# Concepts for integrating electrical energy storage into CFRP laminate structures for aeronautic applications

Frédéric Laurin, Alexander Beutl, Qixiang Jiang, Helmut Kühnelt

► To cite this version:

Frédéric Laurin, Alexander Beutl, Qixiang Jiang, Helmut Kühnelt. Concepts for integrating electrical energy storage into CFRP laminate structures for aeronautic applications. 12th EASN International Conference on "Innovation in Aviation & Space for opening New Horizons", Oct 2022, Barcelone, Spain. pp.012062, 10.1088/1742-6596/2526/1/012062. hal-04570017

HAL Id: hal-04570017

https://hal.science/hal-04570017

Submitted on 6 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Concepts for integrating electrical energy storage into CFRP laminate structures for aeronautic applications

Frédéric Laurin, Alexander Beutl, Qixiang Jiang, Helmut Kühnelt

► To cite this version:

Frédéric Laurin, Alexander Beutl, Qixiang Jiang, Helmut Kühnelt. Concepts for integrating electrical energy storage into CFRP laminate structures for aeronautic applications. 12th EASN International Conference on "Innovation in Aviation & Space for opening New Horizons", Oct 2022, Barcelone, Spain. pp.012062, 10.1088/1742-6596/2526/1/012062. hal-04570017

HAL Id: hal-04570017

https://hal.science/hal-04570017v1

Submitted on 6 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
PAPER • OPEN ACCESS

# Concepts for integrating electrical energy storage into CFRP laminate structures for aeronautic applications

To cite this article: Frédéric Laurin et al 2023 J. Phys.: Conf. Ser. 2526 012062

View the [article online](#) for updates and enhancements.

You may also like

- Structural battery composites: a review
Leif E Asp, Mats Johansson, Göran Lindbergh et al.

- Model of a structural battery and its potential for system level mass savings
Wilhelm Johannisson, Dan Zenkert and Göran Lindbergh

- Multi-physics design optimization of structural battery
Reza Pejman, Emin Caglan Kumbur and Ahmad Raeisi Najafi

<figure><img src="image_2.png" /></figure>

The
Electrochemical
Society

Advancing solid state &
electrochemical science & technology

<figure><img src="image_3.png" /></figure>

DISCOVER
how sustainability
intersects with
electrochemistry & solid
state science research

<figure><img src="image_4.png" /></figure>
PAPER • OPEN ACCESS

# Concepts for integrating electrical energy storage into CFRP laminate structures for aeronautic applications

To cite this article: Frédéric Laurin et al 2023 J. Phys.: Conf. Ser. 2526 012062

View the [article online](#) for updates and enhancements.

You may also like

- Structural battery composites: a review
Leif E Asp, Mats Johansson, Göran Lindbergh et al.

- Model of a structural battery and its potential for system level mass savings
Wilhelm Johannisson, Dan Zenkert and Göran Lindbergh

- Multi-physics design optimization of structural battery
Reza Pejman, Emin Caglan Kumbur and Ahmad Raeisi Najafi

<figure><img src="image_2.png" /></figure>

The
Electrochemical
Society

Advancing solid state &
electrochemical science & technology

<figure><img src="image_3.png" /></figure>

DISCOVER
how sustainability
intersects with
electrochemistry & solid
state science research

<figure><img src="image_4.png" /></figure>
# Concepts for integrating electrical energy storage into CFRP laminate structures for aeronautic applications

Frédéric Laurin¹, Alexander Beutl², Qixiang Jiang³, Helmut Kühnelt²

¹ ONERA, Paris-Saclay University, Châtillon, France.

² AIT Austrian Institute of Technology GmbH, Vienna, Austria.

³ University of Vienna, Vienna, Austria.

frederic.laurin@onera.fr

**Abstract.** The CleanSky2 THT project SOLIFLY is developing further structural batteries for aeronautical applications. This paper presents the concepts of structural integration and the methodology to evaluate the impact of structural battery integration on the mechanical properties of CFRP solid laminates considering the size and shape of the structural battery insert as well as its location through the laminate thickness. A fully parameterised, computationally efficient numerical strategy for finite element simulation has been implemented to evaluate the mechanical properties and, for the first time, the onset of matrix damage as the cell geometry and integration location are varied. The first mechanical characterisation data of SB cell components and cell prototypes were obtained thanks to multi-instrumented tensile tests using digital image correlation and acoustic emission. An initial assessment of the advantages and trade-offs of the SB integration concepts with respect to functionally separated components is discussed.

## 1. Introduction

Electrification of aircraft systems is key for increasing efficiency and reducing climate impact of air transport towards the European net-zero goal. The functional integration of structural capabilities and electrical energy storage in the form of structural batteries (SB) is considered as a low TRL technology with the potential to reduce the impact of battery energy storage on the overall aircraft weight. So far, structural batteries have been mainly studied at material level, largely neglecting their integration into aircraft structures. The CleanSky2 THT project SOLIFLY is developing further structural batteries for aeronautical applications with two cell concepts, one with coated carbon fibres acting as structural and electrically conductive components, and another with laminate structural electrodes. The project puts particular emphasis on their integration into CFRP laminates that are compatible with aeronautical manufacturing processes.

