Effect of lay-up structures on mechanical properties of yarn-level wrapped flax/basalt woven hybrid PLA thermoplastic composites

doi:10.21203/rs.3.rs-8140710/v1

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Research Article

Effect of lay-up structures on mechanical properties of yarn-level wrapped flax/basalt woven hybrid PLA thermoplastic composites

https://doi.org/10.21203/rs.3.rs-8140710/v1

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This study established a systematic experimental research method to investigate the effect of lay-up structure (i.e., fiber hybridization mode) on the mechanical properties of natural-inorganic fiber hybrid polylactic acid (PLA)-based thermoplastic composites. Using flax fibers (natural reinforcement phase) and basalt fibers (inorganic reinforcement phase) as reinforcements and environmentally benign PLA as the matrix, wrapped yarns (with flax/basalt as the core and PLA as the wrapping fibers) were fabricated via a yarn-level wrapping process, which were subsequently woven into flax and basalt plain-woven preforms, and finally six laminated composite specimens with different lay-up structures were prepared by hot-pressing. To clarify the performance discrepancies among composites with different lay-up structures, their flexural, tensile, and low-velocity impact properties were systematically characterized and evaluated, coupled with fracture morphology analysis for mechanism investigation. The results showed that the lay-up structure exerts a influence on the mechanical properties of thec omposites: the pure basalt lay-up composites exhibit the optimal

strength but inferior toughness, while the pure flax lay-up composites display outstanding toughness but relatively lower strength. Among the hybrid lay-up structures, the alternating lay-ups show superior comprehensive mechanical properties compared to the concentrated lay-ups-the former can achieve uniform stress transfer, improve the fiber-matrix interfacial bonding state, and mitigate basalt fiber buckling, while the latter exhibits limited performance enhancement due to stress concentration. Fracture morphology analysis further confirmed that flax fibers can effectively retard the crack propagation process of the composites. This study confirms that through rational fiber hybridization design (especially the alternating lay-up strategy), the strength, stiffness, and toughness of the composites can be effectively balanced, providing important theoretical support and technical reference for the development of high-performance and environmentally benign PLA-based composites.

Flax Fibers

Basalt Fibers

Wrapped Yarns

Thermoplastic Hybrid Composite Materials

Mechanical Properties

Lay-up Structures

Natural fiber-reinforced composites, characterized by environmental friendliness, renewability, and biodegradability, have received extensive research attention in fields such as automotive, construction, aerospace, circuit boards, and healthcare in recent years[1–7]. The increased attention to environmental impact and biodegradability has driven a research upsurge of natural fibers (hemp [8], jute [9], flax [10], sisal [11], bamboo fibers [12], coconut shell fibers [13], etc.) in the field of composites. As a natural fiber with significant specific strength advantages, flax fibers exhibit significantly higher average ultimate stress than hemp fibers, with specific strength far exceeding the latter, and possess complete biodegradability[14–15]. However, natural fiber-reinforced polymer composites generally have shortcomings such as high hygroscopicity and lower mechanical properties (especially modulus) than synthetic fiber composites[16], and single-fiber systems cannot simultaneously meet the demands of "environmental friendliness" and "high performance". Therefore, constructing a "natural fiber-synthetic fiber" hybrid reinforced polymer system to overcome the shortcomings of single-fiber systems through performance complementarity is necessary. As an important branch of synthetic fibers, high-performance fibers are characterized by high strength, high modulus, and high temperature resistance, and are indispensable in military and high-end manufacturing fields[17]. Among them, compared with carbon fibers, basalt fibers not only have similar mechanical properties[18] but also exhibit outstanding environmental stability, with better resistance to aging factors such as ultraviolet radiation, acid-base, and temperature-humidity changes than most synthetic fibers[19]. Most importantly, basalt fibers have abundant raw materials, non-toxic production processes, lower prices than carbon fibers, and complete recyclability[20], which highly aligns with the current manufacturing demands of low carbon, sustainability, and low pollution, making them an ideal alternative to traditional high-energy-consuming synthetic fibers.

