**Hybrid Structures Made of Polyurethane**/**Graphene Nanocomposite Foams Embedded within Aluminum Open-Cell Foam**

#### **Susana C. Pinto 1, Paula A. A. P. Marques 1, Romeu Vicente 2, Luís Godinho <sup>3</sup> and Isabel Duarte 1,\***


Received: 15 May 2020; Accepted: 5 June 2020; Published: 9 June 2020

**Abstract:** This paper focuses on the development of hybrid structures containing two different classes of porous materials, nanocomposite foams made of polyurethane combined with graphene-based materials, and aluminum open-cell foams (Al-OC). Prior to the hybrid structures preparation, the nanocomposite foam formulation was optimized. The optimization consisted of studying the effect of the addition of graphene oxide (GO) and graphene nanoplatelets (GNPs) at different loadings (1.0, 2.5 and 5.0 wt%) during the polyurethane foam (PUF) formation, and their effect on the final nanocomposite properties. Globally, the results showed enhanced mechanical, acoustic and fire-retardant properties of the PUF nanocomposites when compared with pristine PUF. In a later step, the hybrid structure was prepared by embedding the Al-OC foam with the optimized nanocomposite formulation (prepared with 2.5 wt% of GNPs (PUF/GNPs2.5)). The process of filling the pores of the Al-OC was successfully achieved, with the resulting hybrid structure retaining low thermal conductivity values, around 0.038 W·m−1·K<sup>−</sup>1, and presenting an improved sound absorption coefficient, especially for mid to high frequencies, with respect to the individual foams. Furthermore, the new hybrid structure also displayed better mechanical properties (the stress corresponding to 10% of deformation was improved in more than 10 and 1.3 times comparatively to PUF/GNPs2.5 and Al-OC, respectively).

**Keywords:** open-cell foam; polyurethane foam; hybrid structures; graphene-based materials; nanocomposites

#### **1. Introduction**

In recent years, porous materials have attracted a huge interest from both academia and industry because they may find applications in a variety of fields, such as energy storage [1], catalysis [2], drug release [3], sound and thermal insulation [4], environmental remediation [5] and others. Giving the International Union of Pure and Applied Chemistry (IUPAC) definition, porous materials can be categorized based on their pore sizes: microporous (pore size <2 nm), mesoporous (2–50 nm), and macroporous (>50 nm) [6]. According to these categories, the properties of the porous materials and their subsequent applications will differ. Moreover, they can be found in three-dimensional (3D) and two-dimensional (2D) structures. Common 3D porous materials are sponges, foams, wood, and bone. Two-dimensional porous materials include separation membranes, filter paper, textiles, and so on.

Depending on the previously referred characteristics, and additionally based on their chemical composition, the porous materials can present multifunctionality. Multifunctional materials offer multiple characteristics that can be translated into excellent performance in existing applications and also open up avenues for untouched application fields [7]. These can exist naturally or can be engineered, with the latter being usually obtained by combining two or more materials. New functionalities arise from the synergistic combination of the individual materials properties [8].

One interesting example of porous materials are aluminum open-cell foams (Al-OC). This type of foam is characterized by a low weight, high thermal and electrical conductivities, and high internal surface area. Furthermore, they are recyclable and non-flammable [9]. However, they present a low compressive strength, when compared, for example, with closed pore cell foams. To overcome this drawback, Al-OC foams can be combined with other materials, such as silicone [10,11], epoxy [10,12], or polyurethane [13]. Although the mechanical performance of the composite foams is compensated, their final weight is increased, which is not desirable for certain applications requiring lightweight structures. In this context, filling these metallic skeletons with lightweight porous materials can be an interesting alternative to bulk polymers. Reinfried M. et al. [14] explored the concept of hybrid foams, which consist of two different interpenetrating embedded foam-material classes. The idea behind the hybrid foams is to overcome the individual shortcomings of single-material foams by combining foams of two different material classes and therefore achieving synergistic property combinations that are relevant and beneficial for future applications.

Recently, our research group explored the concept of lightweight multifunctional hybrid structures by combining Al-OC foams with cellulose/graphene foams [15]. We reported the impregnation of a cellulose/graphene foam into an Al-OC foam, creating a hybrid structure with higher mechanical properties (increase in stress of 100 times) with respect to the cellulose foam. This multifunctional hybrid foam presented also high sound absorption coefficient (near 1 between 1000–4000 Hz) and low thermal conductivity.

To further explore these types of structures, in the present work we considered the incorporation of polyurethane foams (PUF) into Al-OC ones. PUF are known for their excellent thermal and acoustic insulation properties, low thermal conductivity, good mechanical and chemical stability and low manufacturing cost [16]. The PUF represents three quarters of the production of polyurethane (PU) materials and the major market sectors include insulation materials in buildings, shock absorbers for vehicles, packaging, footwear, and furniture [17,18]. However, due to their high flammability, the improvement of their fire-retardancy properties became crucial [19]. Taking this into account, PUF precursors like polyols have been synthetized with specific chemical functional groups to confer fire retardancy [16,20]. In addition, nanoclays (montmorillonite) [21], titanium dioxide [22], iron oxide magnetic nanoparticles [23], expandable graphite [19,24], and carbon nanostructures [25–29] have also been employed to confer flame retardancy. Often, the combinations of different fillers are used to access improved fire-retardancy behavior due to a synergetic effect [30,31]. Interestingly, the graphene-based materials' addition to polymeric matrices has been reported to provide, besides fire-retardancy, the ability to improve the mechanical properties [32,33] and sound absorption features [34,35].

Pursuing the goal to contribute to the development of lightweight multifunctional materials, this work presents, in a first step, the effect of the addition of two graphene-based materials, graphene oxide (GO) and graphene nanoplatelets (GNPs) on the PUF properties. The focus was on the fire retardancy, mechanical, acoustic, and thermal properties. After the characterization of the different PUF nanocomposites, a selected composition was incorporated in an Al-OC, creating a lightweight multifunctional hybrid structure that was further characterized.

#### **2. Materials and Methods**

#### *2.1. Materials*

The raw materials employed in PUF synthesis were: (i) methylene diphenyl diisocyanate (MDI) (VORANATE M229 from Dow Chemicals, Estarreja, Portugal), with average functionality of 2.7 and

NCO content of 31.1%; and (ii) polyol, with a hydroxyl value of 239 mg KOH/g (VORACOR CR1112 from Dow Chemicals, Estarreja, Portugal).

