Ryoichi Koterazawa

Propagation of microscopic fatigue crack under periodic overstressing in quenched and tempered 0.45%C steel.

Propagation of microscopic fatigue cracks by periodic overstressing was studied with quenched and tempered 0.45% C steel. Acceleration of crack propagation occured in 0.45% C steel as in the case of 0.15% C steel previously reported, when the crack length was more than 50μm. The lower limit of understress that caused such an acceleration was reduced steeply as the crack length became longer. The acceleration in 0.45% C steel was less than in 0.15% C steel in accordance with the previous result on macroscopic cracks in which the acceleration was less in materials of higher strength. In the cases of short cracks (below 200μm) and long cracks (above 200μm) loaded with low understress, the microscopic fracture surface consisted of small facets, which were supposed to be related to the crystal structure of steel, and cracks observed on the specimen surface propagated intermittently, suggesting that microstructure had a significant effect upon crack propagation under intermittent overstressing. In the case of long cracks loaded with high understress, the microscopic fracture surface consisted of large facets and cracks propagated steadily, indicating that microstructure had less effect. No appreciable difference was observed in crack opening behavior before and after overstressing, and the acceleration of crack propagation was not likely to be related to the crack closure phenomenon.

Propagation of microscopic fatigue crack under periodic overstressing. Overstress and understress values and acceleration of crack propagation.

Propagation of microscopic fatigue cracks by periodic overstressing was studied under different overstress and understress conditions using a low carbon steel. When the overstress value was large, a significant acceleration of crack propagation (more than one hundred times) occurred even if the understress value was lower than the threshold stress, similarly as in the case of macroscopic cracks. The lower limit of the understress that caused such a significant acceleration was reduced as the overstress was increased, but it was independent of the crack length as far as the crack was microscopic (less than 50μm). For longer cracks, the lower limit was considerably reduced by increasing the crack length. When the overstress value was low, a significant acceleration in short cracks occurred only when the understress value was nearly equal to the threshold stress, but in longer cracks it took place even the understress value was lower than the threshold stress. The microscopic fracture surface consisted of small facets, which was supposed to be related to crystal structures. Observation of the specimen surface showed that cracks propagated intermittently, hesitating at the boundary of microstructure. These observations indicate that microstructure has a significant effect upon microscopic crack propagation under intermittent overstressing.

The Effect of Material Purity and Temperature on the Static-to-Dynamic Transition in Creep of Metallic Materials

Intermittent stress creep tests were performed of low carbon steel, commercially pure titanium, commercially pure aluminum, high purity aluminum (99.99%), tough pitch copper (99.9%) and high purity copper (99.99%) at various temperatures, in order to get informed of the mechanism of the static-to-dynamic transition in creep. Drastic transitions were observed in the materials of commercial purity, whereas transitions were not apparent in the materials of high purity (99.9% or more). With regard to the effect of temperature, there seems to be a critical temperature for each material above which the transition appears and below which it does not appear. The critical temperature is likely to be related to the strain ageing temperature of the material. There was no difference between the transition behaviors in the range of the high temperature creep and the low temperature creep. According to these results, the transition is possibly related to the interaction of dislocations and solute impurities, appearing at temperatures where the diffusion rate of the solute atoms is high enough to get to dislocations readily. This means also that the interaction of creep and creep recovery, which is the cause of the transition, is related to the solute-dislocation interactions.

The Role of Recovery in the Static-to-Dynamic Transition of Creep(A phenomenological Analysis of the Interaction of Creep and Recovery)

A phenomenological theory was developed to describe the static-to-dynamic transition in creep of metals reported in the preceding report with a good agreement with the experiment. The theory is essentially one of interaction of creep and creep recovery, ant it is based on the following two fundamental features: first, the total strain under creep condition can not be regarded as the sum of the three mutually independent components, i. e. elastic, anelastic (creep recovery) and plastic (creep) strains as has usually been done. There is a strong interaction between the anelastic and the creep strains. Secondly, the creep recovery under load-off condition does not appear to be simply the inverse of the anelastic creep under load-on condition as has so far been thought. The delay time for the load-off condition seems to be much longer than that for the load-on condition. It is also to be noted that the rate of creep recovery immediately after unloading is approximately equal to the creep rate during the load-on period. Much more works are desired to be done on the creep recovery not only because of its importance in connection with the creep behavior under cyclic stress condition but also because it is possibly related with the mechanism of creep in general.

