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epoxy laminates subjected to lightning strike
Composites: Part A 79 (2015) 164–175
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Experimental study of damage characteristics of carbon woven fabric/epoxy laminates subjected to lightning strike Yichao Li a,b, Renfu Li a,b,⇑, Luyi Lu a,b,*, Xianrong Huang a a b
Department of Aerospace Engineering, Huazhong University of Science and Technology, 430074 Wuhan, China Digital Manufacturing Equipment and Technology National Key Laboratory, Huazhong University of Science and Technology, 430074 Wuhan, China
a r t i c l e
i n f o
Article history: Received 12 June 2015 Received in revised form 21 September 2015 Accepted 24 September 2015
Keywords: A. Fabrics/textiles B. Delamination C. Damage mechanics D. Electron microscopy
a b s t r a c t This paper experimentally investigates the damage characteristics of two stacking sequenced ([452/02/ 452/902]s, [302/02/ 302/902]s) carbon woven fabric/epoxy laminates subjected to simulated lightning strike. Characteristics of the damage are analyzed using visual inspection, image processing, ultrasonic scanning and scanning electron microscope. The mechanical properties of post-lightning specimens are then studied. Observations show that as the lightning strike is intensified, an enlarged resin pyrolized area appears majorly along the weft orientation while the delamination region extends equally to both of the warp and the weft direction. The resin/fiber interfacial bonding is severely damaged by a thermal–mechanical effect due to lightning strike infliction. Mechanical testing further shows that the stacking sequence can influence the failure significantly. Compared with prepreg taped material, the restrained damage area due to special designed stacking sequence, lamina thickness and the weft nylon binder make the woven fabric reinforcement a good choice for the fabrication of lightning protection structures. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Carbon fiber reinforced polymer (CFRP) composites have been widely used in aerospace industry for decades due to their high specific stiffness and strength. With the emergence of various affordable carbon fibers and cost-effective manufacturing process, the applications of CFRP composites have been also widely used in automotive, sports and civil infrastructures. In recent years, the support of smart grid and green, sustained energy utility in China and worldwide has attracted an increase interest in the potential replacement of large amount of electrical products by composite structures in the electric power industry [1–3]. As one may easily see most of the structures in electric power industry often work outdoor all year round, damage from lightning strike is one of the primary issues which needs to be addressed before the use of composite materials broadly spreads into the power industry. The phenomenon of lightning strikes on CFRP composites has been investigated in aerospace industry for decades. A large number of studies have been conducted and various protection methods ⇑ Corresponding authors at: Department of Aerospace Engineering, Huazhong University of Science and Technology, 430074 Wuhan, China. E-mail address: [email protected] (R. Li). http://dx.doi.org/10.1016/j.compositesa.2015.09.019 1359-835X/Ó 2015 Elsevier Ltd. All rights reserved.
have been proposed as summarized in [4]. Some representative papers are briefly surveyed in this section. Chemartin et al. [5] proposed a comprehensive study on the direct effects of lightning on composite structures from the formation of lightning arc, the strike to the aircraft structure and the thermal and mechanical constraints during the damage process. They discovered that Joule heating and overpressure are the two main factors for the various damage forms of composites. Feraboli et al. [6,7] investigated the damage response of carbon fiber/epoxy specimens with and without the influence of fastener under lightning strikes. The study showed the damage pattern was dramatically influenced by the fastener with a through-thickness delamination mode. But the damage area cannot be fully demonstrated due to the limited geometry of the specimens. Hirano et al. [8] examined a series of post-lightning graphite/epoxy composite laminates by means of visual inspection, ultrasonic testing and X-ray inspection technique and categorized the damage pattern into fiber damage, resin deterioration and internal delamination modes, which have strong relationships to certain lightning parameters. In the aspect of numerical simulation, Ogasawara et al. [9] proposed a coupled thermal–electrical method to simulate the lightning strike damage of CFRP. Their results suggested that both dielectric breakdown at the interlayer gaps and surface recession govern the damage behavior in the thickness direction. Abdelal and Murphy [10]
