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Investigating the effects of UV light and moisture ingression on low-impact resistance of three different carbon fiber–reinforced composites

  • M. N. Uddin
  • J. M. George
  • V. R. Patlolla
  • R. AsmatuluEmail author
Original Research
  • 54 Downloads

Abstract

In this article, the effects of ultraviolet light (UV) and moisture exposure on the low-velocity impact behavior of three different carbon fiber (CF) laminate composites (unidirectional fibers, plain weave woven fibers, and non-crimp fibers (NCF)) are reported based on the experimental observations. The composite laminate was fabricated by vacuum bagging method following the manufacturing specifications with symmetric and asymmetric stacking, and then the test coupons were extracted for impact testing, C-scan, and surface characterization studies before and after UV light and moisture exposures. A low-velocity impact test was carried out to evaluate the damage resistance and tolerance of the laminate specimens. The test outcomes uncovered that the NCF laminates were far predominant in load-carrying capacity than the woven and unidirectional laminates, with the NCF-asymmetric (NCF-NS) laminate exhibiting the greater load-carrying capacity of 3017.7 kN/m and impact energy of 7.07 kJ/m (NCF-NS). The NCF-symmetric (NCF-S) laminate showing impact energy of 7.0 kJ/m and load-carrying capacity of 2886.8 kN/m with some decrease after UV and moisture exposure for both cases. The ultrasonic C-scan revealed that NCF laminates, both NCF-S and NCF-NS, have the least penetration indicating greater out of plane fracture toughness and damage tolerance. The NCF-NS laminate has the least damage area (33.35 mm2) and dent depth (0.12 mm) as compared with other laminate studied here. The wettability of the panels was similar; however, the woven (baseline) panel showed the highest water contact angle (112°). After the UV and salt fog exposure, the contact angles of the composite panels were reduced between 66° and 58°. In addition, this study also reveals the effect of stacking sequence on the impact properties of the NCF composite laminate.

Graphical abstract

The present study focused on the effect of UV light and moisture exposure on the impact damage resistance of three different laminate composites.

Photographic images of post-impact CF laminate composites damage area: a unidirectional, b woven, and c NCF

Keywords

Non-crimp fibers UV light Low-velocity impact test Wettability Stacking sequence Mechanical properties 

1 Introduction

Carbon fiber-reinforced polymer composites (CFRP) have been extensively used in modern light-weight structural application such as aerospace, automotive, electronics, consumer goods, sporting goods, energy, marine, infrastructure, and defense applications due to their outstanding specific strength and other properties [1, 2, 3, 4]. It can be considered that these composite materials are immune to environmental effects, but the properties of CFRP can be degraded by service environments such as exposure to water, UV light or other corrosive environments, change in temperature, and long-term physical and chemical stability. In addition, even after fabrication, the CFRP can absorb environmental species, blistering, direct chemical reaction between the components of the composite and environmental species, and degrade the properties and performance of the materials [5]. Besides, low-velocity impact usually causes internal damage to the structure, which degrades the performance of the laminated composites. It is expected that during use or assembly, composite laminate face low-velocity impact loadings. Even when the impact damage is barely visible, it can significantly lower the performance of the composite laminates in harsher environments. Therefore, researching the damage mechanisms and energy absorption during impact is critical in the design of laminated composite structures; however, this is very difficult because of their heterogeneous nature.