Graphene oxide (GO) (4 mg/mL aqueous dispersion) was purchased from Graphenea (San Sebastián, Spain) and graphene nanoplatelets (GNPs) in powder were acquired from Cheaptubes (Cambridgeport, MA, USA). Silicone oil was acquired from Sigma-Aldrich (Darmstadt, Germany). The AlSi7Mg0.3 open-cell foams with pore sizes of 10 ppi (pores per inch) were supplied by Mayser GmbH & Co. KG (Lindenberg, Germany).

#### *2.2. Sample Preparation*

Pristine PUF and PUF nanocomposites were prepared by a two-step procedure, as schematized in Figure 1, route A. First, the pre-polymer (polyol), silicone oil (5 wt%), water as blowing agent (5 wt%) and GO or GNPs (1.0, 2.5 and 5.0 wt%) were placed in glass beaker and homogenized for 30 s using a mechanical stirrer at high speed. Next, the proper amount of MDI to obtain a RNCO/OH = 0.80 (ratio between NCO groups of isocyanates and OH groups) was added and the mixture was homogenized again for 10 s. The PUF and PUF nanocomposites were obtained by free expansion in the cup mold at room temperature. The foams are hereafter referred as PUF, PUF/xGNPs and PUF/xGO, where x refers to the carbon nanostructure content.

**Figure 1.** Scheme describing the preparation of: (**A**) polyurethane foam (PUF) nanocomposites; and (**B**) hybrid structures.

For the preparation of the Al-OC hybrid (the interconnected porous metallic foam with the optimized PUF nanocomposite) the metallic foam was placed into the cup containing the nanocomposite mixture right after the addition of MDI. The pores of the Al-OC were filled during the nanocomposite foam expansion (Figure 1, route B), hereafter referred as PUF/GNPs2.5-OC. Before characterization, the samples were settled to rest for 24 h at room temperature to ensure complete reaction.

#### *2.3. Sample Characterization*

Attenuated total reflection–Fourier transformed infrared spectroscopy (ATR-FTIR) spectra were collected using a Perkin Elmer FTIR System Spectrum BX Spectrometer (Buckinghamshire, UK) equipped with a single horizontal Golden Gate ATR cell, in the range 4000 to 500 cm−<sup>1</sup> and running 64 scans with a resolution of 4 cm−1. Scanning electron microscopy (SEM) analysis was performed in a TM 4000 Plus (Hitachi, Tokyo, Japan) scanning electron microscope at accelerating voltage of 15.0 kV. Samples (10 <sup>×</sup> 10 <sup>×</sup> 10 mm3) were analyzed in a X-ray microcomputed tomography (μCT) equipment from SkyScan 1275 (Bruker μCT, Kontich, Belgium) with penetrative X-rays of 30 kV and 125 μA, in high-resolution mode with a pixel size of 8 μm and 450 ms of exposure time. NRecon and CTVox software (Bruker, Kontich, Belgium) were used for 3D reconstruction and CTan software (Bruker, Kontich, Belgium) was used in morphometric analysis (total porosity, pore size distribution and cell-wall thickness). The apparent density and porosity were determined geometrically for three specimens of each PUF nanocomposite.

The thermal conductivity properties were evaluated with a Hot Disk TPS 2500 S instrument (Gothenburg, Sweden), at 20 ◦C; in accordance with the standard ISO 22007-2.2 and ASTM D7984, the specimens were cube shaped with (25 <sup>×</sup> <sup>25</sup> <sup>×</sup> 25 mm3). The value of the sound absorption coefficient was estimated from measurements made with an impedance tube according standard ASTM E 1050 [36] for cylindrical shaped specimens with 37 mm of diameter and 22 mm of thickness. The thermal stability of the nanocomposites foams was assessed by thermogravimetric analysis (TGA) using a thermogravimetric analyzer (Netzsch Jupiter, Selb, Germany) at a scanning rate of 10 ◦C/min, in the temperature range of 30–800 ◦C, under a synthetic air atmosphere (80% N2 and 20% O2). The fire retardancy test was based in the direct observation of the response of the specimens when submitted to a flame. The test consisted in applying an ethanol flame, at the specimen's bottom using the set-up in vertical sample position, for 3 s, plus a subsequent application (3 s) if the specimen self-extinguished. The tests were conducted in half cylindrical shaped specimens, with 30 mm of diameter and 10 mm of thickness.

The mechanical testing machine (Shimadzu MMT, Kyoto, Japan; maximum load 101 N) was used to study the quasi-static compressive response of PUF nanocomposites (10 <sup>×</sup> 10 <sup>×</sup> 10 mm3) under a strain rate of 1 mm/min up. The uniaxial compression test of Al-OCF and hybrid PUF structures were performed in a Shimadzu-AGS-X-10kN (Kyoto, Japan) testing machine at a speed of 6 mm/min.

#### **3. Results**

#### *3.1. PUFs Nanocomposites*

The success of PUF and PUF nanocomposites preparation relies on the appropriate reaction between the precursors, namely the extinction of isocyanate groups through the reaction with hydroxyl groups of the polyol and urethane formation; this was confirmed by FTIR analysis (Figure S1). The addition of GO or GNP, at the used concentrations, did not prevent the progression of foam formation. However, the GO and GNPs did not disperse in the same way in the polymer matrix, as can be easily observed in the left column of Figure 2. The photographs show a homogeneous black color when GNPs were added, suggesting a good interaction between these nanofillers and the polymer matrix. On the contrary, black spots were observed in the PUF when the GO was used. The GO nanosheets were directly obtained from the chemical exfoliation of graphite and contain several oxygen chemical functionalities, which is what makes the GO highly hydrophilic. The GNPs were also obtained from graphite exfoliation, but without the use of chemical oxidants, thus resulting in a non-oxidized surface [37], GNPs being hydrophobic. This difference in the chemical surface structure of the carbon nanostructures determines their dispersion in the polymeric matrix, which have hydrophobic domains enabling GNPs dispersion. This section will provide a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn.

