Effect of stitch and biaxial yarn types on the impact properties of biaxial weft knitted textile composites

2.1 Composites constituents

Five types of BWK fabric were produced on a flat bed-knitting machine (Shima Seiki Mfg., Ltd., Japan). Figure 1 depicts the fabricated BWK reinforcement fabric [glass fiber (GF)-GF-NY]. 575 tex E-glass yarn (Nippon Electric Glass Co. Ltd., Japan) and 330 tex aramid yarn (Kevlar-29, Dupont-Toray Co. Ltd., Japan) were used as biaxial materials. 68 tex E-glass (ECG 75 1/0 1 OZ, Hokuriku Fiber-Glass Co. Ltd., Japan), 44 tex aramid (Kevlar-29, Dupont-Toray Co. Ltd., Japan), and 29 tex nylon were used as stitch yarns. Vinyl ester resin (Ripoxy R-806, Showa High Polymer Co. Ltd., Japan) was used as matrix. Table 1 shows the parameters of the BWK fabric.

Figure 1

BWK fabric, GF-GF-NY.

To be able to characterize the effect of stitch yarns in the composite, the volume of each stitch yarn type was chosen as close as possible to that in BWK. The same sensitivity was also shown during the selection of the biaxial yarns. Additionally, in order to achieve a more reliable comparison of the specimens, the warp and weft yarn densities of each type of specimen were adjusted as near as possible, which are shown in Table 1.

2.2 Fabrication method

Before the fabrication of composites, the BWK preforms were placed in a vacuum heater at 100°C for 6 h. Due to the vacuum-heating process, there was good interaction between the fiber and resin. After vacuum heating, composite panels with six plies were fabricated directly by hand lay-up method. The stacking sequence of six layers was written in a symmetric laminate code such as [0/90/0/0/90/0], where 0° denotes the direction of warp yarns or the wale direction and 90° denotes the direction of weft yarns or the course direction. Volume fractions of the five types of specimen were kept constant to compare the specimens. Spacers were used to control the required specimen thickness. The composite panels were cured at room temperature for 24 h, followed by a 2-h postcure at 100°C. The overall fiber volume fraction was about 41.5% for all types of BWK composite structure, with a thickness of 2.9 mm. A notation system was used to differentiate the name of five specimens. The notation “GF-GF-AR” means that, in that order, warp yarn is GF, weft yarn is GF, and stitch yarn is AR.

2.3 Mechanical characterization

The schematic drawings of the three-point bending impact (Figure 2A and B) and plate bending impact (Figure 2C and D) test set-ups are shown in Figure 2. Three-point bending impact tests were carried out on specimens according to JIS-K7084 standard. The three-point bending impact and plate bending impact damages were inflicted on different specimens in a drop weight test using the universal testing machine type Dynatup 9250HV (Instron, Japan). The drop weight was used as an impactor for both of the tests. The boundary of the plate bending impact specimens was clamped on all sides by a rectangular steel plate with a 76-mm-diameter circular hole. The center of the specimen was impacted by a striker with a narrow hemispherical impactor of 12.9-mm diameter. The rectangular boundary (span length of 30 mm) of the specimens in the three-point bending impact tests was supported by using a platform, and the specimens were attached on the platform by using a double-face tape. The first arrangement in the plate bending impact tests was intended to prevent any motion of the plate boundary, both in-plane and out-of-plane. In the second condition in the three-point bending impact tests, the in-plane motion of the supported boundary points of the specimen was prevented by the platform, and downward motion was allowed in the region (30-mm span length) in the center of the specimen, while only upward displacement of the whole specimen was possible just before finishing the tests.

Figure 2

Schematic drawings of test set-ups: (A, B) three-point bending impact test and (C, D) plate bending impact test.

The weight of the impactor was 6490 g for the three-point bending impact test and 16,430 g for the plate bending impact test. The incident impact energy was 20 J for the three-point bending impact test and 55 J for the plate bending impact test.

The composite coupons had a nominal dimension: (i) 80×10×2.9 mm for the three-point bending impact test and (ii) 100×100×2.9 mm for the plate bending impact test. Three specimens from each type of composite panel were tested in the wale and course directions in the three-point bending impact test. In the plate bending impact test, two specimens were tested.

