Wooseok Ji

On the importance of work-conjugacy and objective stress rates in finite deformation incremental finite element analysis

This paper is concerned with two issues that arise in the finite element analysis of 3D solids. The first issue examines the objectivity of various stress rates that are adopted in incremental analysis of solids. In doing so, it is revealed that large errors are incurred by an improper choice of stress rate. An example problem is presented to show the implications of the choice of stress rate. The second issue addresses the need to maintain work-conjugacy in formulating and solving bifurcation buckling problems of 3D elastic solids. Four popular commercial codes are used to obtain buckling loads of an axially compressed thick sandwich panel, and it is shown that large errors in buckling load predictions are incurred as a result of violating the requirement of work-conjugacy. Remedies to fix the errors in the numerical solution strategy are given.

Progressive Damage and Failure Prediction of Open Hole Tension and Open Hole Compression Specimens

Progressive damage and failure in open hole composite laminate coupons under tensile and compressive loading conditions is modeled using Enhanced Schapery Theory (EST). The input parameters required for EST are obtained using standard coupon level test data and are interpreted in conjunction with finite element (FE) based simulations. The capability of EST to perform the open hole strength prediction accurately is demonstrated using three different layups of IM7/8552 carbon fiber composite. A homogenized approach uses a single composite shell element to represent the entire laminate in the thickness direction and this requires the fiber direction fracture toughness to be modeled as a laminate property. The results obtained using the EST method agree quite well with experimental results.

Face-on and Edge-on Impact Response of Composite Laminates

This paper presents comprehensive experimental and numerical studies on the face-on and edge-on impact behavior of composite laminates. Experimental work is focused on finding the impact energy limits for barely visible impact damage (BVID) when a laminated composite plate is impacted either on its face or an edge. High-fidelity finite element analysis (FEA) model utilizing Enhanced Schapery theory (EST) combined with Discrete Cohesive Zone Model (DCZM) is developed to predict the response of the laminated composite plate subjected to face-on and edge-on impact loading. Predictions from the proposed high-fidelity FEA model are compared against experimental data and it is shown that the numerical results agree well with the test data.

The Temporal Evolution of Buckling in a Dynamically Impacted Column

The time-dependent progressive evolution of transverse displacements of an axially impacted, slender, geometrically imperfect, column is studied here. The analysis is concerned with evaluating the time-history associated with the evolution of the buckling response as a function of the initial geometric imperfection amplitude. The exact solution of the axial stress wave propagation is employed to study the physics of the buckling response with the nonuniform axial strain distribution varying in time and space. The responses of axially impacted columns are examined in light of past experimental results and associated numerical solutions. Results in the present paper are limited to elastic column behavior.

Single Edge Notch Tension Test on Cross-Ply Laminated Composites for Intralaminar Fracture Properties

A systematic yet simple way to measure the fracture toughness value for a Mode I crack that occurs perpendicular to the fiber direction in a unidirectional composite is presented. Cross-ply single edge notch tension (SENT) tests combined with finite element analysis areutilized to obtain the in-plane fracture toughness value under tension in the direction of fibers. The notch length of the SENT specimen is used as a parameter to back out the fracture toughness value in a consistent manner.

