Energy Absorption of Carbon-Fiber-Reinforced Composite Laminates Under Low-Velocity Impacts

Mechanics of Composite Materials - Drop-weight tests were conducted to investigate the impact responses of carbon-fiber-reinforced composite laminates under low-velocity impacts. A finite-element...     

Mesh-Independent Modeling and Moiré Interferometry Studies of Damage Accumulation in Open-Hole Composite Laminates

Mechanics of Composite Materials - A three-dimensional ply-level modeling of multiple matrix cracking near an open hole in a quasi-isotropic composite laminate was performed. A mesh-independent...     

Equivalent Spectral Method to Estimate the Fatigue Life of Composite Laminates Under Random Vibration Loadings

Mechanics of Composite Materials - The power spectral density (PSD) of an equivalent stress is proposed to analyze the fatigue life of composite laminates under random vibration loadings using the...     

The Boundary Finite Element Method for Stress Concentration Problems in Composite Laminates

The Boundary Finite Element Method is presented as a very efficient method for the analyses of stress concentration problems in laminates. Results for the free-edge stress field of symmetric cross-ply laminates show to be in very good agreement with comparative finite element analyses.     

The Effects of Poisson Contraction on Matrix Cracking in Brittle-Matrix Composites

The effects of Poisson contraction on matrix cracking in unidirectional fiber-reinforced brittle-matrix composites are studied in this paper. The fibers, initially held in the matrix by a compressive pressure due to the thermal expansion mismatch, are subjected to frictional slipping over the matrix as soon as a fiber-bridged crack is formed. The friction between the fibers and the matrix is assumed to follow the Coulomb friction law. A shear-lag model, which includes the Poisson contraction and the friction due to the relative fiber/matrix slipping, is adopted to calculate the stress and strain fields in the fibers and matrix. Using the energy balance approach, a relation for the critical matrix cracking stress for propagating of a semi-infinite fiber-bridged crack is derived. The results obtained show that the Poisson contraction has a strong effect on the predicted matrix cracking stress in brittle-matrix composites, especially in composites with a stiff matrix.     

复合材料层板饱和裂纹间距的试凑法(cut-and-try)

本文首次通过三维有限元法计算分析出90°层中横向裂纹的饱和间距值,为预测饱和裂纹间距提供了一条新途径.     

Thermal control of Huygens mock up probe in Earth atmosphere: Thermo‐fluid dynamic simulation and mission data

A mock up of Huygens probe with 76 channels acquisition has been developed at University of Padova and successfully flown with a stratospheric space balloon from Italian Space Agency Base in Trapani on May 30th 2002. Temperature sensors have monitored temperature profiles in critical points of electronics, batteries and structure during raise at 40 km altitude, floating and parachuted descent. A thermal model of the probe has been implemented taking into consideration incoming external fluxes (solar direct, albedo and radiative heat fluxes), internal heat fluxes generation and convective heat transfer with atmosphere as a function of probe altitude. For the evaluation of convective fluxes and probe spinning rates an algebraic turbulent model, developed by the present authors, has been employed. This model is suitable to predict effects such as flow curvature and separation and solid boundary rotations. Simulation results have been utilized during project phase to optimize thermal paths in order to keep critical components in the allowable temperature range and for post flight analysis of mission data. These data show that passive thermal control of the probe has performed as expected contributing to a extremely successful scientific flight.     

Mixed Micromechanics and Continuum Damage Mechanics Approach to Transverse Cracking in [S, 90n]s Laminates

The stiffness reduction in [S, 90 _n ] _s laminates due to transverse cracking in 90-layers is analyzed using the synergistic continuum damage mechanics (SCDM) and a micromechanics approach. The material constants involved in the SCDM model are determined using the stiffness reduction data for a reference cross-ply laminate. The constraint efficiency factor, which depends on the stiffness and geometry of neighboring layers, is assumed to be proportional to the average crack opening displacement (COD). The COD as a function of the constraint effect of adjacent layers and crack spacing is described by a simple power law. The crack closure technique and Monte Carlo simulations are used to model the damage evolution: the 90-layer is divided into a large number of elements and the critical strain energy rate G _c having the Weibull distribution is randomly assigned to each element. The crack density data for a [0₂/90₄] _s cross-ply laminate are used to determine the Weibull parameters. The simulated crack density curves are combined with the CDM stiffness reduction predictions to obtain the stiffness versus strain. The methodology developed is successfully used to predict the stiffness reduction as a function of crack density in [±/90₄] _s laminates.     

