Wieslaw K. Binienda

Analysis of an Interface Crack for a Functionally Graded Strip Sandwiched Between Two Homogeneous Layers of Finite Thickness

Abstract The interface crack problem for a composite layer that consists of a homogeneous substrate, coating and a nonhomogeneous functionally graded interphase was formulated for singular integral equations with Cauchy kernels, which were integrated using the Lobatto–Chebyshev collocation technique. Mixed-Mode Stress Intensity Factors (SIFs) and Strain Energy Release Rates were calculated. The SIFs were compared for accuracy with relevant results previously published. The parametric studies were conducted for the various thickness of each layer and for various nonhomogeneity ratios. Particular application to the Zirconia thermal barrier on steel substrate is demonstrated.

Analysis of the Driving Forces for Multiple Cracks in an Infinite Nonhomogeneous Plate, Part I: Theoretical Analysis

A general methodology is constructed for the fundamental solution of an arbitrarily oriented crack embedded in an infinite nonhomogeneous plate in which the shear modulus varies exponentially with one coordinate. The stress is evaluated as a summation of two states of stresses; one is associated with a local coordinate system in an infinite plate, while the other is associated with the boundaries of finite plate defined in a structural coordinate system. The fundamental solution is used to generate stress intensity factors and strain energy release rates for fully interactive multiple crack problems. Part I of this paper focuses on the analytical development of the solution. In Part II, the numerical technique used in solving singular integral equations obtained in Part I is presented, along with a parametric study.

Finite Element Model for Failure Study of Two-Dimensional Triaxially Braided Composite

A new three-dimensional finite-element model of two-dimensional, triaxially braided composites is presented in this paper. This mesoscale modeling technique is used to examine and predict the deformation and damage observed in tests of straight-sided specimens. A unit cell-based approach is used to consider the braiding architecture and the mechanical properties of the fiber tows, the matrix, and the fiber tow-matrix interface. A 0°/±60° braiding configuration has been investigated by conducting static finite-element analyses. Failure initiation and progressive degradation has been simulated in the fiber tows by using the Hashin failure criteria and a damage evolution law. The fiber tow-matrix interface was modeled by using a cohesive zone approach to capture any fiber-matrix debonding. By comparing the analytical results with those obtained experimentally, the applicability of the developed model was assessed and the failure process was investigated.

Analysis of the driving forces for multiple cracks in an infinite nonhomogeneous plate. Part II : Numerical solutions

In Part I of this work, an analytical model was developed for the fundamental solution for a crack embedded in an infinite nonhomogeneous plate. This fundamental solution is used here to generate the stress intensity factors and strain energy release rates for fully interactive multiple crack problems. Also, a numerical technique used in solving the singular integral equation in Part I is presented, along with a parametric study. The parametric study addresses the influence of crack distance, relative angular orientation, and the coefficient of nonhomogeneity on the crack driving forces. The strain energy release rate is recommended for use as a crack propagation criterion because it depends on the local material properties as well as all the remaining parameters contained in the stress intensity factors.

Analysis of the driving force for a generally oriented crack in a functionally graded strip sandwiched between two homogeneous half planes

The driving forces for a generally oriented crack problem embedded in a Functionally Graded strip sandwiched between two half plane are analyzed using singular integral equations with Cauchy kernels, and integrated using Lobatto-Chebyshev collocation. Mixed-mode Stress Intensity Factors (SIF) and Strain Energy Release Rates (SERR) are calculated. The Stress Intensity Factors are compared for accuracy with previously published results. Parametric studies are conducted for various non-homogeneity ratios, crack lengths, crack orientation and thickness of the strip. It is shown that the SERR is more complete and should be used for crack propagation analysis.

