Shuji Taira

Creep of 13-Cr Steel under Varying Stresses

Oil-quenched and tempered 13-Cr steel SUS-2 was tested at a temperature of 500°C under constant stresses, stepwise changing stresses and periodic stresses of rectangular wave. Behaviours of the material under changing stresses were compared with those under constant stresses.It was found that, strictly speaking, the law of“the mechanical equation of state in solid” was not valid for the material tested. The strain rate after the abrupt increase in stress was higher than that of the reference test under the constant stress equal to that after the stress change, and the strain rate was lower after the abrupt decrease in stress. This tendency was accentuated when the first stress change was delayed from primary creep to secondary creep, and further to tertiary creep. The behaviour of this material may be estimated with a small error by the use of the law of“the mechanical equation of state in solid, ”when the stress change is given during earlier stage of creep. The earlier the time of stress change, the smaller the error induced in the estimation.This is quite the same in the tests under periodic stress. The deformation in primary and secondary creep under periodic stress could be estimated from the data of constant stress creep tests with sufficient accuracy. This is applicable for the prediction of creep limit under varying stresses from the data of constant stress creep tests.

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.

On the Fatigue Strength of 13%Cr-steel under Combined Static Tension and Repeated Bending Stress at Elevated Temperature

Fatigue strength of 13% Cr-steel at elevated temperature was studied by using the newly made high temperature fatigue testing machine, which was specially designed for testing turbine blades under service condition, such as under the combined stresses of static tension and repeated bending. Particular attention was given to the influence of static tension on the fatigue strength of the material at elevated temperature under repeated bending stress. Testing temperature was 450°C.The results obtained from this study were as follows;1) The S-N curves made by taking the maximum stress on the stress axis show that the S-N curves for the tests with the higher static tensile stress lie on the side of the higher fatigue strength.2) As far as the static tension applied is small the condition of the fatigue is affected mainly by the magnitude of the repeated bending stress and for the fatigue test under the higher combined static tension the fracture is destined chiefly by the magnitude of that tensile stress.

Effects of Unloading on the Creep of Low Carbon Steel

The creep tests were carried out on low carbon steel at 450°C. The amount of strain recovery when the stress released was found to be approximately equal to the amount of the elastic strain. The strain rate showed larger value when the stress applied again and decreased gradually with the lapse of time, converging to the value at the test for the virgin state.

On the Fracture of Brittle Coating

The authors have reported in the previous paper that the brittle coatings may break when the maximum principal strain exceeds a critical value and the cracks may spread perpendicularly to the principal strain.The elastic constant and Poissons ratio of coating are different from those of the base material; therefore, the stresses in the coating and those of the surface of specimen are different even when subjected to the same deformation. Standing on such a point of view, the law governing the fracture of coating is necessary to be strictly investigated. The relations of the stress and strain of coating to those of the base material are considered. Utilizing this relation, it has been explained by the experiments of the specimens with a uniaxial and a biaxial stress distribution whether of a few general laws of fracture should be applied.By this experiment, the authors have found that the law of fracture based on the maximum principal strain can be applied with enough accuracy.

Compressive Stress Determination by Brittle Coating

On the stress determination of machine parts, it is necessary for us to know not only tensile stress but also compressive stress. Of the tensile stress determination by brittle coating we have reported in the previous paper. For measuring compressive stress, we utilized the phenomenon that the coating on the surface of machine parts under load relaxes within an hour. That is, the machine parts are loaded in compression and during this condition is maintained the relaxation proceeds in the coating. After the duration of about 50 minutes the coating becomes free from stress. On leaving the load, the machine parts tend to revert to the initial condition and then the tensile stress occurs in the coating. Thus minute cracks break out in the coating as in the case of direct method (tensile stress determination). The mode of qualitative as wel las quantitative analysis of compressive stress is analogous to that in the case of tension. A few of examples experimented are shown.

Stress Analysis of Torsion by Brittle Coating

The experimental analysis of torsional stress is discussed, where the surface stresses of the bars with uniform section of various cross section as well as the bars of stepwise varying section are shown. As the sum of both principal stresses, σ1 and σ2, must be zero on the surface of the twisted bars, we can find the magnitudes of σ1 and σ2, to say nothing of their directions, the value of σ1-νσ2 (ν=Poissons ratio) being obtained by the density of cracks on coating.

On the Yielding of Steel under Bending Moment (Report 1)

Nowadays, one of the most important problems in the field of the strength of materials is the yielding condition under non-uniform stress distribution. This paper deals with the result of investigation on the yielding of bars under bending moment. The stress existing in the yielding baran is measured by means of X-ray and then the yielding mechanism is treated theoretically. When a bar is loaded in bending under an increasing load, the bending moment increases to some extent proportionally to the deflection, as is well known, even after the former excees the moment Ms, which is the moment value when the stress of the outermost layer just reaches the yield stress σs. That is, the yield stress seems to be raised. As the result of present experiment, however, stress value exceeding the definite yield stress σs is never found even at a moment over Ms. The measured stress distribution inside a bent bar subjected to a moment over Ms will be stated. Basing on the experimental results, the yielding mechanism under bending moment is theoretically treated. By the analysis the yielding phenomenon is found to be reasonably explained.

On the Yielding of Mild Steel by Bending

The moment-deflection curve of a mild steel beam remains nearly straight until the applied moment reaches as high as 1.5 times of Ms, where Ms denotes the moment when the stress of outermost fiber becomes the equivalent yielding stress σs in tension test. To investigate the phenomena, the surface stress of uniformly bent bar of 0.09% carbon steel was measured by means of X-ray and the stress distribution within the cross section was found utilizing residual stress measurement by corrosion method. As the results, we concluded that the deformation is perfectly elastic as far as the applied moment does not exceed Ms. And even beyond this limit, the surface stress remains under σs. Some analytical method is then introduced to obtain the stress distribution.

X-ray Inuestigation on the Change of Residual Stress due to Repeated Stress

繰返應力の殘留應力におよぼす影響は,實際上重要なる問題である.従來は,殘留應力は繰返應力によつて變化しないものとして取扱われているが,筆者等は殘留應力を有している軟鋼板形試驗片に繰返曲げ應力を與えて,X線的應力測定法により,應力繰返數の増加に伴なう殘留應力の變化を觀察した結果,殘留應力は繰返數の増加と共に,對數曲線的に減少し,遂にある繰返數において消滅し,そののちは殘留應力の消えたまゝの状態で破壊にいたることを明らかにした.從つて,殘留應力が材料の疲勞強さにおよぼす影響は餘り大なるものでなく,むしろ殘留應力を生ぜしめた加工による材質の變化の影響の方が大であることを明らかにした.