Yozo Kato

Fatigue Crack Thresholds .DELTA.Kth Value of Ferritic Nodular Cast Iron at Elevated Temperatures.

The in-plane bending fatigue tests were carried out on FCD 450 at elevated temperatures up to 723 K. The threshold stress intensity range ΔKth and its effective values ΔKeffth were observed as a function of the temperature T. Plots of ΔKeffth and T revealed that ΔKeffth remained constant up to 623 K and then decreased to a value about 28% lower at 723 K. The ΔKth-T relations showed that ΔKth remained constant in the range of 473 K to 723 K, its value being about l0% lower than that at room temperature. While the ΔKeffth value at 723 K decreased by 28% lower than that at 623 K, there were no significant differences between the ΔKth value at 723 K and that at 623 K. It appears that the rise of closure level Kcl induced by the high-temperature oxide on fracture surface results in the apparent increase in ΔKth value at high temperature such as 723 K.

Small crack growth characteristics of high strength steel under variable loading in distilled water.

Small crack growth characteristics under variable loading were investigated on high strength steel SNCM439 in room air and distilled water. Under repeated two-step variable loading in room air, the crack growth rates of small cracks at lower stress and higher stress levels decreased and increased respectively, as compared with those at constant stress amplitude. In distilled water, on the other hand, the decrease in crack growth rate at lower stress level was not observed below a crack length of 0.5mm, while the crack growth rate at higher stress level decreased as compared with those at constant stress amplitude, which was attributed to crack branching. In order to clarify the effect of crack length on retardation behavior, tests were conducted under Hi-Lo two-step loading at constant stress intensity factor range (ΔK=11MPa√m→7MPa√m). It was found that the retardation cycles in both environments increased markedly with increasing crack length until they reached a constant value corresponding to long crack data, but it was smaller in distilled water than in room air.

Effect of Loading History on Fatigue Crack Propagation Behavior of High Strength Steel in Corrosive Environment

The effects of loading history and corrosive environment on the retardation in crack growth were investigated in rotating bending. The material used was 200°C tempered SNCM439. Tests were made at two-step varying loading, K1→K2, in room air and in 1%NaCl solution. The stress intensity was estimated by K=σmax√πa/2 for convenience sake, where σmax is the stress amplitude and a is the crack length measured circumferentially on the surface.The results obtained are as follows:(1) The number of retardation cycles was influenced by the environment, K2, and the intermediate treatment (ex. hydrogen charge) during the change in K from K1 to K2.(2) The retardation crack length was determined by K1, irrespective of environment.(3) The relationship between the number of retardation cycles and a parameter of varying load, (K1-K2)/K2, was derived for each environment. The number of retardation cycles increased linearly with (K1-K2)/K2.(4) The crack propagation rate in retardation period decreased with an increase in K1 and increased with an increase in K2.

The Effect of Low Temperature Quenching and Quench Ageing on the Fatigue Strength of Notched Specimens of Low Carbon Steels

The fatigue limits, σw1 and σw2, which are critical stresses for initiation and propagation of stage 2 cracks respectively, were compared among annealed, quenched and quench aged steels. To examine the effect of heat treatment on the fatigue life, crack propagation rate, d(2a)/dN, was measured at a stress above σw2. The contribution of quenching and quench ageing to the increase in the fatigue limits is greater for σw2 than for σw1, and the relationship, ασw1sσw0, is kept independent of the heat treatments, where σw0 is the fatigue limit of unnotched specimens. Therefore, the notch sensitivity factor, ( σw0/σw2-1)/(α-1), is reduced by both treatments. The strengthening effect is more remarkable with quench aged steel than with quenched steel. At higher ΔK level, d(2 )/dN is independent of the heat treatment, and is correlated with the streas intensity factor ΔK. The increase in the fatigue life by quenching and quench ageing is due to the decrease in d(2a)/dN at lower ΔK level.

The Effect of Rotatory Bending Stress at High Temperature on the Hardness of Carbon Steel

We measured the micro-Vickers hardness of 0.26 %-carbon steel, which had been subjected to rotatory bending stress at a high temperature. The fatigue test was conducted at the room temperature, 300°C and 550°C mainly, and hardness testing was held under 1000 gram load. Summary of the results is as follows : (l) At 300°C, the increase of the hardness number is greater and the hardening layer is deeper than that at the room temperature. But at 550°C, the hardness change is little or nothing. (2) Moreover, this hardening layer is caused by the repeated stress above about 0.8 σω at the room temperature, but about 0.65 σω at 300°C, where σω is the endurance limit at each temperature.

The Time-Strength of Carbon Steel under Large Rotating Bending Load

We studied the effects of the speed of cyclic stressing and the shape of notches on the time-strength of 0.5per sent carbon steel by means of Onos rotating beam fatigue machine. Unnotched specimens, notched specimens with circumferential semicircular groove and with circumferential 60deg V-shaped groove were tested at 1700 and 63rpm respectively. Rotating-beam fatigue tests were mainly run at 6 different nominal stresses from 80 to 40kg/mm2, which were greater than the yield point of the material as found under static tension. We used 5 specimens at each stress level.Summary of the results is as follows:(1) In notch tests, the average value of the fatigue life of 5 specimens at 63rpm is 25 to 14per sent less than that at 1700rpm and all the test results have fallen within the individual scatter band of the fatigue life, indicating the apparent effect of testing speed.(2) However, in unnotch tests, probably attributing to harmful internal heating, the test results both at 1700 and 63rpm are almost fallen within the same scatter band, indicating no apparent effects of testing speed.(3) Below about 65kg/mm2, the fatigue life decreases as the stress concentration factor becomes greater. However, above 65kg/mm2, the fatigue life of the semicircular notched specimens becomes greater than that of the unnotched specimens. Hence it appears that the effect of notches on the time-strength may not always be harmful.

An Experiment on Fatigue of Low Carbon Steel at High Temperatures

In these days it has become more and more important to study the fatigue of metals at high temperatures. But very little work has been done in rotating beam fatigue at high temperatures because of unavailability and inadequacy of equipments and temperature control problems. This paper reports the experimental results of the fatigue test under the rotating bending stress at high temperatures, using the unnotched and notched specimens with the circumferential groove of a semi-circle of low carbon steel with 0.12% carbon. The conditions of the fatigue test were as follows : (1) temperature ; advancing by 100°C from the room temperature to 600°C. (2) speed of stress reversals ; 1700 rpm.