# Impact behavior of plywood core laminated with flax-epoxy composite skin

John Susainathan, Enrique Barbero, Sonia Sanchez-Saez, Florent Eyma, Arthur Cantarel

► To cite this version:

John Susainathan, Enrique Barbero, Sonia Sanchez-Saez, Florent Eyma, Arthur Cantarel. Impact behavior of plywood core laminated with flax-epoxy composite skin. European Journal of Wood and Wood Products, 2025, 83 (5), pp.172. 10.1007/s00107-025-02324-x. hal-05489319

HAL Id: hal-05489319

https://ut3-toulouseinp.hal.science/hal-05489319v1

Submitted on 2 Feb 2026

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# Impact behavior of plywood core laminated with flax-epoxy composite skin

John Susainathan<fnref n="1" /> · Enrique Barbero<fnref n="2" /> · Sonia Sanchez-Saez<fnref n="2" /> · Florent Eyma<fnref n="3" /> · Arthur Cantarel<fnref n="3" />

Received: 12 February 2025 / Accepted: 19 August 2025 / Published online: 8 September 2025
© The Author(s) 2025

## Abstract

The introduction of sandwich structures made from materials that are environmentally friendly, light, and strong enough to carry the applied loads is one way of reducing the carbon footprint of the transport industry. This work investigates the impact behaviour of a structure composed of a plywood core and flax/epoxy composite skins, as a proposal for a sandwich structure that takes advantage of the high longitudinal strength of raw timber along the transverse direction across the grain. The variation of peak force, indentation, impact bending stiffness and absorbed energy over a wide range of impact energies is studied, comparing the sandwich structure with the plywood alone. The failure modes (matrix in tension, matrix in compression and delamination) that occur in both the plywood and the sandwich structure have also been studied. The sandwich structure shows superior impact resistance, making it a promising alternative to conventional sandwich structures that are both sustainable and lightweight.

## 1 Introduction

Sandwich structures are widely used as lightweight solutions in the transport industry (Meo et al. 2005; Huo et al. 2020), where low carbon emissions and sustainable development are key aims (Cheng and Xiong 2024; Puškár and Kopas 2024). These goals can be further advanced through the incorporation of natural, biodegradable, and/or recyclable materials in the design and fabrication of such structures.

Despite the good mechanical properties of composite sandwich, it is renowned that these structures are vulnerable to localized transverse loads, particularly to impact loads (Sutherland and Guedes Soares 2022; Oliveira et al. 2022; Abrate et al. 2013; He et al. 2019). These loads can produce the appearance of several damage modes in the sandwich structure, such as localised cell wall buckling in honeycomb

core sandwiches, core crushing in foam core sandwiches, or debonding between skins and core (Meo et al. 2005; Shin et al. 2008). Beyond these, typical laminate damage modes such as delamination, fibre failure, and matrix cracking can also occur in the skins (Salehi-Khojin et al. 2007; Khan et al. 2020; Castellanos and Prabhakar 2022). Depending on the impact energy levels and the impactor characteristic, the failure modes may be different. This damage reduces the structural integrity (Meo et al. 2005; Abrate et al. 2013; Shin et al. 2008; Vural and Ravichandran 2003) and the after-impact strength, especially under compression loads (He et al. 2019; Samlal and Santhanakrishnan 2022), even though the damage is barely visible (He et al. 2019). This makes studying the impact load response of a sandwich structure a critical topic. The response to impact loads, especially the energy absorption capability of the sandwich structures, is controlled by the properties of the skins and core (Naghizadeh et al. 2020).

There are several alternatives for core materials such as polymer or metal foams (Yang et al. 2015; Wang et al. 2024), honeycomb cores (Meo et al. 2005; Shin et al. 2008), lattice cores or even corrugated cores (Zheng et al. 2022). Some materials of natural plant origin or made from plant-based natural materials, such as balsa wood, cork and plywood, are candidates for the manufacture of sandwich cores due to their high specific properties, low cost and good energy dissipation capabilities (Sutherland and Guedes 2022; Oliveira

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

<footnote>John Susainathan
johnprakashs@gmail.com</footnote>

<footnote n="1">¹ Department of Mechanical Engineering, University of Galway, Galway, Ireland</footnote>

<footnote n="2">² Department of Continuum Mechanics and Structural Analysis, University of Carlos III of Madrid, Leganés (Madrid), Spain</footnote>

<footnote n="3">³ Institut Clément Ader (ICA), Université de Toulouse, Toulouse, France</footnote>
et al. 2022; Abrate et al. 2013; Shin et al. 2008; Vural and Ravichandran 2003).

