Nguyen, Thanh TuanYoon, Kee BongPark, JaeyeongBaek, Un Bong...
20页
查看更多>>摘要：The strain-controlled low-cycle fatigue behavior of P91 steel is investigated under various strain amplitudes ranging from 0.20% to 1.0% at 25, 538, and 566 degrees C. The fatigue life decreases with an increase in the test temperature; however, a clear reduction is observed at low strain amplitudes, resulting in the degradation of the fatigue limits. The amount of cyclic softening increases with an increase in the strain amplitude and is significantly higher at elevated temperatures. Based on experimental results, Basquin-Coffin-Mason models are established to express the relationship between the strain amplitude-fatigue life curves and the cyclic stress-strain. The transition fatigue life increased from 6677 reversals at 25 degrees C to approximately 12,000 reversals at the elevated temperatures of 538 and 566 degrees C. The more pronounced effect of temperature at the very low strain amplitude condition is attributed to the occurrence of multiple oxidation-enhanced crack initiations after long-term exposure to elevated temperatures.
查看更多>>摘要：Grey model has been several applications on fatigue crack, with prediction on fatigue crack growth life curve often serving as milestones. A typical difficulty in predicting the fatigue crack growth life curve is the retardation of crack growth caused by the overload effect. Overload retardation, resulting in the lack of experiment data points in the retardation interval and the deflection of crack growth curve, is a long-term challenge for predicting fatigue crack growth. We improve the ability of grey model to predict fatigue crack growth life by second order B-spline interpolation, an interpolation for local data points. It combines the trend of local data point to deal with the retardation interval, and interpolates multiple data points in the retardation interval when there is a lack of local data, and uses a metabolic grey model to construct the prediction formula of crack growth. In the study involving variable amplitude load and multiple overload fatigue crack growth experiments, the proposed method is proved to improve, with statistical significance, the predictive ability on the whole range of experiment data. The method is simple and accurate. Consequently, it is conducive to solve the problem of fatigue crack growth life of engineering structure.
查看更多>>摘要：This study was focused on the conical gaskets in ultra-high pressure pipeline sealing devices. Failure analysis of the conical gaskets was carried out by combining gasket experiments and numerical simulations. Macroscopic dimension inspection, miniature specimen tensile tests, micro-hardness tests, metallographic tests, and scanning electron microscope (SEM) experiments were carried out to study the differences between used gaskets and unused gaskets. Results show that the mechanical properties of the used gaskets degenerate and microstructural changes occur, which is caused by frequent pressure fluctuations in the working environment of high temperature and high pressure. Besides, the stress distribution and deformation of the gasket, as well as the effect of pressure fluctuation on the gasket stress, were analyzed by numerical simulation. Meanwhile, an improved layer-variable parameter numerical model is proposed for the numerical simulation of the used conical gaskets. The model was used to compare the elastic resilience of the gaskets before and after service under different preload, respectively. Results show that the simulated results by the model is closer to the experimental results. The elastic resilience of the used gaskets is significantly less than that of the unused gaskets. Therefore, it can be concluded that the mechanical properties degradation and accumulation of plastic deformation of the gaskets caused the lack of the elastic resilience of the gasket, which may be the direct cause of the seal failure.
查看更多>>摘要：During service, thermal barrier coating will generally go through a series of complicated nonlinear phenomena. In this work, a constitutive framework is provided for the sintering and creep of ceramic top-coat under thermal cycling. The influence of the top-coat sintering and creep was investigated using a three-dimensional thermal barrier system model. It is found that the topcoat sintering gives rise to an obvious reduction in the low-temperature out-of-plane stress within the top-coat but hardly has an effect on the high-temperature out-of-plane stress. The bond-coat/ TGO interface is subjected to significant normal stresses upon cooling to ambient temperature, which have greater magnitudes because of the top-coat sintering. Nonetheless, the top-coat creep has very limited effects on the out-of-plane stress within the top-coat and the normal stress across the bond-coat/TGO interface. The cohesive damage model was used to evaluate the delamination of bond-coat/TGO interface under thermal cycling. It is found that the interface delamination could promote the out-of-plane tension of the top-coat and then accelerate the cracking of the topcoat. A static crack is preset in the top-coat to analyze the influence of top-coat crack on the interface delamination. It is found that the top-coat crack hardly affects the normal stresses across the bond-coat/TGO interface and therefore is less likely to facilitate the interface delamination.
Akram, FarooqHameed, AsadNawaz, AliKhan, Haris Ali...
13页
查看更多>>摘要：This study contains the analysis of a cylinder block assembly of an aircraft brake system that failed due to the formation of cracks. The failure analysis was performed using a two-prong strategy (i) experimental, which included the analysis of the fracture surfaces along with microstructure and composition, and (ii) numerical (finite element analysis, FEA), to find the stress state. Visual inspection revealed relatively long cracks emanating from a sharp corner (junction) which traversed 87 mm in one direction and 52 mm in another. At a mirror-symmetric location, another smaller crack (similar to 16 mm) revealed the initial growth stage. During detailed fractography of the region containing long cracks, clear beach marks indicating progressive failure were observed pointing towards the same sharp corner. Microstructure and compositional analysis revealed the cylinder block assembly material to be heat treated Al 2024 having no inherent nonconformities. The hardness was also found to be conformant to the typical of the Al 2024. The stresses are produced as a consequence of (a range of) hydraulic pressures acting on its pistons which were estimated using FEA. The stress was confirmed to be concentrated (having K-t similar to 2.25) at the same junction(s) as identified during fractography. Due to the cyclic nature of the applied loads, the resulting stresses initiated the fatigue crack(s) at the stress concentration locations (junctions). Fatigue life calculation using unnotched SN diagram (K-t = 1, R similar to 0)) showed that failures could potentially occur between 2.83 E5 to 1.12 E5 load cycles, depending on the value of stress amplitude, and it matched with the estimated life in our case. Thus, a combination of cyclic brake applications and the presence of stress concentration locations resulted in fatigue crack initiation and propagation leading to the ultimate rejection of the cylinder block.