Various studies focusing on the impact properties of composites laminate have been reported in the literature [6, 7, 8, 9, 10, 11]. Saito and his co-workers reported the effects of ply thickness on the impact damage mechanisms in carbon fiber-reinforced polymer composites [6]. The laminates were fabricated using 38 μm thick thin-ply prepregs. Their experimental results reveal that thin-ply laminates exhibit 23% greater strength than standard-ply laminates. Besides, a significant decreased in the transverse crack and localized delamination was essentially extended in thin-ply laminates. They recommended the specific ply thickness without severe crack propagation to be less than or equal to 40 μm. Chen and Hodgkinson conducted a detailed investigation of the impact (low velocity and high velocity) behavior of two different fiber-reinforced composites, i.e., non-crimp fabric-reinforced composites and 3D-woven fabric-reinforced composites, to evaluate the damage resistance and tolerance of laminates [7]. Their results showed that 3D-woven composites have the highest damage resistance and tolerance during a low-velocity impact, while NCF-reinforced composites showed better damage resistance during a high-velocity impact. Another group investigated the effects of impact energy and stacking sequence on the damage resistance and compression after impact (CAI) strength of carbon and glass fiber reinforced hybrid laminated composites [8]. Their observation showed that hybrid laminated composites have greater structural efficiency of around 51% when subjected to a 12 J impact and 41% for those subject to an 18 J impact as compared with the regular carbon fiber-reinforced polymer composites laminates. Tanaka et al. investigated the degradation of mechanical properties of CF/polyamide composites due to water absorption [9]. A significant decrease in interfacial shear strength was observed for the absorption of water at 80 °C in the CF/PA6, and CF/PA66 composites. In addition, interfacial shear strength was also reduced for the CF/PA12 model composites because of the absorption of water. However, drying the composites in the oven restore the interfacial shear strength. Gustin and his co-workers used Kevlar/carbon hybrid fiber and fabricated sandwich composites to investigate the effects of low-velocity impact on it [10]. They reported that absorbed energy was improved by the addition of Kevlar fiber to the face sheet that is about 10% as compared with CF. Besides, about 14% improvement was observed in impact force by the addition of Kevlar/carbon hybrid fiber as compared with CF. Nevertheless, modulus of elasticity was decreased by the addition of Kevlar or hybrid fiber to the face sheet. Shyr et al. conducted a low-velocity impact test to study the damage characteristics and failure strengths of composite laminates in different fabric structures with variable thickness of the laminates [11]. They used non-crimp fabric, woven fabric, and discontinuous nonwoven mat to manufacture the composite laminate. Among these, the non-crimp fabric showed greater impact resistance.

The objective of this novel study is to investigate the low-velocity impact phenomena (at an energy level of 6.7 J and impact velocity of 2.96 m/s.) on polymeric composites laminate manufactured using different fibers architecture (e.g., unidirectional fibers, plain weave woven fibers, and NCF) and the effect of UV radiation and moisture absorption on the impact properties of the fabricated laminated composites. Energy absorption during low-velocity impact testing was evaluated for different laminated composites. The ultrasonic C scan and optical microscopy images are used to analyze the damage resistance and tolerance. Test results provide an understanding of how the fibers architecture and stacking sequence, as well as UV light and moisture absorption, can affect the low-velocity impact properties of three different composite laminate.

2 Experiment

2.1 Materials and methods

Unidirectional, plain weave woven, and NCF prepreg lamina was supplied by Cytec Industries (CYCOM® 5320–1) for fabricating the composite panels. The release agent, acetone, and NaCl were purchased from Sigma-Aldrich. These materials were utilized without any further alterations. For out-of-autoclave manufacturing, the CYCOM® 5320–1 prepreg system is most suitable. This prepreg system cures at low temperature and suitable where ease of tooling or vacuum-bag-only curing is required. This prepreg composite having low porosity and high quality is as good as autoclaved cured composites. The mechanical properties of CYCOM® 5320-1 epoxy resin prepreg system with 177 °C post-cure is like the 177 °C autoclave-cured epoxy prepreg systems. The manufacturer recommended processing parameters for this epoxy prepreg system are presented in Table 1.
Table 1

The cure cycles of the prepreg system (CYCOM® 5320-1 toughened epoxy resin prepreg)