This paper presents the concept of structural integration and the methodology for evaluating the effect of structural battery integration on the mechanical properties of CFRP solid laminates considering the size and shape of the structural battery insert as well as its location through the laminate thickness. Section 2 presents a fully parameterised, computationally efficient numerical strategy for finite element simulation to evaluate the elastic properties and, for the first time, the onset of matrix damage when varying the cell geometry and integration location. Section 3 presents the first mechanical characterisation data of SB cell components and cell prototypes obtained with multi-instrumented tensile tests using digital image correlation and acoustic emission. Finally, in section 4, an initial assessment of

<figure><img src="image_5.png" /></figure>
# Concepts for integrating electrical energy storage into CFRP laminate structures for aeronautic applications

Frédéric Laurin¹, Alexander Beutl², Qixiang Jiang³, Helmut Kühnelt²

¹ ONERA, Paris-Saclay University, Châtillon, France.

² AIT Austrian Institute of Technology GmbH, Vienna, Austria.

³ University of Vienna, Vienna, Austria.

frederic.laurin@onera.fr

**Abstract.** The CleanSky2 THT project SOLIFLY is developing further structural batteries for aeronautical applications. This paper presents the concepts of structural integration and the methodology to evaluate the impact of structural battery integration on the mechanical properties of CFRP solid laminates considering the size and shape of the structural battery insert as well as its location through the laminate thickness. A fully parameterised, computationally efficient numerical strategy for finite element simulation has been implemented to evaluate the mechanical properties and, for the first time, the onset of matrix damage as the cell geometry and integration location are varied. The first mechanical characterisation data of SB cell components and cell prototypes were obtained thanks to multi-instrumented tensile tests using digital image correlation and acoustic emission. An initial assessment of the advantages and trade-offs of the SB integration concepts with respect to functionally separated components is discussed.

## 1. Introduction

Electrification of aircraft systems is key for increasing efficiency and reducing climate impact of air transport towards the European net-zero goal. The functional integration of structural capabilities and electrical energy storage in the form of structural batteries (SB) is considered as a low TRL technology with the potential to reduce the impact of battery energy storage on the overall aircraft weight. So far, structural batteries have been mainly studied at material level, largely neglecting their integration into aircraft structures. The CleanSky2 THT project SOLIFLY is developing further structural batteries for aeronautical applications with two cell concepts, one with coated carbon fibres acting as structural and electrically conductive components, and another with laminate structural electrodes. The project puts particular emphasis on their integration into CFRP laminates that are compatible with aeronautical manufacturing processes.

This paper presents the concept of structural integration and the methodology for evaluating the effect of structural battery integration on the mechanical properties of CFRP solid laminates considering the size and shape of the structural battery insert as well as its location through the laminate thickness. Section 2 presents a fully parameterised, computationally efficient numerical strategy for finite element simulation to evaluate the elastic properties and, for the first time, the onset of matrix damage when varying the cell geometry and integration location. Section 3 presents the first mechanical characterisation data of SB cell components and cell prototypes obtained with multi-instrumented tensile tests using digital image correlation and acoustic emission. Finally, in section 4, an initial assessment of

<figure><img src="image_5.png" /></figure>
the advantages and trade-offs of the SB integration concepts with respect to functionally separated components is discussed.

## 2. Numerical integration strategy of battery cells into composite structures

The objective of the work is to establish recommendations for the integration of battery cells into laminated composite structures in order to limit/control the degradation of the mechanical properties of carbon/epoxy composite structures compared to the reference one without battery cell.

### 2.1. *Studied composite material*

The considered composite material is made of AS4/8552 UD ply with an aerial weight of 194g/m², which is widely used in the manufacturing of composite structures in aeronautical industries. Furthermore, this material has been studied in the literature [1] and mechanical properties are available in [2]. A classical quasi-isotropic laminate consisting of 16 plies has been considered. The stacking sequence is [(45/90/-45/0)$_2$]$_s$ and fulfils the recommendations of current design offices [3].

### 2.2. *Studied battery cell concepts*

The SOLIFLY project focuses on the development of structural batteries using two different concepts, the multifunctional Coated Carbon Fibres (CCF) material and the multifunctional Reinforced Multilayer Stack (RMS), illustrated in Figure 1a and Figure 1b respectively. The CCF concept battery cells contain carbon fibres coated with active materials as electrodes. The carbon fibres also act as reinforcement for the battery cell. The reinforced multilayer stack (RMS) consists of conventionally coated anode on Cu foil and cathode on Al foil. The electrolyte consists of a polymer which provides mechanical stiffness and strength and a liquid electrolyte for ion conduction. No additional separator is needed.

<figure><img src="image_6.png" /><figcaption>**Figure 1:** Presentation of a) CCF cell and b) RMS cell concepts, developed in the SOLIFLY project</figcaption></figure>

### 2.3. *Presentation of the virtual testing strategy*

A virtual test campaign on a quasi-isotropic [(45/90/-45/0)$_2$]$_s$ laminate has been performed in order to establish recommendations for the integration of battery cells. As reported in Figure 2, the battery cells are explicitly introduced into the finite element simulation within composite laminates subjected to tensile or compressive or bending loadings, which are the relevant ones when considering industrial composite structures. In both cell concepts, the cell has a thickness approximately corresponding to that of a CF ply or multiples thereof (around 0.2 mm per ply) and can therefore be easily integrated into the stacking sequence as a part of the ply. It should be noted that the cell thickness of the RMS concept can be adjusted to match the ply thickness by adapting the thicknesses of the cathode and anode coating.

For the virtual testing strategy, a mesher tool has been specifically developed in order to mesh any shape of battery cell, from theoretical ones (circular, rectangular) to real complex ones extracted directly from CT-scans. This mesher is based on the level-set method representing complex geometry without needing to specify an explicit formulation of the shape under consideration [4]. Full integration quadratic elements were chosen, which are generic wedge elements called c3d15, used for each unidirectional ply in the laminate.
the advantages and trade-offs of the SB integration concepts with respect to functionally separated components is discussed.

