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Abstract

Within the scope of experiments, five kinds of biaxial weft-knitted (BWK) fabrics with various loop lengths (8.0, 9.2, 10.5, 11.9, and 13.5 mm) were used as reinforcement systems to fabricate thermoplastic composites with polypropylene (PP) resin yarn. Then, the final composite became BWK composite with various loop lengths. The mechanical properties of the composites were investigated by conducting tensile, three-point bending, and three-point bending impact tests on specimens. In all specimens, PP was commingled with glass yarn. Glass was used as the reinforcement material. Fiber volume fraction of weft fibers with the 8.0 mm loop length was the highest compared with the other four types of specimens. Because of the higher volume fraction of the BWK composites with the 8.0 mm loop lengths, tensile, three-point bending, and three-point bending impact properties of the 8.0 mm loop were higher than the other four types (9.2, 10.5, 11.9, and 13.5 mm) of composite structures.

Keywords

Knitting loop length biaxial weft-knitted composites thermoplastic composites tensile bending and impact properties

Introduction

Knitting with advanced fibers, such as glass and aramid, to produce near net shape preforms has in recent years received increasing interest.¹

In order to improve the mechanical properties, such as strength and stiffness, of weft-knitted fabric, straight yarns both in weft and warp directions can be integrated. These types of reinforcements are called biaxial weft-knitted (BWK) structures. Weft and warp yarn layers are held together by a stitching yarn system in BWK fabrics. Reinforcing yarns, for example, glass or aramid fibers, can be used within all yarn systems.² The tensile, bending, and impact properties of the BWK thermoset composites were reported by Demircan et al.³

Thermoplastic composites are being used in various industries such as automotive, wind turbines, and so on. The most important advantages of thermoplastics are their potential for rapid, low cost, and mass production of reinforced composites. On the other hand, thermoplastic composites have very high viscosity (usually 500–5000 Pa s) that makes the processing of thermoplastic matrix composites difficult. Therefore, some techniques, such as commingled yarn, were developed in order to improve processability of thermoplastic composites. The matrix fiber will be mixed with reinforcing fiber in commingled yarn technique and this technique was proven to be a cost-effective method of processing of thermoplastic composites.^4,5 Therefore, the commingled yarn technique was chosen in order to fabricate the BWK preforms. Some research has been carried out to find out consolidation quality of glass fiber (GF)/polypropylene (PP) commingled yarn-based composites.^6,7

Knitted fabric-reinforced thermoplastic composites were studied by some researchers.^8–10 Some researchers studied knitted fabric-reinforced thermoplastic composites with commingled fibers.^11–16 The tensile properties of knitted fabric-reinforced composites made from GF-PP commingled yarn with different loop densities were investigated by Zaixia et al.⁴ They found that the tensile strength of the composites increases followed by light decrease as the loop density of preform increases. High-performance thermoplastic composite from flat-knitted multilayer textile preform using hybrid yarn was investigated by Abounaim et al.¹³ They found that the mechanical properties of two-dimensional composites were greatly affected by different arrangements of reinforcement yarns. And the integration of reinforcement yarns as biaxial inlays (warp and weft yarns) is found to be the best solution for knitting to have the highest mechanical properties compared with the tuck stitch-shaped and unidirectionally arranged reinforcements. Tensile, three-point bending, and impact properties of textile-inserted PP/PP-knitted composites using injection–compression molding were reported by Khondker et al.¹⁷ The tensile properties of weft-knitted composites for energy absorption were studied by Xue et al.¹⁸ They described correlation between fabric structure (e.g. loop height and width, number of wales or courses per unit length, etc), matrix damage, and materials properties. The effect of architecture on the mechanical properties of knitted composites was reported by Anwar et al.¹⁹ They investigated tensile and compression properties of three milano ribs and one-rib weft-knit glass fabric reinforcement. Moreover, only a few numbers of contributions were made about mechanical properties of BWK composite.^20–22 Demircan et al.^23,24 reported bending and impact properties of BWK thermoset and thermoplastic composites.

In the literature, contributions about the mechanical properties of knitted composites were reported, which were explained above. However, only a few numbers of contributions were made about the mechanical properties of the BWK thermoplastic composites. The purpose of this research is to characterize the mechanical properties of the BWK thermoplastic textile composites. Within this study, we investigated tensile, three-point bending, and three-point bending impact properties of the BWK thermoplastic composites with various loop lengths. The obtained results of this study can be used to design new textile preforms during development of different composite materials.

