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Low-velocity impact behavior of intralayer hybrid composites based on carbon and glass non-crimp fabric
Journal Pre-proofs Low-Velocity Impact Behavior of Intralayer Hybrid Composites Based on Carbon and Glass Non-Crimp Fabric Chen Zhang, Yunfei Rao, Wei Li PII: DOI: Reference:
S0263-8223(19)32828-4 https://doi.org/10.1016/j.compstruct.2019.111713 COST 111713
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
27 July 2019 8 October 2019 18 November 2019
Please cite this article as: Zhang, C., Rao, Y., Li, W., Low-Velocity Impact Behavior of Intralayer Hybrid Composites Based on Carbon and Glass Non-Crimp Fabric, Composite Structures (2019), doi: https://doi.org/10.1016/ j.compstruct.2019.111713
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© 2019 Published by Elsevier Ltd.
Low-Velocity Impact Behavior of Intralayer Hybrid Composites Based on Carbon and Glass Non-Crimp Fabric Chen Zhang a, b, Yunfei Rao a, b, Wei Li a, b, c, * a
College of Textiles, Donghua University, NO.2999, Northern RenminRd, Songjiang District Shanghai 201620, China b Center for Civil Aviation Composites, NO.2999, Northern RenminRd, Songjiang District Shanghai 201620, China c Key Lab of Textile Science & Technology, Ministry of Education, NO.2999, Northern RenminRd, Songjiang District Shanghai 201620, China *Corresponding author E-mail address: [email protected]; Tel.: +86-137-6402-2421
Abstract: The low-velocity impact behavior of intralayer hybrid composite laminates made from warp-knitted fabrics with carbon (C) and glass (G) fibers was investigated. The impact testing at varying energy levels was performed using drop weight impact tests. The impact response was evaluated in terms of peak force and absorbed energy, and damage mechanism were characterized by visual observations and fluorescent dye penetration. Furthermore, the damage modes was assessed through quasi-static indentation (QSI) and acoustic emission (AE) techniques. The results from this study indicated that the hybrid structure exhibited a positive hybrid effect in peak force and the absorbed energy, and failure mechanism were significantly affected by the hybrid ratio and intralayer structure. The intralayer hybrid laminate of C:G = 1:1 revealed better impact resistance, and that of C:G = 1:4 exhibited less damage accompanied by the least carbon fiber content, however, more fiber damage appeared in C:G=1:2 laminates. Keywords: Carbon/glass hybrid composites; intralayer hybrid; low-velocity impact behavior; quasi static indention; acoustic emission; nondestructive testing
1. Introduction Composite materials exhibit superior properties primarily owing to their high specific stiffness and strength, which are used to fabricate the primary load bearing structures of Airbus and Boeing aircraft in the last few decades [1]. However, they are susceptible to low-velocity impact loads such as bird strikes or tool drops during service and can result in barely visible impact damage (BVID) [2]. BVID cannot be detected from outside the laminates but can lead to matrix cracking, delamination, and fiber fractures, particularly in composites made of unidirectional fabric, which will cause a substantial reduction in load-bearing capacity and structural integrity of laminates [3]. The impact resistance of composites can be enhanced by improving fiber/matrix interface adhesion, matrix modification, and fiber hybridization [4-6]. Hybrid composites that are created by three or more components may be the most practical composites in the industry, such as wind blade and helmet manufacturing [7]. They can offer a wide range of mechanical properties that cannot be
achieved by one type of fiber. The hybrid structure primarily includes interlayer and intralayer hybrids, the interlayer hybrids consist of stacking layers of the two fiber types onto each other and the intralayer configuration consists of two fiber types that are mixed on the fiber level, typically as co-mingled yarns [8]. The impact properties of hybrid composites can be affected by several factors, such as fiber orientation (hybrid structure), fiber volume (hybrid ratio), and fiber type. For the same volume fraction of the fiber constituents, the impact properties may vary with the different stacking sequence of the fiber layers [9-11]. Jesthi et al. investigated the stacking sequence on impact properties of carbon/glass hybrid composites after seawater absorption. The results indicated that the hybrid composite of [GCG2C]S type exhibited the highest impact strength owing to the stiffer carbon layers adjacent to the glass layers and decreased by 2.9% after seawater aging [12]. Benli and Sayman [13] studied the effect of stacking sequence under low-velocity impact and discovered that it demonstrated a catastrophic effect in terms of damage area and energy absorption. Sevkat et al. [14] reported that the impact properties of a glass/ graphite hybrid composite can be improved when the glass layer is placed on the outer side; however, this increased the laminate weight. Several researchers have studied the effect of hybrid ratio under low-velocity impact. Ismail et al. [15] reported that hybrid composites with a weight percentage ratio of 25% kenaf fiber and 75% glass fiber exhibited better impact properties. Dehkordi et al. [16-17] studied basalt/nylon intralayer hybrid composites and found that at low impact energy, the basalt/nylon fiber content does not affect the impact performance of hybrid composites. With increasing impact energy, the impact performance becomes increasingly dependent on the hybrid ratio. However, damages during impact experiments cannot be detected easily owing to the short test time, it was experimentally proven that damage occurring in composites due to quasi-static indentation (QSI) is similar to that produced by low-velocity impact (LVI) [18-19]. Many researchers have investigated low-velocity impact and equivalent quasi-static indentation loading [20-22]. For example, Castellano [23] compared the differences and similarities between QSI and LVI damage in unidirectional glass fiber-reinforced polymer composites by continuum damage mechanics theory. Kumar [24] studied the low-velocity impact behavior of glass/carbon/basalt hybrid composite laminates by performing QSI tests, the results indicated no distinct difference between low-velocity impact tests and quasi-static indentation tests. Moreover, static test methods provide more data regarding damage initiation and propagation with high accuracy compared with dynamic impact tests [25-26]. Additionally, non-destructive techniques have been widely utilized to elucidate damage evolution in composites, such as shearography, ultrasonic, and acoustic emission (AE) monitoring [27]. The acoustic emission is known as the ultrasonic elastic wave excitated by various fracture in the materials, which may be used for monitoring the damage modes and crack progression of composite materials in real time [28-30]. In addition, many researchers have demonstrated that the AE signals can be used to distinguish different damage mechanism, such as matrix cracking, delamination, and fiber breakage, particularly for amplitude signals. Andreikiv et al. [31] simulated low-velocity impact test results using QSI test with AE monitoring to analyze damage initiation and progression. Fotouhi et al. [32] used a criterion based on amplitude and energy of the AE event signals to investigate the damage modes occurring in thin-ply UD carbon/glass hybrid laminates. Carbon fiber and glass fiber are two typical raw materials for fabricating hybrid composites. The addition of glass fiber has been shown to increase impact resistance in carbon fiber composites owing to its different stiffness and strength properties [33]. Meanwhile, studies on the effect of an intralayer hybrid structure on the impact properties of carbon/glass hybrid composites are scarce. The primary objective of this study is to investigate the low-velocity impact behavior of
carbon/glass intralayer hybrid composite, with three hybrid ratios of non-crimp carbon and glass fabrics. Experimental results from the low-velocity impact tests were studied through the damage characteristics in addition to using force–time and energy–time curves. Furthermore, QSI tests and AE signals were used to evaluate the failure mechanism of different interlayer hybrid laminates. 2. Materials and test methods
2.1 Materials The composite laminates investigated in this study are fabricated with unidirectional carbon fabric, glass fabric and three types of C/G intralayer hybrid fabric, as shown in Table 1. The carbon fiber is provided by TORAY Inc. The glass fiber is from CPIC Inc., and the epoxy resin is provided by SWANCOR Inc. The structures of the intralayer hybrid fabrics are shown in Figure 1.
Fabric Type Carbon Glass C-G C-G-G C-G-G-G
Carbon Fiber 728.3 0 364.2 242.8 145.7
Areal Density (g/m2) Glass Fiber 0 944.9 472.4 629.9 755.9
Ratio of C/G (Hybrid Ratio) 1:0 0:1 1:1 1:2 1:4
Table 1. Hybrid fabric specifications.
