Static and dynamic characteristics of composite structures

Distinct element method (DEM) requires enormous computation time because it needs small time increments that are sufficient to follow high frequency fluctuations in contact dynamic model. In general, the equations of motion of particles are solved by the second order Adams-Bashforth method, which estimates the contact force for the next time step by linear extrapolation, or by multistep methods such as predictor-corrector method. In order to overcome the deficit of conventional methods, we propose a Contact Force Prediction Method that allows us to use larger time increments. The method incorporates the contact force at every contact point obtained as the exact solutions or numerical solutions of differential equations that describe two particle contacts. It is confirmed that the proposed method gives the simulation results for packing and discharge of particles which are consistent with the experimental data, while accelerating DEM computation by 3 to 8 times.

The properties of 13 computational methods for the integration of first-order differential equations in time are studied. Special attention is given to the representation of periodic fluctuations in a simple spectral baroclinic model of the atmosphere. Errors in the energy, three dimensional scale, and frequency for linear and nonlinear oscillations are evaluated. Comparisons of both one-step and two-step methods are made. It is found that the two-step schemes compare favorably with one-step methods only when given the advantage of a smaller time increment. Even then, it is concluded that certain one-step procedures incorporating two or more extrapolations over each constant increment of time produce errors which grow most slowly. With small time increments, these errors are generally made smallest by increasing the number of time extrapolations at each step rather than by decreasing the time increment.

Stepwise crack propagation is evidently observed in experiments both in geomaterials and in hydrogels. Pizzocolo et al. (2012, “Mode I Crack Propagation in Hydrogels is Step Wise,” Eng. Fract. Mech., 97(1), pp. 72–79) show experimental evidence that mode I crack propagation in hydrogel is stepwise. The pattern of the intermittent crack growth is influenced by many factors, such as porosity of the material, the permeability of the fluid, the stiffness of the material, etc. The pause duration time is negatively correlated with the stiffness of the material, while the average propagation length per step is positively correlated. In this paper, we integrate extended finite element method (XFEM) and enhanced local pressure (ELP) method, and incorporate cohesive relation to reproduce the experiments of Pizzocolo et al. in the finite deformation regime. We investigate the stepwise phenomenon in air and in water, respectively, under mode I fracture. Our simulations show that despite the homogeneous material properties, the crack growth under mode I fracture is stepwise, and this pattern is influenced by the hydraulic permeability and the porosity of the material. Simulated pause duration is negatively correlated with stiffness, and the average propagating length is positively correlated with stiffness. In order to eliminate the numerical artifacts, we also take different time increments into consideration. The staccato propagation does not disappear with smaller time increments, and the pattern is approximately insensitive to the time increment. However, we do not observe stepwise crack growth scheme when we simulate fracture in homogeneous rocks.

A scaled boundary finite element formulation for solving plane-strain viscoelastic problems

A large time increment approach for cyclic viscoplasticity

This paper simulates the 2D transient magnetohydrodynamic (MHD) flow in a rectangular duct in terms of the velocity of the fluid and the induced magnetic field by using the radial basis function (RBF) approximation.  The inhomogeneities in the Poisson’s type MHD equations are approximated using the polynomial functions (1+r) and the particular solution is found satisfying both the equations and the boundary conditions (no-slip and insulated walls).  The Euler scheme is used for advancing the solution to steady-state with a time increment and a relaxation parameter which are determined for achieving stable solution.  It is shown that, as Hartmann number increases, the fluid becomes stagnant at the center of the duct, the flow is flattened and boundary layers are developed on the Hartmann and side walls.  These are the well-known characteristics of the MHD duct flow.  The stability analysis is also carried in terms of the spectral radius of the coefficient matrix of the discretized coupled system.  Stable solutions are obtained with RBF by using quite large time increment and suitable relaxation parameters on the expense of explicit Euler time-integration scheme used.

A computationally efficient convolution method, based on discretized impulse response and transition matrix integral formulations, is developed for the transient analysis of complex linear structures interacting through strong local nonlinearities. In the formulation, the coupling forces due to the nonlinearities are treated as external forces acting on the coupled subsystems. Iteration is utilized to determine their magnitudes at each time increment. The method is applied to a generic rotor-housing model representing a turbopump of a space shuttle main engine (SSME). In that model, the local nonlinearity is due to clearances between the rotor bearing outer races and the carrier attached to the housing. As compared to the fourth-order Runge-Kutta numerical integration methods, the convolution approach proved more efficient and robust for the same accuracy requirement. This is due to the closed-form formulation of the convolution approach which allows for the use of relatively larger time increments and for a reduction in the roundoff errors.