Matrix materials are mainly polymer resins, which are primarily classified into thermosetting resins and thermoplastic resins [21–26]. Compared with thermosetting resins, thermoplastic resins exhibit high processing flexibility, strong recyclability, and excellent toughness, making them more suitable for large-scale composite molding [27]. In existing studies, thermoplastic hybrid composites are mostly fabricated via hot-press molding process, and their matrices include polyamide (PA) [28], polypropylene (PP) [29], polyethylene (PE) [30], polyurethane (PU) [31], and polylactic acid (PLA) [32], etc. Among them, as a biodegradable thermoplastic polymer, PLA possesses both good processing fluidity ((melting point 150–170°C, suitable for hot-press molding) and biocompatibility, making it an ideal green matrix to replace traditional petroleum-based thermoplastic resins (e.g., PP, ABS); however, pure PLA-based composites have relatively low mechanical properties and heat resistance, which are difficult to meet the load-bearing requirements of structural components [35].

To improve the performance of thermoplastic matrix composites, researchers mostly adopt the strategy of hybridizing natural fibers with high-performance synthetic fibers; existing studies have confirmed that fiber hybridization, interface optimization, and reasonable lay-up design can synergistically enhance the comprehensive performance of PLA-based composites-through the hybridization of natural fibers with high-performance synthetic fibers, combined with the optimization of material ratio and structural design, the adaptability of the mechanical properties of PLA-based composites can be significantly improved to meet diverse engineering needs[36].

In the field of hybrid fiber composite research, numerous scholars have carried out systematic work. Ravandi et al.[37] systematically investigated the effect of the lay-up structure of carbon/flax fiber hybrid laminates on their impact resistance. Experimental results show that placing the high-modulus carbon fiber layer on the impact contact surface cannot significantly improve the impact resistance of the material; interestingly, when the flax fiber layer is on the impacted side, its impact resistance is better than that of pure flax fiber composites with the same thickness. Sun et al.[38] conducted tensile and flexural tests on carbon/basalt hybrid composites respectively. The test results show that the lay-up sequence has a significant effect on the strength and flexural modulus of the composites. V. Fiore et al.[39] studied different lay-up hybridization methods of basalt/carbon fibers, confirming that fiber hybridization can reduce material costs while maintaining their excellent mechanical properties.

In terms of lay-up structure optimization, existing studies mainly focus on hybrid composite systems of synthetic fibers. In the field of automotive design, improving the safety performance of vehicles in collision accidents is one of the core requirements; in addition, low-velocity impact testing, as an effective method, can evaluate the performance of composite components under collision conditions. However, the mechanical property degradation mechanism and damage evolution process of composites under low-velocity impact are relatively complex, and damage is difficult to identify accurately, which makes the quantitative study of composite impact damage particularly important[40]. In recent years, scholars at home and abroad have carried out a large number of experimental studies and numerical simulation works on the low-velocity impact behavior of composite laminates[41, 42].

Raponi[43] prepared epoxy resin-based composites reinforced with flax fibers, basalt fibers, and their hybrid forms, and systematically studied their performance under low-velocity impact loads. The study found that compared with single flax fiber-reinforced composites, hybrid fiber composites have significant improvement and enhancement in quasi-static mechanical properties and damage tolerance. Dhakal[44] conducted low-velocity impact experiments on flax-basalt/vinyl ester resin hybrid composites, and combined advanced characterization techniques such as scanning electron microscopy (SEM) and X-ray micro-CT to deeply analyze the failure mechanism of the material under different impact energies. The results show that hybrid fiber composites not only have higher impact peak load, but also exhibit excellent energy absorption characteristics. Fragassa[45] conducted a comparative study on the mechanical properties and impact resistance of flax fiber, basalt fiber, and their hybrid form composites, confirming that fiber hybridization can significantly improve the impact resistance of flax fiber composites without increasing the material weight. Wang et al.[46] systematically studied the effect of the lay-up configuration of carbon fibers and flax fibers on the low-velocity impact characteristics and damping behavior of composites. The results show that hybrid composites with a sandwich structure exhibit excellent impact resistance and prominent energy absorption capacity. The study by I.D.G. et al.[47] shows that the appropriate stacking sequence of basalt fiber layers and carbon fiber layers can improve the balance of mechanical properties of hybrid composite laminates. Ashraf et al.[48] combined basalt fibers and flax fibers to make composite circular patches, bonded them with 3 mm thick Al 2024-T3 aluminum plates, and prepared composites with three configurations (asymmetric, symmetric, and sandwich) through a vacuum-assisted bagging process, and analyzed their impact performance by means of drop-weight impact tests. The results show that the asymmetric configuration of basalt/flax fiber composites has better impact resistance and strength.Nema et al.[49] used hand lay-up technology to prepare composites by changing the stacking sequence of flax fibers and carbon fibers. The results show that the stacking sequence has an impact on the above mechanical properties: the storage modulus of hybrid composites with carbon fibers in the outer layer is higher than that of composites with flax fibers in the outer layer.However, systematic research work on PLA-based composites reinforced with flax/basalt fiber hybridization is still relatively scarce, especially the mechanism of how different lay-up structures affect their mechanical properties remains unclear.

