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Low-velocity impact behavior and residual tensile strength of CFRP laminates
Composites Part B 161 (2019) 300–313
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Low-velocity impact behavior and residual tensile strength of CFRP laminates
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Jianwu Zhoua, Binbin Liaob, Yaoyao Shia,∗, Yangjie Zuoc, Hongliang Tuod, Liyong Jiae a
School of Mechanical Engineering, Northwestern Polytechnical University, Xi'an, 710072, China Institute of Process Equipment, Zhejiang University, Hangzhou, 300027, China c School of Aeronautics and Astronautics, Sichuan University, Chengdu, 610065, China d School of Aeronautics, Northwestern Polytechnical University, Xi'an, 710072, China e First Aircraft Institute of Aviation Industry Corporation, Xi'an, 710089, China b
ARTICLE INFO
ABSTRACT
Keywords: Impactor parameters Layup patterns Impact behaviors Tensile strength
This paper experimentally studies the low-velocity impact behaviors and residual tensile strength of carbon fiber reinforced plastics (CFRP) laminates. Firstly, the effects of four factors, i.e. impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, on the impact responses of laminates are studied. The damage characteristics are evaluated by dent depth, delamination damage projection area (DDPA) and energy dissipation. Secondly, the tensile responses of the laminates after impact are investigated based on residual tensile strength (RTS). Finally, the correlations between the four damage evaluation criteria, i.e. dent depth, DDPA, energy dissipation and RTS, are sorted out. Experimental results demonstrate that the four factors have significant effects on the impact behaviors of laminates in different ways. The DDPA is negatively correlated with the dent depth of laminates, and the dent depth can be treated as an important reference for the RTS. Besides, a fracture phenomenon, i.e. a clear band of fiber fracture around the impact area after impact, has been observed and discussed.
1. Introduction
strength was found in all specimens [9]. The similar conclusions in tensile strength reduction after impact were also obtained by Koo et al. [10]. Apart from the impactor shape, Amaro et al. [11], Icten et al. [12] and Evci et al. [13] studied the effect of impactor diameter on lowvelocity impact. These researches almost obtained the same conclusion that impactor diameter had an obvious influence on the impact behaviors of laminates by affecting the target stiffness. In fact, the impact angle also has a certain influence on initial damage and residual performance of laminates. However, the previous researchers focused on the conventional oblique impact [14,15], as shown in Fig. 1(a), and basically no work explored the special impact angle, as shown in Fig. 1(b), which is also likely to occur during service. Moreover, the influence of layup patterns on the impact behaviors of laminates also has been widely studied. References provided contradictive information concerning the influence of stacking sequence on impact resistance. Some researchers claimed that such influence was not significant [16], however, others held the opposite opinion. Czanocki [17] suggested that clustering reinforcement plies of the same orientation produced negative effects regarding the resistance of laminates against impact-induced damage. Lopes et al. [18,19] redesigned
For the high specific strength, high specific stiffness and perfect fatigue resistance [1–3], composite materials have been widely used in some important parts of the turbofan engine [4–6]. However, the aeroengine of plane is close to the ground during taking off and landing, and the fan blade is prone to be damaged by the low-velocity impact caused by inhaling sundries on the runway. As known, the composite fan blade always bears huge centrifugal force for its high-speed revolution and the composite laminates are sensitive to low-velocity impact [5], which causes the reduction of the strength by up to 70% and even more [7]. Therefore, the study of impact behaviors and residual tensile strength of CFRP laminates is necessary to improve the impact resistance and damage tolerance of the composite fan blade. It has been observed that the impactor parameters affect the impact behaviors of laminates significantly. Mitrevski et al. [8] explored with three different impactor shapes and found that the energy dissipation, peak impact force and contact duration of laminates all exhibited sensitivity to the impactor shape. Furthermore, the blunter impactor caused the larger damage area, and a significant reduction in tensile
∗
Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Shi).
https://doi.org/10.1016/j.compositesb.2018.10.090 Received 5 July 2018; Received in revised form 23 September 2018; Accepted 26 October 2018 Available online 28 October 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. Two different types of impact angles: (a) the angle α between the impact direction and the normal direction of the plane; (b) the angle β between the axis of the dent and the 0° direction of the laminate.
types of impactors are designed and used for low-velocity impact test and quasi-static tensile test. In order to comprehensively and effectively evaluate the damage status of laminates, four different damage evaluation criteria, i.e. the dent depth, DDPA, energy dissipation and RTS are adopted, and the correlations between them are discussed.
Table 1 Laminate details. Type
Layup pattern
The proportion of the plies with 0° fiber
T1 T2 T3 T4 T5
[904/04]4T [452/02/-452/902]2S [452/03/-452/90]2S [-452/03/90/452]2S [90/03/452/-452]2S
50% 25% 37.5% 37.5% 37.5%
2. Material and methods 2.1. Composite material and fabrication
the stacking sequence of laminates, and an improvement in the damage resistance was observed whilst keeping the similar in-plane and bending stiffness of laminates. Furthermore, scholars [20–23] improved the low-velocity impact performance and compression strength of laminates after the impact by using a genetic algorithm. Actually, the proportion of ply orientation is an important embodiment of the layup patterns of laminates, which has a certain effect on the overall performance of laminates because of the anisotropy of the composites. However, few kinds of literature mention it, and the further experiments are necessary to provide data for exploring the influence of layup patterns. The damage evaluation criteria (including dent depth, DDPA, energy dissipation, RTS, etc.) are vital for the assessment of the damage status of laminates. Aktaş et al. [24], Evci et al. [25] and Li et al. [26] indicated that the energy dissipated by the laminates was closely related to the damage status, while it had no obvious relation with the DDPA [27,28]. Besides, the dent depth in the impact face was positively correlated with the impact energy [29,30]. Balasubramani et al. [31] and Khan et al. [32] explored the RTS of the laminates after low-velocity impact to assess the internal damage status and attempted to establish a connection with impact energy. The results demonstrated that there was no fixed correlation between the RTS and the impact energy. Similarly, the crack length [33,34] on the impact face was also adopted to assess the damage status. Unfortunately, although the damage evaluation criteria have been studied by some researchers individually, the correlations between them are still ambiguous, which can be used to assess the damage status of laminates effectively through some easy available parameters. This paper studies the influence of four factors, i.e. impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, on the impact behaviors of laminates by using experimental means. Five kinds of laminates with different layup patterns and two
Five kinds of CFRP laminates with different layup patterns were manufactured, as listed in Table 1. The laminates were made of T300/ YH69 unidirectional prepreg by autoclave molding. Each laminate was made of 32 plies with the nominal cured ply thickness of 0.12 mm. The whole lay-up process was carried out at a constant environment condition with a temperature of 25 ± 2 °C and relative humidity of 40 ± 5%. The test specimens of 230 × 50 mm were obtained from the original composite laminates using water-jet cutting, and the average thickness of specimens was 3.75 mm. 2.2. Fixture and impactor The fixture which includes the cover plate, the guide plate and the supporting block is shown in Fig. 2. Two rubber gaskets were placed between the cover plate and specimens to fix the specimens firmly during the impact test. Three impactors were manufactured to study the effects on the impact behaviors of laminates: one strip impactor and two hemispherical impactors. Among them, the strip impactor (Fig. 3(a)) was designed with the length of the impact part equaling to 19 mm. The diameters of the hemispherical impactors (Fig. 3(b) and c) were 10 mm and 16 mm, respectively. 2.3. Experimental investigation The impact test was performed according to ASTM D7136 [35] using the Instron Dynatup 9250HV Impact Testing Machine. The impact energy was calculated by:
E = CE h
(1)
where CE is the specified ratio of impact energy to specimens thickness, 6.7 J/mm; h is the thickness of the specimens, mm. 301
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Fig. 2. The fixture.
Specimens were fixed in the fixture, the anti-rebound device was activated to catch the impactor for preventing the second impact, and the total mass of impact part was 5.607 kg. During the impact, a digital data acquisition unit was used to record the impact force-time curves, the impact velocity and the energy history. The dent depth of each specimen was measured after impact within 10 min through the high-precision CCD laser displacement sensor LK-GD500, as shown in Fig. 4. The specific test arrangements about the four factors are shown in Table 2. The quasi-static tensile test was conducted on the control specimens and the damaged specimens with the tensile rate of 2 mm/min according to ASTM D3039 [36]. Here, the control specimens were the original specimens without impact damage, while the damaged specimens were the specimens after the impact tests with impact damage. All the specimens were tested by the same tensile testing machine called CRIMS DDL300.
