axial directions. Young's modulus of graphite decreased with increasing load. The plastic strain at a given compressive load was calculated from the stress-strain curve and the initial gradient of the unloading curve at the load. The ratio of lateral plastic strain to axial plastic strain for the graphites was less than 0.5, indicating that the volume of the graphites decreased during compressive loading. By assuming that the volume change was caused by contraction of pores, plastic strain associated with contraction of pores was calculated from the axial plastic strain and lateral plastic strain by slips along the basal planes. The plastic strain increased with increasing axial plastic strain and porosity of graphite.
In order to study the characteristics of static strength of ceramics, 3-point bending tests with a constant loading rate were carried out at room temperature in air, by using sintered silicon nitride, partially stabilized zirconia and two kinds of alumina. The effects of specimen geometry and temperature on the strength were also investigated by conducting ring compression tests for the silicon nitride at room temperature and 1300°C in air. In the 3-point bending tests, better fitness for the two-parameter Weibull distribution was found when the data were correlated with the true fracture stress evaluated at the fracture point of specimen, rather than the maximum nominal stress. The effect of specimen geometry on the strength was small for the case investigated in this study. The mean strength and the coefficient of variation of the strength at 1300°C were reduced to about half and one-third of those at room temperature, respectively. The fracture surface of the tested specimens was observed through a scanning electron microscope, and the predominant flaws were identified which were preexisting in the specimens. The equivalent crack length was evaluated taking account of the crack geometry and the stress state associated with the crack location. The fracture stress was correlated with the equivalent crack length. The results showed that the strength decreased with decreasing flaw size, compared with the estimate expected from the linear fracture mechanics concept. The tendency in the relation was explained by the modification that a material constant should be added to the original equivalent crack length. The fractographic observation suggested that the fracture of the silicon nitride at elevated temperatures followed the formation of semi-circular damage-region due to the slow crack growth. Another fracture mechanics approach was required in the analysis of the fracture of ceramics with inelastic behavior at elevated temperature.
A method to express the distribution of the experimental results of fracture toughness of ice was investigated by postulating the Weibull distribution with three parameters. The data were taken from the previous experiments conducted to study the effect of loading rate on the fracture toughness9), 10). Weibull parameters were estimated by several estimation methods11), 12) and the parameters obtained were compared with each other. The effect of loading rate together with that of specimen size on the parameters were discusssd at three loading rate ranges. It was confirmed that the parameters estimated were closely related to the experimental value of fracture toughness.
Statistical fatigue tests on nodular cast iron in rotating bending were carried out by using a number of specimens, and P-S-N characteristics were discussed quantitatively. Fracture surfaces were observed by SEM, with special attention given to the crack initiation site and the effect of graphite morphology on the fatigue life. The main conclusions are summarized as follows; (1) No significant difference was found in the S-N properties among different blocks cast under the same conditions. It was also confirmed that the S-N property is independent of the location within a block from which the specimen was cut. (2) Fatigue life distribution was well represented by a three-parameter Weibull distribution modified by the saturated probability of failure. Based on the stress dependence of parameters in the distribution, statistical fatigue property was successfully determined. (3) Fatigue crack initiation was governed by graphite morphology in the surface layer of the specimen. A correlation was found between fatigue life and the square root of the projected area of the graphite section in the longitudinal direction of the specimen.
Single-edge-notched specimens of a low-carbon steel and an aluminum alloy were fatigued under cyclic in-plane bending. The growth behavior of a short fatigue crack formed at the notch root was discussed on the basis of the measurement of crack closure. For each material, the propagation rate of short fatigue cracks was uniquely related to the effective stress intensity range, and the relation agreed well with that obtained for long cracks. The resistance curve was constructed in terms of the threshold value of the maximum stress intensity factor which was the sum of the threshold effective stress intensity range ΔKeffth and the opening stress intensity factor Kopth. The value of ΔKeffth was constant, while Kopth increased with increasing crack length cnp in accordance with Kopth=Kopth∞[(cnp-c1)/(c2-c1)]1/2 where Kopth∞ is the Kopth value for long cracks, and c1 and c2 are the characteristic crack lengths. The ratio of Kopth∞ to (c2-c1)1/2 was independent of the material. The predicted values of the fatigue limit of crack initiation, the fatigue limit of fracture, and the non-propagating crack length agreed very well with the experimental results. A simplified method of determining the resistance curve was also proposed.
