Role of nanofillers in low speed impact enhancement of composites

Abstract

The current study investigates the impact characteristics of polymeric-based nanocomposites strengthened with carbon nanotubes, nanoclay, aluminum oxide and silicon carbide particles nanofillers. Different weight percentages of each nanofiller were prepared. A dead weight drop mechanism was utilized to compare the impact characteristics of different nanocomposite materials. An X-ray technique was utilized to characterize the formation of microstructural defects of the laminated composites post impact. The results showed that the nanoclay fillers were the best to enhance the impact and mechanical properties of the composite materials with 4.3 wt% of nanoclay being the optimum percentage.

Keywords

Nanofiller impact composite

Introduction

Nanocomposite materials have recently created a revolution in high-performance structures. The widespread use of these composites has attracted the attention of scientists in many engineering applications due to their superior characteristics. The development of such materials enables the circumvention of classic material performance trade-offs by accessing new properties and exploiting unique synergies between materials. This occurs when the length scale of morphology and the fundamental physics associated with a property coincide, i.e., on the nanoscale level. Through control/alteration of the additives at the nanoscale level, one is able to maximize property enhancement of selected polymer systems to meet or exceed the requirements of current military, aerospace and commercial applications. The technical approach involves the incorporation of nanoparticles into selected polymer matrix systems whereby these particles may be surface-treated to provide hydrophobic characteristics.¹

One of the important characteristics of nanocomposites that lead to their wide use in many industrial applications is their improved impact resistance.^2–4 These composites are used in a number of engineering applications across various industries, including such products as automobile airbags, flexible structures like boat sails and parachutes, reinforcement in composites, architectural expressions in building roof structures, protective vests for security circles and protective layers around the body in planes.

The main objective of the current study is to investigate the potential use of different types of nanofillers in woven Kevlar laminated composites to enhance their composite impact energy resistance and mechanical properties. Four types of nanofiller particles were used, namely aluminum oxide (Al₂O₃) carbon nanotubes (CNT), nanoclay (NC) and silicon carbide (SC) particles.⁵ It is shown that the variation of nanofiller percentage leads to different mechanical characteristics of the resulting composite, and an increase of the energy absorption by a high percentage of fillers may accompany tendency to delamination. To investigate such effects, different percentages of fillers were utilized in the conducted experiments. Two sets of samples, for each of the fillers used, were prepared with a different fiber/matrix percentage for each composite. The results were compared to control samples composed of Kevlar plies only. Since the weight of the composite is important as a design parameter in such applications needing high strength-to-weight ratio, the effect of fiber/matrix percentage was also investigated.

To achieve the objectives of this research, an in-house-developed vertical drop-weight experimental setup was utilized to test the prepared sample laminates. This setup was built according to the guidelines given in the standard specifications “ASTM- D 7136”.⁶ The test procedure determines the damage resistance of multidirectional polymer matrix composite laminated plates subjected to a drop-weight impact event. The potential energy of the drop-weight is specified prior to each test, whereas the damage resistance is quantified in terms of the resulting size and type of damage in the sample. An X-ray technique has also been utilized to characterize the changes in the internal structure of the different Kevlar-laminated composites after impact. Front and side views were taken for each type of Kevlar composites, the X-ray films then were exposed to strong illumination source to reveal the image details, then photos were captured using 10 mega pixels digital camera.

Materials used in preparing samples

The test samples were prepared by incorporating the four used additive particles (Al₂O₃, NC, CNT, SiC) into selected polymer matrix systems.¹ All four additives were added in two different percentages to the vinylester (VE) to manufacture two different sets of samples.

The NC additive used is of the Montmorillonite clay, Nanomer I.34TCN type, obtained from the Sigma-Aldrich company (Missouri, USA). This NC contains 25–30 wt% methyl dihydroxyethyl hydrogenated tallow ammonium. The multi-walled CNT particles used in this research was obtained from the Nanolab company (Newton, Massachusetts, USA). The diameter of each single CNT was in the range was 10–30 nm and the length in the range 5–20 µm. The Al₂O₃ particles used are with 50 µm size obtained from FLUKA Company (St Gallen, Switzerland). The SiC particles with 1200 Grit number (15.3 µm particle size) are of the black type and obtained from the PANADYNE Company (Warminster, Pennsylvania, USA).

