Small Artificial Defects Research Articles

Rapid and Sensitive Radioscopy for Fine Ceramics Using an Image‐Subtraction Method

A new microfocus X‐ray testing method has been developed to detect very small defects in ceramic products with high speed and reliability. By using image subtraction, enhanced X‐ray images of defects were extracted from the background noise. Much‐better sensitivity (e.g., 0.1% in Si3N4 that was 20 mm thick) was obtained within a few minutes. This method is considerably superior to the conventional film method. In this study, Si3N4 test pieces (penetrameters) that had very small artificial defects (such as slits and pores) were prepared and examined by using the new method.

Effect of Small Artificial Defects on Fatigue Limit of Carbon Steel at Elevated Temperatures.

The effects of small artificial defects on the fatigue limit of an annealed carbon steel have been investigated in the temperature range of 20°C to 450°C. The experimental results have been compared with those predicted by Murakami's √(area) model. Small drilling holes were used as artificial defects. The diameters of the holes were 0.1, 0.2 and 0.3 mm, respectively and their depths were almost the same as the diameter. The fatigue limits of the drilled specimens as well as the smooth specimens showed a sharp peak caused by dynamic strain aging near 350°C. Vickers hardness, HVs, necessary for the application of the √(area) model was measured in the same temperature range as the fatigue tests. The Vickers hardness, HVs, did not show such a sharp peak as the fatigue limit near 350°C. The fatigue limits obtained from the fatigue tests agreed with those predicted by the model within a scatter band of ±10% at 20°C and 250°C. The experimental values, however, were larger than the predictions at 350°C and 450°C. At 350°C, the deviation was large and its degree became about 35%. The results were attributed to a much greater effect of dynamic strain aging for the fatigue limit than for the hardness. When the hardness, HVc, measured after fatigue loading was applied to the model, the deviation was decreased to 18%. Thereafter modified hardness, HVf, obtained by multiplying fatigue limit ratio, Rf, to the hardness, HVs, was applied to the model, where Rf is the ratio of high temperature fatigue limit to room temperature fatigue limit. Most of the results were in the scatter band of ±10% from the predicted value.

Prediction of fatigue strength in HSS-Co, S55C and the friction welded materials based on statistical evaluation of inclusion size

In this paper, the effects of nonmetallic inclusions on the fatigue strength of medium and high strength steels are quantitatively investigated by considering the relationship between the Vickers hardness, Hv and the maximum size of nonmetallic inclusion,\(\sqrt {(area)_{\max } } \). The maximum size of nonmetallic inclusion defined by the square root of the projected area in an inclusion can be estimated by the statistics of extreme values with a definite number of specimens or structural components. While most of other studies using the parameter\(\sqrt {(area)_{\max } } \) have performed the experiments with the specimens with small artificial defects or notches, this study investigates the possibility of prediction on fotigue strength in the unnotched smooth specimens. The results show that strength of the friction welded joints of HSS-Co to S55C carbon steel is almost equal to that of the weaker material in the optimum welding conditions. Under the limiting condition for the nonpropagating cracks emanating from defects or inclusions, the threshold stress intensity factor range °Kth and the lower limit of fatigue strength σw1 were successfully estimated from the largest inclusion size\(\sqrt {(area)_{\max } } \). From the analysis of SEM fractographs, it can be concluded that the fatigue fractures of the specimens are associated primarily with the inclusions located at the outer periphery of the specimens.

高温におけるＴｉ‐６Ａｌ‐４Ｖ合金の疲労強度に及ぼす人工微小欠陥の影響

The bending fatigue strength of Ti-6Al-4V alloy was investigated in the temperature range of 20°C to 450°C. A small artificial defect was introduced in the form of a small hole drilled in the surface of an unnotched specimen. The diameter of the hole was about 0.1mm and its depth was virtually the same as the diameter. The fatigue strength σF of the unnotched specimen was compared with the σFN of specimens with a small hole. The following results were obtained.(1) The fatigue strength is remarkably decreased even by such a small hole. The decrements of the fatigue strength caused by the hole, (σF-σFN)/σF are about 40%, 32%, 27% and 47% at 20°C, 250°C, 350°C and 450°C, respectively.The values of σFN predicted by the Murakami's √area parameter model are about 3%-17% smaller than the experimental results. The prediction seems to give a conservative estimation.(2) In the temperature range of 350°C to 450°C, both σF and σFN are increased by solution hardening, dynamic strain aging and so on. Since the static yield strength is hardly influenced by such a strengthening factor, the temperature dependence of the yield strength may be used as the reference for evaluating the increments in the fatigue strength. It is concluded that the maximum increments of σF and σFN come to about 60%.

Effects of small defects on fatigue strength of metals

The effect of a small artificial defect (ie a drilled hole, the diameter of which is from 40 to 200 μm) on the fatigue strength of low carbon steel and medium carbon steel was investigated. The fatigue strength of the specimen containing the very small hole was found to be of almost the same order of magnitude as the fatigue limit of the plain specimen - that is, the existence of the very small hole did not weaken the fatigue strength of the holed specimen. This phenomenon had previously been predicted, and explained quantitatively, from a comparison of the hole dimensions with the size of non-propagating cracks which appear in the plain specimen at the stress level of the fatigue limit. The results of the present investigation can be applied to the evaluation of fatigue notch sensitivity of materials with high tensile strength or high hardness, and, from the metallurgical aspect, to the quantitative control of defects or inclusions in metals.