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presented a temperature dependent material modeling procedure to simulate the thermal damage of composite panel with the influence of embedding copper mesh and numerically validated the protection mechanism which is to consume lightning energy on damaging copper mesh itself. A coupled thermal–electrical–structural with element deletion method was reported by Wang et al. [11] to study the ablation damage behavior of carbon fiber/epoxy laminates. It showed that the temperature could be an adequate indicator to describe the damage condition of composites after lightning infliction. Other simulation studies that focused on both thermal–electrical and mechanical damage from lightning can be referred to [12,13]. The current work is part of a funded project which aims to design a series of composite poles subject to various levels of voltages in the remote central and western regions of China. Due to that the poles have to work in harsh environments, the convenience in transportation and the resistance to the local environmental attacks such as lightning strike, moisture and corrosion become the major concerns for the choice of material and design process. As such, carbon woven fabric has been selected as the material candidate for its excellent mechanical property and relative inexpensive cost compared to other materials, for example the prepreg taped ones. However, literature search shows that little attention has been paid to the study of lightning strike on woven fabric materials, which is an essential issue that could determine the acceptance or failure of the CFRP composite pole. For such composites, one primary concern is that the influences from the stacking sequence of the laminas on the damage patterns and the integrity of the composite structures after a lightning strike is inflicted on. Therefore, the present work focuses on the investigation of the basic damage mechanics of two stacking sequenced ([452/02/ 452/902]s, [302/02/ 302/902]s) carbon woven fabric/epoxy laminates. The reason to choose the stacking sequence of [452/02/ 452/902]s is because of its quasi-isotropic property which is required by the components on the pole such as main body and cross arm. In the meanwhile, the stacking sequence of [302/02/ 302/902]s is chosen because it can be used for the design of connections, attachment or fixing components which subject to a major longitudinal loading. The responses of these stacking sequences to lightning strike, as well as its postlightning residual strength are essential for the safety concern of the composite poles. Three types of lightning strikes with amplitude current 38 kA, 32 kA and 22 kA are applied to the two stacking sequenced specimen discussed above. Damage types and properties are analyzed by visual inspection, image processing, ultrasonic testing and field emission scanning electron microscope. The effect of stacking sequence on the formation of various damage forms is discussed and mechanical properties of post-lightning specimens are subsequently studied. The lightning damage mechanism in microscopic level is also explored. 2. Experimental procedure 2.1. Constituent materials UD woven fabrics [14] are materials with a majority of fiber yarns in the warp direction (98%) and a minority of thread yarns in the weft direction. Fiber yarns are designed to withstand unidirectional loadings while the thread yarns provide handling and drape properties for the woven fabric [15]. In this work, TORAY T700SC-12K UD woven fabric is used with single layer density of 300 g/m2 in the warp direction. Nylon binder yarns with single
Table 1 Properties of the carbon woven fabrics. T700SC-12K Longitudinal tensile modulus (GPa) Longitudinal tensile strength (MPa) Transverse tensile modulus (GPa) Transverse tensile strength (MPa) Shear modulus (GPa) Shear strength (MPa) Elongation (%) Volume density (g/cm3 ) Weight per unit length (tex) Width of single fiber yarn (mm) Thickness of single fiber yarn (mm)
250 4900 18 23 7 59 2.1 1.8 800 2.4 0.25
Table 2 Properties of the epoxy resin. 760E/766H Density (g/cm3 ) Viscosity (mPa s, 25 C) Tensile modulus (MPa) Tensile strength (MPa) Flexural modulus (MPa) Flexural strength (MPa) Elongation (%) Heat resistance (°C) Water absorption (7 d at 23 C, %)
1.2 1400 3000 67 3000 105 4.0 71 0.25–0.45
layer density of 30 g/m2 in the weft direction hold the warp yarns from fall-apart. AIRSTONE 760E/766H of Dow Chemical Company is chosen as epoxy resin/hardener. Parameters of the UD carbon woven fabric and resin matrix are presented in Tables 1 and 2, respectively.