**Figure 2.** Flame behavior of: (**a**) PUF, (**b**) PUF/GNPs2.5, and (**c**) PUF/GO2.5.

#### 3.1.1. Thermal Stability

Although the dispersion of both nanofillers (GNPs and GO) was not the same, their flame-retardant action was remarkable. A control test with pristine PUF showed flame propagation at some extension with smoke release, even if no dripping was observed, (Figure 2a). On the contrary, PUF nanocomposites prepared with 2.5 wt%, showed fire retardant properties, with the flame extinguished after 1 s. Importantly, the specimens maintained their shape after burning without dripping or smoke release (Figure 2b,c). Similar behavior was observed for the compositions with 5.0 wt% of the GO or GNPs. For the lower nanofillers amount tested (1 wt%), the results were not as good, since at 3 s there was still flame propagation, mainly for PUF/GNPs1.0, with smoke release in both cases (Figure S2). Carbon nanostructures are known to play a key role either in slowing down the flame propagation or even in providing self-extinction to thermoplastic or thermosetting polymeric matrices [38]. Their positive effect in the thermal stability of PUF nanocomposites may be attributed to the high specific area and layered structure of the nanofillers, which tend to form a dense and continuous char layer acting like a physical barrier at the PUF surface. This barrier becomes an obstacle to the release of the volatile degradation products, preventing or causing the delay of the degradation of the whole composite [39]. It is remarkable that, although the GO nanosheets are not as good dispersed as the GNPs in the polymer foam (photographs in the left side of Figure 2b,c), their effect on the flame retardancy was as efficient or even more efficient than the GNPs. GO's benefits in the fire retardancy efficacy over graphene have been referred to and are attributed to the GO oxygen functionalities that decompose and dehydrate at quite low temperatures. This causes a cool down of the polymer substrate during the combustion process and simultaneously release gaseous species that dilute the oxygen atmosphere near the ignition zone [30,39,40]. Graphene and GO are described mainly as co-flame retardants, at loadings from 1.0 to 10 wt% [41]. Also, functionalized graphene designed and prepared from expandable graphite and phosphorus-containing compounds has been described as flame retardants and smoke suppressor of PUF at 6.1 wt% [30]. It is worth mentioning that in our study, the pristine PUF and PUF nanocomposites with 1 wt% of nanofillers also kept their shape after burning. However, due to the

better results obtained for 2.5 and 5.0 wt%, the PUF with 1.0 wt% of nanofillers were excluded in the further characterization results.

The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves are shown in Figure S3a,b, respectively. The pristine PUF and PUF nanocomposites showed similar TGA curves, suggesting that GNPs and GO do not significantly influence the decomposition of PUF. This finding is most likely related to the small amount of nanofillers used. The initial decomposition temperature corresponding to 5% (T5%), and 50% (T50%) of mass degradation, the maximum-rate degradation temperature (Tmax), and mass residue at 750 ◦C are listed in Table 1. The results show that GNPs had a positive effect on T5% on PUF, while GO was detrimental to the early thermal stability. These results are caused by the earlier degradation of the GO oxygen functionalities at low temperature, thus accelerating the degradation of the PUF matrix. The oxygen functionalities of GO may interact with the PUF precursors during the foam formation thus interfering with the crosslinking reaction, as reported by Gama et al. [19]. In fact, at high temperatures there is the complete burning of GO, proved by the lower percentage of residue at 750 ◦C [42]. The sample PUF/GNPs2.5 presents the higher thermal stability with T50% of 404 ◦C. It was reported by Liu et al. [43] that graphene can act as a heat source and accelerate the decomposition of PUF. As so, the 2.5 wt% seems to have a more positive effect on the thermal properties of PUF than 5.0 wt%.

**Samples T5% (** ◦**C) T50% (** ◦**C) Tmax (** ◦**C) Residue 750** ◦**C (%) PUF** 173.6 349.8 312.3 5.68 **PUF**/**GNPs2.5** 197.7 404.6 314.7 5.18 **PUF**/**GNPs5.0** 188.5 392.9 316.5 5.12 **PUF**/**GO2.5** 165.4 360.7 311.4 2.42 **PUF**/**GO5.0** 149.2 355.6 312.9 1.88

**Table 1.** Experimental data of TGA analysis.

#### 3.1.2. Morphology

SEM images of pristine PUF and PUF nanocomposites show an inhomogeneous open-cell structure composed by quasi-spherical interconnected pores for all specimens. However, depending on the presence or absence of nanofillers, some small differences were noticed. As shown in Figure 3a insets, the GNPs and GO sheets are located, and sometimes wrapped, in the PUF cell walls. It is reported that carbon nanostructures have a nucleating effect during foam formation, thus altering the PUF morphology [44]. Usually, the presence of such fillers decreases the average cell size and increase foam density improving damping properties, flame-retardancy, and mechanical properties [45]. By comparing PUF/xGNPs with PUF/xGO SEM images, GO seems to promote thicker cell walls and joints which can be related with the agglomeration of GO or from the affinity of polyol and MDI with GO. The GO nanosheets are located between adjacent cavities of the foam, and the cell wall sticks together around them, producing thicker cell walls. This effect is accentuated with the increase in the quantity of GO: PUF/GO5.0 presented bigger pore size and thicker cell walls than PUF/GO2.5. On the contrary, the increase of GNP content in the PUF promotes a pore size decrease.

The 3D reconstruction performed by μCT analysis (Figure 3b) confirms the high porosity of the specimens, with porosity values of 94.4% for PUF, 93.8% for PUF/GNPs2.5, 93.5% for PUF/ GO2.5, 91.9% for PU/GNPs5.0, and 91.8% for PUF/GO5.0. The μCT images also demonstrated that the morphology of the samples is heterogeneous with wide range of pore size and wall thickness. The mean cell sizes for pristine PUF, PUF/GNPS2.5, PUF/GO2.5, and PUF/GNPs5.0 are in the range 168–328 μm, while for PUF/GO5.0 these are located between 328–468 μm (Figure 3c). PUF/GO5.0 nanocomposites have also higher percentage of thicker wall cells (Figure 3d). It is worth mentioning that, due to the limitation of resolution of μCT, only pores and cell walls thickness above 8 μm were detected.

**Figure 3.** (**a**) SEM images, (**b**) 3D μCT rendering, (**c**) pore size distribution, and (**d**) cell wall thickness distribution for PUF, PUF/GNPs2.5, PUF/GNPs2.5, PUF/GO2.5, and PUF/GO5.0.