Samples preparation

Samples description

The control samples (170 × 170 mm size and 0.125 mm thickness) were prepared by mounting 15 woven Kevlar 49 layers, provided by Fiber Materials Inc., USA, arranged symmetrically in 0°/45° orientations. The Kevlar plies were bonded together with general-purpose vinyl ester resin (AOC-Resins Co, USA). K-12 hardener with 0.5 wt% was added to insure and accelerate the vinyl ester curing. The percentage of vinylester was determined by the minimum amount of resin required to insure complete saturation of Kevlar layers by resin. The matrix mix was prepared in such a way to insure saturation of each Kevlar layer. The fiber/matrix ratios were calculated after finishing the hot process operation to ensure that these ratios correspond to the composites when used in real applications. It is shown that the resin amount needed to saturate the Kevlar layers was increased by increasing the percentage of fillers. It has been noticed that in spite of the small percentage of an added filler, the change in matrix viscosity due to the filler had a noticeable effect on the composite weight.

The desired composite thickness was built up by placing various layers of fiber and resin mixture. Consolidation of the fiber composites involves two important processes: resin flow through porous media and elastic fiber deformation.^7–12 Heat was supplied during processing to expedite the cure rate of the resin.¹³ Regular painting brush was utilized to distribute the resin evenly over the first Kevlar ply; the second ply was placed in 45° angel over the first ply (0°). Consolidating metallic roller was scrolled over the second ply to force the resin to penetrate the second ply, eliminate air bubbles inside resin and to eliminate cavities between the two Kevlar layers. After that, resin was brushed again over the second ply and the third Kevlar ply was laid down in 0° orientation. The same scenario was repeated to prepare the rest of Kevlar layers; scrolling by the metallic roller, brushing the resin and placing the new Kevlar layer. Finally, composite hot pressing technique was performed to insure plies perfect bonding and unified thickness and to expedite the curing process. This technique requires each sample to be placed inside the hot press machine where the two pressing plates were sprayed by a mold release agent (Formentrennmittel obtained from Weicon, Germany). The laminate system was heated to 175℃ and compressed by a force of 450 kg distributed over 300 × 300 mm area for 15 min. Two sets of control samples were prepared. The first set (15 layers of Kevlar + VE) was prepared by adding 37 g of vinylester to 78 g of Kevlar making a resin percentage of 32 wt%. The second set (15 layers of Kevlar + vinylester) was prepared by adding 29 g of VE to 78 g of Kevlar, making a resin percentage of 27 wt%. The nanofiller samples were prepared as shown in Table 1. It should be noted that the same procedures of preparing the control samples were followed in preparing all the other samples where composite laminates are produced. Moreover, in all samples, K-12 hardener with 0.5 wt% was added to insure and accelerate the VE curing.

Table 1.

Composite samples description.

Nanofiller type		Composition		Nanofiller wt%
Nanofiller type		Nanofiller (g)	VE (g)	Nanofiller wt%
Al₂O₃	Set 1	5.6	143.7	1.31
Al₂O₃	Set 2	26.7	172.4	5.60
NC	Set 1	25.0	223.9	4.30
NC	Set 2	37.5	211.4	9.40
CNT	Set 1	2.5	250.0	0.32
CNT	Set 2	5.0	250.0	0.80
SiC	Set 1	5.7	193.3	0.82
SiC	Set 2	28.6	220.3	4.15

Al₂O₃: aluminum oxide; CNT: carbon nanotubes; NC: nanoclay; SiC: silicon carbide.

Samples coding

A total of five different composite sample groups, where each group has different additive, were prepared. The first group represents the control samples without any additive reinforcements. Each of the remaining four groups contains a different additive; namely, Al₂O₃, NC, CNT and SiC samples. Each group contains two sets of low and high concentrations of the same additive. All samples were constructed out of 15 woven Kevlar layers with size 50 × 50 mm and thickness varying between 3.10 and 5.65 mm. Table 2 shows the description of the sample groups with their corresponding fiber, matrix, weight and additive percentage. For example, DW 3-2 means that this sample is composed of 15 Kevlar layers + VE + Al₂O₃ with 1.31 wt% and represents the statistical trial number 2.

Table 2.

Description of samples.

