查看更多>>摘要：The temperature-related evolution law of friction and wear behaviours of copper-based powder metallurgy friction block on forged steel brake disc under extreme braking conditions are studied. The results indicate that the friction and wear behaviours of the friction block have certain randomness with temperature rise, but the overall trend is definite. The friction coefficient at room temperature is 0.33, starts to rise when the temperature exceeds 170 degrees C, reaches its maximum value of 0.43 at 240 degrees C and remains stable until 400 degrees C. After that, the friction coeffi-cient starts to decrease significantly. The wear rate starts to increase significantly when the temperature reaches about 100 degrees C. When the temperature reaches 500 degrees C, the wear rate increases by an order of magnitude compared with that at room temperature. When the temperature rises from room temperature to nearly 500 degrees C, the wear mechanism gradually changes from abrasive wear to oxidative wear, and finally to adhesive wear.
查看更多>>摘要：Premature failures of TRB in wind turbine remain a significant contributor to high operation and maintenance costs. The analysis of contact characteristics is the premise of the TRB failure investigation. In this paper, a novel five degrees of freedom (5DOF) TRB model which considers bearing clearance and preload, is established. Instead of the Newton-Raphson method, the meta -heuristic optimization is adopted to solve the nonlinear system by the L-2-norm minimization control. This improvement can intelligently judge the load region of TRB, avoid the selection of initial values and improve the convergence speed. The load distribution results are compared to the simulation results of the finite element model, Romax model and Harris model to verify the effectiveness of the proposed model. Besides, the proposed method has a higher efficiency than the existing methods. Finally, the influences of bearing clearance and preload on the contact characteristic and fatigue life are discussed.
查看更多>>摘要：This work addresses the free and nonlinear vibrations of the internal gear transmission. A rigid-elastic coupled model is established using Hamilton's principle and the nonlinear in-extensional assumption, where the cracked time-varying mesh stiffnesses regarding torque-dependent and geometric nonlinearity caused by the heavy torque on the elastic ring gear are considered. Based on the coupled model, the natural frequencies and vibration mode shapes are given, and the distinctive modal properties associated with the torque are investigated by comparing with a rigid model. The differences of the nonlinear behaviors between the two models are presented through bifurcation diagrams, the largest Lyapunov exponent (LLE) diagrams, phase diagrams, Poincare maps, time histories, and FFT spectra. The effects of parameters such as excitation frequency, torque, and crack depth on the dynamic responses are examined. The results show that the nonlinear behaviors are rich, and quasi-periodic motion replaces periodic-1 motion when the elasticity of the ring gear is considered. The dynamic responses indicate that the larger crack depth becomes, the more obvious the sidebands can be found in the frequency domain. As for lower excitation frequency, the main frequencies also include the mesh, elastic vibration, modulation, and natural frequencies.
查看更多>>摘要：In most metal structures, corrosion has been recognized as the main cause of failure, and fracture often occurs in structures subjected to corrosion, e.g., metal pipes. Since the evolution of a corrosion defect into a crack takes the majority of a structure's lifespan, accurate quantification of this transition limit state is significant in assessing the probability of failure. This paper aims to propose a methodology for predicting the occurrence of a crack in corroded structures based on the Physics-of-Failure (PoF) theory. The Stress Intensity Factor (SIF) concept and the J-integral-based finite element analysis are used to derive the solution of the SIFs in the corroded structure. A cast iron pipe with a crack-shape corrosion defect is taken as an example. A model for estimating the SIF for a corrosion pit with high aspect ratio has been proposed. The evolution of SIF is modeled by a stochastic process, and an advanced time-dependent method (i.e., upcrossing method) is used to quantify the probability of failure. It is found that combining the concept of Physics-of-Failure with time-dependent reliability method can provide an effective tool for predicting the failure probability of corroded structures. It is also found from the sensitivity analysis that the internal pressure has the highest effect on the probability of failure, followed by the corrosion pit depth. The results from this paper offer a necessary basis for safety and reliability analysis of corroded metal pipes, providing asset managers a cost-effective tool in planning maintenance works.
