1 Introduction

At present, the development of new technologies is closely linked to the need for reduced emissions to protect the environment. In this scenario, the transportation sector, with a particular reference to the aeronautic field, plays a fundamental role in achieving a strong reduction in pollutant emissions. Civil aviation accounts for 13.2% of carbon dioxide (CO2) emissions from the European transport sector (Ref 1). The 2019 European Aviation Environment Report shows a relative increase of 16% in CO2 emissions compared to the reference year 2005 to 2017. Based on current models, a further 42% growth is expected. Nitrogen oxide (NOx) emissions also showed a sharp increase (Ref 2).

European Flight Path adopted a resolution on these results, aiming for ambitious environmental targets for civil aviation. By 2050, the expectation is to reduce both CO2 and NOx emissions per passenger-km by 75 and 90%, respectively. In this context, hybrid and all-electric aircraft represent a higher step toward green aviation. As of today, some all-electric aircraft have been designed and flown. A concise list of all-electric available aircraft is reported in (Ref 3). Although not listed, noteworthy is the Pipistrel Velis Electro, belonging to the very light aircraft category, which is the only existing all-electric aircraft to be certified. In (Ref 4), the lesson learned from the certification process of this aircraft is thoroughly explained, with a focus on the requirements needed to ensure the airworthiness of the employed battery pack.

The lithium battery of traditional electric means of transport is a great invention but absolutely monopolizes most of the weight of the vehicle. Thus, scholars are focusing on the concept of structural battery, which is quite different because it accumulates energy considering that the structure is a battery (Ref 5).

Structural batteries stand out as a possible solution to improve aircraft electric energy-storing capabilities. These are multifunctional components, manufactured similar to composite materials already used on many aircraft, and capable of replacing stress-supporting parts typically made from metal alloys or carbon fibers. In this way, the increased weight on board the aircraft due to the presence of batteries is mitigated by the multifunctionality of this new technology, acting not only to store energy but also as a load-bearing component.

In literature (Ref 6, 7), two parameters are used to describe the characteristics of such technology: the degree of integration, which refers to the proportion by which the battery is integrated into the structure, and the scale of functionalization, referring to the physical dimension of the elements giving multi-functionality.

The state of the art on the research of a structural battery configuration can be extended on different research paths involving the study of electrode materials (Ref 8), the matrix materials that provide rigidity to the composite while being ionically conductive (Ref 9, 10), assembly, scale-up, demonstration, multiphysics modeling, design, and applications. The possible applications are endless, the casings for mobile phones, laptops (Ref 11), or structural components of cars and airplanes that simultaneously carry loads and store electrical energy, allowing for significant weight savings.

Concerning batteries, lithium-based ones are typically considered, given their better performance in terms of specific energy and specific energy density compared to batteries based on different chemistries. A comparative study is reported in (Ref 12), in which lithium, nickel–cadmium, and lead acid-based batteries are assessed via the aforementioned performance parameters.

Nonetheless, such battery parameters are exactly the limiting factor for electrification in aircraft applications. In (Ref 13), different energy sources are compared in terms of specific energy and specific energy density: it is highlighted that even the most advanced current battery storage systems have worse performance than traditional fuels like Diesel and Kerosene. Once the required energy is fixed, the weight of the lithium battery is about 60 times that of the Kerosene system and the required storage space itself is 18 times bigger. This is obviously a core issue, especially for the heavy aircraft categories, in which the energy request increases. Moreover, pollution remains a problem, since approximately 95% of lithium batteries are landfilled instead of recycled upon reaching end of life (Ref 14, 15), although some interesting solutions are proposed in literature (Ref 16, 17).

In this paper, Carbon Fiber Reinforced Polymer (CFRP) laminates layered to simulate energy-storing functionalization are compared with non-functionalized ones via low-velocity impacts, both performed numerically and experimentally. The aim is to understand if the cause of the initiation and propagation of the damage needs to be taken into account and to understand whether there are any differences in the energy absorption capacity of the structure when it is intended also for another function (e.g., energy storing) besides carrying loads.