Parameter

Cure cycle A

Cure cycle B

Ramp rate

0.6–1.7 °C/min

0.6–2.8 °C/min

Cure temperature

93 ± 6 °C

121 ± 6 °C

Cure time

12 h

3 h

Post-cure

2 h at 177 °C

2 h at 177 °C

2.2 Fabrication of composite laminates

The NCF laminate can be manufactured by various methods such as resin transfer molding (RTM), resin film infusion (RFI), and prepreging followed by autoclave curing. In this study, the vacuum bagging method was used to manufacture the four different composite laminate. The unidirectional composite laminates were fabricated following the ASTM D5687/D5687M. The number of plies was 16 with a stacking sequence of [45/0/− 45/90]. The cured thickness of the unidirectional laminated composite was 0.254 mm. The laminate layup was in such a way that the 0° fiber oriented with the long way dimension. However, in the woven fabric laminate the stacking sequence was [(+ 45/− 45)/(0/90)] followed the same ASTM standard as used in manufacturing unidirectional laminates and a cured thickness close to 0.254 mm. The designations (+ 45/− 45) and (0/90) characterize a single ply of woven fabric having warp and weft fibers oriented at the specified angles. Furthermore, the number of fabric plies, the stacking sequence and ASTM standard applied in NCF laminates (symmetric and asymmetric) were like woven fabric laminate and a cured thickness of 0.254 mm. The mold was sterilized by acetone before layup, and then a release agent was applied to the mold surface. The prepreg materials could thaw when it was removed from the freezer to reached room temperature. A moisture-proof bag was used to preserve the prepreg materials for at least 3.5 h after getting the prepreg from the freezer. After a couple of ply layup, de-bulking by means of a vacuum to remove air packs in the layups was performed to accomplish good compaction. During de-bulking, a vacuum of 635 Torr was applied for 15 min and heated to 38 °C for 10–15 min, but no more than six de-bulking at 38 °C was done on a single layup. In the bagging scheme, one layer of non-porous release fabric was applied over the prepreg layup, followed by a layer of the breather over the non-porous release film, and finally, a nylon-bagging film was applied. Then multiple vacuum connectors were attached through the bag to ensure uniform vacuum pressure was applied throughout the mold.

2.3 Sizing and curing

Inconsistent fiber arrangement will influence the properties and furthermore, the increment of the coefficient of variation. The composite laminates were cured under vacuum by a Grieve® industrial oven. The composite laminates were cured while strictly controlled time, temperature, and vacuum conditions, and it was cured at 135 °C for 2 h. The as-manufactured laminate has uniform cross-section over the entire surface and has a thickness taper lower than 0.08 mm. Furthermore, special care was taken while machining the laminated composite panels to avoid unexpected notches, uneven surfaces, fibers pullout or any delamination. The drop weight impact test specimens were cut from the large panel by 150 mm × 100 mm.

2.4 Impact, C-scan, UV, and moisture tests

Three different test methods are widely employed for impact testing of the materials, i.e., Izod, Charpy, and drop weight test. The low-velocity drop weight impact test method was used here to measure the impact and absorb energy for different laminated composites. The cured woven, NCF, and unidirectional fiber composite panels were tested with the Instron® Dynatup 8250 drop-test tower using ASTM D7136 (impact testing for composites). The impact tester has a drop-weight impactor, and its weight is 3.57 kg, kept constant for all the test and a bounce back catcher and a guide component. The indenter diameter was 25.4 mm. The composite laminates fabricated with three different types of carbon fiber were subjected to low-velocity impact energy of approximately 6.7 J. The laminate was placed horizontally on the rigid base on the specimen fixture of Dynatup 8250 Drop-weight impact tester and drop height was 435 mm, giving impact velocity of 2.96 m/s and corresponding impact energy of 6.7 J. The depth of the damaged area was measured by a depth gauge right after the impact and damage characteristics of the composite laminates were examined. The ultrasonic C-scan nondestructive tests (NDT) were carried out to see the defects in the panels such as inclusions, voids, and delamination before and after the impact tests. A standard C-scan test (Olympus sonic workstation 1000) was used to analyze the areas of the damages. Moreover, the C-scan test reveals two important information; (i) the porosity in the composite panels, and (ii) the impact behavior by analyzing the damaged area. For the UV test, ASTM D4329 and ASTM D4587 were followed to expose the test specimens to ultraviolet radiation. In the UV chamber, the fluorescent lamps emit the radiation spectrum in the ultraviolet range. Therefore, the laminated composite panels were exposed for 20 days to UV light produced by UVA-340 lamps in a chamber which is known as the QUV Accelerated Weathering Tester, manufactured by the Q-Lab. The ASTM D5229/D5229M is the preferred test standard for moisture absorption test and used for this study. After UV exposure, a salt fog chamber was used for moisture absorption (20 days) on the composites. In the salt fog chamber, 3% NaCl solution was used, and the composite panels were placed in the chamber. The impact energy is a measure of the work done to fracture a test specimen. According to ASTM D7136, the impact energy was calculated using the following equation:

$$ \kern0.75em E={C}_Eh $$
(1)
where:
E

potential energy of impactor prior to dropping J,

CE

specified ratio of impact energy to specimen thickness, 6.7 J/mm and

h

nominal thickness of specimen, mm.