In this study, flax fibers and basalt fibers were used as reinforcement phases, and polylactic acid (PLA) filaments were simultaneously used as wrapped yarns and thermoplastic matrix. Four lay-up configurations of flax/basalt hybrid composites (F/F/B/B, B/B/F/F, B/F/B/F, F/B/F/B) were fabricated via hot-pressing molding process. Through flexural, tensile, and low-velocity impact tests, the mechanical behaviors of the above four lay-up composites were comprehensively evaluated, and the influence mechanisms of alternating lay-ups (F/B/F/B, B/F/B/F) and concentrated lay-ups (F/F/B/B, B/B/F/F) on the mechanical properties of the hybrid composites were systematically clarified; the damage evolution behaviors of the composites under different lay-up structures were deeply clarified, aiming to provide a high-performance and low-cost design scheme of flax/basalt hybrid composites for fields such as automotive interior parts and environmentally friendly packaging materials.

2.1 Materials

In this study, flax fibers (F) and basalt fibers (B) were selected as reinforcements, with polylactic acid (PLA) filaments simultaneously serving as wrapped yarns and the thermoplastic matrix. Flax fiber yarns (200 tex) were provided by Shandong Weifang Jinyue Textile Co., Ltd. (China), basalt fiber yarns (200 tex) by Xiangyang Huierjie Co., Ltd. (China), and PLA filaments by Zhejiang Hisun Biomaterials Co., Ltd. (China). The wrapped yarns (with flax/basalt fibers as core yarns and PLA as wrapping yarns) required in this study were prepared by Shengfang Special Fiber (Shanghai) Co., Ltd. The schematic diagram of the wrapping process is shown in Fig. 1a. During the preparation of wrapped yarns, flax fiber yarns and basalt fiber yarns (each with a linear density of 200 tex) were used as independent core yarns, respectively, and double-wrapped with PLA yarns of 62 tex linear density; among them, the fiber parameters are listed in Table 1, and the wrapping twist (100 twists/m) and other processing parameters are listed in Table 2.

Table 1
Specifications of Selected Materials
Material		Tensile Strength /(N·tex-1)	Elongation at Break /%	Density / (g·cm-3)	Fiber Diamete/µm	Linear Density /tex
Flax Yarn	50	0.12	2	1.44	18.4	200
Basalt Filament	79	0.14	2.6	2.7	7	200
Polylactic Acid (PLA) Filament	3.5	0.22	15	1.24	250	62

Table 2
Processing Parameters of Wrapped Yarns
Core Yarn	Wrapped Yarn	Twist Direction	Twist(twist per meter)
Flax Yarn	Polylactic Acid (PLA) Filament	Alternating S/Z Helical Wrapping	100
Basalt Fiber Yarn	Polylactic Acid (PLA) Filament	Alternating S/Z Helical Wrapping	100

2.2 Layup Structure Design of Fabrics

This study designed a plain-woven fabric structure (as shown in Fig. 1d, e), and the fabric adopted a "one-up-one-down" plain interweaving pattern, with both warp and weft yarns using wrapped yarns as the main interweaving components. Among them, the warp density was strictly controlled at 40 yarns/10 cm, and the weft density was set at 60 yarns/10 cm; Fabric thickness: 1.2 mm. This study also designed six hybrid lay-up structures (as shown in Fig. 1), which are specifically as follows: ① Lay-up structure with four layers of basalt fabric, i.e., B/B/B/B; ② Lay-up structure with four layers of flax fabric, i.e., F/F/F/F; ③ Lay-up structure with two flax fabric layers and two basalt fabric layers arranged in sequence, i.e., F/F/B/B and B/B/F/F; ④ Four-layer structure with alternating flax and basalt fabric layers, i.e., F/B/F/B and B/F/B/F, with the lay-up sequence of "flax fabric→basalt fabric→flax fabric→basalt fabric" (the specific lay-up is shown in Fig. 1f).