A Predictive Model for the Compressive Strength of 3D Woven Textile Composites

The compressive response of 3D woven textile composites (3DWTC), that consist of glass fiber tows and an epoxy matrix material, is studied using a finite element (FE) based micromechanics model. A parametric Representative Unit Cell (RUC) model is developed in a fully three-dimensional setting with geometry and textile architecture for modeling the textile microstructure. The RUC model also accoutns for the nonlinear behavior of the fiber tows and matrix. The computational model is utilized to predict the compressive strength of 3DWTC and its dependence on various geometrical and material parameters. The finite element model is coupled with a probabilistic analysis tool to provide probabilistic estimates for 3DWTC compressive strength. I. Introduction hree-dimensional woven textile composites (3DWTCs) are gaining ever-increasing attention from various engineering sectors due to numerous structural advantages of the material system over conventional laminated composites 1-3 . Typical laminated composites have very good in-plane properties in the direction of the fiber tows, but tend to have low strength properties perpendicular to the fiber direction. The interfaces between the layers are the weakest link and delamnation is a common failure mode of laminated composites. Textile composites can have fiber tows in multiple directions and thus can have better properties in all loading directions. Through 3D weaving processes, 3DWTCs can implement Z-yarns woven around warp and weft fiber tows, improving the resistance to delamination failure dramatically. Advanced weaving technologies make the tow architecture tailorable to achieve desired mechanical performance for numerous applications. However, it is not easy to design and tailor 3DWTC for specific applications since it is very difficult to predict the mechanical performance of textile composites due to the complex textile architecture. Currently textile composites, especially with three dimensional architectures, are designed by an expensive build-and-test approach, which is a time consuming and costly process. There exists a stong need for a predictive model that is capable of reliably predicting mechanical performance of textile composites including basic stiffness and strength properties. In the present study, a finite element model for 3DWTCs is developed to predict the mechanical properties, specifically with a focus on compressive strength prediction. The modeling strategy, originally developed by Song et. al. 1 for two-dimensional tri-axially braied textile composites, will be extended here for 3DWTCs. They have shown good correlations with 2D in-plane woven systems for determining stiffness and strength properties, and also in describing progressive damage. The proposed model utilizes the true measured 3D geometry and nonlinear behavior of individual constituents to estimate compressive strength.

Computational modeling of failure in composite structures including uncertainties in material and geometrical properties

This paper is concerned with a progressive failure analysis methodology for fiber reinforced composite laminates combining various analytical models designed for investigating failure mechanisms at different length scales. The methodology here employs a fundamental mechanism based approach to predict failure or damage initiation with strong coupling between the multiple length scales. The discrete cohesive zone model elements are used to model the adhesion and delamination failure at macroscale while Schapery theory, a continuum damage theory based on thermodynamics, is used to model material degradation occurring at the lamina level. Furthermore, the present numerical framework is incorporated with a probabilistic analysis module, based on the NEESUS software, to consider material variability and manufacturing inconsistencies. The combined analysis modules are implemented in a non-linear finite element code for modeling the progressive failure of advanced composite structures. The proposed progressive failure analysis methodology is applied to several cases for validating its capability of predicting the evolution of the interactive failure mechanisms in composite structures.

Modeling Axial Impact Response of Sandwich Panels using Probability-Based Finite Element Analysis

The dynamic response problem of a sandwich structure subjected to axial impact by a falling mass is investigated. A comprehensive set of impact test results of sandwich panels with various configurations is presented. Failure mechanisms and the temporal history of how a sandwich column responds to axial impact are discussed through the experimental results. The experimental results are compared against finite element based simulation of the impact event.

Progressive Failure Analysis Method of a Pi Joint with Uncertainties in Fracture Properties

In this presentation, the structural performance of fiber reinforced composite laminate Pi joints is studied through a progressive failure analysis (PFA) method. The bonded joint area that is the weakest link of the Pi joint structure is modeled using the discrete cohesive zone method (DCZM). The damage growth and failure of the bonded interface is modeled through an exponential decaying traction-separation law that governs the behavior of DCZM elements. This interface model is implemented into a non-linear finite element (FE) code for modeling the progressive failure of the composite Pi joint structures. The present PFA framework is incorporated with a probabilistic analysis module to consider material variability and manufacturing inconsistencies. The proposed PFA methodology is demonstrated for a 2D Pi-shaped laminate composite structure adhesively bonded through a Pi joint, and subjected to a pull-off load. Nomenclature G IC = Mode I critical strain energy release rate G IIC = Mode II critical strain energy release rate C σ = critical cohesive strength of Mode I C τ = critical cohesive strength of Mode II iC δ = critical relative displacement fields ( i =1 and 2) i δ = relative displacement fields ( i =1 and 2) i α = softening rate of a traction separation law ( i =1 and 2) ) ( ~ mn i K = initial stiffness of a DCZM subelement