Experimental procedures

Composites constituents

In all thermoplastic composite panels, glass was used as a reinforcement material and PP was used as the resin yarn. The GF/PP commingled yarns were used as warp (410 tex), weft (410 tex), and stitch yarns (138 tex) in the BWK fabrics. The volume content of the GFs was 52% and that for the PP fibers was 48% in the warp and weft-commingled yarns. The volume content of the GFs was 27% and that for the PP fibers was 73% in the stitch commingled yarns. Schematic drawing of the BWK fabric is shown in Figure 1(a) to (d). Figure 1(a) depicts the warp and weft yarns. The knitted fabric with stitch yarn is shown in Figure 1(b). And, Figure 1(c) and (d) shows the BWK fabric with warp, weft, and stitch yarns. Five types of the BWK plain knitting fabrics with the GF/PP commingled yarns with the various loop lengths (8.0, 9.2, 10.5, 11.9, and 13.5 mm) were produced on a flat bed knitting machine (Shima Seiki Mfg. Ltd., Sakata, Japan). The properties of the BWK fabrics are shown in Table 1. Figure 2(a) to (e) shows the photographs of the BWK fabrics. The schematic drawings of the BWK fabrics are shown in Figure 2(f). All BWK fabrics had plain knitting structure. Plain knitting is a basic knitting stitch in which each loop is drawn through other loops to the right side of the fabric. Plain is the simplest and most economical weft-knitted structure.²⁵

Figure 1.

Schematic drawing of the BWK fabric, (a) the warp and weft yarns, (b) the knitted fabric with stitch yarn, (c) and (d) the biaxial weft-knitted fabric with warp, weft, and stitch yarns. BWK: biaxial weft-knitted.

Figure 2.

Photographs and schematic drawing of the BWK fabrics, (a) 8.0 mm, (b) 9.2 mm, (c) 10.5 mm, (d) 11.9 mm, (e) 13.5 mm, and (f) schematic drawing of the BWK fabric, A = warp, B = weft and C = stitch. BWK: biaxial weft-knitted.

Table 1.

Properties of BWK fabrics.

Loop length of BWK material (mm)	8.0	9.2	10.5	11.9	13.5
Density of weft yarn (end/cm)	7.6	5.8	4.9	3.8	2.8

BWK: biaxial weft knitted.

We made experiments by changing the knitting loop lengths (8.0, 9.2, 10.5, 11.9, and 13.5 mm). For example, in the 13.5 mm (Figure 2(e)), the length of the loop structures in the warp direction was the longest compared with the other four types of knitting (Figure 2(a) to (d)).

Fabrication method

The BWK preforms were stayed in a vacuum heater at 80°C for 6 h before fabrication of composites. Due to vacuum heating process of the BWK preforms, good interaction between fiber and resin could be provided. In BWK composites, 2 and 10 layers were fabricated on hot press compression machine (Figure 3(a)). Figure 3(b) shows the lower and upper mold dies. During fabricating of the BWK composites, the PP resin fibers in the BWK fabric were melted and disappeared and became resin. Only the GF warp, weft, and stitch fibers were stayed in the BWK composites. The stacking sequence of 10 layer composites was written in a symmetric laminate code. Fabrics were cut in the weft direction, attached in a metallic frame, and put in the molding die. The molding pressure, temperature, and time were 3 MPa, 220°C, and 13 min, respectively. Later, the mold was cooled until it comes to the room temperature around 35°C. Fiber volume fractions were found out by performing burnout tests.

Figure 3.

(a) Schematic drawing of the hot press compression machine and (b) lower and upper mold dies.

Table 2 shows the fiber volume fraction and thickness of 2 and 10 layers BWK composites. The volume fraction of weft yarn in composites was increased by changing the length of the knitting yarn. And, 8.0 mm had the highest weft fiber volume fractions in both 2 and 10 layers (16.4% and 16.1%). Whereas, the volume fraction of weft yarns in the BWK composites with 13.5 mm loop length had the lowest (9.7% and 9.9%).

Table 2.

Fiber volume fraction and thickness of composites.