2.3 Fabrication Process of Hybrid Layered Composites

In this study, the prepared plain-woven fabrics were placed in an oven at 30°C for pre-drying for 24 hours to remove moisture from the fabrics. After pre-drying, the hot-pressing process was performed in a plate vulcanizer (schematic diagram shown in Fig. 1g): first, pressure was applied and maintained at 150°C and 5 MPa for 4 minutes, then the pressure was released for 1 minute, and this pressure application-release operation was repeated 3 times to enhance the interfacial bonding performance between fibers and matrix. After the hot-pressing cycle, the temperature of the plate vulcanizer was increased to 170°C and maintained for 7 minutes, then 5 MPa pressure was applied again and maintained for 5 minutes to complete the hot-pressing molding of the composites. Finally, the hot-pressed composites were cut into standard sample sizes for subsequent mechanical property tests.

2.4 Mechanical Property Testing

2.4.1 Flexural Property Testing

For the flexural property test, the determination of sample dimensions and test operation were performed in accordance with ASTM D7264 standard, and a universal testing machine (Shenzhen Wance Testing Machine Co., Ltd.) equipped with a 10 kN load cell was used to conduct the three-point bending test. The test loading rate was set to 2 mm/min, with a span of 80 mm, and the dimensions of the composite sample were 100 mm×13 mm×2.5 mm (length×width×thickness), as shown in Fig. 2(a). To reduce test errors, 5 parallel tests were set up for each lay-up structure.

2.4.2 Tensile Property Test

For the tensile property test, the determination of sample dimensions and test operation were carried out with reference to ASTM D3039 standard, and the same model of universal testing machine as that used in the flexural property test (Shenzhen Wance Testing Machine Co., Ltd.) was employed to conduct the tensile test of hybrid composites; as shown in Fig. 2(b), the dimensions of the clamping areas at both ends of the sample were 170 mm×25 mm×2.5 mm (length×width×thickness), and the effective gauge length was set to 120 mm. To reduce test errors, 5 parallel tests were set up for each lay-up structure.

2.4.3 Low-Velocity Impact Property Test

For the low-velocity impact property test, the determination of sample dimensions and test operation were implemented with reference to ASTM D7136 standard, and a drop-weight impact testing machine (Shenzhen Wance Testing Machine Co., Ltd.) was used to conduct the test; as shown in Fig. 2(c), the dimensions of the impact sample were 75 mm×50 mm×2.5 mm (length×width×thickness), and the impact energies were set to 3 J and 5 J, respectively. During the test, the center of the sample was ensured to be accurately aligned with the center of the impact hammer. To reduce test errors, 5 parallel tests were set up for each lay-up structure.

3.1 Flexural Properties

Figure 3(a) shows the stress-strain curves of hybrid lay-up woven composites, and Fig. 3(b) presents the flexural strength and flexural modulus test results of the composites. It can be seen from the test data in the figures that different hybrid lay-up structures have a impact on the flexural properties of the composites. Among them, the B/B/B/B structure exhibits the highest flexural strength and flexural modulus, while the F/F/F/F structure shows lower flexural strength and flexural modulus. The B/B/B/B structure exhibits excellent flexural properties, which stems from the high modulus and high strength characteristics of basalt fibers—as the main load-bearing phase of the composite, they can improve the overall stiffness of the material. In contrast, the mechanical properties of pure flax fibers are relatively limited, leading to lower load-bearing capacity of the F/F/F/F structure.

The flexural properties of the F/F/B/B and F/B/F/B structures fall between those of the pure basalt fiber lay-up structure (B/B/B/B) and pure flax fiber lay-up structure (F/F/F/F). In terms of specific data, the peak load, flexural strength, and flexural modulus of the B/B/B/B structure are 93.64 N, 106.1 MPa, and 6.93 GPa in sequence; the corresponding performance indicators of the F/F/F/F structure are 15.48 N, 23.36 MPa, and 0.80 GPa.

Among the hybrid lay-up structures, the flexural properties of the F/B/F/B structure are improved, with a peak load, flexural strength, and flexural modulus of 38.25 N, 56.65 MPa, and 3.74 GPa, respectively; compared with the pure flax lay-up (F/F/F/F) structure, the improvement rates are 147.1%, 142.5%, and 366.3% in sequence. The flexural properties of the B/F/B/F structure show the most prominent improvement, with a peak load, flexural strength, and flexural modulus of 48.72 N, 70.53 MPa, and 4.18 GPa, respectively; compared with the pure flax lay-up (F/F/F/F) structure, the improvement rates are 214.7%, 201.9%, and 421.2% in sequence. The flexural properties of the F/F/B/B structure show relatively limited improvement, with a peak load, flexural strength, and flexural modulus of 26.6 N, 39.29 MPa, and 1.46 GPa, respectively; compared with the pure flax lay-up (F/F/F/F) structure, the improvement rates are 71.8%, 68.2%, and 82% in sequence. The peak load, flexural strength, and flexural modulus of the B/B/F/F structure are 31.03 N, 45.82 MPa, and 2.07 GPa, respectively; compared with the pure flax lay-up (F/F/F/F) structure, the improvement rates are 100.5%, 96.1%, and 158.1% in sequence. The strength of F/B/F/B and B/F/B/F is higher than that of F/F/B/B and B/B/F/F, which is attributed to better inter-core adhesion[50].