2.2. Fabrication of specimens The UD carbon woven fabric/epoxy laminates are prepared by using vacuum assisted resin transfer molding (VARTM) method. A schematic layout is shown in Fig. 1(a) and the preparation of specimen fabrication is presented in Fig. 1(b). VARTM [16] is a highly efficient and cost effective molding technique for composite part made from fabrics. A vacuum bag is covered on the mold, and resin is transported through an inlet port on the bag and impregnated into carbon fabrics with the help of a well-permeated resin distribution medium (bleeder), meanwhile, the extra air is evacuated from the vacuum outlet port. The UD carbon woven fabrics are placed on a 500 mm 500 mm sized mold with a stacking sequence of [452/02/ 452/902]s and [302/02/ 302/902]s. The 400 mm 400 mm sized laminate with a total of 16 plies carbon woven fabrics is then fabricated. The weight fraction of carbon fibers is approximately 75%. Epoxy resin and hardener with a mixture ratio of 3:1 by weight is injected into the bag under a pressure of 1 bar. The impregnated carbon fiber is pre-cured for 24 h at a temperature of 30 C, and then the laminate is taken to an autoclave for 8 h post-curing at a temperature of 70 C. The test specimens are cut with a diamond-coated saw to the dimension of 150 mm 100 mm 4.0 mm for lightning strike testing and 250 mm 25 mm 4.0 mm for mechanical testing.
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Fig. 1. Schematic of VARTM (a) and preparation of specimen fabrication (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.3. Lightning strike generator and experimental setup The lightning strike is simulated by a lightning impact highcurrent generator (HRHG(A)-100 kA) produced by WuGao HuaRui Voltage Technology Co. Ltd. (Fig. 2(a)). It consists of 24 high voltage capacitors (100 kV and 4 lF), an adjustable resistor, a charge supply (100 kV and 80 kJ), a control board, a protection system and a test platform. Capacitors can be charged on the control board to generate lightning strike with specific current amplitude. If emergent situations happen, the protection system can force the device to discharge. Resistors are used for generation of different
waveforms, and the amplitude of the lightning current can be achieved by adjusting the voltage charge of the capacitors. The whole device can produce current amplitude ranges from 1 kA to 100 kA. The setup of the test platform is demonstrated in Fig. 2(b). Test specimen is placed on a rectangular copper panel fixed by two red colored columns. A C-shaped aluminum component is installed on the bottom surface of a circular copper plate, and the other end attached to a copper striker by screws. The distance between the striker tip and the specimen is 1.5 mm, which is slightly shorter than the setting in [6,8] (2–3 mm). The reason is to guarantee
Fig. 2. Configuration of lightning impact high-current generator (a) and magnified setup of the test platform (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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the breakthrough of specimen and expect less energy dissipation during the striking process. In order to record the impulse current of the lightning strike, a current transformer (Rogowski coil) is installed on the C-shaped component and an oscilloscope is used to measure the waveform parameters. A copper sheet is connected to the bottom of the specimen to the earth ground. It should be noted that the data obtained from oscilloscope is in voltage form, so data transformation should be applied to get the current waveform. 2.4. Test setup Two types of laminates with stacking sequence of [452/02/ 452/902]s (45 specimen for short) and [302/02/ 302/902] s (30 specimen for short) are prepared for the experiment. The lightning strike current waveform is set as component A, which represents the first return stroke of the lightning infliction according to SAE ARP 5412 [17]. Three types of strike with amplitude current 22 kA, 32 kA and 38 kA are applied to investigate the damage characteristics of the composite specimens. The action R R 2 integral i dt and the charge transfer i dt , which are defined as the temporal integration of the first and second order of the electric current, are two proper parameters for the evaluation of lightning strike effects. The action integral is considered as a representative value to describe the amount of energy transferring into resistive heating in the inflicted material, while the charge transfer is used to measure the damage severity by the heat of lightning current to the surface of the specimens. Another parame_ _ ter, I0:2 peak I10—90% (Ipeak and I 10—90% refers to peak current and rise rate of current from 10% to 90% of peak, respectively) is applied to evaluate the influence of lightning arc acoustic shock [18,19] on the damage of tested specimens. The related experimental parameters are summarized in Table 3. 2.5. Field emission scanning electron microscopy