From the review of the impact performance of sandwich cores with natural cores (Ong and Silva 2024), balsa wood is of great interest as a core material in sandwich panels due to the similarity in micro-buckling of tracheid cells with macro-buckling of honeycomb cell walls (Vural and Ravichandran 2003) and its transversely isotropic behaviour (Yang et al. 2024). Another material of natural origin used as a core in sandwich structures is natural or agglomerated cork (Hachemane et al. 2013; Gomez et al. 2023).

A sandwich structure with a high-density balsa (HD balsa) core has impact behaviour and energy absorption capability comparable to a sandwich structure with an aluminium honeycomb core (Shin et al. 2008; Atas and Sevim 2010; Hachemane et al. 2013) and better than a structure with a polymer foam core (Atas and Sevim 2010). Atas and Sevim (2010) compared the impact behaviour of sandwich structures with different cores such as HD balsa and PVC foam and showed that sandwich structures with HD balsa have better results in terms of energy absorption capability and impact induced damage as compared to sandwich structures with conventional cores such as polymeric foams. Similarly, Shin et al. (2008) analysed the impact response of sandwich structures with different cores such as HD balsa and aluminium honeycomb and confirmed that wood-based sandwich structures have comparable energy absorption to aluminium honeycomb sandwich structures.

Abdalslam et al. (2013) compared the low velocity impact behaviour of end and regular grain balsa wood core sandwich and found that sandwich with regular grain offered better energy absorption and lower impact resistance than the same with an end grain balsa core due to its higher stiffness and also revealed that energy absorption, impact load and failure modes are strongly dependent on the orientation of core grain.

Raw timber is an inexpensive and abundant material that has been used for thousands of years in many applications and products. However, its mechanical strength and toughness are often unsatisfactory for many modern structural applications (Oliveira et al. 2022; Susainathan et al. 2018). It also has an anisotropic behaviour, with a lower strength in the direction perpendicular to the grain. These limitations can be overcome by using plywood, which effectively uses wood superior strength and stiffness along the grain by bonding adjacent timber veneers at 0° and 90° with a suitable adhesive (Naghizadeh et al. 2021). Wood has the highest load carrying capacity in the longitudinal (fibre) direction and higher flexural stiffness due to its inherent orthotropic nature which describes independent mechanical properties in the three mutually perpendicular directions (Susainathan et al. 2018).

The high velocity impact behaviour of plywood core sandwiches has been studied experimentally and numerically by Li and Zhang (2023) impacted using different types of projectiles in sandwiches with plywood core and aluminium skins. Naghizadeh et al. (2020) studied experimentally high-velocity impact behaviour of the sandwiches with plywood cores and carbon fibre skins reinforced by carbon nanotubes. They observed that the incorporation of the nanotubes reduced the damage caused in the sandwich panels.

A detailed review of the challenges associated with using natural and/or renewable material for transport applications can be found in Oliveira et al. (2022). The most recent efforts in the development of eco-friendly sandwich structures by classifying different approaches to the environmental optimisation of structures in the current context of ecological transition are studied by Castanié et al. (2024). Wang et al. (2022) investigates the off-axis mechanical properties of birch plywood through conducting tensile, compressive and shear tests under five different angles to the face grain, (i.e., 0° to 90° to the face grain, with an interval of 22.5°) and revealed that the stress–strain relationships, failure modes, strength and elastic properties of birch plywood are highly influenced by the load-to-face-grain angle.

The behaviour of a structure made with a plywood core and different skins (poplar and okume plies) was studied (Abdalslam 2013; Timbolmas et al. 2024). Plywood with flax composite skins offered better results in terms of specific properties (stiffness and strength) and ecological benefits in the post-impact response due to its lower density (Abdalslam 2013). Jorda et al. (2020) claimed that when moulding complex geometry, flax fibre reinforcement in beech plywood is a potential way of increasing load bearing capacity and stiffness of plywood formed parts by respectively 76% and 38% on average. Jakob et al. (2022) reviews the strength potential inherent to biobased cellulosic materials with either unidirectional or close-to unidirectional orientation in order to ascertain their suitability for potential applications. This includes, amongst others, materials derived from wood or bamboo, natural fibre composites, all-cellulose composites, and nanocellulose-based materials.

A sustainable alternative to the conventional laminates used to make the skins of a sandwich structure are those made with natural fibres. According to Strohrmann and Hajek (2019), the work in the development of sustainable composites should focus on fibres since more energy is consumed in fibre manufacturing than in matrix manufacturing. Thus, using natural fibres with conventional (synthetic) matrices significantly reduces environmental impact while maintaining good mechanical properties. In the manufacturing of sustainable composites, vegetal fibres are the most used (Dicker et al. 2014). Flax, in particular, is widely used
due to its availability and good properties (Yan et al. 2014). Due to their low density, flax fibres have specific strength and stiffness comparable to glass fibre and good corrosion and fatigue resistance (Gurunathan et al. 2015; Cuynet et al. 2018). Another advantage of these fibres is their low cost compared to synthetic fibres, although it is generally necessary to apply treatments to the fibres, for example to improve the matrix-fibre adhesion, which increases their cost (Dicker et al. 2014).