## 2. Numerical integration strategy of battery cells into composite structures

The objective of the work is to establish recommendations for the integration of battery cells into laminated composite structures in order to limit/control the degradation of the mechanical properties of carbon/epoxy composite structures compared to the reference one without battery cell.

### 2.1. *Studied composite material*

The considered composite material is made of AS4/8552 UD ply with an aerial weight of 194g/m², which is widely used in the manufacturing of composite structures in aeronautical industries. Furthermore, this material has been studied in the literature [1] and mechanical properties are available in [2]. A classical quasi-isotropic laminate consisting of 16 plies has been considered. The stacking sequence is [(45/90/-45/0)$_2$]$_s$ and fulfils the recommendations of current design offices [3].

### 2.2. *Studied battery cell concepts*

The SOLIFLY project focuses on the development of structural batteries using two different concepts, the multifunctional Coated Carbon Fibres (CCF) material and the multifunctional Reinforced Multilayer Stack (RMS), illustrated in Figure 1a and Figure 1b respectively. The CCF concept battery cells contain carbon fibres coated with active materials as electrodes. The carbon fibres also act as reinforcement for the battery cell. The reinforced multilayer stack (RMS) consists of conventionally coated anode on Cu foil and cathode on Al foil. The electrolyte consists of a polymer which provides mechanical stiffness and strength and a liquid electrolyte for ion conduction. No additional separator is needed.

<figure><img src="image_6.png" /><figcaption>**Figure 1:** Presentation of a) CCF cell and b) RMS cell concepts, developed in the SOLIFLY project</figcaption></figure>

### 2.3. *Presentation of the virtual testing strategy*

A virtual test campaign on a quasi-isotropic [(45/90/-45/0)$_2$]$_s$ laminate has been performed in order to establish recommendations for the integration of battery cells. As reported in Figure 2, the battery cells are explicitly introduced into the finite element simulation within composite laminates subjected to tensile or compressive or bending loadings, which are the relevant ones when considering industrial composite structures. In both cell concepts, the cell has a thickness approximately corresponding to that of a CF ply or multiples thereof (around 0.2 mm per ply) and can therefore be easily integrated into the stacking sequence as a part of the ply. It should be noted that the cell thickness of the RMS concept can be adjusted to match the ply thickness by adapting the thicknesses of the cathode and anode coating.

For the virtual testing strategy, a mesher tool has been specifically developed in order to mesh any shape of battery cell, from theoretical ones (circular, rectangular) to real complex ones extracted directly from CT-scans. This mesher is based on the level-set method representing complex geometry without needing to specify an explicit formulation of the shape under consideration [4]. Full integration quadratic elements were chosen, which are generic wedge elements called c3d15, used for each unidirectional ply in the laminate.
<figure><img src="image_7.png" /><figcaption>Figure 2: Numerical strategy for the cell integration concept for solid laminate composites</figcaption></figure>

The finite element simulations are performed considering linear elastic behaviour for the composite material and for the battery cells. The composite plies are assumed to be perfectly bonded to each other but also to the battery cells, which is an arguable assumption made for the sake of simplicity. The finite element simulations are performed with the in-house software Z-set developed jointly by ONERA and the Ecole des Mines, using an implicit solver. The boundary conditions applied to the FE simulations consist of fixing the lower free edge and imposing the uniaxial displacement on the upper free edge at a constant displacement rate. Finite element simulations are considered without geometric non-linearities due to the low strain at failure of the carbon/epoxy laminates considered.

A parametric study has been performed considering different positions of the battery cells through the thickness of the laminate (by partially replacing either 0, 90 or ±45-plies, while maintaining the constraint of symmetry of the lay-up) with different sizes of the battery cells (with surface areas betweeb 225mm² and 531mm²), and different shapes (square, circular, rectangular, ellipsoid, rectellipsoid, ...), in order to provide first guidelines for the manufacturing of composite plates with embedded battery cells, as reported in Figure 3.

In order to limit the computation time, the 3D finite element simulations are performed using a decomposition domain strategy since the number of degrees of freedom is very high due to the ply-by-ply mesh (more than 1.2 million degrees of freedom). Considering this strategy, the computation time is about 5 minutes considering a linear elastic behaviour of the constituents, decomposed into 8 subdomains. The control of the computational time is a very important point for the virtual parametric study under consideration.

In order to estimate the onset of damage in the composite material, the ONERA Progressive Failure Model (OPFM), which has been validated on many test configurations for carbon/epoxy laminates [5], was used as a post-processing to the linear elastic simulations at each Gauss point. The stress failure criterion associated with the first ply failure in the interfibre mode (i.e. prediction of the first transverse crack in the laminate) is given in Eq. (1).

$$f_2 = \left(\frac{\sigma_{22}}{Y_f}\right)^2 + \left(\frac{\tau_{12}}{S_c(1-p\sigma_{22})}\right)^2 = 1 \quad \text{with} \quad p = -\frac{1}{Y_c} \qquad (1)$$
<figure><img src="image_7.png" /><figcaption>Figure 2: Numerical strategy for the cell integration concept for solid laminate composites</figcaption></figure>

The finite element simulations are performed considering linear elastic behaviour for the composite material and for the battery cells. The composite plies are assumed to be perfectly bonded to each other but also to the battery cells, which is an arguable assumption made for the sake of simplicity. The finite element simulations are performed with the in-house software Z-set developed jointly by ONERA and the Ecole des Mines, using an implicit solver. The boundary conditions applied to the FE simulations consist of fixing the lower free edge and imposing the uniaxial displacement on the upper free edge at a constant displacement rate. Finite element simulations are considered without geometric non-linearities due to the low strain at failure of the carbon/epoxy laminates considered.

