Virtual Testing of Non-Crimped Fabric Composites

Lead Research Organisation: Imperial College London
Department Name: Aeronautics


This research project is within the framework of the 'Wing of Tomorrow' project, whose key objective is the design and manufacturing of a complete wing structure from advanced carbon based composites and simulated using Predictive Virtual Testing techniques. The initiative of this research is based on the advantages that Non-Crimp Fabric composites (NCF) (e.g. good mechanical properties, improved impact properties compared to pre-impregnated materials (prepregs), low production cost, high drapeability) can offer in aerospace applications, resulting in lighter structures with reduced operational and manufacturing costs.
Non-Crimped-Fabrics have been under development for some time; they provide certain advantages over traditional pre-impregnated (prepreg) composite materials, mainly a reduction in unit cost and manufacturing cost. Specifically, the non-crimped fabric composites can be produced with an out-of-autoclave manufacturing process. For example, resin transfer moulding (RTM) which uses woven laminates, is relatively low cost and takes advantage of easy handling of large sheets of the fabric.
In terms of strength, NCF do have improved through-thickness properties without significant drop in in-plane performance. In addition, NCFs offer new challenging possibilities to designers and manufacturers, given their higher degree of tailorability, greater deposition rate and improved impact performance when compared to well-established prepreg composites.
The objectives/aims could be summarized as follows:
- A significant goal of the project is the development and validation of Finite Element (FE) simulation tools (i.e. Virtual Testing) which can reduce costs and time within the industrial development cycle since full-scale experimental tests are prohibitively expensive and time consuming.

- The failure mechanism in compression of the NCF is going to be further investigated and understood through this project, since NCFs do suffer from low compressive properties due to their internal structure (i.e. fibre waviness).

- Furthermore, the present PhD project aims to implement physically-based constitutive failure criteria, namely LaRC05, in NCF composites. Moreover, the mechanical performance of a typical aircraft structure (e.g. skin-stringer panel as part of the wing box structure) will be analysed, both computationally (Finite Element Methods) and experimentally under several loading cases, such as tension, compression, shear, cyclic load tension, impact and compression after impact.

- The computational analysis is performed with the application of finite element methods through Dassault Systèmes Abaqus commercial software. The validation and development of the constitutive failure criteria is a cornerstone of this research and will be carried out at tow level before proceeding with the structural level analysis. Coupon level testing will be performed to identify and characterise the typical failure mechanisms with the assistance of several inspection techniques, among which fractography is emphasized.

Hence, a material mechanical properties database will be created to validate all the models developed in this project. As structural level are defined, those models developed in the length scale of a laminate, which are ideal for preliminary design and structural optimisation due to their ability to provide quick predictions.
By the end of this project it is believed that the ability to predict the composite performance up to failure and beyond, in a Virtual Modelling environment, will be significantly improved, having a positive influence to a wide range of industrial applications.

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R511961/1 30/09/2017 30/03/2023
2091551 Studentship EP/R511961/1 30/09/2017 30/03/2022
Description The funded work is still on going, hence a final conclusion cannot be drawn, nevertheless the findings, so far, propose a set of failure criteria able to predict the failure onset and propagation in quasi-static conditions of a particular type of composite materials, named non-crimp fabric composites. This type of composite materials is widely applied in the aerospace, automotive and energy sectors. The prediction capabilities of this constitutive failure model can significantly reduce the required number of mechanical testing of specimens which is very costly both for the academia and the industry. A major benefit of this computational model is that it is not limited to failure prediction of specimen level components but it is equally capable to predict accurately the mechanical behaviour of large structural components (e.g. wing structure components).
Exploitation Route Since the project is currently active, there are still further stages of progress which could be achieved. However, the ultimate objective of this research project is the developed numerical model to be extended, aside from the academia, to industrial applications, therefore the industrial collaboration with the AIRBUS UK.
Sectors Aerospace, Defence and Marine,Construction,Energy,Transport

