The high temperature deformation of copper and mild steel has been studied at the homologous temperatures ranging from 0.5Tm (407°C) to 0.9Tm (948°C) for copper and from 0.7Tm (989°C) to 0.9Tm (1350°C) for mild steel under the constant true strain rate from about 10-3/sec to 30/sec in torsion. From the determination of their stress-strain behaviors and optical microstructural observations, the rate controlling mechanisms of the dynamic restoration process responsible for the occurrence of steady state deformation were investigated. The flow curves of the both materials exhibited the maxima, after which the flow stress decreased to a steady state value. It is clear from the above observations that dynamic recrystallization occurs at strains greater than a critical value and the recrystallized grain size depends only on the flow stress. The maximum flow stress data for both materials were plotted against the temperature compensated strain rate, using the activation energy for self diffusion. The form of strain rate dependence of the maximum flow stress in copper was similar to those found in a number of materials, studied over a wide range of strain rate: below a critical stress, that is approximately 10kg/mm2 in this case, a power law was observed with a stress exponent of 6.5, and above this critical stress a steeper stress dependence occurred. The stress exponent for mild steel was about 5.5 in the temperature range studied, as a straight line was obtained when the temperature-compensated strain rate was plotted against the maximum flow stress. The similar dependence was observed for the steady state flow stress when this condition was achieved. The dynamically recrystallized grain size of a copper specimen which deformed in each testing condition was measured by using the line intercept technique, from which the average recrystallized grain size (d) was determined. When the reciprocal grain size (1/d) was plotted against Zener-Hollomom parameter, a straight line was obtained. Therefore, it is confirmed that the operative restoration process during the high temperature deformation of copper is the dynamic recrystallization, since there is a constant relationship between the recrystallized grain size and deformation parameters. From the similarity in the stress-strain behaviors of copper and mild steel and the metallographic observations, it is reasonable to consider that the restoration process of mild steel in γ-range is dynamic recrystallization.
The short time tensile test was conducted on the base metal, TIG weld metal, TIG weld joint and EBW weld joint of Hastelloy X at 700°C, 800°C and 900°C. Their fracture surfaces were observed by means of a scanning electron microscope, and the microstructures by means of both optical and electron microscopes, so as to investigate the relation between the fracture mechanism and microstructure. The results obtained are summarized as follows: (1) The fracture mode of the base metal changed from dimple fracture to intergranular fracture as the temperature was raised from 700°C to 900°C, though the ductility increased gradually. (2) TIG weld metal and TIG weld joint showed minima in their ductility near 800°C due to the fracture mode transition from interdendritic fracture to intergranular fracture, while the ductility of EBW joint increased gradually similar to that of the base metal. The fracture surfaces of EBW joint revealed that the fracture type was almost dimple fracture. (3) Both the dimple fracture of base metal and the interdendritic fracture of TIG weld metal and TIG weld joint were caused by the crack initiation at the precipitated M6C carbide and its propagation.
Notch creep rupture strength under torsion was studied for OFHC copper and S 10 C low carbon steel at 200°C and 450°C, respectively. The torsional loading was selected in this study mainly because of several relative merits over the tensile loading from mechanics point of view, such as no existence of the hydrostatic stress component and the negligibility of change in specimen geometry during finite deformation. The results obtained are summarized as follows: (1) Both of the materials investigated, which had previously been found to show “notch-strengthening” under tension, behaved “notch-weakening” under torsion, as was expected from a fracture-mechanics analysis by the authors of notch creep rupture strength under torsion. (2) The mode of crack propagation for the OFHC copper was transgranular in the short life range and intergranular in the medium and long life ranges. For the S 10 C low carbon steel, cracks grew in the transgranular or mixed trans- and intergranular mode. These were essentially the same as those observed under tension. (3) Comparing the results observed under torsion with those under tension, one can conclude that the mechanical plastic constraint of strains induced by the multiaxial tension in the vicinity of the notch root and the contraction of cross-sectional area due to large deformation, particularly in unnotched specimens, are two main causes responsible for “notch-strengthening”; in the lack of these two causes “notch-weakening” prevails.
