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Predictive Simulation of Defects in Structural Composites

Periodic Reporting for period 3 - PSIDESC (Predictive Simulation of Defects in Structural Composites)

Reporting period: 2020-06-01 to 2021-05-31

Two technical challenges stand out in aviation with respect to the ongoing work done in the PSIDESC project. The first is the environmental impact air travel could have if technologies don’t evolve. The second is related to following up with manufacturing rates of aircrafts while keeping costs under control. Innovation also plays a key role here to ensure that the European aircraft industry will remain competitive with regards to emerging technologies.
Composite materials based on high strength and stiffness reinforcements such as carbon fibers, have the possibility to reduce the overall weight of the structure of an aircraft and improve its service life. Performance of these materials depend on the quality of manufacturing. As zero defects in a manufactured part will make manufacturing and inspection costs soar, managing the effect of defects is key for production.
The PSIDESC project contributes to manage the effects of defects with attention to composite materials manufacturing in and out-of-autoclave.
Cenaero, MSC Benelux and Sonaca, joined efforts to implement and validate this numerical tool at the test specimen scale up to a large scale aeronautical structure defined with Airbus, the topic manager. The goal is to develop a tool capability to enlarge panel acceptance criteria to support composite part manufacturing for next generation aircraft. The consortium aimed to combine:
- The parametric description of defects
- A multiscale approach of the material at -ply & structural levels
- A probabilistic chain
Challenges were faced to be able to apply the approach over a very large range of geometries and manufacturing conditions. A TRL level of 4 was achieved in the project and the partners and topic manager, hope to further introduce such modelling techniques for evaluation in the design cycle and allow for more affordable and lighter structures.
After 36 months of research and development, the major deliverables consist in the test specimen virtual allowable evaluation tool, integrated in the Digimat suite. The second deliverable is the computational probabilistic chain, aimed at using parallel evaluations possible with high performance computing centres, to describe the expected statistical distribution of the response of a structure considering the possible defect inherent to manufacturing conditions.
Three types of defects were selected based on occurrence rates, processes concerned, knock-down on properties, detectability, the expected gain if reduced, the repairability and the amount of data. The defects that are selected are porosity, waviness and gaps/overlaps of tows. Together, they cover more than 75% of the defects typically found in the production of aircraft structural parts.
The numerical description and integration in finite element models as well as sensitivity analysis were completed. This allowed further on to compare and validate the numerical results with representative experimental data of the defects.
Based on data available and description of the defect in terms of size, location and detection thresholds, the representation of the defect at an RVE level, as well as its inclusion in the structural analysis framework, were completed. For porosity, this was done at an elementary level as we can treat it as a change in local material properties, for gaps and waviness, the effect of the defect is intimately linked to its orientation and its geometry. A valid workflow to include these defects was thought up and tested.
Work was carried out to address the presence of defects at the scale of a composite panel. The challenge is to identify how to integrate the defect as well as choose the right distribution of parameters for the defect. The consortium aimed for a possible inclusion of the defect in the structural model without having to fundamentally modify the representation of the reference structure. This will allow to include the effect of the defect on the distribution of forces. The workflow and model implementation tools that were created allow to cover a large scope scenarios.
To extend the impact of the project, further validation with respect data can be sought in the near future. The project reached its main objectives and both the specimen virtual allowable evaluation tool, integrated in the Digimat suite and the probabilistic evaluation chain are major deliverables that will rapidly be exploited through activities at the Topic Manager's premises and through each of the participant's activities. New research project activities are being prepared to further build on the experience gained and reach new levels of technology readiness level. While dissemination activities during the last year of the project were limited due to COVID restriction (conferences, fairs), the partners hope to be able to communicate on project results after the project ending.
From a manufacturing technology perspective, the project will impact two main aspects. The first is design, where better knowledge of the material allowables with respect to design tolerances will help further reduce the weight of structural parts, and the second one is scrap reduction or time for certification/validation of a part presenting a defect. The first aspect could be translated in terms of CO2 emissions. A 5% decrease of weight of the aircraft structure (~38 tonnes for an A320), coming from more adequate design tolerances (identification of critical/non-critical zones) mounts up to a 3% decrease in CO2 emissions. Reducing the dispersion tolerance from a defect showed, at the panel level, a difference of 2% in areas of the panel and 4% in areas of the stringer. While still early to confirm any numbers, bringing this to the level of the full aircraft, this mounts up to a gain of 1000 kg in the weight of a medium range aircraft.

The second impact in terms of validation of parts with defects can reduce costly steps in QA & NDT for composites. Based on estimations and research showing that inspection & repair costs make up 5 to 10 percent of the total manufacturing cost, covering most defect type justifications through simulation tools to reduce a significant portion of those costs (including delays for delivery). A 2% cost decrease in manufacturing could be achieved. Through these contributions, the project can impact the competitiveness of European aircraft manufacturing and contribute to reaching a more sustainable aircraft manufacturing industry objective for 2035 and 2050.

From a numerical technology standpoint, what will differentiate the developments of this project is the capacity to quantify the influence of different defects on structural integrity and therefore take the right design decisions or scrap a part or accept it after a deviation in the manufacturing. In terms of numerical developments the project focus in on statistical material and defect representation.

The ambition and impact of this project concerning numerical simulation were mainly to improve material models and methodologies and perform variability analysis by setting-up a probabilistic toolkit with material and structural models.

Achieving this and validating the toolkit at the level of an experimental demonstrator ensure that these methodologies can help evolve the design and effect of defect paradigm in manufacturing of composite structures. These objectives have been addressed and reached a TRL 4 on a numerical demonstrator.
Example of a gap due to a twisted tow
Through thickness waviness due to improper consolidation near the stiffener
Representation of the effect of statistical distribution of a porosity defect on a panel
Porosities in a composite laminate