查看更多>>摘要：Rectangular reinforced concrete (RC) culverts are widely used in drainage systems of many megacities, which may experience significant deterioration due to the corrosive sewage environment. How to assess the performance of existing RC culverts is crucial to the public safety. Currently, very few studies have been reported on the assessment of the performance of deteriorating RC culverts which are in service. This paper reports the research work on the performance assessment of a drainage culvert in Shanghai, China, which is one of the most populous megacities in the world. Field investigations are first conducted to examine the deterioration of drainage culverts in the real world. The deterioration of the culvert is modeled through a stochastic model, which is calibrated based on field investigation data. The performance of the culvert is then examined through the local design code, which seems to be overconservative for ultimate bearing capacity assessment. To avoid the conservative assumptions in the design code method, a high-resolution finite element model is built to assess the performance of the culvert. During the assessment process, the uncertainty associated with the deterioration is also considered. Based on the findings from the research, recommendations are made regarding the maintenance of the drainage culverts. The research reported in this paper may also provide useful reference for assessing the performance of deteriorating culverts in other regions of the world.
查看更多>>摘要：Mode shape-based method has shown its dominance in failure identification of beams. However, it is a challenge for fault detection in a plate structure. It requires combined information in two directions to determine the damage location. In this study, a damage index, namely mode shape derivative based damage identification (MSDBDI), is applied to localize damage in fixed-free plate structures. Two-dimensional (2D) displacement mode shapes and their derivatives are used to identify the MSDBDI index. It is a fact that this indicator is stuck in indicating the damage severity. Hence, a coupled model between an artificial neural network (ANN) and antlion optimizer (ALO), so-called ALOANN is used to overcome this drawback. In this method, ALO instead of a backpropagation algorithm is used to look for the best initial values of learnable parameters of ANN i.e. weights and biases through mean squared error (MSE). These obtained parameters are added to ANN for damage identification. The efficiency of the proposed approach is tested with two numerical studies of the plate structures with single and multiple damage scenarios. In the first application, damage scenarios in a plate-like structure are detected. In the second application, ALO first is used to build a FE model of a composite structure based on a vibration experiment. Then the slab of the updated model is assumed to be suffered several damage scenarios. In both applications, failures are localized by using damage index. Then, the proposed approach is used to quantify the corresponding extent by means of changes in frequencies and displacement mode shapes. Values of damage index are achieved from modal properties of one or three out of the first five modes of the two considered structures. A conventional ANN also is investigated for comparison. Results of damage identification indicate that the damage indicator coupled with ALOANN show better performance compared with using ANN alone even when a noise level is assigned to modal properties.
查看更多>>摘要：Nickel-based single crystal creep's properties are commonly calculated according to the rupture strength under constant load. The creep deformation of turbine blades is not only occurred under constant loading but also the variable loading. The influence of variable load on creep deformation should also be considered. Based on this background, in the present study, gradient loading creep tests of DD6 nickel-based single crystal are carried out under two modes. Take load and holding times into account, the creep characteristics and microstructure evolution is studied. The results show that the load and holding time have significant effects on the creep properties of nickel-based single crystals. The change of load induces an instantaneous jump on the creep deformation and strain rate. The holding time of each loading stage mainly affects the evolution of the internal microstructure of nickel-based single crystal. The research on creep behavior of nickel-based single crystal under gradient load in this paper not only enriches the service performance evaluation of nickel-based single crystal under complex environment but also provides a reference for material design and performance optimization.
查看更多>>摘要：Rail operations today have become more difficult due to increasing operating speeds and decreasing maintenance times. Today's higher frequency train loads are more detrimental, and prestressed railway sleepers especially are susceptible to these dynamic loads. Laminated carbon fiber reinforced polyurethane (L-CFRPU) reinforced concrete railway sleeper models (LCR concrete sleepers) have the potential to exhibit higher damping and resonance resistance more economically and practically. In this study, experimental modal analysis was performed on a large number of samples produced to understand the damping characteristics of sleepers. According to the results, new LCR concrete sleepers have extremely higher damping ratios of over 50% and are expected to be beneficial in extending the service life of the railway components, reducing maintenance needs and harmful effects on the environment.
