Electron beam welding was used to make joints of SUS 405 steel. Compact tension type specimens with incomplete penetration notches were extracted from the electron beam welds. Fracture toughness tests were carried out at room temperature for the specimens in the as-welded and postweld heat-treated (685°C×2, 4 and 8 hours) conditions. In all cases, cracks propagated in the middle of electron beam weld. Because of the natural complicated crack shape etc., a large scatter appeared in the fracture toughness, Jc. The PWHT tended to improve the fracture toughness. The effect of specimen thickness on fracture toughness Jc was also examined. The statistical model based on a Weibull distribution indicated that the fracture toughness obtained by small specimens can be used to characterize that of large specimens or real structures.
Electron beam welding was used to weld a SUS 405 steel plate with A387 Gr. 22, A204 Gr. B and SS41 steel plates. Compact tension type specimens (0.6T-CT) with incomplete penetration notches were extracted from the electron beam welds. Fracture toughness tests were carried out at room temperature for the specimens in the as-welded and post-weld heat-treated (685°C×2, 4 and 8 hours) conditions. The welded joints of A387-SUS 405 and A204-SUS 405 gave higher fracture toughness when PWHT was applied. Those of SS41-SUS 405 gave lower fracture toughness when PWHT was applied. The fractographic and sectioning observation of specimens indicated that the fracture toughness of the welded dissimilar metal joints was affected by the crack path deflection caused by the heterogeneity of welds and ferrite bands. In the welded joint of A387-SUS 405, the crack propagated right through the weld metal which was tempered by PWHT. When a long time PWHT was applied, the crack deflected from the weld metal toward the ferrite band in both the welded joints of A204-SUS 405 and SS41-SUS 405.
The overall yield strength of rectangular bar specimens with double symmetrical U-shaped notches of unequal depth and/or unequal radius was analysed by the upper and lower bound theorems. The occurance of interference between the two notches was thus studied. The yield strength for double notches became smaller because of the interference than that for a single notch. The amount of decrease in yield strength decreased with an increase of the difference in notch depth; so did the minimum distance between the notches at which the interference occurs. The interference did not occur below a certain distance. The difference in notch depth had stronger influence on the interference than the difference in notch radii. These analytical results can be easily applied to study the interference for double symmetrical notches of equal notch depth and equal notch radius, double symmetrical cracks and also for the combination of an extremely large notch radius and an extremely small notch radius.
A new testing device was developed to study the tensile properties of sintered silicon nitride plate specimens at room and elevated temperatures. An infrared image furnace was used to heat a specimen, and tensile tests were performed at room temperature, 1000, 1200 and 1300°C. Ten specimens were used at each temperature with the purpose of statistical analysis. It was observed that the tensile strength decreased gradually when the temperature was raised from room temperature to 1000°C, and decreased rapidly when the temperature exceeded 1000°C. The Weibull shape parameter was in the range from 11.9 to 16.6 with an average 14.2 for the above test temperature range. At 1300°C, inelastic deformation caused by the viscosity drop of the glassy phase was observed on the load-displacement curve. Mirror-mist-hackle features were clearly observed on the fracture surfaces of specimens tested at room temperature, 1000 and 1200°C. The mirror and mist areas around the fracture origin were determined for each fractured specimen, and it was confirmed that the relation σB·r1/2=const. holds between the mirror or mist radius r and the tensile strength σB in the temperature range from room temperature to 1200°C. But at 1300°C, the fracture surface was dominated by flat and smooth region, and it was difficult to define the mirror and mist areas. Furthermore, a slow crack growth region surrounding the fracture origin was observed on some of the fracture surfaces at 1200 and 1300°C.
Fatigue crack initiation and growth characteristics under mixed mode loading have been investigated on aluminum alloys 2017-T3 and 7075-T6, using a newly developed apparatus for mixed mode loading tests. In 2017-T3, the fatigue crack initiation and growth characteristics from a pre-crack under mixed mode loading were divided into three regions-shear mode growth, tensile mode growth and no growth-on the ΔKI-ΔKII plane. The shear mode growth was observed in the region expressed approximately by ΔKII>3MPa√m and ΔKII/ΔKI>1.6. In 7075-T6, the condition of shear mode crack initiation was expressed by ΔKII>8MPa√m and ΔKII/ΔKI>1.6, and continuous crack growth in shear mode was observed only in the case of ΔKI/ΔKII≈0. The threshold condition of fatigue crack growth in tensile mode was described by the maximum tensile stress criterion, which is given by Δσθmax√2πr≈1.6MPa√m, in both aluminum alloys. The direction of shear mode crack growth approached to the plane in which ΔKI decreases and ΔKII increases towards the maximum with crack growth. The da/dN-ΔKII relation of the curved cracks growing in shear mode under mixed mode loading agreed well with the da/dN-ΔKII relation of a straight crack under pure mode II loading.
