Acrosoma engineering, testing and monitoring methodology by jay dielman

ENGINEERING, TESTING AND MONITORING METHODOLOGY

04 October 2011 V1

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ACROSOMA® 3D STITCHED PANEL The Acrosoma® panel, a composite sandwich panel with a tri-dimensional fibre structure, resulted from a special production method. The fully patented Acrosoma® production machine combines tufting (textile technique) with pultrusion (composite technique). The difference with other existing tri-dimensional production methods is that the Acrosoma® panel machine does not just connect the skins of the panel, but holds them together as well. This technology is patented worldwide and several more patents of this technology and applications are pending. Traditional sandwich construction is prone to delamination. The shear forces in the foam are the highest in the connection areas between the skin and foam. Once the delamination originates there is no mechanical barrier to stop it. Therefore a delamination will progress until the entire sandwich structure collapses. In order to circumvent the delamination of skins and foam, there has been a growing search for a tri–dimensional reinforced sandwich panel. The solution for this consists of “stitching” a reinforcement fibre through the sandwich panel (Z-direction), as shown below. The tri-dimensional Acrosoma® Sandwich Concept has through – the – thickness reinforcements with continuous fibres.

The information contained in this document is the proprietary and exclusive property of ACROSOMA NV except as otherwise indicated. No part of this document, in whole or in part, may be reproduced, stored, transmitted, or used for design purposes without the prior written permission of ACROSOMA NV. The name “ACROSOMA” is a globally protected name and logo

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ACROSOMA FINITE ELEMENT ANALYSIS METHODOLOGY In order to show the advantages of Acrosoma® structures, and to proof the properties, ACROSOMA NV has developed an in – house methodology to simulate the behavior of the Acrosoma® structure. This Finite Element based simulation process starts with the creation of a 3D model in CATIAV5®. The material properties, included the different plies in the lay-up, are modeled with ESAComp®. This is an interface program developed by ESA (European Space Agency). Most FEA models can handle very accurate composite structures, but fail with 3D structures, especially in structures where the density of fibers in one direction is significantly different than the other two, like in Acrosoma® panels. This is the reason why specific interface programs are necessary. ACROSOMA NV adapted this method with the support of ESA. The materials and the 3D model are combined. This combination is then modeled with Finite Elements. Different lay-ups imported from the interface program are assigned to the different geometrical regions. By applying the appropriate loading conditions and boundary conditions to the FEA model, calculations can be done and afterwards the results can be evaluated and compared. The FEA program being used by ACROSOMA NV is ABAQUS®. All of this results in the optimization of the Acrosoma® structure’s lay-up. Design requirements Acrosoma® panel configuration

CAD model (CATIAV5®)

Material model (ESAComp®)

Boundary conditions Loads

FE model (ABAQUS®)

Result analysis

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DIMENSIONAL CONTROL OF STRUCTURES For the dimensional control of structures after production, Acrosoma NV uses a Nikon laser radar. The laser radar is a system that supports non-contact and targetless inspection of large objects. It overcomes the limitations of traditional laser tracker measurement and can speed up inspection drastically. The use of the laser radar offers numerous applications: -

Quality assurance application including CAD to part comparison In process applications e.g. component alignment Tool building and alignment Etc.

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TESTING After the production of Acrosoma® panels or complete Acrosoma® structures, tests are done with several purposes: -

Check performance in a full scale test; Relate test results to FE results to finetune the material properties; Qualification of the production Failure reconstruction for repair.

To do this, Acrosoma NV has several tools: -

A full scale test bench up to 400 kN and 3 Hz

This test bench is used to test large structures at high loads. It is an Acrosoma NV design and specially developed. The test bench is able to do: -

Creep tests (tension and compression) Maximum load tests (tension and compression) Fatigue tests

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An Instron 50kN test machine

This test machine is used to determine properties of the Acrosoma® panels. On this test machine it is possible to do: -

Compression tests 4 – point bending tests o Max. span = 1600mm o Max. loading span = 1000mm 3 – point bending tests o Max. span = 1600mm Fatigue tests up to 1Hz

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MONITORING ACROSOMA NV strongly believes in using fibre optics to monitor the behavior of composite materials. It offers many advantages compared to electrical strain gages. ACROSOMA NV has implemented this technology already in their Acrosoma® panel. Combined with the high tech software (finite element analysis), ACROSOMA NV is able to characterize the behavior of the Acrosoma® panels in detail.