In adhesive bonded lap joints, stress in the adhesive layer is concentrated at the lap end because of difference in elastic moduli of constituents and abrupt thickness change. Hence, in order to improve the joint stength by reducing the stress concentration in the adhesive layer, it is practiced in industry to form it with a fillet of adhesive spew or to round the corners of the adherend. However, there are few studies on the fatigue strength charactristics of these improved adhesive joints. In order to investigate the relation between the fatigue stength of these joints and the local stress distribution near the lap end, a series of fatigue tests were conducted on the adhesive bonded lap joints with an adhesive fillet plus a rounded adherend, and the local stress near the adherend corner was analysed by FEM. Furthermore, the fatigue crack propagation behavior was observed with an optical microscope. As a result, It is confirmed that the fatigue strength of lap joints with different corner radii can be standardized by the maximum value of maximum shear stress near the lap end.
Fatigue crack growth tests under constant amplitude and repeated two-step loadings were carried out on four kinds of materials of a high-strength low alloy Cr-Mo steel, which have different microstructures obtained by various heat treatments. Crack length and crack closure were measured by using the minicomputer-aided unloading elastic compliance method. The results obtained are summarized as follows. The fatigue crack growth rates of as-rolled, and 600°C and 400°C tempered materials were quite similar. However, the behavior of effective stress intensity range ratio, U, was found to be different depending on the microstructures of the materials. For the 600°C and 400°C tempered materials, U showed the maximum value at the Kmax of about 20MPam1/2 and then gradually decreased with increasing Kmax because of cyclic softening. It was found that for the 200°C tempered material, which has martensitic structure and high hardness, the dominant fracture appearance was cleavage facets at low ΔK region and intergranular facets at high ΔK region, and that the growth rate was higher than those of the other materials in the region of ΔK above 20MPam1/2. The U was found to be 0.5-0.7 in the whole region of ΔK tested. This is probably due to the plasticity-induced and roughness-induced crack closure, because the material used had enough ductility and showed rougher fracture surfaces. Even for the materials which showed cyclic softening, the fatigue crack growth rates under repeated two-step loadings could be well predicted in terms of the measured ΔKeff, using the da/dn-ΔKeff relationship obtained from the constant amplitude loading tests. However, the predicted growth rates might be low and unconservative, when ΔKeff was estimated from the constant amplitude loading test results.
The fatigue crack growth behavior of hot work die steel SKD62, which was austenitized at 1025°C, 1100°C, and 1200°C and subsequently quenched and tempered at 560°C-650°C, was investigated. The fatigue test was carried out on a ΔK control servohydraulic test machine, by using compacttension specimens. The impact test was carried out on an instrumented Charpy impact test machine. The results are summarized as follows. (1) For steels with high hardness (about Hv 600) after tempering at 560°C-570°C, the fatigue fracture toughness Kfc was found to increase by raising the austenitizing temperature from 1025°C to 1100°C. The increase of Kfc is presumably attributed to the decrease of undissolved carbides due to increased austenitizing temperature. However, there was no further increase in Kfc for the increase of austenitizing temperature from 1100°C to 1200°C. (2) For steels with low hardness (about Hv 400) after tempering at 640°C-650°C, the Kfc was found to decrease with increasing austenitizing temperature. This may be attributed to the embrittlement of grain boundaries by raising both the austenitizing and the tempering temperatures. (3) At the linear region in the da/dN versus ΔK curve, the crack growth rate was not affected by the austenitizing temperature but decreased slightly with the rise of tempering temperature. The crack growth rate in this region could be correlated with the effective stress intensity factor range ΔKeff, regardless of tempering temperature. (4) The Charpy impact value decreased with the rise of austenitizing temperature, regardless of tempering temperature. This trend did not agree with the Kfc mentioned above, because the Charpy impact value in this case mainly reflected the resistance to crack initiation.
