Micro-mechanical finite element analysis of Z-pins under mixed-mode loading

© 2015 Elsevier Ltd. All rights reserved.This paper presents a three-dimensional micro-mechanical finite element (FE) modelling strategy for predicting the mixed-mode response of single Z-pins inserted in a composite laminate. The modelling approach is based upon a versatile ply-level mesh, which takes into account the significant micro-mechanical features of Z-pinned laminates. The effect of post-cure cool down is also considered in the approach. The Z-pin/laminate interface is modelled by cohesive elements and frictional contact. The progressive failure of the Z-pin is simulated considering shear-driven internal splitting, accounted for using cohesive elements, and tensile fibre failure, modelled using the Weibulls criterion. The simulation strategy is calibrated and validated via experimental tests performed on single carbon/BMI Z-pins inserted in quasi-isotropic laminate. The effects of the bonding and friction at the Z-pin/laminate interface and the internal Z-pin splitting are discussed. The primary aim is to develop a robust numerical tool and guidelines for designing Z-pins with optimal bridging behaviour

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

Enhanced cohesive zone model to predict delamination behavior of carbon/epoxy laminated curved beams

This paper proposes an enhanced Cohesive Zone Model (CZM) for the prediction of delamination in curved beams of epoxy carbon laminates. This model improves the conventional CZM, taking into account the fiber-bridging phenomenon and the variation of the element size among the thickness in the curved zone. The advantages of the enhanced model are underlined when results obtained from the numerical simulations of a four-point-bending test in compliance with ASTM D6415 standard are compared with the corresponding experimental results. The prediction of the post-failure behavior obtained with this model is closer to that obtained experimentally than with the conventional model

Cohesive zone model for facesheet -core interface delamination in honeycomb FRP sandwich panels

The focus of this dissertation is on developing efficient modeling techniques to study facesheet-core interface delamination in honeycomb fiber-reinforced polymer (HFRP) sandwich panels. Delamination problems are usually treated from a fracture mechanics point of view. However, interface delamination is generally very complex in nature and difficult to solve, because it involves not only geometric and material discontinuities, but also the inherently coupled Mode I, II and III fracture in layered material systems attributed to the well-known oscillatory singularity nature of the stress and displacement field in the vicinity of the delamination crack tip. One of the key issues in this research is to determine the best way to characterize interface delamination within the framework of continuum mechanics rather than using ad hoc methods just to facilitate numerical implementations, such as springs across a crack in the finite element method.;The usual requirement of defining an initial crack and assuming self-similar progression of a crack, make traditional fracture mechanics approaches inefficient for modeling interface delamination. To circumvent these difficulties, five most relevant nonlinear crack models are reviewed and compared. It is concluded that by unifying strength-based crack initiation and fracture-based crack progression, the cohesive crack modeling approach has distinct advantages compared to other global methods.;In this study, a cohesive zone model (CZM) with linear-exponential irreversible softening traction-separation law, satisfying empirical mixed-mode fracture criteria, is proposed to represent progressive damage occurring within the interface during the fracture process. The CZM is implemented as a cohesive interface element through a user-defined element subroutine within the general purpose finite element code ABAQUS. The framework and formulation of a three dimensional interface element are presented. Two sets of parameters are required for application of the developed interface element, namely, interfacial strength and fracture toughness. (Abstract shortened by UMI.)

Analysis Methods for Progressive Damage of Composite Structures

This document provides an overview of recent accomplishments and lessons learned in the development of general progressive damage analysis methods for predicting the residual strength and life of composite structures. These developments are described within their State-of-the-Art (SoA) context and the associated technology barriers. The emphasis of the authors is on developing these analysis tools for application at the structural level. Hence, modeling of damage progression is undertaken at the mesoscale, where the plies of a laminate are represented as a homogenous orthotropic continuum. The aim of the present effort is establish the ranges of validity of available models, to identify technology barriers, and to establish the foundations of the future investigation efforts. Such are the necessary steps towards accurate and robust simulations that can replace some of the expensive and time-consuming "building block" tests that are currently required for the design and certification of aerospace structures

