Advanced Composite Integrated Skin Panel Structural Testing – Results from Clean Sky ACID Project

Stiffened panels are required in structures which can be obtained by different processes. They can be made by attaching stiffeners to a thin panel or by producing integrally stiffened panels. An innovating manufacturing process based on Liquid Resin Infusion (LRI) can be employed for obtaining integrally stiffened panels. It is based on moulding a dry NCF (Non Crimp Fabric) pre-form of Carbon fibre plies, which is bonded by a one-shot injection process to high stiffness, pre-cured pre-preg T-section stiffeners. This method presents benefits like lower costs in machining and fewer assembly operations.

 

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The structural behaviour of integrally stiffened panels is normally better than those panels with attached stiffeners, but the difference is difficult to quantify by analysis, and is dependant on the manufacturing technology. Especially, the major interest is to clarify the structural behaviour of the panels, and more specifically their critical mode of failure.

The immediate solution could be to carry on comparative structural tests on different coupons moulded by different manufacturing methods, but it must be taken into account that habitually employed strain and stress measuring systems are limited to specific predefined points or have limited resolution. As the manufacturing process and materials are expensive, and last a long term, few coupons are available. Therefore, carefully combined measurement systems must be employed to obtain as much information as possible during the test, and also recurrent information is desirable to correlate results obtained by different sources.

As an answer to this scenario, the ACID project was launched to explore and analyze some of the previous factors, trying to study comparatively the mechanical properties and behaviours of different panels obtained by different manufacturing processes.

 

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To achieve this goal, a testing matrix was accomplished, based on 3 LRI coupons. Two of them are panels with attached stiffeners and the other one is an integrally stiffened panel. It is expected that the results obtained in the tests help to clarify the panels’ behaviour and allow comparing the mechanical advantages versus economic benefits of the manufacturing processes.

The main objectives of the project were described as follows:

  1. Carry on large scale structural tests for obtaining ultimate properties and failure modes of components manufactured by different processes.
  2. Measure strain and stress information during the test in a recurrent manner to combine and correlate the obtained signals which define the structural behaviour of the panels throughout the test.
  3. Analyze the obtained results, establishing a comparison between the behaviours of panels with attached stiffeners and integrally stiffened panels.
  4. Analyze the obtained results, establishing a qualitative comparison between the mechanical advantages versus economic benefits of the manufacturing processes.

The main achievements of the project were the validation of the novel techniques for composite manufacturing due to the result obtained in the tests. The final mechanical response of the differently implemented panels shows great similarities in the main mechanical characteristics (failure load, stiffness, failure mode).

This validation serves as a starting point for further methodologies development and means a widening of the possible applications or fields of Composite materials.

At the same time, the cross comparison of the measurement devices is useful when deciding the most convenient measurement system for each project. The pros and cons are highlighted and an estimative error between systems is obtained.

The major environmental benefit is the validation of the novel cleaner manufacturing composite methodologies (less energy needed, less wastes, lower costs) against conventional procedures in representative playground.

Final workshop of Cenit TARGET at AIRBUS

On December 5th, the final workshop of the Cenit TARGET project was held at AIRBUS premises in Getafe (Madrid). This research project, focused on smart and environmentally sustainable technologies for the production of structures made of composite materials, has been founded by the CDTI (Centro de Desarrollo Tecnológico Industrial), organism depending of the Spanish Ministry of Economy and Competitiveness.

cenit_targetAirbus facilities in Getafe (Madrid)

 

The event highlighted the main achievements reached by the 12 partner companies: Airbus Operations, Acciona Infraestructuras, Aernnova, Applus, Aciturri, Airbus Defence & Space, Hexel, Idec, MTorres, Sairem Ibérica, Tecnatom y TR Composites, along with 24 research organizations (Universities and Technology Centres).

Among them, CIDAUT has worked jointly with Aciturri in research activities to promote the development of advanced liquid composite moulding processes, seeking to reduce cycle times, energy consumption, scrap material and manufacturing costs associated to the production of structural components in Aeronautics, while improving product quality and increasing process robustness and repetitiveness. Processes that, if automated, would allow the effective production of complex structural components and the optimization of structures assembly.

The TARGET project, which commenced in 2010 with a 30 million Euros budget, was divided in nine work packages with the objective of researching and developing new smart and environmentally sustainable technologies for the production of composite structures. In particular, the research activities are focused on materials and processes that avoid using large autoclaves, and on functions integration; laying ground for new equipment and automation concepts that should lead to major cost savings.

TARGET main outcomes that deserve mention are: the development of advanced compaction and curing technologies of thermoset and thermoplastic materials out of autoclave; nanomaterials processing for their optimized integration in aeronautic laminates; the research in advanced resin infusion technologies and 3D carbon fabrics optimization; the development of new non-destructive inspecting techniques prior to curing; and the definition of integrated production systems, both in situ and in line, for the efficient manufacturing of composite components and structures.

CIDAUT Develops an Accurate Mould Design Methodology for Composite Parts Through the Simulation of Process-Induced Distortions

Within the scope of the Cleansky Programme, CIDAUT leads a research project aimed at the manufacturing of composite rotorcraft blades with very tight tolerances that will incorporate an Active Gurney Flap mechanism. Active Gurney Flap (AGF) systems enable the rotorcraft to safely operate with reduced tip speeds whilst preserving high performances with reduced fuel consumption and noise; something that constitutes one of the most challenging research areas leaded by the Green Rotorcraft Consortium in the Cleansky Programme: the development of Active Rotor Technologies.

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Active Gurney Flap mechanisms scheme.

