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• The complex cracking mechanism of the blade of the 54H60 propeller system is investigated. • A mixed-mode fatigue crack propagation analysis is performed. • The structural reliability analysis supports the rapid liberation of the blade with the presence of IGC. Legacy propeller blades made of aluminum alloy are susceptible to fatigue cracking. Once the crack nucleated, it propagates rapidly because of the high rotational speed of the blade. This study uses mixed-mode fatigue crack analysis to understand the out-of-plane interaction of intergranular crack (IGC) and fatigue crack revealed in a U.S. Marine Corp. C-130 crash in 2017. The cracking mechanism is sequential: IGC followed by fatigue cracking, which eventually causes the liberation of the blade. A finite-element-method-based stress analysis elucidates the transition from IGC to fatigue crack. The result showed that the IGC provided enough driving force for the fatigue crack nucleation. A Monte-Carlo-simulation-based risk analysis is performed using the stress and crack growth analyses. The critical plane approach is utilized to determine the equivalent stress intensity factor for the crack growth analysis. The risk analysis results support the rapid liberation of the blade with consideration of large variation in initial crack size. The results also clearly show the necessity of structural risk analysis for planning inspection and maintenance. [ABSTRACT FROM AUTHOR]

A drill collar with material of 0Cr20Mn21Ni2N serviced in a coastal continental shelf oilfield, has cracked and leaked after a short period of working. Failure analysis of drill collar was performed by a series of experiments, including Chemical analysis, Fractographic analysis, Microstructural analysis, Intergranular attack test, Mechanical property, Residual stress test, EBSD analysis of relationship between crack propagation and crystal orientation. The fractographic observations showed crack initiated from the inner surface and extended to outer surface by the mode of inter-granular and quasi-cleavage trans-granular fracture, many branch cracks were produced during its propagation. Serious corrosion morphology was found both on the inner surface of drill collar and fracture surface, and S and Cl were detected in the corrosion products. The above evidence indicates that the cracking failure mode of drill collar can be described as the Stress Corrosion Cracking (SCC) of austenitic stainless steel. The causes of stress corrosion cracks are attributed to three aspects. Firstly, the microstructures of the materials are sensitized seriously, and there are many precipitated phases at grain boundaries, which result in chromium depletion in the grain boundaries and decrease of corrosion resistance of the materials. Secondly, during service the mud water on the inner surface of drill collar is composed of seawater containing chloride and sulfate radical, which provides corrosive environment for materials. Thirdly, the residual tensile stress on the inner surface of the drill collar reaches 112 MPa, which hinders the formation of passive film on the surface of stainless steel and has adverse effect on the corrosion resistance of the material. In addition, the mechanism of crack formation was studied by EBSD. Corrosion pits of inner-surface of drill collar is intergranular corrosion, which first occurs at random grain boundaries with high misorientation angles, rather than at special grain boundaries such as Σ3. SCC crack initiates from the corrosive pit and propagate along the grain boundary first and gradually transit to transgranular growth with Zigzag fracture morphology along {111}〈110〉 slip system, which proves SCC transgranular crack propagation obeys the slip corrosion mechanism. • The failure for stainless steels of Fe-Cr-Mn-N system has been seldom reported by literature. • The failure is attributed to sensitized microstructure, corrosive environment, and residual tensile stress. • The mechanism of SCC formation is also studied, which proves crack propagation obeys the slip corrosion mechanism. [ABSTRACT FROM AUTHOR]

The influence of pit shape and size on local stress concentration in the tensile specimen and the stress corrosion cracks inception was studied by employing the element remove technique. The maximum stress located in the bottom of pit on FV520B tensile specimen. The location of maximum strain was near the mouth of the pit or the shoulder and plastic strain existed in this region. Stress concentration factor and plastic deformation on four different geometrical shape pits of hemisphere, semi-ellipsoid, bullet and butterfly were numerically investigated, respectively. The simulation results showed that butterfly pit got the biggest stress concentration factor. The plastic strain rate during pit growth was in the sensitivity range of stress corrosion cracks inception, indicating that stress corrosion cracks were more likely to nucleate near the pit tip or the shoulder. [ABSTRACT FROM AUTHOR]