Sample group	Sample code	Composition	Fiber weight (g)	Matrix weight (g)	Additive in wt% of composite	Weight (g)
Control	DW 1	15 Kevlar layers + VE	78	37	0.00	115
Control	DW 2	15 Kevlar layers + VE	78	29	0.00	107
Al₂O₃	DW 3	15 Kevlar layers + VE + Al₂O₃	78	43	1.31	121
Al₂O₃	DW 4	15 Kevlar layers + VE + Al₂O₃	78	58	5.60	136
NC	DW 5	15 Kevlar layers + VE + NC	78	59	4.30	137
NC	DW 6	15 Kevlar layers + VE + NC	78	132	9.40	210
CNT	DW 7	15 Kevlar layers + VE + CNT	78	36	0.32	114
CNT	DW 8	15 Kevlar layers + VE + CNT	78	52	0.80	130
SiC	DW 9	15 Kevlar layers + VE + SiC	78	31	0.82	109
SiC	DW 10	15 Kevlar layers + VE + SiC	78	45	4.15	123

Al₂O₃: aluminum oxide; CNT: carbon nanotubes; NC: nanoclay; SiC: silicon carbide.

It can be seen from Table 2 that the resin amount needed to saturate the Kevlar layers was increased by increasing the percentage of additives; this was due to the increase in viscosity of the matrix mix when the additive percentage increased. It should be noted that the change in fiber/matrix percentage in each sample is reflected on each sample weight. The weight of the composite is, thus, important as a design parameter, especially in applications that need high strength-to-weight ratio materials. It can be noticed that the particles reinforced samples demonstrated an increase in weight, as compared to the control sample Group DW 2, varying from 2% in the composite containing 0.82 wt% SiC to 96% in the composite containing 9.4% of NC. Although the additives’ weights were very low as compared to the weight of the Kevlar and resin, the change in matrix viscosity had a considerable effect on the composite weight.

Experimental setup

The impact energy absorption characteristics of the Kevlar composites were investigated using an impact energy test. This test was carried out by utilizing a vertical drop-weight experimental setup with rails to guide the falling weight during the free fall with known height and weight. This setup, shown schematically in Figure 1, was built according to guidelines given in the ASTM-D 7136.⁶ The main advantage of such a setup is that it is unidirectional with no preferential direction for failure and can be used to test molded samples. Moreover, the samples do not have to shatter to be considered failures. These factors make falling weight testing a better simulation of functional impact exposures, and therefore closer to real-life conditions.

The developed setup is appropriate to test samples with 50 × 50 mm square dimensions with 1–10 mm thickness. Each tested sample was supported over a hollow steel chamber with an inside dimension of 50 × 50 × 10 mm. The cylindrical steel striker has 25.4 mm (one inch) diameter and 50 mm length and is allowed to fall from a height of up to 1.3 m onto the sample.

The test method adopted here determines the damage resistance of multidirectional polymer matrix composite laminated plates subjected to a drop-weight impact event. A flat, rectangular composite plate is subjected to an out-of-plane, concentrated impact using a drop-weight device with a cylindrical impactor. The potential energy of the drop-weight is specified prior to each test. The damage resistance is quantified in terms of the resulting size and type of damage in the specimen. The damage-resistance properties generated by this test method are highly dependent upon several factors including specimen geometry, layup, impactor geometry, impactor mass, impact force, impact energy and boundary conditions. Thus, the results are generally not scalable to other configurations and are particular to the combination of geometric and physical conditions tested.

The test was conducted in the following sequence. The weight was first lifted manually by the rolling handle to the desired level, the sample was placed between the fixing plates (specimen chamber), then the locking screw was placed to prevent any lateral sliding, after that the weight was released toward the sample. After the impact, the weight was lifted again to release the impacted sample, the locking screw was removed and the sample was liberated. Each test was followed by visual investigation to identify failed samples. The number of removed layers and the delamination between the woven Kevlar layers in the lateral direction was noticed and recorded. Similar procedures were adopted by Hallett and Ruiz¹⁴ to study the response of unidirectional reinforced carbon/epoxy beams consisting of alternate 0°/45° plies under low speed impact.

