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Experimental and numerical investigation of interply hybrid composites based on woven fabrics and PCBT resin subjected to low-velocity impact
Accepted Manuscript Experimental and numerical investigation of interply hybrid composites based on woven fabrics and PCBT resin subjected to low-velocity impact Bin Yang, Zhenqing Wang, Limin Zhou, Jifeng Zhang, Wenyan Liang PII: DOI: Reference:
S0263-8223(15)00452-3 http://dx.doi.org/10.1016/j.compstruct.2015.05.069 COST 6487
To appear in:
Composite Structures
Please cite this article as: Yang, B., Wang, Z., Zhou, L., Zhang, J., Liang, W., Experimental and numerical investigation of interply hybrid composites based on woven fabrics and PCBT resin subjected to low-velocity impact, Composite Structures (2015), doi: http://dx.doi.org/10.1016/j.compstruct.2015.05.069
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Experimental and numerical investigation of interply hybrid composites based on woven fabrics and PCBT resin subjected to low-velocity impact Bin Yanga, Zhenqing Wanga*, Limin Zhoub, Jifeng Zhanga, Wenyan Lianga College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin, China b Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong, China a
Abstract: Low-velocity impact response of interply hybrid composites based on glass and carbon woven fabrics as reinforcement and polymerized poly (butylene terephthalate) (PCBT) as matrix was presented in this paper. Experiment and finite element method (FEM) were respectively performed to investigate the hybridization effect on the composites under impact velocity of 3m/s, 5m/s and 7m/s. Specimens used in the experiment were made by vacuum assisted prepregs process (VAPP). All the material parameters used in simulation were determined by experimental test. Three-dimensional finite element models were developed in ABAQUS/Explicit to analyze the damage behavior of interply hybrid composite laminates, and a user subroutine VUMAT was compiled for the woven fabrics reinforced PCBT composites. Experimental results show that hybrid composites at hybrid mass ratio of 37:63 could absorb more energy in the impact event compared with pure composites, and the perforation thresholds enhance significantly. The simulation results are well agreed with experimental results. The damage mode of the interply hybrid composites under low-velocity impact is further discussed. Keywords: Low-velocity impact; interply hybrid composites; woven fabrics; perforation thresholds
*
Corresponding author. Address: College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin, China. E-mail address: [email protected]. Tel./fax: +86-0451-82589364.
1. Introduction Fiber reinforced plastic (FRP) composites are highly susceptible to internal damage, which is generally caused by external dynamic mechanical behaviors such as the low-velocity impact on composite structures [1, 2]. It has been known that the relatively light impact load could cause barely-visible impact damage in FRP composites, and the surface may appear to be undamaged to visual inspection [3-6]. Moreover, integrity of FRP composites after impact is one of the major concerns in the design of composite structures in real engineering field. To achieve wider application scope of FRP composites, it is essential to understand the damage mechanism of the material under low-velocity impact load. In addition, improving the damage-resistance performance of FRP composites is one of the important considerations which have attracted more and more interests in the past few years. Therefore, it is valuable to find an effective approach that could enhance the impact damage resistance and tolerance of FRP composites. Generally, the laminated composites applied in high performance structures are fabricated by unidirectional fiber (UDF) layers. However, UDF layers are highly susceptible to impact damage due to their low transverse tensile strength. One of the ways to improve the anti-impact performance of UDF reinforced composites is to use woven fabric (WF) instead of UDF in the material [7]. Because the transverse tensile strength of WF composites is much higher than UDF composites, WF composites have been recognized to have superior impact resistance characteristics. Woven-fabric is produced by the weaving process in which the fabrics are formed by interlacing warp and fill strands. The integrated nature of the fabric provides the balanced in-plane properties. The transverse tensile strength of the WF composites is much higher than UDF composites. This is one of the possible reasons of the superior impact resistance characteristics of WF composites. In this regards, Naik et al. [8] have studied the woven-fabric laminated composite panels under transverse central low-velocity impact by finite element analysis code. The results indicate that woven-fabric laminates are more resistant to impact damage. In recent years, due to the high specific stiffness and high specific strength, carbon fibers are widely used in real industry fields. Unfortunately, the low toughness of carbon fibers has limited their applied range, and this disadvantage is especially obvious when refer to the apply field where low-velocity impact load may occur [9]. One of the possible ways to solve this problem is the
application of hybrid composites. Hybrid composites are the materials made by combining two or more different fibers in a common matrix. They offer a range of properties that cannot be obtained with a single reinforcement. Hybridization effect allows designers to tailor the composite properties to meet the exact needs of the structure. Many researchers have successfully adopted hybridization approach to enhance the mechanical properties and to improve the damage resistance of composites. Studies on static mechanical characteristics of hybrid composites are generally including tensile and compressive properties after impact. It is found that hybrid composites have higher interlaminar shear, tensile, as well as compression after impact properties by using hybridization approach [10-14]. While researches on dynamical characteristics of hybrid composites are usually comprised of mechanical experiments such as low-velocity impact test [15-29]. In this regards, Hosur et al. [15] carried out the experimental investigation to determine the response of four different combinations of hybrid laminates under low-velocity impact load. Alaattin et al. [16] investigated the effect of stacking sequence on the impact behavior of sequentially stacked hybrid composites. Gonzalez et al. [17] studied the drop-weight impact response of interply hybrid laminates based on polymer matrix composite materials. Metin et al. [18] investigated the impact behavior of hybrid composite plates (glass-carbon/epoxy) experimentally, and then they used an energy profiling method together with load-deflection curves to determine the penetration and perforation thresholds of hybrid composites. Impact, compression after impact, and tensile stiffness properties of carbon fiber and Kevlar combination sandwich composites were investigated by Jeremy et al. [19]. Thanomsilp et al. [20] studied penetration impact resistance of hybrid composites based on commingled yarn fabrics. Aktas et al. [21] investigated the effect of stitch pattern on the impact and post impact behavior of eight ply woven-knit hybrid composite plates. Yan et al. [22] prepared inter/intraply hybrid composites reinforced with Kevlar nonwoven/glass woven fabrics, and impact tests were performed on the prepared panel. Wang et al. [23] tested the impact properties of a lightweight hybrid composite. Manikandan et al. [24] presented a study on the experimental and numerical investigation of metal layered interply hybrid composites subjected to low velocity impact. Sevkat et al. [25] studied the drop-weight impact response of hybrid woven composite plates. Sarasini et al. [26] studied the low-velocity impact behavior of hybrid laminates reinforced with woven aramid and basalt fabrics. In [27], they also investigated the effects of basalt fiber hybridization on quasi-static mechanical
properties and low velocity impact behavior of carbon/epoxy laminates. Meanwhile, they studied the low velocity impact behavior of E-glass/basalt reinforced hybrid laminates in [28]. Tasdemirci et al. [29] investigated the ballistic performance of an interlayer composite experimentally and numerically. In this paper, interply hybrid composite laminates based on plain woven glass and carbon fabrics as reinforcement and polymerized poly (butylene terephthalate) resin (PCBT) as matrix were prepared via vacuum assisted prepregs processing (VAPP). Low-velocity impact tests were carried out on the prepared hybrid composites as well as the pure woven carbon fabrics reinforced PCBT composites. Experiments were performed with focus on estimating the perforation thresholds of the two kinds of composites. Furthermore, based on the experimental results, finite element simulation was carried out in ABAQUS/Explicit with the help of VUMAT subroutine program. Damage details in the composite laminates after impact were given in the simulation results, and damage morphology of hybrid composites was shown as a function of contact time in the impact event. Finally, based on the damage morphology observed in both the experiment and simulation, the failure mode of the materials was analyzed. 2. Experimental details 2.1. Materials and Manufacturing The polymer used as matrix is CBT-100, supplied by Cyclic Corporation. CBT resin has a big-ring paucity of polyester structure with the molecular weight Mw=(220)n (n=2-7)g/mol. It could be transformed into linear high molecular weight engineering thermoplastic poly (butylene terephthalate) (PCBT) plastic at 190oC via entropically-driven ring-opening polymerization mechanism in the presence of catalyst [30, 31]. The catalyst selected in the polymerization reaction of CBT resin is butylchlorodihydroxytin (PC-4101). The resin is mixed with catalyst at the mass ratio 100:0.6. The reinforced material is woven carbon fabric and woven glass fabric with the surface density of 300g/m2 and 700 g/m2, respectively. Fig.1 shows the two-dimensional orthogonal plain woven fabrics used in the experiment. Table 1 lists the fabric parameters of the fibers. In the table, gw, gf are the gap between adjacent strands, and aw, af are the strand width. The superscript w and f indicate the warp and fill direction of the woven fabric, respectively. It should be noted that all the materials need to be dried in a vacuum oven for 12h as pretreatment.