The strengthening mechanism and stress corrosion cracking (SCC) behavior of a new Mg-6Zn-1Y-0.5Cu-0.5Zr alloy (given the name "ZWCK6100″) were investigated in this work. The results showed that the microstructure, mechanical properties and stress corrosion resistance were significantly affected by the combined addition of Cu and Y. The alloy consisted of α-Mg, Zn 2 Zr, CuMgZn, rod-shaped MgZn 2 precipitates and I phase (Mg 3 Zn 6 Y), and displayed a balance between strength and ductility, with the yield strength, ultimate tensile strength and elongation being 320.3 MPa, 351.5 MPa and 19.8%, respectively. The main strengthening mechanisms were found to be grain refinement, second phases and texture. In addition, the stress corrosion resistance was assessed by slow strain rate testing and fractography. These tests showed that ZWCK6100 alloy exhibited improved SCC resistance and low SCC sensitivity in 3.5 wt% NaCl solution. The novel ZWCK6100 alloy may serve as a promising magnesium alloy for industrial applications. • A novel ZWCK6100 alloy was developed by combined additions of Y and Cu. • ZWCK6100 alloy exhibited an excellent balance of strength and elongation. • The alloy also displayed low stress corrosion cracking sensitivity in 3.5 wt% NaCl solution. • The alloy may serve as a promising magnesium alloy for industrial applications. [ABSTRACT FROM AUTHOR]

Abstract: Corrosion behavior of cold-drawn, stress-relieved and as-annealed ZnAl15 alloys in hot and humid environment was investigated. The results show that stress corrosion crack of ZnAl15 alloys occurred in hot and humid environment and the main corrosion products was ZnO with a small amount of Al(OH)3. The as-annealed and stress-relieved ZnAl15 alloys resulted in better corrosion resistance than the cold-drawn, especially the as-annealed. It implies that the residual stress from cold-drawing and the phase transformation stress from decomposition of supersaturated solid solution accelerated corrosion together during corrosion process. [Copyright &y& Elsevier]

Abstract: This paper presents the applicability of induced current potential drop technique to visualize surface morphology of multiple and/or branched cracks. Measurements of potential drop (PD) were performed around intergranular stress corrosion cracks. Data analysis software to extract crack locations from the PD data was developed. The usefulness of the software was investigated by comparing the analyzed results with a photo of the measured crack. The analyzed crack morphology showed good agreement with the photo of the crack though small crack located near greater cracks was not extracted. Moreover, a small crack, which was difficult to be identified from the photo of the crack, was identified with this software. [Copyright &y& Elsevier]

Abstract: In this paper the influence of high-temperature pre-precipitation treatment on corrosion behaviors of 7055 aluminum alloy is investigated. High-temperature pre-precipitation treatment enhances the discontinuing distribution of the coarse precipitates along the grain boundary, and increases the Cu content of grain-boundary precipitates, which obviously improves stress corrosion resistance, suppresses the serve fracture of sub-grain boundary under the stress corrosion and reduces inter-granular corrosion (IGC) susceptibility with the same strength and ductility. The electrochemical corrosion results of inter-granular phases indicate that the Cu-rich, discrete and coarse grain-boundary precipitates and the decreasing potential difference among inter-granular phases in the grain boundary are responsible for the enhancement of corrosion resistance of 7055 alloy. [Copyright &y& Elsevier]

Abstract: Recent studies on stress corrosion cracking properties of 321 bend pipe and pipe with weld, 316NG austenitic stainless steel pipe in ocean environment in virtue of slow strain rate tensile stress corrosion crack test are performed in different concentration of NaCl solution. Elongation and max tensile stress are selected as parameters to evaluate the SCC susceptibility based on the phenomenon that elongation and max stress fall as samples being exposed to NaCl solution environment, the results show that 316NG has the best stress corrosion crack resistance and 321 bend pipe materials behavior oppositely. In addition, the influence of differing strain rate and concentration of NaCl solution on properties of materials are researched, elongation increases along with tensile strain rate enhancement, mechanical properties are immune to variety of Cl- concentration from 3.5% to 26.5%. [Copyright &y& Elsevier]

Abstract: SCC (Stress Corrosion Crack) is a key issue in the nuclear industry of piping section. When weldingthe pipe, heat input occurs to the trend of thermal stress. And heat input affected residual stress to the pipe.In this study, the effect of residual stress for the SCC in the piping was determined.Experimental method is four kinds of specimens(1 pass, 5 pass, 10pass, 15pass) are produced toSCC. ANSYS codeanalyze the temperature gradient forthe specimen. Using the temperature gradient analyzedtothe residual stress in the pipe. According to this result, ascertain the effect of residual stress to SCC [Copyright &y& Elsevier]

