Meisam Jalalvand

Micro/macro approach for prediction of matrix cracking evolution in laminated composites

A computational constitutive model is presented to predict matrix cracking evolution in laminates under in-plane loading. Transverse cracks are treated as separate discontinuities in the micro-model that provides damage parameters for the macro-model. Both micro- and macro-models are implemented using finite element analysis, specifically, ANSYS, to avoid limitation of analytical micro-modeling. The computational cost of the micro-model is limited to constructing a database of micro-model predictions a priori. The macro-model is simply a finite element analysis discretization of the structure using plane stress or shell elements in ANSYS. The macro-model queries the database, which effectively becomes a constitutive model. The damage surfaces in the database are obtained from the results of large number of finite element micro-scale (unit-cell) analyses. The proposed procedure is implemented in ANSYS as a usermaterial subroutine for transverse crack initiation and propagation in symmetric cross-ply and [0r/(θ / −θ)s/0n]s laminates under in-plane loads. This method is also examined to study matrix crack evolution in tensile specimen with open hole, and the results found to be in good agreement with available experimental data.

Numerical modeling of diffuse transverse cracks and induced delamination using cohesive elements

This article is devoted to the modeling of spread kind of damages such as matrix cracking and induced delamination in symmetric and asymmetric cross-ply laminates of composite materials using cohesive elements. For matrix crack modeling, parallel rows of cohesive elements are used between every row of 2D elements in 90° layers. Delamination is also modeled by cohesive elements at the 90°/0° interface. Since matrix cracking is a diffuse kind of damage mechanism, application of cohesive elements is not straightforward, and special techniques are necessary to resolve the modeling difficulties. For this purpose, two techniques of “bisecting” and “random distribution of strength of cohesive elements” are proposed here. Both techniques are applied to various symmetric laminates of [0/903]s and [90n/0]s (n = 1 to 3). The predicted stiffness and damage progresses from both techniques are in good agreement with the experimental results. Then, asymmetric cross-ply laminates of [90n/0] (n = 1 to 3) are analyzed to show the capability of this method in progressive damage analyses. The proposed method is less restricted in comparison with available micromechanical methods and is able to predict damage initiation, propagation and damage-mode transition for any symmetric and asymmetric cross-ply sequence. Therefore, this method can be used for development of “in-plane damage” of constitutive laws especially when specimens are subjected to complex loading and boundary conditions.

Analysis of the damage mechanisms in mixed-mode delamination of laminated composites using acoustic emission data clustering

In this study, acoustic emission (AE) technique is used to investigate different time-to-failure mechanisms of delamination in glass/epoxy composite laminates. Woven and unidirectional layups were subjected to the double cantilever beam, end notch flexure, and mixed-mode bending tests and the generated AE signals were captured. Discrimination of the AE events, caused by different types of the damage mechanisms, was performed using wavelet packet transform (WPT) and fuzzy clustering method (FCM) associated with a principal component analysis (PCA). The FCM and WPT analyses identified three dominant damage mechanisms. Furthermore, different interface layups and different GII/GT modal ratio values (ratio of mode II strain energy release rate per total strain energy release rate) indicated different time-to-failure mechanisms incidence. Additionally, the damaged mechanisms were observed using scanning electron microscopic (SEM) analysis. The results showed that the dominant damage mechanisms in all the specimens are matrix cracking and fiber–matrix debonding. Besides, some fiber breakage appeared during the tests, and the percentage of this damage mechanism in the unidirectional specimens and mode I condition was higher than those in the woven specimens and mode II. SEM observations were also in good agreement with the obtained results. It was found that the presented methods can be utilized to improve the characterization and discrimination of damage mechanisms in the actual occurring modes of delamination in composite structures.

Delamination of Laminates Governed by Free Edge Interlaminar Stresses Using Interface Element

In this paper, interface element with de-cohesive constitutive law is used to predict the delamination progress of laminates in which delamination is the prominent failure mode. For this purpose, a finite element program is developed to perform nonlinear damage analysis. The analyses are carried out based on the interlaminar constitutive law of elastic-plastic-damage proposed before in the literature. Delamination initiation and propagation of several laminates with dominant interlaminar shear stresses at free edges are investigated to find the failure load. It is shown that the difference between the predicted failure loads using the present study and the experimental results are 3.1% to 19.4% for various laminates.

Numerical aspects of delamination modeling using interface elements

In spite of a vast background on cohesive constitutive law and its use for various analyses such as delamination in composite laminates, some numerical aspects of that have been less explored and reported in the literature. The aim of this paper is to study the phenomenon of spurious stress oscillation and also dominant process zone (where damage has its most significant evolution) in delamination modeling. For this purpose, distribution of normal stress and damage parameter of different DCB specimen are analyzed. Distribution of stress around the process zone indicates spurious oscillation just ahead of the delamination tip where damage parameter is zero and no effect of this phenomenon is seen on results of applied load versus opening displacement. Additionally, it is shown that larger values of penalty stiffness lead in smaller length of dominant process zone and very large values of penalty stiffness pushes the distribution of damage parameter to become step-like function. Authors believe that this effect is in fact the main reason of un-converged solution of models with too large penalty values.

Shear-Mode Viscoelastic Damage Formulation Interface Element

In this paper, a viscoelastic-damage cohesive zone model is formulated and discussed. The interface element constitutive law has two elastic and damage regimes. Viscoelastic behaviour has been assumed for the shear stress in the elastic regime. Three element Voigt model has been used for the formulation of relaxation modulus of the material. Shear Stress has been evaluated in the elastic regime of the interface with integration over the history of the applied strain at the interface. Damage evolution proceeds according to the bilinear cohesive constitutive law up to the complete decohesion. Numerical examples for one element model has been presented to see the effect of parameters on cohesive constitutive law.