Search

In this study, a segmented degradation model was established for the first time to predict and evaluate the dynamic degradation characteristics of carbon fibre-reinforced epoxy composite (CFRC) shells accurately by considering temperature variations during heating, maintenance, and cooling. The model is based on the Reddy's high-order shear deformation theory, complex modulus method, and coefficient fitting approach. First, explicit expressions of material parameters and thermal expansion coefficients (TECs) in different thermal degradation stages were proposed, followed by the derivation of the differential equations to solve the structural dynamic degradation characteristics. Moreover, an identification method for the fitting coefficients of the CFRC material was described, which is based on experimental tests and iterative calculations of elastic moduli, loss factors, and TECs at different degradation time points in different thermal degradation stages. Finally, the natural frequencies, damping ratios, and time-domain responses were measured using a novel testing system for the dynamic degradation of specimens subjected to a pulse excitation load to validate the developed model. Theoretical and experimental results indicate that the natural frequencies decreased as the degradation time increases at the heating-up and temperature maintenance stages, whereas they increased at the cooling stage. However, an uptrend in the damping and response behaviours was observed in the first two stages, whereas a downtrend was recognized at the cooling stage.

BACKGROUND: Due to the complexity of cardiac surgery, almost all patients need to be admitted to the intensive care unit (ICU) for postoperative care after surgery. After being discharged from the ICU, some patients need to be readmitted due to disease deterioration during hospitalization. We conducted a meta-analysis of the literature to investigate the incidence of readmission to the ICU in patients undergoing cardiac surgery. METHODS: The PubMed, Medline, and Elsevier databases were searched using the keywords “cardiac surgery,” “readmission,” “intensive care unit,” and “ICU” to retrieve English-language articles published from January 2000 to January 2021. The articles were screened, and their quality was evaluated. A meta-analysis was performed on the outcomes of patients after readmission to the ICU using Stata16.0 software. RESULTS: Ultimately, 9 articles were included in the meta-analysis, comprising 32,825 cardiac surgery cases, of whom 1,302 were readmitted to the ICU. The incidence of readmission to the ICU was 3.97%. Among the direct reasons for readmission to the ICU, respiratory failure accounted for 13.6–48.6%, while hemodynamic instability accounted for 21.6–51.9%. The results of the meta-analysis showed that the mortality rate of patients readmitted to the ICU was significantly higher than that of patients not readmitted to the ICU [risk difference (RD) =8.05, 95% confidence interval (CI): 5.10–12.69, Z=8.965; P

This research proposes a novel analytical model capable of accurately predicting the strain-dependent characteristics of fiber reinforced composite shells (FRCSs) with partial constrained layer damping (CLD) treatment by considering the nonlinearities of fiber reinforced composite and viscoelastic materials simultaneously. The nonlinear material properties are represented based on Jones-Nelson nonlinear theory, energy-based strain energy method, and complex modulus method. Then, the governing equations of motion for FRCSs are developed via Ritz method, and the identification procedure of nonlinear fitting parameters is also presented. By taking a T300 carbon fiber/epoxy resin cylindrical shell with partial CLD patches as an example, a series of experiments are carried out to validate the proposed modeling approach. Finally, the effects of material properties on nonlinear vibration behaviors of FRCSs covered with partial CLD patches are evaluated. Comparisons show that the proposed nonlinear model is more accuracy than that without considering strain dependence, where the maximum errors between the proposed model and measured data for natural frequencies, damping ratios and resonant response are 6.9%, 11.3%, and 11.2%, respectively.

In this paper, an analytical model for fiber reinforced polymer composite cylindrical shells (FRPCCSs) accounting for material and geometric nonlinearities and thermal effect is proposed, which is capable of predicting natural frequencies, damping ratios, and resonant responses under different excitation amplitudes and temperatures. Firstly, the nonlinear Young's and shear moduli and loss factors of fiber reinforced polymer composites considering amplitude and temperature dependent properties are represented based on the Jones-Nelson nonlinear material theory, complex modulus principle, together with polynomial fitting technique. Then, the equations of motion for FRPCCSs are derived accounting for nonlinear strain–displacement relations of the von Karman type. The procedure of determining fitting coefficients for nonlinear Young's and shear moduli and loss factors is also presented in details. Following theoretical studies, a series of thermal vibration experiments on two shell specimens made of CF120 carbon/epoxy composites are carried out to validate the proposed modeling approach as well as evaluating nonlinear vibrations of FRPCCSs experimentally. It has been found that the first three natural frequencies of composite shell specimens are initially decreased and then increased along with the increase of excitation amplitude. This variation trend becomes more significant at high temperature with the maximum variation degree of 8.2%. Meanwhile, the corresponding damping ratios and resonant responses of FRPCCSs show upward trends as the excitation amplitude and environmental temperature increase. Compared to excitation amplitude, environmental temperature has more significant effect on the damping performance of FRPCCSs, of which the relative variation degree is up to 300.5% as the temperature raises from 20 to 200 °C.

In this paper, nonlinear vibrations of fiber-reinforced composite cylindrical shells (FRCCSs) with bolted joint boundary conditions are studied both theoretically and experimentally, where the nonlinear amplitude-dependent material property of fiber-reinforce composites (FRCs) and partial bolt loosening boundary conditions are taken into account. By representing the nonlinear material property via Jones-Nelson theory, Love shell theory is used to calculate the elastic strain energy of shells. The bolt loosening boundary conditions are achieved using artificial spring-damper technique. The Rayleigh-Ritz method is employed to derive the equations of motion for FRCCSs, from which the natural frequencies, damping ratios, and forced response can be obtained. Then, a series of vibration tests are carried out on a FRCCS specimen to validate the modeling approach proposed here. Based on the validated model, vibrations of FRCCS structures accounting for amplitude dependence of FRCs with different partial bolt loosening boundary conditions are investigated. It is found that increasing bolt loosening degree and loose bolt number leads to decrement of natural frequencies, and increment of modal damping ratios and resonant response amplitudes due to the coupling effect of declined boundary stiffness and increased boundary damping at bolted constraint edges. As the excitation level rises, the amplitude-dependent characteristics of natural frequencies and damping parameters of FRCCSs gradually become weak, while the increasing rates of resonant response amplitudes show an upward trend.

The three-dimensional flow around the four-cylinder and five-cylinder arrays are numerically studied at Re = 3900 by large eddy simulation (LES). By selecting eight spacing ratios (L/D = 2.0–7.0) and three incidence angles (α = 0°–45°), the influence of the center cylinder on the flow characteristics and interference effects of the four-cylinder array is discussed. Different spacing ratios can be divided into three categories with respect to flow regions of the multi-cylinder: small gap flow regime (L/D = 2.0–3.5), critical flow regime (L/D = 3.5–5.0) and large gap flow regime (L/D = 5.0–7.0). For cases with small spacing ratios, the influence of the center cylinder is very notable on the flow characteristics of the four-cylinder array. As the ratio increases, the influence of the center cylinder gradually decreases on the surrounding cylinders in the multi-cylinder array. On the other hand, the direction of the incoming flow also affects the flow characteristics of both four-cylinder and five-cylinder arrays. The development of the shear layer is strongly influenced by the center cylinder for the upstream cylinders at α = 45°. Boundary vortex flux components, flow characteristics and fluid force coefficients change significantly with adding the center cylinder for the four cylinders.