When two materials are bonded, free-edge stress singularity usually develops at the intersection of free-surfaces and interface. The order of the free-edge stress singularity p can be obtained as a root of the characteristic equation deduced in terms of Airy's stress function. It is theoretically predicted that logarithmic free-edge singularity develops when the order of the singularity p has double root, but no numerical examination is available in the literature. In this study, stress distribution on the interface of the bonded dissimilar orthotropic materials is calculated by using the boundary element method (B.E.M.) under the condition where the order of the singularity has the double root of the characteristic equation. It was found that the logarithmic free-edge stress singularity developed under mechanical loading. The order of the stress singularity calculated by the nonlinear least squares method from the stress distribution obtained by B.E.M. analyses agreed well with one obtained by using the characteristic equation. The logarithmic free-edge stress singularity developed under thermal stress loading also.
Although the plastic strain induced in materials increases the mechanical strength, it may reduce the fracture toughness. In this study, the change in fracture toughness of SM490 carbon steel due to pre-straining was investigated using a stress-based criterion for ductile crack initiation. The specimens with blunt notch of various radiuses were used in addition to those with conventional fatigue pre-cracking. The degree of applied plastic strain was 5%, 10% or 20%. The fracture toughness was largest when the induced plastic strain was 5%, although it decreased for the plastic strains of 10% and 20%. The stress and strain distributions near the crack tip of fracture toughness test specimens was investigated by elastic-plastic finite element analyses using a well-correlated stress-strain curve for large strain. It was shown that the critical condition at the onset of the ductile crack was better correlated with the equivalent stress than the plastic strain at the crack tip. By using the stress-based criterion, which was represented by the equivalent stress and stress triaxiality, the change in the fracture toughness due to pre-straining could be reasonably explained. Based on these results, it was concluded that the stress-based criterion should be used for predicting the ductile crack initiation.
The method of introduction of the precrack in fracture toughness testing of plastics has been shown to have a significant effect on the measured value of fracture toughness. In this paper, fracture toughness tests of PMMA (Polymethylmethacrylate) and PC (Polycarbonate) have been carried out by CT specimens with two kinds of precracks, fatigue precracking and razor blade slit recommended in ASTM standard D 5045. It was found that the average value of the plane strain fracture toughness, KIc of the razor blade slit specimens was 45% higher than that of the fatigue precracking specimens in the case of PMMA and the former approached to the latter when SR heat treatment was conducted after the razor blade slit. Furthermore, the effect of uncracked ligament, W-a (W : specimen width, a : crack length) of 3 point bend specimens on KIc for PC has been investigated experimentally and analytically. 3-dimensional elastic-plastic finite element analysis of the specimens with various uncracked ligaments has been performed by using a stress-strain curve of PC, in which nonlinear stress-strain behavior yields before the yield stress and strain softening yields after that. It was found that KIc could be obtained even for PC if the size criterion of ASTM D 5045, W-a > 2.5(KQ/σY)2 was satisfied. However, yielding zone at the crack tip of plastics is larger than that of metals when the value of (KQ/σY)2 of both materials is same. Therefore, it is recommended that a larger specimen of 2≤W/B≤4 recognized in ASTM D 5045 is used when the value of 2.5(KQ/σY)2 is nearly equal to W-a.
Two types of free-standing nickel thin films were produced by electrodeposition using sulfamate solution : CC films made under constant current without brightener and CCally films with brightener. The effect of film thickness on the tensile and fatigue properties was studied by using films with 10 and 30μm in thickness. The grain size of CC films determined by X-ray line broadening was about 52nm on the substrate side and increased away from the substrate side resulting in coarser grain size of thicker films on the solution side. The grain size of CCally film was about 9 to 10nm, and the size was slightly larger on the solution side. The fracture strength and yield strength in tension tests were higher in thinner films, and followed the Hall-Petch relation irrespective of film thickness when plotted in terms of the grain size measured on the solution side. The fatigue strength was also higher in thinner films, and the Hall-Petch relation was observed when plotted in terms of the grain size measured on the solution side. The resistance to fatigue crack propagation was higher in thicker films. The threshold stress intensity factor increased linearly with the square root of the grain size irrespective of film thickness. The thickness-effect of thin films on fatigue properties comes from the distribution of the grain size throughout thickness and the coarser grain size made on the solution side controlled the strength of the films. The fatigue fracture surface near the threshold consisted of granular features whose size was larger for films with coarser grains. At high stress intensity factors, striations were observed on the fracture surface of CC films, while only fine granular feature was observed for CCally films.