5 mm
5 mm
5 mm
5 mm
5 mm
CF
GF
CF
GF
GF
a.
C-G
b.
5 mm
5 mm
CF
GF
C-G-G
c.
Figure 1. Schematic structure of three interlayer hybrid fabrics.
Table 2. Stacking configurations of hybrid structures.
Stacking sequences
Nomenclature
C
G
5 mm
5 mm
5 mm
GF
GF
GF
C-G-G-G-G
Stacking sequences
Nomenclature
1-1
1-2
1-4
2.2 Laminates The laminate was laid-up with 4 layers in the [0/45/-45/90] sequences. The composite laminates yielded a nominal thickness of 3.2 mm. Table 2 lists the specimen specifications and nomenclature. The composite laminates were prepared using the vacuum-assisted resin transfer molding (VARTM) process to maintain the fiber volume fraction at 50% [34-36], and the curing condition was 60 °C for 7 h. 2.3 Experiments The low-velocity impact tests were performed on INSTRON Dynatup 9250HV. The samples were cut into 100 mm × 150 mm. The specimens were cut into 150 *100 mm using a jet cutting machine and were clamped on a rigid rectangular platform with an inner/outer diameter of 75/125 mm, and they were restrained by four corner rubber-tipped clamps to prevent slipping during impact test (Figure 2(a)), and the impact energies was design to 20, 30, and 40 J respectively, according to ASTM-D7136. Quasi-static indentation tests were performed in the universal testing machine, the sample size was maintained at 150 mm × 150 mm as per ASTM-6264, and the indentation tests were performed with the velocity of 2 mm/min to analyze the failure process of the intralayer hybrid laminates. A 12.7-mm diameter hemisphere impactor was used throughout the QSI and LVI tests. Five specimens were tested for each configuration sample. An AE monitoring device (Physical Acoustics Corporation) was used to monitor the real-time dynamic damage changes during each QSI test (Figure 2(b)). The two AE transducers were placed 60 mm apart to reduce the attenuation of waves originating from the source. High-sealant vacuum grease (silicon grease) was used as the coupling agent between the sensor and laminates to improve the acoustic coupling between them. The amount of PDT (Peak Definition Time), HDT (Hit Definition Time), and HLT (Hit Lockout Time) was set to 80, 600 and 600 μm, respectively. To remove background noise, 45 dB was considered as the threshold. The AE software for acquisition processing was Micro-II Digital AE System, and the AE events recorded by both sensors were utilized for data processing. The internal damage of the specimens was studied using the C-scan method, (Figure 2(c)), and scanning electron microscopy (SEM) was used to detect smaller damages. The cross-section damage on the specimen was investigated using fluorescent dye penetration through optical microscopy. Zyglo penetrant was used to penetrate the sample for tracing the crack path [37-38].
(a)
(b)
(c)
Figure 2. (a) INSTRON Dynatup 9250HV, (b) QSI and AE system, (c) C-scan instrument.
3. Results and Discussion 3.1 Dynamic response of composite plates under low-velocity impact Figure 3 shows the impact force-displacement histories of five laminates under low-velocity impact at three energy levels, 20J, 30J and 40J. The general trend of the dynamic response is similar for different laminates made of pure or hybrid fabrics. Furthermore, the displacement soars with the increasing impact energy. The initial slope decreases as the glass fiber content increases, this behavior is attributed to the lower stiffness of the glass fiber compared with that of carbon fiber. The load drop or change in slope of the force-displacement curve may indicate nominal stiffness reduction owing to the various damage modes. For a low-impact energy (20J), indentation and slight matrix crack were occurred and the level of curve fluctuation on the loading phase was similar to that on the unloading phase (shown in Figure 3(a)). However, no hysteresis is found in the force-displacement curves under this impact energy except for the C and 1-2 laminates, indicating there is energy absorption through local damage in these two laminates. At higher impact energies (shown in Figure 3(b) and (c)), other damage mechanism were activated. For 40J energy impact, the fluctuation curve appeared after the peak point with longer unloading phase. While the curve experienced shorter fluctuation phase before the peak point on 30J energy impact. It can be assumed that some fiber failure occurred on the 40J energy impact and with impact energy increased, the catastrophic damage started earlier. And it is noteworthy that for the C and 1-2 laminates, a higher curve fluctuation occurred compared to other laminates during every impact event, this could contribute to more fiber fracture and delamination. In addition, the G and 1-4 laminates exhibited analogous curve profiles owing to their close glass fiber volume fraction.
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Figure 3. Force–displacement curves under three impact energy levels (a) 20J, (b), 30J, (c) 40J.
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Figure 4. Fmax of various laminates.
Fmax (peak load) represents the maximum load that a composite laminate can bear before experiencing major damages during the impact, as shown in Figure 4. As stated previously [39], the peak load increase with impact energy. The G laminates exhibit the highest peak load and the C laminates show a decreasing trend with higher impact energy. This is due to the carbon fiber composites is more brittle and less flexible than glass fiber composites, the carbon fiber composites are able to maintain the structural integrity under low impact energy (20 J), however, the fiber in C laminates broken earlier than other laminates as the impact energy increased, the carbon fiber damage can be found in three intralayer hybrid composites as well, while less fiber broken were found in G laminates and the damage will discuss in the next phase. On the other hand, the intralayer hybrid laminates have higher peak load than C laminates under 40J impact energy. As the result of the carbon fiber modulus is higher than that of glass fiber, and the elongation fracture of glass fiber is two times higher than that of carbon fiber. This combination leads to a positive hybrid synergistic effect to improve the impact resistance and the peak force can be improved by this structure. Nevertheless, the peak load exhibits a nonlinear relationship with the hybrid ratio. There is less carbon fiber in 1-4 laminates, so the glass fiber play a major role to undertake the impact force during tests, resulting in the higher peak load than 1-2 laminates. And the most amount of hybrid interfaces in 1-1 laminates leading to a strong hybrid synergistic effect, which is beneficial to prevent the damage growth and resulting in a higher peak force than 1-4 laminates. 3.2. Energy absorbed through damage mechanism Figure 5 shows the energy-time histories of five laminates under three impact energies and the data were obtained from the Instron Dynatup 9250HV. The impact energies can be divided into two parts [40]. Like the C laminates in Figure 5(a), the elastic energy (Eel) returned to the impactor and caused it to rebound as the remaining energy was absorbed by the laminates, this can be represented by the resulting plateau (Ea) in the energy-time curve and is primarily dissipated through laminate damage, the maximum impact energy (Emax) experienced by the composite, which is the combination of Eel and Ea, and the E (t) is the energy absorbed by the composite at any time “t”. The schematic diagram of the
energy-time plot can calculate using equation. (1), where, “V (t)”, “Vi”, “m”, “g” and “ ” are velocity of the impactor at a time “t”, initial velocity, the mass of the impactor, the gravitational acceleration and the deflection, respectively[41]. Furthermore, the values of absorbed energy can be obtained by calculating the enclosed area in the load- displacement charts mentioned previously. It can be seen in the Figure 5, the slope of curve of C laminates is the highest while the G laminates is the lowest, and that value of intralayer hybrid laminates lie between these two structures. The extent of curve dropping became less with increasing impact energy, specifically, the C and 1-2 laminates exhibited a non-elastic energy curve at 40 J impact energy, implying that the impact energy is nearly equal to the absorbed energy and that the laminate has been penetrated [42], this energy is refer to the perforation energy (Eperf). （1）
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Et
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Figure 5. Energy-time curves under three impact energy levels (a) 20J, (b) 30J, (c) 40J.
C 1-1 1-2 1-4 G
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C 1-1 1-2 1-4 G
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Figure 6. Absorbed energy (Ea) of various laminates.