查看更多>>摘要：Hydraulic actuators are an important executive component of modern aircraft and are typically characterized by a reciprocating sealing form. Sealing failure affects the implementation of aircraft missions and flight safety. To determine the main cause for sealing failure of the ACGT-HP combined rod seal in an aircraft actuator, taking the new seal as the control group, compare and analyze the failure and non-failure combined rod seals under the same working conditions for approximately the same time. By means of microscopic examination, physical performance testing and chemical composition analysis, eight ACGT-HP seal failure reasons that act syner-gistically are summarized and preventive improvement measures are proposed. It is concluded that the main reason for the failure of the combined seal is that the wear of the seal is exacerbated by various inappropriate coupling factors, resulting in the rapid reduction of contact stress and seal failure. By putting forward the key points of seal design and assembly, a universal failure analysis method from macro to micro is found, which can be used for various types of combined seals to guide the design and application of the ACGT-HP seals.
Ivorra, SalvadorTorres, BenjaminCarcel, Alfonso C.
15页
查看更多>>摘要：This paper describes the collapse analysis of the historical El Jaume bell, cast in 1429, one of the bells in the Cathedral of Valencia's Micalet bell tower (Spain). In 1992 the yoke was completely restored and given a new steel axis. The failure occurred when the bells were being rung on Christmas Day 2014, fortunately without any casualties. After several field visits to assess the damage and failure "in situ", a diagnosis was made of the causes of the failure. The diagnosis was the result of (1) a detailed visual inspection, (2) a metallographic and fractographic analysis of the fractured steel axis, (3) a fatigue damage analysis by finite element modelling software and (4) an estimation of the bell's remaining fatigue life cycle using Wholer S-N curves. In view of the results obtained it was concluded that the fatigue life cycle should have been reached between 300,000 and 400,000 cycles, while the failure occurred at 346,500 cycles. Although these elements are usually designed to not fail during its life, they can sometimes suffer cracks during cyclic ringing. One of the main causes of the fatigue failure was the incorrect design of the steel gudgeon, which produced a high stress concentration and drastically reduced the fatigue strength at 10(6) cycles. The paper also describes the remedial measures taken to avoid fatigue failure in the other bells and the lessons that have been learned from the experience.
查看更多>>摘要：Side perforated nozzles reducing diameter, subjected to variable-pressure fluids, become vulnerable components, during controlling the fluid direction and dispersion in industrial spray equipment. The damage behavior of certain 405 stainless steel nozzles was evaluated with the proposed damage model, self-excited flow parameter fluctuations causing comb-tooth damage model. The nozzle inner bottom and adjacent wall of the diversion cone were expanded as a nearly-circular section groove with dense parallel micro-ditches and an adjacent bright transition belt above the expanded groove. The investigation suggests a separation design of the oblique diversion deflecting and section reducing for avoiding this model damage.
查看更多>>摘要：Solid expandable tubular (SET) is plastically formed in the well, it needs to undergo the double test of internal and external pressure. The collapse resistance of the SET after expansion is an important guarantee for the wellbore structural integrity reconstruction. In addition, due to the diversity of the material of the SET and the complexity of plastic forming, there is no unified formula for calculating the collapse strength of the SET. Aiming at the above problems, this paper builds a three-dimensional dynamic collapse strength calculation Finite element model (FEM) based on the collapse experiment of SET, and analyzes the influence of expansion rate, wall thickness and eccentricity on the collapse strength of expansion pipes, and fits the theoretical calculation formula of collapse strength. The research results show that: the expansion rate causes the collapse performance of the pipe after expansion to be reduced by 15% to 30%; the wall thickness has almost a linear effect on the collapse performance; the greater the eccentricity, the lower the collapse strength, the more likely it is to " Concave ". Through data fitting, a theoretical prediction formula for the collapse strength of SET with a material of 20G and a diameter-to thickness ratio of less than 30 is obtained. The formula is simple and easy to calculate using parameters. The research results in this paper provide theoretical guidance and methods for the study of the collapse strength of SET.
查看更多>>摘要：A carbon steel octene tube has leaked from the fusion zone and caused economic losses. The failure process and mechanism were investigated in this work, by means of scanning electron microscope, electron backscattered diffraction, energy-dispersive X-ray spectroscopy and Scanning Kelvin probe force microscopy. Results showed the failure was attributed to caustic corrosion cracking. The cracking mechanism is anodic dissolution mechanism, including film rupture, intergranular corrosion and repassivation processes. A less corrosion-resistant chemical composition, a larger residual stress after welding and a higher content of hydroxyl ions synthetically resulted in the cracking synthetically. Hydroxyl ions firstly reacted with passive film and caused initiation of intergranular cracks. Residual stress and intergranular corrosion result in an inter granular propagation of SCC cracks.
NSTL
Elsevier