Previous research has studied the impact behaviour of sandwich structures with flax/epoxy skins and plywood cores (Abdalslam 2013; Susainathan et al. 2018). However, since the response of the structure and the level of damage are highly dependent on impact energy, it is necessary to study a wide range of energies, not just the limited scope seen in the aforementioned studies. Furthermore, impacts at the same energy level but with varying velocities will result in different damage extents (Vieille et al. 2020). It is also noteworthy that damage mechanisms differ between woven and tape laminates, with tape laminates typically exhibiting more extensive damage (Bensadoun et al. 2017). For this reason, the conclusions drawn from a specific combination of plywood and woven laminates, tested under a given impact energy and velocity, cannot be reliably extrapolated to different energy and velocity ranges, particularly when tape laminates are also employed as skins of the structure.

In this paper, the low-velocity impact behaviour of a structure composed of a plywood core and skins made from flax/epoxy plies (eco-structure) is experimentally investigated. The eco-structure results obtained of absorbed energy, peak strength, and impact bending stiffness are compared with those obtained testing plywood alone.

<table><caption>(a)</caption><thead><tr><th></th><th>0°</th><th>1mm</th></tr></thead><tbody><tr><th scope="row">Poplar</th><td>90°</td><td>2mm</td></tr><tr><th scope="row"></th><td>0°</td><td>2mm</td></tr></tbody></table>

<table><caption>(b)</caption><thead><tr><th></th><th>0°</th><th>0.23mm</th></tr></thead><tbody><tr><th scope="row">Flax</th><td>90°</td><td>0.23mm</td></tr><tr><th scope="row"></th><td>90°</td><td>0.23mm</td></tr><tr><th scope="row"></th><td>0°</td><td>0.23mm</td></tr></tbody><thead><tr><th></th><th>0°</th><th>1mm</th></tr></thead><tbody><tr><th scope="row">Poplar</th><td>90°</td><td>2mm</td></tr><tr><th scope="row"></th><td>0°</td><td>2mm</td></tr></tbody><thead><tr><th></th><th>90°</th><th>2mm</th></tr></thead><tbody><tr><th scope="row">Poplar</th><td>0°</td><td>2mm</td></tr><tr><th scope="row"></th><td>90°</td><td>2mm</td></tr><tr><th scope="row">Flax</th><td>0°</td><td>1mm</td></tr><tr><th scope="row"></th><td>0°</td><td>0.23mm</td></tr><tr><th scope="row"></th><td>90°</td><td>0.23mm</td></tr><tr><th scope="row"></th><td>90°</td><td>0.23mm</td></tr><tr><th scope="row"></th><td>0°</td><td>0.23mm</td></tr></tbody></table>

<caption>**Fig. 1** Stacking sequence of the specimens tested in this work: (a) plywood, (b) composite eco-structure</caption>

# 2 Materials and methods

This section presents the specifications of the materials utilized in the experimental tests, in conjunction with a concise description of the testing devices and the experimental procedure.

## 2.1 Specimens

Two types of specimens were utilized in this study. The first type is made of a monolithic plywood, while the second is a composite structure comprising a core made of the same plywood and two laminate skins.

The core material was a poplar plywood made with plies stacked at 0° and 90° (Fig. 1a) using Tannin based natural origin adhesive called Naturbind as commented by the manufacturer (Garnica plywood, Spain). The plywood was purchased from a commercial supplier and was used as received. The plywood thickness is 12 mm.

The skins were made of cross-ply flax/epoxy laminate made of four unidirectional (UD) with the stacking sequence [0°/90°]s (Fig. 1b). This material was selected as representative of the types of skin used in natural sandwich structures and has good mechanical properties. The pre-preg material was purchased from the company ECO-TECHNILIN and has a fibre percentage of 50% with an areal density of 220 g/m². The skin were manufactured by thermos-press moulding at 120°C with a pressure of 0.4 N/mm² for a duration of one hour, following the recommendation of the manufacturer. Boards of 500 × 500 mm² were produced and then cut into specimens of 150 × 100 mm² for testing.

## 2.2 Experimental procedure

Impact tests were performed using a drop-weight tower CEAST Fractovis (Fig. 2a) followed by post-impact visual inspection. The tests were carried out in accordance with the recommendations set out in the ASTM D7136 standard.

An impactor with a mass of 4.861 kg and a hemispherical nose with a diameter of 16 mm was used (Fig. 2a). In each test the specimen was placed in a support device with a cut-out with internal dimensions 125 × 75 mm² (Fig. 2b). A restraint system was employed to prevent that the specimen does not move during or after impact, ensuring that all the impact energy is transferred to the specimen. The drop tower has a pneumatic anti-rebound system to ensure that each specimen receives a single impact.