A parametric study has been performed considering different positions of the battery cells through the thickness of the laminate (by partially replacing either 0, 90 or ±45-plies, while maintaining the constraint of symmetry of the lay-up) with different sizes of the battery cells (with surface areas betweeb 225mm² and 531mm²), and different shapes (square, circular, rectangular, ellipsoid, rectellipsoid, ...), in order to provide first guidelines for the manufacturing of composite plates with embedded battery cells, as reported in Figure 3.

In order to limit the computation time, the 3D finite element simulations are performed using a decomposition domain strategy since the number of degrees of freedom is very high due to the ply-by-ply mesh (more than 1.2 million degrees of freedom). Considering this strategy, the computation time is about 5 minutes considering a linear elastic behaviour of the constituents, decomposed into 8 subdomains. The control of the computational time is a very important point for the virtual parametric study under consideration.

In order to estimate the onset of damage in the composite material, the ONERA Progressive Failure Model (OPFM), which has been validated on many test configurations for carbon/epoxy laminates [5], was used as a post-processing to the linear elastic simulations at each Gauss point. The stress failure criterion associated with the first ply failure in the interfibre mode (i.e. prediction of the first transverse crack in the laminate) is given in Eq. (1).

$$f_2 = \left( \frac{\sigma_{22}}{Y_f} \right)^2 + \left( \frac{\tau_{12}}{S_c(1 - p\sigma_{22})} \right)^2 = 1 \quad \text{with} \quad p = -\frac{1}{Y_c} \qquad (1)$$
where $\sigma$ is the stress tensor, $Y_t$, $S_c$ and $Y_c$ are respectively the transverse tensile strength, the in-plane shear strength and the transverse compressive strength of the UD ply.

<figure><img src="image_8.png" /><figcaption>**Figure 3:** Presentation of the meshed laminated quasi-isotropic plate [(45/90/-45/0)$_2$]s with embedded battery cells in 90-plies with different shapes and surface areas.</figcaption></figure>

## 3. Material properties

### 3.1. Composite material

The elastic properties of the AS4/8552 composite material are extracted from the literature [1] and are given in Table 1. Moreover, for the purpose of predicting the onset of damage in the composite, the transverse tensile and compressive strengths noted respectively $Y_t$ and $Y_c$ and the in-plane shear strength $S_L$ are also given in Table 1.

<table><caption>**Table 1.** Elastic properties of the AS4/8552 composite material (transverse isotropy)</caption><thead><tr><th>Properties</th><th>AS4/8552</th></tr></thead><tbody><tr><td>E<sub>11</sub></td><td>134.0 GPa</td></tr><tr><td>E<sub>22</sub></td><td>10.0 GPa</td></tr><tr><td>G<sub>12</sub></td><td>4.9 GPa</td></tr><tr><td>ν<sub>12</sub></td><td>0.31</td></tr><tr><td>Y<sub>t</sub></td><td>81.0 MPa</td></tr><tr><td>Y<sub>c</sub></td><td>-250.0 MPa</td></tr><tr><td>S<sub>L</sub></td><td>114.0 MPa</td></tr></tbody></table>

### 3.2. Battery cells mechanical properties

It is essential to know precisely the behaviour and the failure properties of the two battery cell concepts developed in SOLIFLY.

For this purpose, six dummy battery cells of the CCF cell concept have been manufactured. The CCF concept consists of coating carbon fibres, combined with a bi-continuous structural electrolyte as a matrix element, to form prepregs that can be further processed to form electrode strips. These strips are
where $\sigma$ is the stress tensor, $Y_t$, $S_c$ and $Y_c$ are respectively the transverse tensile strength, the in-plane shear strength and the transverse compressive strength of the UD ply.

<figure><img src="image_8.png" /><figcaption>**Figure 3:** Presentation of the meshed laminated quasi-isotropic plate [(45/90/-45/0)$_2$]s with embedded battery cells in 90-plies with different shapes and surface areas.</figcaption></figure>

## 3. Material properties

### 3.1. Composite material

The elastic properties of the AS4/8552 composite material are extracted from the literature [1] and are given in Table 1. Moreover, for the purpose of predicting the onset of damage in the composite, the transverse tensile and compressive strengths noted respectively $Y_t$ and $Y_c$ and the in-plane shear strength $S_L$ are also given in Table 1.

<table><caption>**Table 1.** Elastic properties of the AS4/8552 composite material (transverse isotropy)</caption><thead><tr><td>Properties</td><td>AS4/8552</td></tr></thead><tbody><tr><td>E<sub>11</sub></td><td>134.0 GPa</td></tr><tr><td>E<sub>22</sub></td><td>10.0 GPa</td></tr><tr><td>G<sub>12</sub></td><td>4.9 GPa</td></tr><tr><td>ν<sub>12</sub></td><td>0.31</td></tr><tr><td>Y<sub>t</sub></td><td>81.0 MPa</td></tr><tr><td>Y<sub>c</sub></td><td>-250.0 MPa</td></tr><tr><td>S<sub>L</sub></td><td>114.0 MPa</td></tr></tbody></table>

### 3.2. Battery cells mechanical properties

It is essential to know precisely the behaviour and the failure properties of the two battery cell concepts developed in SOLIFLY.