Specimens (composites)	Layers	Weft V _f (%)	Warp V _f (%)	Stitch V _f (%)	Total V _f (%)	Thickness (mm)
8.0 mm	2	16.4	7.9	17.0	41.4	0.93
9.2 mm	2	14.4	8.8	17.5	40.8	0.83
10.5 mm	2	11.9	9.5	16.8	38.3	0.72
11.9 mm	2	10.5	10.2	18.0	38.7	0.67
13.5 mm	2	9.7	12.4	16.9	39.1	0.59
8.0 mm	10	16.1	7.8	16.7	40.7	4.46
9.2 mm	10	14.8	9.1	17.9	41.8	3.89
10.5 mm	10	12.4	9.9	17.4	39.7	3.44
11.9 mm	10	10.8	10.5	18.5	39.8	2.95
13.5 mm	10	9.9	12.8	17.3	40.1	2.52

V _f: fiber volume fraction.

Mechanical characterization

Tensile tests were conducted on two layer BWK composites. Tensile and three-point bending tests were conducted on the specimens according to ASTM-D303 standard in the course (weft yarn) direction. The measurements of tests were performed using universal testing machine (type 55R4206, Instron, Norwood, Massachusetts, USA), under displacement control with speed 1 mm/min. Figure 4(a) shows the geometry of the specimen from tensile test. In this figure, lamina and aluminum thicknesses are shown with t _c and t _Al, respectively. The thickness of aluminum tabs was 0.5 mm. The composite coupons had a nominal dimension 200 × 20 mm² for tensile test. Figure 4(b) shows the geometry of the specimen from three-point bending test. A 10,000-N load cell was used for tensile test that was 500 N for three-point bending test. The tensile and bending measurements were performed at ambient conditions of 23 ± 2°C and 50 ± 5% relative humidity. Because of the different thickness of 10-layer BWK specimens, the specimen span lengths were changed from 42 mm to 72 mm. The width of the specimens was 15 mm.

Figure 4.

Geometry of the specimen, (a) tensile test, (b) three-point bending test, and (c) three-point bending impact test.

Figure 4(c) shows the test set up and geometry of the specimen from three-point bending impact test. The three-point bending impact tests were conducted on 10-layer BWK specimens according to JIS-K7084 standard in the course (weft) direction. The three-point bending impact damages were inflicted on different specimens in a drop weight test using universal testing machine (type Dynatup 9250HV, Instron). The drop weight was used as an impactor for the tests. The weight of the impactor was 6490 g, and the incident impact energy was 20 J for the three-point bending impact test. The composite coupons had a nominal dimension 92 × 15 mm² for three-point bending impact test. Test span length was 72 mm.

Results and discussions

Tensile properties

The stress–strain curves of the BWK composites during tensile test are shown in Figure 5(a). From these curves, it can be seen that tensile stress with the BWK composites increases linearly with increase in the strain and was followed by a sudden drop in a stress value corresponding to the ultimate failure of the composite. The specimen with the 8.0 mm loop length showed the highest tensile stress and the tensile ultimate strain, whereas the 13.5 mm loop showed the lowest tensile stress and the tensile ultimate strain in the weft direction.

Figure 5.

(a) and (b) Tensile test results and (c) and (d) three-point bending test of the BWK composites with various loop lengths (8.0, 9.2, 10.5, 11.9, and 13.5 mm). BWK: biaxial weft-knitted.

Figure 5(b) shows tensile modulus and strength results of the BWK composites. The highest tensile modulus and strength were obtained by the 8.0 mm loop composites in the weft direction (19.1 GPa and 303 MPa). The tensile modulus and strength of the 8.0 mm loop composites were 28% and 38% higher than the BWK composites with the 13.5 mm loop (13.7 GPa and 187 MPa) in the weft direction.

The possible reason of obtained higher tensile strength results with the 8.0 mm loop composites would be higher weft fiber volume fraction (16.4%) than the BWK composites with the 9.2, 10.5, 11.9, and 13.5 mm (14.4, 11.9, 10.5, and 9.7%).