Superior resistance to the stage II fatigue crack growth was obtained on an ultra high carbon steel containing massive eutectic carbide extended in the direction perpendicular to the original crack growth direction by forging. The fatigue crack growth rate obtained in this study was about 1/4 times as low as that of general mild steel without eutectic carbides. Microscopic observations revealed that the fatigue crack path was bent or branched when it encountered the massive carbide. Taking the actual crack configurations affected by the carbide into consideration, the “Effective stress intensity factor (SIFeff)” for fatigue crack growth was estimated, in which significant reduction of the SIFeff was obtained. An analytical study showed that the SIFeff of the bent crack would decrease significantly with increasing the angle θ between the carbide extention direction and the loading direction. The relation between the growth rate and SIFeff was examined on various specimens that have different carbide orientation on the basis of the above analysis. It was found that the growth rate drastically changed in accordance with θ and also found that the dependense of the growth rate on θ was in good agreement with the SIFeff-θ relation. It was made clear that crack growth retardation was due to the reduction of SIFeff resulting from the complicated crack morphology affected by massive carbide.
The rolling contact fatigue tests were carried out on high manganese austenitic steel to investigate the influence of the work hardening phenomenon generated by repeated contact stress on the rolling contact fatigue strength by using a Nishihara-type wear testing machine under a lubricated state with a relative slip ratio of -9%. The results obtained are summarized as follows: (1) In the cases that carbon contents were about 1.0 and 1.2%, high manganese austenitic steel had the same fatigue endurance limit (2035MPa), regardless of carbon content. (2) The rolling contact fatigue endurance limit of high manganese austenitic steel increased remarkably with work hardening generated by repeated contact stress. (3) The rolling contact fatigue endurance limit of high manganese austenitic steel increased remarkably compared with the austenitic stainless steel (SUS304) having the same matrix structure as that of high manganese austenitic steel. (4) A crack of high manganese austenitic steel propagated along the boundary of the work hardening zone and the unwork hardening zone. The depth of crack propagation, at which the crack front became almost parallel to the contact surface, was fairly deeper than the maximum stress region calculated according to the maximum shearing stress theory.
Fifteen sets of S-N data of carbon/epoxy composites obtained by axial-load fatigue tests were collected from published papers and statistically analyzed. For simplicity of the analysis, the analyzing range of S-N observations in a set of S-N data was limited and the form of S-N curve and the distribution of fatigue life were assumed realistically. Fatigue life scatter and fatigue strength scatter were derived from the deviations of S-N observations from the S-N curve of which equation was determined by the least-squares method. It was found that the amount of fatigue strength scatter is practically constant regardless of S-N data obtained at different laboratories and the amount of fatigue life scatter is dependent of the slope of the S-N curve.
The impact fatigue tests for oxygen-free copper and high strength copper were carried out at various temperatures between 100 and 250°C as well as at room temperature. The fatigue strength and plastic deformation due to cyclic creep under impact tensile loads were analyzed in consideration of temperature-dependence and static tensile properties. The impact fatigue strength was found to be expressed by the formula of σ(NfT)m0=D0, where m0 and D0 are the material constants for impact fatigue strength at each high temperature Tθ(K). The values of m0 and D0 depended on the temperature and were expressed by the formulas of m0=m0*+m1*(Tθ-273) and D0=D0*exp(Q/kTθ). The behavior of cyclic creep plastic deformation was expressed by a formula including only plastic strain εp and normalized number of cycles N/Nf, which is independent of impact stress σ, duration time T and temperature Tθ. The values of m0, D0 and main material constants (a1, a2), which appear in the formula expressing the cyclic deformation, had distinct relationships with the ultimate strength σB, reduction of area φ and elongation δ in static tension. Impact fatigue strength and cyclic creep plastic deformation at high temperatures can be well estimated through these relationships from the static tensile properties at room temperature.
This paper describes the initiation and propagation of heat checking and the variation of residual stress on aluminum die casting dies. Detailed observations by SEM were conducted repeatedly after every prescribed number of shots on the cavity surface near the gate of die casting dies made of steel, SKD61 (AISI H13), in actual service, and the origins of heat checking were searched. Using the newly defined crack density, the propagation of heat checking on the cavity surface was discussed quantitatively. The residual stress on the surface of dies, which was produced by cyclic thermal stress, was measured by X-ray, and the variation of residual stress in the early part of the service life was made clear. The following conclusions were drawn from this investigation. (1) Cracks, appearing in the early stage of heat checking, started from three types of origins: First, pits and wavy patterns, which initiated during the use of the die from the clean surface without any defect before use, second, various machining scratches and defects such as indentations, and third, strain concentrated parts of the die surface. (2) The crack density, obtained from the number of surface cracks longer than 0.2mm, increased rapidly in the first period, and then saturated in the latter period of crack propagation. (3) The residual stress of die surface before use, was compressive stress which had been created during the making of the dies, but it changed immediately toward the tensile side and satulated in the early stage of casting process before 50 shots.