For this purpose, six dummy battery cells of the CCF cell concept have been manufactured. The CCF concept consists of coating carbon fibres, combined with a bi-continuous structural electrolyte as a matrix element, to form prepregs that can be further processed to form electrode strips. These strips are
stacked to form battery cells. AS4/LY556 0-plies with coated fibres in some plies, were provided by UNIVIE.

Three specimens, composed of eight 0-plies to facilitate the mechanical testing, were cut to size 150mm x 16mm x 0.83mm. All the specimens were tested at ONERA on an electromechanical testing machine with a maximum capacity of 10kN, see Figure 4a. All tests were multi-instrumented [6] with (i) acoustic emission (2 nano30 sensors associated with the commercial software MISTRAS) to detect damage events during the loading, (ii) digital images correlation (using two 12bit CCD cameras with a resolution of 2048x2048 pixels and the commercial software Vic3D) to measure the strain during the test and to detect cracks, and (iii) SEM analysis after loading to study carefully the failure pattern. The first specimen was loaded to ultimate failure at $\sigma=262$ MPa which was due to delamination, see Figure 4b. No fibre failure was observed during the test, only large delamination cracks. The strength value obtained is therefore quite high, as expected.

<figure><img src="image_9.png" /><figcaption>Figure 4: (a) Experimental device at ONERA and (b) stress/strain curve during a tensile test at failure on a CCF battery cell.</figcaption></figure>

Several loads/unloads were carried out during the tests to demonstrate that there was no hysteresis until the final failure again due to delamination. The scatter associated with the tests was not negligible due to initial defects (discussed below). The Young modulus was measured to evolve between 55GPa and 58GPa. Based on these measurements, it is possible to estimate the other elastic properties of the equivalent unidirectional ply with coated fibres as a transversely isotropic elastic material using homogenisation methods. Knowing the elastic properties of AS4 fibre and LY556 matrix [7] found in the literature, it is possible to estimate the volume fibre ratio of $V_f=24\%$ of the CCF cell concept considering different homogenisation methods, such as the Mori-Tanaka method [8]. Therefore, the elastic properties of the equivalent UD ply were estimated to be $E_{11}=56$GPa, $E_{22}=4.9$GPa, $G_{12}=2.1$GPa, $v_{12}=0.3$. It is important to note that for the CCF cell concept, initial defects were observed prior to testing. In fact, initial delamination cracks were observed on the edges of every CCF concept specimens, which explains the low stress at failure due to premature delamination. Moreover, initial splitting cracks and significant fibre misalignments were optically observed. For CCF concept battery cells, some improvements are still needed to avoid initial defects and to reduce the scattering in the mechanical properties.

Furthermore, dummy RMS battery cells (of a preliminary, but from a mechanical point of view representative version of the RMS battery cell concept) were provided by AIT. Figure 5a presents the microstructure of the RMS battery cell concept. From this large cell, two specimens of 150mm x 20mm x 0.19mm were cut with scissors and tested at ONERA on an electromechanical testing machine with a maximum capacity of 10kN, see Figure 5b. It should be noted that performing mechanical test on extremely thin specimens remains very difficult. Again, the tests were multi-instrumented with acoustic emission and digital images correlation to accurately determine the failure mechanisms.
stacked to form battery cells. AS4/LY556 0-plies with coated fibres in some plies, were provided by UNIVIE.

Three specimens, composed of eight 0-plies to facilitate the mechanical testing, were cut to size 150mm x 16mm x 0.83mm. All the specimens were tested at ONERA on an electromechanical testing machine with a maximum capacity of 10kN, see Figure 4a. All tests were multi-instrumented [6] with (i) acoustic emission (2 nano30 sensors associated with the commercial software MISTRAS) to detect damage events during the loading, (ii) digital images correlation (using two 12bit CCD cameras with a resolution of 2048x2048 pixels and the commercial software Vic3D) to measure the strain during the test and to detect cracks, and (iii) SEM analysis after loading to study carefully the failure pattern. The first specimen was loaded to ultimate failure at $\sigma=262$ MPa which was due to delamination, see Figure 4b. No fibre failure was observed during the test, only large delamination cracks. The strength value obtained is therefore quite high, as expected.

<figure><img src="image_9.png" /><figcaption>Figure 4: (a) Experimental device at ONERA and (b) stress/strain curve during a tensile test at failure on a CCF battery cell.</figcaption></figure>

Several loads/unloads were carried out during the tests to demonstrate that there was no hysteresis until the final failure again due to delamination. The scatter associated with the tests was not negligible due to initial defects (discussed below). The Young modulus was measured to evolve between 55GPa and 58GPa. Based on these measurements, it is possible to estimate the other elastic properties of the equivalent unidirectional ply with coated fibres as a transversely isotropic elastic material using homogenisation methods. Knowing the elastic properties of AS4 fibre and LY556 matrix [7] found in the literature, it is possible to estimate the volume fibre ratio of $V_f=24\%$ of the CCF cell concept considering different homogenisation methods, such as the Mori-Tanaka method [8]. Therefore, the elastic properties of the equivalent UD ply were estimated to be $E_{11}=56$GPa, $E_{22}=4.9$GPa, $G_{12}=2.1$GPa, $v_{12}=0.3$. It is important to note that for the CCF cell concept, initial defects were observed prior to testing. In fact, initial delamination cracks were observed on the edges of every CCF concept specimens, which explains the low stress at failure due to premature delamination. Moreover, initial splitting cracks and significant fibre misalignments were optically observed. For CCF concept battery cells, some improvements are still needed to avoid initial defects and to reduce the scattering in the mechanical properties.

