查看更多>>摘要：The ballistic resistance of a composite ceramic-metal target has been studied by performing three-dimensional finite element computations on ABAQUS/Explicit finite code. The bi-layer alumina 95-aluminium 2024-T3 target has been subjected to impact load by a cylindrical steel projectile at varying incidence velocity in order to carry out a detailed investigation about the behaviour of a composite with a front ceramic layer. First of all the optimum thickness ratio of the front ceramic and backing aluminium layer has been found out. Subsequently, the significance of the eccentricity on the impact response has been studied by hitting the composite target at six different impact locations and the results have been compared with that of the concentric impact condition. The possibility of reducing the areal density of the composite by using the perforated backing aluminium plate has also been studied. The eccentricity in the impact load has played a significant role on the ballistic resistance of the composite. The consideration of perforated backing plate though could not enhance the ballistic efficiency. However, it has been found that a major portion of the energy of the projectile is lost during its interaction with the front ceramic tile leading to significant erosion of the projectile. The backing plate on the other hand played the role of supporting the front layer however its share of energy absorption subsequent to the comminution of ceramic was not very significant. The Johnson-Holmquist (JH-2) constitutive model has been used for simulating the material behaviour of the ceramic and the Johnson-Cook (JC) constitutive model has been used for simulating the behaviour of backing plate and the projectile.
查看更多>>摘要：In recent years, buffer layer failure of high voltage crosslinked polyethylene (XLPE) cables have occurred many times. The typical feature is the appearance of holes surrounded by white powders on the surface of water blocking tape. Although the generation of white powders is possibly related to the moisture of water blocking tape, the failure mechanism of buffer layer is not yet clear. In this paper, the effects of moisture content on the amount of white powders and volume resistivity, relative permittivity as well as thermal conductivity of the tape were studied through experiments. Moreover, based on the test results mentioned above, the electric and temperature field distribution of the buffer layer were calculated by finite element simulation. It has shown that water blocking powders become spherical after absorbing water, which is an important feature to indicate the tape getting wet. Moreover, volume resistivity of the tape also rised with increasing powders. With the increase of white powders, volume resistivity of water blocking tape increases and both relative permittivity as well as thermal conductivity decreases. Additionally, the cause of buffer layer failure is that the precipitation of white powders causes the increasing volume resistivity and decreasing relative permittivity of water blocking tape, which finally leads to the partial discharge between the tape and aluminum sheath.
查看更多>>摘要：This study analyzes the simulation model configuration and its loading conditions in fatigue and subsequent crack growth tests. Following the simulation principles, a full-size 3D finite element analysis of a representative GTE turbine disc was performed. With the aim of reproducing the simulation model, the in-service critical zone of both the stress-strain state and damage accumulation is studied. Based on the parametric computations, the block type loading conditions of the simulation models are determined and verified. Experimental study of the surface crack growth rate in the simulation model under harmonic and block loading at elevated temperature is performed for the fatigue and creep-fatigue interaction conditions. To interpret the experimental data, we calculate the nonlinear stress intensity factor distributions along the semielliptical crack front for flaws growing on the inner surface of the hole in the simulation model using numerical procedures. It is found that in terms of the nonlinear fracture resistance parameters, the sections of the fatigue fracture diagrams form one common curve representing the previous and subsequent increasing stages of block loading in pure fatigue and creep-fatigue interaction loading conditions. The possibility of using the proposed simulation model, tested under variable amplitude loading, to assess the structural integrity of GTE turbine disks is discussed.
查看更多>>摘要：Ever since the 1980s there is a sustained interest in the size effect, as one of the most pronounced consequences of fracture mechanics. In the present study, the investigation of the size effect is focused on estimation of the Weibull cumulative distribution function (CDF) of the critical value of the J-integral (J_c) in the transition temperature region under constraint of a small statistical sample size. Specifically, the J_c experimental data correspond to the C(T) specimen testing of the reactor pressure-vessel steel 20MnMoNi55 at only two geometrically-similar sizes. Thus, a simple approximate scaling algorithm has been developed to tackle the effect of the C(T) specimen size on the J_c CDF under these circumstances. Due to the specific form of the two-parameter Weibull CDF, F(J_c [ β, η), the scaling procedure is applied according to a two-step scheme. First, the J_c-scaling is performed to ensure the approximate overlap of the points that correspond to the CDF value F(J_c =η) = 1 - 1/e ≈ 0.632 for different C(T) specimen widths (W), which assumes η-W~k = const. Second, the F-scaling is performed to ensure the equality of the slopes of the CDF in the scaled (F- W vs. J_c- W~k) space. The objective of the sketched approach is to obtain a size-dependent J_c CDF that encapsulates a reasonably conservative data extrapolation.
查看更多>>摘要：The aim of the paper is to extend and put at disposal to common users a reliable numerical procedure called MPM (Multi-Pier Macro) approach, already presented in-plane, for walls subjected to two way bending. This approach is developed starting from a Multi-Pier MP approach proposed by the authors in the past. The MP approach for the out-of-plane analysis of masonry can be used for practical purposes, but requires a refined discretization with relatively huge computation effort and a specific training for the user. The MPM approach overcomes some well known limitations of the MP one, using a discretization that is totally independent from the actual unit cell dimension considered and totally unrelated to the specific texture of the masonry wall considered. The MPM method relies on a very unrefined discretization of a wall out-of-plane loaded with parallelepiped cells with large edges made by vertical and horizontal beams plus braces, assuming for each element equivalent geometric and mechanical properties that guarantee the elastic, peak and post peak rough equivalence between the original masonry wall and the mono-dimensional discretization proposed. Its main feature is to allow the analysis of large and complex masonry structures, shortening the calculation time and being accessible to practitioners without a specific background on masonry and who own a commercial software equipped only with non linear 1D elements.The procedure is benchmarked on four walls out-of-plane loaded in two way bending, in presence also of perforations for three of the four walls; a good predictivity in the non-linear range is observed when compared with much more sophisticated numerical approaches.