It is worth noting that the flexural properties of the B/F/B/F structure are superior to those of the F/B/F/B structure, which indicates that basalt fibers can more effectively resist flexural stress when placed in the outer layer. This is attributed to the higher surface layer strength[50]. However, the performance improvement of the F/F/B/B structure is limited, possibly because basalt fibers are concentrated in the inner layer and fail to fully exert their high modulus advantage. Furthermore, by comparing hybrid structures with pure flax lay-up structures, it can be concluded that the hybrid lay-up method can effectively improve the flexural properties of the composites.

Figure 4(a) shows the damage images of composites with different lay-up structures after flexural property tests (observed from three perspectives: impact surface, back surface, and side surface). Combined with the 3D microscopic images of the damaged areas in Fig. 4(b), it is found that the flexural surfaces of different hybrid structures exhibit the same damage characteristics: matrix cracking, slight yarn damage, and whitening of the damaged area. By observing the side bending images, it is found that composites containing basalt lay-ups have a larger bending degree compared with pure flax structures. It can thus be inferred that the better the flexural properties of the composite, the stronger its tolerance to bending degree.

The F/F/F/F and B/B/B/B structures have a single damage mode, and their mechanical responses are dominated by the inherent properties of the laminate, with no interference caused by interlayer property differences. The B/B/F/F and F/F/B/B structures induce interfacial stress concentration due to interlayer mechanical property differences, and the damage shows obvious regional characteristics (brittle fracture of the rigid layer and slow damage evolution of the flexible layer), with their mechanical properties co-regulated by lay-up sequence configuration and interfacial behavior. During the mechanical load transfer process of the F/B/F/B and B/F/B/F structures, due to repeated adaptation of interlayer properties, the interface remains in a stress concentration state, the damage morphology shows alternating complex characteristics, and their strength failure mechanism is controlled by the interfacial damage accumulation process, ultimately exhibiting progressive multi-stage failure behavior.

3.2 Tensile Properties

Since there is no front-back difference in tensile tests, the F/F/B/B and F/B/F/B structures do not require tensile tests on both front and back sides. Figure 5(a) shows the tensile stress-strain curves of composites with different lay-up structures, and Fig. 5(b) presents their tensile strength and tensile modulus test results. Tensile test results show that the incorporation of basalt fibers has a strengthening effect on the tensile strength of the composites. Although the pure basalt lay-up (B/B/B/B) has the highest tensile strength and modulus, the stress decreases after the initial linear stage, followed by a plateau period, and finally rapid fracture occurs. This phenomenon can be attributed to local interfacial debonding caused by fiber buckling during the hot-pressing process. In contrast, the pure flax lay-up (F/F/F/F) exhibits more excellent ductility. However, the F/F/B/B and F/B/F/B structures maintain high tensile strength while also exhibiting a certain degree of ductility.

Specific data are as follows: the peak load, tensile strength, and tensile modulus of the B/B/B/B structure are 1378.8 N, 19.16 MPa, and 5.41 GPa, respectively; the corresponding parameters of the F/F/F/F structure are 713.13 N, 9.51 MPa, and 1.26 GPa, respectively. The tensile properties of the F/B/F/B hybrid structure are improved compared with the pure flax lay-up (F/F/F/F) structure; its peak load, tensile strength, and tensile modulus are 1181.88 N, 15.76 MPa, and 3.95 GPa, respectively, which are 65.7%, 65.7%, and 213.5% higher than those of the pure flax lay-up (F/F/F/F) structure. The peak load, tensile strength, and tensile modulus of the F/F/B/B hybrid structure are 944.02 N, 12.59 MPa, and 3.40 GPa, respectively, which are 32.4%, 32.4%, and 170.0% higher than those of the pure flax lay-up (F/F/F/F) structure.

