A theory of steady-state creep is here presented, based on the rate of climbing of piled-up dislocations and the fraction of Frank-Read sources working at a given stress. Grain boundaries are considered as the source to produce equilibrium concentration of vacancies. The theory gives a linear relation between log(ε/σ2) and L/σ, where ε and σ are the creep rate and the applied stress respectively. The experimental creep data with several metals gave a fairly good fitting to the relation. And, from the slope of the plotting the average length of Frank-Read sources can be calculated. It is about 2.1, 1.8 and 0.26 microns for annealed polycrystalline specimens of Cu, Al and an 18-8 stainless steel, respectively.
In order to find the correlation of the torsion or bending creep strength with the tensile creep strength for hardened and tempered 13%Cr steel, creep tests in tension, torsion and pure bending have been made at temperatures 400°C and 500°C. And then, these experimental results were compared with the analytical results by calculation based on von Mises or Tresca criterion. The tensile creep tests were conducted on 8mmφ cylindrical specimens, the torsion creep tests on 10mmφ/8.5mmφ thin-walled and 8mmφ solid cylindrical specimens, and the bending creep tests with 6mmt×10mm plate specimens. The apparatus employed in the torsion or bending creep tests were made in our laboratory. The creep test continued altogether for 400 hours. The results obtained can be summarized as follows: (1) The ratio of the shear creep strength to the tensile creep strength of 13%Cr steel corresponding to a mean creep rate of 0.2 or 1.0 per cent per 1000 hours for 300 to 400 hours is respectively 0.48 for 400°C, and 0.43, 0.46 for 500°C. On the other hand, the ratio derieved from the calculations due to von Mises or Tresca criterion is respectively 0.48 and 0.43 for 400°C, and 0.51, 0.45 for 500°C. (2) The ratio of the torsion creep strength of solid cylindrical specimen to the tensile creep strength for same creep rates is respectively 0.68, 0.70 for 400°C and for 500°C, 0.49, 0.50. (3) The ratio of the bending creep strength to the tensile creep strength for same creep rates is respectively 1.37, 1.55 for 400°C, and 1.47, 1.35 for 500°C, while the ratios obtained by calculation are 1.29 for 400°C and 1.34 for 500°C.
It is the aim of this paper to present evidences for the changes of work hardening phenomena caused during the fatigue test under repeated alternating combined stresses in virgin and tensile cold-worked mild steel specimens. Changing the grade of tensile pre-straining K, the phenomena are measured dynamically with the use of strain gage and statically the dial gage, and then the differences between both measured results are argued. The aspects of slip band produced on the surface of fatigued specimens are studied by means of electron microscope in connection with the changes of work hardening. The authors find that it is not possible to draw the S-N curve by the use of the Schenk type fatigue testing machine and this origin is traced. The main conclusions from this research are as follows: (1) In the case of uniaxial tensile pre-strained steel the work hardening was saturated at an early stage of the fatigue test, and then the strength falls due to the relieving of an excess of internal stress and development of micro-crack was continued slowly up to failure. (2) In the case of annealed steel two work hardening peaks were observed and one bottom appeared between those peaks at three kinds of α respectively. The difference of stresses at the 1-st peak and the bottom decreased by changing α from 0° to 90°. (3) The difference of stresses measured dynamically and statically were remarkable at an early stage of all fatigue tests and it was found that the work hardening phenomenon depended on the cyclic strain rate. (4) The appearance of slip band was due to the softening of fatigue test.
In connection with the structural changes taking place during the fatigue test of metals, the various concepts of micro-cracks do not seem to have been made clear, and various innovations of technical terms have been proposed by different investigators to designate these concepts. Since no systematic studies have been made of the relationships between these terms much confusion has been occasioned in the use of these terms. In this paper these relationships are argued and a new system of nomenclature is proposed in order to make this matter clear. The concepts and appellations argued here are the surface notch (or top), the fissure, the surface disturbance, the persistent slip band, the intrusion (or extrusion) and the sub-microscopic crack. The main conclusions from the present research are as follows: (1) The surface top and notch suggested by Prof. Wood on the metallographic taper section or the approximate taper section of polycrystalline copper specimens are equivalent to the extrusion and the intrusion of small size. Because they cannot be removed by annealing but are connected with the fissure, their occurrence implies the initiation of fatigue damage. (2) In the views of the authors the fissure, the persistent slip band and the deep intrusion are the phenomena that would also appear whatever different things were tried by the same method of observation, and their origination means the occurrence of the decisive fatigue damage. Consequently, the authors suggest that they must be micro-cracks. (3) The block movements observed between the blocks divided by the micro-cracks can be suppressed more or less by silver plating. (4) The assignment of concepts for origination of sub-microscopic cracks is to be made when the phenomena of hardening are saturated. This assignment of concepts is not applicable to the fatigue of copper polycrystal.