3. Results and discussion 3.1. Drop-weight impact analysis 3.1.1. Damage inspection of specimens for different factors The statistical results of dent depth and DDPA of specimens are shown in Table 3. Moreover, the typical pictures of the C-scan test for the delamination area and the impact damage of specimens are shown in Fig. 5 and Fig. 6, respectively. The four factors are studied. Generally, there are two main reasons for delamination during impact. On the one hand, the relative slippage between plies occurs because of the bending of laminates, which resulting the interlaminar shear stress then causes the interlaminar damage [37]. On the other hand, the load propagates in the laminates in the form of the stress wave. When the stress wave reaches the back face of the laminates, the
Fig. 3. The impactors: (a) strip impactor; (b) and (c) hemispherical impactors with the diameter of 10 mm and 16 mm, respectively. 302
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Fig. 4. Methods for measuring dent depth. Table 2 Low-velocity impact test arrangements. Test
Key factor
Type
Impactor parameter
Impact energy
Impact velocity
Times
1
Impact angle
T1
10 J
1.88 m/s
2
Impactor diameter
T2
25.1 J
2.96 m/s
3 4
The proportion of ply orientation Stacking sequence
T3 T4, T5
Strip impactor, ∥ Strip impactor, ⊥ Hemispherical impactor, Hemispherical impactor, Hemispherical impactor, Hemispherical impactor,
25.1 J 25.1 J
2.96 m/s 2.96 m/s
5 5 5 5 5 5
10 mm 16 mm 16 mm 16 mm
Note. Symbol “∥” means that the strip impactor is parallel to the 0° direction of specimens. Symbol “⊥” means that the strip impactor is perpendicular to the 0° direction of specimens.
certain influence on the dent depth and DDPA of laminates. This is possible because the fiber orientation in the 4 plies near the impact face is 90°. During the impact that is parallel to 0° direction of specimens, the fibers in these 4 plies bear the main load, which results in a smaller dent depth. Whereas during the impact that is perpendicular to 0° direction of specimens, the fibers in these 4 plies bear the lesser load and the more severe matrix damage occurs, which lead to a larger dent depth. Furthermore, the different damage status is also reflected in the interlaminar damage. Specifically, the DDPA of the T1∥ is 2217.8 mm2, which is 5.3% larger than that of the T1⊥.
Table 3 Summary of the impact test results. Specimen
Dent depth (0.01 mm)
DDPA (mm2)
EDissipation (J)
EDissipation/EImpact (%)
T1∥ T1⊥ T210 T216 T316 T416 T516
14.2 16.9 92.5 62.6 75.1 72.9 71.7
2117.8 2011.5 2823.7 3178.7 3017.4 3110.3 3195.1
8.78 7.65 21.53 16.15 17.96 17.24 16.88
87.8 76.5 85.8 64.3 71.6 68.7 67.3
3.1.1.2. Impactor diameter. T210-A and T216-A are the typical pictures of impact damage of T2 type of specimens, which are caused by the hemispherical impactors with the diameters of 10 mm and 16 mm, respectively. The results show that the dent depth of T210 is 0.925 mm, which is 47.8% larger than that of T216. This is mainly because the 10 mm impactor and specimens have less contact area and more severe intralaminar damage occurs within the specimens. Moreover, the DDPA of T216 is 3178.7 mm2, which is 12.6% larger than that of T210, and the phenomenon can also be validated in Refs. [12,13]. This is due to the
obvious reflection will occur to produce the tensile wave which resulting the tensile stress [37,38]. The tensile stress maybe accelerates the initiation and evolution of damage at the interface between plies. 3.1.1.1. Impact angle. T1∥-A and T1⊥-A are the typical pictures of impact damage of T1 type of specimens, which are caused by the strip impactor that is parallel and perpendicular to 0° direction of the specimens, respectively. The results indicate that the impact angle has a
Fig. 5. The C-scan results of all types of specimens after impact. 303
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Fig. 6. Impact damage of all types of specimens and the ring fracture.
fact that the impact force of 10 mm impactor is concentrated in the smaller area, which resulting the more severe fiber and matrix damage in this area, hence, the DDPA is smaller. Whereas the delamination could further develop to the larger area during the impact by 16 mm impactor.
comparing with the T316, the three types of specimens have little difference in their dent depth. Exactly, the maximum difference in dent depth is only 4.7%. The reason can also be explained by the hypothesis of mutual support between adjacent plies mentioned earlier, and the reasonability of the hypothesis is verified once again. Moreover, similar to the dent depth, the maximum difference in DDPA is only 5.9%. This indicates that the effect of stacking sequence on the interlaminar performance is limited. However, combined with the analysis of the above four factors, whether the difference in dent depth (or DDPA) is significant or not, they all have the same correlation that the larger dent depth corresponds to the smaller DDPA. On the other hand, a phenomenon can be observed. There always exists a clear band of fiber fracture around the impact area after impact by different sizes of hemispherical impactors, named as “ring fracture” in this paper. The reason could be that the out-of-plane load on specimens increases continuously during the loading of the impactor, and the closer to the center of dent leads to the greater vertical load, as shown in Fig. 8(a). The fiber breakage occurs when the vertical load exceeds the limitation of the fiber, then the matrix begins to broken rapidly. Therefore, the ring fracture is the critical position of a large number of fibers fracture in the impact area, as shown in Figs. 8(b) and Fig. 9. Furthermore, the position of ring fracture after impact by 10 mm impactor is closer to the center of the dent. Since the change of the vertical load to the specimens of 10 mm impactor is more drastic than
3.1.1.3. The proportion of ply orientation. T316-A is a typical picture of impact damage of T3 type of specimens. Compared with the specimens after impact by 16 mm impactor in the T2 type of specimens, the dent depth of the T316 is 0.751 mm, which is 20% larger than that of the T216. It could be that the next ply will support the previous one when the specimens are impacted by the out-of-plane load. If the two adjacent plies are in the different orientation, the support force is dominated by the fibers, as shown in Fig. 7(a). However, if the two adjacent plies are in the same orientation, the proportion of the support force provided by the matrix increases, as shown in Fig. 7(b). However, the matrix is more prone to be damaged than the fiber, therefore, the more continuous plies in the same orientation for T316 laminates provide the weaker mutual support which causes the larger dent depth. Nevertheless, opposite to the dent depth, the DDPA of T316 is 3017.4 mm2, which is 5.1% smaller than the value of T216. 3.1.1.4. Stacking sequence. T416-A and T516-A are the typical pictures of impact damage of T4 and T5 types of specimens, respectively. By
Fig. 7. Two different support: (a) fiber dominated; (b) matrix dominated. 304
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Fig. 9. Scanning electron microscopic (SEM) of ring fracture and fiber breakage.
ability of laminates to resist initiation delamination. Moreover, delamination is the dominant failure mode under out-of-plane load [40], and the bending stiffness of the laminates decreases rapidly because of the delamination [26,41]. Furthermore, the impact force perturbation after FH implies the propagation of the in-plane damage, then a varying degree of reloading to the maximum impact force FMax happens because of the different potential energy which stores in the impactor. As shown in Fig. 11(a), the value of FH,T1∥ is 3761 N, which is 17.1% larger than that of FH,T1⊥. It indicates that a larger load is required for the initial delamination of the laminates when the impact of the strip impactor is parallel to 0° direction of the specimens. However, the value of FMax,T1∥ is 4102 N, which is 8.2% smaller than that of FMax,T1⊥. This may be because when the impact of the strip impactor is perpendicular to 0° direction of the specimens, the impactor contacts with the 90° fibers (parallel to the impactor) first, and the resistance at this stage is relatively small. Then the impactor continues to move downward, the 0° fibers (perpendicular to the impactor) are also involved in the resistance, hence, the resistance at this stage increases obviously. Whereas when the impact of the strip impactor is parallel to 0° direction of the specimens, the situation of resistance by fibers is opposite. Thus, the impactor is subjected to a larger resistance in the early stage, while the resistance can not increase obviously in the latter stage. As shown in Fig. 11(b), the value of FH,T216 is 3241 N, which is 27.7% larger than that of FH,T210. This is because more fibers and matrix are involved in resisting the impact of 16 mm impactor due to the larger contact area. Whereas the impact force of 10 mm impactor is more concentrated, and the more severe damage occurs in specimens, which result in the stiffness degradation of specimens more significant. Hence, the value of FMax,T210 is 6635 N, which is 24.3% smaller than that of FMax,T216. On the other hand, the maximum difference between FH (FMax) in Fig. 11(c) and (d) is 5.7% (1.8%) and 10.8% (13.8%), respectively. The overall fluctuation trend of the curves in Fig. 11(c) and (d) are almost identical except the T416 curve which includes the largest value of FH, the smallest value of FMax and the longest contact duration. This illustrates that the effect of stacking sequence on impact responses of laminates is more significant than that of the proportion of the ply orientation in this paper.