查看更多>>摘要：This paper reports an experimental investigation on bearing behavior and failure mechanism of single-lap bolted composite interference-fit joints under thermal effects. To achieve this target, testing temperatures ranged from -25 to 110 °C were designed to simulate thermal service environments. The unidirectional composite lamina was tested longitudinally and transversely to figure out potential change of material properties induced by thermal loading. The residual effects of stacking sequence, interference-fit size and tightening torque on bearing strength and stiffness of composite joints after exposed to thermal conditions were evaluated. Microscopic studies applying X-ray technique on damage areas to monitor damage evolution were conducted to comprehend the thermal failure mechanism. The results evidence that the bearing performance of composite joints was slightly enhanced by low temperature due to cold hardening of matrix resin but greatly weakened by high temperature owing to the combined effects of heat softening of resin, residual stress relaxation and thermal expansion of joint components. Tightening torque to enhance bearing performance is the most beneficial at room temperature, whereas the interference-fit remains the dominant influence at sub-freezing environments. In conclusion, thermal loading plays an important role in bolted composite interference-fit joints. It is an unneglectable factor that would enlarge damage area, alter failure mode and should be understood thoroughly.
查看更多>>摘要：With the advances in material research and pipe manufacturing, the construction of oil and gas pipelines is developing continuously, and the evaluation of welding joints with defects has become the focus of pipeline safety research. In this study, two full-scale wide plate tensile tests were performed on 01422 mm x 25.7 mm X80 specimens with original and repaired welding joints. By comparing and analysing the experimental load-displacement responses, as well as the synchronous monitoring results of strain gauge and digital image correlation (DIC) system, the variation trend of the crack mouth opening displacement (CMOD) with distal strain, and the strain capacity of welding joints under different critical CMOD conditions were obtained. This way, the difference in bearing capacity between the original and the repaired welding joints was further determined. Under the same load condition, the driving force of the defect crack expansion at the repaired welding joint was smaller. When the critical CMOD was 0.5 mm and 1.0 mm, the strain capacity of the repaired welding joint was increased by 0.12% and 0.35%, respectively. Therefore, the repair operations of the welding joint effectively strengthen its deformation capacity. This research can provide a valuable data basis for the subsequent verification of numerical analysis models, and can also be referenced for the safety evaluation of pipelines with crack defects.
查看更多>>摘要：The teeth of misaligned spline couplings used in power transmission parts of helicopter and aeroengine often suffer serious abnormal wear faults during service. Through the self-made spline coupling test rig, the mechanism of misalignment on the spline coupling wear was explored. The tooth contact domain of the spline coupling with shaft misalignment is close to the addendum, and the tooth contact domain of the spline coupling with angle misalignment is mainly concentrated on the side of the tooth end. The edge contact effect becomes more serious as the amount of misalignment increases, resulting in abnormal wear at specific positions on the tooth surface. The presence of misalignment increases the vibration intensity of the spline coupling based vibration signal monitoring, which will increase the wear of the teeth. In addition, the difference in system vibration response of different misalignment types can be used to identify the misalignment failure mode of the spline coupling. It can be inferred from the characterization of wear debris that fretting wear is the cause of the failure of the misaligned spline coupling. Tooth modification, surface strengthening modification and lubrication design are necessary as potential solutions to improve the wear resistance of misaligned spline couplings.
查看更多>>摘要：The type IV composite pressure vessels (CPVs) are used as a reliable solution for storing compressed gas but still require research and development to reach commercial advancements. The main two challenges in the design of CPVs are: a) debonding of liner and composite shell during the curing process, and b) accurate finite element modeling of the thickness and angle variation of helical layers at the dome regions. In this study, debonding of composite shell from polymeric liner and bosses due to the temperature variations during the curing process is simulated by using bilinear cohesive law. Regarding the second challenge, the Abaqus WCM plugin is used to model the actual geometry of two-liter type TV CPVs domes by defining and controlling manufacturing parameters. The UVARM subroutine is developed to apply stress analysis and predict the damage initiation using the Maximum Stress, Tsai-Wu, Tsai-Hill, Hashin, and Puck failure criteria. Also, a USDFLD subroutine is written to implement progressive failure analysis using the Puck failure criterion with sudden material degradation model. The numerical axial and radial displacements as well as burst pressure results are compared with the experimental results available in the literature. A good correlation is observed between the finite element and experimental results. Also, numerical results showed that the initial debonding gap due to curing does not affect the progressive damage of the composite shell.
查看更多>>摘要：In this study, pitting corrosion was investigated numerically and experimentally under actual operational conditions of a gas turbine compressor. First, the samples cut from a blade made a Custom 450 stainless steel were exposed to different corrosion times in a 3.5 wt% NaCl solution After the creation of pits in the samples, the dimensions of the corrosion pits were measured using a 3D profilometer device. The stoichiometry equations governing the corrosion of the Custom 45 alloy in a chloride solution were extracted and applied to the models. The results of this simu lation were validated by comparing the depth of the simulated pits and the experimental samples Subsequently, simulation was conducted for 48 months on a compressor having pits. There is a good agreement between the simulation results and the results obtained from the Turbo machine laboratory of the Texas A&M University. In addition, pits grew under the influence of corrosion joined together, and made an equivalent pit.
NSTL
Elsevier