The value of dissipated energy was calculated from the history of time-energy curve, as illustrated in Figure 6, the percentage of the Ea increases when increasing the impact energy and the absorbed energy for various laminates exhibit the same trend at each impact energy. The C laminates have the highest absorbed energy, and the 1-1 and 1-2 laminates dissipated almost the same level of energy that is higher than that of 1-4 and G laminates. Additionally, while the G laminates have the minimum absorbed energy. This phenomenon indicates that the internal damage of the C laminates are more
12
14
severe under the same impact energy, followed by those of 1-2 and 1-1 laminates, and the last is the G and 1-4 laminates. In addition, for 20 J impact energy, G and 1-4 laminates have the nearly same absorbed energy, which is due to the similar glass fiber volume fraction of these two structures, whereas at the 30J and 40J impact energy, 1-4 laminates reach a higher level of Ea than G laminates, indicating the 1-4 laminates may suffer more damage than G laminates under higher impact energy. Compared to the G laminates, the energy absorption capabilities of the intralayer hybrid composite laminates were, on average, 10% (1-1), 11% (1-2), and 4% (1-4) higher. 3.3 Indentation The indentation depths were measured at once after impact and subsequently plotted against the impact energy, as shown in Figure 7. A growth trend is shown with the increase in impact energy. It is noteworthy that, for all the intralayer hybrid laminates, the indentation depths are shallower than those of the C laminates. This phenomenon is attributed to the enhanced bond strength and toughness between the layers by intralayer hybrid structure, however, the depths do not follow a linear behavior with the hybrid ratio, and the 1-4 laminates exhibit a similar trend as that of the G laminates, like in absorbed energy field. The indentation depths of the 1-2 laminates with higher absorbed energy values were deeper than those of other intralayer hybrid laminates. Meanwhile, the indentation depths of the C, 1-1, and 1-2 laminates increased rapidly under 40 J impact energy, revealing a critical damage may occur under this impact energy. 5
C 1-1 1-2 1-4 G
Indention Depths (mm)
4
3
2
1
0 20J
30J
40J
Impact energy (J)
Figure 7. Indention depths vs. impact energy during LVI.
4. Analysis of damage mechanism As the damage was barely identified under 20J impact, C-scan results were used to investigate the delamination, as shown in Figure 8. All of the laminates exhibited a peanut pattern with the long axis directed along the fiber orientation of the non-impacted face (defined as back face). The 1-4 laminates displayed a similar damage shape as that of the G laminates, furthermore, a “tail” shape in the C and 1-2 laminates were observed, indicating some carbon fiber debonding in the back face. Additionally, the delamination shape can be analyzed by damaged laminates and C-scan images, the progress is illustrated in Figure 9(a): the laminates were divided into four plies, with each exhibiting the same stacking angle, sequence, and position corresponding to the real four fabric. Subsequently, the C-scan results were cut into the same scale as that of the damaged laminate, drawing the delamination profile along its edges. Finally, combining the picture of the four plies structure and the delamination profile, a
“perspective picture” was obtained. Therefore, the propagation of stress waves and the size of damage area can be discussed based on this method. As shown in Figure 9(b), the delamination growth stopped at the hybrid boundary, particularly at 90 ° direction, indicating the hybrid interface have positive effect to prevent stress wave propagated. The 1-4 laminates own the least hybrid interface with the highest delamination height (24 mm) among the intralayer hybrid laminates, and the 1-1 laminates have the most hybrid interfaces with the smallest size of delamination (15mm*28mm). Furthermore, in the 1-1 and 1-2 laminates, the two sides of the damage profile were sharper than those in the 1-4 laminates. This “triangle” mode (as shown in 1-2) is attributed to the more hybrid interfaces along the ±45° directions, while the stress wave has more opportunities to propagate at ±45° directions for1-4 laminates, leading to a smooth profile of delamination image. The damage area from the C-scan results are shown in Figure 10, the delamination area could be decreased by intralayer structures, and that increased with more glass fiber content. The 1-1 laminates show the least delamination area, while the C laminates have the biggest delamination area, more than twice as much as that of 1-1 laminates. Nevertheless, the absorbed energy exhibited a nonlinear relation with the delamination area, suggesting there are other failures affecting the absorbed energy. 0°
90° C
1-1
1-2
1-4
Figure 8. C-scan results of each laminates after 20J impact.
C G （a）
G
1-1
1-2
1-4
（b） Figure 9. (a) Analysis of delamination area, (b) perspective picture of hybrid laminates.
Damage area (mm2)
800
600
400
200
0 C
1-1
1-2
1-4
G
Figure 10. Damage area from the C-scan of each laminates under 20J impact.
The damage surfaces in front and back sides of the samples under 30J and 40J impact energy were visualized, as presented in Table 2, the damage feature is helpful to explain the different peak load of various laminates. The broken fiber on both sides of the plates represented by the dotted yellow line, and the delamination on the back side can be identified via the red line. Table 2. Photographs of damaged samples on the contact side and back side after (a) 30 J, (b) 40 J impact. (a) 30 J
Front
Back
(b) 40J
Front
Back
sample
C
1-1
1-2
1-4
The unidirectional plies exhibited good mechanical properties in the direction of the fibers, but
G
were weaker in the transverse direction [43]. Hence, a large delamination was resulted after the impact. As shown in Table 2, the C laminates exhibited the largest damage accompanied by stripped carbon fibers on the rear side and the longitudinal fracture can be observed at the bottom carbon fiber. However, just few fibers near the indentation broken in G laminates, leading to the highest peak load. As the impact energy increased, the carbon fiber started separating from the bottom layer in the C laminates (shown in Table 2(b)), and the intralayer hybrid laminates exhibited less carbon fiber separation damage due to the hybrid synergistic effect. The damage morphology of the intralayer hybrid laminates exhibited the similar pattern, such as the carbon fiber broken at the bottom ply while the glass fiber shown better structural integrity with less damage. It is noticeable, the stress waves stopped at the hybrid interface, which is consistent with the result of C scan analysis, that is due to the hybrid interface have contributed to a “fence” effect to prevent the stress waves from spreading, reducing the fiber broken and the delamination damage. Therefore, there are less visualized damage in 1-1 laminates among other intralayer hybrid laminates. Meanwhile, the large amount of matrix cracking on the back side of the 1-2 laminates was obvious, and the 1-4 laminates displayed a similar delamination mode to that of the G laminates attribute to the close glass fiber volume fraction. Furthermore, as shown in Table 3, the damage patterns and the extent of damage were identified from the dye penetration tests through the cross section of the damaged laminates, the damage mode were marked by red label. For the 30J impact energy, severe damage are shown in the C and 1-2 laminates with a large fiber split on the back face. The large scale delamination and fiber broken under the impactor were observed in C laminates, which is consistent with the largest delamination area from C scan result. Table 3. Cross section of each laminate after impact, viewed under a microscope. (a) 30J
C
1-1
1-2
1-4
G
(b) 40J
In addition, severe damage are shown in the 1-2 laminates with large fiber split on the back face compared with 1-1 and 1-4 laminates, in order to investigate the damage mode in 1-2 laminates, the fractured region at the front face of the 1-2 laminates was shown by the scanning electron microscopy (SEM) results (Figure 11), the results displayed a large amount of fiber fractures and the crack propagated through the thickness by cutting the fiber, moreover, matrix-fiber debonding, fiber pull out, and delamination between the hybrid interfaces were observed. Hence, the C and 1-2 laminates were associated with more absorbed energy compared with other laminates. The 1-1 laminates exhibited wide and multilayer delamination areas with fewer broken fibers at the back face, and the glass fiber was protected by the surrounding carbon fibers, leading to the high peak force. As for the 1-4 and G laminates, a similar damage mode with many delamination were found owing to the similar glass fiber volume fraction, nevertheless, carbon fiber-matrix debonding appeared in the back face of the 1-4 laminates, resulting in the higher absorbed energy and lower peak load compared with the G laminates. The damage modes were similar under two impact energy, as can be seen in Table 3(b), the deformation and broken fiber were more serious under 40J impact energy. And the indentation depth were more noticeable in C, 1-1 and 1-2 laminates compared with 1-4 and G laminates, which were consistent with the changing trend in indentation characteristics. In addition, the penetration feature were shown in C and 1-2 laminates, correspond to the no dropping curve in time-energy results.