In the work, PCBT casts are prepared to evaluate the matrix properties. In order to fabricate PCBT casts, melted CBT resin and catalyst were mixed at mass ratio 100:0.6. The mixture was poured into a metal mold and cured for 1h in a vacuum oven at 220oC, and then demoded at 100oC. Vacuum assisted prepregs process (VAPP) is adopted to manufacture the composite laminates. The prepregs are made with fiber to CBT resin mass ratio of 1:1. It is worth noting that the catalyst needs to be mixed with prepregs uniformly prior to the fabrication. Therefore, an approach similar to physical vapor deposition (PVD) method is adopted to deposit catalyst on the prepregs. The specific preparation processing is as follow: firstly, catalyst powder (0.6wt.% of the resin) was added in 200ml isopropanol aqua, and then stirred by magnetic stirrer at 70oC until completely dissolved. The woven fabrics were immersed in the prepared solution in a metal square plate holder. To ensure all the prepregs could blend fully with the solution, the system was kept in an incubator for 3h at room temperature. Finally, the prepregs need to be dried at 140oC to evaporate isopropanol aqua and left catalyst on the surface. Fig.2a indicates the schematic diagram of the VAPP setup used in this work. It is a RTM-like device with a port linked with the vacuum pump to give vacuum pressure in the polyimide bag. In the VAPP process, the preprges were placed on the polyimide film, and then sealed with heat-resistant sealant. The vacuum bag was placed between the upper and bottom panel of a hot press machine. The machine could provide temperature and pressure to satisfy the curing condition of the resin. It should be noted that the curing temperature during the process is a stage-like temperature: cure at 230oC for 1 hour and post-cure at 190oC for another hour. The composite laminates were demoded when the whole system cooled to room temperature. A total of 25-ply woven carbon-glass fabric reinforced PCBT resin interply hybrid composite laminates (CF-GF/PCBT) with each ply thickness of 0.2mm were prepared by VAPP. The hybrid laminate is a panel of alternate carbon layer and glass layer with two carbon layers as surface on both sides. Thus the panel is a symmetric laminate that has 13 layers of woven carbon fabrics and 12 layers of woven glass fabrics, as indicated in Fig.2b. In the following work, we use [C/G]13C-12G to indicate this layer form in hybrid laminates. In the hybrid composite laminates, the carbon to glass fiber hybrid mass ratio is 37:63. 25-ply woven carbon fabric reinforced PCBT composite laminates (CF/PCBT) were also prepared by VAPP. The mass ratio of hybrid composites to pure carbon composites is 100:74. These two kinds of panels with laminate thickness of 5mm will be
used in the drop-weight impact test. To characterize the fundamental mechanical behavior of the prepared materials, 12-ply pure carbon and pure glass fabric reinforced PCBT composite laminates with thickness of 2.4mm were respectively manufactured. Three types of tensile specimens were cut from the square laminate by a low-speed diamond saw blade cutting machine along transverse, longitudinal and 45o direction, respectively. Specimens with pre-crack were performed in the test to calculate the interlaminar strength. 2.2. Static tests In order to determine the material stiffness and strength parameter of CF/PCBT composites and woven glass fiber reinforced PCBT composites (GF/PCBT), tensile tests were respectively accomplished using INSTRON-4505 servo-electric testing machine. The dimension of the transverse and longitudinal tensile specimens is 200×25×2.4mm3 in accordance with the ASTM (D3039/D3039M-08) standard, and the calibrating length is 100mm. Dimension of the 45o tensile specimen is 120×25×2.4mm3 with calibrating length 80mm. To evaluate the through-the-thickness direction properties of the composite laminates, tensile and compression of PCBT casts were performed, respectively. Dimension of the specimen are 60×6×4mm3 and 25×10×10mm3 in tensile and compression tests, respectively. Additionally, double cantilever beam (DCB) and three-point end notched flexure (3ENF) tests were respectively adopted to estimate the interlaminar performance of the obtained composite laminates. Specimens with dimension of 200×25×2.4mm3 and 100×25×2.4mm3 were adopted in the tests according with the standard ASTM-D5528. The pre-crack with length of 25mm is laid in the middle layer of the each specimen, as indicated in Fig.3. DCB and 3ENF test were also performed by an INSTRON-4505 testing machine. In DCB test, a pair of hinges was glued on the upper and lower surface of the specimens by strong glue. During the experiment, they were used as fixture with the upper one carrying the external load and the lower one fixed on the test machine (Fig.3a). 3ENF test was performed in flexural mode as shown in Fig.3b. 3ENF specimen has a pre-crack at the end of the specimens. Tests were performed at ambient temperature (25oC), and five specimens per type. The cross-head speed was 2 mm/min. Fig.4 demonstrates the representative curves of macroscopic stress-strain of CF/PCBT and GF/PCBT composites: (a), (b) and (c), (d) are respectively the tensile properties of CF/PCBT and
GF/PCBT composites; (e), (f) are the tensile and compression properties of PCBT casts, and (g), (h) are the load-displacement curves of CF-GF/PCBT composite laminates obtained in DCB and 3ENF tests. Mechanical properties of the composite laminates determined from Fig.4 were listed in Table 2 and Table 3. In the tables, longitudinal elastic modulus E11, Poisson’s ratio µ 12, longitudinal tensile strengths Xt and transverse elastic modulus E22, transverse tensile strengths Yt were measured by tensile curves in Fig.4a to Fig.4d. It should be noted that the tensile test was respectively along longitudinal and transverse direction of the woven fabric composite specimens [32]. Shear modulus G and strength S were measured from 45o tensile curves in Fig.4b and Fig.4d. The specimens were loaded up to the failure loads in the axial direction. Elastic modulus E11, E22 and G were calculated from the initial slope of the stress-strain curves by the following formula:
E=
S 3m 4bh3
(1)
where E is the modulus, and S is the support span. m is the slope of the load-deflection curve, while b and h are the width and thickness of specimens, respectively. Tensile strengths of the composite laminates, Xt, Yt, and S, are determined by dividing the failure load to the cross-sectional area of the longitudinal, transverse and 45o tensile test specimens, respectively. The longitudinal and transverse compressive strengths, Xc and Yc, are obtained by dividing the failure load to the cross-sectional area of the specimens. Mechanical properties in the normal direction are estimated by compressing PCBT casts to failure. E33, Zt and Zc are determined from stress-strain curves in Fig.4e and Fig.4f. Zt and Zc are also obtained by dividing the failure load to the cross-sectional area of the specimens. Similarly, E33 is defined by Eq.1. Mode I and Mode II interlaminar fracture toughness determined from Fig.4g and Fig.4h can be respectively calculated by the following formula:
GⅠ c = GⅡ c =
3Pcδ c 2ba
9a0 2 Pcδ c 3 2b( l 3 + 3a03 ) 8
(2)
(3)
where GⅠ c and GⅡ c are the interlaminar fracture toughness; a0 and a are the pre-crack length and crack propagating length, respectively. Pc, δc are the load and displacement when the crack
propagates to a specific length, while b and l are the width and length of the specimens, respectively. In our work, Mode I and Mode II interlaminar fracture toughness can be used to evaluate the interlaminar performance of PCBT-based composite laminates, and the two values calculated from DCB and 3ENF curves were also listed in Table 3. 2.3. Drop-weight impact tests Square specimens with dimension of 100×100×5mm3 were cut from the prepared composite laminates, and they were used in the drop-weight impact tests. All impact tests were conducted on a PC-driven Instron/9250HV pneumatic-assisted instrumented drop-weight impact tester, which was equipped with a pneumatic brake to avoid multiple strikes. The square specimen was clamped by the test machine and left a diameter of 80mm area as impact region in the specimen. The impactor nose used in this study had a hemispherical shape with a radius of 6mm and the mass m=9.1445kg. The hemispherical impactor was dropped from a predetermined height to perform the impact test, as depicted in Fig.5. The impact force history was measured using a load cell located just above the impact head, and the impact velocity was recorded by a pair of photoelectric-diodes attached to the base of the test machine. 3. Experimental results 3.1. Perforation thresholds of the composite laminates One of the important considerations in designing the composite structure serviced in real industries is their perforation thresholds. The penetration threshold is defined as the impact energy when the impactor does not rebound from the specimen for the first time [33]. Fig.6 depicts the velocity of the two types of composite laminates subjected to various impact energies as a function of contact time. As can be seen, each curve is of the highest value at the beginning of impact event. With increasing of the contact time, velocity of the hemispherical impactor declines from 3m/s to 0m/s of CF/PCBT composites. Then the curve has negative values (-0.5m/s), which implies the rebounding of the impactor. However, velocity of impactor declines to approximately 1.5m/s from 5m/s. No negative section is observed in the figure, which implies that no rebounding happens and the specimen has been perforated. In case of CF-GF/PCBT hybrid composites, velocity in the curve shows similar tendency. It firstly decreases to 0m/s from 5m/s, and then the impactor is rebounded to 0.5m/s. At V=7m/s, velocity of the impactor decreases to 1.5m/s when
the specimen is perforated. Thus, penetration thresholds of the two composite laminates can be calculated by linear interpolation method from the velocity data in this study. Fig.7 illustrates the fitting results obtained from the initial velocity (Vi) and residual velocity (Vr) in the impact event. It can be seen in the figure that when Vr is zero, the initial velocity of the two laminates is 3.5m/s and 5.5m/s, respectively. Hence, 3.5m/s and 5.5m/s can be considered as penetration threshold of the two composite laminates. Moreover, velocity of the two specimens decreases to the same value (1.5m/s) when they are completely perforated. According to Newton’s law, it can be calculated that CF-GF/PCBT hybrid composites absorbs more impact energy than CF/PCBT composite laminates. Fig.8 illustrates the displacement-time (d-t) curves of the two composite laminates under various impact energies. To illustrate the problem better, curve of a duplicate sample is given in the figures. From the d-t curve, it can be found that deflection of CF/PCBT composites subjected to impact velocity V=3m/s recovers to its initial position after reaches to the maximum deformation with increasing of contact time. In contrast, deflection increases with time until to the penetration of CF/PCBT composite laminates at impact velocity of 5m/s. It worth mention that deflection of CF-GF/PCBT hybrid composite laminates at 7m/s varies the same way as CF/PCBT composites. It increases with contact time until the composite laminates damaged, and no decreasing of deflection is observed in Fig.8b. However, deflection of hybrid composites appears a different tendency at V=5m/s. Instead of declining to zero, it decreases to a special value after impact. This is mainly due to the existing of glass fibers in the hybrid composites. Because toughness of glass fibers is much higher than that of carbon fibers, deformation of CF-GF/PCBT hybrid composites is larger than that of CF/PCBT composites near the perforation thresholds region. As a result, pure carbon fiber based composites could recover to the initial state as soon as possible, while hybrid composites have unrecoverable plastic deformation after impact. Fig.9 shows the four contact force-time (F-t) curves of the composite laminates subjected to different impact energies. Contact force is an important parameter in impact problems and can be generally defined as the reaction force applied by specimen to the impactor [33]. In Fig.9, all the four F-t curves have mountain-like shapes. The maximum contact force increases with the increasing of impact energy acted on CF/PCBT composites. When the impact energy is low, such as 3m/s, the maximum contact force is 6kN. It increases to 8kN when the impact velocity
