Off-Axis Fabric Orientation Angle Effect on the Flexural Characterisation of Mineral Basalt-Fibre-Reinforced Novel Acrylic Thermoplastic Composites

Abstract

A fabric orientation angle has a significant influence on the failure mechanisms at the lamina level. Any change in this angle can lead to a sudden reduction in strength, potentially resulting in catastrophic failures due to variations in load-carrying capacity. This study examined the impact of off-axis fabric orientation angles (0°, 15°, 30°, 45°, 60°, and 90°) on the flexural properties of non-crimp basalt-fibre-reinforced acrylic thermoplastic composites. The basalt/Elium^® composite panels were manufactured using a vacuum-assisted resin transfer moulding technique. The results show that the on-axis (0°) composite specimens exhibited linear stress–strain behaviour and quasi-brittle failure characterised by fibre dominance, achieving superior strength and failure strain values of 1128 MPa and 3.85%, respectively. In contrast, the off-axis specimens exhibited highly nonlinear ductile behaviour. They failed at lower load values due to matrix dominance, with strength and failure strain values of 144 MPa and 6.0%, respectively, observed at a fabric orientation angle of 45°. The in-plane shear stress associated with off-axis angles influenced the flexural properties. Additionally, the degree of deformation and the fracture mechanisms were analysed.

1. Introduction

Basalt fibres, produced by melting and extruding natural basalt rock, are gaining attention for their high strength, thermal stability, and environmental sustainability [1,2,3]. In recent years, the use of environmentally friendly fibres as affordable reinforcement materials in composites has attracted significant interest. Many researchers have reported that basalt-fibre-reinforced polymer (BFRP) composites are a possible replacement for conventional glass-fibre-reinforced polymer (GFRP) composites and carbon-fibre-reinforced polymer (CFRP) composites due to their superior mechanical properties and sustainability [4,5,6]. Basalt fabrics have been used as reinforcement for thermoset matrices, such as epoxy [7,8,9,10,11,12], vinyl ester [13], and polyester [14,15,16], as well as thermoplastic matrices, including polypropylene [17,18], nylon [19,20], and polyethylene [21,22]. All mentioned research reported the mechanical performance of BFRP composites through tensile, compression, flexural, interlaminar shear, and impact responses; relevant comparisons were made with their counterparts, such as GFRP or CFRP. Since the present study focuses on the performance of BFRP composites, the introduction is limited to relevant published research on basalt/Elium composites.

Elium resin is a unique, infusible methyl methacrylate resin (Thermoplastic) in liquid form developed by Arkema in France, with higher ductility and toughness. Elium resins are based on polymethyl methacrylate (PMMA), a liquid thermoplastic. Their liquid state at room temperature makes them highly processable using traditional thermoset composite manufacturing techniques. However, unlike thermosets, which undergo irreversible crosslinking during curing, Elium resins polymerise into linear or lightly branched thermoplastic polymers, which remain reprocessable and recyclable. Unlike conventional thermoplastics, which are typically solid and require high temperatures to mould or process, Elium is unique in being a liquid thermoplastic at room temperature, allowing it to be processed similarly to thermosetting resins [23,24]. Elium^® 191 XO/SA is a three-part resin developed for composite manufacturing processes, such as resin transfer moulding (RTM) and vacuum infusion, which features a low exotherm to produce large parts with thick sections suitable for larger WTBs. Elium^® 191 XO/SA polymerises via the free radical polymerisation of PMMA (50 wt%), which is initiated by peroxides and accelerated by chemical promoters, resulting in a solid thermoplastic composite matrix. The initiator (commonly an organic peroxide-based initiator) decomposes to generate free radicals. The accelerator (50 wt%) (e.g., N,N-dimethylaniline or cobalt salts), which promotes the decomposition of the initiator, is essential for initiating polymerisation under practical processing conditions and enables the resin to cure effectively during manufacturing [25,26,27,28].