Description Predictive virtual testing of NCF composites, resulting in reduction of cost and time within the industrial development cycle.
Geographic Reach Multiple continents/international 
Policy Influence Type Influenced training of practitioners or researchers
Impact The development of simulation tools particularly for NCF composites can positively influence the reduction of cost and time within the industrial development cycle since full-scale experimental tests are prohibitively expensive and time consuming. The advantages of the further development of the NCF composites as well as the appropriate predictive numerical models are focused on low production cost, high drapeability that NCF composites can offer in aerospace applications, resulting in lighter structures with reduced operational and manufacturing costs.
Title An Energy-based damage mechanics failure model for NCF composites. 
Description This constitutive failure model proposes a set of failure criteria, capable to accurately predict the mechanical response of NCF composites under a variety of loading cases. Its novelty regards the identification of influence and the introduction of the mechanical properties of the stitching reinforcement into the tensile properties of NCF composites, which until now are neglected, as well as the further development of compression failure criterion which incorporates both the in- and out-of-plane fibre waviness. Additionally, this damage mechanics approach introduced six damage variables for in plane damage per layer. The damage variables are associated with warp and weft fibre damage in both tensile and compressive failure modes, with an additional damage variable to determine the deterioration of the fibre matrix-interface, and finally with the failure of the stitching reinforcement. The damage variables are directly related to the stiffness degradation within the composite laminae and ultimately within the laminate. The evolution of damage in each mode is controlled via a series of damage-strain equations, thus allowing the total energy dissipated for each damage mode to be set as a material parameter. 
Type Of Material Improvements to research infrastructure 
Year Produced 2019 
Provided To Others? No  
Impact Until the current stage of the project, it is worth mentioning that a very good agreement between the numerical and the experimental results has been achieved in the load cases of in-plane tension, compression and shear. In terms of failure propagation, the outcome of the constitutive failure model analysis revealed good correlation with the respective Compact Tension tests. The benefit of these positive results is that the amount of the mechanical tests in these particular loading cases can be reduced and hence save both material and labour time. 
Title Characterisation of the mechanical properties of carbon Non-Crimp Fabric (NCF) Composites at specimen level. 
Description An array of both standard and non-standard tests was performed. The coupon testing began with tests to characterise the basic elastic behaviour of the NCF composites and evaluate the engineering properties that will determine the design allowables. The mechanical testing included in-plane tension, compression and shear. Then, the interlaminar fracture properties were examined through Mode I (Double Cantilever Beam) & Mode II (4pt-End Notched Flexure) tests, while the translaminar fracture properties were assessed with Compact Tension test. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? No  
Impact The outcome of the mechanical testing of NCF composites revealed that the internal structure of these materials, i.e. fibre waviness in terms of fibre misalignment angles, is directly related to the stiffness and strength reduction both in tension and compression. The fibre waviness is induced to the NCF composite through another structural parameter of NCF composites, which is the stitching reinforcement. This phenomenon is further obvious when the fabric is produced with high stitch tension; however, there are cases where the stitches induce only minimum fibre waviness. 
Description Airbus iCase Award scholar 
Organisation Airbus Group
Department Airbus Operations
Country United Kingdom 
Sector Private 
PI Contribution The contributions of my side towards the partner of the research project are focused on providing detailed information about the development stages of the analytical and computational modelling of NCF composites. Furthermore, on the experimetal side, the results of the mechanical testing are shared to the partner and the conclusions drawn after the postprocessing (e.g. fractographic analysis) are thouroughly discussed.
Collaborator Contribution The partner provides raw material (NCF and resin system) as well as transfer of expertise both in the computational and experimental sides of the project, through weekly meetings. Moreover, the partner financially contributes to researcher's participation in related conferences for the entire duration of the present research project.
Impact Due to a signed Non- Disclosure Agreement, the results of this collaboration cannot be published for now.
Start Year 2017
Title Simulia Abaqus 2017 
Description Simulia Abaqus, provides a numerical enviroment for, but not limited to, predicting the mechanical behaviour of composite and metal structures until failure and beyond, in a wide range of load applications. The simulations which were carried out involved a spectrum of nonlinear static to impact/crash loading conditions in explicit numerical solver. 
Type Of Technology Software 
Year Produced 2017 
Impact This software provides the user the opportunity to implement his own source code and failure models next to the defaults as provided by the software. Hence, an external subroutine (VUMAT) was introduced to the main platform of the software, resulting in more accurate prediction of failure and failure propagation of Non-Crimp Fabric Composites with reduced computational time.