Effect of the Frequency of Superposed Alternating Stress on Dynamic Creep

In recent years, the problems of the dynamic creep have become of increased interest with the use of alloys at high temperature. With a view to predicting the dynamic creep from the static creep data, the analysis based upon the strain hardening theory, was reported in the previous papers by the authors. In the papers, it was found that the analysis could be applied to the materials of relatively stable structure, that is, carbon steels, some ferritic and austenitic steels and commercially pure titanium. In the case of supper alloys of precipitation hardening type, however, considerable discrepancy was observed between the theory and the experiments, and this discrepancy seemed to result from the acceleration of precipitation hardening or the strain retardation due to alternating stress. In the present paper, the dynamic creep tests to elucidate the effects of alternating stress frequency on the strain retardation have been carried out within the frequency range from 20 to 4000 cpm. In the experiments, for the higher frequency range from 800 to 4000cpm the alternating stress has been applied to a specimen by the centrifugal force of rotating eccentric mass as reported previously, and for the lower range of 20 and 80cpm the alternating stress has been applied by an eccentric disc and lever mechanism. In both cases, the application of sinusoidally alternating stress has been accurately attained.

Multiaxial Dynamic Creep of a Carbon Steel under Combined Stresses of Static Tension and Alternating Torsion

Numerous articles have been published on the multiaxial creep problem since 1935 when bailey presented his extensive theoretical work together with some experimental results on this subject. However, these articles are concerned mainly with theories, and experimental works, so far made, are not sufficient to verify the theories. Especially, the experimental works on the nonsteady stress problem are two few, there being only those conducted by the Johnson and his colleagues.This report deals with the two dimensional dynamic creep of thin-walled cylinder under axial static tension and alternating torsion, as a special case of the nonsteady-state multiaxial creep. The tests were conducted with a low carbon steel at the temperature of 450°C. The static tensile load was applied to the specimen by dead weight through a lever mechanism, while the alternating torsion was given by vibrating the end of a lever, attached to the specimen grip perpendicularly to the axis of the specimen, with the aid of an eccentric driven by an induction motor. The vibrating frequency was 1500 cycles per minute. These loading assemblies were realized by modifying the elevated temperature fatigue testing machine for combined tension and bending, which was used by the authors previously in making the combined stress dynamic creep test under static tension and alternating bending. Heating of the specimen was carried out by a conventional heating system consisting of an electric furnace and an automatic temperature controller. The axial elongation of the specimen was measured by dial gauges.The results of the tests revealed that the material employed shows similar creep curves under combined static tension and alternating torsion as those in the case of simple tension creep tests. The magnitude of creep under a combined stress, however, was considerably greater than under the static tensile stress that is equal in magnitude to the static tensile component of the combined stress.These experimental results were discussed from the standpoint of the multiaxial creep theory, and a fairly good agreement was obtained between theory and experiment. In this discussion, the multiaxial creep theories based on von Mises and Tresca criterion for the effective stress were first taken up by using the strain hardening criterion for the stress-strain relation, but they failed to agree with the experimental results. As the second step, the authors used an effective stress, derived from the information of multiaxial static creep tests made with the same material under the combined tension and torsion, and obtained a fairly good agreement between theory and experiment for dynamic creep. Finally, the effect of alternating torsional creep strain on strain hardening was also discussed, and it was shown that a better agreement between theory and experiment can be obtained by taking this effect into account. However, this effect was rather small and may be neglected for practical purpose.

