3.1. Loading Response
Figure 2a–d depict the loading reactions of the composite specimens that were affected by the impactor.
Figure 2a shows the impact loading response of the hybrids and non-hybrids with 20 J of impact energy.
The impact load of PLA + CF + 130D KF was 1.506 kN, which was 1.66 times more than PLA + CF, while PLA + CF + 200D KF resisted the impact load until 1.807 kN, which was two times more than PLA + CF. With 30 J of impact, this difference compared to PLA + CF was 1.45 times and 1.5 times more, respectively, shown in
Figure 2b. Furthermore, the impact loads faced by PLA + CF + 130D KF and PLA + CF + 200D KF were 1.48 times and 1.43 times, respectively, higher than PLA + CF at 40 J of impact energy, which is elucidated in
Figure 2c. When the specimens were subjected to 50 J of impact energy, illustrated in
Figure 2d, again, the impact resistance of the hybrids was 1.36 and 1.43 times higher than non-hybrid PLA + CF. The higher impact loading of the hybrids with less displacement shows that the hybrids are stiffer and stronger than the non-hybrids. In other words, the impact resistance of the hybrids was better than all the non-hybrids. Carbon fibers proved to be a viable fiber for improving their performance [
28]; herein, the non-hybrid PLA + CF specimens showed high-impact loads compared to the other non-hybrids, which is why the impact strength of the hybrids was discussed according to the PLA + CF composites.
3.2. Material Damage and Impact Resistance
For all the hybrids and non-hybrids, the impact damage was different, and it became more visible with the increase in impact energy from 20 J to 50 J. The specimens right for printing are provided in
Figure 1c.
Figure 3a–f shows the damage and impact resistance of the hybrid and non-hybrid composites with 20 J of impact energy. In
Figure 3b, cracks are visible in both the front and back sides; in the Kevlar-containing specimens, the delamination was obvious, with damage on the hitting side and back side, showing that the fibers were broken with the stretch, as shown in
Figure 3c,d, while, in the hybrids, no damage could be found, just slight indentation marks apparent on the hitting side of the specimens. Moreover, the damage on the back sides the of the PLA + CF + 130D KF hybrids was 83.592% and PLA + CF + 200D KF 90.486% less than the PLA + CF composites. The impact resistive force of these hybrids was higher than the non-hybrids, with minimum damage, which is evidence that the hybrids are more impact resistive than non-hybrids, as shown in
Figure 3a,e,f. Further values are listed in
Table 2.
Considering the sum of all resistive forces of the hybrid and non-hybrid composites is 100%, PLA + CF + 200D KF showed the highest values. Moreover, both hybrids were able to occupy 58.4% of the pie chart in
Figure 4, which suggests that the impact resistance of the ductility and brittleness together are superior compared to the impact-resistive property of the single sort of non-hybrid composites.
Figure 5a–f elucidate the damage and impact resistance of the hybrid and non-hybrid composites with 30 J of impact energy. In
Figure 5b, cracks are aggravated and more visible in both the front and back sides; in the Kevlar-containing specimens, the delamination was obvious, with damage on the hitting side and back side showing that the fibers were broken with the stretch, deboning between the layers, and deeper penetration of the impactor, as shown in
Figure 5c,d, while, in the hybrids, just slight indentation marks were apparent on the hitting sides of the specimens. Moreover, the damage on the back sides of the PLA + CF + 130D KF and PLA + CF + 200D KF hybrids was almost 79.69% and 88.16% less than PLA + CF non-hybrids, respectively, which was slightly bigger than the hybrids of 20 J due to the increase in impact energy. Again, the impact forces of the PLA + CF + 130D KF and PLA + CF + 200D KF hybrids were higher than the non-hybrids, with minimum damage, which shows their high strength against impacts, as shown in
Figure 5a,e,f. All the concerning values of the hybrid and non-hybrids against 30 J of impact energy are listed in
Table 3.
Figure 6 depicts the dominance of the hybrid composites compared to the non-hybrids in a pie chart with 30 J of impact energy. The PLA + CF + 130D KF was 27.1%, and the PLA + CF + 200D composite was 28.03% of the pie chart. Moreover, the damage areas of these hybrids were also 79.69% and 88.16% less than the PLA + CF composites, as listed in
Table 3.
Figure 7a–f elucidate the damage and impact resistance of the hybrid and non-hybrid composites with 30 J of impact energy. In
Figure 7b, the cracks are aggravated and more visible in both the front and back sides but without evidence of the impactor’s penetration; in the Kevlar specimens, the delamination was clearer, with damage on the hitting side and back sides showing that the fibers were broken with the stretch, and the deboning between layers were more visible because of the deeper penetration of the impactor, as shown in
Figure 7c,d. On the other hand, in the hybrids, damage was visible in the PLA + CF + 130D KF composites, but still, not very deep indentation marks were apparent on the hitting side of the PLA + CF + 200D KF specimens. Moreover, the damage on the back sides of these hybrids was 78.82% and 88.16% less than the PLA + CF non-hybrids, which were slightly bigger than the hybrids of 30 J. Again, the impact resistance forces of the PLA + CF + 130D KF and PLA + CF + 200D KF hybrids were higher than the non-hybrids, with minimum damage, which show their high strength against impacts, as seen in
Figure 7a,e,f and
Table 4. All the concerning values of the hybrid and non-hybrids against 40 J of impact energy are listed in
Table 4.
On the other hand, in
Figure 8, the percentages of the resistive impacts of the hybrids were again higher than the non-hybrids, which suggests that the hybrid composites were stable against variations in the impact energies, with minimum damage areas.