Several studies in the literature have explored the integration of various fibres with Elium and compared the mechanical performance of basalt/Elium composites with basalt/epoxy composites [28,29,30,31]. The tensile strength was 24% and 16% higher than that of basalt/epoxy and glass/epoxy composites, respectively [32]. However, the flexural strength of basalt/Elium was 28% lower than that of basalt/epoxy, while the flexural modulus was 5% higher. Further studies by Taheri and Llanos [33] and Bandaru et al. also reported a similar 29% increase in the tensile strength of basalt/Elium compared to basalt/epoxy composites [34]. The flexural performance of basalt/Elium was compared with that of other performance fibres, such as Kevlar and ultrahigh molecular polyethene (UHMWPE), and it was found that the flexural strength of basalt/Elium was 103–246% higher due to its good consolidation compared to the latter. All these studies on basalt/Elium reported the mechanical performance of unidirectional (UNI) or woven composites.

The fibre orientation angle influences the failure mechanisms in composite materials under bending at the lamina level. Any alteration in this angle can significantly affect the material’s strength, leading to considerable variations in load-carrying capacity, which may result in catastrophic failures of the composite structure. Shah et al. [35] investigated the off-axis bending behaviour of 3D orthogonal woven E-glass-fibre-reinforced composites using Elium and epoxy with a specific surface weight of 5200 gsm. They found that on-axis specimens exhibited a brittle response, while off-axis composites demonstrated a nonlinear ductile response. Their study showed that the flexural strength of E-glass/Elium was 418 MPa, and the maximum strain occurred at a 45° angle due to the matrix-dominant mechanism in both Elium and epoxy-based composites. Similarly, Zhang et al. [36] observed comparable flexural behaviour in polyamide-based 3D fabric/epoxy composites. Regarding compressive failures in UNI fabric-reinforced polymer composites with different loading angles, recent studies revealed that the compressive strength remained relatively unchanged between an on-axis 0° loading and an off-axis 10° loading, with no significant alterations in the failure mechanism, which resulted in out-of-plane kink band formations. However, for angles greater than 10°, a noticeable decrease in compressive strength was observed, along with a transition in the failure mechanism to in-plane kinking. Understanding this transition in failure mechanisms is crucial when developing structural designs and determining fibre angles in multi-directional laminate lay-up sequences.

Elium is the world’s first fully recyclable liquid thermoplastic resin. It is renowned for its good processability and toughness, making it a promising material for developing advanced FRPs. However, research on Elium composites remains limited, especially regarding alternative fibres such as plant-based and eco-friendly basalt. Basalt fibre, derived from abundant igneous rocks, is biodegradable and possesses mechanical properties comparable to those of glass and carbon FRPs. Despite its potential for structural applications in industries such as automotive and aerospace, studies on basalt fibre composites using Elium resin are scarce, particularly concerning different mechanical load conditions, including flexural strength. One significant gap in the existing literature is the effect of off-axis fabric orientation on the flexural performance of basalt/Elium^® 191 XO/SA composites, which has not been thoroughly explored. To the authors’ knowledge, discussions on the off-axis bending behaviours and failure mechanisms of quasi-UNI composites are rare. Therefore, generating reliable off-axis bending data for design is essential. This novel study investigated the influence of off-axis fibre orientation angles on the flexural performance of basalt/Elium^® 191 XO/SA composites. Laminates were manufactured using the vacuum infusion method, and samples were prepared at various fibre orientations, ranging from 0° to 90°. A scanning electron microscopy (SEM) test was conducted on the upper (compression) surface of the specimens to examine failure features at different fibre orientation angles.

2. Materials and Methods

2.1. Materials

A novel resin, Elium^® 191 XO/SA from Arkema, Colombes, France, was used for the infusion process to manufacture the composite laminates.

Table 1. Physical and mechanical characteristics of the Elium^®191 XO/SA resin [23].

Property	Elium®191 XO/SA
Viscosity (cP) at 25 °C	100
Density (g/cm3) at RT	1.01
Gel time (min) at 25 °C	210–260
Curing process at RT	24 h infusion and cure
Post-curing	2 h at 80 °C
Tensile strength (MPa)	47.10
Tensile modulus (GPa)	2.68
Flexural strength (MPa)	80.51
Flexural modulus (GPa)	2.77

presents the typical physical and mechanical properties of Elium^® (Manufacturer’s datasheet), including the gel time, curing, and post-curing information. Basaltex (NV), Nieuwpoort, Belgium, supplied a quasi-UNI non-crimp basalt fabric featuring multi-compatible sizing. The basalt fabric (578 g/m²) comprises 90% of its total areal weight in fibres aligned along the 0° direction, 8.6% along the 90° direction, and 1.4% polyester stitching. The stitching provides structural stability for the fabric and helps prevent the misalignment of the 0° fibres.