#### 3.1.3. Mechanical Properties

The mechanical properties of the foams, the apparent density, the compressive strength at 10% of compression, and the compressive modulus are summarized in Table 2. The results indicate that the foams' densities increase with either GO and GNPs loading. In addition, the compressive strengths and compressive modulus steadily grow with the increase of both nanofillers content. It is reported that the addition of nanofillers to cellular materials can have different effects depending on several aspects, the loading content, the size and shape, the compatibility between the nanofillers and the matrix [33,46]. For the same fillers loading, the GO provides higher compressive modulus and higher resistivity to undergo load without total collapse of pores (higher stress plateau) than the GNPs. From the SEM images it was observed that GO induced thicker cell walls, while PUF/GNPs were similar to PUF. The improvement of compressive modulus and strength by the addition of carbon nanostructures to PUF was also reported by other experimental works, where phosphorus-functionalized GO [30] and expandable graphite [19] were used. Furthermore, MDI has isocyanate terminal groups (NCO) that can react with carboxylic and hydroxyl groups of GO leading to the chemical cross-linking between GO and PU matrix via the formation of amides or carbamate esters, thus improving the mechanical response [47].

From the average stress–strain compressive curves (Figure 4a), the foams exhibit the typical behavior of cellular materials. An elastic region at low strain values (5% of strain), where the stress–strain curve is linear, followed by a near constant stress until 55–60% of strain, is designated the stress plateau. Finally, densification takes place, with the complete collapse of the cells and the formation of a compact material, like a thin film, which is characterized by an abrupt increase of stress.


**Table 2.** Apparent density, compressive modulus, and compressive strength for 10% of deformation (n = 5).

**Figure 4.** (**a**) Average stress–strain compressive curves; (**b**) the sound absorption coefficient of the different PU based composite foams.

#### 3.1.4. Sound Absorption

Figure 4b shows the sound absorption coefficient between 100 and 4000 Hz for the PUF and PUF with 2.5 and 5.0 wt% of GNPs and GO. Globally, the sound absorption coefficient increases with frequency until 1500 Hz, followed by a small decrease and finally a subsequent stabilization between 2000 and 4000 Hz. The sound absorption coefficient is strongly influenced by the morphology of the foams, namely the density (associated to cell-wall thickness) and pore features (size, quantity, interconnectivity, tortuosity) [48]. It was reported that a smaller interconnected pore structure gives a better sound absorption coefficient due to the high airflow resistivity provided by cell walls [49]. Furthermore, higher porosity, associated with low density offers less resistance to sound-wave dissipation which results in a low sound absorption coefficient. Many factors contribute or influence the sound absorption, and therefore the overall values are a balance of all factors [17,48,50,51]. The incorporation of GNPs and GO increases the compressive modulus (with the later presenting higher values), and thus the cells have greater propension to undergo cell stretching, bending, and buckling without deformation. Also, by SEM and μCT analysis it was observed that the incorporation of GNPs and GO fillers decreases the porosity and pore size. The PUF/GO2.5 has the higher sound absorption curve between 1250 and 1750 Hz, reaching the value of 1 and absorbing more than approximately 50% than PUF. In fact, between 1000 and 1750 Hz, all the PUF nanocomposites have superior sound absorption coefficient comparatively to pristine PUF, which covers the sensitive frequency region of the human ear [34]. One parameter often used to describe sound absorption is the noise reduction coefficient (NRC). This parameter corresponds to the average of the sound absorption coefficients at the octave bands of 250, 500, 1000, and 2000 Hz, and rounding the result to the nearest multiple of

0.05. However, the use of NRC has its limitations since equal NRC values do not necessarily translate the same curve profile. Thus, it is worthwhile to use all the available data, especially for sound absorption at frequencies below 250 Hz or above 2000 Hz. The NRC values are gathered in Figure 4b and results showed that, although PU/GO2.5 and PU/GO5.0 have the same NRC that PU/GNPs5.0, they have distinct profiles of sound absorption vs. frequency, in this case, with PU/GO2.5 having better performance between 1200–2000 Hz. Gama et al. [17] obtained similar NRC values around 0.40–0.45 for PUF with a similar density (around 40 kg/m3) and a higher thickness (approximately 40 mm).

#### 3.1.5. Thermal Conductivity

To evaluate the possible application of these types of foams as thermal insulation materials, their thermal conductivity was evaluated (Table 3). A good thermal insulation material should present low thermal conductivity. The results obtained for all specimens (0.035–0.037 W·m−1·K<sup>−</sup>1) are in the range of most widely used commercial insulation materials, 0.030–0.040 W·m−1·K−<sup>1</sup> as reported in the literature [18,52,53]. The thermal diffusivity decreases with the addition of carbon nanostructures, except for the PUF/GO5.0, as listed on Table 3. This could be due to the barrier effect created by the nanofillers that difficult the heat flow.


**Table 3.** Thermal conductivity, thermal diffusivity, and specific heat (n = 5).

From the combination of different materials, it was expected to create different multifunctional structures with high strength and reduced weight as a result of the synergetic effect of the individual materials which could be applied in different fields. In this sense, following the characterization of PUF nanocomposites regarding their fire retardancy, mechanic, acoustic, and thermal insulator properties, the formulation PUF/GNPs2.5 was selected to be incorporated in the Al-OC skeleton as a filling material and denoted as PU/GNPs2.5-OC. However, it is worth mentioning that all the developed PUF nanocomposites were suitable to be incorporated inside Al-OC.

#### *3.2. Hybrid Structures*

The effect of filling the voids of Al-OC foams with bulky polymers has been reported [10–12,54], showing enhanced mechanical properties due to the pore filling which improves the compressive strength and energy absorption capacity of the hybrid foams. However, the use of bulky polymers to fill the voids of the Al-OC foams can be disadvantageous when lightweight structures are required. Here, we wanted to explore the properties of the hybrid structure resulting from filling the Al-OC foam with a porous one, thus not compromising the lightness of the final structure. An easy and simple process to prepare this hybrid was followed (Figure 1, route B).

#### 3.2.1. Structure

Figure 5a,b show the Al-OC and the PU/GNPs2.5-OC photographs, respectively, and in Figure 5c a 3D reconstruction of the hybrid structure obtained by μCT analysis is presented. It is worth mentioning that the weight ratio of Al-OC and PUF/GNPs 2.5 in the hybrid structure was 48.2% and 51.8%, respectively. It was observed that, during the PUF nanocomposite expansion, when in contact with Al-OC structs, some of the foam cells started to collapse. Because of that, the PUF/GNPs2.5 structure inside the Al-OC foam presented a different porosity and morphology from the optimized one. Although the pores were not uniform and the control of the porosity during the foaming was not possible, it ensured that the procedure were performed under the same operating conditions. In addition, the hybrid structures were reproducible as a whole, taking into account not only the filling material but the also the Al-OC. Though no chemical bonding between the metal surface and polymer was observed or even expected, an excellent form-fitting connection of the PUF/GNPs2.5 to the rough metal surface was visible by direct observation. The PUF/GNPs2.5 density was increased in the vicinity of the metal surface, visible in the color scheme (green color, Figure 5c).