The control samples were used to evaluate the minimum energy required to penetrate the composite specimens. This amount of energy is considered to be a constant parameter during the entire research. The different composition composites were impacted with the same amount of energy. The height was varied and the failure mode was noticed. Complete penetration took place at heights of 850 mm and above. With a load mass of 30 kg, this is equivalent to 250 J potential energy.

Results of the impact test

The main focus was to investigate the impact response of the samples and study their failure modes. Beside the penetration resistance capability, the failure mechanism of all samples are discussed. Since the recorded data are relatively large, the detailed results will be presented here for only two representative sample groups; namely, the Control and Al₂O₃ groups. However, the results for all tested samples of all groups will be discussed and summarized. The number of samples tested for all sample groups (except DW 1) was five. It should be noted that the two control sample groups (DW 1 and DW 2) were tested under different impact energies to predict the minimum energy required to penetrate the samples. That minimum energy level was determined as 250 J and was then fixed for the rest of the samples with nanofillers.

Control samples – no reinforcement

The results of the penetration resistance capability and the failure mechanism of the different control samples are recorded in Table 3. It can be observed that DW 1-1 to DW 1-9 showed considerable variation in thickness, which was due to the manual preparation process, and inclination of one of the hot press machine plates to eliminate this variation.

Table 3.

Impact test results of the control sample group (DW 1).

Sample	Thickness (mm)	Height (cm)	Impact energy (J)	Response
DW 1-1	3.25	115	338	Complete penetration + delamination^a
DW 1-2	3.30	111	327	Complete penetration + delamination^a
DW 1-3	3.40	110	324	Complete penetration + delamination^a
DW 1-4	3.15	110	324	Complete penetration + delamination^a
DW 1-5	3.30	104	306	Complete penetration + delamination^a
DW 1-6	3.15	94	277	Complete penetration + delamination^a
DW 1-7	3.50	94	277	Complete penetration
DW 1-8	3.50	85	250	Complete penetration + delamination^a
DW 1-9	3.30	75	221	Partial penetration + delamination^a

Delamination was noticed by visual inspection.

In addition to the fiber fracture and matrix cracking, delamination was noticed in all tested samples. Such delamination is one of the most common types of damage in laminated fiber-reinforced composites due to their relatively weak interlaminar strength. The tested DW 2-1 to DW 2-5 samples showed homogeneity in thickness, which was due to the placement of the composite between two aluminum plates when compressed by the hot press machine. The results of this set are summarized in Table 4 where the delamination and fiber fracture modes in this set of samples were similar to the control sample set DW 1.

Table 4.

Impact test results of the control Samples (DW 2).

Sample	Thickness (mm)	Height (cm)	Impact energy (J)	Response
DW 2-1	3.10	89	262	Complete penetration + delamination^a
DW 2-2	3.10	87	256	Complete penetration + delamination^a
DW 2-3	3.10	86	253	Complete penetration + delamination^a
DW 2-4	3.10	85	250	Complete penetration + delamination^a
DW 2-5	3.05	83	244	Partial penetration (two plies remain) + delamination^a

Delamination was noticed by visual inspection.

Al₂O₃-reinforced samples

The first set of the Al₂O₃ group (DW 3) contained 1.31 wt% of Al₂O₃. All samples had 3.55 mm thickness. The additives percentage was low, so no increase in the penetration resistance was noticed and the impactor was able to penetrate the composite samples in all three trials. The visual inspection of the impacted samples did not show evidence of the presence of any delamination through the composite thickness as is clear in Table 5.

Table 5.

Impact test results of the aluminum oxide (1.31 wt%) samples (DW 3).

Sample	Thickness (mm)	Height (cm)	Impact energy(J)	Response
DW 3-1	3.55	85	250	Complete penetration + no delamination^a
DW 3-2	3.55	85	250	Complete penetration + no delamination^a
DW 3-3	3.55	85	250	Complete penetration + no delamination^a

Delamination was noticed by visual inspection.

The second set of this group (DW 4) contained 5.60 wt% of Al₂O₃. This relatively high percentage contributed to the impact resistance of the samples and the impactor was able to partially penetrate the composite samples in all three trials. The visual inspections of the impacted samples showed high tendency of samples to delaminate as indicated in Table 6.

Table 6.

Impact test results of the aluminum oxide (5.60 wt%) samples (DW 4).

