The effects of overload on the threshold stress intensity factor of delayed fracture (KIHE) for spring steel were studied. Firstly, tensile overload was applied to a wedge engraved SUP9 specimen (Wedge Opening Load; WOL specimen), then delayed fracture tests were carried out to determine the resultant KIHE. It developed that the value of KIHE increased together with the stress intensity factor KOV by applying tensile overload. The effects of tensile overloading on KIHE were evaluated based on the theory of fracture mechanics. The KIHE correlates with the KOV, and the predicting equation has been established.
Thin stainless steel sheet has been widely used for the small sized and high strength mechanical parts. The materials are initially experienced either the TA (Tension Annealing) or heat treatment after cold rolling, then processed for the final products. Not only stainless steel but thin sheet materials basically have anisotropy in their physical and mechanical properties because of unidirectional rolling history. A few results were reported on the effect of the annealing temperature on those properties. In this paper, the effect of temperature annealing in the air on the anisotropy of mechanical properties of stainless steel thin sheet such as surface roughness, W-bending, Vickers hardness, Young's modulus, spring bending elastic limit and pulley-bendingtype fatigue limit were described.
The shape of a stabilizer bar for automobile suspension systems is usually designed from the standpoint of avoiding physical interference with other components. Also, the diameter of the bar is usually pre-selected and fixed to achieve a desired vehicle anti-roll stiffness. Therefore, engineers/designers have little design flexibility to adjust the resultant stress that a given bar will experience in use. In order to improve the durability of stabilizer bars, there are conventional approaches such as a material approach (for example using high strength materials) or a manufacturing approach (for example using efficient shot peening process). It is also known that increasing the radius of bend portions will reduce stress since high stress usually occurs at bends, but this approach is not always possible when interference to other components becomes an issue or when the clamping position of the bar is difficult to change. Unlike conventional methods mentioned above, this paper provides a method to reduce the stress at a bend by slightly modifying the bend shape. By properly adjusting the bend shape from an arc (constant radius) to a non-constant radius curve (Bezier curves are used in this study), stress reduction could be as much as 8.8%, thereby extending the fatigue life of the bar approximately 1.7-1.8 times. This is accomplished by decreasing curvature in high stress areas and increasing curvature in low stress areas, resulting in flat stress distributions. This paper shows that the stress reduction depends on the bar shape and the stress distribution of the original bend shape. It is also shown that a stress reduction can be achieved by creating the bend, not only with continuously changing radius, but also with a series of arcs with different radius.
The production of coil spring requires the precise dimension control of coil diameter and free length. Friction between coiling pin and wire material has the key role in controlling the dimension of coil spring products. In this research, experiments on the effect of newly developed coiling pin unit with supersonic vibration being applied. The obtained result shows that the machine capability has improved significantly.
In this study, a new and convenient mechanical testing method (Own-Weight Multi-layered Cantilever Method) is provided for measuring Young's modulus of each layer in a thin flexible multi-layered material (thin plate, rod, wire). The method is based on a nonlinear theory that takes into account large deformation behaviors of multi-layered materials. By means of measuring the horizontal displacement or the vertical displacement at the free end of the cantilever, Young's modulus of each layer can be easily obtained for various thin and long multi-layered materials. Measurements were carried out on a two-layered material consisting of SUS (a stainless steel material) and PVC (a high-polymer material). The results confirm that the new method is suitable for thin flexible multi-layered materials. In the meantime, the new method proposed in this paper can be applied widely to measure Young's modulus of every thin layers formed by PVD, CVD, Electrodeposition, Coating, Paint, Cladding, Lamination, and others.
In spring factory, large size springs are manufactured in hot forming process. In many cases, the hardness of hot formed springs is inspected by Brinell hardness test. The quality agreement between customer and manufacturer on hardness is often established by other hardness like Vickers and Rockwell hardness, though the spring hardness quality control in shop floor is done by Brinell hardness. Therefore, hardness conversion is often carried out. SAE-J417 has been used for the hardness conversion in the spring industry for this purpose. However, it is often said that the hardness conversion value is not adequate in the area where the hardness is higher than conventional area. This committee started on investigating the difference between SAE-J417 conversion data and various spring hardness test data on June 2007. The purpose of this committee is to offer the new conversion data between various hardness tests for the spring steel and to establish JSMA standard. As a result of three years activity, following findings were obtained. (1) It has been understood that the correlation in Brinell hardness, Rockwell hardness and Vickers hardness is not necessarily correct in SAE-J417 data as the result of various hardness tests on SUP9 spring steel. (2) The dispersion was ±1.5% in maximum when measuring the hardness by Brinell hardness tester, Rockwell hardness tester and Vickers hardness tester. (3) The dispersion was about one percent as a result of investigating the influence of testing load in Vickers hardness measuring. (4) The correlation between various hardness tests using several grades of spring steel did not show clear difference by steel grades. (So, the committee made a new conversion table for spring steel by treating all the data from different steel grades as data from one grade.)
The committee members conducted the research of spring elastic limit for spring materials. Those testing instruments used in this research were APT type tester, Siemens tester and a newly developed prototype tester. The first two traditional testers are the possessions of member companies. The measurement by APT type and Siemens tester were in good correlation. The prototype tester displayed almost equal level reading with Siemens tester as far as the spring elastic limit is concerned. Modulus of elasticity reading by the prototype tester, however, were 10∼20 GPa higher than Siemens tester reading. The newly developed prototype tester has capability for the display of load-deflection diagram of thin specimen down to 0.05mm as well as measuring spring elastic limit.
A series of cooperative research was conducted in order to know what extent the shot peening could increase fatigue strength of spring steel. The specimen containing a drilled hole with various depths of 0.05-0.4 mm was shot-peened at different intensities using steel shots of 0.3 and 1.0 mm in diameter. It was proved that the shot peening improves substantially the fatigue strength of specimens containing an artificial small flaw. One of the findings in this research is that the correlation of the acceptable surface defect size and shot size is observed.