The performance of the F/B/F/B structure is superior to that of the F/F/B/B structure; this difference indicates that when basalt fibers are uniformly dispersed, the stress transfer efficiency is higher—uniformly distributed fibers can bear loads more continuously and suppress local stress concentration. When basalt fibers are concentrated in the inner layer (F/F/B/B structure), the early fracture of outer flax fibers causes load overload of inner basalt fibers, resulting in the failure to fully release their strengthening potential.

Figure 6 presents the damage morphologies of composites with different lay-up structures after tensile tests. Analysis of the damage morphologies shows that: the B/B/B/B lay-up structure exhibits obvious fiber buckling, with large-area cracks in the matrix in the damaged area, and buckling causes stepped fracture of basalt fibers; fiber pull-out and resin plastic deformation can be observed in the damaged area of the pure flax lay-up (F/F/F/F) structure; the damaged area of hybrid structures exhibits a mixed fracture mode; the addition of flax lay-ups reduces the buckling degree of basalt fibers during the hot-pressing process, and the rough surface of flax fibers improves the wetting effect of PLA resin, with no macroscopic delamination observed.

3.3 Low-Velocity Impact Properties

Figure 7 shows the impact response characteristics of composites with different fiber hybridizations. Test results show that under 3 J and 5 J impact energies (as shown in Fig. 7(a), (b)), the force-time curves exhibit a typical three-stage characteristic: Stage Ⅰ is a rapid load rise period with no obvious decline; Stage Ⅱ shows a load decline accompanied by fluctuations; Stage Ⅲ shows a sudden load drop due to impactor rebound or specimen perforation. It is worth noting that with the introduction of basalt lay-ups, the peak load and initial curve slope of flax/basalt hybrid composites show a faster increasing trend compared with the pure flax lay-up (F/F/F/F). The analysis results show that the load fluctuation in Stage Ⅱ mainly originates from the asynchronous fracture of flax fibers and basalt fibers.

Impact displacement change directly reflects the deformation capacity of materials, and the displacement characteristics of different lay-up structures differ. As shown in Fig. 7(c), (d), the pure flax lay-up (F/F/F/F) exhibits the maximum displacement, which is 25.3 mm and 14.75 mm under 3 J and 5 J impact energies, respectively, consistent with the ductile fracture characteristics endowed by the high elongation at break of flax fibers. In contrast, the pure basalt lay-up (B/B/B/B) has the smallest displacement, which is 5.63 mm and 7.16 mm under 3 J and 5 J impact energies, respectively. This is mainly due to the high modulus characteristic of basalt fibers, which makes them more prone to brittle fracture under impact and limits their plastic deformation capacity. Hybrid lay-up structures achieve controllable adjustment of displacement characteristics through synergistic effects, and their displacements are all between the pure flax lay-up (F/F/F/F) and pure basalt lay-up (B/B/B/B). Compared with the pure flax lay-up (F/F/F/F), the displacement of hybrid lay-ups decreases by 22.85% and 56.2% under different impact energies, respectively, reflecting the effective restriction of basalt lay-ups on the deformation of flax lay-ups. The displacement stability of alternating lay-ups (F/B/F/B, B/F/B/F) is better, while the displacement change of concentrated lay-ups (F/F/B/B, B/B/F/F) is more affected by structural discreteness, further confirming the important influence of lay-up sequence on deformation coordination.

Peak impact force is a key indicator to measure the initial impact resistance of materials, and test results show that the peak forces of different lay-up structures differ. As shown in Fig. 7(e), (f), the pure flax lay-up (F/F/F/F) has the lowest peak impact force, which is 253.28 N and 245.10 N under 3 J and 5 J impact energies, respectively, consistent with the inherent mechanical properties of natural fibers. The pure basalt lay-up (B/B/B/B) exhibits an extremely high peak force, which is 1362.74 N and 1699.14 N under 3 J and 5 J impact energies, respectively, increasing by 438% and 593% compared with the pure flax lay-up (F/F/F/F), highlighting the advantages of high strength and high modulus of basalt fibers. Hybrid lay-up structures achieve optimized adjustment of peak force through fiber synergistic effect. Taking the B/F/B/F lay-up as an example, its peak force is 508 N and 1158.2 N under 3 J and 5 J impact energies, respectively, increasing by 100.57% and 372.54% compared with the pure flax lay-up (F/F/F/F), but only 37.3% and 68.2% of the pure basalt lay-up (B/B/B/B), indicating that the introduction of flax lay-ups can effectively alleviate stress concentration at the initial stage of impact. The influence of lay-up sequence on peak force is particularly: alternating lay-ups (F/B/F/B, B/F/B/F) perform best in terms of mechanical property balance; when basalt is used as the outer layer (B/F/B/F lay-up), its peak force increases by 12.4% and 18.7% compared with the F/B/F/B lay-up under 3 J and 5 J impact energies, respectively, which is related to the direct bearing effect of outer high-strength fibers on impact load; however, concentrated lay-ups (F/F/B/B, B/B/F/F) exhibit higher damage dispersion due to interfacial stress concentration, further verifying the advantage of alternating lay-ups in stress distribution uniformity.

