The dependence of slip line occurrence at the periphery of microcircular holes formed by photoetching in an electrodeposited copper foil upon the number of cycles and cyclic stress magnitude for plane bending and cyclic torsion is examined. The results show that the principal stresses in the element under biaxial stress, which are undetectable by the conventional copper electroplating method, can be evaluated using the equation based on the probability process of slip line occurrence, which takes into account both cyclic stress magnitude and the number of cycles.
Torsional fatigue tests were conducted for circumferentially notched bars of austenitic stainless steel (JIS SUS316L) and carbon steel (JIS SGV410) under completely reversed cyclic torsion without static tension (Case A) and with static tension (Case B). The propagation behavior of cracks formed at the notch root was examined from a viewpoint of fracture mechanics. The length of crack was evaluated by the electrical potential method under the assumption of concentrical cracks propagating inward on the minimum cross section of specimens. For Case A of both steels, the crack propagation rate decreased with crack length, and turned to increase after taking the minimum rate point. Sharper notches gave rise to a slower rate for SUS 316L, while to a faster rate for SGV410. At the same stress amplitude, the crack propagation rate in SUS316L is lower than in SGV410. For Case B, the crack propagation rate was monotonically increased with crack length. Similar tendency was observed when the crack propagation rate was correlated to the stress intensity range. The J integral range was estimated from the hysteresis loop between torque and twist angle. For Case B, the crack propagation rate was expressed as a power function of the J integral range. For Case A, the crack propagation was lower than that for Case B, because of retardation due to the sliding contact between crack faces. The amount of retardation was larger for sharper notches at lower stress amplitudes in SUS316L, while blunter notches gave larger retardation in SGV410. The amount of retardation in Case A was closely related to the roughness or the acuteness of the factory-roof fracture surface.
The scattering of tensile strength results from various causes. For example, it arises from the non-uniformity of metallurgical structures caused by imperfect mixing of constituent materials, nonmetallic inclusions and the change in cooling rate. Furthermore, the scattering due to the difference in shape and size of test specimens depends upon the degree of improvement of plants and machines and the human efforts (carefulness, etc.). Therefore, it may be possible to determine the scattering range from a comparatively small number of data if the impovement is proper. So we considered theoretically a method of estimating the mean value, the maximum or the upper value and the minimum or the lower value.The result of our theoretical consideration shows that the mean value σM of tensile strength for specimens having the same shape and size can be estimated from (TS)max and (TS)min by the following equation.σM=√(TS)max·(TS)min (1)where (TS)max and (TS)min are the maximum and minimum values of tensile strength for the sample. Then, the upper value (σ∞)r, the lower value (σl)r, the maximum value (σmax)r and the minimum value (σmin)r can be obtained by the following equations(σ∞)r=σM(δ∞)r (σl)r=σM/(δ∞)r} (2)(σmax)r=σM(δmax)r (σmin)r=σM/(δmax)r} (3)where (δ∞)r and (δmax)r are coefficients which depend on the appraisement quantity q for the tensile strength of specimens, the degree of human improvement r and the quality of samples.In order to verify the above theoretical consideration, the estimated values were calculated from the experimental data on a few specimens (12 pieces) by means of eqs. (1) and (3), and compared with the estimated values by usual statistical techniques. The results of such comparative investigation on many specimens of different lengths (l=5.2, 13, 26, 39, 52, 78 and 104mm) are summarized as follows:(1) As shown in Table II, the mean value σM for all samples except the ones with l=52mm lies within the 95% confidence interval for the arithmetic mean value σ. Consequently, σM and σ are regarded equal in practical applications.(2) The values of (σmax)r and (σmin)r obtained from eq. (3) are quite comparable to those of σ+3s and σ-3s, where s is the standard deviation.(3) The scattering range of the tensile strength tends to decrease as l becomes longer or shorter than 52mm.
In this study, the fatigue tests for some engineering steels under the condition of stress rate R=-1 were conducted by the ultrasonic fatigue testing machine. The frequency of this machine was 20kHz. But net frequency f_ was lower than 20kHz. Because the intermittent operation with air cooling were conducted in order to suppress the heat generation by high speed test. The relationship between crack propagation rate and stress intensity factor range was compared with that of conventional fatigue test for low frequency (20kHz). The relation of ultrasonic was good agreement with that of conventional fatigue test. In-situ observation of a crack initiation and propagation was conducted. Striations were observed on the fatigue fracture surface.
The FPB (Fine Particle Bombarding) process attracts attention as a new surface-preparation technique. The influence of surface modification by the FPB process on various characteristics of SUP10 was investigated. The surface roughness of FPB treatment materials decreased more than shot peening treatment materials. As a result of comparing the shot time of FPB process, it proved that surface roughness increased as the shot time became long. On every projection conditions of FPB process, the hardness near the surface rose higher than the shot-peening. Hardness was the maximum at the time of shot time 20 seconds, and residual stress was the maximum at the time of 5 seconds. The maximum compressive residual stress was a value equivalent to a shot-peening process. The fatigue strength of the FPB material rose than shot-peening material by low surface roughness and high surface hardness. These results showed that a FPB process was effective in the improvement in fatigue strength of spring steel.