2.5 Water contact angle measurements

The hydrophobicity and hydrophilicity of a solid surface are mainly determined by the water contact angle measurements. A contact angle larger than 90° indicates that the surface is hydrophobic, meaning the wettability is limited to reduce degradation by moisture. When the water contact angle is less than 90°, the surface is hydrophilic; hence, the surface is wettable, and moisture can get into structures quickly and degrade the material. An optical goniometer (KSV CAM 100) was used to measure the surface wettability of the composite panels. For measuring water contact angle, a small syringe was employed to place a small droplet of DI water (about 5 μl) on the surface of the composite panels. Two factors namely surface chemistry and surface morphology dominate the water contact angle of a material.

3 Results and discussion

3.1 Surface wettability of composite laminates

Water contact angle tests are usually used to estimate surface energy or surface wettability. The water contact angle determines whether the surface is hydrophobic or hydrophilic, wetting or non-wetting, and polar or non-polar. The hydrophobicity or hydrophilicity of surfaces depends on both their chemical composition and surface geometrical structure (micro and nanoscale roughness). A hydrophobic surface is one where the water contact angle is greater than 90o [12, 13]. A surface is hydrophilic if it tends to absorb water or be wetted by water droplets by making a static contact angle less than 90°, where the physical damage is expected to be higher on laminate and honeycomb composites [14, 15, 16]. While surface roughness and delamination in composite materials influence the dimensional condition and performance of a component, a rough or contaminated surface generally forms a high static contact angle due to the asperities present on the surface. Table 2 summarizes the water contact angle of CF laminates before and after UV and moisture treatment for 20 days. All the composite panels are hydrophobic before UV and moisture treatment, whereas after treatment, they are hydrophilic. The woven laminate has a contact angle of 112.16°, whereas NCF-NS carbon fibers exhibited a contact angle of 96.10°. The unidirectional and NCF-S laminate displayed almost similar wettability. After UV and moisture treatment of 20 days, all samples showed hydrophilic behaviors, wherein the contact angles were reduced to between 66 and 58°. This indicates that composite laminates are more hydrophilic in nature after UV and moisture treatment. The hydrophilic composite laminates are prone to degrade the mechanical properties as well as lifetime due to environmental effects.
Table 2

Water contact angle of CF composite laminates before and after UV and moisture treatments for 20 days

Specimens

Water contact angle (°) (before UV and moisture exposure)

Water contact angle (°) (after UV and moisture exposure)

Unidirectional

94.6 ± 1.66

57.8 ± 2.8

Woven

112.16 ± 2.01

58.62 ± 3.12

NCF-S

94.33 ± 1.02

63.09 ± 1.68

NCF-NS

96.10 ± 1.92

66 ± 1.9

The UV and moisture exposures showed significant surface degradation, whereby the resin disintegrated and formed voids between the fibers and the panel and formed major surface cracks. The wettability was drastically increased in all composite panels, and moisture seeped into the voids and altered the contact angle measurement. After UV and moisture exposures, the water contact angle decreased due to surface treatment and surface chemistry changes (radicalization, oxidation, and cross-linking). Figure 1 shows the unidirectional composite specimen before and after 20 days of UV and moisture exposure, clearly showing surface cracks and morphological changes on the surfaces of the composite panels.
Fig. 1

Optical microscopy images of unidirectional composite specimens: a before and b after 20 days of UV and moisture exposures on composite panels (surface cracks clearly visible on UV and moisture treated samples)

3.2 Low-velocity impact results

The impact characteristics of the fiber-reinforced composite materials depend on several factors such as matrix materials type, fibers, and type of fillers, geometry, and testing conditions. The manufactured composite panels with three different types of CF laminate were low-velocity impact tested (before and after UV and moisture exposures) and test data such as time, load, impact, and absorbed energy were recorded. It is assumed that the resin structure of fiber composites has a profound effect on the impact energy absorption of laminated composites. However, for the NCF and unidirectional laminates, the thickness, drop height, drop weight, and tup size were constant but for the woven laminates, the thickness was different than the NCF and unidirectional laminates. These laminates were manufactured according to the company specifications with various thicknesses and parameters, compared with the other manufactured laminates. The test data was normalized to compare among the four different composite panels. The representative impact load-time curves of the baseline and UV and moisture-treated CF laminates are shown in Fig. 2.
Fig. 2

Impact load-time curves for baseline and post-UV and moisture-treated CF laminates