Nowadays, not only referring to the aeronautic field (Ref 18), but many kinds of structural components have also been realized by composite materials, due to their energy absorption capability. Low-velocity impact tests are fundamental to characterize the material in terms of damage tolerance and energy-absorbing capabilities. Therefore, numerous are the applications on this topic, aiming to enhance the comprehension of composites structural dynamics when subjected to low-velocity impact tests.

Guida et al. (Ref 19) showed the results of an impact test campaign performed on laminated composite structures of different geometries related by different scale factors in accordance with ASTM-D7136 (Ref 20). This work was used to evaluate the scaling effects (Ref 21) since problems related to the absorption of energy concern various applications in the aeronautical sector up to full-scale demonstrators.

More recently, Zhao et al. (Ref 22) developed a multiscale modeling framework to enhance the comprehension of low-velocity impact and compression after impact behaviors of plain-woven CFRP laminates. A good correspondence between experimental and numerical simulations is reached, revealing the decrease in the residual compressive strength of impacted CFRP laminates, but also the reliability of the adopted methodology.

Relevant is also the study performed by Wang et al. (Ref 23), in which the delamination of CFRP laminates under ice impact is investigated. Low-velocity impact tests are performed on CFRP laminates of different thicknesses by ice balls of different diameters using a gas gun system. This study has been considered as a reference to elucidate the delamination mechanism, investigated both numerically and experimentally.

Kaware et al. (Ref 24) studied the crush resistance to low-velocity impact tests of several composite structures by varying different design parameters (e.g., stacking sequence, boundary conditions, projectile energy, dart geometry, and impact angle).

The objective of the paper is to take a step forward in improving the mechanical behavior of structural batteries with the study of new interfacial treatments for battery panels, as well as to promote the use of alternative solid-state batteries with improved mechanical properties that can positively contribute to the development of the battery structure of the future. Once the maturity of the solution is reached, the evaluation of the internal capacity and resistance of the battery after the impact will be fundamental for the tuning, but for this it is necessary to reach the final failure.

2 Investigated Solution about Structural Battery: Model of a Carbon Fiber Laminate

Structural batteries are basically classified into two categories namely, respectively, multifunctional structures and multifunctional materials. In the first category, different materials within the structural battery perform a single function, the energy storing or load-bearing one; however, the overall compound performs both functions. In the latter, all materials adopt multiple functions, being contemporary load bearing and energy storing components. A more detailed classification can be found in (Ref 25), whereas structural batteries are classified in integrated conventional storage, integrated thin-film energy storage, structural laminate capacitors, and microscaled fiber capacitors.

Integrated conventional storage is obtained by embedding commercial lithium batteries into dedicated structural elements. The weight savings related to this structural battery type is limited to the replacement of the battery cover, e.g., with composite fiber material laminates. In (Ref 26), a comparative study is performed on two examples belonging to this structural battery type, obtained, respectively, by embedding lithium batteries in a composite laminate and in a sandwich structure. The core challenge for this type of structural batteries is to find the right compromise between mechanical and electrochemical performances (Ref 27), since an inverse relation between them has been observed: the more cells are integrated, the higher the energy density achieved, but the worse the mechanical properties.

Integrated thin-film energy storage represents a very similar approach to type-I since thin-film batteries are embedded in the structural elements instead of conventional ones. It has the advantage of minimizing the impact of the battery on the mechanical properties of the composite structure. On the other hand, higher costs are expected since thin-film batteries are significantly more expensive than conventional cells. A detailed investigation of such an approach is presented in (Ref 28).

Structural laminate capacitors (type-III) represent the passage from multifunctional structures to multifunctional materials, since this approach is based on the substitution of the conventional battery components, with load-bearing elements (Ref 29), to get higher weight savings. An example is given in Fig. 1, in which a laminated compound that also serves as a battery is shown.

Fig. 1
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Reproduced from Aerospace (Ref 7) under the CC BY 4.0 license

Example of type-III structural battery.

