The specimens of S15C, S25C, S35C, S45C, S55C, SK5 and SK3 carbon steels were ground under various grinding conditions and their residual stresses were measured. And the mechanism of the residual stress occurrence in the specimens with pearlitic structure was investigated. Usually, it is considered that the residual stress by grinding results from plastic deformation by thermal stress which depends on heat generated during grinding. However, the effect of plastic deformation caused directly by mechanical load during grinding has not been fully investigated. Under this circumstance, the residual stress and the plastic distorsion at the subsurface of each part along the wheel-work contact zone were measured by taking out the specimens successively during the grinding process, and the correlation between the residual stress and the plastic distorsion was discussed.
The effect of 60°V-type circumferential notch having various notch root radii on the fracture behavior under tension was investigated for a commercial 18Ni maraging steel. Regardless of notch root radius, the V-type notched specimens (as solution-treated) were found to fracture in a ductile manner and consequently to show cup and cone fracture, resulting in notch strengthening. On the other hand, in the solution-treated and aged specimens with V-type notch, the fracture mode was macroscopically brittle without producing notch weakening. However, the fracture surface was mostly cup and cone type as in the case of the as solution treated specimens. In both of the fracture surfaces mentioned above, the following tendency was found: the width of shear lip region in fracture surface increased with notch root radius, and the size of dimple found there was smaller than that in normal rupture region and decreased with decreasing notch root radius, while the size of dimple in normal rupture region remained almost unchanged with notch root radius. Discussions were finally made on these results on the basis of both distributions of stress and strain at notch by FEM.
Rotating bending fatigue tests were made on a quenched and tempered 0.45% carbon steel in the temperature range from room temperature to 450°C at a machine speed of 3600rpm. The fatigue strength σf at 4×104 to 3×107 cycles reached a maximum at about 350°C due to cyclic strain aging in the same manner as the annealed material. For the quenched and tempered material the fatigue limit was clearly confirmed up to 200°C but not observed at the peaking temperature of 350°C, while for the annealed material the fatigue limit had been confirmed even at 350°C. The fatigue strength of the quenched and tempered material was higher than that of the annealed material at every temperature, but the effect of cyclic strain aging on fatigue strength of the former was less than that of the latter. This suggests that this material shows different effect of cyclic strain aging at high temperatures when heat treated differently. The observation of fatigue crack propagation behavior revealed that the strain aging at 350°C is more effective for the quenched and tempered material in preventing crack initiation rather than inhibiting crack propagation, contrary to the results of the annealed materials, which shows that strain aging is more effective in inhibiting crack propagation.
Fatigue tests have been conducted on the cruciform flat specimens of a weldable structural steel and a stainless steel with a center crack by using a high-cycle biaxial fatigue test facility, in order to investigate the effect of biaxial stress on the fatigue crack growth properties ΔKI-da/dN and the threshold conditions ΔKIth. From these comprehensive experiments, no effect of biaxial stress was found on both ΔKI-da/dN relation and ΔKIth, if the crack was large and the stress level was low. However, a significant effect of biaxial stress appeared when the crack was small and the stress level was high. The critical region where the effect of biaxial stress appears was clarified quantitatively, based on the crack length measured and the stress level used in the present biaxial fatigue tests as well as those of other experimental works. Moreover the elastic-plastic FEM analysis was carried out on a crack in a cruciform specimen under biaxial stresses. The effects of biaxial stress on the plastic region and the crack opening displacement at a crack tip were discussed in comparison with the effect on the fatigue crack growth.
In order to examine the fatigue life distributions, statistical fatigue tests were carried out at two stress levels by using a number of specimens having only one surface defect or having two surface defects. Theoretical interpretation was attempted on the distribution characteristics of the fatigue life from the following two view points. (1) The fatigue life distribution of the specimens with a few surface defects is derived from that of the specimens having only one defect based on the concept of extremes distribution. (2) The total fatigue life is separated into crack initiation and crack propagation lives, and, the distribution of total life is derived by convolution integral of the above two componental distributions. Finally, it was shown that the experimental life distribution was successfully explained by either one of these analyses.
The fatigue crack growth behavior of SUS 304 stainless steel when one slow-fast strain cycle was introduced to the fast-fast strain cycles was investigated at the temperature of 873K. It was found that the fatigue crack growth rate in the fast-fast strain cycles was accelerated by the introduction of one slow-fast strain cycle. It was also found that there was a linear relationship between the size of the region where the above acceleration occurred and the value of creep J-integral range which was produced just when one slow-fast strain cycle was introduced. The distribution of hardness just below the fracture surface, where the above acceleration occurred, was similar to the one in the pure slow-fast strain cycle fatigue. From the results thus obtained, it was thought that the one slow-fast strain cycle affected the stress-strain field at the crack tip and contributed to the acceleration of crack growth rate.
