Effects of Z-pin on moisture absorption property and damage mode under flexural load for carbon fiber composite

2.1 Materials

Unidirectional prepregs used for manufacturing laminates were composed of carbon fiber and epoxy resin, which had a resin content of 31 wt% and a fiber areal density of 145 g/m². Carbon fiber tows with the filament number of 240 and 6,000 were prepared for Z-pins with the diameters of 0.1 and 0.5 mm, respectively. Diglycidyl ether Bisphenol-A (DGEBA) epoxy resin was then impregnated into CCF800 carbon fiber tows, and Z-pins were fabricated after the epoxy resin cured at 120°C for 3 h. The fabricated Z-pins were a continuous rod-like component and cut to 20 cm long for easy operation. The diameters of Z-pins with the nominal diameters of 0.1 and 0.5 mm were measured at 106 ± 7 and 505 ± 12 μm in practical. The DGEBA epoxy resin was purchased from China Bluestar Co., Ltd, with the tradename of WSR618. The CCF800 carbon fiber tows were purchased from Weihai TuoZhan Fiber Co., Ltd. The prepregs, with the brand name of UIN10000/9A16, were purchased from Weihai Guangwei Composites Co., Ltd. Release films, breathers, vacuum bag, and sealant tapes for the autoclave process were purchased from Airtech Advanced Materials Group.

2.2 Preparation and characterizations

As for the preparation of Z-pin composite laminate, Z-pins were implanted into the prepregs at the center region of 12.5 mm (width direction) × 10 mm (length direction), as shown in Figure 1a. The prepregs were unidirectionally laid on a heating table at 30 ± 1°C to make them soft. Capillary tubes with an inner diameter of 0.15 and 0.55 mm were implanted into the soften prepregs vertically. Subsequently, the continuous Z-pin was placed into the capillary tubes. Z-pin remained inside composites after the capillary tube was pulled out. Then, the rest of the Z-pins above the top surface of the prepregs was cut off; 2 mm-thickness composite laminates and Z-pin reinforced composite laminates were fabricated using an autoclave process with 120°C curing temperature.

Figure 1

(a–b) Scheme of Z-pin plantation; (c) illustration of self-designed fixture with samples before compressing spring; (d) illustration of self-designed fixture with samples after compressing spring; (e) composite samples under flexural load; and (f) composite samples under moisture coupled with the flexural load.

The dimension of the sample was 80 mm × 12.5 mm × 2 mm. In order to avoid moisture diffusion along the in-plane direction, the samples were sealed with epoxy resin at four edges. The moisture condition was set as 5 times the concentration of simulated seawater at room temperature 23 ± 2°C according to ASTM D1141. Besides, a self-designed fixture was used to implement a three-point flexural load in combination with moisture conditions. The three-point flexural stress applied to the composite sample can be adjusted by controlling the compression degree of a calibrated spring. At least five specimens were used for each case.

Moisture properties of composite and Z-pinned composite were evaluated after moisture-only condition and moisture-coupled flexural load condition. The applied load was 60% of the flexural strength of the composite. The moisture content was calculated according to ASTM D5229, as shown in equation (1), and was determined with three significant digits. Microstructures inside composite and Z-pinned composite after different moisture conditions were detected through X-ray μCT (Nano Voxel TS17139). The resolution for X-ray μCT is 11.86 μm in each direction. Since the ratio of width to thickness is around 6, six specimens can be stacked together for X-ray μCT while ensuring the resolution. The X-ray μCT zone covers the center 12.5 mm (width direction) × 15 mm (length direction) part of composite specimens. The voltage used in the X-ray μCT test is 120 kV, and the current is 80 μA. The exposure time is 0.45 s, and the resolution is 11.8 μA. Figure 2 shows that the slice at different positions can be taken from three directions after X-ray μCT. The three-point flexural test was conducted using a universal testing machine (Instron, 3382), according to ASTM D7264. The macroscopic failure morphology on the lateral surface of the composite and Z-pinned composite was observed after a mechanical test by the optical camera. The internal microstructure after the mechanical test was detected through X-ray μCT.

Figure 2

Illustration of slice selection from X-ray μCT experiment, where the red slice parallels to the fiber direction and vertical to the thickness direction of composite, noted as the first direction; the green slice parallels to the fiber direction and thickness direction of composite, noted as the second direction; the blue slice vertical to the fiber direction and parallels to the thickness direction of composite, noted as the third direction.