3. Results and discussions 3.1. Post-lightning damage inspection 3.1.1. Visual damage inspection Tests of the simulated lightning strike in compliance with the conditions in Table 3 have been conducted on 12 specimens, 6 for [452/02/ 452/902]s and 6 for [302/02/ 302/902]s. A violent explosive sound is heard with noticeable smoke seen on the surface of the specimen near the lightning attachment area. It is noted that a significant fire flame appeared on both of the 45 and 30 specimen under 38 kA and 32 kA current level, while no distinct fire and spark is spotted for the 22 kA case. Fig. 3 is a snapshot of the surface burning from the 30 specimen under the 38 kA lightning strike. It can be clearly seen that the fiber yarns in the lightning zone are broken and lifted up. The damaged fiber tip is ignited with the fire expanding along the fiber direction. A large area of resin is degraded on the specimen surface, leaving the bare UD fiber yarns to the outside. Fig. 4(a)–(f) is a top view of specimen surfaces after inflicted by three types of lightning strikes, and a magnified view on the surface of 45 and 30 specimen after 38 kA is presented in Fig. 5. Using the image processing technique, typical damage profiles for the two stacking sequenced specimens are demonstrated in Fig. 6. From the visual inspection of the post-lightning specimens, the damage forms can be classified as fiber damage and resin pyrolized damage. To comprehensively evaluate the damage response inflicted from lightning strike, visual damage forms of the present specimens (45 specimen) are compared with two other lightning strike literatures [8,20] which conducted in unprotected and protected carbon fiber/epoxy laminates, respectively. For [8], lightning strikes are applied to prepreg taped (IM600/133, 0.15 mm) graphite/epoxy laminate manufactured by autoclave molding with a stacking sequence of [45/0/ 45/90]4s. Paper in [20] proposes a ‘‘CFRP laminate – epoxy adhesive – buckypaper (MWCNTs, 70 lm)”
Cross sections are cut off from the damaged specimens and dried for 24 h in an oven with a temperature of 50 C. Then, their surfaces are coated with gold and examined by a field emission scanning electron microscope (SEM) Nova NanoSEM 450. The device has a magnification range of 40–400,000 and an imaging resolution of 1.6 nm (1 kV) in high vacuum mode. 2.6. Ultrasonic inspection All post-lightning specimens are inspected by throughtransmission method on a C-scan ultrasonic flaw detector device. A transmitter and a receiver controlled by a three-axis bridge are placed on the opposite side of the specimen and a jet of water is accompanied when the ultrasound is emitted. Considering the significant attenuation of ultrasound signal in woven fabric composites, a 0.5 MHz transducer is used as the transmitter. The signal data is acquired and processed by NDTS-90 (NDT Systems).
Fig. 3. Snapshot of surface burning for 30 specimen under 38 kA strike. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 3 Test conditions and lightning strike parameters for the current experiment. Stacking sequence
Waveform (ls)
Current amplitude (kA)
Action integral (A2 s)
Charge transfer (C)
0:2 _ I10—90% Ipeak
[452/02/ 452/902]s [302/02/ 302/902]s
6.8/24 6.8/24
22 22
13230 13230
0.58 0.58
2.1e10 2.1e10
[452/02/ 452/902]s [302/02/ 302/902]s
6/22.5 6/22.5
32 32
18830 18830
0.71 0.71
3.4e10 3.4e10
[452/02/ 452/902]s [302/02/ 302/902]s
6.4/22.8 6.4/22.8
38 38
22660 22660
0.82 0.82
4.2e10 4.2e10
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(a) [452/02/-452/902]s 38 kA
(b) [302/02/-302/902]s 38 kA
(c) [452/02/-452/902]s 32 kA
(d) [302/02/-302/902]s 32 kA
(e) [452/02/-452 /902]s 22 kA
(f) [302/02/-302 /902 ]s 22 kA
Fig. 4. Top view of the specimens after lightning strike damage. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Close-up view of the surface damage on 45 specimen (a) and 30 specimen (b) after lightning strike. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
configuration for lightning protection. The adhesive contains three types which are: pure epoxy resin (neat adhesive), CNTs (1.0 wt%) embedded epoxy resin (conductive adhesive) and boron nitride (20 wt%) embedded epoxy resin (insulating adhesive). The laminate has the same constituents and manufacturing process with [8] and its lay-up sequence is [45/0/ 45/90]2s.