Figure 11. SEM images of 1-2 laminates under 30J impact.
In order to investigate the damage process of the intralayer hybrid laminate, QSI with AE tests were performed on the laminate. The displacement parameter of QSI tests were set to 8mm, 9mm and 12mm corresponding to maximum displacement measured under 20J, 30J and 40J impact energy [44-45], and the cross section of laminates under three displacements were revealed by dye penetration tests as shown in Table 4. The AE signal was measured by the data acquisition system in real time, and clustering analysis was performed using multivariable principal component analysis (PCA), while the optimum number of clusters were estimated by the Daviese-Bouldin index [46]. Furthermore, the amplitude was classified by the K-means algorithm into three classes. Only mathematical considerations were applied in this classification to obtain the best separation of the AE data, while the physics were not considered.
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Figure 12. Distribution of amplitude versus times of AE signal clustering under QSI tests for all laminates.
Matrix cracking is characterized by an amplitude in the range of 45-52 dB. Delamination is characterized by an amplitude between 53 and 63 dB. The fiber pull-out and fiber breakage exhibit an amplitude range of 64–100 dB. Figure 12 illustrates the distribution of the amplitude of the AE signals versus time during the tests, and the evolution of the applied load was superposed in the same plot. During the QSI tests, matrix cracking was the first damage mechanism that appeared in the chronology and it propagated until the specimen’s final failure. Subsequently, delamination appeared after matrix cracking, followed by fiber pull-out and fiber breakage. As depicted in time-load curves, the 1-2 laminates exhibited longer contact duration at the fluctuation phase compared with other intralayer hybrid laminates. Conversely, the 1-4 laminates showed the shortest fluctuation phase with a “cliff” load descent trend resembling that of the C laminates. The 1-1 laminates show a stable curve with few fluctuations until the broken down, indicating this structure have a good potential to prevent the damage growth. This difference may cause by diverse damage mode. Figure 13 presents the ratio of cumulative counts for each damage mode on different laminates to describe the percentage of every damage event [47]. A greater ratio of broken fiber was shown in the C and G laminates, while less broken fibers was shown in the intralayer hybrid laminates indicating a positive hybrid effect. It is noteworthy that the 1-2 laminates exhibited more fiber damage compared with the 1-1 and 1-4 laminates.
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Matrix Cracking Delamination Fiber Damage
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C 80 60 40
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Figure 13. Percentage of cumulative number of AE counts under QSI tests for all specimens.
Furthermore, the contribution of each damage mechanism to global failure was evaluated for the different hybrid ratio. For each damage mechanism, the damage contribution ( ) is obtained by the ratio of the cumulative AE energy of the given mechanism ( ) by the cumulative AE signal energy at failure (
) [48-49], as shown in equation (2), where i = matrix cracking, delamination, fiber damage
and n is the total damage mechanism number, n = 3. （2） Figure 14 presents the relative contribution to the final failure of each damage mechanism. As shown, the most significant energy mechanism is fiber breakage, it constitutes more than 80% for the C and G laminates. For the intralayer hybrid structure, the 1-1 and 1-4 laminates exhibited a high percentage of delamination and relatively lower ratio of fiber fracture. Although the cumulative number of AE counts for delamination is nearly the same as that of fiber broken in the 1-4 laminates, delamination contributed less to the damage of laminates. In addition, the
of fiber pull-out and fiber
breakage was greater in the 1-2 laminates, indicating that the 1-2 laminates contained more absorbed energy among the three intralayer hybrid materials during the tests. 100
Matrix Cracking
Delamination
Fiber Damage
Damage contribution (%)
80
60
40
20
0 C
1-1
1-2
1-4
G
Figure 14. Damage mechanism contribution of five laminates. Table 4. Cross section of hybrid laminates after QSI tests, (a) 1-1 laminates, (b) 1-2 laminates, (c) 1-4 laminates.
(a)
8mm
12mm 9mm
Displacement
(b)
(c)
8 mm
9 mm
12 mm
This consequence can be illustrated by the cross section profile, as shown in Table 4. The crack paths in the delaminated samples were traced using Adobe Photoshop CS6 to map out a skeleton of the cracks [50]. The delamination and fiber broken are distinguished by the red and blue areas, respectively. More delamination and fiber broken were found under deeper displacement. As it shown, the failure process of the 1-1 laminates primarily included the multilayer delamination and fiber broken on the upper side of the laminates. For the 1-2 laminates, the fiber fracture appeared earlier (in 9 mm) with the carbon fiber broken under the impactor in contrast with 1-1 and 1-4 laminates and the glass fiber exhibited a similar damage trend under the LVI tests. On the other hand, the carbon fiber on the bottom layer and near the impactor of the 1-4 laminates revealed the fiber broken features, as shown by the blue region, and the multilayer delamination were observed like 1-1 laminates. In general, the intralayer hybrid laminates demonstrated high potential in decreasing failure propagation, specifically, the 1-1 and 1-4 laminates exhibited less fiber cracks in comparison with the 1-2 laminates. 5. Conclusions
The low-velocity impact behavior of intralayer hybrid composite laminates was investigated. Additionally, the damage modes and failure process of the laminates were determined by QSI and AE tests. The primary conclusions from this study are summarized as follows: The intralayer hybrid structure exhibited a positive effect on the low-velocity impact owing to the hybrid effect. The 1-2 intralayer structure exhibited a relatively low peak force and high absorbed energy, while the 1-1 and 1-4 intralayer structures exhibited higher peak forces with lower absorbed energy. The C-scan and SEM results and the cross section of damaged laminates were used to investigate the impact damage. The C and 1-2 laminates exhibited more fiber damages that were related to more absorbed energy, meanwhile, the 1-4 laminates revealed a similar damage mode to that of the G laminates as a result of their close glass fiber volume fraction. The delamination area exhibited a growing trend with a decrease in carbon content, while the 1-1 laminate exhibited the least delamination area owing to more hybrid interface. The damage mode on different intralayer hybrid was studied by QSI and AE tests. The AE signal indicated a good relation to the damage process, and three types of damage cluster were used to describe the damage process. According to the cumulative counts of these damage modes, the intralayer hybrid exhibited a positive effect in decreasing the fibers broken and delamination damage. Furthermore, the fiber failure occupied larger ratio of damage contribution in 1-2 laminates compared with other intralayer hybrid laminates per the
theory, while the 1-1 and 1-4 laminates revealed
fewer broken fibers by the cross section images. In conclusion, the low-velocity impact property and crack resistance could be improved by the intralayer hybrid structure, and was highly influenced by the hybrid ratio. The 1-1 laminates demonstrated the better impact resistance among the intralayer hybrid structures owing to more hybrid interface, and the 1-4 laminates exhibited a smaller delamination area and lower damage degree by the least amount of carbon fiber, while more fiber broken may happen in 1-2 laminates. Therefore, choosing an appropriate hybrid ratio in intralayer hybrid structures is necessary for optimizing the impact property.
Conflict of Interest The authors declared that they have no conflicts of interest to this work.
