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Spline couplings are commonly used to transfer rotary motion in rotating structures, such as low-pressure rotor systems in dual-rotor aero-engines. The misalignment introduced by spline coupling assembly error will seriously affect the safe operation of rotor system. In this paper, a method is proposed to calculate the spline meshing stiffness under arbitrary misalignment conditions, considering the static and dynamic misalignment. The accurate meshing parameters of spline couplings under parallel and angular misalignments were determined by computing the effective meshing region of misaligned spline teeth. Furthermore, a time-varying stiffness-damping spline model is established, considering friction on the spline teeth surfaces, to derive the dynamic model of the spline-rotor system under arbitrary misalignment conditions. The Newmark-β method, in conjunction with the Newton–Raphson method, was used to solve the dynamic response of the system, and the correctness of the model was verified by experiments. Finally, the variation of spline meshing stiffness and force caused by static and dynamic misalignment is described, and the effects of parallel and angular misalignment, as well as the phase difference in unbalance, on the vibration response and self-excited vibrations are also investigated. [ABSTRACT FROM AUTHOR]

Maneuverability is one of the important tactical and technical indexes of fighter aircraft. In this paper, the finite element method is used to establish a dynamic model of the rotor system that can consider arbitrary maneuvering flight form and rotor speed variation during the flight, and the vibration characteristic of the dynamic model is investigated in detail. In addition, the nonlinear forces caused by bearings and oil film are also considered. The Newmark-β method combined with Newton-Raphson method is adopted to solve the dynamic equations. The influences of speed variation, rolling, pitching, and yawing maneuver loads on the vibration responses of the rotor system are also evaluated. [ABSTRACT FROM AUTHOR]

In this paper, a flexible tether-net system is applied to capture the space debris and a numerical framework is established to explore its nonlinear dynamic behaviors, which comprises four principal phases: folding, spreading, contacting, and closing. Based on the discretization of the whole structure into multiple nodes and connected edges, elastic force vectors and associated Jacobian matrix are derived analytically to solve a series of equations of motion. With a fully implicit method applied to analyze the nonlinear dynamics of a slender rod network, the involved mechanical responses are investigated numerically accounting for the interactions. Contact between the deformable net and a rigid body is handled implicitly through a cost-effective modified mass algorithm while the catenary theory is utilized to guide the folding process (from planar configuration to origami-like pattern). The dragging and spreading actions for the folded hexagon net could be realized by shooting six corner mass toward a specific direction; next, the six corners would be controlled to move along a prescribed path producing a closing gesture, when touch between the flying net and the target body is detected, so that for the space debris could be captured and removed successfully. We think the established discrete model could provide a novel insight in the design of active debris removal (ADR) techniques and promote further development of the model-based control of tether tugging systems. [ABSTRACT FROM AUTHOR]

A double-well Duffing oscillator with nonlinear damping is identified in this paper, using the nonlinear subspace identification (NSI) technique in conjunction with a novel virtual input perturbation approach. This kind of system exhibits a negative stiffness characteristic, which has become popular in engineering applications such as vibration absorbers. The studied device exhibits strong nonlinear elastic and damping behaviors. In particular, the latter results in a very challenging identification, and the applicability of the proposed method to nonlinear damping is therefore discussed. In this context, the method is first tested considering simulations with quadratic friction. The experimental investigation is eventually undertaken considering a broadband random excitation, with a friction model depending on both velocity and displacement. Results show that NSI is capable of estimating the nonlinear model associated with stiffness and damping nonlinearities with a high level of confidence. This is particularly true in the case of complex nonlinear behaviors related to the double-well characteristics, using the proposed virtual input perturbation technique. This is confirmed also by the proposed experimental application and enlarges the range of applications of the NSI technique. [ABSTRACT FROM AUTHOR]

In-plane compression experiments are performed on 3D fine weave pierced C/C composite at a wide strain rate range of 0.0001/s–1000/s. The in-plane compressive failure mechanism of the composite at quasi-static and high strain rates is analyzed by a scanning electron microscope. The results show that the in-plane compressive modulus, maximum stress and the corresponding strain increase with increasing strain rate. The quasi-static in-plane compressive failure mode of the 3D fine weave pierced C/C composite is characterized by the shear failure at the angle of 45 ∘ and the local buckling of the x-direction fiber bundles. In comparison, the high strain rate in-plane compression failure mode of the composite is characterized by the compressive fracture of the interlaminar matrix and the progressive compression failure of the x-direction fiber bundles. A strain-rate-dependent in-plane compressive constitutive model is proposed to predict the dynamic in-plane compressive response of the composite. The proposed constitutive model is verified by experimental data. [ABSTRACT FROM AUTHOR]