From each test, the contact force between the impactor and the specimen was recorded by a piezoelectric transducer. Additionally, the impact velocity was measured using a photoelectric cell. Utilizing these data and the mass of the impactor, it is possible to calculate its deceleration.
<figure><img src="image_3.png" /><figcaption>**Fig. 2** Description of experimental devices used: (a) image of the impactor, (b) details of supporting systems: clamping devices and specimen</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 3** Force-time curves for the plywood and eco-structure: (a) impact energies 10 J and 15 J, (b) impact energies 20 J and 25 J</figcaption></figure>

By double integration of the acceleration-time curve, the impactor displacement was calculated. Assuming that the impactor and the specimen are in contact throughout the test, this displacement was considered equivalent to that of the specimen. This procedure enabled the determination of the force-displacement curve and from the area under this curve, the absorbed energy. The equations to be used in this process of successive integration are defined according to the reference (Sanchez-Saez et al. 2005).

Impact tests were performed on three specimens at each impact energy levels (10 J, 15 J, 20 J and 25 J). The most representative curves with points ($F_i$, $x_i$) is drawn through calculating the average values of tested parameter such as force ($F_i=\Sigma F_j$/n where j=1,2,..,n) at each point of time or displacement points ($x_i$) for each energy levels. The number of specimens tested (n) is postulated as three in this study. This range of energies was selected to represent scenarios commonly encountered in industrial applications, such as a

tool falling onto the structure in an assembly or maintenance operation (Haridharan et al. 2020). The highest impact energy was selected as the one that produces the saturation limit of the peak force.

# 3 Results and discussions

## 3.1 Global response analysis

The global analysis of the impact response for each configuration (plywood and eco-structure) is performed by studying the force-time and force-displacement curves. The most representative curves obtained through calculating the average values at each point of the three curves for each energy level are plotted in Figs. 3 and 4, respectively. The force-displacement curves for both configurations show a loading and an unloading region. However, there are significant
<figure><img src="image_5.png" /><figcaption>**Fig. 4** Force-displacement curves for plywood and eco-structure: (a) impact energies 10 J and 15 J, (b) impact energies 20 J and 25 J</figcaption></figure>

differences in these curves for both configurations including a different post-peak softening, drop in force with a rapid increase in displacement and lack of de-kinking which refers to the absence of de-densification.

Only the specimens impacted at 10 J, show comparable behaviour in both configurations (Figs. 3a and 4a). The force increases until it reaches a maximum, which coincides approximately with the maximum displacement and then decreases to zero (rebound of the impactor). After the force drops to zero, a residual displacement remains. This residual displacement, which is measured instantly at the end of contact between the impactor and the specimen, can be considered an approximation of the indentation produced in the specimen. However, it should be noted that a dimensional recovery may occur later, which would not be captured by this measurement. In this study, the instantaneous measurement of residue 1 displacement following the test will be referred to as 'indentation' (Fig. VII). This type of curve is associated with a reduced level of damage in the specimen. However, although curves of the eco-structure impacted at 10 J are similar in shape to those of the plywood, the values of stiffness, peak force, maximum displacement and residual displacement are significantly different. The higher Young modulus and mechanical strength of the flax/epoxy skins in the eco-structure can explain the higher bending stiffness and peak load and lower maximum displacement shown in Fig. 4a.

Very slight oscillations are observed before the peak force in the eco-structure impacted at 10 J and 15 J (Fig. 3a), which could be due to the onset of damage progression starting at 1.94 ms (Fig. 4a). Finally, the impactor rebounds slightly faster than for plywood (Figs. 3a and 4a). This behaviour can be explained by the lower friction between the impactor and the upper ply of the composite skin during impactor rebound. For the eco-structure impacted at

10 J, a small post-peak softening appears at approximately 2.38 ms after the loading phase in the force-displacement curve (point A in Fig. 3a). A negligible drop in the force was observed, indicating a sudden increase in the extent of damage progression. By contrast, the curves in the plywood structure show no post-peak softening or oscillations when impacted at 10 J (point B in Fig. 3a) indicating a gradual increase in the extent of damage progression.

In the curves of the plywood specimens impacted at 10 J, it can be observed that the force increases until it reaches a maximum, which coincides approximately with the maximum displacement. It then decreases to zero during the unloading phase (rebound of the impactor), and leaves a residual displacement (Fig. 3).

In the curves of higher impact energies, this trend is less evident due to the increase in damage progression. Similar curves have been observed in impact tests with other plywood materials, as alder (Naghizadeh et al. 2021) and poplar-okume (Susainathan et al. 2018). However, higher peak forces and bending stiffness have been observed in the plywood studied in this work due to their higher Young modulus.