Sample	Thickness (mm)	Height (cm)	Impact energy (J)	Response
DW 4-1	4.00	85	250	Partial penetration (7 plies removed) + delamination at different places
DW 4-2	4.00	85	250	Partial penetration (7 plies removed) + Delamination at different places^a
DW 4-3	4.00	85	250	Partial Penetration (7 plies removed) + Delamination at different places^a
DW 4-4	4.00	85	250	Partial Penetration (6 plies removed) + Delamination at different places^a

Delamination was noticed by visual inspection.

NC reinforced samples

The first set of the NC group (DW 5) contained 4.30 wt% of NC. This percentage was enough to increase the penetration resistance of the composite and the impactor was able to partially penetrate the composite samples in all trials. The visual inspection of the impacted samples did not show any evidence of delamination. The second set of this group of samples (DW 6) contained 9.40 wt% of NC. This high percentage enhanced the impact resistance of the samples in a noticeable way. In fact, the impactor was not able to penetrate the composite samples in all four trials. The visual inspections of the impacted samples showed high tendency of the samples to delaminate and showed almost complete delamination in some samples.

CNTs-reinforced samples

The first set of the CNT group (DW 7) contained 0.32 wt% of CNT. This low percentage was not able to increase the penetration resistance of the composite and, as a result, the impactor was able to penetrate the composite samples in all three trials. However, the visual inspection of the impacted samples did not show any evidence of presence of any delamination. The second set of samples of this group (DW 8) contained a percentage of 0.80 wt% of CNT, which enhanced the impact resistance of the samples in an insufficient way. But the impactor was still able to penetrate the composite samples in the four trials. The visual inspection of the impacted samples did not show evidence of presence of any delamination.

SC reinforced samples

The first set of this group (DW 9) contained a low percentage of 0.82 wt% of SiC, which was not able to increase the penetration resistance of the composite. As a result, the impactor was able to penetrate the composite samples in the three statistical trials. The visual inspection of the impacted samples did not show any evidence of presence of any delamination. The second set of samples (DW 10) contained a high percentage of 4.15 wt% of SiC, which enhanced the impact resistance of the samples in a noticeable way and the impactor was able to partially penetrate the composite samples in the four statistical trials. The visual inspection of the impacted samples did not show evidence of presence of any delamination.

Summary of the impact test results

The low speed impact test results of the four group of samples with different additive reinforcement in addition to the results of the second set of the Control group are summarized in Figure 2. This figure clearly demonstrates that the Al₂O₃ composites with 1.31 wt% did not enhance the Kevlar composite resistance, but it did change the failure mode by enhancing the lateral direction resistance since no delamination was noticed by visual inspection. Samples containing 5.6% of Al₂O₃ increased the impact resistance of the Kevlar composite to the extent that the impactor could not completely penetrate the Kevlar layers, but this was at the expense of the delamination resistance achieved by the low percentage of the Al₂O₃.

Figure 2 also shows that the NC-reinforced composite samples were the best additive as regarding the impact resistance. Samples with 4.3 wt% were able to relatively resist the impactor where only 5 out of 15 layers were removed with no delamination noticed, while samples with 9.4 wt% of NC resisted the impactor in such a way that only 2 layers were removed. This great response of energy absorption, however, was at the expense of the delamination resistance shown in the composites that contain 4.3% of NC.

CNT-reinforced composites were not able to resist the impactor penetration. This could be due to the random distribution and the low percentage of the CNT used. Although the SiC composites with 0.82 wt% were not able to enhance the Kevlar composite resistance, it changed the failure mode by enhancing the lateral direction. Therefore, no delamination was noticed by visual inspection. Samples with 4.15% of SC increased the impact resistance of the Kevlar composite so that the impactor could not penetrate more than one third of the Kevlar layers and at the same time no delamination was noticed by visual inspection.

X-ray results

An X-ray technique has been utilized to characterize the changes in the internal structure of the different Kevlar laminated composites after impact. Photos of the front and side views were taken for each of the control sample group DW 2 and the other four groups with additive reinforcement DW 3–DW 10. The X-ray films then were exposed to strong illumination source to reveal the image details, then photos were captured using 10 mega pixels digital camera.