Three-point bending properties

Figure 5(c) and (d) shows the results of the three-point bending test with various loop lengths of knitting fibers. The flexural strength yielded different trend compared with the tensile test results in the weft direction. The tensile modulus and strength of the BWK composites (Figure 5(b)) were higher than the flexural modulus and strength of the BWK composites (Figure 5(d)) in the course direction. Different failure mechanism could be responsible for this different trend. A measure of the resistance to deformation of the composite in bending is called flexural modulus. Flexural strength and stiffness are mainly controlled by the strength of the reinforcement fibers. The BWK composites with the 8.0 mm loop length exhibited superior flexural modulus and strength (9.3 GPa and 216 MPa) compared with other tested specimens. The BWK composites with the 8.0 mm loop exhibited 43% and 22% higher flexural modulus and strength than that was with the 13.5 mm loop (5.3 GPa and 169 MPa) in the weft direction.

Because of the higher fiber volume fraction of the BWK composites with the 8.0 mm loop in the weft direction, the flexural properties with the 8.0 mm loop had the highest compared with the other tested BWK composites.

Energy results from three-point bending test are exhibited in Table 3. The area under load–displacements curves gives the absorbed energy during three-point bending test. Initiation energy was found out to calculate the area under load–displacement curve until maximum load and that for propagation energy after maximum load. Because composites with the 8.0 mm loop had higher maximum load (0.62 kN) than that was with the other loop lengths (0.52, 0.45, 0.36, and 0.28 kN), the total absorbed energy with the 8.0 mm loop (4.8 J) was higher than that was with the other loop lengths (3.5, 3.2, 2.7, and 1.9 J) in the course direction. The ductile index (DI) is defined as the ratio of the propagation energy and initiation energy.^26,27 The 8.0 mm loop specimens had higher propagation energy (4.8 J) compared with the other specimens, resulting in a higher DI with standard deviation (1.7 ± 0.76) compared with the other specimens (1.8 ± 0.23, 1.8 ± 0.10, 1.3 ± 0.65, and 1.1 ± 0.17).

Table 3.

Energy results after three-point bending test with standard deviation.

Type of samples	Maximum load (kN)	Initiation energy (J)	Propagation energy (J)	Total absorbed energy (J)	Total absorbed energy per millimeter (J)	DI
8.0 mm	0.62 ± 0.03	1.95 ± 0.61	2.88 ± 0.44	4.83 ± 0.17	1.08 ± 0.04	1.72 ± 0.76
9.2 mm	0.52 ± 0.01	1.26 ± 0.06	2.20 ± 0.18	3.46 ± 0.12	0.89 ± 0.03	1.76 ± 0.23
10.5 mm	0.45 ± 0.01	1.15 ± 0.02	2.04 ± 0.07	3.19 ± 0.05	0.93 ± 0.01	1.78 ± 0.10
11.9 mm	0.36 ± 0.01	1.26 ± 0.34	1.41 ± 0.38	2.67 ± 0.03	0.90 ± 0.01	1.29 ± 0.65
13.5 mm	0.28 ± 0.01	0.90 ± 0.10	1.00 ± 0.03	1.90 ± 0.07	0.75 ± 0.03	1.14 ± 0.17

DI: ductile index.

Three-point bending impact properties

Figure 6 shows three-point bending impact test results of the BWK thermoplastic composites, load–displacement curves during three-point bending impact test. The load– displacement curves showed that the highest maximum load was achieved by the 8.0 mm loop (0.9 kN), whereas the 13.5 mm loop had the lowest maximum load about 0.2 kN in the weft direction.

Figure 6.

Three-point bending impact test results, load–displacement curves during three-point bending impact test.

Table 4 shows energy results after three-point bending impact test. The 8.0 mm loop had the highest impact properties and total absorbed energy (12.7 J) compared with the other four specimens (9.1, 8.2, 5.8, and 4.0 J). Changing the structures of knitting makes the fabric stronger in three-point bending impact tests. Since the density and volume fraction of weft yarn were increased in the BWK composites with the 8.0 mm loop-sized knitting, the strength and the capacity of impact shock absorption of the BWK composite were improved. With the various knitting structure techniques, strength design in the fabric could be controlled. The 8.0 mm loop type of composites had about three times higher total impact energy (12.7 J) compared with the plain knitting (4.0 J; Table 4). The propagation energy result was higher than the initiation energy result in all the specimens. This result indicates that most of the energy was absorbed after maximum load. Furthermore, total energy results were recalculated with same thickness of specimens and found the 8.0 mm loop absorbed more total energy (2.8 J/mm) than the other four types of composites (2.3, 2.4, 2.0, and 1.6 J/mm), as shown in Table 4. Additionally, the 8.0 mm loop type of composites had the highest DI (2.5) compared with the other specimens (2, 2.2, 2.2, and 1.7).