Methods for efficient modelling of progressive failure in laminated fibre-reinforced composites

To meet increasing demands on reduced CO2 emissions, the automotive industry is currently very active in research to reduce vehicle weight by incorporating laminated composites (primarily carbon fibre-reinforced polymers) into structural components.Historically, composite materials have mainly been used in the aerospace industry, whereby CAE-based design and development tools for composite structures have been developed primarily to the specific needs and requirements in this industry. In general, the crashworthiness of aerospace structures is only assessed to a small extent compared to that of automotive structures. Consequently, no suitable numerical simulation tools, capable of assessing the crashworthiness of composite automotive structures, have been developed.The fracture process of laminated composites is more complicated than that of metals, the dominant class of materials used in automotive crash protection systems today. Thus, numerical models developed for metals cannot be used to accurately predict the crashworthiness of composite structures.\ua0Instead, high-fidelity models that can resolve the complicated fracture process must be used.\ua0However, these models require excessive computational times, making industrial crash simulations infeasible.\ua0It is therefore crucial to develop computationally efficient numerical tools, which are able to accurately predict the crashworthiness performance of composite structures.In this thesis, I will present a route towards full-scale vehicle crash simulations using a computationally efficient adaptive method. The method is based on an equivalent single-layer shell model which, during the analysis, is adaptively transformed to a high-fidelity model in areas where higher accuracy is required.\ua0This way, the increased computational cost, associated with the analysis of progressive damage in laminated composites, can be limited both in time and to the pertinent areas of the model.The adaptive modelling method\ua0can successfully reproduce the same level of accuracy as a high-fidelity model, at lower computational cost. Consequently, this method can help to enable computationally efficient crash simulations of laminated structures, which in the long run will allow composite materials to have a widespread use in future automotive vehicles

Manufacturing and Characterization of Continuous Nanofiber-Reinforced Composites

Fiber-reinforced composite laminates are some of the most advanced structural materials available. However, delamination remains a critical challenge due to its prevalence in structures and ability to cause catastrophic failure. Recently, high-temperature composites are at the forefront of polymer-matrix composites research, but they are prone to microcracking followed by delamination. Nanoreinforcement of interfaces by continuous nanofibers has been proposed earlier at UNL and produced increased interlaminar fracture resistance in conventional advanced composites. However, no studies have yet been conducted on emerging high-temperature composites. Also, there is insufficient information on the translatability of observed modes I and II interlaminar fracture toughness improvements to the structural performance level. The main objectives of this dissertation were to explore feasibility of nanofiber-based delamination suppression in high-temperature laminates and to study translation of delamination suppression via nanofiber-interleaving to the performance of composite structural volumes. Unidirectional carbon/epoxy and carbon/cyanate ester composites were reinforced with continuous nanofiber interleaves electrospun from polyacrylonitrile or polyimide, and their fracture mechanics performance was characterized and compared. Significant improvements in modes I and II fracture resistance were demonstrated with the high-temperature material for the first time. The improvements in material properties were also translated to the structural performance of laminates with and without holes and L-shaped composites. Nanofiber-reinforced specimens continued to perform better than pristine specimens, and the high-temperature material showed greater improvements. To mimic the controlled anisotropy and high fiber volume fraction of traditional advanced laminates, laminated nanocomposites reinforced with aligned, continuous nanofibers were fabricated and characterized. Results prove the feasibility of manufacturing nanolaminates with distinct oriented plies, high nanofiber volume fractions, and improved properties. Lastly, feasibility of nanofiber structure tailoring with graphene nanoribbons and MXenes was explored. It was shown that incorporation of MXene nanoparticles can lead to significant improvements in the graphitic structure of the templated carbon nanofibers. Overall, this dissertation provides novel results on continuous nanofiber-reinforcement of high-temperature composites and advanced composite structures. The knowledge gained will contribute to the extension of electrospun nanofibers from the laboratory to industrial applications. Advisor: Yuris Dzeni