查看更多>>摘要：Tribological failures, like wear, scuffing and pitting, etc, often occur in aero ball bearings due to large dry contact area, high friction-flash temperature and stress concentration at the interface between the contacting ball and race. In this paper, a numerical tribo-dynamic analysis procedure is developed for the predictions of dynamic performance, lubrication state, friction-temperature, and subsurface stress of the aero ball bearing with 3D measured roughness. The numerical model is verified by comparing the friction coefficients with the testing results, which show well agreements and proves the presented model is available. The dynamic performance of bearing at different azimuth angle are studied, which indicate the increasing radial load and speed result in more asymmetric distributions of the contact load, stress and slide-roll ratio, the maximum values and the unloaded-region become larger. With the increase of the inner race groove coefficient and the contact angle, thepv value and the radial stiffness decrease clearly. The tribology behaviors of the ball bearing are also investigated, the obtained results indicate that the heavy-load and low-speed lead to large dry contact areas, high friction-temperature, and high local stress peaks, the lubrication state become worse, which may further cause the failures of mass wear, scuffing and micro-pitting.
查看更多>>摘要：The paper presents an assessment of fatigue-based reliability in measuring failure random vibration loadings due to unexpected loads under various road conditions. Random loads were the main cause of fatigue failure in a component durability analysis. Acceleration signals were measured during a road test held on rural and highway road surfaces. These signals were captured from an accelerometer placed on a vehicle's lower arm suspension system in a locally made sedan-type automobile. This arrangement enhanced the capacity for collecting acceleration signals using multi-body dynamics simulation, which would minimise the need for actual acceleration data measurements. Modal analysis was conducted to determine the natural frequencies and mode shapes of the geometry of a coil spring so that strain signals based on each mode shape could be obtained. As modal parameters occurred within the frequency range of road excitation, fatigue life prediction was not affected by instances of dynamic behaviour in the components in time domain. Fatigue life was analysed using the Dirlik, Lalanne and narrowband approaches in frequency domain. The Dirlik method predicted a good correlation for the measured acceleration and generated strain signals at 2.32 x 10~8, 2.55 x 10~8 and 1.51 x 10~8 blocks to failure, indicating that a coil spring could withstand the uneven road surface without failing prematurely. Hence, fatigue-based reliability can evaluate the hazards of rate-reliability, based on predictions of fatigue life data on component durability.
查看更多>>摘要：Bolts tend to show different fatigue behavior with respect to the size or shape of a bolt and the level of preload though it is the same material. This makes it difficult to predict the fatigue life of bolts. In this study, a novel method to construct a unified stress-life (S-N) curve was proposed for the three different bolts with different sizes and lengths subjected to various external axial loading conditions and preloads, ranging from 44 % to 88% of the ultimate tensile stress. A total of 133 experiments for 46 different fatigue conditions were conducted and finite element analysis was also performed to calculate multiaxial stress components at the root of the thread. The fatigue characteristics of the bolts were investigated depending on various loading conditions, and the life prediction performance with fitted S-N curves was compared with respect to four mean stress correction models. The unified S-N curve showed high accuracy in life prediction; 15 % mean absolute error without exceeding ± 50 % for finite life loading conditions.