Fig. 8. (a) Sketch of the impactor loading; (b) Sketch of the impactor unloading; (c) Sketch of the different position of ring fracture.
16 mm impactor due to the bigger curvature, which leads to the position of ring fracture after impact by 10 mm impactor is closer to the center of the dent, as shown in Fig. 8(c). 3.1.2. Impact force-time history The impact force-time curves during impact are shown in Fig. 10. The method of fast Fourier transform (FFT) is used to filter the impact force oscillation. All types of laminates are almost consistent in impact force-time history, and the coefficient of the variation of the impact force measurements is less than 10%. Some differences are observed from the impact force responses of these specimens, which can be influenced by their damage mechanisms [34]. The Fig. 11(a), (b), (c) and (d) is the typical impact force-time curves of specimens, which focuses on the impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, respectively. In the elastic region, the impact force linearly increases until it reaches a characteristic impact force FH at which the stiffness changes due to the Hertzian failure [25,27,39]. Therefore, the value of FH is regarded as a pointer of the
3.1.3. Impact force-central displacement response The typical impact force-central displacement curves during impact are demonstrated in Fig. 12(a), (b), (c) and (d), which focuses on the impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, respectively. The curves can generally be divided into three zones of A, B and C, which is according to the damage 305
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Fig. 10. Impact force-time curves: (a) impact of strip impactor that is parallel to 0° direction of T1 type of specimens; (b) impact of strip impactor that is perpendicular to 0° direction of T1 type of specimens; (c) impact of 10 mm hemispherical impactor to T2 type of specimens; (d), (e), (f), (g) impact of 16 mm hemispherical impactor to T2, T3, T4, T5 type of specimens, respectively. 306
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Fig. 11. Typical impact force-time curves which focus on: (a) impact angle; (b) impactor diameter; (c) the proportion of ply orientation; (d) stacking sequence.
initiation of specimens, damage evolution of specimens and specimens rebounding. However, the Fig. 12(a) shows a significant difference from the others. This may be related to the impactor shape and impact energy. Zone A implies the elastic responses of the specimens until the impact force reaches the damage initiation threshold. In Zone B, the damage initiates and evolves during the impact, and the impact force perturbations are associated with the delamination, fiber and matrix damage within plies, i.e. damage propagation [30,39]. Finally, zone C indicates the specimens rebounding while the impactor is returned by the stored elastic energy in the laminates until it fully rebounds. The corresponding central displacement at the end of zone C indicates the residual, permanent depression of the laminates to some extent [34]. There is an obvious boundary of zone A and B only during the 10 J impact by strip impactor in Fig. 12(a), it indicates that the impactor shape and impact energy have great influences on the damage propagation of laminates. However, the other factors have little effects on the phenomenon of the zone boundary in this paper. Generally, the larger zone B is an indication of more damage evolution within the laminates [34,42]. The difference of the maximum central displacement is 0.11 mm, 0.66 mm, 0.17 mm and 0.39 mm in Fig. 12(a), (b), (c) and (d), respectively. Particularly, the maximum central displacement of impact by 10 mm impactor is 5.84 mm, which is 12.7% larger than that of the 16 mm one. The result shows that the more severe damage is caused by the 10 mm impactor during the impact. Then the severe damage leads to a smaller elastic deformation that stores the rebound energy, hence, the residual central displacement is larger. Furthermore, it can be inferred that the more severe damage in T210 than that in T216 is mainly
reflected in the fiber and matrix damage but not the delamination by combining with the results of the dent depth and DDPA after impact. Besides, the deflection curves which are influenced by the proportion of ply orientation and stacking sequence show the high consistency, respectively. This is because the deflection of laminates is closely related to the out-of-plane load and the bending stiffness of themselves, meanwhile, the phenomena coincide with the corresponding impact force-time curves in Fig. 11(c) and (d). 3.1.4. Transferred energy-time history The total energy dissipated by the specimens with time is determined by calculating the area under the impact force-central displacement curve [34,43]. The transferred energy-time curves are given in Fig. 13(a), (b), (c) and (d), which focuses on the impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, respectively. When the impactor starts to contact with the specimens, the energy dissipation starts to appear and the specimens deform by absorbing the kinetic energy of the impactor. When the impactor reaches the lowest end, all kinetic energy has been transformed partly into the strain energy and partly into the dissipated energy. Then the impactor rebounds and elastic unloading occurs, and all recoverable strain energy of the specimens transforms into the kinetic energy of the impactor until separation. The final energy value demonstrates the total amount of energy dissipated by specimens largely through damage evolution. The peak energy moments of specimens exhibit significant differences in Fig. 13(b) and (c). Specifically, the difference in peak energy 307
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Fig. 12. Typical impact force-central displacement curves which focus on: (a) impact angle; (b) impactor diameter; (c) the proportion of ply orientation; (d) stacking sequence.
moment influenced by the impactor diameter and the proportion of ply orientation is 0.56 ms and 0.26 ms, respectively. This is because the laminates are damaged by impactor during impact, and the damage reduces the transverse combined stiffness of laminates which directly determines the acceleration of the impactor during impact [44,45]. Therefore, the greater damage difference of laminates results in the greater difference at the moment when the velocity of impactor decelerates to zero (equal to the peak energy moment). As shown in Table 3, the impactor diameter and the proportion of ply orientation have greater influences on the damage difference of laminates than other factors, thus the difference in peak energy moment is more obvious in Fig. 13(b) and (c). In addition, the concept of energy dissipation rate (EDissipation/EImpact, expressed in percentage, where the EDissipation and EImpact is the total energy dissipation and impact energy, respectively) is introduced to state the results more intuitively. The difference in energy dissipation rate of specimens in Fig. 13(a), (b), (c) and (d) is 11.3%, 21.5%, 7.3% and 4.3%, respectively. The results indicate that the four factors affect the energy dissipation of laminates. This is because the final energy dissipation is the sum of energy dissipated by various damages in the laminates. However, the four factors affect the final energy dissipation in different ways. The impact angle affects the final energy dissipation by causing the different fiber damage status because of the difference in the quantity of contact between the impactor and fibers in the surface plies. The impactor diameter affects the final energy dissipation by causing the different intralaminar damage status because of the difference in the contact area between the impactor and laminates. Moreover, for the proportion of ply
orientation, the more continuous plies in the same orientation provide the weaker mutual support between adjacent plies, which resulting the more severe damage during impact. The interface angle (defined as the angular difference in fiber orientation between two plies) affects the damage resistance [20,21,46]. Especially, when the interface angle is larger than 75°, the larger angle leads to the weaker interlaminar performance [47]. Furthermore, the difference of the number of the interface angle is reflected in the stacking sequence (the T3, T4 and T5 type of specimens have 0, 6 and 8 pairs of 90° interface angles, respectively), hence, the stacking sequence affects the final energy dissipation. 3.2. Residual tensile performance analysis 3.2.1. Damage inspection of specimens for tensile fracture The phenomena of all specimens are almost consistent in the process of the tensile failure. Some typical pictures of tensile fracture are shown in Fig. 14, and the T1C-A, T2C-A, T3C-A, T4C-A, T5C-A is the control specimen of T1, T2, T3, T4, T5 type of specimens, respectively. The following two points can be summarized after the quasi-static tensile test. (1) There are some differences in the form of fracture of specimens after impact by different energy, which is mainly related to the damage status within the specimens. The damage of specimens is not severe after 10 J impact. Therefore, the fracture does not extend to both sides symmetrically along the strip dent axis during the 308
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Fig. 13. Typical transferred energy-time curves which focus on: (a) impact angle; (b) impactor diameter; (c) the proportion of ply orientation; (d) stacking sequence.
tensile process, such as T1∥-A. However, the damage of specimens is more severe after 25.1 J impact, and the material performance in the area reduces significantly. Thus, the fracture extends to both sides symmetrically during the tensile process, such as T210-A. (2) Failure phenomena in both faces of specimens are different, which are related to the distribution characteristics of intralaminar damage within specimens after impact. The main damage mode near the impact face is the intralaminar damage (Fig. 15(a)), and the delamination area is mainly near the back face. Thus, the phenomenon of fiber-matrix debonding (Fig. 15(b)) is more obvious near the back face during the tensile process, such as T216-A. Besides, the viewpoint is further proved by the failure phenomena in both faces of the control specimens, such as T5C-A, which are without intralaminar damage.