Rolling bearing and squeeze film damper will introduce structural nonlinearity into the dynamic model of aeroengine. Rubbing will occur due to the clearance reduction design of the engine. The coupling of structural nonlinearity and fault nonlinearity will make the engine present rich vibration responses. This paper aims to analyze the nonlinear vibration behavior of the whole aeroengine including rolling bearing and squeeze film damper under rubbing fault. Firstly, the dynamic model of a turboshaft engine with nonlinear support and rubbing fault is established; The rolling bearing force, the oil film force and the rubbing force are introduced into a dual-rotor–casing model with six support points. Secondly, the linear part of the model is verified by the dynamic characteristics of the three-dimensional finite element model. Finally, the varying compliance vibration, the damping effect and the bifurcation mechanism are analyzed in detail in which the bearing clearance, speed ratio and rubbing stiffness are considered. Results show that the rubbing fault in the nonlinear support case will excite more significant varying compliance vibration in the low-speed region and expand the rotating speed range of the chaotic region in the high-speed region compared with that in the linear support case. [ABSTRACT FROM AUTHOR]

Engineering structures made of ceramic matrix composites (CMCs) usually suffer from cyclic loads during service, which could lead to disastrous failures. This work focuses on the fatigue behavior of a 2.5D C/SiC composite under tension–tension cyclic loading. Experiments of the 2.5D C/SiC composite are firstly carried out to determine the fatigue lifetime of the material at different stress levels. The fracture surfaces examined by a scanning electronic microscope indicate that the damage mechanisms under cyclic loading are closely related to crack propagation, fiber/matrix interfacial degradation, and fiber breakage. Considering the damage evolution of fibers and interfacial resistance, a micromechanical model is adopted to describe the fatigue behavior of 2.5D C/SiC composite, and the numerical results are compared with the experimental results. Further, a sensitivity analysis is performed as a function of the interfacial shear stress, fiber Weibull modulus, and fiber strength. The calculation of sensitivity factors shows that the variations of the fiber Weibull modulus and fiber strength have the most significant influence and, thereafter, the variation of interfacial shear stress. [ABSTRACT FROM AUTHOR]

An improved finite element modeling method is developed for isotropic curvilinearly stiffened plates under a thermal environment. The existing modeling method for curvilinearly stiffened plates avoids the difficulty of node overlap, which is suitable for plates with different thicknesses. By introducing the influence of temperature on material parameters and thermal stress on additional stiffness, the existing method is improved and extended to the study of thermodynamics. The proposed method is verified by modal test at normal temperature and commercial finite element software at the thermal environment. The variation of dynamic characteristics with temperature under different boundary conditions was studied. Results show that when the boundary conditions are asymmetric, the influence of temperature on the thermal mode shape is more significant than the case of symmetrical. [ABSTRACT FROM AUTHOR]

The effect of geometrical nonlinearities due to pressure load on the thermal buckling and dynamic characteristics of composite plates are investigated in this paper, which is the main contribution of this research work. The mechanical behavior of the plate is described with the first-order shear deformation theory. The geometrical nonlinearity due to both thermal effect and pressure load is introduced in the finite element model of the plate via additional stiffness matrices. Thermal buckling and modal analysis of a four-sided simply supported rectangular composite plate under different pressure fields are conducted. Numerical results show that both the mode frequencies and critical buckling temperature of the plate rise with the increase of the pressure. The vibrational mode shapes change with the gradient pressure load field. The maximum buckled deflection point moves from the center to the place where is easier to reach compressive stress state under uniform thermal load. The pressure distribution has a significant effect on the buckling mode shapes of the plate. [ABSTRACT FROM AUTHOR]

As research works of the transient statistical energy analysis (TSEA) and transient local energy approach (TLEA) mostly focus on simple structures, TSEA and TLEA are adopted to quantify the transient response of a complex vibro-acoustic system at the mid-high frequency range in this paper. Numerical examples of a coupled oscillator system, an L-shaped plate, and a launch vehicle fairing model are conducted to demonstrate the effectiveness and accuracy of TSEA and TLEA. The computational precision of TSEA and TLEA is verified by the analytical solution and finite element method. Furtherly, the transient energy responses of subsystems with different coupling ratios between subsystems are investigated. Results show that TLEA has a better performance than TSEA. With the increasing coupling ratio between subsystems, the rise time and peak energy of transient energy response of subsystems decrease gradually. Both ratios of rise time and peak energy predicted by TLEA to these of the TSEA increase as the rising of the coupling ratio. [ABSTRACT FROM AUTHOR]