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S0263-8223(19)32828-4 https://doi.org/10.1016/j.compstruct.2019.111713 COST 111713
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
27 July 2019 8 October 2019 18 November 2019
Please cite this article as: Zhang, C., Rao, Y., Li, W., Low-Velocity Impact Behavior of Intralayer Hybrid Composites Based on Carbon and Glass Non-Crimp Fabric, Composite Structures (2019), doi: https://doi.org/10.1016/ j.compstruct.2019.111713
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Low-Velocity Impact Behavior of Intralayer Hybrid Composites Based on Carbon and Glass Non-Crimp Fabric Chen Zhang a, b, Yunfei Rao a, b, Wei Li a, b, c, * a
College of Textiles, Donghua University, NO.2999, Northern RenminRd, Songjiang District Shanghai 201620, China b Center for Civil Aviation Composites, NO.2999, Northern RenminRd, Songjiang District Shanghai 201620, China c Key Lab of Textile Science & Technology, Ministry of Education, NO.2999, Northern RenminRd, Songjiang District Shanghai 201620, China *Corresponding author E-mail address: [email protected]; Tel.: +86-137-6402-2421
Abstract: The low-velocity impact behavior of intralayer hybrid composite laminates made from warp-knitted fabrics with carbon (C) and glass (G) fibers was investigated. The impact testing at varying energy levels was performed using drop weight impact tests. The impact response was evaluated in terms of peak force and absorbed energy, and damage mechanism were characterized by visual observations and fluorescent dye penetration. Furthermore, the damage modes was assessed through quasi-static indentation (QSI) and acoustic emission (AE) techniques. The results from this study indicated that the hybrid structure exhibited a positive hybrid effect in peak force and the absorbed energy, and failure mechanism were significantly affected by the hybrid ratio and intralayer structure. The intralayer hybrid laminate of C:G = 1:1 revealed better impact resistance, and that of C:G = 1:4 exhibited less damage accompanied by the least carbon fiber content, however, more fiber damage appeared in C:G=1:2 laminates. Keywords: Carbon/glass hybrid composites; intralayer hybrid; low-velocity impact behavior; quasi static indention; acoustic emission; nondestructive testing
1. Introduction Composite materials exhibit superior properties primarily owing to their high specific stiffness and strength, which are used to fabricate the primary load bearing structures of Airbus and Boeing aircraft in the last few decades [1]. However, they are susceptible to low-velocity impact loads such as bird strikes or tool drops during service and can result in barely visible impact damage (BVID) [2]. BVID cannot be detected from outside the laminates but can lead to matrix cracking, delamination, and fiber fractures, particularly in composites made of unidirectional fabric, which will cause a substantial reduction in load-bearing capacity and structural integrity of laminates [3]. The impact resistance of composites can be enhanced by improving fiber/matrix interface adhesion, matrix modification, and fiber hybridization [4-6]. Hybrid composites that are created by three or more components may be the most practical composites in the industry, such as wind blade and helmet manufacturing [7]. They can offer a wide range of mechanical properties that cannot be
achieved by one type of fiber. The hybrid structure primarily includes interlayer and intralayer hybrids, the interlayer hybrids consist of stacking layers of the two fiber types onto each other and the intralayer configuration consists of two fiber types that are mixed on the fiber level, typically as co-mingled yarns [8]. The impact properties of hybrid composites can be affected by several factors, such as fiber orientation (hybrid structure), fiber volume (hybrid ratio), and fiber type. For the same volume fraction of the fiber constituents, the impact properties may vary with the different stacking sequence of the fiber layers [9-11]. Jesthi et al. investigated the stacking sequence on impact properties of carbon/glass hybrid composites after seawater absorption. The results indicated that the hybrid composite of [GCG2C]S type exhibited the highest impact strength owing to the stiffer carbon layers adjacent to the glass layers and decreased by 2.9% after seawater aging [12]. Benli and Sayman [13] studied the effect of stacking sequence under low-velocity impact and discovered that it demonstrated a catastrophic effect in terms of damage area and energy absorption. Sevkat et al. [14] reported that the impact properties of a glass/ graphite hybrid composite can be improved when the glass layer is placed on the outer side; however, this increased the laminate weight. Several researchers have studied the effect of hybrid ratio under low-velocity impact. Ismail et al. [15] reported that hybrid composites with a weight percentage ratio of 25% kenaf fiber and 75% glass fiber exhibited better impact properties. Dehkordi et al. [16-17] studied basalt/nylon intralayer hybrid composites and found that at low impact energy, the basalt/nylon fiber content does not affect the impact performance of hybrid composites. With increasing impact energy, the impact performance becomes increasingly dependent on the hybrid ratio. However, damages during impact experiments cannot be detected easily owing to the short test time, it was experimentally proven that damage occurring in composites due to quasi-static indentation (QSI) is similar to that produced by low-velocity impact (LVI) [18-19]. Many researchers have investigated low-velocity impact and equivalent quasi-static indentation loading [20-22]. For example, Castellano [23] compared the differences and similarities between QSI and LVI damage in unidirectional glass fiber-reinforced polymer composites by continuum damage mechanics theory. Kumar [24] studied the low-velocity impact behavior of glass/carbon/basalt hybrid composite laminates by performing QSI tests, the results indicated no distinct difference between low-velocity impact tests and quasi-static indentation tests. Moreover, static test methods provide more data regarding damage initiation and propagation with high accuracy compared with dynamic impact tests [25-26]. Additionally, non-destructive techniques have been widely utilized to elucidate damage evolution in composites, such as shearography, ultrasonic, and acoustic emission (AE) monitoring [27]. The acoustic emission is known as the ultrasonic elastic wave excitated by various fracture in the materials, which may be used for monitoring the damage modes and crack progression of composite materials in real time [28-30]. In addition, many researchers have demonstrated that the AE signals can be used to distinguish different damage mechanism, such as matrix cracking, delamination, and fiber breakage, particularly for amplitude signals. Andreikiv et al. [31] simulated low-velocity impact test results using QSI test with AE monitoring to analyze damage initiation and progression. Fotouhi et al. [32] used a criterion based on amplitude and energy of the AE event signals to investigate the damage modes occurring in thin-ply UD carbon/glass hybrid laminates. Carbon fiber and glass fiber are two typical raw materials for fabricating hybrid composites. The addition of glass fiber has been shown to increase impact resistance in carbon fiber composites owing to its different stiffness and strength properties [33]. Meanwhile, studies on the effect of an intralayer hybrid structure on the impact properties of carbon/glass hybrid composites are scarce. The primary objective of this study is to investigate the low-velocity impact behavior of
carbon/glass intralayer hybrid composite, with three hybrid ratios of non-crimp carbon and glass fabrics. Experimental results from the low-velocity impact tests were studied through the damage characteristics in addition to using force–time and energy–time curves. Furthermore, QSI tests and AE signals were used to evaluate the failure mechanism of different interlayer hybrid laminates. 2. Materials and test methods
2.1 Materials The composite laminates investigated in this study are fabricated with unidirectional carbon fabric, glass fabric and three types of C/G intralayer hybrid fabric, as shown in Table 1. The carbon fiber is provided by TORAY Inc. The glass fiber is from CPIC Inc., and the epoxy resin is provided by SWANCOR Inc. The structures of the intralayer hybrid fabrics are shown in Figure 1.
Fabric Type Carbon Glass C-G C-G-G C-G-G-G
Carbon Fiber 728.3 0 364.2 242.8 145.7
Areal Density (g/m2) Glass Fiber 0 944.9 472.4 629.9 755.9
Ratio of C/G (Hybrid Ratio) 1:0 0:1 1:1 1:2 1:4
Table 1. Hybrid fabric specifications.
5 mm
5 mm
5 mm
5 mm
5 mm
CF
GF
CF
GF
GF
a.
C-G
b.
5 mm
5 mm
CF
GF
C-G-G
c.
Figure 1. Schematic structure of three interlayer hybrid fabrics.
Table 2. Stacking configurations of hybrid structures.