As an illustration, the photos of the X-ray results are shown, for only the control group DW 2 and the two sets of samples of the Al₂O₃ group, in Figures 3, 4 and 5, respectively. The X-ray results of the samples of all groups are summarized, however, Figure 6 shows the delaminated percentage of composite areas. The photos in Figures 4 –6 show variation in the light intensity which is due to the delaminated areas. The ability of X-ray to penetrate empty places (delaminated) is higher than its ability in solid regions (non-delaminated). The bright areas represent the delaminated areas while the dark areas represent the remained bonded areas. One can notice considerable match between the front and side view for each sample. Image-processing software was used to modify the contrast and brightness of each photo. This allowed easy prediction of the border line around the delaminated areas.

Figure 6 demonstrates that the control samples show high tendency to delaminate where 37% of the sample area was delaminated. The delamination of the control sample started at the circular impacted disk circumference and extended to one of the sample corners. Reinforcement with 1.3 wt% of Al₂O₃ did not show any evidence of delamination by visual inspection. However, the X-ray results show a decrease in the delaminated area to 31%, while enhancement with a high percentage of Al₂O₃ (5.6 wt%) increased the delaminated area to 50% of the total area. NC with 4.3 wt% played an effective rule in delamination resistance where it can be seen that the delaminated area reduced to 22%. The increase in NC percentage to 9.4 wt% produced a contrary effect on the results and almost complete delamination happened. The X-ray results showed that 71% of the total area was deboned. Multi-walled CNT with 0.32 wt% decreased the delaminated area to 24% while 0.8 wt% did not change the delaminated percentage neither delaminated places in the control sample. SC particles with 0.82 wt% enhanced the delamination resistance to 26% instead of 37% in the control sample, while 4.15 wt% from the same material resulted 34% delamination percentage. The results of this section revealed enhancement in delamination resistance in all additives at low percentages of additives, while the high percentage aggravated the delamination in all types of additives. The NC with a low percentage (4.3 wt%) had shown the best results in delamination resistance.

Conclusions

The effect of adding small amounts of various fillers on the impact resistance of Kevlar/VE composites has been studied in this work. The research identified methods to optimize the impact resistance of polymeric composites reinforced with laminated woven Kevlar layers. It was demonstrated that the amount of the additive reinforcements exerts significant influence over the energy absorption capability of polymer-based composites. The factors contributing to the enhancement of the energy absorption and failure modes of composites, such as the type of additive and its amount, were investigated. The effect of four different additive particles, namely Al₂O₃, NC, Multiwall CNTs, and SC, was investigated. Two set of samples with low and high concentration for each additive were prepared and tested. The results were compared to control samples composed of Kevlar plies only. A vertical drop-weight experimental setup was utilized to test the prepared sample laminates.

An X-ray imaging method has also been utilized to characterize the changes in the internal structure of the different Kevlar-laminated composites after impact. Front and side views were taken for each type of Kevlar composites, the X-ray films then were exposed to strong illumination source to reveal the image details. The results revealed enhancement in the delamination resistance at low percentages of NC additives, while the high percentage of NC aggravated the composite delamination. It has been concluded that the NC fillers were the best to enhance the impact and mechanical properties of the composite materials, with 4.3 wt% of NC being the optimum percentage. Among the obtained experimental data, the NC with a low percentage (4.3 wt%) had also shown the best results in delamination resistance. The composite material forms used in this research are limited to continuous-fiber reinforced polymer matrix composites. The properties generated by the low speed impact test method are highly dependent upon several factors, which include specimen geometry, layup, impactor geometry, impactor mass, impact force, impact energy and boundary conditions. Thus, the results are generally not scalable to other configurations, and are particular to the combination of geometric and physical conditions tested.

Footnotes

Acknowledgment

This research received partial funding from the United Arab Emirates University.

Conflict of interest

None declared.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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Role of nanofillers in low speed impact enhancement of composites

Abstract

Keywords

Introduction

Materials used in preparing samples

Samples preparation

Samples description

Samples coding

Experimental setup

Results of the impact test

Control samples – no reinforcement

Al2O3-reinforced samples

NC reinforced samples

CNTs-reinforced samples

SC reinforced samples

Summary of the impact test results

X-ray results

Conclusions

Footnotes

Acknowledgment

Conflict of interest

Funding

References

Al₂O₃-reinforced samples