查看更多>>摘要：The failure analysis of 316H steel welded joints in sodium-cooled fast reactors at 550 ? under creep-fatigue load with different hold times has been systematically investigated through this work. For the first time, the reason for the difference of cracking failure mechanism between base metal and weld metal was illustrated from the perspective of microstructure evolution. Under various hold times, different damage mechanisms led the fracture surfaces of the samples to exhibit different degrees of tunneling. Meanwhile, due to the existence of a delta-ferrite network in the weld metal, the cracks were deflected, resulting in higher resistance to crack propagation and a more tortuous crack propagation path than those in the base metal. In addition, the nucleation and growth of voids at the grain boundaries were the primary reasons that base material exhibited intergranular crack propagation, and the evolution of ferrite in the weld was a critical reason for the formation of intergranular pores in the welded metal.
查看更多>>摘要：It is well known that the deformation and failure during the slope excavation often depends on the excavation height and slope rate. Based on the theory of loading and unloading response ratio, the present study established an evaluation model of the excavation dynamic load-increasing displacement response ratio and created the instability criterion. During the excavation, the stability law of the Chaqishan ancient landslide was analyzed using the Midas GTS NX finite element software, and four slope excavation schemes were redesigned. The results show that, the variation law of the excavation dynamic load-increasing displacement response ratio was consistent with the evolution law of stability coefficient, and there was an obvious negative correlation between them. Therefore, the stability variation law in the slope excavation could be evaluated using the evaluation model of excavation dynamic load-increasing displacement response ratio.
查看更多>>摘要：The perforation failure occurs frequently in the pipeline system of the atmospheric distillation tower. In this paper, the methods of technological process and computational fluid dynamic (CFD) were used to reveal the failure reason of the reducing pipe and provide theoretical guidance. The results of X-ray diffraction analysis shows that the corrosion products of circulation line reducing pipe is FeS and other corrosive substances. The corrosion mechanism was studied and the process was described. Besides, it was found that the valve in the circulation line was not fully opened in the early maintenance. The CFD result shows that the increase of the valve opening will aggravate the corrosion risk of the reducing pipe bottom wall.
查看更多>>摘要：In this paper, investigations into the premature failure of gearbox overdrive gears were done in three passenger car models. The premature failure occurred when a replaced gear model was used instead of the original gear. A series of experiments were carried out on gears made by two manufacturers, including failed gear after an average operation of 100,000 km and gear with partial failure but usable with an average operation of 200,000 km. Numerical and experimental results showed that sharp hardness slope (reduction of 300 HV in 0.15 mm), high porosity, and Hertz stress distribution of about 90% of the allowable contact stress sigma Ha are the factors affecting premature failure. In addition, replacement gear samples with low density compared to original gear have low yield and ultimate strength due to high porosity and interconnected structure of pores. Pores with sharp corners act as the initiation points of the cracks, resulting in final fracture.
查看更多>>摘要：Aiming at the deformation and fracture of slope elevator caused by the cyclic alternating load in service, analyzes and studies the failure reasons and mechanisms from microstructure, physical and chemical properties, and service state. The microstructure of the elevator was lower bainite that preserves the intact of the laths orientation, with high P element content, a certain amount of low harmful low-melting-point elements, and presence of D-type non-metallic inclusions, led to low impact toughness. It indicated that failure of the elevator can be attributed to dual effects of load and improper heat treatment, caused dislocation glide to occur in the crystals, eventually resulting in a mixed-crystal-type brittle fatigue fracture. It was advised to strictly control the content of trace elements, rationally optimize the heat treatment process, and select micro-structure with good resistance to fatigue impact and alternating load, to improve the deformation resistance and critical fracture toughness of the elevator.
查看更多>>摘要：Field observations demonstrate that rapid unloading can induce intensive dynamic failure of a tunnel in a jointed rockmass. In this study, we demonstrate the characteristics of the dynamic failure and establish a series of validated discrete element models to investigate the mechanism of the dynamic failure. Our results indicate that the excessive release of strain energy and the complex nonlinear behavior of joints together resulted in the dynamic failure. Primarily, joints can enlarge the unloading disturbance zone (UDZ) and lead to the excessive release of strain energy, especially for a model with smaller joint spacing and higher unloading rate. Secondly, joints can significantly decrease the dependence of the dynamic effect on the unloading rate and expand the unloading time threshold that can induce the dynamic effect, more importantly, the nonlinear variation of joint stiffness not only induces vibration localization but also increases vibration amplitude and reduces vibration frequency.
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Elsevier