Periodic Reporting for period 1 - IFT-MultiLam (Enabling Interlaminar Fracture Testing of MULTIdirectional composite LAMinates for safe and efficient structures)
Reporting period: 2023-02-01 to 2025-01-31
Reducing human related environmental impact is a major societal challenge. In this context, minimisation of the aviation sector’s impact is paramount, and indeed the key goal of the European Partnership for Clean Aviation. Among the strategies to achieve such goal, one is improving structural efficiency, which would lead to weight savings, cheaper operations, and better fuel efficiency. The adoption of composite materials, notably of fibre-reinforced polymers, has already led to positive outcomes. Still, the full potential of these materials is far from being fully exploited. This is due to the complexity of the mechanical behaviour of these materials, that still need to be fully understood, often resulting in oversized structures.
To improve this situation, an in-depth knowledge of composites damage mechanisms is crucial. Among these, interlaminar fracture, or delamination, is one of the most critical, so that an accurate evaluation of interlaminar fracture toughness (IFT) is key to design safe and efficient structures. As of today, standard tests to quantify IFT are restricted to specimens with unidirectional (UD) layups and where delamination is propagated parallel to the fibres direction. On the flipside, most structures are built using multidirectional (MD) laminates, where delamination may appear at any interface and propagate in any direction. In these situations, IFT may differ from that obtained by standard tests. The reason for the lack of practices to evaluate IFT in MD laminates is the complexity associated with testing MD specimens and the technical challenges associated with this. The main consequence of this is that structural performance predictive capabilities remain limited, in turn leading to design uncertainties and oversized, inefficient structures.
The IFT-MultiLam Project (Enabling Interlaminar Fracture Testing of MULTIdirectional composite LAMinates for safe and efficient structures) aims to develop the knowledge and the tools needed for an accurate evaluation of IFT in MD laminates. To do so, a recently developed set of MD specimens, called Fully-Uncoupled Multidirectional (FUMD), is exploited. While these specimens have already been proven to be extremely promising, further steps toward widespread adoption are still needed. These steps constitute the objectives of the IFT-MultiLam project, and are:
(i) the obtainment of a complete set of FUMD delamination specimens;
(ii) the exploration of the actual testing possibilities offered by such a large set, e.g. regarding interfaces that can be tested;
(iii) the development of analytical/numerical tools for the selection of the most suitable specimens for actual testing; and
(iv) the experimental validation of the approach developed.
To improve this situation, an in-depth knowledge of composites damage mechanisms is crucial. Among these, interlaminar fracture, or delamination, is one of the most critical, so that an accurate evaluation of interlaminar fracture toughness (IFT) is key to design safe and efficient structures. As of today, standard tests to quantify IFT are restricted to specimens with unidirectional (UD) layups and where delamination is propagated parallel to the fibres direction. On the flipside, most structures are built using multidirectional (MD) laminates, where delamination may appear at any interface and propagate in any direction. In these situations, IFT may differ from that obtained by standard tests. The reason for the lack of practices to evaluate IFT in MD laminates is the complexity associated with testing MD specimens and the technical challenges associated with this. The main consequence of this is that structural performance predictive capabilities remain limited, in turn leading to design uncertainties and oversized, inefficient structures.
The IFT-MultiLam Project (Enabling Interlaminar Fracture Testing of MULTIdirectional composite LAMinates for safe and efficient structures) aims to develop the knowledge and the tools needed for an accurate evaluation of IFT in MD laminates. To do so, a recently developed set of MD specimens, called Fully-Uncoupled Multidirectional (FUMD), is exploited. While these specimens have already been proven to be extremely promising, further steps toward widespread adoption are still needed. These steps constitute the objectives of the IFT-MultiLam project, and are:
(i) the obtainment of a complete set of FUMD delamination specimens;
(ii) the exploration of the actual testing possibilities offered by such a large set, e.g. regarding interfaces that can be tested;
(iii) the development of analytical/numerical tools for the selection of the most suitable specimens for actual testing; and
(iv) the experimental validation of the approach developed.
The design of FUMD specimens requires the use of unconventional stacking sequences, obtained from a set of solutions to the equations describing some of the desired constraints on the specimens, called quasi-trivial (QT) quasi-homogeneous solutions. While QT solutions have enormous potential for laminate design, their understanding and availability are still limited. Hence, the first project activity was the development of a novel formal framework to describe and manipulate QT solutions. This enabled to reveal their mathematical nature in a new way, and to discover properties that were not known before. The insight and the tools developed allowed us to devise a novel and efficient strategy to obtain QT solutions, which exploits a recursive algorithmic procedure. Hence, we were able to create a huge QT database, that will be soon made open access.
With the newly established foundations on QT solutions, the second project activity was the creation of a database of sequences for FUMD delamination specimens. Firstly, an in-depth analysis was conducted to formalise, better than in the past, the design routes to obtain FUMD specimens from QT quasi-homogeneous solutions. This was instrumental to devise and develop routines to find, extract, and (when needed) process those solutions in the QT database that can be used to obtain sequences for FUMD specimens. As a result of this, we created the desired, comprehensive database of sequences for FUMD specimens.