Abstract: During the last couple of decades, lots of researches on structural integrity assessment and leak rate estimation have been carried out to prevent unanticipated catastrophic failures of pressure retaining nuclear components. However, from the standpoint of leakage integrity, there are still some arguments for predicting the leak rate of cracked components due primarily to uncertainties attached to various parameters in flow models. The purpose of present work is to suggest a leak rate estimation method for thin tubes with artificial cracks. In this context, 23 leak rate tests are carried out for laboratory generated stress corrosion cracked tube specimens subjected to internal pressure. Engineering equations to calculate crack opening displacements are developed from detailed three-dimensional elastic-plastic finite element analyses and then a simplified practical model is proposed based on the equations as well as test data. Verification of the proposed method is done through comparing leak rates and it will enable more reliable design and/or operation of thin tubes. [Copyright &y& Elsevier]

Abstract: Degradation of polymers is usually manifested in a reduction of molecular weight, increase of crystallinity in semicrystalline polymers, increase of material density, a subtle increase in yield strength, and a dramatic reduction in toughness. Stress corrosion cracking (SCC) results from strongly coupled thermo-mechano-chemical processes, and is sensitive to material composition and morphology. The individual crack propagation stage is critical in determining the lifetime of pipe. Based on author''s previous works, crack layer (CL) theory model is adopted in this study to describe the individual stress corrosion (SC) crack propagation kinetics and the time interval from crack initiation to instability and break through. The effect of localized chemical degradation at the crack tip on SC crack growth kinetics is addressed. Typical SC crack growth is presented and discussed as a step-wise manner based on the proposed model. In addition, scanning electron microscopy (SEM) observation and Fourier transform Infrared spectroscopy (FTIR) analysis of failed samples obtained by accelerated SCC tests are applied to validate the proposed model. SEM is useful to identify the change of fracture mechanisms from chemically driven crack to mechanically driven crack by the formation of visible striations. FTIR analysis enables tracking of the accumulation of chemical degradation by detecting the amount of carbonyls on the crack surface. Carbonyl index is defined to compare the amount of chemical degradation quantitatively. The purpose of this paper is to continue to develop the technical theory and understanding behind SCC phenomena to facilitate all polymer pipe industries and in particular the polyethylene pipe industry to design better resins and piping systems. [Copyright &y& Elsevier]

Abstract: Discovery of stress corrosion cracking (SCC) in the blade attachment region of nuclear power plant low-pressure steam turbine discs leads to industry concern about this problem. Reliable inspection methods for the detection of SCC in steam turbine blade attachment area are essential to the rotor run/repair/retire decision making process. The conventional broad beam ultrasonic inspection technique usually has inadequate resolution for flaw sizing and tendency for false calls. But the phased array technique by improved detection capability and resolution could increase coverage of the examination areas with less scanning required. Using phased array ultrasonic technique to straddle mount-type low-pressure turbine disc in nuclear power plant located in Korea was studied in this paper. [Copyright &y& Elsevier]

Abstract: In general, a crack found in a steam generator tube has an elliptical shape, and as soon as it penetrates through the wall, it is assumed to be rectangular for conservatism. However, the leak and crack growth behavior have not been clearly understood after the elliptical crack penetrates the tube wall. One steam generator tube failure showed the tube rupture behavior without any significant prior indication. Several experiments using steam generator tubes with a trapezoidal crack were performed under constant pressure loading by Argonne National Laboratory and exhibited crack growth behavior much faster than expected. It has been found to be caused by time-dependent fatigue crack growth. No K I solution is available for a trapezoidal crack which can be used for fatigue crack growth analysis. The objective of this study is, therefore, to develop a new K I solution for a trapezoidal crack using FEM. [Copyright &y& Elsevier]

Abstract: A partial-contact stress corrosion crack (SCC) is electrically modeled as a crack region with non-zero conductivity in eddy current testing (ECT). This partial-contact effect is excluded by an optimally designed crack-conductivity-insensitive depth characterization signal function (DCSF), and consequently the master curves obtained from electric-discharge machining (EDM) notches can be utilized directly in the depth sizing of SCCs. Furthermore, a crack conductivity independent artificial neural network (ANN) is constructed so that the entire depth profile can be reconstructed regardless of the crack conductivity. These two approaches are numerically validated and applied to the characterization of SCCs in SUS304 from measurement ECT signals. The average depth of each SCC is fast estimated from the DCSF, and the detailed depth profile is reconstructed from ANN. The ECT depth-sizing results show reasonable agreement with UT-TOFD measurement. [Copyright &y& Elsevier]