Papers by Keyword: Intergranular Cracking

Abstract: The 7000 series alloys have the highest strength in the aluminum alloys, but lower fatigue properties than 2000 series alloys. Thus, 7000 series alloys are not applied to a large proportion of the aircraft components. However, the mechanism for this has not been elucidated yet. In humid air, hydrogen embrittlement based on intergranular cracking has been known to occur in 7000 series alloys. To date, in order to explain the difference in the fatigue crack growth behavior in the two series alloys, the effect of the test environment on the fatigue crack growth of the two series alloys has been investigated in this study. A 7075-type as well as 2024-type alloy with relatively coarse equi-axed grains was T6-tempered and subjected to fatigue crack growth test in humid and dry environments. Crack growth rate at low ΔK level seemed to be larger in the 7075-type alloy than the 2024-type alloy in the humid air, when assessed by means of gradually decreasing K method. In order to clarify this result, crack growth rate of the two alloys was assessed by means of gradually increasing K method as well as decreasing K method. Crack growth rate of the 7075-type alloy in moist air was concluded to be the largest in consistent with the previous study. Thus, the large fatigue crack growth rate of the 7075-type alloy is attributable to hydrogen embrittlement.

Abstract: The effects of applied stress and test temperature on the environment-induced cracking (EIC) behavior of carbon steel in 5 kmol/m3 NH4NO3 solution were investigated by using constant load method. The three parameters (time to failure (tf), steady state elongation rate (lss) and transition time to time to failure ratio (tss/tf)) were obtained from a corrosion elongation curve. The relationships between applied stress and the three parameters were divided into three regions; the stress-dominated, the EIC-dominated and the corrosion-dominated regions. Under an applied stress of 225 MPa (in the EIC-dominated region), the test temperature dependences of the three parameters were investigated. Although tf decreased with increasing test temperature above 323 K, it became constant independent of test temperature under 323 K. Whenever the EIC takes place, the relationship between log tf and log lss become a good straight line with a slope of -1.1 irrespective applied stress and test temperature, which means that lss becomes a parameter for predicting tf. In addition, the fracture appearance in the EIC-dominated region was intergranular. From the results obtained, the intergranular cracking of carbon steel was discussed in terms of film formation, dissolution and hydrogen permeation.

Abstract: In order to estimate the long life integrity of vessel steels with considering various material compositions and irradiation conditions, it is necessary to understand physical mechanisms of the degradation of mechanical properties. In this research, chemical composition effects were investigated for Reactor Pressure Vessel Steels (RPVS) to apply small specimen test technique to surveillance test method. All specimens used in this study were machined from the A533B plate material, which are standard, low Mn, high Cu, high P, and high Cu and high P steels. Tensile strength is increased by phosphorous and copper additions. Charpy tests were carried out at temperature from 73 K to 473 K. The ductile to brittle transition temperature (DBTT) is shifted to higher temperatures with phosphorus additions accompanied by the reduction of the upper shelf energy (USE). The fracture mode of P-added A533B steels at temperatures in the lower shelf energy (LSE) region is intergranular cracking. Test results were discussed in view of the differences on elements of Cu, Mn and P.

Abstract: The SSRT behavior in hydrogen dissolved hot water was investigated for cold worked SUS316L at a strain rate of 5 x 10-7/sec. The cold work to 75% thickness reduction of the as-annealed steel resulted in the hardness increase from 150 HV to 420 HV. The tensile yield stress of the cold worked specimens (CW=75%) was about 1000 MPa and the total tensile elongation was significantly reduced from 0.8% of annealed specimen to 0.14% of the 75%CW specimen. The results of EPR tests on SUS316L steel indicated that the EPR-DOS increased with increasing sensitization period at 700°C and decreased with increasing degree of cold work or reduction in thickness. In the water with hydrogen dissolution of 0.4 ppm, many IGSCC type cracks were nucleated on the specimen side surfaces, while the fractured surface was almost TGSCC. No such a SCC as observed in hydrogen dissolved water was observed after the test in oxygen dissolved water. The susceptibility to SCC increased with increasing hydrogen content in hot water. Cold work caused the reduction of the number of surface cracks and disappearance of IGSCC.

Abstract: A model for intergranular damage propagation in polycrystalline materials is proposed, based on Markov Chain theory, Monte Carlo simulation and percolation concept. The model takes into account crack branching and coalescence, multiple crack nucleation sites, crack-turning etc., as well as the effect of grain boundary plane orientations with respect to the external stress direction. Both honeycomb and voronoi microstructures were utilized as the input microstructures. The effect of multiple crack nucleation sites has been found to have great influence on the crack propagation length. It has been observed that percolation threshold reported in the literature based on hexagonal microstructure is not applicable when the effect of external stress direction on the susceptibilities of grain boundaries is considered. The successful integration of voronoi algorithm with the Markov Chain and Monte Carlo simulations has opened up the possibilities of evaluating the intergranular crack propagation behaviour in a realistic manner.

Abstract: Fatigue damage behavior under repeated electric loading was investigated on two kinds of PZT ceramics with discontinuous electrodes. Intergranular cracking was observed at the electrode edge in soft PZT under electric fields greater than ±400 V/mm. However, under the same loading conditions, no damage was observed in hard PZT. When cracking occurred, permittivity of specimens decreased with the number of cycles corresponding to the amount of mechanical damage. FEM analyses of the electroelastic field of the specimens showed that cracking due to cyclic electric loading was related to 180˚ domain switching caused by concentrated electroelastic field.

Abstract: Interstitial free (IF) steels are widely used as thin sheet in the automobile industries because of their many favorable properties. Although, fatigue properties of IF steels do not have significant importance to auto body makers, however, they are very concerned about the tensile and fatigue strength of the steels used for structural purposes to ensure safety of passengers. So, fatigue results of this steel might help researchers to understand the behaviors of high strength steels. In this study cyclic and static properties have been studied at room temperature in the air. Initiative has also been taken to observe the fatigue fracture morphology of this steel. Experimental results show that the fatigue limit is corresponding to about 40% of tensile strength and 80% of the yield strength of this steel. Fractographic observations reveal a mixed type of fracture mode (intergranular and transgranular cracking) fractures.