For the eco-structure, the maximum force coincides with the maximum displacement at impact energies of 10 J and 15 J. At higher energies (20 J and 25 J), this does not occur. In these cases, a region appears in which the force decreases significantly while the displacement increases. This region is marked with the letters D, H and F in Fig. 4. This phenomenon can be due to an increase in damage progression in the form of fibre breakage and crushing at higher energy levels.

The force-time curves of the plywood core at 10 J have quasi-sinusoidal shape (Fig. 3a), implying a low level of damage and very small indentation (Fig. VIIIa), whereas the curves of the remaining energy levels (15 J, 20 J, and 25 J)
for the plywood core have a characteristic shape (non-quasi-sinusoidal) as shown in Fig. 3, consisting of loading, post-peak softening followed by a greater drop in peak force with increasing displacement, which implies a slightly higher level of damage caused by partial penetration of the impactor into the plate and a slightly higher indentation than that of 10 J (Fig. 4). For the eco-structure, the force-time curves for all energy levels tested (Fig. 3) have a quasi-sinusoidal shape, which implies very low level of damage and less indentation as compared to that of plywood (Fig. 3).

The force-time and force-displacement curves of 15 J for the eco-structure show the pattern consisting of loading with the onset of negligible oscillations at 2.095 ms (Fig. 3a) as indicated by the point C in Fig. 3a and followed by the unloading phase. However, the curves of the plywood at 15 J show the non-sinusoidal shape consisting of loading, post-peak softening (Fig. 3a) and the notable peak force drop with an increasing displacement between 2.29 ms and 4.88 ms (Fig. 4a) as indicated by D in Fig. 3a.

The eco-structure impacted at 10 J exhibits higher bending stiffness and peak forces than other composite structures made from flax/epoxy skins and core made from other plywood (Ozdemir et al. 2018; Walsh et al. 2017) which suggests that the core has a relevant effect on the impact response.

Similarly, agglomerated Cork core/Carbon fibre sandwich panels with 10 mm thickness tested in reference (Walsh et al. 2017), result in two peaks (1.75kN and 1.5kN) connected with a heavy drop in peak force and ascending plateau which represents post-peak hardening. In the case of thermo-plastic composite skin and balsa core sandwich panel with 15 mm thickness tested in reference (Ozdemir et al. 2018), it is observed with peak force of 3.2kN and zero plateau. The peak force and the characteristics of the plateau observed for the sandwich panels tested at 10 J from the two references mentioned above are quite smaller and is found with a longer plateau than that of the sandwich panels impacted at 10 J in the present study. Whereas from the sandwich panel tested in reference (Ozdemir et al. 2018), it is found with a zero plateau and smaller peak force than that of the sandwich panels tested at 10 J in this study. A balsa core sandwich panel of 11.5 mm thickness with glass epoxy skin tested in reference (Atas and Sevim 2010) and balsa or birch wood core sandwich panel with Carbon/epoxy composite skin tested in reference (Basha et al. 2022), result in a minor visible plateau and peak load (2.5kN). The eco-structure panel studied in the present study results in higher impact bending stiffness (1.262 N/mm) and peak load (4.59kN) and also results in kinking up to the peak load. This can be as a result of reduction in in-plane stiffness caused by the presence of smaller number of longitudinal (fiber at 0°) thick plies in the plywood studied in the references.

For an impact energy level of 20 J, the force-displacement curves of eco-structure show a moderate post-peak softening with a small oscillation between 2.01 ms and 3.2 ms (Fig. 4b) with longer duration than that of 15 J as indicated by point E in Fig. 3b. For plywood, the similar curves show a post-peak softening followed by a higher drop in peak force between 1.86ms and 6.32 ms (Fig. 3b) with an increasing displacement before the unloading region as indicated by F in Fig. 4b are observed.

For an impact energy of 25 J, the force-displacement curves of the eco-structure show a higher post-peak softening with moderate oscillation between 2.01 ms and 3.64 ms (Fig. 4b) with quite longer duration than the observed softening at 15 J and 20 J as shown in (G) in Fig. 3b and also result in a typical unloading section. However, the similar curves of the plywood core show a post-peak softening, and a highest amount of peak force drop between 1.58 ms and 6.1 ms with an increasing displacement as shown in region H in Fig. 4b. This is followed by a small amount of anti-kinking (de-densification) between 6.16 ms and 7.17 ms (Fig. 3b) as shown as region I in Fig. 3b and then the unloading section. The above phenomenon can be attributed due to the greater amount of fibre fracture, debonding and the fibre crushing, or cell walls collapse at the impact zone in the upper plies as shown in Fig. Xb. The above failure scenario can be explained by the predominance of local shear stresses caused by impactor induced shear and crushing. This results in a higher unrecoverable displacement due to higher inelastic energy dissipation and leads to an increase in the energy absorbed by internal damage. Comparing all energy levels, plywood starts to reach the level of saturated peak force even at 15 J, whereas the eco-structure reaches the level of saturated peak force at the energy levels of 20 J and 25 J with a post-peak hardening as the displacement increases due to the presence of higher strength than plywood, leading to better damage resistance.