3.1 Moisture properties of composite and Z-pin composite after different moisture conditions

Table 1 provides the moisture content of composite and Z-pinned composite after moisture-only condition and moisture coupled with flexural load condition. The moisture content of composites is 0.187 ± 0.010 and 0.211 ± 0.025% after immersion for 7 days with and without flexural load, respectively. For the composites reinforced by 0.5 and 0.1 mm pins, the moisture contents of Z-pinned composites after moisture for 7 days reach 0.232 ± 0.017 and 0.221 ± 0.010%, which are higher than composites without pins. It is noted that the moisture content of Z-pinned composites is higher than that of the composite without Z-pins. In addition, the moisture contents of Z-pinned composites are around 0.200% after moisture coupled with load condition, less than the cases without load. During flexural loading, the part above the middle plane of composites suffers from compressive stress and the part below bears tensile stress. And the area in contact with the loading nose is subjected to compressive stress concentration. It was reported that tensile stress promoted moisture absorption and compressive stress inhibited moisture absorption [40,41]. Therefore, the moisture content in the composite with flexural load is less than that of just moisture condition.

Figure 3 depicts microstructures inside composites through thickness direction after different moisture conditions, where the slices are taken through the third direction from X-ray μCT. Morphology inside the composite after moisture for 7 days is shown in Figure 3a, the structure of which is uniform. The morphology after moisture coupled with flexural load is as uniform as that in Figure 3a. Morphologies inside Z-pinned composites after different moisture conditions are presented in Figure 3b–e. As shown, the background outside the composite is black, and the area nearby Z-pin is darker compared with other areas inside the composite. For X-ray μCT technology, the color difference presented in the sample is based on the density difference of material constituents. The smaller the density, the darker the color it presents [42,43]. The background during X-ray μCT is air, which presents as black edge around samples. The density of seawater is around 1.0 g/cm³, which is lower than 1.5 g/cm³ of carbon fiber composites. Therefore, it can be deduced that the dark area at rich-resin pocket around Z-pin in Figure 3b–e is caused by moisture intake. Comparing Figure 3b and c with Figure 3d and e, the moisture absorption is more serious for Z-pinned composite after moisture conditions without load. Therefore, the conclusion can be drawn that the Z-pin implantation contributes to more moisture intake of the composite under moisture conditions. In addition, flexural load helps elevate the moisture intake under moisture conditions.

Figure 3

Microscopic morphology through the third direction of (a) composite after moisture for 7 days, (b and c) Z-pinned composite after moisture for 7 days, and (d and e) Z-pinned composite after moisture coupled with flexural load for 7 days.

Similarly, the slice through the second direction presents the morphology inside the sample of the in-plane direction. As depicted in Figure 4a, multiple dark strips appeared inside the Z-pinned composite after moisture for 7 days. And the strips are less significant on Z-pinned composite after moisture-coupled flexural load condition, as shown in Figure 4b. This also illustrates that the flexural load elevates the moisture absorption of Z-pinned composites.

Figure 4

Microscopic morphology through the second direction of Z-pinned composite after (a) moisture for 7 days, and (b) moisture coupled with flexural load for 7 days.

3.2 Crack initiation of composite and Z-pin composite after moisture coupled with flexural load

The morphology characteristic was further analyzed to study the effect of applied flexural load on Z-pinned composites. The pictures were taken at different locations through the second direction of composite from X-ray μCT. As shown in Figure 5a, a crack appears along the interface of the rich-resin pocket brought by Z-pinned implantation. This is because the interface of the rich-resin pocket is a weak point for defect generation. In addition, crack initiates and propagates across the rich-resin pocket as illustrated in Figure 5b–d. Since the pictures were taken at different positions in the thickness direction, it can be deduced that the crack initiation and propagation happened along both in-plane and thickness directions. This implies that the implanted Z-pin is a source of crack generation and then yields crack propagation by the applied mechanical loads. It demonstrates that applying flexural load causes crack inside Z-pinned composite, either at the interface or across the resin pocket zone.

Figure 5

Microscopic morphology at different locations through the second direction of Z-pinned composites after moisture coupled with flexural load for 7 days. (a) Shows crack at interface; (b, c and d) show crack across resin pocket.