increases to 5m/s. For CF-CF/PCBT hybrid composites, the contact force is of approximately a constant peak value around 8kN both at impact velocity of 5m/s and 7m/s, as seen in Fig.9b. In addition, in the F-t curve of hybrid composites, the load increases to its maximum value slower and then decreases to zero sharply during the impact process. This leads to the larger area closed by F-t curve, and further leaves a platform on the F-t curve with increasing of contact time. However, in terms of CF/PCBT composites, the contact force increases to the maximum value with higher rise rate and decreases to zero slowly, resulting in pointy shape curves in Fig.9a. 3.2. Deflection based curves in the impact test Contact force-deflection (F-d) curve under various impact energies is a typical characteristic of composite material under impact load. Fig.10 denotes the F-d curves of CF/PCBT and CF-GF/PCBT composites subjected to different velocities. Like F-t curves presented in Fig.9, all the four curves collectively have a mountain-like shape. Individually, however, there are two basic curve types: closed curve and open curve. Fig.10a and Fig.10c are closed type curves, while Fig.10b and Fig.10d represents for the open curve. When the impact energy is lower, F-d curve is a closed type and the entire descending section consists of rebounding. In these cases, impact load dose not result in a serious damage to specimens. When the impact velocity is bigger, sharply decrease of impact force is found at the end of the impact event. Fig.11 shows the variation of impactor velocity versus deflection (v-d) under various impact energies. As seen in the figures, the velocity is of the highest value right before the contact happens between impactor and specimens. Magnitude of the velocity decreases in a parabolic shape and becomes zero after the maximum displacement is reached. Afterwards, the velocity has negative values and its absolute value increases with the decreasing of displacement. Then velocity reaches to a constant value, which implies the rebounding of impactor. Fig.11a, c and Fig.11b, d represent for the specimens undergoing partial perforation and complete perforation, respectively. The ratio of Vr/Vi could be used to further describe the material’s behavior under impact load. Generally, the magnitude of Vr/Vi could be positive or negative ranges from -1 to 1. When Vr/Vi is negative between -1 to 0, rebounding of the impactor takes place. If Vr/Vi equals 0 or is almost zero, the laminate is at the threshold moment of perforation. When Vr/Vi is positive between 0 to 1, it means that the material is completely perforated. The Vr/Vi ratio calculated from Fig.11 is shown in Fig.12. It is clear that for a given laminate, velocity ratio Vr/Vi various from
negative to positive with the increasing of impact energy. Additionally, for a given initial energy (e.g. V=5m/s), Vr/Vi of CF/PCBT composites is positive, whereas Vr/Vi of CF-GF/PCBT composites is negative. From the above discussion, it can be concluded that CF-GF/PCBT hybrid composites at hybrid mass ratio of 37:63 have better impact resistance performance compared with CF/PCBT composites. Addition of glass fabrics in carbon fabric reinforced PCBT composites has positive correlation. Due to the higher toughness of glass fabrics, woven glass fabrics have higher breaking extensibility than carbon fabrics in hybrid composite laminates. Hence, glass fabrics in hybrid composites could protect the whole panel from penetrating by its large deformation under low-velocity impact load. In addition, mainly due to the mismatch of deformation in glass and carbon fabrics in the event, opportunity of delamination between glass layers and carbon layers in hybrid composites is improved. Delamination could absorb some of the impact energy in the impact event. All the aforementioned factors could enhance the perforation thresholds of hybrid composites, and the enhancement effect will be further discussed in this paper according to the finite element method (FEM) simulation results. 4. FEM Simulation The finite element numerical approaches appear to be the best technique for analyzing the behavior of composite laminates since they are fairly accurate, less expensive and less time consuming. As discussed, the hybrid composite laminates could absorb more impact energy and have better impact resistance performance. Therefore, simulation work is mainly performed on CF-GF/PCBT hybrid composites to explain the detailed failure progress. In this work, all simulations are performed by ABAQUS software under boundary conditions similar to those in the real impact test. As known, the impact problem is a three-dimensional problem, however traditional composite laminate is treated as shell, which cannot solve the contact problem well [34]. In our work, a finite element models is developed to simulate the failure in composite laminates under impact load. Based on the three-dimensional finite element method, 3D Hashin criteria is adopted in the progress failure models. Results obtained in the experiments and simulations are compared. 4.1. Finite element model
The finite element model of the impact system with detail of hybrid composite laminates is illustrated in Fig.13. In the FEM model, the spherical shaped impactor is defined as a rigid body with radius of 12mm. To save the computation time, coarser meshes are used in the model far away from the maybe contact region. Fine meshes are applied right under the impactor to capture a relatively accurate result. Besides, unlike real experiment, the impactor is placed right on the composite panel with the given velocity of 5m/s and 7m/s, respectively. To make a reference, simulation work is also done on CF/PCBT composite laminates at V=5m/s. It should be mentioned that all element type is C3D8R in the simulation. All the material stiffness and strength parameters used in the FEM model are inputted according to the experimental results listed in Table 2 and Table 3. Hashin damage criteria is developed in 1980 which is used to simulate the damage evolution in unidirectional fiber reinforced composites [35]. Based on Hashin’s theory, user subroutine VUMAT is compiled to calculate the mechanical behavior of woven fabric composites in this paper. 3D Hashin failure criteria including five different failure modes (warp and fill fiber tensile failure, compression failure, and matrix compression failure) is as follow: Fiber tensile damage in warp direction:
e 2ftw = (
σ 11 Xt
)2 + (
σ 12 S12
)2 + (
σ 13 S13
)2 ≥ 1
(4)
Fiber compression damage in warp direction:
e 2fc w = (
σ 11 Xt
)2 ≥ 1
(5)
Fiber tensile damage in fill direction:
e 2ftf = (
σ 11 Yt
)2 + (
σ 12 S12
)2 + (
σ 23 S 23
)2 ≥ 1
(6)
Fiber compression damage in fill direction:
e 2fcf = (
σ 22 Yt
)2 ≥ 1
(7)
σ 13
(8)
Matrix compression damage:
edc2 = (
σ 23 S 23
)2 + (
S13
)2 ≥ 1
In the formulas, σ is the stress applied on the element along various directions of fiber reinforced composites, while S stands for the shear strength of the element. Xt, Yt are respectively the tensile strength along fill, warp and thickness directions of composite material, and e is defined as the coefficient to judge the failure of element. In the model, elements failure happens when any of the failure criteria listed in Eq.(4) to Eq.(8) is satisfied (e≥1), then the stiffness of the element will reduce to a special percentage of its original value according to the rule shown in Table 4 [36]. When fiber tensile damage in both the warp and fill direction appears together (eftw≥1 and eftf≥1), the element in the model will be deleted and cannot further carry load during the impact event. In terms of the interlamination mechanical behavior of composites, surface-based cohesive behavior (SBCB) is adopted between each glass and carbon fiber layer. SBCB uses the linear elastic traction-separation theory, which is similar to the cohesive element adopted by numerous authors [37-39]. In SBCB, delamination initiation happens when a quadratic interaction function involving the contact stress ratios (as defined in the Eq.9) reaches a value of one. This function is as follow: 2
2
2
⎧ tn ⎪⎫ ⎧ ts ⎫⎪ ⎧ tt ⎪⎫ ⎨ o ⎬ + ⎨ o ⎬ + ⎨ o ⎬ =1 ⎩ tn ⎪⎭ ⎩ ts ⎪⎭ ⎩ tt ⎪⎭
(9)
While damage evolution is based on total energy release rate, as follow:
Gn + Gs + Gt =1 Gc
(10)
Here, tn, ts, and tt are the normal and shear components of tractions, and t no , t so , tto are the corresponding peak values of the contact stress when the separation is purely in the normal or shear direction, respectively. Gn, Gs, and Gt are the work done by the normal and shear tractions, and Gc is the critical total energy release rate of the material. 4.2. Simulation results and discussion 4.2.1. Results comparison and damage process of hybrid composites in the impact event In Fig.14, F-t curves obtained in the experiment and FEM simulation at impact velocity V=5m/s and V=7m/s are compared. It can be concluded from the figure that remarkably close agreement is obtained between simulation and experiment. Compared with experiment results, error of contact force of model developed in ABAQUS/Explicit is firstly small and then large as the function of contact time. Discrepancies between numerical and experimental results in Fig.14
can be interpreted as follow: at the beginning of the impact event, no element in the model is damaged and contact is just existed between the upper surface and impactor, and good match between experiment and simulation results is achieved. However, since the impactor is considered as rigid body, the energy absorbed by it is neglected. As a result, stiffness degrades faster under higher energy with increasing of contact time, and the curves appear as sharply decline at the moment when the element failed. Fig.15 and Fig.16 show the simulation results of hybrid composites subjected to various impact velocities as a function of contact time. It can be found that deformation of hybrid composites increases with contact time until rebounding occurs at V=5m/s, while it increases until the hybrid laminates are completely penetrated at V=7m/s. At V=5m/s, damage of the laminates firstly come into being at t=6ms, and the laminates are half perforated at t=8ms (Fig.15). When the impact energy is higher (V=7m/s), the laminate damaged rapidly. As seen in Fig.16, damage of hybrid composites emerges at t=1ms. The cross-section of the damaged CF-GF/PCBT composite laminates in the experiment is also shown in Fig.15 and Fig.16. As seen in the figures, damage morphology of the impacted specimens in the experiment and simulation matches well. It also should be noted that damage area in both cases is not high, and damage location is around the contact region between the impactor and panel in the center of the specimens. Moreover, it can be observed from the cross-section of the damaged specimens that the damage morphology includes warp and fill fiber breaking and delamination between two layers at V=5m/s in Fig.15. However at V=7m/s, fiber breaking is the main damage mode. Progressive failure of glass and carbon fabrics in interply hybrid composites as a function of layer position is shown in Fig.17. Here V=5m/s, field variable is used to represent damage of laminate in the explicit model. It can be found in Fig.17 that the critical damage in hybrid composites in ply-16. In other words, the layers located up to ply-16 are damaged, whereas under ply-16 are undamaged under the velocity of 5m/s. 4.2.2. Damage modes in the simulation and experiment of different composite laminates Fig.18 shows the damage pattern on the contact surface in different composite laminates after impact in the experiments. It can be found the dent and hole clearly on the impacted laminates. For CF-GF/PCBT panel, at velocity of 5m/s, impact damage is initiated firstly as matrix crack in the materials, and then this crack cut some layers of carbon and glass fabrics on the contact surface, as shown in Fig.18c and Fig.15. This damage process is accompanied by delamination
between glass and carbon layers, as verified by the cross-section of damaged specimens in Fig15. Thus, impact energy is mainly absorbed by fiber breaking and delamination between layers in hybrid composites. At V=7m/s, the energy available is high enough to cut through the fabrics in both fill and warp direction. Hence, the hybrid composite laminates are completely penetrated as shown in Fig.18d and Fig.16. In case of CF/PCBT composites, at lower impact velocity (V=3m/s), the primary damage mode is indentation-induced matrix cracking on the impact surface (Fig.18a). To study the detailed damage mode of CF/PCBT composites at impact velocity V=5m/s, we give the simulation results in Fig.19. As seen in Fig.18b and Fig.19, CF/PCBT laminates are completely penetrated at V=5m/s. Fiber breaking is the main damage mode, and the damage morphology matches well in the experiment and simulation. In Fig.19, we also compared the damage morphology of CF/PCBT and CF-GF/PCBT composite laminates after impact at V=5m/s in the simulation. In the figure, carbon fabrics in CF/PCBT break easily under dynamic loads due to their brittle nature, while they are difficult to be broken under the protection of glass fabric layers in the hybrid composites. In CF-GF/PCBT at V=5m/s, because the impact energy is mostly absorbed by carbon and glass fabric breaking and delamination under the contact surface, carbon and glass fabric layers under the non-contact surface are not broken. As a result, the hybrid composites show better impact damage resistance and tolerance. 5. Conclusions In this study, hybrid effect on low-velocity impact response of interply hybrid composites prepared by vacuum assisted prepregs process (VAPP) is investigated by experiments and finite element methods. From the study, the conclusions can be summarized as follows:
Specimens based on fiber/CBT prepregs are successfully manufactured by VAPP in this work, and the fundamental mechanical parameters of the materials are obtained experimentally.