Plastic strain-controlled low cycle fatigue tests and cyclic tension (C.T.) tests with various plastic strain ratio Rε (Rε=Δεm/Δεp: Δεm being a tensile plastic strain per cycle and Δεp a controlled plastic strain range) were carried out on rod specimens with a small hole which were machined from austenitic stainless steel 316L. The effects of the Rε value on fatigue deformation properties, final fracture mode and surface crack growth behavior were investigated from the C.T. test results. In the C.T. tests (Rε=0-0.667), the stress amplitude in the cyclic work hardening curve showed a notable decrease after a peak stress, and a cyclic strain exponent n' in the cyclic stress-strain curve was lower (about n'=0.17) than that of a constant amplitude test (n'=0.50). In the case of the low Rε value (Rε<0.25), the specimen was fractured with the surface fracture mode by fatigue, while in the high Rε value (Rε≥0.25) it was fractured with the internal fracture mode by the effect of accumulative tensile deformation, and the estimated Δεp-Nf relation from the linear accumulation of the fatigue damage Df and the tensile damage Dt indicated fairly good agreement with the experimental data for the range of these C.T. tests. Moreover, in the range of the low Rε value, the surface crack growth rate da/dN was evaluated successfully by plotting against the parameter of modified J integral range ΔJ*(ΔJ*=ΔJ/(1-Rε)n, where n is a constant), after considering the effect of Rε value.
Near-threshold fatigue crack growth tests on SUS 316L stainless steel were conducted at liquid helium temperature (4K) and a reference temperature of 300K. The crack length and the crack closure point in CT-specimens in a cryostat were automatically calculated by a 16-bit computer system based on the compliance theory. The initial ΔK(ΔKo) and the normalized ΔK gradient during ΔK-decreasing were varied on reference to the ASTM E647 standard. Constant ΔP tests, or ΔK-increasing tests, were also conducted to find the practical and appropriate ΔK-decreasing rule. It was found possible to use higher ΔKo and smaller increment of crack length (Δa) in the 4K tests. However, the tests at 300K raised some problems concerning the ASTM standard. Through the ΔK-range tested, two curves of the da/dnvs. ΔK relation at R=0.4 and 0.7 lied almost parallel at both temperatures of 4 and 300K, but they showed a complex temperature dependence. With a decrease of temperature from 300 to 4K, the ΔKth, corresponding to da/dn=10-7mm/cycle, increased from 3.4MPa√m to 6.5MPa√m at R=0.4, and did from 2.8 MPa√m to 5.5MPa√m at R=0.7. By taking the crack closure effect into consideration, the da/dnvs. ΔKeff relation was also discussed. These values of ΔKth and ΔKeff, th for SUS 316L were compared with those of other 300-series ausenitic stainless steels. SEM fractographs and micrographs of cross-sectional view indicated that the fracture surface appearing at 4K was much smoother than that at 300K.
Low-cycle fatigue tests were conducted in order to investigate the effects of strain wave shape, strain hold time and mean stress on the low-cycle fatigue properties of modified 9Cr-1Mo steel at 550°C in air. The life reduction of fast-slow and compressive-strain hold cycling was observed at the same degree as slow-fast and tensile-strain hold cycling, respectively. But the life of both tensile- and compressive-strain hold cycling with the same hold time approximately equaled that of fast-fast cycling. We concluded that the life reduction was caused by the time-dependent damage which was induced by the difference of the tension going period from the compression going period. However, the mean stress effect may be another cause for the life reduction, because the positive mean stress was noticeable for fast-slow cycling. A creep-fatigue damage model connected with the overstress was applied to the life prediction. By analyzing the stress-strain response under unloading, the degree of overstress was experimentally determined as a function of the inelastic-strain rate. Based on our test results, the time-dependent damage parameter of the damage model was modified. The predicted lives by this modified damage model agreed well with the experimental results.