**Figure 5.** Specimens (**a**) Al-OC; (**b**) PUF/GNPs2.5-OC; and (**c**) μCT PUF/GNPs2.5-OC.

#### 3.2.2. Mechanical Properties

The stress–strain curve is shown in Figure 6a and the energy absorption (EAD) and specific energy absorption (SEA) are illustrated in Figure S4. As described earlier for pristine PUF and PUF nanocomposites, these hybrid structures also present the typical behavior of foams, with three distinct regions: elastic, stress plateau, and densification [10]. Comparing the PU/GNPs2.5-OC compressive response with Al-OC, a similar behavior can be observed; however, with higher stress peak and stress plateau values and the densification occurring earlier. The EAD is higher for hybrid structures, with improvements of 27% comparative to the Al-OC foam and 13 times lower than the PUF/GNPs2.5. However, the SEA is lower, suggesting that the increase in EAD does not compensate the increase in weight, as found by Reinfried [14], that combined a steel open-cell foam with an expanded polystyrene foam. The values of apparent density, stress peak, EAD and SEA, and thermal conductivity are gathered in Table 4.

**Figure 6.** (**a**) Average stress–strain compressive curves; (**b**) sound absorption of Al-OC, PUF/GNPs2.5 and PUF/GNPs2.5-OC.


**Table 4.** Apparent density, stress peak, energy absorption, specific energy absorption, and thermal conductivity (n = 3).

Contrary to the hybrid structures composed by the Al-OC impregnated with dense polymers, epoxy [12] and polydimethylsiloxane [11], whose compressive mechanical behavior is governed by the dense materials, in this case it is the metal skeleton (Al-OC) that controls the compressive response under loading, as reported in a similar study describing the Al-OC filled with bacterial cellulose nanocomposite foam [15]. However, it should be noted that the weight of these structures is much lighter, and a good compromise between weight and strength can be achieved.

#### 3.2.3. Sound Absorption

The sound absorption ability of the hybrid structure (evaluated by the sound absorption coefficient) present higher sound absorption values (peak at 1200 Hz with a value of 0.8 and followed by a nearly constant value of 0.7 for higher frequencies) (Figure 6b) when compared with the filling material. The increase of stiffness provided by the Al-OC structure can improve the absorption at low frequencies. Increased sound absorption can be obtained for thick specimens [55,56]. The NRC value for PUF/GNPs2.5-OC was 0.45, three times higher than for pristine Al-OC. These values are comparable with other cellular materials reported in the literature with similar thickness (around 22 mm) and density (120 kg/m3) [57].

#### 3.2.4. Thermal Conductivity

The values of thermal conductivity obtained for the hybrid PUF/GNPs2.5-OC are similar to those obtained for PUF and its nanocomposites, around 0.038 (W·m−1·K−1), suggesting that Al-OC contribution is negligible in the overall thermal conductivity value. This may be due to the small (about 1 mm) layer of filler foam (PUF and PUF/GNPs2.5) surrounding the metal foam that blocks the heat transfer. Another aspect is related to the highly porous structure of Al-OC, so the amount of solid material (Al-SiO3) that would effectively increase the conductivity value is very small. The same trend was observed in our previous works [11,12,15], in which the contribution of Al-OC was small regarding the filling materials. The polymer filler (e.g., polydimethylsiloxane [11], epoxy [12] and cellulose nanocomposites [15]) acts as an insulator, preventing effective heat transfer. On the other hand, some authors [58,59] have demonstrated that the open-cell metal foams can be used to enhance the low thermal conductivity of the pure phase change materials (PCM), composite PCMs and paraffin for application to many situations (e.g., latent heat thermal energy storage system, heat sink and heat exchanger). Furthermore, Fiedler et al. reported [60] that the polymeric adhesives exhibit a low thermal conductivity, and thus form thermal barriers between advanced pore morphology (APM) foam elements (sphere-like closed-cell aluminum foam), resulting a distinct decrease of the thermal conductivity of adhesively bonded APMs. Globally, hybrid PUF/GNPs2.5-OC can be used as a thermal insulation material [18,56].

#### **4. Conclusions**

In the present study, hybrid structures were prepared by impregnating an Al-OC foam with other cellular material, a PUF nanocomposite. Prior to the incorporation into the open structure of Al-OC foams, the standalone PUF nanocomposites were prepared with different amounts of GNPs and GO and fully characterized. The presence of the carbon nanostructures in the PUF nanocomposites provided PUF with excellent fire retardancy, better mechanical strength, and thermal and acoustic insulating properties. The formulation with 2.5 wt% of GNPs was considered the more promising one. The process of filling the Al-OC open structure with the optimized PUF nanocomposite was successfully achieved, and the resulting hybrid structure maintain a low thermal conductivity value (0.038 W·m−1·K<sup>−</sup>1), high sound absorption coefficient specially at mid to high frequencies (NRC 0.45) and better mechanical behavior (the stress corresponding to 10% of deformation was improved in more than ten times). Therefore, these hybrid structures are thoroughly multifunctional materials with potential applications in the construction, automotive and, aeronautic sectors.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2075-4701/10/6/768/s1: Figure S1: Normalized FT-IR spectra of MDI, Polyol, PUF, PUF/GNPs2.5 and PUF/GO2.5; Figure S2: Flame response of PUF/GNPs1.0 and PUF/GO1.0; Figure S3: (a) Thermogravimetric (TG) and (b) derivative thermogravimetric (DTG) curves of PUF with different graphene based materials (GNPs and GO) additives under oxidative atmosphere; Figure S4: (a) EAD and (b) SEA curves of Al-OC; PUF/GNPs2.5 and PUF/GNPs2.5-OC.

**Author Contributions:** Conceptualization, S.C.P., P.A.A.P.M., I.D. and R.V.; methodology, S.C.P., I.D., P.A.A.P.M. and L.G.; formal analysis, S.C.P., I.D., P.A.A.P.M.; investigation, S.C.P., I.D., P.A.A.P.M., L.G. and R.V.; writing—original draft preparation, S.C.P. and P.A.A.P.M.; writing—review and editing, S.C.P., I.D., R.V. and P.A.A.P.M.; supervision, I.D.; P.A.A.P. and R.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Foundation for Science and Technology: UIDP/00481/2020-FCT and SFRH/BD/111515/2015, and Centro Portugal Regional Operational Program (Centro2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund: CENTRO-01-0145-FEDER-022083.