At hybrid mass ratio of 37:63, intraply hybrid laminates with layer form [C/G]13C-12G could enhance the impact-resistance performance of CF/PCBT composites significantly. Addition of glass fabrics in carbon fabric reinforced PCBT composites has positive correlation. By adding glass fibers in woven carbon fabric reinforced PCBT resin laminates, perforation thresholds enhanced from 3.5m/s to 5.5m/s.
The progressive failure model developed in ABAQUS using VUMAT subroutine can be used to predict the failure of interply hybrid composites based on glass and carbon woven fabrics,
and the failure mode observed in the simulation and experiment matches well. Acknowledgements This work was financially supported by the Chinese National Natural Science Foundation (No.11472086) and the project of cooperation between Hong Kong and Chinese Ministry of Science and Technology (No. S2014GAT013). References [1] Naik NK, Meduri S. Polymer-matrix composites subjected to low-velocity impact: effect of laminate configuration. Compos Sci Technol 2001; 61:1429-1436. [2] Rokbi M, Osmani H, Benseddiq N, Imad A. On experimental investigation of failure process of woven fabric composites. Compos Sci Technol 2011; 71:1375-1384. [3] Christoforou AP, Yigit AS. Scaling of low-velocity impact response in composite structures. Compos Struct 2009; 91:358-365. [4] Zhu SQ, Chai GB. Low-velocity impact response of fiber-metal laminates: experimental and finite element analysis. Compos Sci Technol 2012; 72:1793-1802. [5] Lopes CS, Seresta O, Coquet Y, Gürdal Z, Camanho PP, Thuis B. Low-velocity impact damage on dispersed stacking sequence laminates. Part I: experiments. Compos Sci Technol 2009; 69:926-936. [6] Lopes CS, Camanho PP, Gurdal Z, Maimi P, Gonzalez EV. Low-velocity impact damage on dispersed stacking sequence laminates. Part II: numerical simulations. Compos Sci Technol 2009; 69:937-947. [7] Karaofjlan L, Noor AK. Frictional contact impact response of textile composite structures. Compos Struct 1997; 37:269-280. [8] Naik NK, Sekhe YC, Meduri S. Damage in woven-fabric composites subjected to low-velocity impact. Compos Sci Technol 2000; 60:731-744. [9] Davies G, Zhang X. Impact damage prediction in carbon composite structures. Int Impact Engno 1995; 16:149-170. [10] Dehkordi MT, Nosraty H, Shokrieh MM, Minak G, Ghelli D. The influence of hybridization on impact damage behavior and residual compression strength of intraply basalt/nylon hybrid composites. Mater Des 2013; 43:283-290. [11] Bhatia NMH. Strength and fracture characteristics of graphite glass intraply hybrid composites. Compos Mater Test Des 1982; 22:183-199. [12] Park R, Jang J. The effect of hybridization on the mechanical performance of aramid polyethylene intraply fabric composites. Compos Sci Technol 1998; 58:1621-1628. [13] Pegoretti A, Fabbri E, Migliaresi C, Pilati F. Intraply and interply hybrid composites based on E-glass and poly(vinyl alcohol) woven fabrics: tensile and impact properties. Polym Int 2004; 53:1290-1297. [14] Akhbari M, Shokrieh MM, Nosraty H. A study on buckling behavior of composite sheet reinforced by hybrid woven fabrics. Trans CSME 2008; 32:81-89. [15] Hosur MV, Adbullah M, Jeelani S. Studies on the low-velocity impact response of woven hybrid composites. Compos Struct 2005; 67:253-262. [16] Alaattin A, Mehmet A, Fatih T. The effect of stacking sequence on the impact and post-impact behavior of woven/knit fabric glass/epoxy hybrid composites. Compos Struct 2013; 103:119-135. [17] Gonzalez EV, Maimi P, Sainz JR, Cruz P, Camanho PP. Effects of interply hybridization on the damage resistance and tolerance of composite laminates. Compos Struct 2014; 108:319-331.
[18] Metin S, Numan BB, Onur S. An experimental investigation on the impact behavior of hybrid composite plates. Compos Struct 2010; 92:1256-1262. [19] Jeremy G, Aaran J, Mohammad M, James Stone. Low velocity impact of combination Kevlar/carbon fiber sandwich composites. Compos Struct 2005; 69:396-406. [20] Thanomsilp C, Hogg PJ. Penetration impact resistance of hybrid composites based on commingled yarn fabrics. Comp Sci Technol 2003; 63:467-482. [21] Aktas A, Aktas M, Turan F. Impact and post impact (CAI) behavior of stitched woven–knit hybrid composites. Compos Struct 2014; 116:243-253. [22] Yan R, Wang R, Lou CW, Lin JH. Low-velocity impact and static behaviors of high-resilience thermal-bonding inter/intra-ply hybrid composites. Compos Part B 2015; 69:58-68. [23] Wang Q, Chen ZH, Chen ZF. Design and characteristics of hybrid composite armor subjected to projectile impact. Mater Des 2013; 46:634-639. [24] Manikandan P, Chai GB. A layer-wise behavioral study of metal based interply hybrid composites under low velocity impact load. Compos Struct 2014; 117:17-31. [25] Sevkat E, Liaw B, Delale F. Drop-weight impact response of hybrid composites impacted by impactor of various geometries. Mater Des 2013; 52:67-77. [26] Sarasini F, Tirillò J, Valente M, Ferrante L, Cioffi S, Iannace S, Sorrentino L. Hybrid composites based on aramid and basalt woven fabrics: Impact damage modes and residual flexural properties. Mater Des 2013; 49: 290-302. [27] Sarasini F, Tirillo J, Ferrante L, Valente M, Valente T, Lampani L, Gaudenzi P, Cioffi S, Iannace S, Sorrentino L. Drop-weight impact behavior of woven hybrid basalt–carbon/epoxy composites. Compos Part B 2014; 59:204-220. [28] Sarasini F, Tirillo J, Valente M, Valente T, Cioffi S, Iannace S, Sorrentino L. Effect of basalt fiber hybridization on the impact behavior under low impact velocity of glass/basalt woven fabric/epoxy resin composites. Compos Part A 2013; 47:109-123. [29] Tasdemirci A, Tunusoglu G, Guden M. The effect of the interlayer on the ballistic performance of ceramic/composite armors: Experimental and numerical study. Int Impact Engno 2012; 44:1-9. [30] Mader E, Gao SL, Plonka R, Wang J. Investigation on adhesion, interphases, and failure behavior of cyclic butylene terephthalate (CBT)/glass fiber composites. Compos Sci Technol 2007; 67:3140-3150. [31] Amiya R, Tripathy, Chen WJ, Kukureka SN, William JM. Novel poly(butylene terephthalate)/poly(vinyl butyral) blends prepared by in situ polymerization of cyclic poly(butylene terephthalate) oligomers. Polymer 2003; 44:1835-1842. [32] Zhou S, Wang ZQ, Zhou JS, Wu XD. Experimental and numerical investigation on bolted composite joint made by vacuum assisted resin injection. Compos Part B 2013; 45:1620-1628. [33] Atas C, Sayman O. An overall view on impact response of woven fabric composite plates. Compos Struct 2008; 82:336-345. [34] Hou JP, Petrinic N, Ruiz C, Hallett SR. Prediction of impact damage in composite plates. Compos Sci Technol 2000; 60:273-281. [35] Hashin Z. Failure criteria for unidirectional fiber composites. J Appl Mech 1980; 47:329-334. [36] Reddy YS, Reddy JN. Three dimensional finite element progressive failure analysis of composite laminates under axial extension. Compos Sci Technol 1993; 15:73-87. [37] Lei HS, Wang ZQ, Zhou B, Tong LY, Wang XQ. Simulation and analysis of shape memory alloy fiber reinforced composite based on cohesive zone model. Mater Des 2012; 40:138-147. [38] Lei HS, Wang ZQ, Tong LY, Zhou B, Fu J. Experimental and numerical investigation on the macroscopic
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Figure caption Fig.1. Schematics of the plain woven fabrics. Fig.2. Schematic diagram of the manufacturing process and hybrid form of the composite laminates. Fig.3. Schematic diagram of DCB test and 3ENF test used in the experiment. Fig.4. Typical curves used to determine the strength and modulus of the materials: (a), (b) and (c), (d) are the tensile stress-strain curves of CF/PCBT and GF/PCBT composite laminates; (e) and (f) are tensile and compression curves of PCBT casts; (g) and (h) are the load-displacement curves of CF-GF/PCBT composite laminates obtained in DCB and 3ENF tests. Fig.5. Drop-weight impact setup used in the impact test of PCBT resin based composites. Fig.6. Comparison of velocity-time curves of different composite laminates in impact tests. Fig.7. Penetration threshold of the materials obtained by linear interpolation method. Fig.8. Comparison of deflection-time curves of different composite laminates (including a duplicate sample). Fig.9. Typical contact force history of different composite laminates at various impact velocities. Fig.10. Variation of contact force of different composite laminates as a function of deflection (including a duplicate sample). Fig.11. Velocity of impact head as a function of deflection of specimens (including a duplicate sample). Fig.12. Ratio of residual velocity to initial velocity of various specimens under different impact energies. (In the figure, N denotes negative, while P denotes positive value.) Fig.13. FEM model of the composite laminates and hemispherical impactor. Fig.14. Comparison between experiment and simulation results of hybrid composite laminates Fig.15. Damage cross section of CF-GF/PCBT hybrid composites as a function of contact time at V=5m/s (the impactor is removed for easy observation). Fig.16. Damage cross section of CF-GF/PCBT hybrid composites as a function of contact time at V=7m/s. Fig.17. Damage process as a function of layer position with impact velocity V=5m/s. Fig.18. Contact surfaces of different specimens at different velocities in the experiment. Fig.19. Comparison of damage morphology in CF/PCBT and CF-GF/PCBT laminates after impact at V=5m/s.
Fig.1. Schematics of the plain woven fabrics.
(a) Vacuum bag-assisted hot-press processing
(b) Structural style of the hybrid composite laminates Fig.2. Schematic diagram of the manufacturing process and hybrid form of the composite laminates.