Furthermore, dummy RMS battery cells (of a preliminary, but from a mechanical point of view representative version of the RMS battery cell concept) were provided by AIT. Figure 5a presents the microstructure of the RMS battery cell concept. From this large cell, two specimens of 150mm x 20mm x 0.19mm were cut with scissors and tested at ONERA on an electromechanical testing machine with a maximum capacity of 10kN, see Figure 5b. It should be noted that performing mechanical test on extremely thin specimens remains very difficult. Again, the tests were multi-instrumented with acoustic emission and digital images correlation to accurately determine the failure mechanisms.
<figure><img src="image_10.png" /><figcaption>**Figure 5:** (a) Presentation of the RMS cell concept microstructure, (b) experimental device at ONERA and (c) stress/strain curves during a tensile test with unloading on the RMS concept specimen.</figcaption></figure>

The first specimen tested was subjected to tensile loading up to the final failure, which occurs in the area of interest and is associated with delamination between the layers constituting the battery cell. The failure stress is equal to 40 MPa. No significant acoustic event was recorded prior to the failure, but a non-linear behaviour up to the failure was observed. In order to understand the source of this non-linearity, another test was carried out and stopped before failure (around $\sigma=33$ MPa) and then unloaded. A plastic behaviour is clearly observed in Figure 5c since the Young's moduli, equal to $E=10.2$ GPa (a value close to the transverse rigidity of the ply considered, which will limit the overstressing in the 90 plies), measured during the loading and the unloading are similar and a permanent strain remains at zero stress. The Poisson's ratio was also measured to be $\nu=0.35$, as reported in table 2.

<table><caption>**Table 2.** Elastic properties of the RMS battery cell concept (isotropy)</caption><thead><tr><td>Properties</td><td>RMS Concept</td></tr></thead><tbody><tr><td>E</td><td>10.2 GPa</td></tr><tr><td>ν</td><td>0.35</td></tr></tbody></table>

A plastic behaviour with an isotropic hardening will be considered in future work to describe the non-linear behaviour of the RMS battery cell concept. Moreover, the scattering associated with the tests was rather low. Therefore, the RMS battery cell concept seems to be mature enough to be included in the final demonstrator planned at the end of the project.

## 4. Influence of the integration of battery cells in composite structures

Having determined the mechanical properties of the composite material and of the battery cells, the objective is to define some guidelines for the introduction of battery cells in the composite structure, based on a virtual test campaign considering different sizes and shapes of RMS battery cells, and also considering their integration in different plies (0, 90 or $\pm45$-plies). The macroscopic stiffness and also the damage initiation will be studied in the following.

### 4.1. Influence on the global rigidity

The influence of the introduction of battery cells on the macroscopic stiffness was carefully analysed. The quasi-isotropic laminate subjected to a tensile load was considered considering the different sizes and shapes of battery cells shown in Figure 3. Firstly, it is obvious that battery cells must not be introduced in 0-ply, to avoid a drastic reduction in stiffness (-60% by replacing the 0-ply with battery cells). Secondly, as reported in Figure 6a, for a given volume of battery cells, the shape of the cells does not significantly affect the reduction in macroscopic stiffness. In fact, as reported in Figure 6b, the
<figure><img src="image_10.png" /><figcaption>**Figure 5:** (a) Presentation of the RMS cell concept microstructure, (b) experimental device at ONERA and (c) stress/strain curves during a tensile test with unloading on the RMS concept specimen.</figcaption></figure>

The first specimen tested was subjected to tensile loading up to the final failure, which occurs in the area of interest and is associated with delamination between the layers constituting the battery cell. The failure stress is equal to 40 MPa. No significant acoustic event was recorded prior to the failure, but a non-linear behaviour up to the failure was observed. In order to understand the source of this non-linearity, another test was carried out and stopped before failure (around $\sigma=33$ MPa) and then unloaded. A plastic behaviour is clearly observed in Figure 5c since the Young's moduli, equal to $E=10.2$ GPa (a value close to the transverse rigidity of the ply considered, which will limit the overstressing in the 90 plies), measured during the loading and the unloading are similar and a permanent strain remains at zero stress. The Poisson's ratio was also measured to be $\nu=0.35$, as reported in table 2.

<table><caption>**Table 2.** Elastic properties of the RMS battery cell concept (isotropy)</caption><thead><tr><td>Properties</td><td>RMS Concept</td></tr></thead><tbody><tr><td>E</td><td>10.2 GPa</td></tr><tr><td>ν</td><td>0.35</td></tr></tbody></table>

A plastic behaviour with an isotropic hardening will be considered in future work to describe the non-linear behaviour of the RMS battery cell concept. Moreover, the scattering associated with the tests was rather low. Therefore, the RMS battery cell concept seems to be mature enough to be included in the final demonstrator planned at the end of the project.

## 4. Influence of the integration of battery cells in composite structures

Having determined the mechanical properties of the composite material and of the battery cells, the objective is to define some guidelines for the introduction of battery cells in the composite structure, based on a virtual test campaign considering different sizes and shapes of RMS battery cells, and also considering their integration in different plies (0, 90 or $\pm45$-plies). The macroscopic stiffness and also the damage initiation will be studied in the following.