In Fig. 2, the impact load-time curve is bell-shaped. The unidirectional and woven laminate (before and after UV and moisture exposure) exhibited a sharp load drop followed by high amplitude oscillations. The damage (delamination, interface failure, matrix cracking, elastic and plastic deformations, etc.) occurred in the laminate is represented by the oscillation in the load-time curve. More oscillations are observed in unidirectional and woven laminate before and after UV and moisture exposure. This is because of the transient through-thickness stress waves. When the impactor hit the surface, this stress waves propagate from the impactor surface contact point, and then reflect by the rear face and diminish [17]. However, NCF laminates (both NCF-NS and NCF-S) exhibit a smooth curve with little or no oscillation before and after UV and moisture exposure. This represents little damage occurring during the impact in the specimens. Besides the highest peak force was determined from each curve which is the point of maximum bending of the specimens during the impact test. NCF laminates (both NCF-NS and NCF-S) show sharp and highest peak force before and after UV and moisture exposure as compared with unidirectional and woven laminate.

In the woven composite laminates, the fabric weave facilitates the restraint of the impact of damage or delamination. However, the impact region has permanent damage. The impact load-time curves for NCF-S and NCF-NS laminates have a similar pattern. The difference between NCF-S and NCF-NS laminates is in stacking sequences of prepregs. Figure 3 compares the normalized impact load before and after UV and moisture exposure of the laminated composite. As shown in Fig. 3, the normalized impact load-carrying capacity of the UV and moisture treated specimens decreased marginally compared with the baseline specimens. The UV and moisture treatment degrade the performance of CF composites panels. Moreover, comparing the four different types of CF composite laminate, NCF laminates showed a higher impact on load-carrying capacity even after UV and moisture exposures. In addition, after UV and moisture exposure, environmental degradation was minimum in the NCF laminates compared with the other composite panels. This is because of their heterogeneous structure and multi-axial fiber placement; therefore, fibers are closely packed at the impact zone. In general, UV light and moisture exposure could reduce the impact loads of the fiber composites between 6 and 12%, which is significant for the composite aircraft.
Fig. 3

Normalized impact load before and after UV and moisture exposure of the laminated composite

The impact energy of different CF composites laminate before and after UV and moisture treatment is presented in Fig. 4. For the low-velocity impact testing, the energy and load histories provide various information regarding damage ignition and growth. The absorbed energy is the energy dissipated by different fracture mechanism such as fibers fracture, matrix cracking, plastic deformations, and core failure. In addition, absorbed energy represents the energy at the maximum load point deducted from the total energy. The energy dissipated by force oscillations due to impactor ringing such as sound, heat, and friction energy is represented by the drop from the maximum load point [18]. However, when there is no damage to the materials by the impact loading, the material behaves quasi elastically, and the impact energy is returned to the tup. As a result, the energy-time curve looks like a bell-shaped curve. Therefore, the amount of energy absorbed is negligible even though some energy is always lost in the experimental set-up. Moreover, some part of the impact energy is absorbed, and other part returned to the tup forming a smaller rebound when the material is damaged by the impact loading.
Fig. 4

The impact energy of baseline and UV and moisture-treated CF laminates

Figure 5 compares the normalize impact energy of the CF laminate before and after UV and moisture treatment. As shown in Fig. 5, before UV and moisture treatment, the NCF laminate exhibit the greater impact energy of 7.07 kJ/m (NCF-NS) and 7.0 kJ/m (NCF-S). After the UV and moisture exposure, the impact energy was reduced accounting 6.96 kJ/m (NCF-NS) and 6.94 kJ/m (NCF-S). However, the impact energy drop is quite significant for the woven and unidirectional laminates by the UV and moisture exposure. Besides, woven and unidirectional laminate has lower impact energy, indicating that unidirectional/woven laminate absorbed low energy and underwent a considerable amount of damage. Additionally, the NCF laminate has lower tensile strength compared with unidirectional laminates due to heterogeneous structure, complex geometry, and reduction in elastic modulus, interface de-lamination, out-of-plane stress, and misalignment of fibers and so on. Moreover, there was a considerable reduction in impact energies after UV and moisture exposure since a degradation environment is a function of time and temperature, which leads to voids between fibers. NCF laminates showed more damage tolerance compared to woven and unidirectional laminates.
Fig. 5