The solution of the general heat transfer problem for typical aircraft-type structures usually involves a finite element approach involving the solution of a large number of simultaneous equations. This may be performed by calculation using a digital computer or with an analogue simulator. For calculations involving both spatial and time intervals considerable care must be exercised to obtain solutions which are not divergent or oscillatory and this usually involves very small time increments and a large total computation time.

Experimental investigation into amplitude-dependent modal properties of an eleven-span motorway bridge

A synthesis of peak picking method and wavelet packet transform for structural modal identification

Structural health monitoring assesses structural integrity by processing the measured responses of structures. One particular group in the structural health monitoring research is to conduct the operational modal analysis and then to extract the dynamic characteristics of structures from vibrational responses. These characteristics include natural frequencies, damping ratios, and mode shapes. Deviations in these characteristics represent the changes in structural properties and also imply possible damage to structures. In this study, a new stochastic system identification is developed using multivariate time-frequency distributions. These time-frequency distributions are derived from the short-time Fourier transform and subsequently yield a time-frequency matrix by stacking them with respect to time. As the derivation in the data-driven stochastic subspace system identification, the future time-frequency matrix is projected onto the past time-frequency matrix. By exploiting the singular value decomposition, the system and measurement matrices of a stochastic state-space representation are derived. Consequently, the dynamic characteristics of a structure are obtained. As compared to the time-domain stochastic subspace system identification, the proposed method utilizes the past and future matrices with a lower dimension in projection. A spectral magnitude envelope can be applied to the time-frequency matrix to highlight the major frequency components as well as to eliminate the components with less influence. To validate the proposed method, a numerical example is developed. This method is also applied to experimental data in order to evaluate its effectiveness. As a result, performance of the proposed method is superior to the time-domain stochastic subspace system identification.

Monitoring of civil structures is crucial for their proper functioning. Any crack in a structure changes its static and dynamic behaviours. To detect the damage/crack at the initiating time itself is challenging task in modern time. This paper describes an experimental study to extract the dynamic characteristics of a frame structure using piezo‐electric ceramic (PZT) transducers. Tests are conducted on steel frame to extract the natural frequencies and the experimental mode shapes. Free vibration response is first acquired in the time domain and then transformed into frequency domain using Fast Fourier Transforms (FFT) analyser. Only single PZT patch is sufficient to extract the first nine modes shape of the steel frame .By using numerical model, mode shapes are extracted corresponding to each identified natural frequency. After determining natural frequencies and experimental mode shape, damages can be located by method of Naidu and Soh (2004). This approach can be used for damage/crack detection at ver...

Complex operating environment makes excavator working device become the main wearing parts ,In this paper,builds three-dimensional models of the excavator working device using Pro/E. And selects the working device typical operating conditions which is in the maximum discharge height ,then imports it into the finite element analysis software ANSYS - Workbench module. For the purpose of acquiring its natural frequencies and mode shapes, it conducts the modal analysis by setting the material properties, meshing and boundary conditions,which provided foundation for the optimal design of hydraulic excavator overall vibration analysis and structural dynamic characteristics.

This contribution focuses on the use of a new method to reduce the computational demands of fatigue damage computations using continuum damage mechanics. The LArge Time INcrement (LATIN) method incorporates a model order reduction approach namely the proper generalised decomposition (PGD). LATIN has been extended to tackle damage problems. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper presents the system identification technique by applying PZT actuators and PVDF sensors. An array of finite-length PVDF films is equally spaced and distributed over a cantilever beans acting as sensors. Two finite-length PZT patches are bonded symmetrically and excited 180°out-of-phase for pure bending excitation. The theoretical base of modal analysis for system identification of piezoceramic beams is presented to derive the frequency response functions (FRFs) in conventional modal format. The physical interpretation of P11 and PVDF mode shape functions are characterized respectively. The FRFs between the PZT actuators and PVDF sensors are experimentally measured. By the operation of a series of FRFs and the application of modal parameter extraction method, the modal parameters. including system natural frequencies, mode shapes and modal damping ratios, can be obtained. Both theoretical and experimental approaches agree very well. The system information can be used for control applications and also be applicable to structural failure diagnosis.