In recent years, fatigue limit estimation based on energy dissipation has been getting considerable attentions. In this method, temperature change due to irreversible energy dissipation is measured by infrared thermography for various levels of stress amplitude. It is known that the dissipated energy increases with increasing stress levels, and a certain stress level where the change in dissipated energy shows sharp increase coincides with fatigue limit. However, cause and effect relationship between energy dissipating mechanism and fatigue damage has not been investigated well. In this study, effect of phase transformation on fatigue limit estimation based on dissipated energy is investigated for austenitic stainless steel. Fatigue limit estimation based on dissipated energy and conventional fatigue test were conducted for JIS type 304 and type 316L austenitic stainless steel. It was found from experimental studies that fatigue limit of 316L stainless steel obtained from dissipated energy coincided with that by conventional fatigue test; on the other hand fatigue limit of type 304 stainless steel estimated by dissipated energy measurement gave conservative value compared with that by conventional fatigue test. In the case of type 304 stainless steel, plastic deformation led to a phase transformation from austenite into martensite. As the result, amount of emitted dissipated energy decreased since some amount of irreversible plastic strain energy was consumed for phase transformation, and this affected on the fatigue limit estimation based on energy dissipation.
Stress corrosion cracking (SCC) has been observed near heat affected zone (HAZ) of primary loop recirculation pipes made of low-carbon austenitic stainless steel type 316L in the nuclear power plants. For the non-sensitization material, residual stress is the important factor of SCC, and it is generated by machining and welding. In the actual plants, welding is conducted after machining as manufacturing processes of welded pipes. It could be considered that residual stress generated by machining is varied by welding as a posterior process. This paper presents residual stress variation due to manufacturing processes of pipes using X-ray diffraction method. Residual stress distribution due to welding after machining had a local maximum stress in HAZ. Moreover, this value was higher than residual stress generated by welding or machining. Vickers hardness also had a local maximum hardness in HAZ. In order to clarify hardness variation, crystal orientation analysis with EBSD method was performed. Recovery and recrystallization were occurred by welding heat near the weld metal. These lead hardness decrease. The local maximum region showed no microstructure evolution. In this region, machined layer was remained. Therefore, the local maximum hardness was generated at machined layer. The local maximum stress was caused by the superposition effect of residual stress distributions due to machining and welding. Moreover, these local maximum residual stress and hardness are exceeded critical value of SCC initiation. In order to clarify the effect of residual stress on SCC initiation, evaluation including manufacturing processes is important.
We have tried to apply a pressureless sintered silicon carbide radiant tube to heat treatment in a metal furnace using a high temperature over 1250K. The tube was a 2-meter single-end type radiant tube, where both outer and inner tubes were cantilevered at each flange without any other support. In this study we researched fundamental mechanical and thermal characteristics when using silicon carbide for a radiant tube. Bending strength does not decrease in temperatures less than 1580K, which is about the maximum temperature for a burner head made of metal. The safety factor calculated from the Weibll probability of failure is sufficient for use. We found no problem on either oxidation or thermal shock resistance tests through burning. On a simulation of the radiant-tube furnace, the stress that occurred because of the difference between the temperatures of the upper and lower surfaces was found to be not large enough to cause fractures; this, because of high thermal conductivities. Pressureless sintered silicon carbide radiant tubes have been in use in such industrial furnaces as carburizing furnaces for more than 10 years. The long life of radiant tubes contributes to a low running cost for customers, and has zero emissions. We recognize the practical use for pressureless sintered silicon carbide radiant tubes.
Tensile and fatigue failure behavior of C/C composites with fine woven fiber-cloth laminates was investigated in several configurations of specimens. A 3.2mm thick plate, which has the quality of machine-ability, was used for testing material. During the machining process of specimens, care was taken that the fiber directions of 0°/90° and -45°/45° orientation were set against the loading direction. Tensile and fatigue tests were performed under load control techniques. Notches were made on some specimens, and their fracture behavior was observed. Some different notch shapes were used to investigate the effect of fiber orientation on the fracture behavior of the material. The results showed that the critical fracture stresses on the specimen were affected by fiber orientation and notch shape. In addition, the shear stress conditions affected the fracture behavior.