In the ACCUBLADE project, CIDAUT researches on the development of a robust and very accurate moulding process for the manufacturing of carbon fibre model blades that will be used for validation of the AGF systems through wind tunnel tests. Due to the small scale of the model blades, the dimensional requirements for the manufacturing are very tight, less than +/- 0,1mm on the aerodynamic profile. In order to fulfil the tight tolerances, not only the mould must be machined with high precision means, but also the cavity design must be defined with special consideration for minimizing any distortion during the process.

The analysis of potential process-induced distortions, such as warping or spring-in, has been carried out by means of process simulations based in accurate material parameters identified through laboratory material characterisation tests. The influence of different material and processing parameters including the weave pattern, ply stacking, mould interaction, curing temperature and pressure has been experimentally characterised and correlated with the simulation models. These have been developed for the prediction of the distortions that would appear during the processing of the model blades. Using this methodology, the design of the cavity can be defined to fulfil the required tolerances.

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Mould design optimization methodology by means of process simulations.

During the near to follow validation stage, CIDAUT will partake in the processing of the model blades and the non destructive and destructive inspection tests that will be carried out to demonstrate the functionality of the tools and to validate the design methodology.

In particular, the following outcomes from the ACCUBLADE project will stand out among the technological expected results:

  • An optimization methodology for tooling and moulds design based in accurate distortion simulations accounting for different thermal coefficients of the composite materials.
  • A reduction in current development costs and lead-times for aeronautic composite parts, that usually need expensive and long lasting trial and error procedures to be carried out before a suitable mould cavity design is experimentally determined.

 

Wasis at the European Congress on Composite Materials

Wasis took part last 25th June in Seville to the internationally recognised 16th European Congress on Composite Materials known as the largest congress dedicated to composites materials in Europe. In this occasion more than 1.200 experts from the industry and academia joined the event coming from all areas of the world.

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In agreement with the ECCM16 technical committee in charge of organising this relevant event, a session dedicated to lattice structures was organised, in which Wasis and other international initiatives as well as EU funded research projects shown their work in this field.

Directly related to WASIS, there were 6 publications, summarised in the table below:

Title Authors Summary
FABRICATION OF COMPOSITE CYLINDERS WITH INTEGRATED LATTICE STRUCTURE USING FILAMENT WINDING J.CERQUEIRA, H.FARIA (INEGI), R.FUNCK In WASIS project, a wafer-like integrative concept was developed for an innovative aircraft fuselage. A fuselage section was manufactured through filament winding. Increasingly complex tools, process kinematics and control for accurate prototypes manufacturing were developed. Feasibility analysis is also addressed.
MANUFACTURING AND TESTING OF COMPOSITE WAFER COMPONENTS WITH DUAL-PURPOSE INTEGRATED SEMI-LOOP JOINTS S.KRYVENDA, F.GAGAUZ, M.SHEVTSOVA, L.SMOVZIUK (NATIONAL AEROSPACE UNIVERSITY “KHAI”), I.TARANENKO Combination of wafer reinforcement concept and hybrid metal-to-composite joints provides sufficient reduction of aircraft fuselage weight. Within this work manufacturing process for curved CFRP wafer panel with integrated dual-purpose semi-loop hybrid joints is presented along with panels testing results discussion.
TESTING AND ANALYSIS OF GRID-STIFFENED COMPOSITE STRUCTURAL PANELS A.HAJDAEI, S.GIANNIS (ELEMENT MATERIALS TECHNOLOGY), V.MATEJAK, A.TOULITSIS A composite fuselage structure has been developed in WASIS project based on the lattice stiffening concept. To understand the mechanical performance and validate the design methodology a testing programme involving structural parts of different levels of complexity was executed.
ON THE SAFETY ASSESSMENT AND DAMAGE TOLERANCE OF COMPOSITE LATTICE TYPE AEROSPACE STRUCTURES A.KOTZAKOLIOS (UNIVERSITY OF PATRAS), D.VLACHOS, K.ANTONIADIS, V.KOSTOPOULOS In this work, impact and damage response of composite wafer structures is numerically modelled. The damage created on the structure is identified and the reduction of the airworthiness of the structure is investigated. Comparisons are made with conventional composite structures.
PREPREG LAY-UP TECHNOLOGY FOR MANUFACTURING OF LATTICE STRUCTURE FUSELAGE SECTIONS J.MACK (INSTITUT FUER VERBUNDWERKSTOFFE GMBH), O.MCGERGOR, P.MITSCHANG The manufacturing of isogrid lattice structures for a fuselage section with the AFP process was evaluated. Different designs for rib crossing points are proposed and experimentally analyzed. Solutions for convex as well as concave tooling concepts were tested
LATTICE COMPOSITE STRUCTURE DEVELOPMENT FOR SMALL AIRCRAFT; WASIS R.CORDERO (CIDAUT), M.AVERSANO Development of a vibroacoustic model for prediction of the CFRP fuselage section is described, emphasising on experimental characterisation of largest project prototypes and parameters identified to feed finite element and statistical energy analysis models

Two project prototypes were shown at the congress, the 0.5m and 1m in diameter scaled down fuselage sections, which attracted the attention of many participants to the congress.

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Besides presentations from complementary projects like ALASKA and MAXIMUS were given.
During the lattice structures session, experts exchanged their experience in design and manufacturing, sharing information in open debates and questions sessions after each presentation. It was very interesting to compare the achievements of different initiatives in the aeronautical and aerospace sectors, obtained in European and Asian initiatives.
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For more information and Wasis publications, please visit: http://www.wasis.eu/