2.2. Manufacturing

This work presents a novel and accurate VARTI process as follows: The BF layers, peel ply, infusion mesh, and vacuum bag were placed on a stainless-steel plate, which was covered with a Teflon layer (Figure 1). The curing of Elium resin was carried out following the manufacturer’s guidelines. The process parameters (temperature, pressure, and time) were carefully controlled to ensure a complete curing process. The three parts of the Elium resin were mixed for 5 min, and degassing was conducted for 10 min at a vacuum level of −900 mbar at room temperature (RT) (19 ± 2 °C). The vacuum system draws liquid resin from the pot into the bag, facilitating thorough impregnation and consolidation with the basalt fibre reinforcement. Due to Elium’s low density, a low vacuum infusion level of −700 mbar at room temperature (RT) was optimised and selected until the reactivity of Elium (gel time, approximately 3 h) was achieved. This allows for slow resin flow and prevents voids, bubbles, or cavities from forming inside or on the surface of the cured laminate (compared to about −900 mbar used for high-viscosity epoxy-based composites).

After impregnation, the BF layers and Elium reached gel behaviour (curing process). The inlet pipe was closed while the outlet pipe was left open for 18 h, controlling the vacuum to a high level of −900 mbar (Figure 2) during this process to prevent losing vacuum pressure; hence, Elium has low reactivity under vacuum conditions. The last step was post-curing for two hours at 80 °C.

Once the consolidation was complete, samples for the flexural test were prepared to have various fibre orientations, including 0°, 15°, 30°, 45°, 60°, and 90°.

Table 2. The structure of the BF/Elium composite samples.

Composite	Basalt Fibre Structure	Polymer Matrix
		Elium®191 XO/SA, three parts:
BF/Elium	BAS-UNI 550: - 0° 520 g/m2 - 90° 50 g/m2 - Stitching 8 g/m2	191 XO acrylic resin (50 wt%), 191 SA accelerator (50 wt%), and methyl ethyl ketone peroxide (MEKP) initiator (2 wt%)

details the basalt fabric structure and the three parts of the Elium resin mixing ratios.

2.3. Flexural Testing Procedures

The flexural tests were conducted at RT using the Zwick Roell machine with a 10 kN load cell (ZwickRoell GmbH & Co. KG, Ulm, Germany). The flexural test samples with fibre directions of 15°, 30°, 45°, 60°, and 90° were manufactured and tested according to ASTM D790 [37], with a size of 130 mm × 13 mm. The laminate was manufactured from six plies of BAS-UNI 550 for the BF/Elium composites. The test specimens were cut using a precision diamond saw under continuous water cooling to minimise thermal and mechanical damage to the edges. This method was selected to reduce edge-induced stress concentrations, which can influence bending behaviour. After cutting, all specimens were inspected to ensure clean and consistent edges. The samples’ depth was 2.46 ± 0.05 mm, and the crosshead speed was 4 mm/min. At least four samples were tested for each group, and the average value was dependent. Figure 3 presents the flexural samples with various fibre angle orientations during the test.

The flexural span-to-thickness ratio was 32 to generate a strain rate of 0.01, as preferred in the ASTM D790 standard [37] according to Equation (1). The flexural stress (σ), strain (ε), and modulus (E) were determined from the equations below [38,39]:

$$Z=6Rb/L^2\left(1\right)$$

(1)

$$\sigma ={\displaystyle \frac{{3P}_{\mathit{max}}L}{{2bh}^2}}\left[1+6{\left({\displaystyle \frac{D}{L}}\right)}^2-4\left({\displaystyle \frac{h}{L}}\right)\left({\displaystyle \frac{D}{L}}\right)\right]$$

(2)

$$\epsilon ={\displaystyle \frac{6\mathrm{D}\mathrm{h}}{{\mathrm{L}}^2}}$$

(3)

$$E={\displaystyle \frac{m{L}^3}{{4bh}^3}}$$

(4)

where L, b, and h represent the span, width (mm), and thickness (mm) of the specimen, respectively; m denotes the slope of the tangent to the initial linear portion of the load–displacement curve; D is the maximum deflection prior to failure (mm); P is the applied load (N) at a specific point on the load–deflection curve.