Some Problems in the Analyses on Dynamic Creep

The authors have conducted some analyses on the dynamic creep for several years with the aim to estimate the dynamic creep strength from the informations on static creep, and verified the applicability of the analyses to several materials. In these studies, the analyses were confined to the case of the dynamic creep under an axial varying stress which was uniformly distributed over the cross-section of specimen. In the actual service conditions, however, there would be the cases of non-uniform stress distribution such as occurring in the members subjected to bending or torsion and also in the turbine blade, which is assumed to be under the combined stress state of axial static tension and alternating bending. Taking up the above-mentioned combined stress condition for turbine blade as a typical example of the cases of non-uniform stress distribution, the authors have carried out experiments as well as analyses, which brought the possibility of predicting the dynamic creep strength under this combined stress condition from static creep data. In this paper, the analyses are extended to the cases of bending dynamic creep, torsional dynamic creep and also of dynamic stress relaxation.The outline of the analyses is as follows:The assumptions which were used and verified in the previous study on the dynamic creep under combined static tension and alternating bending is utilized in this study also, that is, (1) The equivalent static stress σe introduced for the purpose of predicting the axial dynamic creep strength in the transient stage of creep is also applicable to the second stage of creep.(2) The distribution of alternating stress σa may be regarded as elastic, since the alternating component of strain is almost purely elastic as a result of the sufficiently high speed of stress alternation.(3) The distribution of the equivalent, static stress σe under a dynamic stress condition is the same as that of the static stress which would produce the same creep.If these assumptions are adequate the distribution of the dynamic stress which will produce the creep strain same as that under any static stress condition can be determined easily in the following way. The distribution of the equivalent static stress σe is obtained as a static creep problem according to the assumption (3). The distribution of the alternating stress σa is known as an elastic stress distribution from the assumption (2). On the other hand, the relation of σe, σa and σm is obtained from static creep data, and, finally, the distribution of the mean stress σm is determined from the relation, by inserting the above obtained value of the equivalent stress σe and the alternating stress σa.According to the results of analyses on the bending dynamic creep and torsional dynamic creep, the relation between Mm/Me and Ma/Me in these cases (where Mm, Ma and Me denote the mean moment, the alternating moment and the equivalent static moment, respectively, as in the case of stress) is similar. to the relation between σm/σe and σa/σe, provided that the latter relation for the case of the dynamic creep under an axial stress condition may be expressed approximately in a straight line within a sufficiently wide range of the variables.In the case of stress relaxation, the differential equation for dynamic stress relaxation becomes as1/Edσm/Idt+f(σm, e0-σm/E)=0,

Dynamic Creep and Fatigue of Commercially Pure Titanium at Room and Elevated Temperatures

Dynamic creep and fatigue tests were conducted with a commercially pure titanium at room temperature and at elevated temperatures of 200°C and 300°C. The material showed some interesting behavior, that is, it did not creep at 300°C whereas marked creep phenomenon was noted at room temperature. At the intermediate temperature of 200°C, remarkable creep phenomenon was observed in the same manner as at room temperature during few hours after loading, and then a transition to a very small rate of creep occurred. The transition appeared earlier under dynamic stress than under static stress. These results were discussed from the standpoint of the analysis to be used for the prediction of dynamic creep strength from static creep data, which was established by the authors for the case of a low carbon steel and a 13 Cr steel. It was proved that this method of prediction can be used also in this case of titanium, provided some modification is made in regard to the peculiar phenomena of this material.

Dynamic Creep Testing Machine and Some of the Test Results with low Carbon Steel

Details of a newly developed dynamic creep testing machine are presented. It has been designed to conduct research into dynamic creep and rupture properties of materials at elevated temperatures under the axial load ranging from the static tensile to the perfectly reversed alternating load.This machine is of the type that applies the static load to the specimen with a pair of coil springs and the alternating load with rotating eccentric masses. The capacity of the machine is as follows:Maximum static load 1tonMaximum alternating load ±1tonAlternating speed 1800-5600cpmMaximum test temperature 800°CThe static load is maintained constant automatically with microswitch and relay mechanism so that it is not influenced by the creep of specimen. The load (magnitude and wave form) can be observed in the cource of tests by means of a load measuring apparatus constructed with wire strain gauges. The specimens are heated by electric furnace and the temperature is controlled to be within ±1°C by an automatic temperature controller. As the measuring apparatus of elongation of specimen is used the combination of dial gauge and dash-pot.Fatigue tests at a room temperature and dynamic creep tests at an elevated temperature (450°C) were made with regard to low carbon steel, using these apparatuses, and some of the results are presented.