### 4.1. Influence on the global rigidity

The influence of the introduction of battery cells on the macroscopic stiffness was carefully analysed. The quasi-isotropic laminate subjected to a tensile load was considered considering the different sizes and shapes of battery cells shown in Figure 3. Firstly, it is obvious that battery cells must not be introduced in 0-ply, to avoid a drastic reduction in stiffness (-60% by replacing the 0-ply with battery cells). Secondly, as reported in Figure 6a, for a given volume of battery cells, the shape of the cells does not significantly affect the reduction in macroscopic stiffness. In fact, as reported in Figure 6b, the
decrease in global stiffness as a function of the battery cell volume is linear, due to the homogenisation of the elastic mechanical properties of the UD ply and the battery cells. The introduction of cells in 90-plies or ±45-plies seems to be rather equivalent.

<figure><img src="image_11.png" /><figcaption>**Figure 6:** Numerical estimation of the reduction of macroscopic stiffness due to the integration of battery cells in different plies (90 or ±45 plies), with different cell surfaces, and different shapes.</figcaption></figure>

The conclusions established here are applicable for tension or compression loadings. Additional work needs to be carried out for bending loads where the order of the ply orientation through the thickness has an influence.

## 4.2. Influence on the onset of damage

Moreover, unlike many studies in the literature [9], the onset of damage within the laminate was also considered. Again, it is not permissible to introduce battery cells in 0-plies in the laminate as these plies ensure the integrity of the laminate. Firstly, as shown in Figure 7, it appears viable to introduce battery cells in 90-plies in order to delay the damage cause by the introduction of battery cells, which is approximately -20% compared to the reference case.

Contrary to the stiffness, the size of the battery cell does not seem to influence the onset of damage. In fact, the transverse crack occurs perpendicular to the tensile load, where the effective width is the smallest for each cell shape, similar to the weakest link in the structure. When battery cells are introduced in ±45-plies, the first ply failure occurs again in 90-plies due to the local load report (about -40%). In this case, the shape of the battery cell has an influence on the first transverse crack, which occurs at one edge of the battery cell (the width of the rectangular cell or the top of the ellipsoid).

<figure><img src="image_12.png" /><figcaption>**Figure 7:** Numerical estimation of the onset of damage due to integration of battery cells in different plies (90 or ±45 plies), different volumes of battery cells, and different shapes.</figcaption></figure>

## 4.3. Proposed guidelines for the integration of battery cells in composite structure

Some recommendations have been made based on the finite element simulations where the battery cells are explicitly introduced. It has been shown that the volume of battery cells linearly decreases the macroscopic stiffness of the laminate when integrated in 90-plies and ±45-plies. The decrease in damage
decrease in global stiffness as a function of the battery cell volume is linear, due to the homogenisation of the elastic mechanical properties of the UD ply and the battery cells. The introduction of cells in 90-plies or ±45-plies seems to be rather equivalent.

<figure><img src="image_11.png" /><figcaption>**Figure 6:** Numerical estimation of the reduction of macroscopic stiffness due to the integration of battery cells in different plies (90 or ±45 plies), with different cell surfaces, and different shapes.</figcaption></figure>

The conclusions established here are applicable for tension or compression loadings. Additional work needs to be carried out for bending loads where the order of the ply orientation through the thickness has an influence.

## 4.2. Influence on the onset of damage

Moreover, unlike many studies in the literature [9], the onset of damage within the laminate was also considered. Again, it is not permissible to introduce battery cells in 0-plies in the laminate as these plies ensure the integrity of the laminate. Firstly, as shown in Figure 7, it appears viable to introduce battery cells in 90-plies in order to delay the damage cause by the introduction of battery cells, which is approximately -20% compared to the reference case.

Contrary to the stiffness, the size of the battery cell does not seem to influence the onset of damage. In fact, the transverse crack occurs perpendicular to the tensile load, where the effective width is the smallest for each cell shape, similar to the weakest link in the structure. When battery cells are introduced in ±45-plies, the first ply failure occurs again in 90-plies due to the local load report (about -40%). In this case, the shape of the battery cell has an influence on the first transverse crack, which occurs at one edge of the battery cell (the width of the rectangular cell or the top of the ellipsoid).

<figure><img src="image_12.png" /><figcaption>**Figure 7:** Numerical estimation of the onset of damage due to integration of battery cells in different plies (90 or ±45 plies), different volumes of battery cells, and different shapes.</figcaption></figure>

## 4.3. Proposed guidelines for the integration of battery cells in composite structure

Some recommendations have been made based on the finite element simulations where the battery cells are explicitly introduced. It has been shown that the volume of battery cells linearly decreases the macroscopic stiffness of the laminate when integrated in 90-plies and ±45-plies. The decrease in damage
initiation in the laminate can be minimised by considering battery cells in 90-ply and the shape does not seem to be as influential. These conclusions can be applied to the two battery cell concepts developed in the project. It is important to note that the damage and failure of the battery cell (considered as a linear elastic material) is not considered in this preliminary study. The plasticity of the battery cells, but also the introduction of the possibility of delamination between the battery cell and the composite material due to a probable low fracture toughness (to be determined soon) will be considered.

## 5. Conclusions and Perspectives

In this paper, a numerical methodology was presented to simulate the integration of battery cells in composite structures and to estimate the decrease in the global stiffness of the laminate (essential to predict the buckling of the large stiffened panel that constitutes the final demonstrator of the project) and the decrease in the onset of damage (strongly related to the fatigue lifetime of the considered composite structure). The mechanical properties of the battery cells were determined thanks to multi-instrumented tests, while those of the composite material have been extracted from the literature. It seems that the global stiffness of the laminate with embedded RMS battery cell concept is influenced by the volume of the cells integrated in the composite structures, but its influence remains limited considering their integration in 90 or ±45-plies. For the onset of damage, the size of the battery cell is not a major issue, as opposed to the shape and position of the battery shape. It seems more relevant to introduce battery cells in 90-plies to limit the decrease of the damage onset. The first composite laminate with embedded RMS battery cell concept will be manufactured soon, considering the proposed recommendations.