Comparing the present 38 kA strike result with the 40 kA case in [8,20], one can observe that the break and lift up of carbon fiber is all initiated from the lightning attachment area for the three studies. However, the fiber damage pattern presents differently. Both Fig. 8(a) of [8] and Fig. 6 (a1 ) of [20] show that carbon fiber dissipated along the fiber orientation to the upper and bottom
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Fig. 6. Damage profile for post-lightning 45 specimen (a) and 30 specimen (b) under 38 kA strike. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
boundary of their specimens, while in the present test (45 specimen), the fiber damage region is restricted to a circle with diameter of about 22 mm (Fig. 6(a)). Extensive fiber lift up along the ply direction is shown in Fig. 8(b) of [8], while only a few fibers around the lightning attachment area are seen to break up in the present work (Fig. 5(a)). This big disparity is due to the different stacking sequence, lamina thickness and reinforcement structure of these three works. For the present test, the [452/02/ 452/902]s stacking sequence can be seen as a [45/0/ 45/90]s type with each layer consists of two original carbon woven fabric, resulting in a lamina thickness of 0.5 mm. This ply thickness is more than three times of the value in [8,20] (0.15 mm), which greatly strengthens its lightning resistance capability. Besides, for the present material, the existence of nylon binder yarns in the weft direction can restrain the out-of-plane freedom of carbon fiber. This also attributes to the decreased damage area comparing to the unprotected specimens in [8,20]. When carbon laminate is equipped with insulating adhesive and buckypaper as shown in Fig. 8 (c1 ) of [8], its visual damage is significantly minimized to the top protection medium, outperforming the present work. However, the cost for the addition of special designed conductive layers and adhesive, as well as the more complicated fabrication technique will potentially prevent its widespread promotion. Compared to the 45 specimen, the fiber damage area in the 30 specimen of the present study exhibits a diamond shaped appearance with the longer diagonal axis lies along the ply orientation (Fig. 6(b)). Large bundle of fiber break up and sublimation can be observed several layers deep around the lightning attachment area (Fig. 5(b)). One may further observe that although the fiber damage zone of the 30 specimen is larger than the 45 one, it does not extend to the specimen boundary which is shown in Fig. 8(a) of [8], instead, the damage size has a limited length along the fiber direction (L2 in Fig. 6(b)). These observations demonstrate that the different stacking sequence will have different responses to the lightning strike. Around fiber damage region, the white area with bare fiber yarns unprotected from resin matrix is resin pyrolysis area (Fig. 4). It can be seen that the area of the resin damage exhibits a diamond shape for both of the 45 and 30 specimens (Fig. 6). If L1 and L2 are defined as the length of the longer and shorter diagonals of the resin pyrolized zone, respectively, L1 is found to be 2 times of L2 for both of the two type specimens under 22 kA, 32 kA and 38 kA strike. Besides, the longer diagonal axis of the diamond shape just lies in the vertical direction of the fiber orientation and extends a large distance from the lightning attachment area. This observation is different to the results in [8,20], in which resin deterioration extended primarily along the fiber direction. This disparity of resin pyrolysis pattern comes from the differences in material types used in the corresponding experiments.