The effect of sensitization on stress corrosion cracking (SCC) of prior cold-worked SUS 304 steel was investigated in boiling 35% MgCl2 solution, and the results were compared with the behavior of intergranular corrosion obtained by Strauss test. The mechanical properties of cold-worked specimens were affected markedly by annealing temperature as compared with annealing time. The cold-worked specimen showed the maximum mechanical strength by annealing at 637K, above which the strength gradually decreased with increasing annealing temperature. The region of sensitization in T-T-S curves shifted to the lower temperature side with increasing prestrain. The susceptibility to intergranular corrosion became maximum at 20% prestrain, but it decreased at 40% prestrain. The susceptibility to SCC of the prestrained specimens increased with increasing annealing temperature and time, and this tendency was consistent with that in the above-mentioned mechanical strength. The susceptibility to intergranular cracking in boiling 35% MgCl2 solution decreased for the specimens treated in the region of sensitization, and this result was not consistent with that of intergranular corrosion behavior in Strauss test solution.
In order to know the reaction between cement and naphthalenesulfonate type superplasticizer (NSNa) quantitatively, Ca(OH)2 and ethyl silicate or aluminum isopropoxide were reacted in the presence of NSH (sulfonic acid form of NSNa). The hydrates obtained were investigated by chemical analysis, XRD and SEM. The effect of the addition of hydrates containing NS on the drying shrinkage of cement mortar was also measured. The results obtained were as follows. (1) NS reacted with silicate and Ca(OH)2. The mole ratio of NS:Ca:Si in the hydrates depended on the mole ratio of Ca/Si in the initial composition. (2) NS also reacted with aluminate and Ca(OH)2. Calcium aluminate hydrates containing NS formed at the mole ratio of approximately NS:Ca:Al=2:2:1 in the absence of gypsum. Ettringite mainly formed in the presence of gypsum. (3) Hydrates containing NS consisted of fine particles (approximately 0.2μm). The addition of hydrates containing NS to cement mortar, in which NS content was kept in a normal dosage, did not influence much on the drying shrinkage of mortar. The hydration of calcium silicate or calcium aluminate under the presence of NS was explained on the basis of the reaction between polyelectrolyte and polybasic acids under the presence of Ca(OH)2.
The thermal expansion behavior of the hybrid materials built up from many thin Al (aluminum IN30) sheets and unidirectional CFRP prepreg layers (fiber volume content 60%) was investigated theoretically and experimentally. The linear thermal expansion coefficients in three mutual directions, parallel to fibers (αXH), perpendicular to fibers and parallel to laminate (αYH), perpendicular to laminates (αZH), were derived from analyzing the two-layer structural model. These anisotropic linear thermal expansion coefficients were measured by the interferometric technique from room-temperature to 150°C. αXH and αYH had little temperature dependent behavior and their values were almost constant. The value of αZH was also almost constant between room-temperature and 80°C, but at higher temperatures than 80°C they increased remarkably with increasing temperature. The constant values of αXH, αYH, αZH were about 7×10-61/°C, 28×10-61/°C, 34×10-61/°C, respectively. The average value of αXH was less than 30% that of monolithic aluminum. The estimated values agreed well with the experimental ones. But in the case of a thickly layered hybrid material, it is necessary to consider a shearing deformation of adhesive layers for the analysis of thermal expansion behavior.
A new type X-ray microbeam equipment which is capable of three-dimensional oscillation at an X-ray radiation position was used to measure the residual stress of pure iron powder and bending plate specimens as well as that in the vicinity of fatigue crack tip of 1/2 CT specimen. The results obtained are summerized as follows. (1) Diffraction profiles of pure iron powder with φ0.02-0.03μm particle size obtained by the present microbeam technique in the beam area of 80μm in diamether and measuring time more than 800sec showed Kα1 and Kα2 peaks clearly, so that these profiles are good enough for stress measurement. (2) The measured stress of pure iron powder with φ10-25μm particle size in the beam area of 80μm in diameter was not equal to 0MPa. The measured stress varied greatly and was independent of measuring time. (3) The measured stress of pure iron powder with φ0.02-0.03μm particle size in the beam area of 80μm in diameter was almost 1MPa, which shows the good accuracy of this X-ray microbeam equipment. (4) The relation between the X-ray measured stress and the mechanically applied stress showed good agreement. (5) The distribution of residual stress at the vicinity of fatigue crack tip showed that the residual stress at the fatigue crack tip was compression and the residual stress in front of crack tip was tension. This tensile residual stress in front of crack tip existed in the area from 0.02mm to 2mm. (6) The above results show that the present equipment is advantageous in the investigation of local stress, shortening the measuring time without sacrificing the accuracy.