because the overall stiffness degradation of the laminates caused by partial fibers breakage is not obvious in the period of the low tensile load. Hence, the specimens with impact damage do not show significant stiffness degradation even in the case of the ultimate load. The RTS of laminates can be reduced by 5.6%–9.3% and 31.3%–45.5% after 10 J and 25.1 J impact, respectively, as shown in Table 4 for details. It is different from the conventional opinion that the tensile performance of composite laminates is insensitive to low-velocity impact. References [7,32] show that the RTS of the laminates mainly depends on the damage status of the fiber whose orientation is in accordance with the loading direction. The impact of the strip impactor from two angles cause different damage status of fiber and matrix in the surface four plies, and the corresponding energy dissipation is different. Therefore, the damage status of the 0° fibers in the internal plies is distinguishing, which results the difference in the RTS. Rather, the RTS of T1∥ is 4.1% larger than that of T1⊥.On the other hand, the intralaminar damage of laminates is the most severe after impact by 10 mm impactor due to the smallest contact area, which results the largest (45.4%) decrease in the RTS. Moreover, the proportion of ply orientation mainly reflects in the number of plies in 0° orientation, hence, the RTS of T316 is 33.5% greater than that of T216 under the same impactor parameters. However, the stacking sequence only has little effect on the RTS in this study, which is mainly due to the small differences of impact damage in T3, T4, T5 types of specimens. Actually, if the stacking sequence is mainly related to the plies in 0° orientation, the stacking sequence may have a great influence on the RTS of laminates, and it will be discussed in the future research.
3.2.2. Tensile strength-displacement response The tensile strength against displacement curves of the specimens are illustrated in Fig. 16(a), (b), (c) and (d), which focuses on the impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, respectively. The T1C, T2C, T3C, T4C, T5C is the control specimen of T1, T2, T3, T4, T5 type of specimens, respectively. It is clear that the approximate slopes of these curves are almost constant, which demonstrates that mainly elastic deformation of the specimens occurs during the tensile process. Moreover, the tensile strength decreases rapidly when the specimens reach the maximum tensile strength. It is worth noting that the tensile stiffness degradation of all specimens nearly does not happen during the whole tensile test, this is 309
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Fig. 14. Typical tensile failure results of specimens.
3.3. Correlation of damage evaluation criteria
impact energy is determined. (2) The total energy dissipation (caused by the initiation and evolution of all damage during impact) is determined by the comprehensive status of the dent depth and DDPA. Unilaterally, the energy dissipation increases with the increase of the DDPA or dent depth. Therefore, that which one is more closely related to the total energy dissipation depends on that which energy dissipation caused by intralaminar or interlaminar damage evolution is dominant. For the 10 J impact, the total energy dissipation increases by 1.13 J when the DDPA increases from 2011.5 mm2 to 2117.8 mm2, and the corresponding dent depth decreases from 16.9 mm to 14.2 mm. This is most likely because that the energy dissipation by interlaminar damage evolution is dominant, i.e. the total energy dissipation is more closely related to the DDPA. However, for the 25.1 J impact, the total energy dissipation increases when the dent depth increases and the corresponding DDPA decreases in the case of single factor variable. This indicates that the energy dissipation by intralaminar damage evolution is dominant, i.e. the total energy dissipation is more closely related to the dent depth. Thus, there is no separate
Four damage evaluation criteria including the dent depth, DDPA, energy dissipation and RTS are used to assess the damage characteristics of laminates after impact in this study. The following correlations between the damage evaluation criteria can be sorted out by summarizing all the test results which are listed in Tables 3 and 4 (1) The dent depth is negatively correlated with the DDPA after impact in the case of single factor variable, and the correlation is independent of the impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, as shown in Fig. 17(a). This is chiefly because the laminates have two main ways to dissipate energy during impact, i.e. the intralaminar damage evolution (including matrix cracking, fiber failure and fiber-matrix debonding) and interlaminar damage evolution (delamination). The dent depth mainly reflects the intralaminar damage status, and the DDPA mainly reflects the interlaminar damage status. Thus, one damage status increases with the decreasing of another when the
Fig. 15. Scanning electron microscopic (SEM) of fracture: (a) fiber fracture and matrix cracking and (b) fiber-matrix debonding. 310
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Fig. 16. Typical tensile strength-displacement curves which focus on: (a) impact angle; (b) impactor diameter; (c) the proportion of ply orientation; (d) stacking sequence.
impact angle, impactor diameter and stacking sequence, as shown in Fig. 17(d). (4) The measurement of the dent depth and the DDPA is effortless, while they usually only reflect the intralaminar or interlaminar damage status of laminates unilaterally. On the contrary, the energy dissipation and the RTS can reflect the damage status of laminates more comprehensively, whereas they are difficult to measure, especially the RTS, only the destructive test can obtain the results. It is noteworthy that the finds reported above prove the correlations between the four damage evaluation criteria to some extent, among them, the dent depth can be treated as an important reference for the RTS, which has a great significance to effectively evaluate the residual tensile performance of laminates after impact.
Table 4 Summary of the tensile test results. Specimen RTS (MPa)
T1C 921.6
T1∥ 870.1
T1⊥ 836.2
T2C 608.5
T210 331.4
T216 400
Specimen RTS (MPa)
T3C 824.6
T316 533.9
T4C 837.1
T416 540.2
T5C 813.7
T516 559.4
fixed correlation between the total energy dissipation and dent depth or DDPA when the impact energy is uncertain, as shown in Fig. 17(b). (3) The RTS mainly depends on the quantity and damage status of the plies whose fiber orientation is in accordance with the loading direction. For the 10 J impact, the total energy dissipation is most likely to be dominated by the interlaminar damage evolution, however, the RTS is insensitive to interlaminar damage, thus, the correlation between RTS and energy dissipation is not significant. However, for the 25.1 J impact, the total energy dissipation is most likely to be dominated by the intralaminar damage evolution, and the RTS is sensitive to intralaminar damage, therefore, the RTS is negatively correlated with energy dissipation in the case of single factor variable, and the correlation is independent of the impactor diameter and stacking sequence, as shown in Fig. 17(c). Besides, the dent depth mainly reflects the intralaminar damage status, thus, the RTS is also negatively correlated with the dent depth in the case of single factor variable, and the correlation is independent of the
4. Conclusions This paper presents an experimental investigation of CFRP laminates, which is beneficial to deepen the understanding of the impact and tensile responses of laminates, and to promote the applications of carbon fiber/epoxy composites. Through systematic study, the following conclusions can be drawn. (a) The impact angles and impactor diameters have important influences on both the impact responses and damage characteristics of the laminates by changing the contact conditions between the impactors and the laminates. When the impact of the strip impactor is perpendicular to the fibers in the surface plies, larger impact force 311
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Fig. 17. The correlations of (a) DDPA vs. dent depth; (b) dent depth or DDPA vs. energy dissipation; (c) energy dissipation vs. RTS; (d) dent depth vs. RTS.
and more severe fiber damage can be observed. Thus, more energy is dissipated. Moreover, the impact of 10 mm hemispherical impactor produces smaller impact force, more severe intralaminar damage, and thus needs more energy dissipation. (b) The impact responses and damage characteristics of the laminates significantly depends on both the proportion of ply orientation and stacking sequence of the laminates. The more continuous plies in the same orientation lead to the weaker mutual support between adjacent plies. As a result, more severe damage and more energy dissipation occur. Besides, more number of 90° interface angle between plies result in more severe interlaminar damage due to the fact that the overall interlaminar performance of laminates is weak. (c) The RTS of the laminates is closely related to the impact energy, and decreases rapidly at the large impact energy such as 25.1 J. Furthermore, the RTS of the laminates mainly depends on the quantity and the damage status of the fibers in the loading direction. Hence, the plies with the fibers in the loading direction are suggested to be placed near the center of the laminates to reduce the effect of low-velocity impact on the RTS. (d) The dent depth is negatively correlated with the DDPA in the case of single factor variable, and the dent depth can be treated as an important reference for the RTS. Meanwhile, the total energy dissipation is determined by the comprehensive status of the dent depth and DDPA. However, for small impact energy, the total energy dissipation is more closely related to the DDPA. Nevertheless, for large impact energy, the total energy dissipation is more closely associated with the dent depth.
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Acknowledgments The authors would sincerely like to acknowledge the support of the National Natural Science Foundation of China [grant number 51475377]. The authors also thank Prof. Xitao Zheng, Prof. Chao Zhang, Dr. Ming Luo, Dr. Xiaoyu Zhang, Dr. Yanpei Wang, Dr. Pan Zhao, Dr. Tao Zhao and Miss Yuzhi Huang, for their suggestions and fruitful discussions.