Efficient Algorithms for Large‐Scale Quadratic Matrix Equations

Quadratic matrix equations and in particular symmetric algebraic Riccati equations play a fundamental role in systems and control theory. Classically, they are solved via methods using their connection to Hamiltonian eigenproblems. Due to the ever‐increasing complexity of the models describing the underlying control problems, new methods are needed that can be used for large‐scale problems. In particular, sparsity of the coefficient matrices, obtained, e.g., from semi‐discretizing partial differential equations to describe the physical process to be controlled, need to be addressed. We briefly review recent approaches based on Krylov subspace methods and discuss a new approach employing a sparse implementation of Newton's method for algebraic Riccati equations.     

Mathematical Modeling of Powder‐Snow Avalanche Flows

Powder‐snow avalanches are violent natural disasters which represent a major risk for infrastructures and populations in mountain regions. In this study we present a novel model for the simulation of avalanches in the aerosol regime. The second scope of this study is to get more insight into the interaction process between an avalanche and a rigid obstacle. An incompressible model of two miscible fluids can be successfully employed in this type of problems. We allow for mass diffusion between two phases according to the Fick's law. The governing equations are discretized with a contemporary fully implicit finite volume scheme. The solver is able to deal with arbitrary density ratios. Several numerical results are presented. Volume fraction, velocity, and pressure fields are presented and discussed. Finally, we point out how this methodology can be used for practical problems.     

Polynomial motion of non‐holonomic mechanical systems of chained form

In this study a simple general motion planning approach for non‐holonomic mechanical systems of chained form is proposed, in which the intricate motion planning problem (steering the system from an initial state to a final state) is converted to a simple curve‐fitting problem (satisfying a set of end‐point conditions). By means of this approach, other geometric constraints, such as passing a channel and avoiding collision with obstacles, and minimizing some cost functions, such as the minimum path length, can be easily handled, while the control inputs can be derived directly from the smooth path planned. For verifying the effectiveness of the proposed approach, a number of simulations are conducted for various task require ments and environments, with respect to a four‐wheel mobile chart (one‐chain system) and a three‐input firetruck (two‐chain system). Copyright © 1999 John Wiley & Sons, Ltd.     

Multi‐Trace Boundary Integral Formulation for Acoustic Scattering by Composite Structures

We study the scattering of an acoustic wave by an object composed of several adjacent parts with different material properties. For this problem we derive a new integral equation formulation of the first kind. This formulation involves two Dirichlet data and two Neumann data at each point of each material interface of the diffracting object. It is immune to spurious resonances, and it enjoys a stability property that ensures quasi‐optimal convergence of conforming Galerkin boundary element discretization. In addition, the operator of this formulation satisfies a relation similar to the standard Calderón identity. © 2012 Wiley Periodicals, Inc.     

Quasi‐Periodic Solutions of Nonlinear Evolution Equations Associated with a 3 × 3 Matrix Spectral Problem

A hierarchy of new nonlinear evolution equations associated with a 3 × 3 matrix spectral problem with three potentials is proposed. With the aid of the characteristic polynomial of Lax matrix for the hierarchy, we introduce an algebraic curve of arithmetic genus m ? 1 , and discuss in detail the properties of the associated Baker–Akhiezer function and meromorphic function. On the basis of the theory of algebraic curves, we obtain the explicit theta function representations of the Baker–Akhiezer function, the meromorphic function, and, in particular, that of solutions for the entire hierarchy of nonlinear evolution equations.     

Generalized Dirichlet‐to‐Neumann Map in Time‐Dependent Domains

We study the heat, linear Schrödinger (LS), and linear KdV equations in the domain l(t) < x < ∞ , 0 < t < T , with prescribed initial and boundary conditions and with l(t) a given differentiable function. For the first two equations, we show that the unknown Neumann or Dirichlet boundary value can be computed as the solution of a linear Volterra integral equation with an explicit weakly singular kernel. This integral equation can be derived from the formal Fourier integral representation of the solution. For the linear KdV equation we show that the two unknown boundary values can be computed as the solution of a system of linear Volterra integral equations with explicit weakly singular kernels. The derivation in this case makes crucial use of analyticity and certain invariance properties in the complex spectral plane. The above Volterra equations are shown to admit a unique solution.