This paper deals with a mechanical analysis of the notch creep rupture behavior under the longitudinal or mode III shear stress condition. The crack initiation, propagation and rupture lives (nondimensional) of prismatic bodies with double-edged hyperbolic notches were predicted using the results of stress and strain analysis of these bodies under the steady-state creep condition, as combined with a fracture criterion, in which a crack was assumed to grow by a characteristic distance, ρs, when the strain at the distance, ρs, ahead of the crack reached a critical value. The effects of several geometrical and material parameters on the crack initiation life of the notched prismatic bodies and also on the rupture life and the crack propagation rate of a deep-cracked prismatic body were predicted. It was found that “notch-weakening” in creep rupture strength prevails under the longitudinal shear stress condition, and that the crack propagation rate changes approximately linearly in a log-log diagram against the net section stress or the elastic stress intensity factor of the prismatic body. The physical interpretation and the extension of the fracture criterion used were discussed. The prediction was extended to the tensile or mode I notch creep rupture behavior, using an analogue between the modes I and III, which is empirically accepted in fracture mechanics. Qualitative agreement was observed between the predicted values and the experimental mode I results available in the literature.
In order to estimate fatigue life under long periodic loading, varying temperature, varying load or the combination of all of them, the increase of fatigue damage should be first well understood. Fatigue tests on a Nb containing austenite stainless steel (SUS 347) were conducted at various temperature levels. Some of the tests were interrupted at predetermined lapses of time before failure for the examination of crack growth. The results obtained in this study are as follows: (1) The temperature levels can be divided into two ranges, each having its own value for the exponential β in the Manson-Coffin rule Δεp·Nfβ=C, and its own mode of stress response with respect to the number of cycles. The surface micro-crack formation and speed of macrocrack propagation in each of them are different from the other. The dividing temperature is about 650°C for SUS 347, and 300°C for 0.16%C steel. (2) The geometrical mean of the depth of the longest surface crack dmax and the sum total of the depths of all the micro-cracks in the gauge length D are equal to the depth of equivalent crack of which area is equal to the sum total area of transversal sectional areas of micro-cracks per unit gauge length. So it can be the most realistic parameter representing the amount of damage.
Nickel-base high temperature superalloys having three different micro-structures have been tested by a rotating fatigue machine at the rate of 3500r.p.m. at 815°C. The test was carried out on the specimens both without and with a 60° notch. The fatigue life of the cast one with a notch was considerably shorter than those of other specimens, but the notch sensitivity was eliminated by the solution heat treatment followed by aging. Fractographic observations were conducted on fracture surfaces. Large MC type carbides have a marked influence in addition to the notch effect on fatigue life. High temperature fatigue fracture surface showed four distinctive features. The planar region including the origin of fracture was the first pattern appearing along the main fatigue crack propagation direction and was very similar to the Stage 1 fracture surface at room temperature in fatigued single crystal of nickel-base superalloy. The second region was the shear mode pattern. The third was the characteristic region containing both transgranular and intergranular fractures. Well defined striations were observed in the transgranular fracture region except the cast one with a notch which showed dull striations. The flat plane, which may be attributed to crystallographic fracture mode, was also observed. The final was the overloading fracture region.
To investigate the influence of the surface layer rataining the roll effect on the low cycle impact fatigue properties of pure aluminum plate, the tensile impact fatigue tests were carried out within 100 impact blows using the two types of specimens. The specimen of the first type had the surface layer which retained the roll effect, and the specimes of the second type were fully annealed ones and were consisted of the grains of about 30μ diameter. Observations were made on the impact stress pattern and the elongation of the specimens during the tests. The surface deteriorations due to the plastic strain were also examined by means of an optical microscope and by an X-ray diffractometer. From these results, the impact fatigue behaviors of both types of specimens were compared and discussed from various view points.
The environmental delayed fracture process in high polymeric solids consists of three stages, which are crack initiation, crack growth and final fracture. Unfortunately, there have been few systematic studies theoretical and experimental about each stage, although they are important to clarify the delayed fracture mechanism of high polymeric solids. The present work was carried out to investigate the crack initiation and growth behaviours by using specimens with and without a notch. The results obtained are summarized as follows: (1) The existence of a critical notch root radius ρcrit. may be suggested from the observation of the fracture mode transition. (2) The environmental crack growth process consists of three stages. At the first stage of initial crack growth, the crack growth curve may be expressed as a function of loading time and initial stress intensity factor. At the second stage of stable crack growth, the crack growth rate is normalized by stress intensity factor.
In the compression test of rock-like materials such as rocks and concretes, so many cracks initiate from initial flaws, and the propagated cracks form a crack pattern responsible for the failure or collapse of the specimen. In order to study the failure process of rolk-like materials, that is the process from crack initiation to failure, the stochastic distribution of initial flaws in the specimen was assumed. The original and modified (by McClintock and Walsh) Griffith theory were employed as the crack initiation criteria. The results obtained explain well the experimentally revealed failure process of concrete or cement mortar specimens under compressive load, and also explain the complicated failure process arising from the inhomogeneity of the material.