查看更多>>摘要：Most of the existing buildings in seismic prone regions have been built before the publication of modern design provisions against earthquakes, resulting in the need for structural retrofitting. Furthermore, some of those buildings are also subjected to additional hazards that may be either triggered by earthquakes (e.g., landslides, soil liquefaction, tsunamis) or associated with other natural or anthropogenic events, such as floods, vehicle collision, blast, and fire. A multi-hazard performance assessment of building structures is thus of paramount importance to implement integrated retrofit strategies, which otherwise would not be economically sustainable if oriented to structural risk mitigation against a single hazard. While retrofit strategies to improve the seismic performance of reinforced concrete (RC) structures have been widely investigated, structural retrofitting against progressive collapse has received very little attention. Within this context, the present paper illustrates a numerical investigation on the influence of seismic retrofitting on structural robustness of a four-storey, five-bay, RC frame building designed only to gravity loads. Seismic performance and structural robustness were respectively evaluated in OpenSees through pushover and pushdown analyses of a fibre-based finite element model. Structural robustness was evaluated under two relevant column-removal scenarios, ie., the sudden loss of a central and a corner column, whereas earthquake resistance was assessed according to the N2 method, evidencing the need for seismic retrofitting. A retrofit measure based on carbon fibre reinforced polymers was then considered to avoid premature brittle failures. Analysis results show that this retrofit strategy was able to increase both seismic safety and structural robustness. Subsequently, a parametric analysis was carried out in order to evaluate the impact of beam span length and shear strength of the retrofitting system.
查看更多>>摘要：The harsh operating environment, and the extreme working condition, of the steam turbine blades have led to the use of alloys with special characteristics. Of these properties are high corrosion resistance and excellent rupture strength. The most widely used alloys in this category are heat treatable martensitic stainless steels. They provide a wide range of mechanical properties. In that sense, this research is focused on one of the most commonly used alloys of these series; i.e. 410 alloy. In this study, a turbine blade made of 410 stainless steel was under operation for 72,000 h, and its possible damage mechanisms were explored. To define the damage causes, microstructural analyses were performed by Optical Microscopy (OM), and a Field Emission Scanning Electron Microscopy (FESEM) instrument equipped with Energy Dispersive Spectroscopy (EDS) detector. As well, mechanical properties evaluation was performed using hardness testing. Results showed that the existence of foreign particles in the environment of the turbine has led to the initiation of damage in the blade through the erosion mechanism. Alternatively, such a damage was intensified by corrosion attack due to the presence of the fuel impurities. Such damages eventually led to the formation of fatigue cracks on the trailing edge of the blade. The latest phenomenon substantially reduced the fatigue strength of the blade. Finally, it is predicted that the reduced fatigue strength would ensue shortened service life for the blade.
查看更多>>摘要：Considering the susceptibility of sandwich structures to impact events and the increasing environmental awareness due to pollution, the present work provides a thorough understanding of the ballistic impact behavior of agglomerated cork and of the resulting green sandwich structures produced with polypropylene (PP) skins reinforced with a flax/basalt intraply hybrid fabric. The effect of density on the agglomerated cork response was evaluated (NL10 p = 0.14 g/cm3, NL20 p = 0.20 g/cm3 and NL25 p = 0.25 g/cm3) and a comparison with commercial polyvinyl(chloride) foams was provided (HP130 p = 0.13 g/cm3, HP200 p = 0.20 g/cm3 and HP250 p = 0.25 g/cm3). The effect of a maleic anhydride coupling agent on the mechanical properties of the skins and of the overall sandwich structures was also investigated. The results highlighted a compromising effect of the weak interface between cork granules and polymeric binder on the impact resistance of the agglomerated cork, but a clear improvement of its performance was observed when embedded as core material between the two skins. Indeed, the two classes of sandwich structures produced with neat PP skins and with the two cores with the same density, i.e. agglomerated cork NL10 and PVC foam HP130, displayed the same ballistic limit of 171 m/s confirming that cork integration in the overall structures allows to approach PVC foam performance. The high-velocity impact response of one agglomerated cork (NL25) and one PVC foam (HP130) was also subjected to finite element analysis employing the CRUSHABLE FOAM model available in ABAQUS obtaining a good fitting with the experimental data.
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Elsevier