Fortran programs were presented for estimation of fatigue life under a constant stress and also under program load. One of the algorythms introduced is based on JSME's method which appears in the publication from the Japan Society for Mechanical Engineers and is widely used in Japan. Another one is based on SAE's method from Fatigue Design Handbook 1968 edited by Society of Automobile Engineers, Inc., in USA. The authors expect that using these programs, as a black box, one can solve some fatigue problems even if he is not familiar to complicated hypotheses to estimate the fatigue life, and it might be a little contribution to the popularlization of design based on fatigue strength.
As an approach to the mixed alkali effect in glass, the optical absorption spectra of Cr3+ were determined in mixed alkali silicate glasses. The values of the crystal field strength Dq and the Racah parameters B and C for the glasses were calculated from the observed band positions. The crystal field strength Dq is plotted in Fig. 1 against alkali concentration ratio, R1/(R1+R2) where (R1, R2)=(Li, Na), (Na, K) or (K, Li). The curves in Fig. 1 show that Dq deviates downwards from linearity predicted by combining the values for the end members, i.e., there is a minimum for Dq. The behavior in the glasses shown by curves in Fig. 1 may be interpreted based on the presence of mixed alkali effect.
The distributed element model, which had been proposed previously by the authors, was applied to the analysis of cyclic shear stress-shear strain behavior which changes with the number of cycles. In order to examine the applicability of the model, cyclic torsion tests were carried out under a constant shear strain amplitude. From the comparison of experiments with the theory, it appears that the model is suitable for describing the cyclic shear stress-shear strain behavior, and that the deformation behavior of materials under cyclic loading may be predicted reasonably well by the model. It is also shown that the cyclic hardening of materials can be explained based on the increase in the rate of hardening, Ht, and the distribution shift toward high yield strain with the number of cycles.
The distributed element model to analyze the cyclic stress-strain relation had been proposed by the authors. In the previous paper, the validity of the model was confirmed in the cyclic torsion tests as well as in the cyclic bending tests under a constant strain amplitude. In this paper, the correlation of the stress-strain relations between cyclic bending and cyclic torsion is examined on the basis of the model. The analysis is made with two yield functions; von Mises and Freudenthal. Through the comparison of analyses and experimental results, it is shown that the Freudenthal's yield function is suitable for the analysis of the cyclic stress-strain relation.
In the temperature range from R. T. to 500°C, the characteristic curves of high temperature fatigue strength of mild steel SM41A showed the maximum stress at 400°C (so called the“peaking temperature”) and the minimum ones at 150°C and 500°C. Based on these results, the following four model conditions were adopted as the temperature variations. In model A, the specimens were held Nh cycles at 500°C and then dropped to 150°C, while in model B Nh cycles at 150°C and then raised to 500°C. Model C was a convex type such as the specimens were held at 150°C in the first 30 minutes, then raised to 500°C, held there for Nh cycles and downed to 150°C again. Model D was the same type as model A except the initial temperature was held at the peaking temperature instead of 500°C. The rotary bending fatigue tests were made under these temperature conditions at three stress levels. By taking the cumulative cycle ratio as the abscissa and log Nh/log N500 as the corresponding ordinate, we constructed log Nh/log N500 versus Σn/N diagrams expressing the relation among repeated stress, hold-time and fatigue life simultaneously. The main results obtained from these diagrams were as follows: (1) The fatigue life at the second temperature was higher than those at the constant temperature both 150°C and 500°C for model A, B and C. (2) The fatigue life at the second temperature in model A was appreciably higher than that in model B. (3) For model D, with increasing hold-cycles Nh the fatigue strength reached the maximum value at the peaking temperature. (4) According to the microphotographs of specimens, a great deal of etch pits and slip bands were found in model D, while in the case of model A and C, they were not so much. They were hardly observed in model B.
A fractographic study was carried out on the creep-fatigue interaction during the crack propagation process of a 304 stainless steel under repeated stress at 650°C. In general, the crack started as a transgranular fatigue crack and changed to an intergranular creep crack at a certain crack length. The transition point moved to the later stage of crack propagation as the period of repeated stress increased. This transition phenomenon could be explained by assuming that both of the crack propagation processes, the transgranular fatigue crack and the intergranular creep crack, are possible under repeated stress condition and that the one with a higher rate actually occurs. The linear fracture mechanics was applicable to explain this phenomenon in spite of the fact that the experiments were conducted under creep condition.