**Acknowledgments:** This work was supported by the projects UIDB/00481/2020 and UIDP/00481/2020-FCT-Portuguese Foundation for Science and Technology and CENTRO-01-0145-FEDER-022083—Centro Portugal Regional Operational Programme (Centro2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund and FCT scholarship grant SFRH/BD/111515/2015.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Fabrication and Mechanical Properties of Rolled Aluminium Unidirectional Cellular Structure**

#### **Matej Vesenjak 1,\*, Masatoshi Nishi 2, Toshiya Nishi 3, Yasuo Marumo 3, Lovre Krstulovi´c-Opara 4, Zoran Ren 1,5and Kazuyuki Hokamoto <sup>6</sup>**


Received: 12 May 2020; Accepted: 5 June 2020; Published: 9 June 2020

**Abstract:** The paper focuses on the fabrication of novel aluminium cellular structures and their metallographic and mechanical characterisation. The aluminium UniPore specimens have been manufactured by rolling a thin aluminium foil with acrylic spacers for the first time. The novel approach allows for the cheaper and faster fabrication of the UniPore specimens and improved welding conditions since a lack of a continuous wavy interface was observed in the previous fabrication process. The rolled assembly was subjected to explosive compaction, which resulted in a unidirectional aluminium cellular structure with longitudinal pores as the result of the explosive welding mechanism. The metallographic analysis confirmed a strong bonding between the foil surfaces. The results of the quasi-static and dynamic compressive tests showed stress–strain behaviour, which is typical for cellular metals. No strain-rate sensitivity could be observed in dynamic testing at moderate loading velocities. The fabrication process and the influencing parameters have been further studied by using the computational simulations, revealing that the foil thickness has a dominant influence on the final specimen geometry.

**Keywords:** unidirectional cellular structure; porosity; fabrication; explosive compaction; metallography; computational simulation; experimental tests; mechanical properties

#### **1. Introduction**

There is an ever-increasing demand for new multifunctional lightweight materials in advanced applications in engineering, transportation, and medicine, which can often be met by cellular metals. Their behaviour can be tailored [1] by combining the base material, porosity, morphology (size and shape of the cells, connectivity between cells) and topology (distribution of the cells within the material) according to [2]. Additionally, the behaviour can be tuned either by the partial [3] or full [4] infiltration of polymer filler into the cellular structure. These parameters and the manufacturing procedure [5] have to be carefully chosen to achieve required physical properties (e.g., stiffness, strength, energy absorption, conductivity) of cellular metals [6]. The main advantages of cellular materials and structures are a lightweight design, fire retardancy, efficient energy absorption, isolation and damping [5]. They can be

used in various industrial applications (e.g., as sandwich structures, filters, heat exchangers, isolators, dampers, bearings, and energy absorbers [7]) due to their advantageous physical [8], biocompatible [9], and ergonomic characteristics [10].

Nevertheless, the production costs of cellular metals are high in general due to some technological problems that have yet to be solved. The current research and development trends are presently centred on the development of foam formation and stabilisation mechanisms, the investigation of advanced blowing constituents (agents), the optimisation of the production and the decrease of their market value [11]. The technological problems are mainly related to the control of the material structure since most existing technologies do not allow for precise control of the shape, size, and distribution of pores. This results in a scatter of physical and other characteristics of these materials and components. Some of the already existing manufacturing methods for different types of cellular structures—e.g., Metallic Hollow Sphere Structures [12], Kagome structures [13], auxetic structures [14], additively manufactured open-cell structures [15], Lotus-type [16] or Gasar [17] and UniPore structures [18], and syntactic foams [19,20]—allow for a higher level of regularity and reproducibility.

The recent development of unidirectional UniPore structures [21] enabled the production of unidirectional cellular metals with a nearly constant size of cells and the intercellular wall thickness through the length of the specimens. Furthermore, the cells are completely isolated, without gaps between each other [18] and advanced mechanical properties [22]. The fabrication method [23] is based on explosive welding phenomena of metal (e.g., copper [18], aluminium [24]) cylindrical pipes with circular cross-section assembly. The transversely isotropic UniPore cellular structure exhibits a promising combination of mechanical [25] and thermal behaviour [26]. The original fabrication of UniPore structures has shown only moderate welding conditions not forming a continuous wavy interface at some interface sections because of the changing collision angle [18]. Additionally, the fabrication consists of a tedious filling of expensive thin inner pipes (with a pipe diameter smaller than 3 mm and its wall thickness of approx. 0.2 mm) with a polymer to avoid complete compaction and its removal after fabrication. Due to these shortcomings, new fabrication methods have been considered. A new procedure to manufacture UniPore structures was proposed [27]. It consists of rolling a non-expensive copper foil with equally spaced spacer bars (made of acryl) placed on the foil and subsequent compaction by explosive detonation.

Herein, the fabrication and properties of novel rolled aluminium UniPore structures were analysed. The rolling of the non-expensive aluminium foil improved welding conditions and decreased handling time of the specimens before and after fabrication. Various rolled UniPore geometries have been fabricated and characterised by metallographic analysis and (quasi-static and dynamic) mechanical compressive testing for the first time. Additionally, the fabrication process was analysed in detail with computational simulations based on the finite element analysis.

#### **2. Fabrication Method and Specimens**

The fabrication method of the rolled aluminium UniPore structures is a convenient and cheaper method for manufacturing the UniPore cellular structures with unidirectional pores by using the aluminium foil (A1100-O). It consists of the following steps: (i) preparation of acrylic spacer bars by cutting the acrylic resin plate into rectangular shaped bars, (ii) positioning of the acrylic spacer bars on the aluminium foil in a uniform pattern with an offset of approximately 3 mm, (iii) tight rolling of the aluminium foil with acrylic resin spacer bars around the aluminium bar as the centre (core), (iv) insertion of the rolled foil into the outer aluminium pipe, (v) central insertion of the aluminium pipe with rolled foil into the PVC round container (height: 270 mm and diameter: 83 mm), (vi) filling the void space between the central aluminium pipe and container wall with the primary explosive (750 g), (vii) explosive ignition by an electric detonator (booster) to achieve explosive compaction of aluminium pipe and foil, (viii) removal of the acrylic bars by heating the recovered specimens.