Stacking sequences
Nomenclature
C
G
5 mm
5 mm
5 mm
GF
GF
GF
C-G-G-G-G
Stacking sequences
Nomenclature
1-1
1-2
1-4
2.2 Laminates The laminate was laid-up with 4 layers in the [0/45/-45/90] sequences. The composite laminates yielded a nominal thickness of 3.2 mm. Table 2 lists the specimen specifications and nomenclature. The composite laminates were prepared using the vacuum-assisted resin transfer molding (VARTM) process to maintain the fiber volume fraction at 50% [34-36], and the curing condition was 60 °C for 7 h. 2.3 Experiments The low-velocity impact tests were performed on INSTRON Dynatup 9250HV. The samples were cut into 100 mm × 150 mm. The specimens were cut into 150 *100 mm using a jet cutting machine and were clamped on a rigid rectangular platform with an inner/outer diameter of 75/125 mm, and they were restrained by four corner rubber-tipped clamps to prevent slipping during impact test (Figure 2(a)), and the impact energies was design to 20, 30, and 40 J respectively, according to ASTM-D7136. Quasi-static indentation tests were performed in the universal testing machine, the sample size was maintained at 150 mm × 150 mm as per ASTM-6264, and the indentation tests were performed with the velocity of 2 mm/min to analyze the failure process of the intralayer hybrid laminates. A 12.7-mm diameter hemisphere impactor was used throughout the QSI and LVI tests. Five specimens were tested for each configuration sample. An AE monitoring device (Physical Acoustics Corporation) was used to monitor the real-time dynamic damage changes during each QSI test (Figure 2(b)). The two AE transducers were placed 60 mm apart to reduce the attenuation of waves originating from the source. High-sealant vacuum grease (silicon grease) was used as the coupling agent between the sensor and laminates to improve the acoustic coupling between them. The amount of PDT (Peak Definition Time), HDT (Hit Definition Time), and HLT (Hit Lockout Time) was set to 80, 600 and 600 μm, respectively. To remove background noise, 45 dB was considered as the threshold. The AE software for acquisition processing was Micro-II Digital AE System, and the AE events recorded by both sensors were utilized for data processing. The internal damage of the specimens was studied using the C-scan method, (Figure 2(c)), and scanning electron microscopy (SEM) was used to detect smaller damages. The cross-section damage on the specimen was investigated using fluorescent dye penetration through optical microscopy. Zyglo penetrant was used to penetrate the sample for tracing the crack path [37-38].
(a)
(b)
(c)
Figure 2. (a) INSTRON Dynatup 9250HV, (b) QSI and AE system, (c) C-scan instrument.
3. Results and Discussion 3.1 Dynamic response of composite plates under low-velocity impact Figure 3 shows the impact force-displacement histories of five laminates under low-velocity impact at three energy levels, 20J, 30J and 40J. The general trend of the dynamic response is similar for different laminates made of pure or hybrid fabrics. Furthermore, the displacement soars with the increasing impact energy. The initial slope decreases as the glass fiber content increases, this behavior is attributed to the lower stiffness of the glass fiber compared with that of carbon fiber. The load drop or change in slope of the force-displacement curve may indicate nominal stiffness reduction owing to the various damage modes. For a low-impact energy (20J), indentation and slight matrix crack were occurred and the level of curve fluctuation on the loading phase was similar to that on the unloading phase (shown in Figure 3(a)). However, no hysteresis is found in the force-displacement curves under this impact energy except for the C and 1-2 laminates, indicating there is energy absorption through local damage in these two laminates. At higher impact energies (shown in Figure 3(b) and (c)), other damage mechanism were activated. For 40J energy impact, the fluctuation curve appeared after the peak point with longer unloading phase. While the curve experienced shorter fluctuation phase before the peak point on 30J energy impact. It can be assumed that some fiber failure occurred on the 40J energy impact and with impact energy increased, the catastrophic damage started earlier. And it is noteworthy that for the C and 1-2 laminates, a higher curve fluctuation occurred compared to other laminates during every impact event, this could contribute to more fiber fracture and delamination. In addition, the G and 1-4 laminates exhibited analogous curve profiles owing to their close glass fiber volume fraction.
8
8
6
4
（ c） 40J
C 1-1 1-2 1-4 G
10
Force (KN)
4
（ b） 30J
C 1-1 1-2 1-4 G
8
Force (KN)
6
Force (KN)
（ a） 20J
C 1-1 1-2 1-4 G
6
4
2
2
0
2
0
0 0
2
4
6
Displacement (mm)
8
0
2
4
6
8
10
0
2
4
Displacement (mm)
Figure 3. Force–displacement curves under three impact energy levels (a) 20J, (b), 30J, (c) 40J.
6
8
10
Displacement (mm)
12
14
C 1-1 1-2 1-4 G
Fmax (KN)
10
8
6
20J
30J
40J
Impact energy (J)
Figure 4. Fmax of various laminates.
Fmax (peak load) represents the maximum load that a composite laminate can bear before experiencing major damages during the impact, as shown in Figure 4. As stated previously [39], the peak load increase with impact energy. The G laminates exhibit the highest peak load and the C laminates show a decreasing trend with higher impact energy. This is due to the carbon fiber composites is more brittle and less flexible than glass fiber composites, the carbon fiber composites are able to maintain the structural integrity under low impact energy (20 J), however, the fiber in C laminates broken earlier than other laminates as the impact energy increased, the carbon fiber damage can be found in three intralayer hybrid composites as well, while less fiber broken were found in G laminates and the damage will discuss in the next phase. On the other hand, the intralayer hybrid laminates have higher peak load than C laminates under 40J impact energy. As the result of the carbon fiber modulus is higher than that of glass fiber, and the elongation fracture of glass fiber is two times higher than that of carbon fiber. This combination leads to a positive hybrid synergistic effect to improve the impact resistance and the peak force can be improved by this structure. Nevertheless, the peak load exhibits a nonlinear relationship with the hybrid ratio. There is less carbon fiber in 1-4 laminates, so the glass fiber play a major role to undertake the impact force during tests, resulting in the higher peak load than 1-2 laminates. And the most amount of hybrid interfaces in 1-1 laminates leading to a strong hybrid synergistic effect, which is beneficial to prevent the damage growth and resulting in a higher peak force than 1-4 laminates. 3.2. Energy absorbed through damage mechanism Figure 5 shows the energy-time histories of five laminates under three impact energies and the data were obtained from the Instron Dynatup 9250HV. The impact energies can be divided into two parts [40]. Like the C laminates in Figure 5(a), the elastic energy (Eel) returned to the impactor and caused it to rebound as the remaining energy was absorbed by the laminates, this can be represented by the resulting plateau (Ea) in the energy-time curve and is primarily dissipated through laminate damage, the maximum impact energy (Emax) experienced by the composite, which is the combination of Eel and Ea, and the E (t) is the energy absorbed by the composite at any time “t”. The schematic diagram of the
energy-time plot can calculate using equation. (1), where, “V (t)”, “Vi”, “m”, “g” and “ ” are velocity of the impactor at a time “t”, initial velocity, the mass of the impactor, the gravitational acceleration and the deflection, respectively[41]. Furthermore, the values of absorbed energy can be obtained by calculating the enclosed area in the load- displacement charts mentioned previously. It can be seen in the Figure 5, the slope of curve of C laminates is the highest while the G laminates is the lowest, and that value of intralayer hybrid laminates lie between these two structures. The extent of curve dropping became less with increasing impact energy, specifically, the C and 1-2 laminates exhibited a non-elastic energy curve at 40 J impact energy, implying that the impact energy is nearly equal to the absorbed energy and that the laminate has been penetrated [42], this energy is refer to the perforation energy (Eperf). （1）
25
Emax
30
Elastic Energy=Eel
Et
10
20 15
(c) 40J
C 1-1 1-2 1-4 G
40
Energy (J)
Energy (J)
25
15
（ b） 30J
C 1-1 1-2 1-4 G
30
Eperf 20
10
Disspated = Ea
10
5
5 0
0
0 0
2
4
6
8
10
12
14
0
2
Time (ms)
4
6
8
10
12
0
14
2
4
6
8
10
Time (ms)
Time (ms)
Figure 5. Energy-time curves under three impact energy levels (a) 20J, (b) 30J, (c) 40J.
C 1-1 1-2 1-4 G
40
30
Ea (J)
Energy (J)
20
50
35
（ a） 20J
C 1-1 1-2 1-4 G
20
10 20J
30J
40J
Impact energy (J)
Figure 6. Absorbed energy (Ea) of various laminates.