(a) DCB test
(b) 3ENF test
Fig.3. Schematic diagram of DCB test and 3ENF test used in the experiment.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Fig.4. Typical curves used to determine the strength and modulus of the materials: (a), (b) and (c), (d) are the tensile stress-strain curves of CF/PCBT and GF/PCBT composite laminates; (e) and (f) are tensile and compression curves of PCBT casts; (g) and (h) are the load-displacement curves of CF-GF/PCBT composite laminates obtained in DCB and 3ENF tests.
Fig.5. Drop-weight impact setup used in the impact test of PCBT resin based composites.
Fig.6. Comparison of velocity-time curves of different composite laminates in impact tests.
Fig.7. Penetration threshold of the materials obtained by linear interpolation method.
(a) (b) Fig.8. Comparison of deflection-time curves of different composite laminates (including a duplicate sample).
(a)
(b)
Fig.9. Typical contact force history of different composite laminates at various impact velocities.
(a)
(b)
(c) (d) Fig.10. Variation of contact force of different composite laminates as a function of deflection (including a duplicate sample).
(a)
(b)
(c) (d) Fig.11. Velocity of impact head as a function of deflection of specimens (including a duplicate sample).
Fig.12. Ratio of residual velocity to initial velocity of various specimens under different impact energies. (In the figure, N denotes negative, while P denotes positive value.)
Fig.13. FEM model of the composite laminates and hemispherical impactor.
Fig.14. Comparison between experiment and simulation results of hybrid composite laminates.
Fig.15. Damage cross section of CF-GF/PCBT hybrid composites as a function of contact time at V=5m/s (the impactor is removed for easy observation).
Fig.16. Damage cross section of CF-GF/PCBT hybrid composites as a function of contact time at V=7m/s.
Fig.17. Damage process as a function of layer position with impact velocity V=5m/s.
Fig.18. Contact surfaces of different specimens at different velocities in the experiment.
Fig.19. Comparison of damage morphology in CF/PCBT and CF-GF/PCBT laminates after impact at V=5m/s.
Tables Table 1. Dimension of plain woven fabric structure.
Material
aw (mm)
gw (mm)
af (mm)
gf (mm)
Carbon fiber
3
0.2
2.9
0.6
Glass fiber
5
1
4
1.2
Table 2. Stiffness properties of woven fabric reinforced PCBT composites from experimental tests.
Material
E11 (GPa)
E22 (GPa)
E33 (GPa)
G12 (GPa)
G13 (GPa)
G23 (GPa)
µ 12
µ 13
µ 23
GF/PCBT CF/PCBT
14.73 25.7
14.73 25.7
10.9 15.9
1.789 3.5
1.43 1.43
1.43 1.43
0.25 0.2
0.5 0.35
0.5 0.35
Table 3. Material strength data of fiber reinforced PCBT composites (MPa).
Material
Xt
Xc
Yt
Yc
Zt
Zc
S12
S13
S23
Mode I kJ/m2
Mode II kJ/m2
GF/PCBT CF/PCBT
356.53 400.8
300 300
320 387.5
280 280
50 50
230 230
25 32
13 15
13 15
1.5 1.5
1.23 1.23
Table 4. Stiffness degradation rule in VUMAT. E1
E2
E3
G12
G13
G23
μ12
μ13
μ23
Tensile break of warp fiber
0.1
—
—
0.1
0.1
—
0.1
0.1
—
Compress damage of warp fiber
0.2
—
—
0.2
0.2
—
0.2
0.2
—
Tensile break of fill fiber
—
0.1
—
0.1
—
0.1
0.1
—
0.1
Compress damage of fill fiber
—
0.2
—
0.2
—
0.2
0.2
—
0.2
Tensile break in z direction
—
—
0.1
—
0.1
0.1
—
0.1
0.1
Compress damage in z direction
—
—
0.2
—
0.2
0.2
—
0.2
0.2
S0263-8223(15)00452-3 http://dx.doi.org/10.1016/j.compstruct.2015.05.069 COST 6487
To appear in:
Composite Structures
Please cite this article as: Yang, B., Wang, Z., Zhou, L., Zhang, J., Liang, W., Experimental and numerical investigation of interply hybrid composites based on woven fabrics and PCBT resin subjected to low-velocity impact, Composite Structures (2015), doi: http://dx.doi.org/10.1016/j.compstruct.2015.05.069
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Experimental and numerical investigation of interply hybrid composites based on woven fabrics and PCBT resin subjected to low-velocity impact Bin Yanga, Zhenqing Wanga*, Limin Zhoub, Jifeng Zhanga, Wenyan Lianga College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin, China b Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong, China a
Abstract: Low-velocity impact response of interply hybrid composites based on glass and carbon woven fabrics as reinforcement and polymerized poly (butylene terephthalate) (PCBT) as matrix was presented in this paper. Experiment and finite element method (FEM) were respectively performed to investigate the hybridization effect on the composites under impact velocity of 3m/s, 5m/s and 7m/s. Specimens used in the experiment were made by vacuum assisted prepregs process (VAPP). All the material parameters used in simulation were determined by experimental test. Three-dimensional finite element models were developed in ABAQUS/Explicit to analyze the damage behavior of interply hybrid composite laminates, and a user subroutine VUMAT was compiled for the woven fabrics reinforced PCBT composites. Experimental results show that hybrid composites at hybrid mass ratio of 37:63 could absorb more energy in the impact event compared with pure composites, and the perforation thresholds enhance significantly. The simulation results are well agreed with experimental results. The damage mode of the interply hybrid composites under low-velocity impact is further discussed. Keywords: Low-velocity impact; interply hybrid composites; woven fabrics; perforation thresholds
*
Corresponding author. Address: College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin, China. E-mail address: [email protected]. Tel./fax: +86-0451-82589364.
1. Introduction Fiber reinforced plastic (FRP) composites are highly susceptible to internal damage, which is generally caused by external dynamic mechanical behaviors such as the low-velocity impact on composite structures [1, 2]. It has been known that the relatively light impact load could cause barely-visible impact damage in FRP composites, and the surface may appear to be undamaged to visual inspection [3-6]. Moreover, integrity of FRP composites after impact is one of the major concerns in the design of composite structures in real engineering field. To achieve wider application scope of FRP composites, it is essential to understand the damage mechanism of the material under low-velocity impact load. In addition, improving the damage-resistance performance of FRP composites is one of the important considerations which have attracted more and more interests in the past few years. Therefore, it is valuable to find an effective approach that could enhance the impact damage resistance and tolerance of FRP composites. Generally, the laminated composites applied in high performance structures are fabricated by unidirectional fiber (UDF) layers. However, UDF layers are highly susceptible to impact damage due to their low transverse tensile strength. One of the ways to improve the anti-impact performance of UDF reinforced composites is to use woven fabric (WF) instead of UDF in the material [7]. Because the transverse tensile strength of WF composites is much higher than UDF composites, WF composites have been recognized to have superior impact resistance characteristics. Woven-fabric is produced by the weaving process in which the fabrics are formed by interlacing warp and fill strands. The integrated nature of the fabric provides the balanced in-plane properties. The transverse tensile strength of the WF composites is much higher than UDF composites. This is one of the possible reasons of the superior impact resistance characteristics of WF composites. In this regards, Naik et al. [8] have studied the woven-fabric laminated composite panels under transverse central low-velocity impact by finite element analysis code. The results indicate that woven-fabric laminates are more resistant to impact damage. In recent years, due to the high specific stiffness and high specific strength, carbon fibers are widely used in real industry fields. Unfortunately, the low toughness of carbon fibers has limited their applied range, and this disadvantage is especially obvious when refer to the apply field where low-velocity impact load may occur [9]. One of the possible ways to solve this problem is the
application of hybrid composites. Hybrid composites are the materials made by combining two or more different fibers in a common matrix. They offer a range of properties that cannot be obtained with a single reinforcement. Hybridization effect allows designers to tailor the composite properties to meet the exact needs of the structure. Many researchers have successfully adopted hybridization approach to enhance the mechanical properties and to improve the damage resistance of composites. Studies on static mechanical characteristics of hybrid composites are generally including tensile and compressive properties after impact. It is found that hybrid composites have higher interlaminar shear, tensile, as well as compression after impact properties by using hybridization approach [10-14]. While researches on dynamical characteristics of hybrid composites are usually comprised of mechanical experiments such as low-velocity impact test [15-29]. In this regards, Hosur et al. [15] carried out the experimental investigation to determine the response of four different combinations of hybrid laminates under low-velocity impact load. Alaattin et al. [16] investigated the effect of stacking sequence on the impact behavior of sequentially stacked hybrid composites. Gonzalez et al. [17] studied the drop-weight impact response of interply hybrid laminates based on polymer matrix composite materials. Metin et al. [18] investigated the impact behavior of hybrid composite plates (glass-carbon/epoxy) experimentally, and then they used an energy profiling method together with load-deflection curves to determine the penetration and perforation thresholds of hybrid composites. Impact, compression after impact, and tensile stiffness properties of carbon fiber and Kevlar combination sandwich composites were investigated by Jeremy et al. [19]. Thanomsilp et al. [20] studied penetration impact resistance of hybrid composites based on commingled yarn fabrics. Aktas et al. [21] investigated the effect of stitch pattern on the impact and post impact behavior of eight ply woven-knit hybrid composite plates. Yan et al. [22] prepared inter/intraply hybrid composites reinforced with Kevlar nonwoven/glass woven fabrics, and impact tests were performed on the prepared panel. Wang et al. [23] tested the impact properties of a lightweight hybrid composite. Manikandan et al. [24] presented a study on the experimental and numerical investigation of metal layered interply hybrid composites subjected to low velocity impact. Sevkat et al. [25] studied the drop-weight impact response of hybrid woven composite plates. Sarasini et al. [26] studied the low-velocity impact behavior of hybrid laminates reinforced with woven aramid and basalt fabrics. In [27], they also investigated the effects of basalt fiber hybridization on quasi-static mechanical
properties and low velocity impact behavior of carbon/epoxy laminates. Meanwhile, they studied the low velocity impact behavior of E-glass/basalt reinforced hybrid laminates in [28]. Tasdemirci et al. [29] investigated the ballistic performance of an interlayer composite experimentally and numerically. In this paper, interply hybrid composite laminates based on plain woven glass and carbon fabrics as reinforcement and polymerized poly (butylene terephthalate) resin (PCBT) as matrix were prepared via vacuum assisted prepregs processing (VAPP). Low-velocity impact tests were carried out on the prepared hybrid composites as well as the pure woven carbon fabrics reinforced PCBT composites. Experiments were performed with focus on estimating the perforation thresholds of the two kinds of composites. Furthermore, based on the experimental results, finite element simulation was carried out in ABAQUS/Explicit with the help of VUMAT subroutine program. Damage details in the composite laminates after impact were given in the simulation results, and damage morphology of hybrid composites was shown as a function of contact time in the impact event. Finally, based on the damage morphology observed in both the experiment and simulation, the failure mode of the materials was analyzed. 2. Experimental details 2.1. Materials and Manufacturing The polymer used as matrix is CBT-100, supplied by Cyclic Corporation. CBT resin has a big-ring paucity of polyester structure with the molecular weight Mw=(220)n (n=2-7)g/mol. It could be transformed into linear high molecular weight engineering thermoplastic poly (butylene terephthalate) (PCBT) plastic at 190oC via entropically-driven ring-opening polymerization mechanism in the presence of catalyst [30, 31]. The catalyst selected in the polymerization reaction of CBT resin is butylchlorodihydroxytin (PC-4101). The resin is mixed with catalyst at the mass ratio 100:0.6. The reinforced material is woven carbon fabric and woven glass fabric with the surface density of 300g/m2 and 700 g/m2, respectively. Fig.1 shows the two-dimensional orthogonal plain woven fabrics used in the experiment. Table 1 lists the fabric parameters of the fibers. In the table, gw, gf are the gap between adjacent strands, and aw, af are the strand width. The superscript w and f indicate the warp and fill direction of the woven fabric, respectively. It should be noted that all the materials need to be dried in a vacuum oven for 12h as pretreatment.