Furthermore, the internal microstructure of composite reinforced by 0.1 mm Z-pin was detected. As shown in Figure 6, the Z-pins with a diameter of 0.1 mm are relatively harder to detect than those with 0.5 mm by X-ray μCT. This is because the finer Z-pin causes a smaller rich-resin pocket. There are some relatively long cracks near the 0.1 mm Z-pins and to a certain distance from pins. Compared to the composite reinforced with 0.5 mm pin, the cracks are longer and the amounts of cracks are higher for the composite reinforced with 0.1 mm pin. This is because the implantation of each pin is the weak point for crack and stress concentration [44]. And the number of 0.1 mm pin is higher than that of 0.5 mm pin with the same volume fraction of pin, so more pins result in more weak points for defect generation. And the higher amounts of cracks make the crack connection between adjacent cracks easier, which will contribute to the formation of long cracks. This implies that the diameter and number of Z-pin influence the crack generation inside composite after moisture conditions.

Figure 6

Microscopic morphology at different locations through the second direction of composite reinforced by 0.1 mm Z-pin after moisture coupled with flexural load for 7 days. (a and b) Show crack near pins; (c and d) show crack away from pins.

3.3 Three-point failure morphology of composite and Z-pin composite after moisture coupled with load

Macroscopic morphology after the failure of composite and Z-pin composite after moisture for 7 days is shown in Figure 7. As shown in Figure 7a, the main fracture develops with multiple interlayer cracks on the composite without Z-pin. Those interlayer cracks distribute along the thickness direction. While the cracks on the composite reinforced with 0.5 mm Z-pin after the same moisture condition are not as many as that in Figure 7a, Figure 7b shows the interlayer crack mainly concentrated on the area close to the surface. The moisture might concentrate on the surface of the Z-pinned composite, which is also observed from the result of X-ray μCT, as shown in Figure 3b. All these results conclude that Z-pin can inhibit the development of interlayer cracks by reducing moisture absorption, but the surface moisture is enhanced.

Figure 7

Macroscopic failure process through third direction of (a) composite and (b) Z-pin composite after moisture condition for 7 days.

Macroscopic failure morphology on the lateral surface of composite and Z-pin composite after moisture coupled with flexural load for 7 days were observed, as shown in Figure 8. For the composite after moisture with flexural load, it fails with multiple defects under the three-point flexural test as depicted in Figure 8a. Aside from the main fracture, side fracture, interlayer crack, and secondary fractures are also generated due to failure. The main fracture is caused by the stress concentration at the loading nose during the three-point flexural test. The applied flexural load on moisture conditions might contribute to side fracture and secondary fracture formation. It is because the property of the area subjected to flexural load is weakened, inducing more opportunity for defect generation. In addition, the cracks along the interlayer imply that the interlayer properties of composites are weakened under the moisture condition. As shown in Figure 8b, the failure morphology characteristics are obviously different on Z-pinned composites after the same moisture condition. The main fracture of Z-pinned composite has less width and depth than that of composite without Z-pin. And only similar secondary fractures can be observed on the Z-pinned composite. It can be deduced that Z-pin plays a role in inhibiting more fracture development during loading, i.e., suppressing main fracture and interlayer cracks.

Figure 8

Macroscopic failure process through lateral surface of (a) composite and (b) Z-pin composite under three-point flexural load after 7 day immersion with 60% flexural load.

Further analysis was conducted on the microscopic morphology inside the composite after failure. The morphologies at the center position of composite and Z-pinned composite through the third direction are presented in Figure 9. As shown in Figure 9a, the main fracture bursts out from the loading position inside the composite without Z-pin. Accompanied with that, multiple cracks run across the interlayer direction from the main fracture. Besides, there are some interlayer cracks near the surface not generating from the main fracture. For the Z-pinned composite, the main fracture appears around the loading position which is not that disastrous as in Figure 9a. Multiple cracks are derived around Z-pin, which cross along the interface of Z-pin and laminate and the interface of laminate or run through thickness direction. Meanwhile, cracks not generating from the main fracture are more notable than Figure 9a. It can be deduced that Z-pin suppresses the main fracture to some extent, but derives more microcracks around it. Thus, Z-pin changes the failure morphology obviously.

Figure 9

Microscopic morphology through the third direction of (a) composite and (b) Z-pin composite under three-point failure after moisture coupled with flexural load for 7 days.