The value of dissipated energy was calculated from the history of time-energy curve, as illustrated in Figure 6, the percentage of the Ea increases when increasing the impact energy and the absorbed energy for various laminates exhibit the same trend at each impact energy. The C laminates have the highest absorbed energy, and the 1-1 and 1-2 laminates dissipated almost the same level of energy that is higher than that of 1-4 and G laminates. Additionally, while the G laminates have the minimum absorbed energy. This phenomenon indicates that the internal damage of the C laminates are more
12
14
severe under the same impact energy, followed by those of 1-2 and 1-1 laminates, and the last is the G and 1-4 laminates. In addition, for 20 J impact energy, G and 1-4 laminates have the nearly same absorbed energy, which is due to the similar glass fiber volume fraction of these two structures, whereas at the 30J and 40J impact energy, 1-4 laminates reach a higher level of Ea than G laminates, indicating the 1-4 laminates may suffer more damage than G laminates under higher impact energy. Compared to the G laminates, the energy absorption capabilities of the intralayer hybrid composite laminates were, on average, 10% (1-1), 11% (1-2), and 4% (1-4) higher. 3.3 Indentation The indentation depths were measured at once after impact and subsequently plotted against the impact energy, as shown in Figure 7. A growth trend is shown with the increase in impact energy. It is noteworthy that, for all the intralayer hybrid laminates, the indentation depths are shallower than those of the C laminates. This phenomenon is attributed to the enhanced bond strength and toughness between the layers by intralayer hybrid structure, however, the depths do not follow a linear behavior with the hybrid ratio, and the 1-4 laminates exhibit a similar trend as that of the G laminates, like in absorbed energy field. The indentation depths of the 1-2 laminates with higher absorbed energy values were deeper than those of other intralayer hybrid laminates. Meanwhile, the indentation depths of the C, 1-1, and 1-2 laminates increased rapidly under 40 J impact energy, revealing a critical damage may occur under this impact energy. 5
C 1-1 1-2 1-4 G
Indention Depths (mm)
4
3
2
1
0 20J
30J
40J
Impact energy (J)
Figure 7. Indention depths vs. impact energy during LVI.
4. Analysis of damage mechanism As the damage was barely identified under 20J impact, C-scan results were used to investigate the delamination, as shown in Figure 8. All of the laminates exhibited a peanut pattern with the long axis directed along the fiber orientation of the non-impacted face (defined as back face). The 1-4 laminates displayed a similar damage shape as that of the G laminates, furthermore, a “tail” shape in the C and 1-2 laminates were observed, indicating some carbon fiber debonding in the back face. Additionally, the delamination shape can be analyzed by damaged laminates and C-scan images, the progress is illustrated in Figure 9(a): the laminates were divided into four plies, with each exhibiting the same stacking angle, sequence, and position corresponding to the real four fabric. Subsequently, the C-scan results were cut into the same scale as that of the damaged laminate, drawing the delamination profile along its edges. Finally, combining the picture of the four plies structure and the delamination profile, a
“perspective picture” was obtained. Therefore, the propagation of stress waves and the size of damage area can be discussed based on this method. As shown in Figure 9(b), the delamination growth stopped at the hybrid boundary, particularly at 90 ° direction, indicating the hybrid interface have positive effect to prevent stress wave propagated. The 1-4 laminates own the least hybrid interface with the highest delamination height (24 mm) among the intralayer hybrid laminates, and the 1-1 laminates have the most hybrid interfaces with the smallest size of delamination (15mm*28mm). Furthermore, in the 1-1 and 1-2 laminates, the two sides of the damage profile were sharper than those in the 1-4 laminates. This “triangle” mode (as shown in 1-2) is attributed to the more hybrid interfaces along the ±45° directions, while the stress wave has more opportunities to propagate at ±45° directions for1-4 laminates, leading to a smooth profile of delamination image. The damage area from the C-scan results are shown in Figure 10, the delamination area could be decreased by intralayer structures, and that increased with more glass fiber content. The 1-1 laminates show the least delamination area, while the C laminates have the biggest delamination area, more than twice as much as that of 1-1 laminates. Nevertheless, the absorbed energy exhibited a nonlinear relation with the delamination area, suggesting there are other failures affecting the absorbed energy. 0°
90° C
1-1
1-2
1-4
Figure 8. C-scan results of each laminates after 20J impact.
C G （a）
G
1-1
1-2
1-4
（b） Figure 9. (a) Analysis of delamination area, (b) perspective picture of hybrid laminates.
Damage area (mm2)
800
600
400
200
0 C
1-1
1-2
1-4
G
Figure 10. Damage area from the C-scan of each laminates under 20J impact.
The damage surfaces in front and back sides of the samples under 30J and 40J impact energy were visualized, as presented in Table 2, the damage feature is helpful to explain the different peak load of various laminates. The broken fiber on both sides of the plates represented by the dotted yellow line, and the delamination on the back side can be identified via the red line. Table 2. Photographs of damaged samples on the contact side and back side after (a) 30 J, (b) 40 J impact. (a) 30 J
Front
Back
(b) 40J
Front
Back
sample
C
1-1
1-2
1-4
The unidirectional plies exhibited good mechanical properties in the direction of the fibers, but
G
were weaker in the transverse direction [43]. Hence, a large delamination was resulted after the impact. As shown in Table 2, the C laminates exhibited the largest damage accompanied by stripped carbon fibers on the rear side and the longitudinal fracture can be observed at the bottom carbon fiber. However, just few fibers near the indentation broken in G laminates, leading to the highest peak load. As the impact energy increased, the carbon fiber started separating from the bottom layer in the C laminates (shown in Table 2(b)), and the intralayer hybrid laminates exhibited less carbon fiber separation damage due to the hybrid synergistic effect. The damage morphology of the intralayer hybrid laminates exhibited the similar pattern, such as the carbon fiber broken at the bottom ply while the glass fiber shown better structural integrity with less damage. It is noticeable, the stress waves stopped at the hybrid interface, which is consistent with the result of C scan analysis, that is due to the hybrid interface have contributed to a “fence” effect to prevent the stress waves from spreading, reducing the fiber broken and the delamination damage. Therefore, there are less visualized damage in 1-1 laminates among other intralayer hybrid laminates. Meanwhile, the large amount of matrix cracking on the back side of the 1-2 laminates was obvious, and the 1-4 laminates displayed a similar delamination mode to that of the G laminates attribute to the close glass fiber volume fraction. Furthermore, as shown in Table 3, the damage patterns and the extent of damage were identified from the dye penetration tests through the cross section of the damaged laminates, the damage mode were marked by red label. For the 30J impact energy, severe damage are shown in the C and 1-2 laminates with a large fiber split on the back face. The large scale delamination and fiber broken under the impactor were observed in C laminates, which is consistent with the largest delamination area from C scan result. Table 3. Cross section of each laminate after impact, viewed under a microscope. (a) 30J
C
1-1
1-2
1-4
G
(b) 40J
In addition, severe damage are shown in the 1-2 laminates with large fiber split on the back face compared with 1-1 and 1-4 laminates, in order to investigate the damage mode in 1-2 laminates, the fractured region at the front face of the 1-2 laminates was shown by the scanning electron microscopy (SEM) results (Figure 11), the results displayed a large amount of fiber fractures and the crack propagated through the thickness by cutting the fiber, moreover, matrix-fiber debonding, fiber pull out, and delamination between the hybrid interfaces were observed. Hence, the C and 1-2 laminates were associated with more absorbed energy compared with other laminates. The 1-1 laminates exhibited wide and multilayer delamination areas with fewer broken fibers at the back face, and the glass fiber was protected by the surrounding carbon fibers, leading to the high peak force. As for the 1-4 and G laminates, a similar damage mode with many delamination were found owing to the similar glass fiber volume fraction, nevertheless, carbon fiber-matrix debonding appeared in the back face of the 1-4 laminates, resulting in the higher absorbed energy and lower peak load compared with the G laminates. The damage modes were similar under two impact energy, as can be seen in Table 3(b), the deformation and broken fiber were more serious under 40J impact energy. And the indentation depth were more noticeable in C, 1-1 and 1-2 laminates compared with 1-4 and G laminates, which were consistent with the changing trend in indentation characteristics. In addition, the penetration feature were shown in C and 1-2 laminates, correspond to the no dropping curve in time-energy results.