Comprehensive analysis results show that hybrid lay-up structures have a impact on the impact resistance of composites. In terms of energy absorption, hybrid lay-ups are generally superior to the pure basalt lay-up (B/B/B/B), among which the B/F/B/F alternating lay-up performs best, while concentrated lay-ups have abnormal performance fluctuations. In terms of displacement characteristics, hybrid lay-ups achieve controllable adjustment of displacement between the pure flax lay-up (F/F/F/F) and pure basalt lay-up (B/B/B/B) through the synergistic effect of basalt and flax fibers, and the displacement stability of alternating lay-ups is better. In terms of peak force, although the pure basalt lay-up (B/B/B/B) has an extremely high peak force, hybrid lay-ups can effectively alleviate stress concentration while improving peak force through reasonable lay-up design, among which alternating lay-ups (especially the B/F/B/F lay-up with basalt as the outer layer) perform best in terms of mechanical property balance. In general, alternating lay-up structures (F/B/F/B, B/F/B/F) can effectively exert the synergistic advantages of hybrid lay-ups and exhibit good impact resistance under both 3 J and 5 J impact energies; however, concentrated lay-ups have the disadvantage of high damage dispersion due to issues such as interfacial stress concentration.

Low-velocity impact test results show that composites with different lay-up structures have differences in energy absorption performance. As shown in Fig. 7(g), (h), the energy absorption value of the pure flax lay-up (F/F/F/F) is at a medium level, but its energy absorption value under 5 J impact energy is 72.2% higher than that under 3 J impact energy. This fluctuation is mainly related to the performance discreteness of natural fibers, which may originate from fiber orientation deviation or uneven resin wetting during sample preparation. The energy absorption value of the pure basalt lay-up (B/B/B/B) is relatively low, which is 3.62 J and 5.84 J under 3 J and 5 J impact energies, respectively, closely related to its brittle fracture characteristics. The energy absorption performance of hybrid lay-up structures is generally superior to that of the pure basalt lay-up (B/B/B/B), with an improvement range of 2.6% and 20.2%, which is mainly attributed to the progressive development of delamination damage and the plastic energy dissipation mechanism of flax fibers. Among them, the B/F/B/F alternating lay-up maintains the highest energy absorption value under both 3 J and 5 J impact energies, which are 4.35 J and 6.14 J, respectively, indicating that alternating lay-up structures can effectively extend the energy dissipation path. It is worth noting that the energy absorption of F/F/B/B and B/B/F/F concentrated lay-ups increases abnormally under 5 J impact energy, with increases of 2.64% and 1.69%, respectively. This phenomenon may be related to the change in local energy dissipation mechanism caused by interfacial defects.

Figure 8(a) shows the damage morphologies of composites with different fiber hybridizations under 3 J and 5 J impact energies. Impact surface, back surface, and side surface views show that the pure flax lay-up (F/F/F/F) has a large damage range after impact, with obvious cracking and fiber pull-out observed on both the impact surface and back surface, and delamination visible on the side surface; the damage of the pure basalt lay-up (B/B/B/B) shows brittle characteristics, manifested as concentrated cracks with fast propagation rate, and fine cracks distributed on the impact surface and back surface. 3D microscopic images (corresponding areas in Fig. 8(b)) further reveal the microscopic damage characteristics: in the F/F/F/F lay-up, due to the high elongation at break of flax fibers, the damage is dominated by ductile fracture, manifested as fiber tearing and scattered resin matrix cracking; in the B/B/B/B lay-up, the high modulus characteristic of basalt fibers makes its damage exhibit brittle fracture characteristics, with cracks propagating rapidly along the fiber direction and penetrating the entire structure.