The schematic illustration of the fabrication method and the explosive compaction (cylindrical) assembly is presented in Figure 1, while the physical properties of the components and the assembled

specimens are listed in Table 1. The porosity of the specimens could be altered by changing the thickness of the outer pipe and by reducing the diameter of the inner aluminium bar.

**Figure 1.** (**a**) Schematic preparation of the specimens and (**b**) experimental production assembly.


**Table 1.** Physical properties of the prepared aluminium UniPore specimens.

The ammonium-nitrate based ANFO-A with the detonation velocity of 2.3 km/s and the bulk density of 530 kg/m<sup>3</sup> was used as the primary explosive for manufacturing the rolled aluminium UniPore structure. The primary explosive was electrically detonated using a booster (10 g SEP explosive). Ignition of the primary explosive caused propagation of the detonation wave through the primary explosive. The detonation gas uniformly radially accelerated the outer aluminium pipe towards the centre of the specimen. The achieved velocity was high enough to allow for welding between surfaces of the outer pipe and aluminium foil. Stable welding conditions and inclination angle were similar to the already known explosive welding mechanism [28]. Furthermore, the explosive welding of clads is being characterised as cold pressure welding. The annealing effect, which would decrease the hardness, does not usually appear, and the increase in hardness is considered to be the result of the work hardening [22]. Figure 2 presents the cross-sections of the recovered specimens, while their geometrical properties (dimensions and porosity) are given in Table 2.

**Figure 2.** Cross-sections of the fabricated specimens.


**Table 2.** Properties of the recovered specimens.

The shape and pore topology can be easily varied and adjusted for specific and individual applications by using the above-described fabrication method porosity (e.g., via wall thickness), dimensions (e.g., diameter).

#### **3. Computational Analysis of the Fabrication Process**

#### *3.1. Computational Model*

The computational simulations of the high-strain-rate deformation mechanism during fabrication of the aluminium UniPore structures were carried out to analyse the outer pipe's acceleration during the fabrication and the deformation of the recovered specimens in more detail. The computational simulations of all four specimen types (Figure 3) were performed based on the following assumptions and simplifications: (i) two-dimensional computational models were used, (ii) the aluminium pipe, bar and foil were compressed without joining, and (iii) the geometry of the rolled foil was modelled with multiple concentric circles.

**Figure 3.** A two-dimensional computational model of the aluminium UniPore specimens.

*Metals* **2020**, *10*, 770

The aluminium and acrylic resin were discretised by the Lagrangian mesh, while the ANFO-A explosive has been modelled with the Eulerian finite elements. The computational simulation was based on the Euler–Lagrange interaction within the engineering code AUTODYN. To assure reliable results and reasonable computational times a finite element mesh convergence study was performed. The results showed the appropriate element size of 0.2 mm and 0.5 mm for the foil and aluminium bar, respectively. The Johnson–Cook constitutive equation [29] was applied for the aluminium (A1100-O) because it takes into account the strain-rate sensitivity and hardening effects. The relation between the pressure and volume of the aluminium and acrylic resin at a specific temperature was defined using the Mie–Gruneisen equation of state [30]. It is based on the shock Hugoniot equation and can be expressed as [31]:

$$P = p\_H + \Gamma \left. \rho(\boldsymbol{\varepsilon} - \boldsymbol{\varepsilon}\_H) \right. \tag{1}$$

$$p\_H = \frac{\rho\_0 \, c\_0^2 \, \mu \, (1 + \mu)}{\left[1 - (s - 1) \, \mu\right]^2} \tag{2}$$

$$
\omega\_H = \frac{1}{2} \frac{p\_H}{\rho\_0} \left(\frac{\mu}{1+\mu}\right) \tag{3}
$$

where *P* represents the pressure, Γ the Gruneisen coefficient, ρ the density, *e* the internal energy and μ = ρ/ρ<sup>0</sup> − 1. The shock velocity (*Us*)

$$
\hbar L\_s = c\eta + s\cdot\mu\_s \tag{4}
$$

changes linearly (represented by the Hugoniot relation) with the particle velocity (*up*). The parameters *s* and *c*<sup>0</sup> are experimentally determined material constants [31]. Parameters of the Mie–Gruneisen equation of state applied in the computational analysis are given in Table 3.


**Table 3.** Values for the Mie–Gruneisen equation of state.

In the computational simulations, the ANFO-A was described as highly pressurised gas with an initial pressure of 0.939 GPa, a detonation velocity of 2.3 km/s, and a density of 530 kg/m3 [32].

#### *3.2. Computational Results*

The diagrams in Figure 4 shows the of the outer pipe velocity *V* changes with time. The results based on the acrylic resin bar of 0.5 mm in thickness are represented with dotted lines, while the results based on the acrylic resin bar of 1 mm in thickness are represented with solid ones. It can be observed from the diagrams that the collision velocity is higher than 300 m/s, which is sufficient to obtain explosive welding [33]. The outer pipe velocity strongly depends on the aluminium foil thickness, while the influence of acrylic resin bar thickness is minimal.

The computationally estimated deformation process during the explosive compaction of all four specimen types is shown in Figure 5. The highly localised deformation during the explosive compaction can be observed. The primary effective plastic deformation occurs in areas around the acrylic bars, where the thin aluminium foil is locally bent.

**Figure 4.** Velocity variation of the outer aluminium pipe during explosive compaction: (**a**) foil thickness 0.2 mm and (**b**) foil thickness 0.4 mm.

**Figure 5.** Simulation of the deformation process during explosive compaction of the four aluminium UniPore samples (Table 1) with annotated effective plastic strain.

The deformed shapes of the UniPore structures obtained by the computational simulations are presented in Figure 6. The computational results are in an excellent agreement with the actual specimens in terms of deformed shapes, which are shown in Figure 2. The deformation mechanism up to the impact between the foil surfaces and acrylic resin bars was thoroughly investigated by conducted computational analyses. However, it should be noted that the metal jet formation was not directly considered in the computational simulations.

**Figure 6.** Computationally predicted final deformed shapes of the four analysed specimen configurations (Table 1).

#### **4. Metallographic Analysis**

Metallographic analysis of the recovered specimens has been performed to determine the quality and suitability of the fabrication method. The specimens were prepared according to the standard metallographic methods (embedding in the epoxy resin, grinding, polishing, chemically etching). The microstructure of two perpendicular (longitudinal and transversal) cross-sections was analysed by the light microscopy using the optical microscope Nikon LV150N (Nikon, Tokyo, Japan). Figures 7 and 8 show the metallographic images of the longitudinal and transversal cross-section, respectively.