3.1 Microstructural observation of as-fabricated composites

The cross-sectional photographs and schematic drawings of as-fabricated composites are shown in Figure 3A–D. It can be seen that there are no voids in the cross-section of the as-fabricated composites. However, there are some voids in the fracture area of impacted specimens, which resulted from the preparation of the microstructure specimens through the pouring of the resin to the fracture area, so some air bubbles were entrapped. This was indicated by comparing the morphology microstructures of the as-fabricated and the impact-tested composites.

Figure 3

Cross-sectional photographs of as-fabricated composites and schematic drawings: (A, B) GF-GF-AR and (C, D) AR-AR-AR.

3.2 Impact properties of composites based on the three-point bending impact test

The impact properties of composites based on the three-point bending impact test were investigated according to the effect of the stitch and biaxial yarns. Composites with the stitch yarns, such as aramid, nylon, and glass, and the biaxial yarns, such as aramid and glass, were compared according to the three-point bending impact test. Initiation energy was determined to calculate the area under load-displacement curve until maximum load, which was used for propagation energy after maximum load.

3.2.1 Effect of the stitch yarn

The impact properties of the BWK composite specimens during the three-point bending impact test in the course and wale directions with different stitch yarns are exhibited in Figure 4A and B and Table 2. The GF-GF-AR composites had an average peak load of 1153 N in the course direction and 1051 N in the wale direction, which was higher than that of other specimens. The GF-GF-AR specimens with the aramid stitch yarn had the highest total absorbed energy in the course and wale directions, 12.8 and 7.35 J, respectively. The ductile index (DI) is defined as the ratio of the propagation energy and initiation energy [16, 17]. The GF-GF-AR specimens had a higher propagation energy compared with the other specimens in both test directions, resulting in a high DI of 10.4 and 3.94.

Table 2

Three-point bending impact test results of BWK composites in the course (C) and wale (W) directions: effect of stitch and biaxial yarns.

Samples	Maximum load (N)	Initiation energy (J)	Propagation energy (J)	Total energy (J)	DI
GF-GF-AR (C)	1153±105	1.13±0.20	11.6±3.36	12.8±3.51	10.4±1.99
GF-GF-NY (C)	1005±9.76	1.23±0.02	9.50±2.02	10.7±2.03	7.69±1.54
GF-GF-GF (C)	1149±58.2	1.20±0.50	5.69±1.91	7.69±1.85	3.13±1.34
GF-GF-AR (W)	1051±46.7	1.64±0.27	5.70±3.03	7.35±2.76	3.94±2.82
GF-GF-NY (W)	937±7.97	2.04±0.33	4.06±2.62	6.10±2.37	2.23±1.60
GF-GF-GF (W)	1032±8.98	1.40±0.05	3.04±0.45	4.90±0.96	2.16±0.27
AR-AR-AR (C)	946±21.5	1.81±0.12	4.71±1.92	6.52±1.81	1.81±0.12
AR-GF-AR (C)	1017±73.4	1.41±0.40	8.35±1.22	9.76±1.40	6.22±1.38
AR-AR-AR (W)	739±10.6	1.35±0.11	3.88±0.36	5.23±0.31	2.90±0.47
AR-GF-AR (W)	989±38.4	1.24±0.09	2.99±0.07	4.23±0.13	2.43±0.17

Figure 4

Load-displacement curves of the BWK composites during the three-point bending impact test – effect of stitch yarn: (A) course (C) direction and (B) wale (W) direction.

3.2.2 Effect of the biaxial yarn

The three-point bending impact test results in the course and wale directions with different biaxial yarns are shown in Figure 5A and B and Table 2. Generally, the specimens showed 1.2–2.3 times higher total energy results in the course direction than in the wale direction. This was caused by the higher weft yarn density than the warp yarn density (2.5–2.9 times) in the BWK, which is shown in Table 1. The GF-GF-AR composites with the glass and glass biaxial yarns had higher peak load and total absorbed energy than did the other specimens in both testing directions. The AR-AR-AR specimens with the aramid and aramid biaxial yarns showed the lowest total absorbed energy in the course direction (6.52 J). The GF-GF-AR specimens showed (a) 88% higher total absorbed energy than the AR-AR-AR specimens in the course direction and (b) 85% higher total absorbed energy than the AR-GF-AR specimens in the wale direction.