Among hybrid lay-ups, the damage morphology of alternating lay-ups (F/B/F/B, B/F/B/F) is easier to control, and the damage degree of their impact surface and back surface is between the pure flax lay-up (F/F/F/F) and pure basalt lay-up (B/B/B/B); basalt lay-ups can restrict the excessive deformation of flax lay-ups, while flax lay-ups can delay the crack propagation of basalt lay-ups, for example, the damage distribution of the F/B/F/B lay-up is more scattered; concentrated lay-ups (F/F/B/B, B/B/F/F) have high damage dispersion due to interfacial defects under 5 J impact energy, and interfacial stress concentration will cause local severe cracking. The above results show that the lay-up method has a impact on the damage mode of composites after impact, among which alternating lay-ups perform better in terms of damage control and performance balance.

This work focuses on the tensile, flexural, and low-velocity impact behaviors of yarn-level hybrid composites composed of flax fiber (F)/basalt fiber (B) reinforcements and polylactic acid (PLA) thermoplastic matrix. The hybrid wrapped yarns were designed with F and B as core yarns, while PLA filaments were used for double wrapping (acting as both wrapping yarns and matrix). Plain-woven fabrics with controlled warp and weft densities were produced from these wrapped yarns, and six hybrid composites with distinct lay-up sequences were successfully fabricated by hot-press molding following pre-drying.

The lay-up sequence is confirmed to have a significant effect on the mechanical properties of the composites: pure basalt fiber (B/B/B/B) lay-ups demonstrate outstanding strength and stiffness but poor toughness and energy absorption, whereas pure flax fiber (F/F/F/F) lay-ups possess excellent ductility but relatively low strength. Hybrid lay-ups achieve remarkable synergistic improvements in tensile, flexural, and impact properties, where alternating lay-up structures (F/B/F/B and B/F/B/F) significantly outperform concentrated lay-up structures (F/F/B/B and B/B/F/F). The superior performance of alternating lay-ups is attributed to their ability to promote uniform stress transfer, alleviate local stress concentration and fiber buckling, optimize fiber-matrix interfacial adhesion, and thereby retard crack propagation.

The impact damage modes of the composites are strongly dependent on the lay-up sequence: pure flax fiber lay-ups are characterized by a wider damage range, pure basalt fiber lay-ups display typical brittle fracture features, and alternating lay-ups exhibit more controllable damage morphologies along with a better balance between strength and toughness. In contrast, concentrated lay-ups exhibit higher damage discreteness, which is mainly caused by interfacial defects-induced uneven stress distribution.

Overall, this study confirms that rational optimization of the lay-up sequence is an effective strategy to balance the strength, stiffness, and toughness of PLA-based F/B hybrid composites. Academically, this work fills the gap in understanding the lay-up-dependent mechanical behaviors and damage mechanisms of yarn-level natural-inorganic hybrid thermoplastic composites; practically, it provides key theoretical guidance for the structural design and engineering application of such green composites in fields like automotive interiors and eco-friendly packaging.

Ethical considerations This article does not contain any studies with human or animal participants.

Consent to participate

Not applicable

Consent for publication

Not applicable

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

Author Contribution

**Xiaonan Wang:** Investigation, Methodology, Formal analysis, Writing-Original draft preparation. **Yiwei Ouyang:** Investigation, Formal analysis, Writing-Reviewing and Editing. **Yiran Han:** Investigation, Methodology, Formal analysis. **Xin Sun:** Investigation, Methodology, Formal analysis. **Xiaoke Huang:** Investigation, Methodology, Formal analysis. **Yang Liu:** Writing-Reviewing and Editing. **Xiaozhou Gong:** Investigation, Formal analysis, Writing-Reviewing and Editing.

ACKNOWLEDGMENTS

This work was financially supported by a grant from the scientific research project of Hubei provincial department of education,China (Project: Q20201705).

Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Effect of lay-up structures on mechanical properties of yarn-level wrapped flax/basalt woven hybrid PLA thermoplastic composites

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Abstract

Figures

1. Introduction

2. Materials and Methods

2.1 Materials

2.2 Layup Structure Design of Fabrics

2.3 Fabrication Process of Hybrid Layered Composites

2.4 Mechanical Property Testing

2.4.1 Flexural Property Testing

2.4.2 Tensile Property Test

2.4.3 Low-Velocity Impact Property Test

3. Results and Discussion

3.1 Flexural Properties

3.2 Tensile Properties

3.3 Low-Velocity Impact Properties

4. Conclusion

Declarations

Author Contribution

ACKNOWLEDGMENTS

Data Availability

References

Additional Declarations

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