- DESCRIPTION OF FIBRE BRAGG GRATING TECHNOLOGY Fibre Bragg Gratings are made by laterally exposing the core of a single-mode fibre to a periodic pattern of intense ultraviolet light. The exposure produces a permanent change in the refraction index of the fibre's core, creating a fixed index modulation according to the exposure pattern. This fixed index modulation is called a grating. At each periodic refraction change a small amount of light is reflected. All the reflected light signals combine coherently to one large reflection at a particular wavelength when the grating period is approximately half the input light's wavelength. This is referred to as the Bragg condition, and the wavelength at which this reflection occurs is called the Bragg wavelength. Light signals at wavelengths other than the Bragg wavelength, which are not phase matched, are essentially transparent. This principle is shown in the figure below. Therefore, light propagates through the grating with negligible attenuation or signal variation. Only those wavelengths that satisfy the Bragg condition are affected and strongly backreflected. The ability to accurately preset and maintain the grating wavelength is a fundamental feature and advantage of Fibre Bragg Gratings. Imput Imput

intensity

intensity intensity

Imput

Output Output

Output

wavelength wavelength

wavelength

Output

wavelength wavelength

intensity

intensity intensity

Output wavelength

wavelength wavelength

wavelength

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- ADVANTAGES OF FIBRE OPTIC SENSORS Fibre optical sensors offer several significant advantages over conventional electrical sensors. The most important advantages are: They are rugged passive components resulting in a high life time (20 years) They form an intrinsic part of the fibre optic cable that can transmit the measurement signal over several tens of kilometers They show no interference with electromagnetic radiation, so they can function in many hostile environments where conventional sensors would fail They do not use electrical signals what makes them explosion safe They have the ability to multiplex many sensors using only one optical fibre, driving down the cost of complex control systems FOS&S’ technologies are all based on fibre optical sensing and can be divided into two main categories: Fibre Bragg Grating Technology Stimulated Brillouin Scattering technology

FBG CHARACTERISTICS

The central wavelength of the reflected component satisfies the Bragg relation: λrefl = 2 n Λ, with n the index of refraction and Λ the period of the index of refraction variation of the FBG. Due to the temperature and strain dependence of the parameters n and Λ, the wavelength of the reflected component will also change as function of temperature and/or strain, see figure below. This dependency is well known, what allows determination of the temperature or strain from the reflected FBG wavelength. Besides temperature and strain, FBGs can be used to measure a variety of other physical parameters such as humidity, pressure, displacement, water leakage ... This can be achieved using smart transduction mechanisms that convert the physical parameter into a strain value onto the FBG.

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- MULTIPLEXING The response of different FBG sensors can be monitored using only one optical fibre. This is achieved by putting different FBGs with different wavelengths in a series configuration, see figure below. Each reflected peak corresponds to a FBG. The wavelength responses of the different FBGs are recorded using a special designed fibre optic measurement system, operating in the C-band (1530 nm -1570 nm), L-band (1570 nm -1610 nm) or C+L band. The larger the wavelength window, the more sensors can be interrogated in a series configuration.

intensity

incident spectrum

intensity

Bragg grating n°1

transmitted spectrum

Bragg grating n°2

Bragg grating n°3

reflected spectrum

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- MONITORING PRINCIPLES There are two fundamental approaches for monitoring the FBG responses. The first approach is shown in the figure below. It makes use of a broadband light source that couples the light through a 2 by 2 coupler into the fibre where the FBGs will reflect different components. This same coupler guides the reflected light, coming from the different FBGs, into an Optical Spectrum Analyser (OSA) module where the different peak wavelengths are calculated. If more then one sensing fibre is used, an additional optical switch is needed to make the interrogation of the different fibres possible. The control of the measurement system as well as the wavelength to parameter conversion is established using a graphical user interface that can be run from a laptop or desktop P.C.

In the second technique, a narrowband tunable laser is swept across the appropriate spectral region, and a reflected signal is observed with a broadband detector only when the laser is precisely tuned to the sensor’s reflectivity.

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- IMPLEMENTING IN ACROSOMA® PANEL A simple 3-point bending test has been done on an Acrosoma® panel. The test sample was equipped with fiber optic sensors, thus during the test the strains were measured. Afterwards, a simulation of the 3-point bending test is made with ABAQUS®. The goal was to compare the strains measured during the test and the strains resulting from the simulation. This gave an insight on how well our Acrosoma® panel is modeled in other simulations. The dimensions of the panel were 2000 x 300 mm². The panel was equipped with 3 fiber optic sensors. The panel and the location of the sensors are shown schematically in the figure below.

On the next page, the real test setup is shown. The load that is tested is a load of 75kg. This load was applied in the center of the panel, uniform across the width.

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Optical fiber sensors

PC: read & process data MicronOptics unit: data logging

Page | 14 Each optical fiber sensor has a wavelength ( 0) that it reflects. When the fiber deforms due to the loading of the structure where it is glued onto, this wavelength shifts to a higher wavelength ( ). This shift is directly related to the strain (Δε) in direction of the fiber:

1 ln S 0 0.78 E 6

The results of the sensors are listed in the table. Sensor

Strain (%)

Middle

0.105

Left

0.046

Right 0.049 The test of the previous paragraph was simulated in ABAQUS®. The results are analyzed and compared to the test results. The figure below shows the strain values of the panel in the longitudinal direction (corresponding direction of optical fiber measurement).

The maximum strain in the middle of the panel is 0.101%. Compared to the strain measured by the optical fibers (0.105%) this gives a difference of only 4%. The measured values correspond to the calculated values.