An approach on determining elastic parameters and Coefficient of thermal expansion (CTE) of a 2.5-dimensional (2.5D) braided composites is proposed in this paper, by adopting mesoscopic mechanics integrated Finite element (FE) modeling. According to the geometric features of meso-structure, Representative volume cell (RVC) models of composite for predicting thermo-elastic parameters are established. On the basis of the models, homogenized parameter prediction is carried out in three steps: Firstly, equivalent elastic properties is predicted subject to periodic displacement boundary conditions; secondly, the equivalent thermal modulus is determined by using the periodic non-adiabatic temperature boundary conditions; thirdly, using the obtained elastic parameters and thermal modulus to calculate the equivalent coefficient of thermal expansion. A multiscale finite element analysis is conducted: The thermo-elastic parameters of yarn is calculated using the RVC model; subsequently, the equivalent parameters of the yarn are substituted into the RVC of 2.5D braided C/SiC composites, to predict the thermo-elastic parameters. Results indicate that the CTE determined by homogenized parameter prediction of 2.5D C/SiC composites shows good agreements with the experimental results. At last, the refined FE model and the equivalent homogeneous model are employed to verify the effectiveness of the predicted parameters in terms of effective modeling. After the comparative analysis on thermal modal data between refined model and equivalent model, the acquired results demonstrate the effectiveness of the proposed method in determining thermo-elastic parameters. [ABSTRACT FROM AUTHOR]

The panel-type structures used in aerospace engineering can be subjected to severe high-frequency acoustic loadings in service. This paper evaluates the frequency-dependent random fatigue of panel-type structures made of ceramic matrix composites (CMCs) under acoustic loadings. Firstly, the high-frequency random responses from the broadband random excitation will result in more stress cycles in a definite period of time. The probability density distributions of stress amplitudes will be different in different frequency bandwidths, though the peak stress estimations are identical. Secondly, the fatigue properties of CMCs can be highly frequency-dependent. The fatigue evaluation method for the random vibration case is adopted to evaluate the fatigue damage of a representative stiffened panel structure. The frequency effect through S-N curves on random fatigue damage is numerically verified. Finally, a parameter is demonstrated to characterize the mean vibration frequency of a random process, and hence this parameter can further be considered as a reasonable loading frequency in the fatigue tests of CMCs to obtain more reliable S-N curves. Therefore, the influence of vibration frequency can be incorporated in the random fatigue model from the two perspectives. [ABSTRACT FROM AUTHOR]

Substructure method has been widely applied in dynamic analysis of complex structures due to high computational efficiency. On the basis of Residual flexibility mixed-boundary (RFMB) substructure method, a model updating approach is proposed in this paper. Four major steps of the RFMB model updating method are summarized as: 1) Substructuring: Dividing the whole structure into residual part and reduced part according to the junction surface; 2) reduction: Using the RFMB component mode synthesis approach to reduce the order of each substructures; 3) assembly: Residual structure analysis by using the reduced assemble matrix; 4) updating: Model updating by solving the optimization problem. Numerical simulation is conducted to verify the effectiveness by adopting a cantilever plate in case I. In case II, the proposed method is applied to identify the elastic parameter of interface of a bolted joint structure using experimental data. After parameter identification, the maximum error between numerical results and the experimental data decreases to 2.44 %. And three component mode synthesis model updating methods: Craig-Bampton (CB), Mixed-boundary (MB) without considering residual flexibility and RFMB model updating approach, are applied to update the same bolted joint structure for comparing the accuracy. For comparing the computational efficiency, the RFMB model updating approach is applied to the complicated aero-engine casing structure. In case III, the average time-consuming of the Whole finite element model (WFEM) is 5.15 times to the Residual finite element model (RFEM) in the single updating iteration. Results show that the proposed approach has better performance in the finite element model updating. [ABSTRACT FROM AUTHOR]

Runyang Bridge is a newly built cable-supported bridge that crosses the Yangtze River in China. The bridge is composed of one suspension bridge and one cable-stayed bridge. During the construction of the bridge, a structural health monitoring system (SHMS) was installed, which was designed by the Southeast University. Since the bridge was open to traffic, quantities of structural ambient responses have been recorded by the SHMS. And, it’s really important to extract structrural information from these records for health monitoring. This paper presents the study on modal identification of the bridge. The dynamic properites including modal frequencies, mode shapes and damping ratios are extracted from the ambient responses. Two identification methods are employed including the enhanced frequency domain decomposition and the stochastic subspace identification. The identified modal parameters from the two methods are compared. Results show that modal frequencies and mode shapes from the two methods are almost the same while the damping ratios are different. [ABSTRACT FROM AUTHOR]