This above response can be explained due to the presence of the flax/epoxy skin and its damage resistance capability facilitate the mitigation of non-linear fatal shear damage progression that caused by greater plastic strain in matrix component of wood, into the plywood core at higher impact energy. Thus, it also causes additional ability to surpass the threshold load for the onset of damage and leads to higher peak force in the case of sandwich panels.

From the curves shown in Figs. 3 and 4 it is possible to estimate several key variables for the plywood structure and the eco-structure to analyse the response to impact loads. Directly from the force-displacement curves it is possible to estimate the peak force, the impact bending stiffness and residual (permanent) displacement. The area enclosed by these curves was used to estimate the absorbed energy.
## 3.2 Analysis of the impact parameters and damage variables

### 3.2.1 Impact parameters

This section presents the impact response of the eco-structure and plywood in terms of key impact parameters, such as energy and specific energy absorption, peak force and impact bending stiffness.

### 3.2.2 Energy and specific energy absorption

Figure 5 shows the curves of absorbed energy and absorbed energy ratio respectively as a function of the impact energy. The absorbed energy indicates the amount of kinetic energy of the projectile that is transferred to the plate during the impact. This energy includes the energy due to the elastic deformation of the plate and the energy absorbed by the damage generation. It is therefore an indication of the damage caused to each configuration. The absorbed energy ratio is defined as the ratio between the absorbed energy and the impact energy.

As expected, the absorbed energy increases with the increasing impact energy. For all the impact energies studied, the absorbed energy and energy ratio are higher in plywood than in the eco-structure (Fig. 5), due to the higher level of damage in each ply of the plywood caused by fibre crushing and fibre fracture. This behaviour is due to the low stiffness and strength of this material. These failure modes (fibre crushing and fracture) have a greater influence on

the energy absorption capacity of each ply of the plywood structure than on the eco-structure due to the presence of the flax/epoxy skin. Higher differences in absorbed energy (over 40%) (Fig. 5) between plywood and the eco-structure are found at the impact energy levels between 10 J and 20 J. However, at the impact energy of 25 J, the percentage of difference in the absorbed energy between these two configurations decreases to around 30%. The phenomenon occurs because the ratio of absorbed energy in plywood is almost 100% (Fig. 5b). Therefore, an increase in absorbed energy requires a greater increase in impact energy.

### 3.2.3 Peak force and impact bending stiffness

The impact response in terms of peak force and impact bending stiffness and the comparison between these two structures are described in this subsection. The curves showing the variation of these variables with the impact energy are shown in Fig. 6. Peak force is defined as the maximum contact force between the impactor and the specimen. At all impact energies, the peak force for the eco-structure was higher than that for the plywood specimens. It was also higher than the values for several aramid honeycomb core sandwich structures (Liu et al. 2016; Susainathan et al. 2018; Xue et al. 2019). The difference in peak force between plywood and the eco-structure reaches a maximum value of 54% at the impact energy of 20 J (Fig. 6a). This indicates that the eco-structure is highly resistant to damage thanks to its greater strength and stiffness. This is due to the flax/

<figure><img src="image_6.png" /><figcaption>**Fig. 5** Comparisons of different impact energy variables for the plywood and eco-structure: (a) absorbed energy versus impact energy, (b) absorbed energy ratio versus impact energy</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 6** Comparisons of different impact strength variables for the plywood and eco-structure: (a) peak force versus impact energy, (b) impact bending stiffness versus impact energy</figcaption></figure>

epoxy composite skin of the eco-structure which also prevents severe damage to the plywood core.

As the impact energy increases, so does the peak force. Larger percentage differences in peak force between the plywood and the eco-structure, are observed at the impact energy levels up to 20 J and reaching the maximum of 54% (Fig. 6a). However, at 25 J, the increase in the amount of applied impact energy is dissipated by a greater level of damage progression in the form of post-peak softening and crushing of plies of the plywood core. This reduces the percentage difference in peak force between these two configurations to 45%. Regarding the eco-structure when compared to some aramid honeycomb sandwich (Liu et al. 2016; Xue et al. 2019), the peak force at 10 J and 15 J is found to be higher than in the conventional sandwich and is observed due to its higher level of damage resistance than the conventional sandwich. This is evident even at the energy levels of 10 J and 15 J, where there is a very minimal indentation without perforation in the eco-structure (Fig. 3). However, the aramid honeycomb sandwich shows a severe damage in the form of perforation, indicating a significant loss of its structural integrity due to the penetration of the impactor on the plate. Specifically, in the case of plywood structure, there is a decrease in peak force between the impact energy levels of 10 J and 20 J as shown in Fig. 6a, due to its low resistance for the damage progression. However, at 25 J, the percentage decrement in peak force between the impact energy levels results in an increment of 9%, allowing it to remain similar to the peak force measured at 10 J (Fig. 6a). This means that the available damage resistance is sufficient to offset most of the increase in impact energy with a small increase in peak force and the rest of the impact energy is insufficient to develop the damage further. For the eco-structure, the increase in peak force (%) between the energy levels decreases as the impact energy increases. This can be explained by the fact that more of the increase in energy is absorbed and transformed into damage progression,

resulting in only a smaller proportion of the impact energy being supported by the residual strength of the structure.