3. Results and Discussion

Figure 4a presents the load–deflection behaviour of flexural samples with different fibre orientations. The nonlinear load–deflection response observed in this figure is indicative of shear-driven interfacial mechanisms. This behaviour accounts for the more ductile and progressive damage observed in off-axis specimens, in contrast to the predominantly brittle failure seen in 0° oriented configurations. The results indicate that the on-axis (0°) direction of the UNI composite specimens exhibits linear stress–strain behaviour and quasi-brittle failure. The quasi-brittle behaviour of on-axis (0°) specimens is due to fibre-dominant characteristics, and Zhang et al. [36] made a similar observation. On the other hand, the behaviour of off-axis fibre direction samples (15°, 30°, 45°, 60°, and 90°) is highly nonlinear, and ductile failure behaviour is observed with lower maximum load values. The flexural strength and failure strain (elongation at break) values are shown in Figure 4 and

Table 3. The flexural properties of on-axis and off-axis composite samples.

Fibre Orientation (Deg.)	Max. Load (N)	Flexural Strength (MPa)	Variation (%)	Failure Strain (%)	Variation (%)	Flexural Modulus (GPa)	Variation (%)
0°	806.0 ± 4.5	1128.3 ± 5.90	-	3.86 ± 0.42	-	36.60 ± 1.49	-
15°	396.6 ± 2.1	594.8 ± 2.49	−47.4	5.79 ± 0.02	50.1	31.78 ± 0.80	−13.2
30°	172.2 ± 1.8	254.2 ± 2.12	−77.5	6.00 ± 0.23	55.6	18.16 ± 0.36	−50.4
45°	95.4 ± 12.1	143.4 ± 14.68	−87.3	6.04 ± 0.32	55.7	12.59 ± 0.05	−65.6
60°	96.8 ± 5.8	145.0 ± 6.13	−87.1	4.77 ± 0.05	23.7	11.94 ± 0.86	−67.4
90°	107.1 ± 14.4	165.2 ± 16.99	−85.4	2.58 ± 0.36	−33.1	13.14 ± 1.19	−64.1

. The maximum load, strength, and failure strain values of the on-axis (0°) samples were 805 N, 1128 MPa, and 3.85%, respectively. It is observed from the curves of the off-axis specimens that the load and strength values reach a minimum at 45°, with 95.4 N and 143.4 MPa, respectively, resulting in an 87.3% reduction in strength compared to the on-axis 0° strength. These values were slightly increased after 45° to reach maximum values at 90° with loads of 107.1 N and strengths of 165.2 MPa, respectively. Obviously, with only a 15° off-axis angle, the strength reduction was 50.1%.

The opposite behaviour was observed for flexural failure strain values (Table 3 and Figure 4b), as they increased with increasing fibre angles up to 45°, reaching a maximum value of 6.04% due to the maximum shear effect at this angle. It decreased after 45°, reaching a minimum failure strain of 2.57% at 90° due to the reduced shear effect and the increased matrix failure effect. For 90° samples, the matrix characteristics become dominant, despite 10% of the fibre being oriented at 90°. This behaviour is consistent with studies on thermoplastic composites under off-axis compressive loading, which have documented significant plastic deformation and higher failure strains, particularly at off-axis angles between 30° and 90° compared to on-axis specimens [40].

Figure 5 represents the flexural modulus at various fibre orientation angles of the basalt/Elium composites. The stiffness values almost followed the load and strength behaviour. Hence, it was reduced to 11.94 GPa at 60° and then increased to 13.14 GPa at 90°, with decrements of 67.4% and 64.1%, respectively. It was observed that the stiffness of the 15° off-axis composite was reduced by 13.2%.

