The spot welded specimen employed in the present study was made of mild steel sheet 1.6mm thick and lap-jointed with 2 spot welds in one row. The stress distribution on the surface of the spot welded specimen under uniform bending was investigated with brittle coating and semiconductor strain gages. The stress measured at the situation adjacent to the welds in longitudinal direction was 1.3 times higher than the nominal stress calculated on the single sheet section. The reversed plane bending fatigue tests were made with the spot welded specimen and the plain specimen of the same sheet. The fatigue fracture of as-welded specimen was found to initiate from the inner surface adjacent to the welds. The endurance limit of as-welded specimen was 1/1.4 of the plain specimen. After the cold press working around the welds with rings and punches, the location of fatigue fracture moved to the single sheet part and the fatigue strength became equal to that of the plain specimen. However, in the cold worked specimen which fractured at the single sheet part, the non-propagated crack was also found at the situation adjacent to the welds. By the X-ray stress measurement, the residual stress at the part adjacent to the welds of as-welded specimen was recognized as tension having the value near the yield stress of the sheet. After the cold press working, the residual stress in the same situation was converted into compression, not inducing the work hardening of the material around the welds. The fatigue cracks adjacent to the welds were initiated at the same cycles regardless of the sign and quantity of residual stress. The residual stress, however, played an important role upon the propagation of the cracking. Under the compressive residual stress by cold press working, the rate of crack propagation was found to be suppressed extensively.
When Mn-Cr-B spring steel is ausformed in the stable austenite phase the deformation of the material will concern its fatigue properties. It is the aim of the present study to clarify what are the effects that the deformation will have with its temperature and at its rate on the fatigue properties of the material. The steel specimens were ausformed at 800°, 850° and 900°C up to nearly 50% reductions in thickness and tempered at 480°C for 1 hour. Then, their fatigue properties have been investigated under completely reversed plane bending stresses and compared with those of the specimens treated conventionally and processed by the thermomechanical treatment of cooling it continuously. The results obtained within this scope of experiment are summarized as follows. (1) The average endurance limit of the ausformed specimens within the range of 22-40% reductions shows an increase of nearly 29% over that of the specimen treated conventionally. (2) The fatigue lives of the ausformed specimens too are remarkably prolonged as compared with the conventionally treated specimens. (3) The fatigue properties at the higher rate of deformation are unreliable because of the slight improvement of the fatigue limit and the great variation of the data. (4) There is not much difference between the fatigue strengths of the ausformed specimens and those specimens produced by the thermomechanical treatment of cooling continuously at the equal rate of deformation, but the hardness levels of the former are by Rc 1-2 numbers lower than those of the latter. (5) The authors recommend the range of 850°-900°C as the appropriate deformation temperature and reductions in thickness up to 40% for this spring steel.
In a series of studies of polycrystalline metallic materials with respect to the influence of their strain history on their plastic deformation, one of the problems concerns their yield, and so the studies have been performed on their behavior of plastic deformation as they are subjected to low-cycle pulsating stresses in connection with their strain history. The present study, being the continuation of the previous study, consists of experimental examination of low-cycle pulsating stress tests under combined axial tension and torsion in which the principal axes under stress must be either rotated or fixed. From the present analytical and experimental studies, the following conclusions have been obtained. It is evident that there is larger plastic deformation of the material under the cycle of stress when the principal axes are in rotation than when they are fixed. This is due to the difference in the stress passage giving rise to variant change of the structure of the material. It is found also that the numerical value of anisotropic parameter of the material and that of parameter of the Bauschinger effect during the cycle of stress are larger when the principal axes are in rotation than when they are fixed. It is suggested moreover that the development of the structural change of the material under the rotation of the principal axes of stress, recognizing correlation between the variation Δσij of the microscopic yield stress from the mean stress and the fluctuation Δ(DE)ij of increased microscopic strain, makes much of coexistence of Δσij and Δ(DE)ij as factors to bring about changes in the location of yield surface of the material.