Figure 11. SEM images of 1-2 laminates under 30J impact.
In order to investigate the damage process of the intralayer hybrid laminate, QSI with AE tests were performed on the laminate. The displacement parameter of QSI tests were set to 8mm, 9mm and 12mm corresponding to maximum displacement measured under 20J, 30J and 40J impact energy [44-45], and the cross section of laminates under three displacements were revealed by dye penetration tests as shown in Table 4. The AE signal was measured by the data acquisition system in real time, and clustering analysis was performed using multivariable principal component analysis (PCA), while the optimum number of clusters were estimated by the Daviese-Bouldin index [46]. Furthermore, the amplitude was classified by the K-means algorithm into three classes. Only mathematical considerations were applied in this classification to obtain the best separation of the AE data, while the physics were not considered.
10000
8000 90
6000
70 4000 60
4000
G
80
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70 2000
Energy (ms*mv)
80
Amplitude (dB)
C Load (N)
Amplitude(dB)
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Matrix Cracking Delamination Fiber Damage
100
Load (N)
Matrix Cracking Delamination Fiber Damage
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60
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1000
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50
0 0
50
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300
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Time (s)
Time (s)
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Time (s)
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1500 50
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4500 80
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1-2
0
50
100
150
200
250
300
350
400
40
0 0
50
100
Time (s)
150
200
Time (s)
Figure 12. Distribution of amplitude versus times of AE signal clustering under QSI tests for all laminates.
Matrix cracking is characterized by an amplitude in the range of 45-52 dB. Delamination is characterized by an amplitude between 53 and 63 dB. The fiber pull-out and fiber breakage exhibit an amplitude range of 64–100 dB. Figure 12 illustrates the distribution of the amplitude of the AE signals versus time during the tests, and the evolution of the applied load was superposed in the same plot. During the QSI tests, matrix cracking was the first damage mechanism that appeared in the chronology and it propagated until the specimen’s final failure. Subsequently, delamination appeared after matrix cracking, followed by fiber pull-out and fiber breakage. As depicted in time-load curves, the 1-2 laminates exhibited longer contact duration at the fluctuation phase compared with other intralayer hybrid laminates. Conversely, the 1-4 laminates showed the shortest fluctuation phase with a “cliff” load descent trend resembling that of the C laminates. The 1-1 laminates show a stable curve with few fluctuations until the broken down, indicating this structure have a good potential to prevent the damage growth. This difference may cause by diverse damage mode. Figure 13 presents the ratio of cumulative counts for each damage mode on different laminates to describe the percentage of every damage event [47]. A greater ratio of broken fiber was shown in the C and G laminates, while less broken fibers was shown in the intralayer hybrid laminates indicating a positive hybrid effect. It is noteworthy that the 1-2 laminates exhibited more fiber damage compared with the 1-1 and 1-4 laminates.
250
300
Energy (ms*mv）
1-1
Matrix Cracking Delamination Fiber Damage
100
6000
Load (N)
90
Load (N)
Amplitude (dB)
90
Matrix Cracking Delamination Fiber Damage
6000
Energy（ ms*mv）
Matrix Cracking Delamination Fiber Damage
100
Matrix Cracking Delamination Fiber Damage
C 80 60 40
G
1-1
20 0
1-4
1-2
Figure 13. Percentage of cumulative number of AE counts under QSI tests for all specimens.
Furthermore, the contribution of each damage mechanism to global failure was evaluated for the different hybrid ratio. For each damage mechanism, the damage contribution ( ) is obtained by the ratio of the cumulative AE energy of the given mechanism ( ) by the cumulative AE signal energy at failure (
) [48-49], as shown in equation (2), where i = matrix cracking, delamination, fiber damage
and n is the total damage mechanism number, n = 3. （2） Figure 14 presents the relative contribution to the final failure of each damage mechanism. As shown, the most significant energy mechanism is fiber breakage, it constitutes more than 80% for the C and G laminates. For the intralayer hybrid structure, the 1-1 and 1-4 laminates exhibited a high percentage of delamination and relatively lower ratio of fiber fracture. Although the cumulative number of AE counts for delamination is nearly the same as that of fiber broken in the 1-4 laminates, delamination contributed less to the damage of laminates. In addition, the
of fiber pull-out and fiber
breakage was greater in the 1-2 laminates, indicating that the 1-2 laminates contained more absorbed energy among the three intralayer hybrid materials during the tests. 100
Matrix Cracking
Delamination
Fiber Damage
Damage contribution (%)
80
60
40
20
0 C
1-1
1-2
1-4
G
Figure 14. Damage mechanism contribution of five laminates. Table 4. Cross section of hybrid laminates after QSI tests, (a) 1-1 laminates, (b) 1-2 laminates, (c) 1-4 laminates.
(a)
8mm
12mm 9mm
Displacement
(b)
(c)
8 mm
9 mm
12 mm
This consequence can be illustrated by the cross section profile, as shown in Table 4. The crack paths in the delaminated samples were traced using Adobe Photoshop CS6 to map out a skeleton of the cracks [50]. The delamination and fiber broken are distinguished by the red and blue areas, respectively. More delamination and fiber broken were found under deeper displacement. As it shown, the failure process of the 1-1 laminates primarily included the multilayer delamination and fiber broken on the upper side of the laminates. For the 1-2 laminates, the fiber fracture appeared earlier (in 9 mm) with the carbon fiber broken under the impactor in contrast with 1-1 and 1-4 laminates and the glass fiber exhibited a similar damage trend under the LVI tests. On the other hand, the carbon fiber on the bottom layer and near the impactor of the 1-4 laminates revealed the fiber broken features, as shown by the blue region, and the multilayer delamination were observed like 1-1 laminates. In general, the intralayer hybrid laminates demonstrated high potential in decreasing failure propagation, specifically, the 1-1 and 1-4 laminates exhibited less fiber cracks in comparison with the 1-2 laminates. 5. Conclusions
The low-velocity impact behavior of intralayer hybrid composite laminates was investigated. Additionally, the damage modes and failure process of the laminates were determined by QSI and AE tests. The primary conclusions from this study are summarized as follows: The intralayer hybrid structure exhibited a positive effect on the low-velocity impact owing to the hybrid effect. The 1-2 intralayer structure exhibited a relatively low peak force and high absorbed energy, while the 1-1 and 1-4 intralayer structures exhibited higher peak forces with lower absorbed energy. The C-scan and SEM results and the cross section of damaged laminates were used to investigate the impact damage. The C and 1-2 laminates exhibited more fiber damages that were related to more absorbed energy, meanwhile, the 1-4 laminates revealed a similar damage mode to that of the G laminates as a result of their close glass fiber volume fraction. The delamination area exhibited a growing trend with a decrease in carbon content, while the 1-1 laminate exhibited the least delamination area owing to more hybrid interface. The damage mode on different intralayer hybrid was studied by QSI and AE tests. The AE signal indicated a good relation to the damage process, and three types of damage cluster were used to describe the damage process. According to the cumulative counts of these damage modes, the intralayer hybrid exhibited a positive effect in decreasing the fibers broken and delamination damage. Furthermore, the fiber failure occupied larger ratio of damage contribution in 1-2 laminates compared with other intralayer hybrid laminates per the
theory, while the 1-1 and 1-4 laminates revealed
fewer broken fibers by the cross section images. In conclusion, the low-velocity impact property and crack resistance could be improved by the intralayer hybrid structure, and was highly influenced by the hybrid ratio. The 1-1 laminates demonstrated the better impact resistance among the intralayer hybrid structures owing to more hybrid interface, and the 1-4 laminates exhibited a smaller delamination area and lower damage degree by the least amount of carbon fiber, while more fiber broken may happen in 1-2 laminates. Therefore, choosing an appropriate hybrid ratio in intralayer hybrid structures is necessary for optimizing the impact property.
Conflict of Interest The authors declared that they have no conflicts of interest to this work.
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