## Acknowledgment

This project has been funded by the Clean Aviation Joint Undertaking under the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 101007577.

## References

[1] A.H. Baluch, O. Falco, J.L. Jimenez, B.H. Tijs, C.S. Lopes, An efficient numerical approach to the prediction of laminate tolerance to Barely Visible Impact Damage, Compos. Struct. 225 (2019) 111017.

[2] Hexcel, HexPly_8552_eu_DataSheet, (2020). https://www.hexcel.com/user_area/content_media/raw/HexPly_8552_eu_DataSheet.pdf.

[3] F.X. Irisarri, Stratégies de calcul pour l'optimisation multiobjectif des structures composites, doctoral thesis of University Paul Sabatier Toulouse, 2009.

[4] P.-O. Persson, Mesh Generation for Implicit Geometries, doctoral thesis of Massachusetts Institute of Technology, 2005.

[5] F. Laurin, N. Carrere, J.F. Maire, A multiscale progressive failure approach for composite laminates based on thermodynamical viscoelastic and damage models, Compos. Part Appl. Sci. Manuf. 38 (2007) 198-209.

[6] F. Laurin, J.-S. Charrier, D. Leveque, J.-F. Maire, A. Mavel, P. Nunez, Determination of the properties of composite materials thanks to digital image correlation measurements, Procedia IUTAM. 4 (2012) 106-115.

[7] P.D. Soden, M.J. Hinton, A.S. Kaddour, Lamina properties, lay-up configurations and loading conditions for a range of fibre-reinforced composite laminates, Compos. Sci. Technol. 58 (1998) 1011-1022.

[8] T. Mori, K. Tanaka, Average stress in matrix and average elastic energy of materials with misfitting inclusions, Acta Metall. 21 (1973) 571-574.

[9] T.J. Adam, G. Liao, J. Petersen, S. Geier, B. Finke, P. Wierach, A. Kwade, M. Wiedemann, Multifunctional Composites for Future Energy Storage in Aerospace Structures, Energies. 11 (2018).
initiation in the laminate can be minimised by considering battery cells in 90-ply and the shape does not seem to be as influential. These conclusions can be applied to the two battery cell concepts developed in the project. It is important to note that the damage and failure of the battery cell (considered as a linear elastic material) is not considered in this preliminary study. The plasticity of the battery cells, but also the introduction of the possibility of delamination between the battery cell and the composite material due to a probable low fracture toughness (to be determined soon) will be considered.

## 5. Conclusions and Perspectives

In this paper, a numerical methodology was presented to simulate the integration of battery cells in composite structures and to estimate the decrease in the global stiffness of the laminate (essential to predict the buckling of the large stiffened panel that constitutes the final demonstrator of the project) and the decrease in the onset of damage (strongly related to the fatigue lifetime of the considered composite structure). The mechanical properties of the battery cells were determined thanks to multi-instrumented tests, while those of the composite material have been extracted from the literature. It seems that the global stiffness of the laminate with embedded RMS battery cell concept is influenced by the volume of the cells integrated in the composite structures, but its influence remains limited considering their integration in 90 or ±45-plies. For the onset of damage, the size of the battery cell is not a major issue, as opposed to the shape and position of the battery shape. It seems more relevant to introduce battery cells in 90-plies to limit the decrease of the damage onset. The first composite laminate with embedded RMS battery cell concept will be manufactured soon, considering the proposed recommendations.

## Acknowledgment

This project has been funded by the Clean Aviation Joint Undertaking under the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 101007577.

## References

[1] A.H. Baluch, O. Falco, J.L. Jimenez, B.H. Tijs, C.S. Lopes, An efficient numerical approach to the prediction of laminate tolerance to Barely Visible Impact Damage, Compos. Struct. 225 (2019) 111017.

[2] Hexcel, HexPly_8552_eu_DataSheet, (2020).
https://www.hexcel.com/user_area/content_media/raw/HexPly_8552_eu_DataSheet.pdf.

[3] F.X. Irisarri, Stratégies de calcul pour l'optimisation multiobjectif des structures composites, doctoral thesis of University Paul Sabatier Toulouse, 2009.

[4] P.-O. Persson, Mesh Generation for Implicit Geometries, doctoral thesis of Massachusetts Institute of Technology, 2005.

[5] F. Laurin, N. Carrere, J.F. Maire, A multiscale progressive failure approach for composite laminates based on thermodynamical viscoelastic and damage models, Compos. Part Appl. Sci. Manuf. 38 (2007) 198-209.

[6] F. Laurin, J.-S. Charrier, D. Leveque, J.-F. Maire, A. Mavel, P. Nunez, Determination of the properties of composite materials thanks to digital image correlation measurements, Procedia IUTAM. 4 (2012) 106-115.

[7] P.D. Soden, M.J. Hinton, A.S. Kaddour, Lamina properties, lay-up configurations and loading conditions for a range of fibre-reinforced composite laminates, Compos. Sci. Technol. 58 (1998) 1011-1022.

[8] T. Mori, K. Tanaka, Average stress in matrix and average elastic energy of materials with misfitting inclusions, Acta Metall. 21 (1973) 571-574.

[9] T.J. Adam, G. Liao, J. Petersen, S. Geier, B. Finke, P. Wierach, A. Kwade, M. Wiedemann, Multifunctional Composites for Future Energy Storage in Aerospace Structures, Energies. 11 (2018).