Impact energy of CF laminates before and after UV and moisture treatment

The elastic recuperation of the composite laminates before and after UV and moisture exposure is depicted in Fig. 6. From Fig. 6, it is observed that the NCF laminate both NCF-NS, and NCF-S exhibit lesser recuperation as compare with woven and unidirectional laminate. The comparison of normalized absorbed energies of different composite laminates before and after UV, and moisture exposure is summarized in Table 3. The UV and moisture exposure cause a reduction in absorbed energies for all the laminated composites studied here. These environmental factors that are introduced into the laminated composites result in breakage of the carbon-hydrogen chemical bonds, the formation of oxide species, and internal and external stresses and lowering the average molecular weight of the polymer. This development accelerates the aging process, produces fatigue cracks, and reduces the overall mechanical properties and lifetime of the composites. The organic molecules in CF laminates occupy most of the volume. The space between molecules allows larger molecules to absorb into and diffuse throughout the material. Therefore, environmental species can interact with polymer molecules, reinforcement, fibers, etc. Furthermore, most of the polymeric materials absorbed moisture, and it influences the mechanical properties of the composites. Presence of moisture lowers the strength, the modulus of elasticity, and the glass transition temperature of the polymer as well as CF-laminated composites. The UV light may accelerate these degradation reactions [19].
Fig. 6

Recuperation of composite laminates (before and after UV and moisture exposure)

Table 3

The normalized absorbed energy of CF laminates before and after UV and moisture treatment

Specimens

Absorbed energy (kJ/m) (before UV and

moisture exposure)

Absorbed energy (kJ/m) (after UV and

moisture exposure)

% reduction

Unidirectional

1.43

1.13

20.98

Woven

2.09

1.72

17.70

NCF-S

3.75

3.71

1.06

NCF-NS

4.33

4.27

1.39

Before UV and moisture exposure, the NCF-NS shows the higher absorbed energy of 4.33 kJ/m followed by NCS-S of 3.75 kJ/m. The woven and unidirectional laminates absorbed the energy of 2.09 kJ/m and 1.43 kJ/m respectively. This exhibits that the NCF-NS laminate has higher damage tolerance than the woven and unidirectional laminates. On the other hand, the unidirectional laminate has the lowest absorbed energy indicating that the unidirectional laminate suffered most by the impact loading. However, the reduction in absorbed energies of the NCF laminates before and after UV, and moisture treatment was quite insignificant. The percentage reduction of absorbed energies before and after UV and moisture-treatment for the unidirectional and woven laminates was 20.98% and 17.70% respectively which also indicates that unidirectional laminate is the least energy-absorbed panel followed by the woven laminate. The impact and absorb energy were reduced by UV and moisture treatment. The NCF laminate demonstrated more damage tolerance compared with the woven and unidirectional laminates.

3.3 Damage analysis of the composite panels

Ultrasonic inspection is the most efficient method used for quality control and quality inspection of materials in the composite industry [20]. The ultrasonic C-scan technique is used to characterize artificial delamination, detect impact damage, characterize the distribution, size, and shape of voids in composite materials and reveal some special features of the fiber/matrix interface [21, 22, 23]. In the case of low-velocity impact, the damage is hidden inside the composite structure. This impact caused various damages such as matrix cracking, delamination, de-bonding, surface splitting, and fibers breakage causing a reduction in strength and performance of the composite materials. Figure 7 illustrates the pre-impact and post-impact C-scan images of the three different CF laminates. The damaged zone is represented by the dark black region at the center of each image. The C-scan images provide the damaged areas, and the dent depth was measured using a depth gauge instantaneously after the impact. The characteristics of out-of-plane drop-weight impact loading on a laminate panel depend on several factors such as impactor geometry and impactor mass, impact energy and velocity, wave propagation, and vibrations in the specimen, and support fixture during the impact occurrence, laminate thickness, ply thickness, stacking sequence, environment, geometry, and boundary conditions, etc. In Fig. 7, it is observed that the NCF composite panels have the least damage areas, followed by the woven laminates. When compare with unidirectional laminates, the NCF composite panel exhibits higher damage tolerance under impact loading. Moreover, in woven laminate, the fibers are placed one over another, and there is no reinforcement along the thickness direction. While damage upon impact is stress concentrated, and the fibers are unable to carry the distributed load, unlike NCF laminates, which can accommodate the distribution of load produced by impact along their multi-axial fibers and reinforcements, damage can be visually seen over a large area of the impacted unidirectional and woven laminate. The damaged area in the unidirectional laminate shows an elliptical shape with the shape of the impactor and the major axis parallel with the fiber orientation. However, in the NCF laminate, the stitching matrix restricted the growth of delamination. The C-scan images of UV and moisture treated after impact NCF (both NCF-NS and NCF-S) composite panels were presented in Fig. 8. Here, we can see that NCF-NS and NCF-S laminates have the greater capability of damage recovery compared with baseline woven panel. It is observed that woven laminate has superior impact resistance than unidirectional laminate. The dent values of the composite laminate before and after UV and moisture exposure changed because of the UV, and moisture exposure causes possible resin degradation because of the dent-relaxation phenomenon.
Fig. 7