In the work, PCBT casts are prepared to evaluate the matrix properties. In order to fabricate PCBT casts, melted CBT resin and catalyst were mixed at mass ratio 100:0.6. The mixture was poured into a metal mold and cured for 1h in a vacuum oven at 220oC, and then demoded at 100oC. Vacuum assisted prepregs process (VAPP) is adopted to manufacture the composite laminates. The prepregs are made with fiber to CBT resin mass ratio of 1:1. It is worth noting that the catalyst needs to be mixed with prepregs uniformly prior to the fabrication. Therefore, an approach similar to physical vapor deposition (PVD) method is adopted to deposit catalyst on the prepregs. The specific preparation processing is as follow: firstly, catalyst powder (0.6wt.% of the resin) was added in 200ml isopropanol aqua, and then stirred by magnetic stirrer at 70oC until completely dissolved. The woven fabrics were immersed in the prepared solution in a metal square plate holder. To ensure all the prepregs could blend fully with the solution, the system was kept in an incubator for 3h at room temperature. Finally, the prepregs need to be dried at 140oC to evaporate isopropanol aqua and left catalyst on the surface. Fig.2a indicates the schematic diagram of the VAPP setup used in this work. It is a RTM-like device with a port linked with the vacuum pump to give vacuum pressure in the polyimide bag. In the VAPP process, the preprges were placed on the polyimide film, and then sealed with heat-resistant sealant. The vacuum bag was placed between the upper and bottom panel of a hot press machine. The machine could provide temperature and pressure to satisfy the curing condition of the resin. It should be noted that the curing temperature during the process is a stage-like temperature: cure at 230oC for 1 hour and post-cure at 190oC for another hour. The composite laminates were demoded when the whole system cooled to room temperature. A total of 25-ply woven carbon-glass fabric reinforced PCBT resin interply hybrid composite laminates (CF-GF/PCBT) with each ply thickness of 0.2mm were prepared by VAPP. The hybrid laminate is a panel of alternate carbon layer and glass layer with two carbon layers as surface on both sides. Thus the panel is a symmetric laminate that has 13 layers of woven carbon fabrics and 12 layers of woven glass fabrics, as indicated in Fig.2b. In the following work, we use [C/G]13C-12G to indicate this layer form in hybrid laminates. In the hybrid composite laminates, the carbon to glass fiber hybrid mass ratio is 37:63. 25-ply woven carbon fabric reinforced PCBT composite laminates (CF/PCBT) were also prepared by VAPP. The mass ratio of hybrid composites to pure carbon composites is 100:74. These two kinds of panels with laminate thickness of 5mm will be
used in the drop-weight impact test. To characterize the fundamental mechanical behavior of the prepared materials, 12-ply pure carbon and pure glass fabric reinforced PCBT composite laminates with thickness of 2.4mm were respectively manufactured. Three types of tensile specimens were cut from the square laminate by a low-speed diamond saw blade cutting machine along transverse, longitudinal and 45o direction, respectively. Specimens with pre-crack were performed in the test to calculate the interlaminar strength. 2.2. Static tests In order to determine the material stiffness and strength parameter of CF/PCBT composites and woven glass fiber reinforced PCBT composites (GF/PCBT), tensile tests were respectively accomplished using INSTRON-4505 servo-electric testing machine. The dimension of the transverse and longitudinal tensile specimens is 200×25×2.4mm3 in accordance with the ASTM (D3039/D3039M-08) standard, and the calibrating length is 100mm. Dimension of the 45o tensile specimen is 120×25×2.4mm3 with calibrating length 80mm. To evaluate the through-the-thickness direction properties of the composite laminates, tensile and compression of PCBT casts were performed, respectively. Dimension of the specimen are 60×6×4mm3 and 25×10×10mm3 in tensile and compression tests, respectively. Additionally, double cantilever beam (DCB) and three-point end notched flexure (3ENF) tests were respectively adopted to estimate the interlaminar performance of the obtained composite laminates. Specimens with dimension of 200×25×2.4mm3 and 100×25×2.4mm3 were adopted in the tests according with the standard ASTM-D5528. The pre-crack with length of 25mm is laid in the middle layer of the each specimen, as indicated in Fig.3. DCB and 3ENF test were also performed by an INSTRON-4505 testing machine. In DCB test, a pair of hinges was glued on the upper and lower surface of the specimens by strong glue. During the experiment, they were used as fixture with the upper one carrying the external load and the lower one fixed on the test machine (Fig.3a). 3ENF test was performed in flexural mode as shown in Fig.3b. 3ENF specimen has a pre-crack at the end of the specimens. Tests were performed at ambient temperature (25oC), and five specimens per type. The cross-head speed was 2 mm/min. Fig.4 demonstrates the representative curves of macroscopic stress-strain of CF/PCBT and GF/PCBT composites: (a), (b) and (c), (d) are respectively the tensile properties of CF/PCBT and
GF/PCBT composites; (e), (f) are the tensile and compression properties of PCBT casts, and (g), (h) are the load-displacement curves of CF-GF/PCBT composite laminates obtained in DCB and 3ENF tests. Mechanical properties of the composite laminates determined from Fig.4 were listed in Table 2 and Table 3. In the tables, longitudinal elastic modulus E11, Poisson’s ratio µ 12, longitudinal tensile strengths Xt and transverse elastic modulus E22, transverse tensile strengths Yt were measured by tensile curves in Fig.4a to Fig.4d. It should be noted that the tensile test was respectively along longitudinal and transverse direction of the woven fabric composite specimens [32]. Shear modulus G and strength S were measured from 45o tensile curves in Fig.4b and Fig.4d. The specimens were loaded up to the failure loads in the axial direction. Elastic modulus E11, E22 and G were calculated from the initial slope of the stress-strain curves by the following formula:
E=
S 3m 4bh3
(1)
where E is the modulus, and S is the support span. m is the slope of the load-deflection curve, while b and h are the width and thickness of specimens, respectively. Tensile strengths of the composite laminates, Xt, Yt, and S, are determined by dividing the failure load to the cross-sectional area of the longitudinal, transverse and 45o tensile test specimens, respectively. The longitudinal and transverse compressive strengths, Xc and Yc, are obtained by dividing the failure load to the cross-sectional area of the specimens. Mechanical properties in the normal direction are estimated by compressing PCBT casts to failure. E33, Zt and Zc are determined from stress-strain curves in Fig.4e and Fig.4f. Zt and Zc are also obtained by dividing the failure load to the cross-sectional area of the specimens. Similarly, E33 is defined by Eq.1. Mode I and Mode II interlaminar fracture toughness determined from Fig.4g and Fig.4h can be respectively calculated by the following formula:
GⅠ c = GⅡ c =
3Pcδ c 2ba
9a0 2 Pcδ c 3 2b( l 3 + 3a03 ) 8
(2)
(3)
where GⅠ c and GⅡ c are the interlaminar fracture toughness; a0 and a are the pre-crack length and crack propagating length, respectively. Pc, δc are the load and displacement when the crack
propagates to a specific length, while b and l are the width and length of the specimens, respectively. In our work, Mode I and Mode II interlaminar fracture toughness can be used to evaluate the interlaminar performance of PCBT-based composite laminates, and the two values calculated from DCB and 3ENF curves were also listed in Table 3. 2.3. Drop-weight impact tests Square specimens with dimension of 100×100×5mm3 were cut from the prepared composite laminates, and they were used in the drop-weight impact tests. All impact tests were conducted on a PC-driven Instron/9250HV pneumatic-assisted instrumented drop-weight impact tester, which was equipped with a pneumatic brake to avoid multiple strikes. The square specimen was clamped by the test machine and left a diameter of 80mm area as impact region in the specimen. The impactor nose used in this study had a hemispherical shape with a radius of 6mm and the mass m=9.1445kg. The hemispherical impactor was dropped from a predetermined height to perform the impact test, as depicted in Fig.5. The impact force history was measured using a load cell located just above the impact head, and the impact velocity was recorded by a pair of photoelectric-diodes attached to the base of the test machine. 3. Experimental results 3.1. Perforation thresholds of the composite laminates One of the important considerations in designing the composite structure serviced in real industries is their perforation thresholds. The penetration threshold is defined as the impact energy when the impactor does not rebound from the specimen for the first time [33]. Fig.6 depicts the velocity of the two types of composite laminates subjected to various impact energies as a function of contact time. As can be seen, each curve is of the highest value at the beginning of impact event. With increasing of the contact time, velocity of the hemispherical impactor declines from 3m/s to 0m/s of CF/PCBT composites. Then the curve has negative values (-0.5m/s), which implies the rebounding of the impactor. However, velocity of impactor declines to approximately 1.5m/s from 5m/s. No negative section is observed in the figure, which implies that no rebounding happens and the specimen has been perforated. In case of CF-GF/PCBT hybrid composites, velocity in the curve shows similar tendency. It firstly decreases to 0m/s from 5m/s, and then the impactor is rebounded to 0.5m/s. At V=7m/s, velocity of the impactor decreases to 1.5m/s when