This investigation has been undertaken to determine the influence of microstructural factors (non-metallic inclusions, prior austenite grain boundaries, laths of martensite, bundle boundaries) on the characteristics of low-cycle fatigue microcracks (initiation sources, initiation period, propagating paths, propagation rate) by conducting optical microscopic observations of the surface of S 38 C smooth specimens tempered at 200°C. The changes in dislocation density during the low-cycle fatigue test have also been studied. The fatigue tests were carried out at 20°C under constant total strain range of ±1.3% by alternating bending. The results obtained are as follows: (1) There were found early fatigue microcracks more than 10μ at the stage of about 10% of the total life (2∼3×102 cycles). (2) In one and the same specimens, many microcracks occurred mainly in non-metallic inclusions, lath boundaries (α'B), prior austenite grain boundaries (γB) and bundle boundaries (βB). (3) The microcracks along α'B originate at the distorted resions like slip bands, but the microcracks along γB seem to have originated in the manner of brittle fracture in grain boundaries. (4) The propagation rate (other than the joining rate) of microcracks along α'B is almost equal to that along γB, and that across the lath boundaries is slower than that along γB. (5) The microcracks that occurred at γB tend to grow into larger cracks than what occurred at α'B, which often remains non-propagating. (6) In the extended cracks (by growing and joining), the paths along α'B are almost equal in length to that along γB, and the additional length of their paths occupies about 80-90% of the whole crack length. (7) During the low-cycles fatigue, dislocation dencity decreases very rapidly in a few cycles, and after that, decreases slowly as the cycles increase in number, and this changing mode is inverse to the increasing distorsion resions.
In view of the fact that in spite of the detailed investigations that have been made of the dynamical behavior of plastics since Maxwell and Voigt presented their models, no dynamical model of FRP (Fiberglass Reinforced Plastics) has yet been developed, it is felt required that such a new model of FRP will have to be established. The researches, if they were attempted, would be limited to creep and relaxation of Maxwell or Voigt models. When FRP material is vibrated or is fatigued, such peculiar phenomena are recognized as change in dynamical elasticity under vibration and increase in rigidity under fatigue. These phenomena can not be explained by Maxwell and Voigt models. So we have constructed a dynamical model of FRP with design that internal friction will take place, based upon the fact that FRP consists of two layers of resin and glassfiber. Therefore a slider is involved in the model as vital factor in order to explain these phenomena. In this paper the authors present a new dynamical model with a slider to investigate the peculiarity of the phenomena, which are theoretically interpreted.
The present paper is concerned with the effects of couple stress and loading width on the stress distributions in an indentation test specimen. An analytical solution was obtained by the Fourier expansion method. Results are as follows. (1) The effect of couple stress is predominant. The larger the material parameter implying the micro-bending rigidity is, the larger decrease there is in the magnitude of stress generally so but especially in the tensile stress in the central region of the specimen. (2) The effect of the loading width on the stress distribution is secondary, as compared with that of couple stress and is limited to the vicinity of the loaded boundary. (3) As the Poisson's ratio increases, the magnitude of both compressive and tensile stresses generally decreases. The effect of the Poisson's ratio, however, is secondary as compared with that of couple stress.
There has been but little study so far made on the low-cycle torsional fatigue on cold drawn wires of small diameters under actual loads. In order to pursue studies on this problem, therefore, we designed and manufactured a low-cycle torsional fatigue testing machine suitable for the test of these wires. There are many types of torsional fatigue testing machines, but the one designed by the authors has a merit in that by using a photo cell the change of the load can be conducted easily and exactly. By using this testing machine we carried out the low-cycle torsional fatigue tests of piano wires under manifold repeated load on two stress levels at room temperature. The main results obtained are as follows: (1) The cumulative cycle ratio Σn/N obtained from the test results were not in agreement with the unit, giving values greater than unity for large stress ratio of τA/τw and those less than the unit for small stress ratio. (2) The time strength at 103 cycles for piano wiresrises whenever it is subjected to multiple repeated loads. (3) The test results show that the plots of total plastic strain versus the number of stress cycles to failure deviate from the linear relation between them which holds under a constant stress amplitude.