Once the database of sequences for FUMD specimens had been created, we investigated in-depth aspects of practical interest for testing purposes. Firstly, we explored if and how the sequences in the database would enable testing of different types of delamination interfaces (different orientations between embedding layers, different orientations with respect to the specimen length direction, among other aspects), and established precisely which test interfaces would be possible for any given total number of plies in the specimen. We then investigated also the possibility to obtain stiff enough specimens by including a sufficient number of 0º layers.
Next, in view of the large number of feasible FUMD specimens, instruments for an informed selection of the most appropriate configurations for testing are needed. For this purpose, we developed analytical expressions allowing to quickly analyse large sets of FUMD configuration based on their main design features and visualize important properties, such as the expected thermal residual stresses, the tendency to suffer finite-width effect, and others. These tools enable a fast preliminary screening of configurations for testing purposes. Additionally, we also developed finite element models capable to provide much more detailed information about specific configurations, at the expense of a longer time needed to analyse multiple configurations. While the tools developed have already proven useful in the selection of FUMD configurations for testing, further developments are ongoing, and it is expected that they will be perfected in the coming future.
Finally, multiple experimental activities to validate the approach developed and to obtain a large experimental dataset will be carried out. Some of these activities are already underway, while others are being prepared currently.
With the newly established foundations on QT solutions, the second project activity was the creation of a database of sequences for FUMD delamination specimens. Firstly, an in-depth analysis was conducted to formalise, better than in the past, the design routes to obtain FUMD specimens from QT quasi-homogeneous solutions. This was instrumental to devise and develop routines to find, extract, and (when needed) process those solutions in the QT database that can be used to obtain sequences for FUMD specimens. As a result of this, we created the desired, comprehensive database of sequences for FUMD specimens.
Once the database of sequences for FUMD specimens had been created, we investigated in-depth aspects of practical interest for testing purposes. Firstly, we explored if and how the sequences in the database would enable testing of different types of delamination interfaces (different orientations between embedding layers, different orientations with respect to the specimen length direction, among other aspects), and established precisely which test interfaces would be possible for any given total number of plies in the specimen. We then investigated also the possibility to obtain stiff enough specimens by including a sufficient number of 0º layers.
Next, in view of the large number of feasible FUMD specimens, instruments for an informed selection of the most appropriate configurations for testing are needed. For this purpose, we developed analytical expressions allowing to quickly analyse large sets of FUMD configuration based on their main design features and visualize important properties, such as the expected thermal residual stresses, the tendency to suffer finite-width effect, and others. These tools enable a fast preliminary screening of configurations for testing purposes. Additionally, we also developed finite element models capable to provide much more detailed information about specific configurations, at the expense of a longer time needed to analyse multiple configurations. While the tools developed have already proven useful in the selection of FUMD configurations for testing, further developments are ongoing, and it is expected that they will be perfected in the coming future.
Finally, multiple experimental activities to validate the approach developed and to obtain a large experimental dataset will be carried out. Some of these activities are already underway, while others are being prepared currently.
Novel important results, beyond the state of the art, were obtained regarding QT solutions. As mentioned, a novel formal framework to describe and manipulate them was developed, leading to the discovery of unknown properties and to the formulation of an efficient search algorithm. Notably, these developments are not limited to QT quasi-homogeneous solutions (those used for FUMD specimens) but apply to other types of QT solutions as well (uncoupled, bending-extension homogeneous). Furthermore, a comprehensive database of QT solutions was created and will be made available in open access. While these results were fundamental within the IFT-MultiLam project, their impact is expected to be further-reaching, due to the huge advantages offered by QT solutions for laminate design in general. Specifically, the availability of a novel formal framework and of the QT solutions database are expected to boost the adoption of QT solutions in laminate design, and thus impact the whole composite community.
Novel results were obtained as well regarding FUMD delamination specimens. These results include: novel insight on the design of FUMD specimens using QT quasi-homogeneous solutions and on their properties; an extensive database of sequences for FUMD specimens, along with detailed information about aspects of practical interest for the use of such sequences in actual experimental tests; a number of tools, analytical and numerical, for the selection of the most appropriate FUMD configurations. On the one hand, these results provide the much-needed insight and tools for a widespread adoption of FUMD delamination specimens as a standard choice in IFT tests. On the other hand, the design strategy for FUMD specimens and the database of sequences created have the potential to be much further reaching: FUMD specimens are, in essence, composite laminates with thermoelastic properties desirable in most structural applications; as such they are of much more general interest.
To ensure further uptake and success, further dissemination activities, including publication of multiple manuscripts, will be carried out. Additionally, further research activities are undergoing and new ones will be launched, if possible, to foster further development.
Novel results were obtained as well regarding FUMD delamination specimens. These results include: novel insight on the design of FUMD specimens using QT quasi-homogeneous solutions and on their properties; an extensive database of sequences for FUMD specimens, along with detailed information about aspects of practical interest for the use of such sequences in actual experimental tests; a number of tools, analytical and numerical, for the selection of the most appropriate FUMD configurations. On the one hand, these results provide the much-needed insight and tools for a widespread adoption of FUMD delamination specimens as a standard choice in IFT tests. On the other hand, the design strategy for FUMD specimens and the database of sequences created have the potential to be much further reaching: FUMD specimens are, in essence, composite laminates with thermoelastic properties desirable in most structural applications; as such they are of much more general interest.
To ensure further uptake and success, further dissemination activities, including publication of multiple manuscripts, will be carried out. Additionally, further research activities are undergoing and new ones will be launched, if possible, to foster further development.