All the specimens, hybrid and non-hybrids, were damaged with 50 J of impact energy, as shown in
Figure 9b–f. According to
Figure 9a,b, the PLA + CF specimens were completely damaged with 1.255 kN of impact force because of the brittle nature of the carbon fibers. The damage patterns in PLA + 130D KF and PLA + 200D KF were a little different than the carbon fiber specimens, because in these specimens, the delamination was aggravated on the hitting side and deboning was also more visible with the naked eye, but its impact resistance force was less than the carbon fiber specimens, as depicted in
Figure 9a,c,d. On the other hand, in the PLA + CF + 130D KF and PLA + CF + 200D KF hybrids, the impactor penetrated, but some unbroken and stretched fibers were still present on the back sides of the specimens. The damage was clearer, but the propagation of future cracks was hard to see with the naked eye. The impact force was still higher than the non-hybrids. In other words, the hybrids were stronger and more impact cracks-resistive than the non-hybrids, because in hybrids, stiffness and ductility act at the same time to protect the specimen.
Figure 10 and
Table 5 illustrate the behaviors of he hybrid and non-hybrid composites against 50 J of impact energy. On the other hand, in the literature, single sorts of composites have been used to study the impact resistance of different CF, KF, and GF composites [
29]. At room temperature, the failure modes included fiber pull-out and fiber tearing. In the case of fiberglass, the failure mechanism was matrix cracking, while, for HSHT, it involved delamination and matrix cracking. In our case, these phenomena were very less, e.g., no fiber pull-out and very less delamination.
3.3. CT Scanning
To see the level of damage caused by impacts with energies of 20 J, 30 J, 40 J, and 50 J, photographs of the damaged hybrid and non-hybrid specimens were captured. Photographs of the front and back were taken.
Figure 3,
Figure 5,
Figure 7, and
Figure 9 depict comparable damage patterns that reveal the extent of the damage and the presence of cracks. Furthermore, the specimens were evaluated using a computed tomography (CT) scan to look more closely at the damaged areas near the impact zone and the blind side of the impact damage. Additionally, these scans showed the quality assurance of the specimens, with strong connections of the matrix and fibers. That was why the CT scans were very smooth from the front and back sides.
As can be seen above in
Figure 3,
Figure 5,
Figure 7, and
Figure 9, most of the non-hybrid specimens had visible cracks and damage after impact. This is why
Table 1 contains the CT scans of only the hybrid fiber composites, and the rest of the CT scans of the non-hybrids are provided in the
Supplementary Materials (SM) for further verifications.
According to the CT scan images presented in
Table 6, the PLA + CF + 130D KF hybrids had more microcracks than PLA + CF + 200D KF. With 20 J of impact energy, PLA + CF + 130D KF had cracks around the hitting area, and these cracks were aggravated with the increase in impact energy. Moreover, these specimens were punctured with 40 J of impact, and the damage was more severe with 50 J, but there were no further microcracks seen. On the other hand, PLA + CF + 200D KF did not show any microcracks on the front side of the hitting area. The cracks were aggravated more with the increase in impact energy. The aggravation of the cracks in the PLA + CF + 200D KF hybrids was less than PLA + CF + 130D KF, which could be seen with 40 J of impact, where the PLA + CF + 130D KF hybrids were punctured while PLA + CF + 200D KF still only had minor cracks. Furthermore, PLA + CF + 200D KF was fully punctured with 50 J, with less damaged areas, compared to PLA + CF + 130D KF. In result, it would be right to say that the lesser presence of microcracks is also evidence that these hybrids have the ability to resist impacts. Additionally, the rhombus shape of the damage that can be seen in the hybrids and non-hybrids (CT scans are in the
Supplementary Materials (SM)) may be because of the 0°/90° printing orientations, so different orientations may have different shapes of damage.
3.5. Hybrid Effect
One of the best ways to improve composites’ ability to absorb energy and resist penetration is through hybridization. Here, hybrid effects of the drop hammer impact test specimens stated above are computed as follows:
Using the rule of mixture (
ROM), the hybrid effect of the printed composites was examined to evaluate the variations in absorbed energy [
30,
31] using Equation (1) and the ensuing hybrid effect (
) (2).
In this case,
EROM denotes the energy absorption mixture rule for those other than the hybrid composites.
,
and
are the absorbed energy values from the non-hybrid composites that are shown in
Figure 11 and
Table 2,
Table 3,
Table 4 and
Table 5.
where
Eh denotes the absorbed energy of the hybrid composites, and
he denotes the hybrid effect. According to Equation (2), the hybrid effect may have either a positive or negative impact.
If “he > 0”, this indicated a positive hybrid effect.
If he < 0, a negative hybrid effect was seen due to less than zero values.
Figure 12a–d illustrate the hybrids effects with 20 J, 30 J, 40 J, and 50 J impact energies. The hybrid effects with 20 J were negative because of low energy absorption compared to the non-hybrids, which did not mean that these hybrids were bad with low energy levels, because if comparing these results, combined with the crack resistance and fractures between the hybrid and non-hybrid specimens, then PLA + CF + 130D KF and PLA + CF + 200D KF were two to three times less damaged, as shown in
Figure 3,
Figure 5,
Figure 7, and
Figure 9 and
Table 2,
Table 3,
Table 4 and
Table 5. Moving forward with higher impact energies, the hybrid effects were positive. Overall, the presented hybrids had good impact resistance and were less damaged, with high-energy absorbers with positive hybrid effects, which are all evidence that hybrid composites are better than non-hybrids in every aspect. Therefore, in the future, the presented composites could be a good option for structural applications in the fields of automobiles and aerospace.