Key Engineering Materials Vols. 353-358

Abstract: Fatigue crack growth (FCG) tests have been performed in an X70 steel with various microstructures (respectively in the as-received and the normalized condition). The effect of room temperature creep (RTC) on FCG behavior has been investigated by comparing with single wave overloads (SWOL). The as-received X70 pipeline steel has high FCG rate at the near-threshold region. While at the Paris region, FCG rate seems insensitive to the microstructure. In both conditions, time-dependent deformation is observed at crack tips (i.e., RTC), which increases with increasing stress-intensity-factor. And this deformation has a high value in the normalized state, under identical testing conditions. Both RTC and SWOL can bring subsequent fatigue crack growth a very short initial acceleration before deceleration, whereas the former induces more serious deceleration and retardation, which attributes to more significant crack closures.

Abstract: This paper deals with the fatigue behavior and its statistical properties of SM490A steel at various temperatures, which is utilized in the railway vehicle. For these goals, the tensile ad fatigue tests were performed by using a servo-hydraulic fatigue testing machine at three temperatures: +20°C, -10°C and -40°C. The static strength and fatigue limits of SM490A steel were increased with decreasing of test temperature. The probabilistic properties of fatigue behavior are investigated by means of probabilistic stress-life (P-S-N) curve and they are well in conformance with the experimental results regardless of temperature. Also, based on P-S-N curves, the variation of fatigue life is investigated and as the temperature decreases, the variation of fatigue life increases moderately.

Abstract: In this study, the plastic η -factors of the SA508Cl.1a narrow-gap welding part, which is used for the primary piping system in a nuclear power plant were obtained by using finite element analysis and the modified fracture toughness testing method was suggested for the narrow-gap welding part. Also, we have performed the fracture toughness test for the SA508Cl.1a narrow-gap welding part by applying the new testing method and then we compared the results with those from the ASTM fracture toughness test.

Abstract: The study aimed at the fatigue life estimation for leakage of refrigerant used in household air conditioner. The leakage of brazed joints was affected by the manufacturing faults with flaws and service condition which is at the internal pressure and at the excitation. Therefore, the four point bending tests were performed to estimate effects of internal pressure and excitation for the fatigue life of brazed joints. In addition, to investigate effect of internal pressure and excitation on the fatigue life, the orthogonal array table in Design of Experiment is adopted.

Abstract: Fatigue crack propagation behavior near the fusion line between SA508 ferritic steel and Ni-based buttering metal was studied to assess the integrity of dissimilar metal welded zone in reactor pressure vessels. Ni-based filler metal has been used as a buttering or filler metal to weld the ferritic steel to the Ni-alloy or austenitic stainless steel. The J integral value and stress field at the crack tip in a simulated small-CT welded specimen model was calculated by using the commercial FE calculation code to anticipate the effect of the yield strength differences between dissimilar metals. If the Ni-based buttering metal has lower yield strength, which means the decrease of material constraint by the weld metal, the J integral value of the crack tip in the base metal near the fusion line was calculated higher than that of the base metal. The fatigue crack propagation behavior near the fusion line was measured by using the small-CT welded specimens of 5 mm thickness. The relationships between da/dN and )K were measured in the base metal and the HAZ near fusion line. The yield strength of the weld metal including microstructure at the joint can be considered more effective than the material constraint on explaining the behavior of fatigue crack propagation near the fusion line.

Abstract: Under extra compressive stress, some phenomena of rock spallings and fractures often exist on rock mass located in sidewalls of underground house and tunnels. It is the reason that the crack growth and coalescence initiation from original flaws (or faults) in rock mass. In the previous studies, many researchers took a flaw as a through flaw (2-dimentional model), but the flaws are not always through the whole rock mass in fact, most of them are only near the surface of rock mass, These are so named as surface flaws. They belong to three dimensional (2-D) flaws. Now, the reports on initiation and growth of 3-D surface flaw are few. So, for the investigation on growth patterns of 3-D surface flaw, a series of samples containing a surface flaw were carried out using frozen casting resin material at about -30°C temperatures. The surface flaw was made of a polyester film was used to model a single closed flaw on rock mass. The experimental results show that the wrapping wing crack (Mode I) initiated at the ends (or tips) of surface flaw first, and then formed a kinking zone (mixed crack zone) at a certain place at the middle of surface flaw region. Some petal cracks (Mode III) and shell-shaped cracks (Mode III) would grow at the middle place of flaw. A big fin crack (Mixed Mode) also emerged in middle of flaw and grown along loading direction. Finally, a team of large cracking curved faces deformed inside the resin specimen; the whole specimen would be splitted off by the initiation and growth of the cracks. The reasons lead to the fracture patterns of 3-D closed surface flaw were provided with brittle fracture mechanics theory in the article, preliminarily.

Abstract: Peening method using cavitation impacts have been developed. In this peening method, cavitation bubbles were generated by a cavitating jet. In order to increase peening intensity, a low speed water jet was injected around a cavitating jet, as the water jet swept away residual bubbles, which weaken the cavitation impact. In the present paper, improvement of fatigue strength of stainless steel by using a cavitating jet with an associated water jet was investigated compared with the results of conventional cavitating jet. It was concluded that the cavitating jet with an associated water jet can peen the stainless steel more effectively compared with the conventional cavitating jet.

Abstract: The effect of different continuous material parameter functions in the non-homogeneous region on the eigen-functions of crack tip fields is studied by means of the semi-analytical numerical method based on the wedge shaped non-homogeneous sector model. The cases of the different opening angles of the non-homogeneous region, the different dissimilar material ratios, the different configurations, etc. are analyzed systematically. The result shows that the effect becomes smaller as the opening angle of the wedge shaped non-homogeneous sector becomes smaller. The valuable conclusion is drawn that the differences among the eigen-functions corresponding to the cases can be neglected as long as the opening angle is small enough for the engineering application, which facilitates the engineering application of the non-homogeneous region model.

Abstract: Effect of mean stress on fatigue strength at N=107 of non-combustible magnesium alloy AMX602B(X=Ca) was investigated. Rotating bending fatigue test and tension-compression fatigue test were carried out on specimens with a small hole or crack. It was clarified that the fatigue strength at N=107 of the specimen with the small hole was about 30-150% higher than that of the specimen with the small crack within the range of σm=0~100MPa. This is the reason why the fatigue strength at N=107 of the specimen with the small hole can be not threshold condition for crack propagation but crack initiation. The fatigue strength at N=107 of the specimens with the small hole decreased within the range of σm=100~195MPa due to a static small crack initiating from the small hole at first loading. The effect of mean stress on fatigue strength at N=107 both oh the specimens with the small hole and crack could be estimated using modified Goodman diagram.

Abstract: In order to investigate the effects of testing frequency on the fatigue crack growth rate of 6061-T6 aluminum alloy in hydrogen gas environment, fatigue crack growth tests were carried out on specimens with small artificial holes in 0.18 MPa hydrogen gas or in 0.18 MPa nitrogen gas. It takes long time to test at low testing frequency, so in this study an accelerated test method was proposed and fatigue tests were carried out using this method. The fatigue crack growth rate in hydrogen gas environment accelerates compared with in nitrogen gas environment. The crack growth rate at lower testing frequency tends to higher.