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Low-velocity impact behavior and residual tensile strength of CFRP laminates
T
Jianwu Zhoua, Binbin Liaob, Yaoyao Shia,∗, Yangjie Zuoc, Hongliang Tuod, Liyong Jiae a
School of Mechanical Engineering, Northwestern Polytechnical University, Xi'an, 710072, China Institute of Process Equipment, Zhejiang University, Hangzhou, 300027, China c School of Aeronautics and Astronautics, Sichuan University, Chengdu, 610065, China d School of Aeronautics, Northwestern Polytechnical University, Xi'an, 710072, China e First Aircraft Institute of Aviation Industry Corporation, Xi'an, 710089, China b
ARTICLE INFO
ABSTRACT
Keywords: Impactor parameters Layup patterns Impact behaviors Tensile strength
This paper experimentally studies the low-velocity impact behaviors and residual tensile strength of carbon fiber reinforced plastics (CFRP) laminates. Firstly, the effects of four factors, i.e. impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, on the impact responses of laminates are studied. The damage characteristics are evaluated by dent depth, delamination damage projection area (DDPA) and energy dissipation. Secondly, the tensile responses of the laminates after impact are investigated based on residual tensile strength (RTS). Finally, the correlations between the four damage evaluation criteria, i.e. dent depth, DDPA, energy dissipation and RTS, are sorted out. Experimental results demonstrate that the four factors have significant effects on the impact behaviors of laminates in different ways. The DDPA is negatively correlated with the dent depth of laminates, and the dent depth can be treated as an important reference for the RTS. Besides, a fracture phenomenon, i.e. a clear band of fiber fracture around the impact area after impact, has been observed and discussed.
1. Introduction
strength was found in all specimens [9]. The similar conclusions in tensile strength reduction after impact were also obtained by Koo et al. [10]. Apart from the impactor shape, Amaro et al. [11], Icten et al. [12] and Evci et al. [13] studied the effect of impactor diameter on lowvelocity impact. These researches almost obtained the same conclusion that impactor diameter had an obvious influence on the impact behaviors of laminates by affecting the target stiffness. In fact, the impact angle also has a certain influence on initial damage and residual performance of laminates. However, the previous researchers focused on the conventional oblique impact [14,15], as shown in Fig. 1(a), and basically no work explored the special impact angle, as shown in Fig. 1(b), which is also likely to occur during service. Moreover, the influence of layup patterns on the impact behaviors of laminates also has been widely studied. References provided contradictive information concerning the influence of stacking sequence on impact resistance. Some researchers claimed that such influence was not significant [16], however, others held the opposite opinion. Czanocki [17] suggested that clustering reinforcement plies of the same orientation produced negative effects regarding the resistance of laminates against impact-induced damage. Lopes et al. [18,19] redesigned
For the high specific strength, high specific stiffness and perfect fatigue resistance [1–3], composite materials have been widely used in some important parts of the turbofan engine [4–6]. However, the aeroengine of plane is close to the ground during taking off and landing, and the fan blade is prone to be damaged by the low-velocity impact caused by inhaling sundries on the runway. As known, the composite fan blade always bears huge centrifugal force for its high-speed revolution and the composite laminates are sensitive to low-velocity impact [5], which causes the reduction of the strength by up to 70% and even more [7]. Therefore, the study of impact behaviors and residual tensile strength of CFRP laminates is necessary to improve the impact resistance and damage tolerance of the composite fan blade. It has been observed that the impactor parameters affect the impact behaviors of laminates significantly. Mitrevski et al. [8] explored with three different impactor shapes and found that the energy dissipation, peak impact force and contact duration of laminates all exhibited sensitivity to the impactor shape. Furthermore, the blunter impactor caused the larger damage area, and a significant reduction in tensile
∗
Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Shi).
https://doi.org/10.1016/j.compositesb.2018.10.090 Received 5 July 2018; Received in revised form 23 September 2018; Accepted 26 October 2018 Available online 28 October 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. Two different types of impact angles: (a) the angle α between the impact direction and the normal direction of the plane; (b) the angle β between the axis of the dent and the 0° direction of the laminate.
types of impactors are designed and used for low-velocity impact test and quasi-static tensile test. In order to comprehensively and effectively evaluate the damage status of laminates, four different damage evaluation criteria, i.e. the dent depth, DDPA, energy dissipation and RTS are adopted, and the correlations between them are discussed.
Table 1 Laminate details. Type
Layup pattern
The proportion of the plies with 0° fiber
T1 T2 T3 T4 T5
[904/04]4T [452/02/-452/902]2S [452/03/-452/90]2S [-452/03/90/452]2S [90/03/452/-452]2S
50% 25% 37.5% 37.5% 37.5%
2. Material and methods 2.1. Composite material and fabrication
the stacking sequence of laminates, and an improvement in the damage resistance was observed whilst keeping the similar in-plane and bending stiffness of laminates. Furthermore, scholars [20–23] improved the low-velocity impact performance and compression strength of laminates after the impact by using a genetic algorithm. Actually, the proportion of ply orientation is an important embodiment of the layup patterns of laminates, which has a certain effect on the overall performance of laminates because of the anisotropy of the composites. However, few kinds of literature mention it, and the further experiments are necessary to provide data for exploring the influence of layup patterns. The damage evaluation criteria (including dent depth, DDPA, energy dissipation, RTS, etc.) are vital for the assessment of the damage status of laminates. Aktaş et al. [24], Evci et al. [25] and Li et al. [26] indicated that the energy dissipated by the laminates was closely related to the damage status, while it had no obvious relation with the DDPA [27,28]. Besides, the dent depth in the impact face was positively correlated with the impact energy [29,30]. Balasubramani et al. [31] and Khan et al. [32] explored the RTS of the laminates after low-velocity impact to assess the internal damage status and attempted to establish a connection with impact energy. The results demonstrated that there was no fixed correlation between the RTS and the impact energy. Similarly, the crack length [33,34] on the impact face was also adopted to assess the damage status. Unfortunately, although the damage evaluation criteria have been studied by some researchers individually, the correlations between them are still ambiguous, which can be used to assess the damage status of laminates effectively through some easy available parameters. This paper studies the influence of four factors, i.e. impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, on the impact behaviors of laminates by using experimental means. Five kinds of laminates with different layup patterns and two
Five kinds of CFRP laminates with different layup patterns were manufactured, as listed in Table 1. The laminates were made of T300/ YH69 unidirectional prepreg by autoclave molding. Each laminate was made of 32 plies with the nominal cured ply thickness of 0.12 mm. The whole lay-up process was carried out at a constant environment condition with a temperature of 25 ± 2 °C and relative humidity of 40 ± 5%. The test specimens of 230 × 50 mm were obtained from the original composite laminates using water-jet cutting, and the average thickness of specimens was 3.75 mm. 2.2. Fixture and impactor The fixture which includes the cover plate, the guide plate and the supporting block is shown in Fig. 2. Two rubber gaskets were placed between the cover plate and specimens to fix the specimens firmly during the impact test. Three impactors were manufactured to study the effects on the impact behaviors of laminates: one strip impactor and two hemispherical impactors. Among them, the strip impactor (Fig. 3(a)) was designed with the length of the impact part equaling to 19 mm. The diameters of the hemispherical impactors (Fig. 3(b) and c) were 10 mm and 16 mm, respectively. 2.3. Experimental investigation The impact test was performed according to ASTM D7136 [35] using the Instron Dynatup 9250HV Impact Testing Machine. The impact energy was calculated by:
E = CE h
(1)
where CE is the specified ratio of impact energy to specimens thickness, 6.7 J/mm; h is the thickness of the specimens, mm. 301
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Fig. 2. The fixture.
Specimens were fixed in the fixture, the anti-rebound device was activated to catch the impactor for preventing the second impact, and the total mass of impact part was 5.607 kg. During the impact, a digital data acquisition unit was used to record the impact force-time curves, the impact velocity and the energy history. The dent depth of each specimen was measured after impact within 10 min through the high-precision CCD laser displacement sensor LK-GD500, as shown in Fig. 4. The specific test arrangements about the four factors are shown in Table 2. The quasi-static tensile test was conducted on the control specimens and the damaged specimens with the tensile rate of 2 mm/min according to ASTM D3039 [36]. Here, the control specimens were the original specimens without impact damage, while the damaged specimens were the specimens after the impact tests with impact damage. All the specimens were tested by the same tensile testing machine called CRIMS DDL300.
