mprovement in the threshold stress intensity factor by tensile overload in large crack was investigated. The crack retarded zone and the non-propagating crack size was validated based on the fracture surface observation. It was observed that crack propagation was retarded by the overload during the fatigue tests. The size of non-propagating cracks and the crack retarded zone corresponded well to the calculated value. Moreover, in order to evaluate the overload effects on the small crack like defect, bending fatigue tests were carried out and discussed the effect of crack size on the improvement of the ΔKth.
To increase the fatigue limit of the automobile parts, especially, the following two items need to be improved: 1) Increase hardness near surface. 2) Introduce highly compressive residual stress near surface. Shot peening is useful technique to improve above items, economically. However, there are few studied evaluated the influences of shot peening method on the rotating bending fatigue limit, systematically. Therefore, the authors conducted a study by using carburized specimen treated four shot peening methods, and analyzed the correlation between the fatigue limit and the sum of yield stress(σY) and maximum compressive residual stress(σrmax). As the result, the fatigue limit was given with an equation as follows:σW=0.3891(σY+σrmax)
To study fatigue strength design method for suspension coil springs for automobile, especially as the fundamental study of fatigue strength for spring steel, torsional fatigue and rotating bending fatigue tests were conducted for shot-peened spring steel of SUP7 having various Vickers hardness through tempering temperature variation. The effect of shot peening on the torsional fatigue strength and rotating bending fatigue strength were studied. Furthermore, the difference between the morphology of torsional fatigue failure and rotating bending fatigue failures were also studied. As a result, following findings came to light. (1) The torsional fatigue limit of peened specimen exceeded that of non-peened one in any hardness condition. On the other hand, as for rotating bending fatigue limit, there was no difference between non-peened and peened specimen, with the exception of specimen with maximum hardness. (2) The relationship between torsional fatigue limit and rotating bending fatigue limit for non-peened specimen was well explained by the shear strain energy theory. On the other hand, as for peened specimen, the relationship was explained by the combination of the maximum principal stress and the shear strain energy theories. (3) The fatigue failure morphology for non-peened specimen due to torsional fatigue indicated that shear fatigue failure initiated parallel to the axial line and then the failure propagated toward principal stress direction. As for peened specimen, the fatigue failure initiated along the 45 degree direction to the axial line (principal stress direction), and then propagated. On the other hand, as for all specimen due to rotating bending fatigue tests, the fatigue failure initiated toward principal stress direction, and then propagated.
Torsional fatigue tests were conducted for both shot-peened and non-peened spring steel, SUP7, with different Vickers hardness by changing tempering temperature. As a result, the torsional fatigue strength of shot peened specimen exceeded that of non-peened ones for the whole hardness level. Furthermore, higher Vickers hardness leads to higher torsional fatigue strength. Also, for the shot peened specimen with the hardness of 620HV, there was no decrease in fatigue limit, as seen in the non-peened specimen. In addition, the results of the measured residual stress for pre-fatigue and post-fatigue tests indicated that there was decay of residual stress after 107cycles fatigue test. The quantity of decay can be predicted by the residual stress before fatigue test and applied stress and yield strength of the specimen. Finally, we proposed the estimating equation for torsional fatigue limit of shot peened specimen. According to our estimation formula for torsional fatigue limit, we can predict accurately the torsional fatigue strength of shot -peened specimen with a hardness of up to 550HV by using the slope of the approximation curve in the modified Goodman diagram and values of residual stress for both specimens after 107 cycles fatigue test.
Effects of shot peening on the bending fatigue strength of spring steel SUP9A (470HV) containing an artificial small hole were investigated. Shot peening (SP) and stress shot peening (SSP) were carried out with the specimens containing an artificial drilled hole 0.1, 0.2 and 0.4 mm depth. Then, bending fatigue tests were carried out with the specimens. The fatigue strengths of specimens containing an artificial small hole were increased by shot peening. Stress shot peening (SSP) was effective in improving fatigue strength. It was found that an artificial drilled hole under 0.1 mm depth and 0.2 mm depth could be made acceptable by SP and SSP. The acceptable defect size by SP and SSP were evaluated based on fracture mechanics.