There have been many published papers about stress corrosion cracking (SCC) of austenitic stainless steels in chloride aqueous solutions. By paying too much importance to acceleration of cracking, however, many of these studies were made at stress levels high enough to cause plastic deformation. Thus, few were concerned in the nucleation mechanism and propagation characteristic of stress corrosion cracking at low stress levels that were practically important in considering the materials resistance in corrosive environments of austenitic stainless steels used for structural materials. Furthermore, to elucidate the actual mechanism of fracture, it is necessary to have a good understanding of the process of failure itself. But, for example, the fracture morphology that is related to such process has not been studied fully systematically. Moreover, in the previous studies concerning the fracture morphology, the extremely early initial stage of stress corrosion cracking that is substantially important has not been fully discussed and many points are left to be studied in future. Therefore in this paper, stress corrosion cracking behavior at the initial stage of austenitic stainless steel SUS 304 was investigated in boiling 42% magnesium chloride aqueous solution. Nucleation and initial propagation of stress corrosion micro cracks were analyzed both in original surface and in fracture surface from microstructural aspect in a wide stress range from near threshold stress to 0.2% proof stress, where no plastic deformation occurred macroscopically. The main results were summarized as follows: (1) Stress corrosion cracking behavior at the initial stage was dependent upon the stress level. Namely, at higher stress levels, intercrystalline cracking originating from triple grain boundaries was observed, but as the stress level was lowered, it was replaced with transgranular one accompanied with corrosion grooves. (2) Micro crack growth from the small corrosion grooves was an important factor determining the value of the threshold stress of material used. (3) At a stress level near 0.2% proof stress, the stress corrosion crack nucleates from the broken point of the passive film on triple grain boundaries where the material is concentrically strained and it propagates so as to interconnect triple grain boundaries.
The water sorption and fracture toughness characteristics of epoxide resins cured with aliphatic diamines and aliphatic dicarboxylic acid were investigated. The concentration of network chains, ν, in the cured resins was governed by the number of methylene linkages in the curing agents. The cured resin with high ν exhibited low diffusion coefficient, D, and high eqilibrium water uptake, M∞. M∞ of the diamine cured system was 2.9 wt%∼1.5%, while that of the dicarboxylic acid cured system was 2.2wt%∼1.0%. The dicarboxylic acid cured system was very sensitive to water and showed larger toughness loss after immersion in water than the diamine system. The fracture toughness, Kc, of the diamine cured resin with high ν increased after immersion in water for 600hrs. The water sorption induced plastic flow at the crack tip, causing an increase in Kc On the other hand, Kc of the dicarboxylic acid cured resin with low ν decreased remarkably after immersion in water for 600hrs (and 2hrs). It seems that the network chains at the crack tip are degraded by ester hydrolysis, resulting in an decrease in Kc.
In order to clarify the notch effects on the fracture strength and fatigue strength of alumina particulate-filled epoxide composite, four-point bend tests were conducted on the notched specimens with various root radii. The fractographic features were correlated to fracture mechanics parameters. The results obtained are summarized as follows; (1) Under monotonic or repeated loading, a crack initiated at the notch root by debonding the epoxy-alumina interface and grew stably along the interface. During unstable fracture the crack propagated around the alumina particles. (2) An approximate method for predicting the fracture stress of notched parts under monotonic loading from KIc and tensile strength of smooth specimens was presented. (3) The crack initiation stage was the major part of fatigue life. The crack propagation stage was negligibly short. (4) The KIc criterion holds for the onset of unstable fracture not only under monotonic loading but also under fatigue with non-zero stress ratio.
The effects of stress ratio R on fatigue crack propagation rate and fatigue fracture surface appearance of polycarbonate at low stress intensity factor range ΔK level were investigated. Fatigue crack closure behavior of these specimens was also examined. From these experiments, it was found that the dependence of fatigue crack propagation rate on R, which was observed in the relation between crack propagation rate and ΔK, was considerably masked by using the effective stress intensity factor range ΔKeff. But, the effect of R on crack propagation mechanism was observed, and so the data of crack propagation rate at each R were not perfectly well expressed even in terms of ΔKeff. Discontinuous growth bands (D. G. B.) were observed only when R was negative, and as R was decreased, D. G. B. seemed to be more easily formed. Fatigue crack closure also seemed to contribute to D. G. B. formation. These results suggested that the cyclic compressive stress at the crack tip played an important role for D. G. B. formation. This was considered to be due to the easy initiation of a single craze at the crack tip which was responsible for D. G. B. formation when cyclic tensile and compressive stress existed at the crack tip.