The process of resin damage can be regarded as dielectric surface discharge due to the insulation property of resin matrix [19,21,22,18,23,24]. The dielectric breakdown model [18], which assumes the local discharge direction is determined by the potential gradient at that point, can be used to explain the resin damage shape in the present study. One can figure out that the potential gradient perpendicular to the fiber orientation is tremendously larger than that along with the fiber direction due to the coupled interaction of strong orthotropic electrical behavior of carbon/ epoxy laminate and dielectric property of nylon binder yarns in the weft direction. Thus, the great potential gradient leads to the relatively bigger resin pyrolized damage in the direction perpendicular to the fiber orientation. One benefit for the large resin damage area is that it consumes a large amount of energy from lightning infliction, leaving small portion of energy to make damages to the fiber yarns embedded in the matrix. 3.1.2. Microscopic damage inspection The microscopic inspection is conducted to unveil the damage mechanisms on the microstructural point of view which is helpful for the understanding of the internal damage induced from lightning strike. Three samples, with the locations showing in Fig. 7, are cut off from the 45 specimen (38 kA strike damage) for microscopic inspection. Fig. 8 shows a SEM image of a delamination area at cross section B. It can be seen that a major delamination happens on the first two sub-layers with its propagation along the 45 /45 interface (Fig. 8(a)). The resin matrix between the two layers is completely pyrolized with scattered carbon filaments hanging on
C 5
75 B A
20
50
10
45 90 0
10mm
Fig. 7. Location of the cross sections for microscopic inspection (45 specimen, 38 kA). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. SEM micrograph of the delamination area (a) and a single carbon filament (b) at cross section B (45 specimen, 38 kA).
the edge of the fiber bundles. A secondary branch [25] can also be observed between the second and third sub-layer (45 /0 ). It propagates along the resin/fiber interface and branches into two paths with that one goes into the interval space between two adjacent fiber bundles. Fig. 8(b) plots a magnified view on the surface of a single carbon filament after lightning strike. It is noted that the carbon fiber is intact but its surface is wrapped with spherical shaped residues. These substances are the pyrolized resin remnants and most of which agglomerated into large particles under the high temperature environment. Presented in Fig. 9 is the configuration of fiber bundles before and after lightning strike, located at cross section C and A, respectively. Fibers are shown closely bonded by resin matrix before lightning infliction (Fig. 9(a)), while significant detachment is discovered on the interface between fiber filaments and resin matrix (Fig. 9(b)) after strike occurs. Fig. 10 shows the damage pattern of a fiber bundle near a delamination area (cross section A). Most of the resin matrix has been pyrolized and vaporized, leaving the individual fiber filaments un-bonded. After magnifying a certain area of the fiber bundles, one can clearly see that there is a big gap between two carbon fibers and the resin matrix in between is cracked, which is possibly due to a compression force generated from the thermal stress inside resin matrix. This observation indicates that the internal resin damage during the lightning strike process not only is induced by the pyrolysis, but also by the combined thermal and mechanical interaction.
3.1.3. Internal damage inspection Besides visual and microscopic inspection of damage forms, the internal damages caused by lightning strike will be studied next by ultrasonic scanning technique. Fig. 11(a)–(f) plots the C-scan results for the two stacking sequenced specimens under the three current amplitudes. The dark area on each of the sub-figures demonstrates the delamination zone. The profile of the resin pyrolysis area in the corresponding panel is also overlaid on each sub-figure for comparison. The two dark columns in the vertical direction are iron wires, which support the specimen when the scanning is under going. The adoption of through-transmission method guarantees the accuracy of the results with a less strict requirement on the transducer alignment to the surface of the test specimen [26]. It can be seen from Fig. 10(a), (c), and (e), a gradual enlarged delamination area appears as the lightning strike level is intensified for the 45 specimen. The damage shape extends simultaneously along the fiber and the vertical direction within the resin damage profile. This result is different from the C-scan result in [8] (Fig. 10) which exhibits the shape of a pair of fans along the fiber direction and the quadrilateral shape presented in [20] (Fig. 6 (a4 )). The reason for this difference is also due to the stacking sequence and lamina thickness. From the B-scan results in [8,20], it can be seen that the damage depth is about 1.2 mm and 0.9 mm, respectively, indicating that the damage extends throughout the first [45/0/ 45/90] unit for the two studies. Therefore, delamination propagation along diverse ply direction in each layer
Fig. 9. SEM micrograph of undamaged fiber bundles at cross section C (a) and after 38 kA lightning strike damage at cross section A (b).