Ultrasonic C-scan images of before and after impact CF laminated composites, (top row) unidirectional; (middle row) woven; (bottom row) NCF

Fig. 8

C-scan images of UV and moisture treated after impact a NCF-AS and b NCF-S-laminated composites

The photographic images of the damaged panels are presented in Fig. 9. The unidirectional laminate displayed a weak impact resistance compared with woven and NCF laminates. Besides, in the unidirectional laminate, the presence of damage, such as dents, depressions, and a combination of splits/cracks, fiber breakage, and material puncture is obviously noticeable. Besides, surface ply is splitting with some associated delamination. However, the woven laminate showed good resistance to impact, exhibiting dents and depressions with only slight splits or cracks and delamination.
Fig. 9

Photographic images of post-impact CF laminate composites damage area, a unidirectional, b woven, and c NCF

The average damage area and dent depth of the four different composite panels are summarized in Table 4. The NCF-NS panel exhibits the least damaged area as well as dent depth followed by NCF-S showing superior impact damage resistance. However, the unidirectional laminated panel has greater damage area and dent depth displaying weaker impact damage resistance as compared with other composite panels studied here. However, the NCF laminate demonstrated a superior impact response, indicating bruised surfaces with negligent depressions, splits, or fiber breakage. Post-UV and moisture treated panels also presented similar damage, whereby the dent depth may reduce with time or upon exposure to different environmental conditions.
Table 4

Average damage area and dent depth of different composite panels

Composite panel

Average damage area

(mm2)

Average dent depth (mm)

NCF-NS

33.35

0.12

NCF-S

48.58

0.16

Woven

86.44

0.61

Uni

176.19

1.32

In general, UV light has two entities (rays and photons), in which the rays act like the knife to cut the polymer (matrix) chains, while photons have full of energy to donate them to the broken chains. This process (also called the aging process) primarily causes oxidation and molecular weight changes, where surface chemistry and energy of the fiber composites are completely altered. High surface energy surface absorbs more water, becomes brittle, and thus weakening the composite structures in the long run. In addition to the types of prepreg composites and stacking sequences, the UV light and moisture exposures also have significant effects on the impact damages of the composite panels.

4 Conclusions

The four different laminated composites were prepared, and low-velocity impact tests were performed by falling drop weight at a velocity of 2.96 m/s and corresponding impact energy of approximately 6.7 J. The effects of UV light and moisture absorption on the impact resistance of four distinctive CF reinforced composites were investigated. It can be concluded that both NCF-NS and NCF-S laminates are least affected by UV light and moisture absorption in comparison to the other composite panels studied here. Prior to UV and moisture exposure, all-composite laminate samples displayed approximately similar impact energy. However, the absorbed energy values were significantly varied in the composite samples. The NCF laminates showed a higher load-carrying capacity (NCF-NS 3017.7 kN/m and NCF-S 2886.8 kN/m) and absorbed energy (NCF-NS 4.33 kJ/m and NCF-S 3.75 kJ/m) before UV and moisture exposure. After UV and moisture exposure, these properties are degraded marginally. The impact damage is less in the NCF laminate followed by the woven laminates. However, the NCF laminates (both NCF-AS and NCF-S) exhibit greater capability of impact damage tolerance. Moreover, the woven laminate showed greater hydrophobicity with a contact angle of 112.16°, whereas the NCF-NS laminate showed a contact angle of 96.1°. The unidirectional and NCF-S laminates have similar wetting properties. Following UV and moisture treatment, all samples showed reduced water contact angles because of the change in composite surface chemistry, which can drastically reduce the composite lifetime.

Notes

Acknowledgments

The authors gratefully acknowledge Wichita State University and National Institute for Aviation Research for technical and financial support of the present research studies.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Department of Mechanical EngineeringWichita State UniversityWichitaUSA

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