It was previously found that material removal in orthogonal cutting of ceramics was characterized by extension of a macro crack: it initiates in a region near the tool tip, then unstably extends slightly downwards following the minimum tensile stress trajectory. Its further extension is affected by material crashing ahead of the tool rake face so that it turns upwards to emarge out at the upper free surface (chip formation type) or stops in the material (crack arrest type) or successively proceeds downwards (crack proceed type). This paper reports the experimental investigation of the effects of depth of cut, cutting velocity and tool rake angle on the macro crack extension. The specimens of glass-ceramics (Macor, Corning Glass Works), mullite and soda-lime glass were cut in some length just at the incipient stage of orthogonal cutting when the depth of cut was varied, and they were cut in full length when the cutting velocities and tool rake angles were varied. The main results obtained are as follows. (1) Increase in depth of cut changes the crack type from chip formation type to crack proceed type through crack arrest type. There appeare distinct differences among these types when interpreted in terms of l/Dt ratio (crack length/depth of cut ratio). The ratio is smallest for the chip formation type and largest for the crack proceed type. Macor and mullite have a wider range of depths of cut for attaining chip formation type and smaller values of l/Dt than soda-lime glass. (2) Decrease in cutting velocity makes the crater of finished surface smaller and shallower. This phenomenon is caused by a decrease in fracture strength of the work which is usually observed at such relatively low strain rate as is attained in the present cuttings. (3) The crater of finished surface becomes smallest when the tool rake angle is 15°, where the most frequent contact between the tool rake face and the work front face occurs to stimulates material crashing ahead of the tool rake face.
The elasticity of cancellous bone constituting the proximal of the femur is considered to be influenced by the degree of trabecular bone orientation and the bone volume fraction. For the purpose of quantifying this influence, the degree of orientation and the distribution of the volume fraction of trabeculae were measured by means of an image analysis. The elasticity and its anisotropy of cancellous bone were also measured by loading tests using small cubic bone specimens. Then, the extent of influence of each factor was examined experimentally. As the result, the following equation was proposed in order to express the elasticity of cancellous bone. E=E0*·Vα*·exp(-b*·P) where V is the degree of orientation, P(=1-Vf) is the porosity and α, b and are constants.
The effect of static contact load on the wear behavior of a carbon steel accompanied with ultrasonic vibration has been studied in unlubricated conditions using a pin-on-disk type wear testing machine. A remarkable increase in wear rate at a critical contact load Pocr was observed in the wear under vibration as in the case of usual wear, and the transition from mild to severe wear appeared. Pocr increased with the amplitude of ultrasonic vibration. In the severe wear region the wear rate was almost proportional to (α/2π)2(P/pm)(1-Θ)2. This value is considered to be an apparent area of real contact, where P is the mean dynamic contact load, α the contacting time (rad) of specimens during one cycle of vibration, and (1-Θ) the ratio of freshly sheared surface of asperities.
An ultrasonic method for measuring JIc has been studied by using compact type specimens of three kinds of high strength steels. The onset of crack extension was found to be accurately detectable by both ultrasonic through transmission and angle beem reflection methods. The latter method was useful to reveal the microscopic change of configuration of precrack tip. In an HT80 steel, the JIc value obtained by the ultrasonic method agreed with that obtained by the R-curve method. However, in A533B and SM50 steels the JIc values obtained by the ultrasonic method were larger than those obtained by the R-curve method. This disagreement of JIc value was attributed to the formation of void or secondary crack before the onset of crack extension.
Toughness of steels for automobile components is usually evaluated by Charpy impact tests using standard U-notch specimens. However, for components heat-treated to high hardness levels, it is desirable to use C-notch specimens having a similar notch depth but a larger notch root radius than the U-notch specimens. In the present paper, the impact behaviour of C-notch specimens with notch radii of 2R, 5R and 10R was investigated in Charpy impact tests and compared with that of U-notch ones. Then, discussion was made about suitable notch radius for each impact specimen with different hardness, and the relation of impact properties between C-notch and U-notch specimens was examined. The results obtained are as follows: (1) It is recommended to select a suitable combination of notch radius and hardness of a specimen in such a way that the propagating crack may be formed at the maximum load in the load-time diagram during the impact test. (2) The relationship of the total impact value between C-notch and U-notch specimens for various materials and testing conditions was approximately represented by a bent line having a knee. The straight line under the knee corresponds almost to the relationship of the impact values for crack formation, and the straight line above the knee to that of the impact values for crack propagation. (3) A 1:1 relation was found to exist for the maximum plastic stress between C-notch and U-notch specimens except those heat-treated to high hardness levels above HV640. (4) The relationship of time period preceding propagating crack formation between C-notch and U-notch specimens was similar to that of impact value for crack formation between the both types. (5) With increase of thee notch radius of specimens, the ductile-brittle transition temperature shifted to lower levels, and the transition temperature range became broadened.