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Fig. 10. SEM micrograph of resin/fiber interfacial damage after 38 kA lightning strike damage at cross section A (a) and the magnified view of a resin/fiber interface (b).
(a) [452/02/-452 /902]s 38 kA
(b) [302/02/-302/902]s 38 kA
(c) [452/02/-452/902] s 32 kA
(d) [302/02/-302/902 ] s 32 kA
(e) [452/02 /-452 /902] s 22 kA
(f) [302/02/-302 /902]s 22 kA
Fig. 11. Overview of the C-scan image combined with resin pyrolysis area.
contributes to the fan shaped and quadrilateral shaped profile. However, damage only penetrates throughout the first two 45 layers for the present work. As such, the damage profile extends regularly along the fiber direction. The reason for the vertical extension is unknown. It may also attribute to the nylon yarns in the weft direction which deserves further study. Under the protec-
tion configuration, the CFRP laminate is almost intact from Fig. 8 (c4 ) of [20], showing the best lightning strike prevention ability. These observations suggest that although the present specimens can restrain the strike damage to some extent, its effect cannot be comparable to the laminates which have special designed protection constituent.
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Apart from Fig. 11(a), (c), and (e), results in Fig. 11(b), (d), and (f) show that, under the same level of lightning strike, a notable larger delamination area exists for each of the 30 specimens than that of the 45 ones. The delamination shape is also observably different for the two types. The cause for the delamination damage is majorly due to Joule heating effect. When lightning strike inflicts, the temperature of the lightning attachment area is up to 3500 K [9] which is high enough to pyrolize the surface resin matrix and sublimate the internal fiber yarns. Subsequent lightning current is delivered inside and generates Joule heating along the fiber direction due to the good conductivity of carbon fiber. In this process, the interfacial bonding of the matrix with its adjacent fibers is severely damaged and the resin between sub-layers is broken up into small pieces under the influence of thermal–mechanical interacting stresses (Figs. 8–10). It agglomerates and pyrolizes to the gaseous substances which are entrapped and accumulated in the interlaminar zone. Finally, under the high temperature, high pressure and limited volume space environment, the entrapped gas explodes and causes massive delamination damage. 3.1.4. Discussion The different delamination and fiber damage pattern for the two kind specimens is largely due to the stacking sequence arrangement. From the above analysis, it can be seen that the gases produced from Joule heating is the essential factor to induce the extension of internal damage. At the same level of lightning strike, the energy transferred into Joule heating (action integral) is considered to be the same, which generates equal amount of gases in the two stacking sequenced specimens. For the 30 specimen, fibers are distributed to form small cells around the 0 direction so that the gases produced from Joule heating effect are trapped in a small and narrow area. Therefore, due to the quick elevation of local temperature and pressure, damage is seriously increased. For the 45 one, however, the larger angle between sub-layers disperses the Joule heating to a greater region so that the damage produced by the pyrolized gases is restrained. Fiber damage can be seen as an external demonstration of gas explosion inside specimen. It is controlled by a coupled effect of surface recession [9] from fiber sublimation, shockwave and resistive heating. When lightning strike intensity is low, the impact of gas explosion is not enough to break up fiber yarns, thus only delamination damage is created. As the strike level increases, the energy of gas explosion not only produces delamination, but also extends its impact to the surface and makes the fiber damage. As such, the different appearances of fiber damage for the two typed specimens can also attribute to the influence of stacking sequence. The restricted effect of pyrolysis gases on the quasi-isotropic ply orientation determines the fiber damage area to be a circular shape for the 45 specimen, while the narrowly distributed gases prone to 0 direction intensifies fiber breakage along its ply orientation significantly for the 30 one. It should be noted that the unique woven fabric structure, the nylon binder yarns in the weft direction, greatly influences the fiber damage pattern. Although the temperature at the lightning attachment area is up to 3500 K, the temperature for the remaining zone is below 1000 K, which leaves the majority of nylon yarns undamaged. When lightning strike inflicts on the specimens, they bind and protect fiber yarns from massive break up, thus restrain the fiber damage area to a limited region. 3.2. Effect of waveform parameters on damage forms The results of the relationship between waveform parameters and the damage patterns for the two stacking sequenced specimen can be revealed in Fig. 12. Fig. 12(a) shows that a linear relationship between the charge transfer and the resin damage area exists