A critical examination was made on the applicability of linear fracture mechanics to creep and fatigue crack propagation at elevated temperatures. It was concluded that the stress intensity factor can be used in the analysis of crack propagation under creep condition so far as the applied load is tensile and the fracture is a plane strain intergranular type. In the case where bending load exists in addition to tensile load, the effect of redistribution of bending stress due to creep complicates the situation and the effective stress intensity becomes significantly lower than the elastic one. Consequently, a precaution has to be taken to select a proper specimen geometry in which no bending stress appears, when one tries to obtain the crack rate-stress intensity relation as fundamental data for crack analysis under creep condition.
The relationship between transition behaviour and temperature has been examined on a low carbon steel at various tensile stress rate. The results obtained are summarized as follows: (1) The stress σB at the point where a curve of fracture stress against temperature meets with that of yield stress against temperature seems to be a constant value independent of tensile stress rate. (2) The relation between the transition temperature TB at σB and the tensile stress rate V is represented by the following equation, 1/TB=a-blogV where a and b are constants. (3) Micro-cracks may be formed by applying the stress above σB in the temperature range above TB. (4) It is considered that a yield slip is required for the nucleation of micro-cracks.
An experiment has been made to investigate the effect of stress distribution on the upper yield of low carbon steel. Tension, torsion and bending tests were carried out on solid cylinders of low carbon steel and tension test on notched plates. It has been found that the upper yield point increases as the stress concentration increases. A statistical approach is tried to combine the Cottrell's model of dislocation pinning with the weakest link concept.
Cathodic protection and damages due to hydrogen embrittlement under excess cathodic current have been discussed on the basis of experiments on fatigue lives of Al-Zn-Mg alloy in saline. Cathodic potentials increase the fatigue lives, the optimum protection being obtained under E=-1.3∼1.4V against a saturated calomel electrode. On the contrary the lives decrease under more negative potentials than E=-1.4V. Cathodic potentials between corrosion potential and E=-1.3∼1.4V suppress electrochemical corrosion, which leads to the delayed crack initiation and propagation. Fatigue striations are reformed from a brittle type to a ductile one by the application of the cathodic potentials, striation spacing also being decreased at the same time. Under the potentials more negative than E=-1.4V, the hydrogen embrittlement due to excess H+ brings the enhanced crack initiation and the second appearance of a brittle type striation. Macroscopic fracture morphology also varies with the cathodic potentials, a shear mode macroscopic crack extention being replaced with a tensile one under the cathodic potentials.
The detailed fundamental experiments about heat drying of tobacco shreds were carried out in order to obtain the effect of heat drying condition on the filling capacity of tobacco shreds which were heated in the dryer used in the process of the cigarettes manufacturing industry. The variables of heat drying condition chosen were the temperature of heated wall (80-240°C), the temperature of heated air (40-190°C), the drying time (7-360sec), the moisture content of the shreds before drying (14-22% WB), their specific volume (1.5-13.0cc/g) and their size determined by sieving (1-4mm) etc. The varieties of tobacco used were the flue-cured and the domestic. The important results obtained are as follows: (1) When the wall temperature and the air temperature became higher, and when the drying time was longer, the filling capacity of tobacco shreds increased. (2) In order to increase the filling capacity of shreds, the drying with heated wall was more effective than that with heated air. (3) By means of an analysis of multiple regression formula among the filling capacity and other parameters representing the magnitude of thermal effects on tobacco, (that is, the temperature of tobacco shreds and their moisture content at the last moment of drying), it was clear that the influence of shreds temperature on filling capacity was greater than that of the moisture content of shreds. (4) The effects of both moisture content and specific volume of shreds before drying on filling capacity were smaller than that of the wall temperature. The tendency of variation of filling capacity with the change of size of shreds in the flue-cured variety differed from that in the domestic variety.