Figure 5

Load-displacement curves of the BWK composites during the three-point bending impact test – effect of biaxial yarn: (A) course (C) direction and (B) wale (W) direction.

3.3 Impact properties of composites based on plate bending impact test

The impact properties of composites based on the plate bending impact test were investigated according to the effect of the stitch and biaxial yarns. Composites with the stitch yarns, such as aramid, nylon, and glass, and the biaxial yarns, such as aramid and glass, were compared according to the plate bending impact test.

3.3.1 Effect of the stitch yarn

Impact resistance is an important aspect for comparing the properties of BWK composite samples. In the literature, high energy absorption of the knitted composites due to the loop structure has been reported [3]. Figure 6A and Table 3 demonstrate the impact properties for the mentioned samples with different stitch yarns. The highest peak load was achieved in the GF-GF-AR specimens (7225 N); the GF-GF-NY specimens had a slightly lower peak load than did the GF-GF-AR specimens (7196 N) and the GF-GF-GF specimens (6458 N). The GF-GF-AR specimens with the aramid stitch yarn absorbed the highest total energy (52.4 J), followed by the GF-GF-NY specimens (the nylon stitch yarn) (51.1 J) and the GF-GF-GF specimens (the glass stitch yarn) (46.0 J). In addition, the value of 46.0 J for the GF-GF-GF specimens is also more than 3 times higher than the result of Pandita et al. [4], whose total absorbed impact energy of glass weft knitted fabric composite fabricated by the autoclave method is 13.9 J.

Table 3

Plate bending impact test results of BWK composites: effect of stitch and biaxial yarns.

Samples	Maximum load (N)	Initiation energy (J)	Propagation energy (J)	Total energy (J)	DI
GF-GF-AR	7225	26.9±1.32	25.5±1.22	52.4±0.10	0.95±0.10
GF-GF-NY	7196	23.6±0.65	27.5±0.25	51.1±0.40	1.17±0.04
GF-GF-GF	6458	20.9±0.79	25.1±0.30	46.0±1.09	1.20±0.03
AR-GF-AR	6801	27.7±2.77	22.2±3.62	49.9±0.85	0.80±0.21
AR-AR-AR	4616	17.2±2.12	22.5±1.67	39.7±0.45	1.31±0.26

Figure 6

Load-displacement curves of the BWK composites during the plate bending impact test: (A) effect of stitch yarn and (B) effect of biaxial yarn.

3.4 Fracture aspects of specimens and effect of hybridization

The fracture aspects of the reverse side of the plate bending impact-tested specimens are demonstrated in Figure 7. The number of cracks in the course direction was much more than in the wale direction because of the higher weft yarn density than warp yarn density. Especially in the first three specimens (GF-GF-GF, GF-GF-NY, and GF-GF-AR), the longer crack propagation length in the wale direction can be seen. This indicated that the composites with the glass reinforcement yarns are more brittle than those with the aramid reinforcement yarns. The number of cracks and energy absorption of the GF-GF-AR composites were higher than those of the other tested specimens. The AR-AR-AR composites had the lowest number of cracks.

Figure 7

The fracture aspects of the reverse side of the plate bending impact-tested specimens.

When the glass weft yarn was used instead of the aramid weft yarn, the AR-AR-AR composites were hybridized and became the AR-GF-AR composites. By hybridizing with the GF, the total absorbed energy of the AR-AR-AR composites was improved by 26% in the plate bending impact test because of the increasing initiation energy. This was increased by 50% in the three-point bending impact test because of increasing propagation energy. A possible reason for the obtained better plate bending impact and three-point bending impact properties of the AR-GF-AR specimens compared with the AR-AR-AR specimens would be the higher number of transverse cracks and the longer delaminations.

3.5 Fracture damage characterization

The cross-sectional photographs and schematic drawings of the GF-GF-AR and the AR-AR-AR specimens after the three-point bending impact test are shown in Figure 8A–D. The GF-GF-AR, GF-GF-GF, GF-GF-NY, and AR-GF-AR specimens demonstrated the same fracture manner, which was observed during the performance of the microstructural characterizations. The AR-AR-AR specimens exhibited a clearly different fracture manner. Therefore, the GF-GF-AR and the AR-AR-AR specimens were selected for comparison of fracture behaviors. The cross-section of these composites was detected under an optical microscope in the course direction. In these photographs, the impact load was applied from the upper side of the specimens. The highest intensity of the fiber breakages was observed near the impact point.