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A further step toward multifunctional materials is obtained with microscaled fiber capacitors (type-IV). In literature, two approaches are proposed, the coaxial (Ref 30) and the layered (Ref 31) ones. Unfortunately, despite scientific effort, type-IV structural batteries with appreciable performance have not yet been obtained.

In light of the above, the aim was to realize a type-III structural battery since promising performances have been already reached. The theorized configuration is shown in Fig. 2.

Fig. 2
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Schematization of the assumed structural battery configuration

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The structural battery consists of positive and negative electrodes, made of carbon fiber fabrics, already used for aeronautical applications, since characterized by high stiffness and strength, also allowing lithium ions intercalation by functionalizing it with active materials such as LiFePO4.

The solid electrolyte is made by polypropylene, typically used as a matrix in thermoplastic composite structural laminates, in which electrical percolation conditions are reached by dispersing lithium salts. Such ingredients have to be melt mixed and the product compression molded into thin films for the use as lithium conductive layers to be inserted in the anode and cathode portions of the structural battery sample. Clearly, for this research study it was not necessary considering its functionalization since the mechanical properties of the laminate are not influenced for this aspect (Ref 7).

The separator is made by a glass fiber fabric, while the current collectors are obtained by aluminum and copper foils (respectively, for the anode and the cathode), so as to avoid galvanic corrosion.

The aim of the performed activity was to manufacture, test, and simulate such a structural battery model just from a mechanical point of view, thus disregarding the electric functionalization and the energy storing function. The experimental activities are carried out by a drop test machine to verify the behavior of composite laminates and the sample response to impact propagation when subjected to low-velocity impact tests.

As schematically described in Fig. 3, two different samples were considered, with the aim of comparing a CFRP laminate, consisting of five carbon fiber fabrics, with a total thickness of 1.82 mm (left) with a type-III structural battery model (right). For the latter, on the upper and lower surfaces two carbon fiber fabrics have been adopted to simulate the structural battery electrodes, while a glass fiber fabric is placed at the center of the stacking sequence, so as to simulate the separator. The total thickness of such a laminate is of 1.71 mm. Notice that for both the samples polypropylene has been used: in the first case, thin polypropylene films, with a thickness of 10 µm, have been used just to serve as matrix for the laminate; in the second case, thicker polypropylene layers (150 µm) are used not only as matrix, but also to model the solid battery electrolyte layers, described in the above-mentioned configuration.

Fig. 3
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Considered arrangements for experimental tests

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Concerning the manufacturing, for both the arrangements hand lay-up has been applied, as shown in Fig. 4(a). Thereafter, a laboratory hydraulic press is used for the compaction, setting process conditions typically involved in the development of thermoplastic composite structural laminates. These conditions relate to the pressure and temperature time histories to which the laminate is subjected during the compaction phase, as shown in Fig. 4(b).

Fig. 4
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Hand lay-up (a) and processing conditions for the preparation of the laminates (b)

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2.1 Experimental Activity

Once achieved, the samples have been subjected to impact tests by a small free-fall tower, as shown in Fig. 5. It consists of four rectified columns fixed on a metal base. Each column is characterized by a guide that ensures the falling frame (equipped by eight steel wheels) to move only in the vertical direction, thus limiting oscillations. A lift/release system has been obtained by a belt, a tensioner, and a hook, to make it possible to regulate the impact height (max. 1 m) of the falling frame. The sample to be tested is clamped at a ground support. Realized uniformly to the standard (Ref 20), the latter consists of a metal frame, with a rectangular hole at the center that allows the specimen deflection when subjected to the impact test. To obtain the desired boundary conditions, four clamps were installed on the ground frame so that the sample could be locked at the edges.

Fig. 5
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Test setup

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The drop tower is instrumented with a quartz piezoelectric force sensor, which is directly exposed to the force at impact and is capable to measure high impact force with fast rise time.

Two impact energy levels were applied to study the samples response to impact propagation at low speeds, as reported in Table 1.