**Figure 7.** Metallographic images of the longitudinal cross-section of UniPore specimen. (**a**) lower magnification; (**b**) higher magnification.

A wavy interface between two colliding surfaces of the aluminium foil is represented in Figure 7, which assures a strong connection and good bonding between surfaces [33]. From the metallographic analysis, it can be concluded that the foil surfaces were welded at a sufficiently high velocity, despite a few places, where the surfaces might not be bonded completely.

The metallographic images in Figure 8 also show good bonding between the foil surfaces in the transversal cross-section and that the pores are separated and isolated between each other.

**Figure 8.** Metallographic images of the transversal cross-section of UniPore specimen. (**a**) lower magnification; (**b**) higher magnification.

#### **5. Compressive Experiments**

#### *5.1. Experimental Set-Up*

The mechanical behaviour of manufactured rolled aluminium UniPore structures was evaluated in transversal direction by the quasi-static and dynamic (one specimen of the type No. 1, 2 and 3) compressive experimental tests using the universal testing machine (Instron 8801, Instron, Norwood, MA, USA). The velocity of the cross-head during the quasi-static and dynamic loading cases was set to 0.1 mm/s and 284 mm/s, respectively. Displacements and loading forces were measured during the compressive tests. At the same time, the deformation mechanism was captured by an HD video camera Sony HDR-SR8E (Sony, Tokyo, Japan) during the quasi-static tests, and the middle-wave infrared (IR) thermal camera FLIR SC 5000 (FLIR Systems, Wilsonville, OR, USA) during the dynamic tests. The IR thermography allows following the yielding, cracking and failure during the dynamic loading [34]. It has been already successfully implemented for studying the response of various cellular structures.

#### *5.2. Experimental Results*

The compressive deformation behaviour of the four specimen types subjected to quasi-static loading conditions is illustrated in Figure 9. A similar deformation behaviour can be noted for all cases. Initially, the porous part of the specimen is compressed, followed by the deformation of the specimen's core. A strong interface bonding can be observed for the specimen No. 1 (aluminium foil thickness: 0.2 mm and acryl bar thickness: 0.5 mm). The bonds between the foil surfaces of the other three specimen types failed at larger strains, especially in case of the specimen No. 4 (aluminium foil thickness: 0.4 mm and acryl bar thickness: 1 mm).

**Figure 9.** Transversal compressive testing of rolled aluminium UniPore structure.

Figure 10 shows the IR images of the deformation mechanism of specimens (No. 1, No. 2 and No. 4) during the dynamic tests. The interface between the foil surfaces tends to fail again in the specimen No. 4. However, the deformation mechanism seems to be similar for all specimen types. The porous structure starts to yield below and above the aluminium core with the plastification zone spreading through the specimen up to full densification. Furthermore, the explosive welding of clads is characterised as cold pressure welding.

**Figure 10.** IR thermography images of the dynamic loading sequence (strain increment: ~0.15): (**a**) Specimen No. 1; (**b**) Specimen No. 2; (**c**) Specimen No. 4.

The mechanical response in terms of force-displacement diagrams is shown in Figure 11. A compressive relationship typical for the cellular metals [2] can be observed in all cases. After the initial quasi-elastic region, the yield stress is reached, followed by a short stress plateau region. Then the force starts to build up gradually and reaches the densification displacement at approximately 14 mm (equal to the strain of 0.54), after which the force drastically increases (the porous structure above and below the core is wholly densified).

The measurements show a very consistent response (almost no deviation between specimens of the same type) up to the end of the plateau region. The deviation becomes prominent during the gradual force increase, which can be attributed to bending, buckling, and collapsing of the foil (intercellular walls) and the separation of the bonds between the foil surfaces.

Finally, the quasi-static and dynamic tests provided similar results. Thus, no strain-rate sensitivity was noted for the tested moderate strain-rates.

**Figure 11.** Compressive force-displacement curves in the transversal direction: (**a**) foil thickness 0.2 mm and (**b**) foil thickness 0.4 mm.

.

#### **6. Conclusions**

The existing UniPore structures with unidirectional pores have shown two main disadvantages: (i) moderate welding conditions, which result in a lack of a continuous wavy interface at some bonded sections, and (ii) the fabrication consists of filling the expensive thin inner pipes with a polymer to avoid complete compaction and its removal after fabrication. Explosive compaction has been applied for the first time to fabricate the rolled aluminium UniPore specimens. After rolling an aluminium foil with acrylic spacers (no additional filling and removal step of the polymer was required), the assembly was subjected to explosive compaction, which—due to the mechanism of explosive welding—resulted in a unidirectional cellular structure with longitudinal pores. The fabrication process and the influencing parameters have been studied in detail by use of computational simulations, revealing that the foil thickness has a dominant influence on the finial specimen geometry (shape). The computational results compare well to the actual specimens in terms of the specimens' final shape. The metallographic analysis confirmed a strong bonding between the foil surfaces, which could be observed through the wavy interface, typical in the successful explosive welding. The compressive experiments showed a typical cellular metal response with excellent repeatability and a low deviation up to the end of the plateau region. Due to the moderate loading velocity, the dynamic compressive tests revealed negligible strain-rate sensitivity.

The future work should be focused on analysing a higher porosity by changing the thickness of the outer pipe and the rolled foil layer, and the diameter of the inner aluminium bar. Furthermore, it would be meaningful to perform a full strain-rate sensitivity study of the UniPore structures.

**Author Contributions:** Conceptualisation, K.H., Y.M. and Z.R.; methodology, K.H., Y.M. and Z.R.; software, M.N.; validation, M.N.; investigation, K.H., T.N., L.K.-O. and M.V.; data curation, M.N., T.N. and M.V.; writing—original draft preparation, K.H., Y.M., M.N., L.K.-O., Z.R. and M.V.; writing—review and editing, K.H., Y.M., M.N., L.K.-O., Z.R. and M.V.; supervision, K.H., Y.M. and Z.R.; project administration, K.H., Y.M. and Z.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by the Slovenian Research Agency–ARRS, research core funding No. P2-0063 and the bilateral research project No. BI-HR/18-19-012.

**Acknowledgments:** The authors acknowledge the financial support from the Slovenian Research Agency–ARRS.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
