The experimental study and predictive virtual testing of biaxial Non-Crimp Fabric (NCF) composites
File(s)
Author(s)
Gouskos, Dimitris
Type
Thesis
Abstract
For many years, NCF composites are under development due to their superior mechanical properties in particular areas when compared against their unidirectional pre-preg taped counterparts. They offer advanced interlaminar and impact properties, reduced manufacturing cost, high level of drapeability but more importantly low production cost with non-autoclave methods (RTM, VI). These advantages have resulted in a grown industrial interest for NCF composites and nowadays in an extensive application to the aerospace, automotive and wind energy sectors. However, NCF composites do have certain disadvantages; due to their internal structure, they exhibit low longitudinal stiffness and compressive strength.
This research project is part of the framework of the Airbus Project, 'The Wing of Tomorrow' and aims to characterise the internal structure, identify the source of structural uncertainties, and comprehend the mechanical response of biaxial NCF composites under quasi-static and impact conditions. An extensive experimental study with deliberate imperfections on NCF composite coupons is performed and includes complete in-plane and interlaminar testing, investigation of damage propagation and low velocity impact (LVI) tests.
Ultimately, the scope of this work is to incorporate the experimental findings to a set of physically based failure criteria, able to predict the damage onset and propagation under quasi-static and impact conditions of the NCF coupons. The proposed criteria account for structural flaws, namely the induced in- and out-of-plane fibre misalignment. Within this study, a novel approach to obtain the effective longitudinal stiffness of the composite, as a function of the in- and out-of-plane fibre misalignment, is presented. Additionally, a new exponential law is introduced during element failure in fibre tension, aiming to capture the load progress in conjunction with the energy release rate while stable damage evolves. Essential influence appears to have in fibre dominated failure modes as in Compact Tension (CT). The failure criteria are also validated against LVI cases with deliberately misaligned and split NCF blankets for energy levels between 30J to 70J and predicted well the mechanical response and the damage modes.
The numerical models are employed in Abaqus/Explicit and the constitutive failure criteria are introduced as a VUMAT subroutine, while they follow the energy-based damage mechanics approach. During computational analyses the mesh objectivity of the model is also studied for every loading scenario, to ensure that acceptable results can be obtained with coarser meshes.
This research project is part of the framework of the Airbus Project, 'The Wing of Tomorrow' and aims to characterise the internal structure, identify the source of structural uncertainties, and comprehend the mechanical response of biaxial NCF composites under quasi-static and impact conditions. An extensive experimental study with deliberate imperfections on NCF composite coupons is performed and includes complete in-plane and interlaminar testing, investigation of damage propagation and low velocity impact (LVI) tests.
Ultimately, the scope of this work is to incorporate the experimental findings to a set of physically based failure criteria, able to predict the damage onset and propagation under quasi-static and impact conditions of the NCF coupons. The proposed criteria account for structural flaws, namely the induced in- and out-of-plane fibre misalignment. Within this study, a novel approach to obtain the effective longitudinal stiffness of the composite, as a function of the in- and out-of-plane fibre misalignment, is presented. Additionally, a new exponential law is introduced during element failure in fibre tension, aiming to capture the load progress in conjunction with the energy release rate while stable damage evolves. Essential influence appears to have in fibre dominated failure modes as in Compact Tension (CT). The failure criteria are also validated against LVI cases with deliberately misaligned and split NCF blankets for energy levels between 30J to 70J and predicted well the mechanical response and the damage modes.
The numerical models are employed in Abaqus/Explicit and the constitutive failure criteria are introduced as a VUMAT subroutine, while they follow the energy-based damage mechanics approach. During computational analyses the mesh objectivity of the model is also studied for every loading scenario, to ensure that acceptable results can be obtained with coarser meshes.
Version
Open Access
Date Issued
2022-04-15
Date Awarded
2023-03-01
Advisor
Iannucci, Lorenzo
Sponsor
Engineering and Physical Sciences Research Council
Airbus Industrie
Grant Number
EP/R511961/1
Publisher Department
Aeronautics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)