3. Results and discussion 3.1. Drop-weight impact analysis 3.1.1. Damage inspection of specimens for different factors The statistical results of dent depth and DDPA of specimens are shown in Table 3. Moreover, the typical pictures of the C-scan test for the delamination area and the impact damage of specimens are shown in Fig. 5 and Fig. 6, respectively. The four factors are studied. Generally, there are two main reasons for delamination during impact. On the one hand, the relative slippage between plies occurs because of the bending of laminates, which resulting the interlaminar shear stress then causes the interlaminar damage [37]. On the other hand, the load propagates in the laminates in the form of the stress wave. When the stress wave reaches the back face of the laminates, the
Fig. 3. The impactors: (a) strip impactor; (b) and (c) hemispherical impactors with the diameter of 10 mm and 16 mm, respectively. 302
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Fig. 4. Methods for measuring dent depth. Table 2 Low-velocity impact test arrangements. Test
Key factor
Type
Impactor parameter
Impact energy
Impact velocity
Times
1
Impact angle
T1
10 J
1.88 m/s
2
Impactor diameter
T2
25.1 J
2.96 m/s
3 4
The proportion of ply orientation Stacking sequence
T3 T4, T5
Strip impactor, ∥ Strip impactor, ⊥ Hemispherical impactor, Hemispherical impactor, Hemispherical impactor, Hemispherical impactor,
25.1 J 25.1 J
2.96 m/s 2.96 m/s
5 5 5 5 5 5
10 mm 16 mm 16 mm 16 mm
Note. Symbol “∥” means that the strip impactor is parallel to the 0° direction of specimens. Symbol “⊥” means that the strip impactor is perpendicular to the 0° direction of specimens.
certain influence on the dent depth and DDPA of laminates. This is possible because the fiber orientation in the 4 plies near the impact face is 90°. During the impact that is parallel to 0° direction of specimens, the fibers in these 4 plies bear the main load, which results in a smaller dent depth. Whereas during the impact that is perpendicular to 0° direction of specimens, the fibers in these 4 plies bear the lesser load and the more severe matrix damage occurs, which lead to a larger dent depth. Furthermore, the different damage status is also reflected in the interlaminar damage. Specifically, the DDPA of the T1∥ is 2217.8 mm2, which is 5.3% larger than that of the T1⊥.
Table 3 Summary of the impact test results. Specimen
Dent depth (0.01 mm)
DDPA (mm2)
EDissipation (J)
EDissipation/EImpact (%)
T1∥ T1⊥ T210 T216 T316 T416 T516
14.2 16.9 92.5 62.6 75.1 72.9 71.7
2117.8 2011.5 2823.7 3178.7 3017.4 3110.3 3195.1
8.78 7.65 21.53 16.15 17.96 17.24 16.88
87.8 76.5 85.8 64.3 71.6 68.7 67.3
3.1.1.2. Impactor diameter. T210-A and T216-A are the typical pictures of impact damage of T2 type of specimens, which are caused by the hemispherical impactors with the diameters of 10 mm and 16 mm, respectively. The results show that the dent depth of T210 is 0.925 mm, which is 47.8% larger than that of T216. This is mainly because the 10 mm impactor and specimens have less contact area and more severe intralaminar damage occurs within the specimens. Moreover, the DDPA of T216 is 3178.7 mm2, which is 12.6% larger than that of T210, and the phenomenon can also be validated in Refs. [12,13]. This is due to the
obvious reflection will occur to produce the tensile wave which resulting the tensile stress [37,38]. The tensile stress maybe accelerates the initiation and evolution of damage at the interface between plies. 3.1.1.1. Impact angle. T1∥-A and T1⊥-A are the typical pictures of impact damage of T1 type of specimens, which are caused by the strip impactor that is parallel and perpendicular to 0° direction of the specimens, respectively. The results indicate that the impact angle has a
Fig. 5. The C-scan results of all types of specimens after impact. 303
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Fig. 6. Impact damage of all types of specimens and the ring fracture.
fact that the impact force of 10 mm impactor is concentrated in the smaller area, which resulting the more severe fiber and matrix damage in this area, hence, the DDPA is smaller. Whereas the delamination could further develop to the larger area during the impact by 16 mm impactor.
comparing with the T316, the three types of specimens have little difference in their dent depth. Exactly, the maximum difference in dent depth is only 4.7%. The reason can also be explained by the hypothesis of mutual support between adjacent plies mentioned earlier, and the reasonability of the hypothesis is verified once again. Moreover, similar to the dent depth, the maximum difference in DDPA is only 5.9%. This indicates that the effect of stacking sequence on the interlaminar performance is limited. However, combined with the analysis of the above four factors, whether the difference in dent depth (or DDPA) is significant or not, they all have the same correlation that the larger dent depth corresponds to the smaller DDPA. On the other hand, a phenomenon can be observed. There always exists a clear band of fiber fracture around the impact area after impact by different sizes of hemispherical impactors, named as “ring fracture” in this paper. The reason could be that the out-of-plane load on specimens increases continuously during the loading of the impactor, and the closer to the center of dent leads to the greater vertical load, as shown in Fig. 8(a). The fiber breakage occurs when the vertical load exceeds the limitation of the fiber, then the matrix begins to broken rapidly. Therefore, the ring fracture is the critical position of a large number of fibers fracture in the impact area, as shown in Figs. 8(b) and Fig. 9. Furthermore, the position of ring fracture after impact by 10 mm impactor is closer to the center of the dent. Since the change of the vertical load to the specimens of 10 mm impactor is more drastic than
3.1.1.3. The proportion of ply orientation. T316-A is a typical picture of impact damage of T3 type of specimens. Compared with the specimens after impact by 16 mm impactor in the T2 type of specimens, the dent depth of the T316 is 0.751 mm, which is 20% larger than that of the T216. It could be that the next ply will support the previous one when the specimens are impacted by the out-of-plane load. If the two adjacent plies are in the different orientation, the support force is dominated by the fibers, as shown in Fig. 7(a). However, if the two adjacent plies are in the same orientation, the proportion of the support force provided by the matrix increases, as shown in Fig. 7(b). However, the matrix is more prone to be damaged than the fiber, therefore, the more continuous plies in the same orientation for T316 laminates provide the weaker mutual support which causes the larger dent depth. Nevertheless, opposite to the dent depth, the DDPA of T316 is 3017.4 mm2, which is 5.1% smaller than the value of T216. 3.1.1.4. Stacking sequence. T416-A and T516-A are the typical pictures of impact damage of T4 and T5 types of specimens, respectively. By
Fig. 7. Two different support: (a) fiber dominated; (b) matrix dominated. 304
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Fig. 9. Scanning electron microscopic (SEM) of ring fracture and fiber breakage.
ability of laminates to resist initiation delamination. Moreover, delamination is the dominant failure mode under out-of-plane load [40], and the bending stiffness of the laminates decreases rapidly because of the delamination [26,41]. Furthermore, the impact force perturbation after FH implies the propagation of the in-plane damage, then a varying degree of reloading to the maximum impact force FMax happens because of the different potential energy which stores in the impactor. As shown in Fig. 11(a), the value of FH,T1∥ is 3761 N, which is 17.1% larger than that of FH,T1⊥. It indicates that a larger load is required for the initial delamination of the laminates when the impact of the strip impactor is parallel to 0° direction of the specimens. However, the value of FMax,T1∥ is 4102 N, which is 8.2% smaller than that of FMax,T1⊥. This may be because when the impact of the strip impactor is perpendicular to 0° direction of the specimens, the impactor contacts with the 90° fibers (parallel to the impactor) first, and the resistance at this stage is relatively small. Then the impactor continues to move downward, the 0° fibers (perpendicular to the impactor) are also involved in the resistance, hence, the resistance at this stage increases obviously. Whereas when the impact of the strip impactor is parallel to 0° direction of the specimens, the situation of resistance by fibers is opposite. Thus, the impactor is subjected to a larger resistance in the early stage, while the resistance can not increase obviously in the latter stage. As shown in Fig. 11(b), the value of FH,T216 is 3241 N, which is 27.7% larger than that of FH,T210. This is because more fibers and matrix are involved in resisting the impact of 16 mm impactor due to the larger contact area. Whereas the impact force of 10 mm impactor is more concentrated, and the more severe damage occurs in specimens, which result in the stiffness degradation of specimens more significant. Hence, the value of FMax,T210 is 6635 N, which is 24.3% smaller than that of FMax,T216. On the other hand, the maximum difference between FH (FMax) in Fig. 11(c) and (d) is 5.7% (1.8%) and 10.8% (13.8%), respectively. The overall fluctuation trend of the curves in Fig. 11(c) and (d) are almost identical except the T416 curve which includes the largest value of FH, the smallest value of FMax and the longest contact duration. This illustrates that the effect of stacking sequence on impact responses of laminates is more significant than that of the proportion of the ply orientation in this paper.