The average value of impact bending stiffness as a function of impact energy level is plotted in the Fig. 6b. This variable is estimated as the slope of the initial linear portion of the rising part of the force-displacement curve prior to the peak force. At all the energy levels, the percentage difference in impact bending stiffness is greater for the eco-structure than for the plywood structure as well due to the higher stiffness of the eco-structure caused by its flax/epoxy composite skins. Compared to the plywood structure, this difference for the eco-structure reaches the maximum value of 81% at 20 J and then decreases to 49% at 25 J (Fig. 6b) which can be explained by the increase in the proportion of energy absorbed for an increase in damage extension rather than damage resistance (Fig. 7).

The impact bending stiffness of the eco-structure is higher than the aramid honeycomb core sandwiches. This can be attributed to the lower level of damage progression in the eco-structure due to its flax/epoxy skin (Figs. 8 and 9). It is evident that this impact bending stiffness always has a significant effect on the level of increase in the extent of damage.

### 3.2.4 Analysis of the damage variables

#### 3.2.4.1 Impact indentation ratio
The residual displacement obtained from the force-displacement curves at the end of the contact between the impactor and the specimen is an estimation of the indentation, The ratio of this indentation to the specimen thickness is defined as the impact indentation ratio (Fig. 7a). The structure with the lowest indentation ratio obviously indicates a lower level of damage with the
<figure><img src="image_8.png" /><figcaption>**Fig. 7** Comparisons of different impact damage variables for the plywood and eco-structure: (a) indentation ratio versus impact energy, (b) ratio - absorbed energy to indentation versus impact energy</figcaption></figure>

minor partial penetration. This can be seen by a clear inden-
tation on the impacted zone (Figs. 8 and 9).

The indentation ratio of the eco-structure (Fig. 7a) is lower than that of the plywood structure at all impact energy levels. This phenomenon implies a lower level of damage due to the higher stiffness and strength of the flax/epoxy skin of the eco-structure than that of the plywood. The previously cited conventional sandwich with an aramid honeycomb core has an impact ratio of almost 100% at 20 J, implying total penetration of the impactor with full perforation. This indicates that the eco-structure studied in this work exhibits good impact behaviour at this energy level.

For the plywood structure, the increase of the indentation
ratio between the energy levels increases with the increase
of the impact energy, reaching a maximum of around 70%
at 25 J (Fig. 7a). In the eco-structure, the increment of the
indentation ratio between the energy levels increases up to
the impact energy of 25 J and reaches the maximum value
of 20% (Fig. 7a). Within the impact energy intervals con-
sidered in this study, any incremental energy applied is
insufficient to suppress the higher damage resistance of
the eco-structure due to the higher strength provided by its
flax/epoxy skin and unable to reach the maximum damage
threshold.

3.2.4.2 Ratio - Energy to indentation In this section, the ratio between the absorbed energy ratio and the indentation ratio is studied for each configuration and impact energy. This variable is used to select the structure that best meets the requirements of absorbed energy and indentation, both related to damage generation (Fig. 7b). For both configurations, this ratio decreases as the impact energy increases. For example, it diminishes from 7.8 for an impact at 10 J on the eco-structure to 3 for an impact at 25 J. A significant

difference in this ratio is observed for the eco-structure com-
pared to the plywood.

The higher ratio in the eco-structure is due to its lower
indentation, caused by its greater strength and stiffness,
derived from its flax/epoxy composite skin. The percentage
of difference in the ratio between plywood and eco-structure
attains maximum value of 82% at the impact energy of 20 J
(Fig. 7b).

**3.3 Failure modes in eco-structure**

From the images of the cross section, the failure modes that
occur in both the eco-structure and the plywood are esti-
mated. In the plywood, the predominant failure modes are
fibre fracture, fibre splitting, fibre crushing and transverse
shear. In the eco-structure, in addition to the above failure
modes corresponding to the core, the delamination between
skin and core also occurs.

Initially, the onset of transverse shear cracking in the
plywood core was always found in the vicinity of damage
caused by peeling cracks due to the process of peeling the
wood plies from the log. These peeling cracks occur regu-
larly in the radial/longitudinal (RL) plane of the plywood
and may either progress in the same plane or turn to propa-
gate in the tangential/longitudinal (TL) plane.