Table 4.

Energy results after three-point bending impact test with standard deviation.

Type of samples	Maximum load (kN)	Initiation energy (J)	Propagation energy (J)	Total absorbed energy (J)	Total absorbed energy per millimeter (J)	DI
8.0 mm	0.98 ± 0.0	3.61 ± 0.19	9.12 ± 0.05	12.73 ± 0.24	2.85 ± 0.06	2.53 ± 0.12
9.2 mm	0.72 ± 0.01	3.03 ± 0.19	6.06 ± 0.55	9.08 ± 0.73	2.34 ± 0.19	2.00 ± 0.06
10.5 mm	0.52 ± 0.02	2.54 ± 0.06	5.62 ± 0.46	8.16 ± 0.52	2.37 ± 0.15	2.21 ± 0.13
11.9 mm	0.35 ± 0.02	1.83 ± 0.24	3.99 ± 0.42	5.82 ± 0.66	1.97 ± 0.22	2.18 ± 0.06
13.5 mm	0.25 ± 0.01	1.55 ± 0.32	2.42 ± 0.38	3.97 ± 0.06	1.57 ± 0.02	1.69 ± 0.60

DI: ductile index.

Relationship between total absorbed energy from three-point bending and three-point bending impact tests

The relationship between the total absorbed energy from three-point bending test and the three-point bending impact test is shown in Figure 7. The total energy was the total value of the initiation and propagation energy. This graph showed that there was a good relationship between the total absorbed energy from the three-point bending test and the three-point bending impact test. The total absorbed energy from the three-point bending test increased with increasing of the total absorbed energy from the three-point bending impact test. The total absorbed energy from the three-point bending impact tests was much higher than that was from the three-point bending tests. Because of the good relationship between the three-point bending test and the three-point bending impact tests, the fracture behavior of specimens during conducting of both tests could be similar.

Figure 7.

Relationship between total absorbed energy from three-point bending test and three-point bending impact test.

Relationship between energy after maximum load and fiber volume fraction of composites

Relationship between energy after maximum load from the three-point bending impact tests and fiber volume fraction of composites in the weft direction is shown in Figure 8. Total energy after maximum load increased with increasing of volume fraction of composites in the weft direction. The 8.0 mm loop had the highest fiber volume fraction (16.1%) and energy after maximum load (9.1 J) in the weft direction. Whereas the 13.5 mm loop had the lowest fiber volume fraction (9.9%) and energy after maximum load (2.4 J) in the weft direction.

Figure 8.

Relationship between energy after maximum load from the three-point bending impact tests and fiber volume fraction of composites.

Conclusion

This study showed that the mechanical properties of the BWK composites could be improved by changing the knitting loop length. The 8.0 mm loop had the highest tensile, three-point bending, and three-point bending impact properties compared with the 9.2, 10.5, 11.9, and 13.5 mm loop length specimens. Because of the highest weft fiber volume fraction in the BWK composites with the 8.0 mm loop, the mechanical properties were improved compared with the other four kinds of specimens in the weft direction. The good agreements between the total absorbed energy from three-point bending test and three-point bending impact test validated our test results. The relationship between energy after maximum load and volume fraction of composites in the weft direction was also studied and the total energy after maximum load increased with increasing of the volume fraction of composites in the weft direction. In future study, we will try to produce three-dimensional BWK composites using WHOLEGARMENT^® technology, which is available on weft-knitting machines.

Footnotes

Funding

The authors especially thank Prof. Hiroyuki Hamada and Assoc. Prof. Mohamed S Aly-Hassan, Kyoto Institute of Technology, Japan, for their help and support.

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Effect of various loop lengths on mechanical properties of biaxial weft-knitted thermoplastic composites

Abstract

Keywords

Introduction

Experimental procedures

Composites constituents

Fabrication method

Mechanical characterization

Results and discussions

Tensile properties

Three-point bending properties

Three-point bending impact properties

Relationship between total absorbed energy from three-point bending and three-point bending impact tests

Relationship between energy after maximum load and fiber volume fraction of composites

Conclusion

Footnotes

Funding

References