Although annealing at around 700°C (below A1) is commonly recommended as the intermediate annealing during cold forming processes of steels, sometimes it does not necessarily gain satisfactory results, because the cold forming is generally heterogeneous deformation of steel. In this investigation, cold hubbed S15CK, S30C, S50C, SK5 and SNC21 steels were annealed at 700°C and 950°C, and their structure ferrite grain size and hardness were examined. As for the 700°C-annealing of mild steel (S15CK) and low carbon alloy steel (SNC21), it was found that cementite was spheroidized, but the annealed steels showed much heterogeneities among severely deformed region and slightly deformed region in the structure, the ferrite grain size and the hardness. By the 950°C-annealing, disadvantage from a small amount of lamellar pearlite colonies was negligible, homogeneities of structure and hardness were excellent and. the grain size was refined and homogenized. The 950°C-annealing was, therefore, more desirable than the 700°C-annealing for mild steel and low carbon alloy steels. With carbon content of steel exceeding 0.3%, the heterogeneities grew a little even by the 700°C-annealing, because the cementite particles could obstruct the grain coarsening. Also, the cementite was spheroidized and the hardness lowered. In such cases, the 700°C-annealing was desirable.
There have been many papers and publications affirming that the moduli of elasticity of plastics vary according to the type of loading-uniaxial tension, uniaxial compression or simple bending. The main purpose of the present study is to clarify whether or not such inconsistencies have been brought about by the intrinsic character of the mechanical behavior of plastics. In the first place, the moduli of elasticity in bending Eb and in tension Et have been measured in a common but more precise manner, and their comparative studies have been performed. In the second place, whether or not there is any difference between Et and the moduli of elasticity in compression Ec has been examined by bending the composed beam of plastics and metal. The following results have been obtained for various kinds of plastics sheets as conclusion. The differences among Et, Ec and Eb as shown in many publications have been caused not by the intrinsic characteristic of the mechanical behavior of plastics but by some other factors. In determing Eb, care should be taken that the measurment should be carried out by the use of specimens with suita-ble width.
In the previous paper the author made an analysis of the power of hydraulic fatigue testing machine under pulsating tension and described the method to reduce the power. This paper treats the analysis of power under completely reversed tension and compression by applying the results of the previous paper. In this case, the power required to afford the tensile stress to the specimen is transmitted directly by the pulsator and the power to afford the compressive stress to specimen is transmitted by discharging the energy stored in the accumulator which utilizes the compressibility of hydraulic oil. In other words, in the case of pulsating tension the twisting moment of the driving shaft is approximately zero in the range 5/6π∼7/6π of rotating angle of the shaft and reverses slowly in the neighborhood of both the end points, and reverses rapidly at 2π. However, in the case of completely reversed tension and compression the moment reverses rapidly at π and 2π. Accordingly, the torque in the case of tension and compression fluctuates rapidly compared with pulsating tension, and large energy fluctuation occurs. Consequently, under the same load amplitude, the larger power is needed in the case of tension and compression than the pulsating tension, and results in the decrease of energy efficiency. This paper carried out the analysis of the energy of hysterisis loop of the specimen in the case of tension and compression, and clarified the basis to design the fly wheel in order to raise the efficiency of energy of the fatigue testing machine. The energy losses of the testing machine, excepting the true energy to afford the stress to specimen are as follows. (1) The resistance of hydraulic system. (2) The energy losses caused by the hydraulic pressure expansion of several parts. (3) The oil leakage between the ram and cylinder, etc. (4) Mechanical losses. By calculating the above values (1∼4) the author could get the ratio of these values to the total consumption of power, and applying these values, he obtained the nomograph which shows the relation between the dynamic load, deflection of the specimen, the oil volume in hydraulic system, and the pulsator capacity which will afford the convenience in calculating the power.