Table 1 Test parameters
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2.2 Numerical Modeling Procedure

The experimental impact tests have been replicated numerically using the LS-DYNA software, to obtain enhanced information on the structural dynamic differences between the two considered configurations. Solid elements have been used to model both the hemispherical dart and the plies of the composite laminates. The dimensions of the mesh elements have been set to have the right compromise between computational time and the accuracy of the expected results.

The laminate is clamped at the edges, while the dart (whose mass has been increased to account for the whole falling frame weight) is free to move along the vertical direction, subjected to gravity acceleration, with a null initial velocity.

Notice that both the carbon and the glass fiber fabrics have been implemented by considering the MAT-022 material card, typically used for orthotropic-elastic material models meshed through solid elements. In Table2, the mechanical properties, both for the carbon and glass fiber fabrics, are schematically reported (Ref 18).

Table 2 Carbon and glass fiber mechanical properties at ply level, Ref 18
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In Fig. 6, the realized numeric model is shown, highlighting the differences lying between the two considered arrangements for the through-the-thickness direction. On the left, the configuration adopted for the CFRP laminate is shown, while on the right, the configuration adopted for the structural battery model is focused.

Fig. 6
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Numerical model developed in LS-DYNA software. Two through-the-thickness modeling arrangements: CFRP laminate (left) and type-III structural battery model (right)

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Notice that for both the arrangements polypropylene layers are considered, modeling them with solid elements. MAT-001 material card has been used, considering a density ρ = 0.91 g/cm3 and a Young modulus E = 1400 MPa (Ref 32).

3 Results

In the considered application, two heights of fall of the dart are considered, corresponding to the energy levels indicated in Table 1. The experimental results, dealing with the force released at the impact, have been compared with the numerical simulation outputs, as proved in Fig. 7. Notice that the outputs of the experimental tests have been averaged and treated by a Butterworth filter. Another aspect to consider concerns the absence of an anti-rebound system for the drop tower; thus, only the first peak value of each curve is considered.

Fig. 7
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Numerical–experimental comparison of the impact force on the two specimens: CFRP laminate (top) and type-III structural battery model (bottom)

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Notice that the numerical model effectively reproduces the impact dynamics and time history of the impact force, but the finite element model is to be improved with more experimental tests. The main purpose of such a global analysis is to try to gain an understanding of the behavior of vibro-impacting systems and the situation, thanks to these preliminary experimental tests, it is clear cut.

Besides that, an interesting indication is obtained: for both considered energy levels, the force peaks, the experimental and numerical ones, are higher for the composite laminate with respect to the type-III structural battery model. Thus, the composite laminate seems to be stiffer, whereas the structural battery model is characterized by a through-the-thickness discontinuity, given by the presence of the glass fiber fabric. Such an indication is confirmed also in Fig. 8, in which the numerical displacement of the impacted zone for both considered arrangements and energy levels is shown. The obtained results confirm that the higher stiffness of the composite laminate, since the displacement is smaller for both the considered energy levels.

Fig. 8
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Comparison of the numerical displacement at the impacting zone on the two specimens: CFRP laminate (top) and type-III structural battery model (bottom)

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4 Conclusions

The main points concerning applications of multifunctional structural batteries in the aeronautic field arising from the most relevant state-of-the-art literature available publications have been presented and discussed.

A focus on low-velocity impact tests is presented, to compare a type-III structural battery model with a carbon fiber laminate. The manufacturing process and the experimental test setup are deeply described. The numerical analysis, implemented by the LS-DYNA software, is performed to enhance the information gathered from the experimental side. The impact energy feature sets of the impact signals on the two different specimens at 1.3 J, 2.6 J energy were established, and the energy recognition verification experiment was conducted. The most relevant result obtained is that the composite laminate appears to be more rigid than the model of the structural battery, while a discontinuity in the thickness is given by the presence of the glass fiber fabric, which acts as a separator. In conclusion, this work allows to focus on the importance of the distance of the carbon layers and the presence of an electrolyte; these results suggest that with the same impact energy it is possible to witness an increase in the carbon fiber load which can improve the impact resistance.