Thermal deformation compensation of a composite beam using piezoelectric actuators

Maintaining the surface shape of precision structures such as spacecraft antenna reflectors has been a challenging task. Surface errors are often introduced by thermal distortions due to temperature differences. This paper presents numerical and experimental results of active compensation of thermal deformation of a composite beam using piezoelectric ceramic actuators. To generate thermal distortion of the composite beam, two film heaters are bonded to only one side of the beam using thermally conductive materials. To correct thermal deformation caused by the film heaters, PZT (lead zirconate titanate), a type of a piezoelectric ceramic material, is used in the form of patches as actuators. These PZT patches are bonded on the other side of the beam. First, finite-element analyses are conducted with consideration of the coupled effects of structural, electric and thermal fields on the composite beam. These analyses include static coupled field modeling of the beam deformation with PZT actuation, transient modeling of the beam under thermal loading, and static coupled field modeling of the composite beam with thermal distortion and simultaneous PZT actuation to correct this distortion. Then, experiments are conducted to study the thermal effect, the PZT actuation effect and active thermal distortion compensation using PZT actuators with a proportional, integral and derivative feedback controller. Finite-element modeling and experimental results agree well and demonstrate that the proposed method can actively perform structural shape control in the presence of thermal distortion.

Analysis of bent crack in unidirectional fibre reinforced composites

This paper presents a fracture analysis for a bent crack in an infinite orthotropic plate subjected to a far-field uniform tensile stress. To determine parameters relevant to the mixed-mode fracture conditions at the tip of the bent crack, the problem is formulated in terms of singular integral equations with generalized Cauchy kernels. The resulting system of equations is then solved numberically employing a Gaussian quadrature and the collocation method. Stress intensity factors, k1 and k2, and the strain energy release rates, GI and GII at the tip of the bent crack are obtained for various values of fibres direction θ and L2/L1 ratios. Extensive results for a graphite-epoxy unidirectional composite laminate are presented.

Effects of hygrothermal cycling on the chemical, thermal, and mechanical properties of 862/W epoxy resin

The hygrothermal aging characteristics of an epoxy resin were characterized over a one-year period, which included 908 temperature and humidity cycles. The epoxy resin quickly displayed evidence of aging through color change and increased brittleness. The influence of aging on the material’s glass transition temperature (T g) was evaluated by Differential Scanning Calorimetry and Dynamic Mechanical Analysis. The T g remained relatively constant throughout the year-long cyclic aging profile. Chemical composition was monitored by Fourier Transform Infrared spectroscopy, where evidence of chemical aging and advancement of cure was noted. The tensile strength of the resin was tested as it aged and this property was severely affected by the aging process in the form of reduced ductility and embrittlement. Detailed chemical evaluation suggests many aging mechanisms are taking place during exposure to hygrothermal conditions.

Analytical Model and Numerical Analysis of the Elastic Behavior of Triaxial Braided Composites

AbstractThis paper is concerned with elastic behavior of a triaxial braided composite by using a three-dimensional analytical model and mesoscale finite-element (FE) analysis, in conjunction with experimental observations. The analytical method and FEM take into account the actual fabric structure by considering the fiber undulation and actual architecture parameters. A representative unit cell model of the triaxial braided architecture is first identified based on fiber volume ratio, specimen thickness, and microscopic image analysis. Detailed geometric parameters for axial and bias fiber bundles are obtained, which provide precise information to enable the development of analytical and FE models. A general three-dimensional analytical model based on realistic architecture is developed with consideration of axial and bias fiber undulation. A typical study on the effect of axial fiber undulation is presented through the analytical model and axial tensile test. The prediction of effective elastic constants...

Modification of a Macromechanical Finite Element-Based Model for Impact Analysis of Triaxially Braided Composites

Abstract A macro level finite element-based model has been developed to simulate the mechanical and impact response of triaxially-braided polymer matrix composites. In the analytical model, the triaxial braid architecture is simulated by using four parallel shell elements, each of which is modeled as a laminated composite. For the current analytical approach, each shell element is considered to be a smeared homogeneous material. The commercial transient dynamic finite element code LS-DYNA is used to conduct the simulations, and a continuum damage mechanics model internal to LS-DYNA is used as the material constitutive model. The constitutive model requires stiffness and strength properties of an equivalent unidirectional composite. Simplified micromechanics methods are used to determine the equivalent stiffness properties, and results from coupon level tests on the braided composite are utilized to back out the required strength properties. Simulations of quasi-static coupon tests of several representative braided composites are conducted to demonstrate the correlation of the model. Impact simulations of a represented braided composites are conducted to demonstrate the capability of the model to predict the penetration velocity and damage patterns obtained experimentally.