the specimen is perforated. Thus, penetration thresholds of the two composite laminates can be calculated by linear interpolation method from the velocity data in this study. Fig.7 illustrates the fitting results obtained from the initial velocity (Vi) and residual velocity (Vr) in the impact event. It can be seen in the figure that when Vr is zero, the initial velocity of the two laminates is 3.5m/s and 5.5m/s, respectively. Hence, 3.5m/s and 5.5m/s can be considered as penetration threshold of the two composite laminates. Moreover, velocity of the two specimens decreases to the same value (1.5m/s) when they are completely perforated. According to Newton’s law, it can be calculated that CF-GF/PCBT hybrid composites absorbs more impact energy than CF/PCBT composite laminates. Fig.8 illustrates the displacement-time (d-t) curves of the two composite laminates under various impact energies. To illustrate the problem better, curve of a duplicate sample is given in the figures. From the d-t curve, it can be found that deflection of CF/PCBT composites subjected to impact velocity V=3m/s recovers to its initial position after reaches to the maximum deformation with increasing of contact time. In contrast, deflection increases with time until to the penetration of CF/PCBT composite laminates at impact velocity of 5m/s. It worth mention that deflection of CF-GF/PCBT hybrid composite laminates at 7m/s varies the same way as CF/PCBT composites. It increases with contact time until the composite laminates damaged, and no decreasing of deflection is observed in Fig.8b. However, deflection of hybrid composites appears a different tendency at V=5m/s. Instead of declining to zero, it decreases to a special value after impact. This is mainly due to the existing of glass fibers in the hybrid composites. Because toughness of glass fibers is much higher than that of carbon fibers, deformation of CF-GF/PCBT hybrid composites is larger than that of CF/PCBT composites near the perforation thresholds region. As a result, pure carbon fiber based composites could recover to the initial state as soon as possible, while hybrid composites have unrecoverable plastic deformation after impact. Fig.9 shows the four contact force-time (F-t) curves of the composite laminates subjected to different impact energies. Contact force is an important parameter in impact problems and can be generally defined as the reaction force applied by specimen to the impactor [33]. In Fig.9, all the four F-t curves have mountain-like shapes. The maximum contact force increases with the increasing of impact energy acted on CF/PCBT composites. When the impact energy is low, such as 3m/s, the maximum contact force is 6kN. It increases to 8kN when the impact velocity
increases to 5m/s. For CF-CF/PCBT hybrid composites, the contact force is of approximately a constant peak value around 8kN both at impact velocity of 5m/s and 7m/s, as seen in Fig.9b. In addition, in the F-t curve of hybrid composites, the load increases to its maximum value slower and then decreases to zero sharply during the impact process. This leads to the larger area closed by F-t curve, and further leaves a platform on the F-t curve with increasing of contact time. However, in terms of CF/PCBT composites, the contact force increases to the maximum value with higher rise rate and decreases to zero slowly, resulting in pointy shape curves in Fig.9a. 3.2. Deflection based curves in the impact test Contact force-deflection (F-d) curve under various impact energies is a typical characteristic of composite material under impact load. Fig.10 denotes the F-d curves of CF/PCBT and CF-GF/PCBT composites subjected to different velocities. Like F-t curves presented in Fig.9, all the four curves collectively have a mountain-like shape. Individually, however, there are two basic curve types: closed curve and open curve. Fig.10a and Fig.10c are closed type curves, while Fig.10b and Fig.10d represents for the open curve. When the impact energy is lower, F-d curve is a closed type and the entire descending section consists of rebounding. In these cases, impact load dose not result in a serious damage to specimens. When the impact velocity is bigger, sharply decrease of impact force is found at the end of the impact event. Fig.11 shows the variation of impactor velocity versus deflection (v-d) under various impact energies. As seen in the figures, the velocity is of the highest value right before the contact happens between impactor and specimens. Magnitude of the velocity decreases in a parabolic shape and becomes zero after the maximum displacement is reached. Afterwards, the velocity has negative values and its absolute value increases with the decreasing of displacement. Then velocity reaches to a constant value, which implies the rebounding of impactor. Fig.11a, c and Fig.11b, d represent for the specimens undergoing partial perforation and complete perforation, respectively. The ratio of Vr/Vi could be used to further describe the material’s behavior under impact load. Generally, the magnitude of Vr/Vi could be positive or negative ranges from -1 to 1. When Vr/Vi is negative between -1 to 0, rebounding of the impactor takes place. If Vr/Vi equals 0 or is almost zero, the laminate is at the threshold moment of perforation. When Vr/Vi is positive between 0 to 1, it means that the material is completely perforated. The Vr/Vi ratio calculated from Fig.11 is shown in Fig.12. It is clear that for a given laminate, velocity ratio Vr/Vi various from
negative to positive with the increasing of impact energy. Additionally, for a given initial energy (e.g. V=5m/s), Vr/Vi of CF/PCBT composites is positive, whereas Vr/Vi of CF-GF/PCBT composites is negative. From the above discussion, it can be concluded that CF-GF/PCBT hybrid composites at hybrid mass ratio of 37:63 have better impact resistance performance compared with CF/PCBT composites. Addition of glass fabrics in carbon fabric reinforced PCBT composites has positive correlation. Due to the higher toughness of glass fabrics, woven glass fabrics have higher breaking extensibility than carbon fabrics in hybrid composite laminates. Hence, glass fabrics in hybrid composites could protect the whole panel from penetrating by its large deformation under low-velocity impact load. In addition, mainly due to the mismatch of deformation in glass and carbon fabrics in the event, opportunity of delamination between glass layers and carbon layers in hybrid composites is improved. Delamination could absorb some of the impact energy in the impact event. All the aforementioned factors could enhance the perforation thresholds of hybrid composites, and the enhancement effect will be further discussed in this paper according to the finite element method (FEM) simulation results. 4. FEM Simulation The finite element numerical approaches appear to be the best technique for analyzing the behavior of composite laminates since they are fairly accurate, less expensive and less time consuming. As discussed, the hybrid composite laminates could absorb more impact energy and have better impact resistance performance. Therefore, simulation work is mainly performed on CF-GF/PCBT hybrid composites to explain the detailed failure progress. In this work, all simulations are performed by ABAQUS software under boundary conditions similar to those in the real impact test. As known, the impact problem is a three-dimensional problem, however traditional composite laminate is treated as shell, which cannot solve the contact problem well [34]. In our work, a finite element models is developed to simulate the failure in composite laminates under impact load. Based on the three-dimensional finite element method, 3D Hashin criteria is adopted in the progress failure models. Results obtained in the experiments and simulations are compared. 4.1. Finite element model
The finite element model of the impact system with detail of hybrid composite laminates is illustrated in Fig.13. In the FEM model, the spherical shaped impactor is defined as a rigid body with radius of 12mm. To save the computation time, coarser meshes are used in the model far away from the maybe contact region. Fine meshes are applied right under the impactor to capture a relatively accurate result. Besides, unlike real experiment, the impactor is placed right on the composite panel with the given velocity of 5m/s and 7m/s, respectively. To make a reference, simulation work is also done on CF/PCBT composite laminates at V=5m/s. It should be mentioned that all element type is C3D8R in the simulation. All the material stiffness and strength parameters used in the FEM model are inputted according to the experimental results listed in Table 2 and Table 3. Hashin damage criteria is developed in 1980 which is used to simulate the damage evolution in unidirectional fiber reinforced composites [35]. Based on Hashin’s theory, user subroutine VUMAT is compiled to calculate the mechanical behavior of woven fabric composites in this paper. 3D Hashin failure criteria including five different failure modes (warp and fill fiber tensile failure, compression failure, and matrix compression failure) is as follow: Fiber tensile damage in warp direction:
e 2ftw = (
σ 11 Xt
)2 + (
σ 12 S12
)2 + (
σ 13 S13
)2 ≥ 1
(4)
Fiber compression damage in warp direction:
e 2fc w = (
σ 11 Xt
)2 ≥ 1
(5)
Fiber tensile damage in fill direction:
e 2ftf = (
σ 11 Yt
)2 + (
σ 12 S12
)2 + (
σ 23 S 23
)2 ≥ 1
(6)
Fiber compression damage in fill direction:
e 2fcf = (
σ 22 Yt
)2 ≥ 1
(7)
σ 13
(8)
Matrix compression damage:
edc2 = (
σ 23 S 23
)2 + (
S13
)2 ≥ 1
In the formulas, σ is the stress applied on the element along various directions of fiber reinforced composites, while S stands for the shear strength of the element. Xt, Yt are respectively the tensile strength along fill, warp and thickness directions of composite material, and e is defined as the coefficient to judge the failure of element. In the model, elements failure happens when any of the failure criteria listed in Eq.(4) to Eq.(8) is satisfied (e≥1), then the stiffness of the element will reduce to a special percentage of its original value according to the rule shown in Table 4 [36]. When fiber tensile damage in both the warp and fill direction appears together (eftw≥1 and eftf≥1), the element in the model will be deleted and cannot further carry load during the impact event. In terms of the interlamination mechanical behavior of composites, surface-based cohesive behavior (SBCB) is adopted between each glass and carbon fiber layer. SBCB uses the linear elastic traction-separation theory, which is similar to the cohesive element adopted by numerous authors [37-39]. In SBCB, delamination initiation happens when a quadratic interaction function involving the contact stress ratios (as defined in the Eq.9) reaches a value of one. This function is as follow: 2
2
2
⎧ tn ⎪⎫ ⎧ ts ⎫⎪ ⎧ tt ⎪⎫ ⎨ o ⎬ + ⎨ o ⎬ + ⎨ o ⎬ =1 ⎩ tn ⎪⎭ ⎩ ts ⎪⎭ ⎩ tt ⎪⎭
(9)
While damage evolution is based on total energy release rate, as follow:
Gn + Gs + Gt =1 Gc
(10)
Here, tn, ts, and tt are the normal and shear components of tractions, and t no , t so , tto are the corresponding peak values of the contact stress when the separation is purely in the normal or shear direction, respectively. Gn, Gs, and Gt are the work done by the normal and shear tractions, and Gc is the critical total energy release rate of the material. 4.2. Simulation results and discussion 4.2.1. Results comparison and damage process of hybrid composites in the impact event In Fig.14, F-t curves obtained in the experiment and FEM simulation at impact velocity V=5m/s and V=7m/s are compared. It can be concluded from the figure that remarkably close agreement is obtained between simulation and experiment. Compared with experiment results, error of contact force of model developed in ABAQUS/Explicit is firstly small and then large as the function of contact time. Discrepancies between numerical and experimental results in Fig.14