Fig. 8. (a) Sketch of the impactor loading; (b) Sketch of the impactor unloading; (c) Sketch of the different position of ring fracture.
16 mm impactor due to the bigger curvature, which leads to the position of ring fracture after impact by 10 mm impactor is closer to the center of the dent, as shown in Fig. 8(c). 3.1.2. Impact force-time history The impact force-time curves during impact are shown in Fig. 10. The method of fast Fourier transform (FFT) is used to filter the impact force oscillation. All types of laminates are almost consistent in impact force-time history, and the coefficient of the variation of the impact force measurements is less than 10%. Some differences are observed from the impact force responses of these specimens, which can be influenced by their damage mechanisms [34]. The Fig. 11(a), (b), (c) and (d) is the typical impact force-time curves of specimens, which focuses on the impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, respectively. In the elastic region, the impact force linearly increases until it reaches a characteristic impact force FH at which the stiffness changes due to the Hertzian failure [25,27,39]. Therefore, the value of FH is regarded as a pointer of the
3.1.3. Impact force-central displacement response The typical impact force-central displacement curves during impact are demonstrated in Fig. 12(a), (b), (c) and (d), which focuses on the impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, respectively. The curves can generally be divided into three zones of A, B and C, which is according to the damage 305
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Fig. 10. Impact force-time curves: (a) impact of strip impactor that is parallel to 0° direction of T1 type of specimens; (b) impact of strip impactor that is perpendicular to 0° direction of T1 type of specimens; (c) impact of 10 mm hemispherical impactor to T2 type of specimens; (d), (e), (f), (g) impact of 16 mm hemispherical impactor to T2, T3, T4, T5 type of specimens, respectively. 306
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Fig. 11. Typical impact force-time curves which focus on: (a) impact angle; (b) impactor diameter; (c) the proportion of ply orientation; (d) stacking sequence.
initiation of specimens, damage evolution of specimens and specimens rebounding. However, the Fig. 12(a) shows a significant difference from the others. This may be related to the impactor shape and impact energy. Zone A implies the elastic responses of the specimens until the impact force reaches the damage initiation threshold. In Zone B, the damage initiates and evolves during the impact, and the impact force perturbations are associated with the delamination, fiber and matrix damage within plies, i.e. damage propagation [30,39]. Finally, zone C indicates the specimens rebounding while the impactor is returned by the stored elastic energy in the laminates until it fully rebounds. The corresponding central displacement at the end of zone C indicates the residual, permanent depression of the laminates to some extent [34]. There is an obvious boundary of zone A and B only during the 10 J impact by strip impactor in Fig. 12(a), it indicates that the impactor shape and impact energy have great influences on the damage propagation of laminates. However, the other factors have little effects on the phenomenon of the zone boundary in this paper. Generally, the larger zone B is an indication of more damage evolution within the laminates [34,42]. The difference of the maximum central displacement is 0.11 mm, 0.66 mm, 0.17 mm and 0.39 mm in Fig. 12(a), (b), (c) and (d), respectively. Particularly, the maximum central displacement of impact by 10 mm impactor is 5.84 mm, which is 12.7% larger than that of the 16 mm one. The result shows that the more severe damage is caused by the 10 mm impactor during the impact. Then the severe damage leads to a smaller elastic deformation that stores the rebound energy, hence, the residual central displacement is larger. Furthermore, it can be inferred that the more severe damage in T210 than that in T216 is mainly
reflected in the fiber and matrix damage but not the delamination by combining with the results of the dent depth and DDPA after impact. Besides, the deflection curves which are influenced by the proportion of ply orientation and stacking sequence show the high consistency, respectively. This is because the deflection of laminates is closely related to the out-of-plane load and the bending stiffness of themselves, meanwhile, the phenomena coincide with the corresponding impact force-time curves in Fig. 11(c) and (d). 3.1.4. Transferred energy-time history The total energy dissipated by the specimens with time is determined by calculating the area under the impact force-central displacement curve [34,43]. The transferred energy-time curves are given in Fig. 13(a), (b), (c) and (d), which focuses on the impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, respectively. When the impactor starts to contact with the specimens, the energy dissipation starts to appear and the specimens deform by absorbing the kinetic energy of the impactor. When the impactor reaches the lowest end, all kinetic energy has been transformed partly into the strain energy and partly into the dissipated energy. Then the impactor rebounds and elastic unloading occurs, and all recoverable strain energy of the specimens transforms into the kinetic energy of the impactor until separation. The final energy value demonstrates the total amount of energy dissipated by specimens largely through damage evolution. The peak energy moments of specimens exhibit significant differences in Fig. 13(b) and (c). Specifically, the difference in peak energy 307
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Fig. 12. Typical impact force-central displacement curves which focus on: (a) impact angle; (b) impactor diameter; (c) the proportion of ply orientation; (d) stacking sequence.
moment influenced by the impactor diameter and the proportion of ply orientation is 0.56 ms and 0.26 ms, respectively. This is because the laminates are damaged by impactor during impact, and the damage reduces the transverse combined stiffness of laminates which directly determines the acceleration of the impactor during impact [44,45]. Therefore, the greater damage difference of laminates results in the greater difference at the moment when the velocity of impactor decelerates to zero (equal to the peak energy moment). As shown in Table 3, the impactor diameter and the proportion of ply orientation have greater influences on the damage difference of laminates than other factors, thus the difference in peak energy moment is more obvious in Fig. 13(b) and (c). In addition, the concept of energy dissipation rate (EDissipation/EImpact, expressed in percentage, where the EDissipation and EImpact is the total energy dissipation and impact energy, respectively) is introduced to state the results more intuitively. The difference in energy dissipation rate of specimens in Fig. 13(a), (b), (c) and (d) is 11.3%, 21.5%, 7.3% and 4.3%, respectively. The results indicate that the four factors affect the energy dissipation of laminates. This is because the final energy dissipation is the sum of energy dissipated by various damages in the laminates. However, the four factors affect the final energy dissipation in different ways. The impact angle affects the final energy dissipation by causing the different fiber damage status because of the difference in the quantity of contact between the impactor and fibers in the surface plies. The impactor diameter affects the final energy dissipation by causing the different intralaminar damage status because of the difference in the contact area between the impactor and laminates. Moreover, for the proportion of ply
orientation, the more continuous plies in the same orientation provide the weaker mutual support between adjacent plies, which resulting the more severe damage during impact. The interface angle (defined as the angular difference in fiber orientation between two plies) affects the damage resistance [20,21,46]. Especially, when the interface angle is larger than 75°, the larger angle leads to the weaker interlaminar performance [47]. Furthermore, the difference of the number of the interface angle is reflected in the stacking sequence (the T3, T4 and T5 type of specimens have 0, 6 and 8 pairs of 90° interface angles, respectively), hence, the stacking sequence affects the final energy dissipation. 3.2. Residual tensile performance analysis 3.2.1. Damage inspection of specimens for tensile fracture The phenomena of all specimens are almost consistent in the process of the tensile failure. Some typical pictures of tensile fracture are shown in Fig. 14, and the T1C-A, T2C-A, T3C-A, T4C-A, T5C-A is the control specimen of T1, T2, T3, T4, T5 type of specimens, respectively. The following two points can be summarized after the quasi-static tensile test. (1) There are some differences in the form of fracture of specimens after impact by different energy, which is mainly related to the damage status within the specimens. The damage of specimens is not severe after 10 J impact. Therefore, the fracture does not extend to both sides symmetrically along the strip dent axis during the 308
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Fig. 13. Typical transferred energy-time curves which focus on: (a) impact angle; (b) impactor diameter; (c) the proportion of ply orientation; (d) stacking sequence.
tensile process, such as T1∥-A. However, the damage of specimens is more severe after 25.1 J impact, and the material performance in the area reduces significantly. Thus, the fracture extends to both sides symmetrically during the tensile process, such as T210-A. (2) Failure phenomena in both faces of specimens are different, which are related to the distribution characteristics of intralaminar damage within specimens after impact. The main damage mode near the impact face is the intralaminar damage (Fig. 15(a)), and the delamination area is mainly near the back face. Thus, the phenomenon of fiber-matrix debonding (Fig. 15(b)) is more obvious near the back face during the tensile process, such as T216-A. Besides, the viewpoint is further proved by the failure phenomena in both faces of the control specimens, such as T5C-A, which are without intralaminar damage.