The off-axis specimens of the thermoplastic basalt/Elium composite exhibit a plateau in their deflection curves, analogous to the plastic plateau typically observed in ductile metals. This behaviour arises from the matrix’s and fibres’ progressive failure, leading to increased strain at final failure, greater bending deflection, reduced bending stiffness, and lower peak loads compared to on-axis specimens. These characteristics can be attributed to multiple failure mechanisms occurring across different length scales, including fibre pull-out and matrix cracking at the constituent level and fibre kinking, fibre tow misalignment, and shear failure at the lamina level. Furthermore, fibre kinking can manifest in either in-plane or out-of-plane morphologies. Figure 6 shows that various failure behaviours are observed in the on-axis (0°) and off-axis (15° to 90°) fractured specimens under flexural testing. In off-axis flexural tests, the fibre orientation relative to the loading axis introduces significant in-plane shear stress between adjacent plies and along the fibre/matrix interface. These stress concentrations, especially at mid-span and near the neutral axis, promote interfacial sliding as a key precursor to failure in angle-ply laminates.

The observed failure patterns in off-axis specimens suggest that interfacial debonding and localised shear zones are dominant mechanisms, consistent with shear-driven failure theories in angle-ply laminates. As reported in [35,41], the Elium matrix can exhibit localised plastic deformation under high shear, promoting crack initiation along the fibre–matrix interface. Our findings align with these models, indicating that the mismatch in stiffness and the off-axis orientation amplify the shear stress concentration, leading to premature failure in those zones. Since the local shear stress exceeded the interface’s cohesive strength, shear separation increased. Sliding at the fibre–matrix interface leads to energy dissipation, crack initiation, and localised damage zones. When interfacial sliding becomes unstable during the flexural test, it triggers macro-scale phenomena such as delamination, matrix cracking, or fibre pull-out (Figure 6). The nonlinear load–deflection response is often a manifestation of such shear-driven interfacial processes (Figure 4a). This mechanism explains why off-axis specimens exhibit more ductile or progressive damage behaviour compared to brittle failures in 0° orientation [42,43,44].

In order to evaluate the micromechanics of the fracture surfaces, the failed specimens were examined using Mira SEM (Tescan, Oxford Instruments, Cambridge, UK). Figure 7 presents the SEM fractography of the specimens’ upper surface (compression side) at various fibre orientation angles, which was taken to observe the failure mechanism at the microstructural level. It was observed that the failure of the 0° on-axis sample was initiated with fibre pull-out and matrix fragmentation at the upper surface when the load reached its maximum value. Additionally, as seen in the images of Figure 3 and Figure 6 and the SEM image of the 0° on-axis sample, the failure occurred at the upper surface (compression side), and there are no signs of failure on the lower surface (tension side). Lamina delamination occurred near the upper and lower fractured surfaces following the failure of the upper surface. While the failure mechanisms of the off-axis samples differed, it was observed that the samples twisted with increasing flexural loading during flexural testing at 15° and 30° (Figure 3). The fibres were pulled out, and matrix fracture occurred when the load reached the maximum at the upper and lower side for the 15° sample, while at angle 60°, the behaviour was similar due to the increasing shear; hence, small fibres were broken from the lower compression side of the sample, and there is no clear broken fibre or matrix for the 60° sample. According to the ASTM D790 standard [45], the support span-to-depth ratio must be selected to ensure that failure occurs in the outer fibres of the specimen solely due to bending moments. Furthermore, certain materials that do not fail at strains up to 5% may exhibit a load–deflection response characterised by a yield point, where the load ceases to increase despite continued strain.

4. Conclusions

This experimental study was conducted on basalt-reinforced Elium composites with varying off-axis angles. Basalt/Elium laminates were manufactured and prepared for flexural testing at angles ranging from 0° to 90°. The flexural tests demonstrated the significant impact of off-axis angles on the performance characteristics, including flexural strength, flexural strain, and flexural modulus. The results showed that flexural strength decreased by 87% when the angle changed from 0° to 45°, highlighting the influence of shear resulting from the angle variation. After reaching 45°, flexural strength increased by 15% at 90°, indicating a transition from shear-dominated behaviour to matrix-dominant properties. In contrast, flexural strain exhibited an opposite trend to that of flexural strength. From 0° to 45°, flexural strain increased by 56%, but beyond that point, it decreased by 57%. These findings suggest that in-plane shear stress caused by off-axis loading significantly influences the flexural properties of the composites.