can be interpreted as follow: at the beginning of the impact event, no element in the model is damaged and contact is just existed between the upper surface and impactor, and good match between experiment and simulation results is achieved. However, since the impactor is considered as rigid body, the energy absorbed by it is neglected. As a result, stiffness degrades faster under higher energy with increasing of contact time, and the curves appear as sharply decline at the moment when the element failed. Fig.15 and Fig.16 show the simulation results of hybrid composites subjected to various impact velocities as a function of contact time. It can be found that deformation of hybrid composites increases with contact time until rebounding occurs at V=5m/s, while it increases until the hybrid laminates are completely penetrated at V=7m/s. At V=5m/s, damage of the laminates firstly come into being at t=6ms, and the laminates are half perforated at t=8ms (Fig.15). When the impact energy is higher (V=7m/s), the laminate damaged rapidly. As seen in Fig.16, damage of hybrid composites emerges at t=1ms. The cross-section of the damaged CF-GF/PCBT composite laminates in the experiment is also shown in Fig.15 and Fig.16. As seen in the figures, damage morphology of the impacted specimens in the experiment and simulation matches well. It also should be noted that damage area in both cases is not high, and damage location is around the contact region between the impactor and panel in the center of the specimens. Moreover, it can be observed from the cross-section of the damaged specimens that the damage morphology includes warp and fill fiber breaking and delamination between two layers at V=5m/s in Fig.15. However at V=7m/s, fiber breaking is the main damage mode. Progressive failure of glass and carbon fabrics in interply hybrid composites as a function of layer position is shown in Fig.17. Here V=5m/s, field variable is used to represent damage of laminate in the explicit model. It can be found in Fig.17 that the critical damage in hybrid composites in ply-16. In other words, the layers located up to ply-16 are damaged, whereas under ply-16 are undamaged under the velocity of 5m/s. 4.2.2. Damage modes in the simulation and experiment of different composite laminates Fig.18 shows the damage pattern on the contact surface in different composite laminates after impact in the experiments. It can be found the dent and hole clearly on the impacted laminates. For CF-GF/PCBT panel, at velocity of 5m/s, impact damage is initiated firstly as matrix crack in the materials, and then this crack cut some layers of carbon and glass fabrics on the contact surface, as shown in Fig.18c and Fig.15. This damage process is accompanied by delamination
between glass and carbon layers, as verified by the cross-section of damaged specimens in Fig15. Thus, impact energy is mainly absorbed by fiber breaking and delamination between layers in hybrid composites. At V=7m/s, the energy available is high enough to cut through the fabrics in both fill and warp direction. Hence, the hybrid composite laminates are completely penetrated as shown in Fig.18d and Fig.16. In case of CF/PCBT composites, at lower impact velocity (V=3m/s), the primary damage mode is indentation-induced matrix cracking on the impact surface (Fig.18a). To study the detailed damage mode of CF/PCBT composites at impact velocity V=5m/s, we give the simulation results in Fig.19. As seen in Fig.18b and Fig.19, CF/PCBT laminates are completely penetrated at V=5m/s. Fiber breaking is the main damage mode, and the damage morphology matches well in the experiment and simulation. In Fig.19, we also compared the damage morphology of CF/PCBT and CF-GF/PCBT composite laminates after impact at V=5m/s in the simulation. In the figure, carbon fabrics in CF/PCBT break easily under dynamic loads due to their brittle nature, while they are difficult to be broken under the protection of glass fabric layers in the hybrid composites. In CF-GF/PCBT at V=5m/s, because the impact energy is mostly absorbed by carbon and glass fabric breaking and delamination under the contact surface, carbon and glass fabric layers under the non-contact surface are not broken. As a result, the hybrid composites show better impact damage resistance and tolerance. 5. Conclusions In this study, hybrid effect on low-velocity impact response of interply hybrid composites prepared by vacuum assisted prepregs process (VAPP) is investigated by experiments and finite element methods. From the study, the conclusions can be summarized as follows:
Specimens based on fiber/CBT prepregs are successfully manufactured by VAPP in this work, and the fundamental mechanical parameters of the materials are obtained experimentally.
At hybrid mass ratio of 37:63, intraply hybrid laminates with layer form [C/G]13C-12G could enhance the impact-resistance performance of CF/PCBT composites significantly. Addition of glass fabrics in carbon fabric reinforced PCBT composites has positive correlation. By adding glass fibers in woven carbon fabric reinforced PCBT resin laminates, perforation thresholds enhanced from 3.5m/s to 5.5m/s.
The progressive failure model developed in ABAQUS using VUMAT subroutine can be used to predict the failure of interply hybrid composites based on glass and carbon woven fabrics,
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Figure caption Fig.1. Schematics of the plain woven fabrics. Fig.2. Schematic diagram of the manufacturing process and hybrid form of the composite laminates. Fig.3. Schematic diagram of DCB test and 3ENF test used in the experiment. Fig.4. Typical curves used to determine the strength and modulus of the materials: (a), (b) and (c), (d) are the tensile stress-strain curves of CF/PCBT and GF/PCBT composite laminates; (e) and (f) are tensile and compression curves of PCBT casts; (g) and (h) are the load-displacement curves of CF-GF/PCBT composite laminates obtained in DCB and 3ENF tests. Fig.5. Drop-weight impact setup used in the impact test of PCBT resin based composites. Fig.6. Comparison of velocity-time curves of different composite laminates in impact tests. Fig.7. Penetration threshold of the materials obtained by linear interpolation method. Fig.8. Comparison of deflection-time curves of different composite laminates (including a duplicate sample). Fig.9. Typical contact force history of different composite laminates at various impact velocities. Fig.10. Variation of contact force of different composite laminates as a function of deflection (including a duplicate sample). Fig.11. Velocity of impact head as a function of deflection of specimens (including a duplicate sample). Fig.12. Ratio of residual velocity to initial velocity of various specimens under different impact energies. (In the figure, N denotes negative, while P denotes positive value.) Fig.13. FEM model of the composite laminates and hemispherical impactor. Fig.14. Comparison between experiment and simulation results of hybrid composite laminates Fig.15. Damage cross section of CF-GF/PCBT hybrid composites as a function of contact time at V=5m/s (the impactor is removed for easy observation). Fig.16. Damage cross section of CF-GF/PCBT hybrid composites as a function of contact time at V=7m/s. Fig.17. Damage process as a function of layer position with impact velocity V=5m/s. Fig.18. Contact surfaces of different specimens at different velocities in the experiment. Fig.19. Comparison of damage morphology in CF/PCBT and CF-GF/PCBT laminates after impact at V=5m/s.
Fig.1. Schematics of the plain woven fabrics.
(a) Vacuum bag-assisted hot-press processing
(b) Structural style of the hybrid composite laminates Fig.2. Schematic diagram of the manufacturing process and hybrid form of the composite laminates.
(a) DCB test
(b) 3ENF test
Fig.3. Schematic diagram of DCB test and 3ENF test used in the experiment.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Fig.4. Typical curves used to determine the strength and modulus of the materials: (a), (b) and (c), (d) are the tensile stress-strain curves of CF/PCBT and GF/PCBT composite laminates; (e) and (f) are tensile and compression curves of PCBT casts; (g) and (h) are the load-displacement curves of CF-GF/PCBT composite laminates obtained in DCB and 3ENF tests.
Fig.5. Drop-weight impact setup used in the impact test of PCBT resin based composites.
Fig.6. Comparison of velocity-time curves of different composite laminates in impact tests.
Fig.7. Penetration threshold of the materials obtained by linear interpolation method.
(a) (b) Fig.8. Comparison of deflection-time curves of different composite laminates (including a duplicate sample).
(a)
(b)
Fig.9. Typical contact force history of different composite laminates at various impact velocities.
(a)
(b)
(c) (d) Fig.10. Variation of contact force of different composite laminates as a function of deflection (including a duplicate sample).
(a)
(b)
(c) (d) Fig.11. Velocity of impact head as a function of deflection of specimens (including a duplicate sample).
Fig.12. Ratio of residual velocity to initial velocity of various specimens under different impact energies. (In the figure, N denotes negative, while P denotes positive value.)
Fig.13. FEM model of the composite laminates and hemispherical impactor.
Fig.14. Comparison between experiment and simulation results of hybrid composite laminates.
Fig.15. Damage cross section of CF-GF/PCBT hybrid composites as a function of contact time at V=5m/s (the impactor is removed for easy observation).
Fig.16. Damage cross section of CF-GF/PCBT hybrid composites as a function of contact time at V=7m/s.
Fig.17. Damage process as a function of layer position with impact velocity V=5m/s.
Fig.18. Contact surfaces of different specimens at different velocities in the experiment.
Fig.19. Comparison of damage morphology in CF/PCBT and CF-GF/PCBT laminates after impact at V=5m/s.
Tables Table 1. Dimension of plain woven fabric structure.
Material
aw (mm)
gw (mm)
af (mm)
gf (mm)
Carbon fiber
3
0.2
2.9
0.6
Glass fiber
5
1
4
1.2
Table 2. Stiffness properties of woven fabric reinforced PCBT composites from experimental tests.
Material
E11 (GPa)
E22 (GPa)
E33 (GPa)
G12 (GPa)
G13 (GPa)
G23 (GPa)
µ 12
µ 13
µ 23
GF/PCBT CF/PCBT
14.73 25.7
14.73 25.7
10.9 15.9
1.789 3.5
1.43 1.43
1.43 1.43
0.25 0.2
0.5 0.35
0.5 0.35
Table 3. Material strength data of fiber reinforced PCBT composites (MPa).
Material
Xt
Xc
Yt
Yc
Zt
Zc
S12
S13
S23
Mode I kJ/m2
Mode II kJ/m2
GF/PCBT CF/PCBT
356.53 400.8
300 300
320 387.5
280 280
50 50
230 230
25 32
13 15
13 15
1.5 1.5
1.23 1.23
Table 4. Stiffness degradation rule in VUMAT. E1
E2
E3
G12
G13
G23
μ12
μ13
μ23
Tensile break of warp fiber
0.1
—
—
0.1
0.1
—
0.1
0.1
—
Compress damage of warp fiber
0.2
—
—
0.2
0.2
—
0.2
0.2
—
Tensile break of fill fiber
—
0.1
—
0.1
—
0.1
0.1
—
0.1
Compress damage of fill fiber
—
0.2
—
0.2
—
0.2
0.2
—
0.2
Tensile break in z direction
—
—
0.1
—
0.1
0.1
—
0.1
0.1
Compress damage in z direction
—
—
0.2
—
0.2
0.2
—
0.2
0.2