because the overall stiffness degradation of the laminates caused by partial fibers breakage is not obvious in the period of the low tensile load. Hence, the specimens with impact damage do not show significant stiffness degradation even in the case of the ultimate load. The RTS of laminates can be reduced by 5.6%–9.3% and 31.3%–45.5% after 10 J and 25.1 J impact, respectively, as shown in Table 4 for details. It is different from the conventional opinion that the tensile performance of composite laminates is insensitive to low-velocity impact. References [7,32] show that the RTS of the laminates mainly depends on the damage status of the fiber whose orientation is in accordance with the loading direction. The impact of the strip impactor from two angles cause different damage status of fiber and matrix in the surface four plies, and the corresponding energy dissipation is different. Therefore, the damage status of the 0° fibers in the internal plies is distinguishing, which results the difference in the RTS. Rather, the RTS of T1∥ is 4.1% larger than that of T1⊥.On the other hand, the intralaminar damage of laminates is the most severe after impact by 10 mm impactor due to the smallest contact area, which results the largest (45.4%) decrease in the RTS. Moreover, the proportion of ply orientation mainly reflects in the number of plies in 0° orientation, hence, the RTS of T316 is 33.5% greater than that of T216 under the same impactor parameters. However, the stacking sequence only has little effect on the RTS in this study, which is mainly due to the small differences of impact damage in T3, T4, T5 types of specimens. Actually, if the stacking sequence is mainly related to the plies in 0° orientation, the stacking sequence may have a great influence on the RTS of laminates, and it will be discussed in the future research.
3.2.2. Tensile strength-displacement response The tensile strength against displacement curves of the specimens are illustrated in Fig. 16(a), (b), (c) and (d), which focuses on the impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, respectively. The T1C, T2C, T3C, T4C, T5C is the control specimen of T1, T2, T3, T4, T5 type of specimens, respectively. It is clear that the approximate slopes of these curves are almost constant, which demonstrates that mainly elastic deformation of the specimens occurs during the tensile process. Moreover, the tensile strength decreases rapidly when the specimens reach the maximum tensile strength. It is worth noting that the tensile stiffness degradation of all specimens nearly does not happen during the whole tensile test, this is 309
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Fig. 14. Typical tensile failure results of specimens.
3.3. Correlation of damage evaluation criteria
impact energy is determined. (2) The total energy dissipation (caused by the initiation and evolution of all damage during impact) is determined by the comprehensive status of the dent depth and DDPA. Unilaterally, the energy dissipation increases with the increase of the DDPA or dent depth. Therefore, that which one is more closely related to the total energy dissipation depends on that which energy dissipation caused by intralaminar or interlaminar damage evolution is dominant. For the 10 J impact, the total energy dissipation increases by 1.13 J when the DDPA increases from 2011.5 mm2 to 2117.8 mm2, and the corresponding dent depth decreases from 16.9 mm to 14.2 mm. This is most likely because that the energy dissipation by interlaminar damage evolution is dominant, i.e. the total energy dissipation is more closely related to the DDPA. However, for the 25.1 J impact, the total energy dissipation increases when the dent depth increases and the corresponding DDPA decreases in the case of single factor variable. This indicates that the energy dissipation by intralaminar damage evolution is dominant, i.e. the total energy dissipation is more closely related to the dent depth. Thus, there is no separate
Four damage evaluation criteria including the dent depth, DDPA, energy dissipation and RTS are used to assess the damage characteristics of laminates after impact in this study. The following correlations between the damage evaluation criteria can be sorted out by summarizing all the test results which are listed in Tables 3 and 4 (1) The dent depth is negatively correlated with the DDPA after impact in the case of single factor variable, and the correlation is independent of the impact angle, impactor diameter, the proportion of ply orientation and stacking sequence, as shown in Fig. 17(a). This is chiefly because the laminates have two main ways to dissipate energy during impact, i.e. the intralaminar damage evolution (including matrix cracking, fiber failure and fiber-matrix debonding) and interlaminar damage evolution (delamination). The dent depth mainly reflects the intralaminar damage status, and the DDPA mainly reflects the interlaminar damage status. Thus, one damage status increases with the decreasing of another when the
Fig. 15. Scanning electron microscopic (SEM) of fracture: (a) fiber fracture and matrix cracking and (b) fiber-matrix debonding. 310
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Fig. 16. Typical tensile strength-displacement curves which focus on: (a) impact angle; (b) impactor diameter; (c) the proportion of ply orientation; (d) stacking sequence.
impact angle, impactor diameter and stacking sequence, as shown in Fig. 17(d). (4) The measurement of the dent depth and the DDPA is effortless, while they usually only reflect the intralaminar or interlaminar damage status of laminates unilaterally. On the contrary, the energy dissipation and the RTS can reflect the damage status of laminates more comprehensively, whereas they are difficult to measure, especially the RTS, only the destructive test can obtain the results. It is noteworthy that the finds reported above prove the correlations between the four damage evaluation criteria to some extent, among them, the dent depth can be treated as an important reference for the RTS, which has a great significance to effectively evaluate the residual tensile performance of laminates after impact.
Table 4 Summary of the tensile test results. Specimen RTS (MPa)
T1C 921.6
T1∥ 870.1
T1⊥ 836.2
T2C 608.5
T210 331.4
T216 400
Specimen RTS (MPa)
T3C 824.6
T316 533.9
T4C 837.1
T416 540.2
T5C 813.7
T516 559.4
fixed correlation between the total energy dissipation and dent depth or DDPA when the impact energy is uncertain, as shown in Fig. 17(b). (3) The RTS mainly depends on the quantity and damage status of the plies whose fiber orientation is in accordance with the loading direction. For the 10 J impact, the total energy dissipation is most likely to be dominated by the interlaminar damage evolution, however, the RTS is insensitive to interlaminar damage, thus, the correlation between RTS and energy dissipation is not significant. However, for the 25.1 J impact, the total energy dissipation is most likely to be dominated by the intralaminar damage evolution, and the RTS is sensitive to intralaminar damage, therefore, the RTS is negatively correlated with energy dissipation in the case of single factor variable, and the correlation is independent of the impactor diameter and stacking sequence, as shown in Fig. 17(c). Besides, the dent depth mainly reflects the intralaminar damage status, thus, the RTS is also negatively correlated with the dent depth in the case of single factor variable, and the correlation is independent of the
4. Conclusions This paper presents an experimental investigation of CFRP laminates, which is beneficial to deepen the understanding of the impact and tensile responses of laminates, and to promote the applications of carbon fiber/epoxy composites. Through systematic study, the following conclusions can be drawn. (a) The impact angles and impactor diameters have important influences on both the impact responses and damage characteristics of the laminates by changing the contact conditions between the impactors and the laminates. When the impact of the strip impactor is perpendicular to the fibers in the surface plies, larger impact force 311
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Fig. 17. The correlations of (a) DDPA vs. dent depth; (b) dent depth or DDPA vs. energy dissipation; (c) energy dissipation vs. RTS; (d) dent depth vs. RTS.
and more severe fiber damage can be observed. Thus, more energy is dissipated. Moreover, the impact of 10 mm hemispherical impactor produces smaller impact force, more severe intralaminar damage, and thus needs more energy dissipation. (b) The impact responses and damage characteristics of the laminates significantly depends on both the proportion of ply orientation and stacking sequence of the laminates. The more continuous plies in the same orientation lead to the weaker mutual support between adjacent plies. As a result, more severe damage and more energy dissipation occur. Besides, more number of 90° interface angle between plies result in more severe interlaminar damage due to the fact that the overall interlaminar performance of laminates is weak. (c) The RTS of the laminates is closely related to the impact energy, and decreases rapidly at the large impact energy such as 25.1 J. Furthermore, the RTS of the laminates mainly depends on the quantity and the damage status of the fibers in the loading direction. Hence, the plies with the fibers in the loading direction are suggested to be placed near the center of the laminates to reduce the effect of low-velocity impact on the RTS. (d) The dent depth is negatively correlated with the DDPA in the case of single factor variable, and the dent depth can be treated as an important reference for the RTS. Meanwhile, the total energy dissipation is determined by the comprehensive status of the dent depth and DDPA. However, for small impact energy, the total energy dissipation is more closely related to the DDPA. Nevertheless, for large impact energy, the total energy dissipation is more closely associated with the dent depth.
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Acknowledgments The authors would sincerely like to acknowledge the support of the National Natural Science Foundation of China [grant number 51475377]. The authors also thank Prof. Xitao Zheng, Prof. Chao Zhang, Dr. Ming Luo, Dr. Xiaoyu Zhang, Dr. Yanpei Wang, Dr. Pan Zhao, Dr. Tao Zhao and Miss Yuzhi Huang, for their suggestions and fruitful discussions.
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