In general, the occurrence of failure modes such as fibre
fracture, fibre crushing, fibre splitting, and the depth of per-
manent indentation is more severe in the plywood structure
than in the eco-structure see (Figs. 8a-c). For plywood, the
fibre breakage and indentation depth propagate up to 58%
of its thickness up to the bottom ply with the increasing
impact energy (Fig. 8). Fibre crushing is observed up to the
middle section of plywood core. Fibre splitting with partial
penetration (no perforation) is observed at the bottom face
of the plywood core for the case of energy levels (20 J and
<figure><img src="image_9.png" /><figcaption>**Fig. 8** Post-impact damage pattern of plywood (PW) at different impact energy levels: (a) sectional view, (b) top view, (c) bottom view</figcaption></figure>

25 J) (Fig. 8a). While comparing the results with a plywood made from okume/poplar, complete perforation is observed even at the impact energy of 15 J. This can be explained as a result of the smaller in-plane stiffness caused by the existence of a small number of longitudinal (fibre at 0°) thick plies in the plywood studied in the reference.

In the eco-structure, the delamination between skin and core, fibre fracture, fibre crushing, and transverse shear are identified as its main failure modes (Fig. 9a and b). Fibre breakage and indentation depth increase by up to 26% of the thickness towards the bottom flax/epoxy ply as the impact

energy increases. Fibre breakage appears up to a quarter of the plywood core (Fig. 9a and b). Penetration did not occur at impact energies of 10 J and 15 J due to there being insufficient impact energy to initiate it. However, penetration of skin and core occurs at higher energy levels (20 J and 25 J impact energy). In the eco-structure, none of these failures of modes occur in the bottom of the structure at any energy levels. This is because the flax/epoxy composite skin in the eco-structure plays an important role in preventing the progression of damage to perforation in the plywood core due to its higher resistance to transverse shear. In the case of a
<figure><img src="image_10.png" /><figcaption>Fig. 9 Post-impact damage pattern of eco-structure (plywood/flax) at different impact energy levels: (a) sectional view. (b) top view</figcaption></figure>

plywood structure, penetration starts at lower energy levels (10 J and 15 J) due to the lower resistance to transverse shear in the wood plies. This is followed by full perforation, fibre fracture and splitting (Fig. 9a and b) at the bottom plies of the plywood, starting at higher energy levels (20 J and 25 J). Additionally, in comparison to the eco-structure,

the permanent indentation and crushing is observed up to one fourth section of the sandwich panels studied in the reference whereas similar permanent indentation and crushing is found up to one sixth section of the sandwich panels tested in the present study. This can be due to the presence of a greater number of longitudinal thicker plies that cause
higher in-plane stiffness than that of sandwich panel tested in reference. This eco-structure panel could be a potential alternative material for aramid honeycomb sandwich structures, which are currently used in cargo-bay floor panel of the aircraft.

## 4 Conclusion

This work experimentally analysed and compared the impact behaviour of a sandwich structure composed of a plywood core and flax/epoxy composite skins (eco-structure) with that of plywood alone. The plywood and eco-structure showed a similar trend in force-displacement curves only at the lowest impact energy (10 J). However, at higher energies, the curves differed significantly due to post-peak softening in the plywood, which also experienced partial penetration and greater damage. Fibre breakage, crushing, and transverse shear became more severe at higher energies (20 and 25 J) for both structures.

While the aramid sandwich data from the scientific literature indicated higher energy absorption, the eco-structure exhibited better damage tolerance with no complete perforation and less damage extension. The eco-structure also exhibits higher peak force and impact bending stiffness than the plywood, likely due to the stiffness and strength of its flax/epoxy skins. Furthermore, the eco-structure showed a significantly lower indentation ratio.

These results suggest that the eco-structure is a viable alternative to conventional sandwiches, offering moderate energy absorption, good specific energy, low indentation, and minimal loss of structural integrity.

Future research should focus on assessing the residual mechanical properties of the eco-structure after impact, such as compression after impact (CAI) strength. This will provide a more comprehensive understanding of its long-term performance and suitability for structural applications. Additionally, the impact damage and residual properties of the damaged structures will be further studied using a previously validated numerical model.

**Author contributions** Susainathan wrote the main manuscript text and carried out test and validation of model. Enrique and Sonia participated in experimental testing and reviewed the manuscript. Florent and Arthur carried out manufacturing of samples and reviewed the manuscript.

**Funding** Open Access funding provided by the IReL Consortium. This work is part of the R&D&I project PID2021-12717OB-100, funded by MCIN/AEI/https://doi.org/10.13039/501100011033 and by European Regional Development Fund (ERDF) A Way for Europe.

**Data availability** No datasets were generated or analysed during the current study.

## Declarations

**Competing interests** The authors declare no competing interests.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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