Figure 8

Cross-sectional photographs of the specimens after the three-point bending impact test and schematic drawings: (A, B) GF-GF-AR and (C, D) AR-AR-AR.

Figure 9A–D shows the enlarged parts and schematic drawings of Figure 8A–D. These photographs show that transverse cracks occurred through the 90° fiber bundles, and long delaminations, through the 0° fiber bundle. According to these observations, the entire energy mechanism was contributed mainly by delaminations and transverse fractures, and the highest numbers of transverse cracks and long delaminations were observed during the three-point bending impact test of the GF-GF-AR specimens. The lengths of delaminations from the impacted side to the reverse side with the GF-GF-AR specimen were 7.4, 5.8, and 8.0 mm, and with the AR-AR-AR specimen, 7.2, 7.2, and 6.0 mm.

Figure 9

Enlarged view of cross-sectional photographs of the specimens after the three-point bending impact test and schematic drawings: (A, B) GF-GF-AR and (C, D) AR-AR-AR; transverse cracks are indicated by circles.

The cross-sectional photographs of the GF-GF-AR and the AR-AR-AR specimens after the plate bending impact test and schematic drawings are shown in Figure 10A–D. Because of the above-mentioned reasons, only the GF-GF-AR and the AR-AR-AR specimens were selected for comparison of the fracture behaviors. The specimens were cut in the wale direction, and the cross-section of these composites was detected under an optical microscope in the course direction.

Figure 10

Cross-sectional photographs of the specimens after the plate bending impact test and schematic drawings: (A, B) GF-GF-AR and (C, D) AR-AR-AR.

Figure 11A–D shows the enlarged parts and schematic drawings of Figure 10A–D. In the GF-GF-AR specimen, the long delaminations occurred near the impacted side and reverse side of the specimens. In contrast to the GF-GF-AR specimen, the long delaminations occurred only near the impacted surface of the AR-AR-AR specimen. The highest numbers of transverse cracks and long delaminations were observed on the GF-GF-AR specimens during the plate bending impact test. The length of delamination with the GF-GF-AR specimen was 5.9 mm, and with the AR-AR-AR specimen, 5 mm.

Figure 11

Enlarged view of cross-sectional photographs of the specimens after the plate bending impact test and schematic drawings: (A, B) GF-GF-AR and (C, D) AR-AR-AR; transverse cracks are indicated by circles.

3.6 Agreements of the impact properties

The total absorbed energy values from the plate bending impact and three-point bending impact tests are exhibited in Figure 12A. For the three-point bending impact test, the results of course direction were used in the graphic. This graphic shows that the total absorbed energy from the three-point bending impact test increased with increasing total absorbed energy from the plate bending impact test. These results support the observation that the fracture behavior of both tests was similar. The relationship of the total absorbed energy from the three-point bending test [12] and from the three-point bending impact test in the course direction is shown in Figure 12B. This graphic shows that there was a good relationship between both tests.

Figure 12

Total absorbed energy (A) from the plate bending impact test and from the three-point bending impact test and (B) from the three-point bending test and from the three-point bending impact test.

3.7 Effect of specimen dimensions and boundary conditions on impact properties

The relationship of total absorbed energy and propagation energy from the plate bending impact test and from the three-point bending impact test is shown in Figure 13. With propagation energy being same, a larger amount of energy was absorbed by the clamped specimens than the supported ones. In other words, the 76-mm clamped specimens absorbed larger impact energy than did the 30-mm supported specimens. When we further compared the same material in both tests with the same propagation energy (GF-GF-AR), it can also be clearly seen that the total energy of the 76-mm clamped tests was higher than that of the 30-mm supported tests. These results indicate that total absorbed impact energy was influenced by changing specimen dimensions and boundary conditions.

Figure 13

Relationship of total absorbed energy and propagation energy from the plate bending impact test and from the three-point bending impact test.