In recent years weight reduction of suspension springs has been increasingly required from the viewpoints of improvement in fuel efficiency of automobiles and global environmental concerns. To realize further weight saving of the suspension spring, it is important to increase its fatigue strength and it is imperative to understand shot peening techniques, which affect improvement in the fatigue strength. However, the relation between surface roughness, residual stress distribution, which affects the fatigue strength, and shot peening conditions has yet to be sufficiently clarified while it is qualitatively comprehended. Moreover, the quantitative relation between the surface roughness, the residual stress distribution, and the fatigue strength has also not been fully understood. If the fatigue strength can be predicted quantitatively from the shot peening conditions, shot peening can be used as a tool for the optimal design of the suspension spring. This quantitative prediction will also be very useful in knowing processing conditions to improve the fatigue strength. In this study, various shot peening conditions, such as shot size and spring steel hardness, were examined to obtain a regression formula to estimate the residual stress and the surface roughness. Further, a method to predict the fatigue limit from the regression formula and the fracture mechanics was examined, and its practicality was verified.
In application of flexible thin materials, it is very important to evaluate mechanical properties of these materials in both analytical and technological interests. In this study, a new and convenient mechanical testing method (Own-Weight Cantilever Method) is provided for measuring Young's modulus in flexible materials (thin plate, or wire). The method is based on a nonlinear theory that takes into account the large deformation behavior of flexible materials. By means of measuring horizontal or vertical displacement at the free end of a cantilever, Young's modulus can be easily obtained for various thin plate or wire materials. In this study, measurements were carried out on a thin piano wire. The result reveals that the new method is suitable for thin flexible materials. In the meantime, the new "Own-Weight Cantilever Method" proposed in this paper is quite a promising method and can be extended to measure Young's modulus of every thin layer in a flexible multilayered material formed by PVD, CVD, Electrodeposition, Coating, Paint, Cladding, Lamination, and others.
The large pitch angle of a coil spring required by higher design stress generates multi axial stress condition. As a result, the correctness of Wahl's stress equation in large pitch angle coil springs seems to be questionable to conduct a fatigue life assessment. It seems urgent to establish the characteristic stress suitable for fatigue life assessment as well as to study the effect of residual stress and surface roughness. The research committee conducted a variety of fatigue tests and stress evaluation using finite element analysis in order to investigate followings. 1) Which stress, such as maximum principal stress, maximum shear stress or Misses stress, is most suitable for estimating fatigue life? 2) How can we evaluate the effect of residual stress? The test results come to the conclusion that the correlation of fatigue life and maximum principal stress is higher than the other stress condition.
The quality of springs, as machine elements, is important to maintain and improve the high quality of Japanese industrial products. These days, dissemination of personal computer makes it easy to evaluate the design of springs when all the fact data (tested values) are provided. The characteristic values of various materials, as a result of research and investigation, are being reported in technical papers issued every year in Japan and overseas. If these values are used for the fact data, we anticipate that the database for spring material becomes useful and covers a widespread area. Therefore, the committee has been working to collect data and to turn them into a database for the benefit of general spring designers. We picked up the data of JIS (Japanese Industrial Standards) materials from a total of 878 technical papers, read off the figures from the diagrams of main characteristics, and then tried to unify the data from multiple papers. Through this work, we found lots of variations in the data due to the differences of testing and sample making conditions. We also found lots of papers without the reference of testing and sample making conditions. The importance of the testing and sample making conditions to be described in all technical papers should be deeply engraved in researchers' mind. Finally, we hope that the data mapping tables made through this activity will be used as a reference database for finding the whereabouts of material characteristics and technical papers which contain them.