In plastic packaged integrated circuit (IC) devices, silicon chips are molded with epoxide resins containing about 60% by volume of silica particles. Recently, the size of chips mounted in a package has increased rapidly with advances in large-scale integration technology. This trend creates a problem of increased mechanical stress in the package, which sometimes causes cracking in the encapsulation resin under temperature cycling and other conditions. Hence thorough understanding of the fracture properties of these materials has become an important issue in package design. In this study, static fracture, fatigue and creep properties of silica particulate filled epoxide resins for IC encapsulation were studied at various temperatures between -55°C and 150°C by using smooth plate specimens. The fatigue tests were carried out with different mean stresses and frequencies. Comparison was made between the properties, and their failure mechanism was discussed. The gradients of both S-N and creep rupture curves were very small. The test results suggest that the fracture of these materials is mainly controlled by stress rather than strain. Scanning electron microscopy of the fracture surfaces indicates that a crack propagates by debonding of filler particle/matrix interfaces at high temperatures around the glass transition temperature, whereas it propagates through filler particles at lower temperatures. Most features of the test results can be attributed to the brittle fracture of the particles.
An investigation has been carried out concerning the influence of water absorption on the tensile and fatigue properties of angle-ply composites of Kevlar 49 (Dupont)/epoxy and a Technola (HM50, Teijin)/epoxy. The influence of water on the fracture mechanism was observed by a SEM and an acoustic microscope. The weight gain of both composites immersed in distilled water at 80°C leveled off at about 2% after two months. For the Kevlar 49 composites water absorption increased tensile strength as well as fracture strain, while the tensile strength of the HM50 composites was decreased by water absorption. For both composites the specimens in water at room temperature and in hot water exhibited a shorter fatigue life than that in air. Water absorption causes plasticization of epoxy resin, thereby reducing interlaminar residual stress of laminates. Also water absorption lowers the interfacial strength between fiber and resin as well as lateral strength of fibers, thereby reducing intra-and interlaminar strength of laminates. These factors influence respectively the tensile and fatigue strength of laminates in water. We also point out the usefulness of a scanning acoustic microscope to examine the internal damage of the composites.
In the last decade, the use of shotcrete in tunnel construction has rapidly increased and the shotcrete has become an important element of the modern tunnel support techniques. Nevertheless, the properties of shotcrete have not been sufficiently investigated. This paper presents the results of an experimental program to evaluate the properties of shotcrete relevant to the tunnel support systems. In the experiments the cubic specimens were prepared and tested to obtain the mechanical properties, especially the strength and Young's modulus in relation to curing time. From these results, the relationship among the uniaxial compressive strength, Young's modulus, curing time, failure strain and ultrasonic velocity are proposed and their relevancy are discussed.
The electrochemical study for CO2 corrosion was carried out on various chromium steels under high temperature and pressure. The results are as follows. The corrosion potentials in the CO2 solution were about 0.2V higher than those in deaerated solution. Passivity was not observed in the anodic polarization curves for carbon and low Cr content steels. FeCO3 films were formed on the surface in the CO2 solution under 100°C, but above 150°C, Fe3O4 and Fe2O3 films were formed in deaerated and CO2 solutions, respectively, for low Cr steels. NiO and Cr2O3 films were observed on the surface in high Ni and Cr steels at high temperature. The contribution of an alloying element for the CO2 corrosion increased with increasing chromium content.
A vesicant polyurethane sheet is widely used for polishing silicon wafers and glass parts. It is believed that the visco-elastic property of vesicant polyurethane sheet and the geometrical form of vesicant layer may have a great influence on the polishing characteristics of workpiece. The present study was focused both on the development of a visco-elasticity measuring setup and on the measurements of visco-elastic property of surface layer in vesicant polyurethane polisher subjected to creep loading. It was found from the experiments that the newly-developed setup was suitable to measure the visco-elastic property of polishers and that the deformation of surface layer of polisher depended on the applied load. The deformation of polisher associated with loading in wet environment was greater than that in dry one.