(a) Charge transfer vs. Resin pyrolysis area
(b) Action integral vs. Internal damage area
(c) Acoustic shock vs. Fiber damage area Fig. 12. Relationship between lightning parameters and areas of different damage patterns for stacking sequence [452/02/ 452/902]s and [302/02/ 302/902]s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
for both of the 45 and 30 specimen. One may also see that the influence from stacking sequence is relatively small on the resin
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damage area under the same lightning strike level. For resin damage is largely a process of surface discharge interaction with specimen, the damage region is mainly determine by the intensity (charge transfer) of the lightning strike. Comparing with the results in [8] (Fig. 21), it can be seen that the present area is larger (2800 mm2 vs. 2300 mm2 ) under the same electrical charge (0.82 C), suggesting that more energy is spent on the formation of surface resin damage for the present specimen, so that less energy is left to make damages to the fiber yarns embedded in the matrix. Plotted in Fig. 12(b) is the relationship between the action integral and the internal damage area. For the 45 specimen, the delamination area linearly increases with the action integral, while a nonlinearly increase pattern is shown for the 30 sample. The main trigger of the internal damage is the Joule heating generated from lightning current and high electrical resistance of composite laminates. As such, different stacking sequence may produce inconsistent distribution of Joule heating that subsequently pyrolizes resin matrix around fiber yarns and causes delamination damage in the test specimens. Here, the present delamination area is not only compared with results in [8,20], but also with another literature [27] which is a traditional copper embedded aerospace composite laminate fabricated from tape (Hexcel HexPly 8552/ AS4, 0.15 mm), fabric (Hexcel HexPly 8552/A193-PW, 3K-70-PW, 0.2 mm) and coated with a typical aerospace primer and paint finishing scheme with vacuum bag molding. A layer made up of Cytec Surfacemaster 905C Composite Surfacing Film and Dexmet 3CU7100FA expanded copper foil is attached on the top of the laminate for protection ([(0/90F)(45/90/ 45/0)2]s). First in comparison with the unprotected specimens in [8] (Fig. 22), it shows that the delamination area is smaller (1800 mm2 vs. 3000 mm2 ) for the present work under the same action integral (22,660 A2 s). This result is also smaller than the unprotected case in [20,27], which are 3380 mm2 and 3677 mm2 , respectively. Second, comparing with the protected ones, the present results is also smaller than the copper embedded one in [27] (1800 mm2 vs. 2735 mm2 ), but is much larger than the special protected laminates in [20] (1800 mm2 vs. 0 mm2 ). In summary, the delamination area follows the order: [20] (protected case) < current study < [27] (protected case) < [8] (unprotected case) < [20] (unprotected case) < [27] (unprotected case). It should be noted that the present work is only subjected to 38 kA strike level. Results in [20,27] also tested lightning strike to 100 kA, they found that the effect of their protection medium is more remarkable under 100 kA strike. However, the damage behavior of the present specimens under high level strike is unknown, which deserves further study. The influence of acoustic shock on the fiber damage size is shown in Fig. 12(c) in which a similar trend to that in Fig. 12(b) can be observed. These two observations indicate that the structure of the stacking sequence has great influence on the damage characteristics of the fibers. With increase of the acoustic shock generated from lightning infliction, it deteriorates the fiber–resin matrix bond and facilitates the fiber damage and internal delamination. The area of fiber damage for the present 38 kA results (45 specimen) are comparable to the ones in [8] (Fig. 19), which are 480–500 mm2 , suggesting a less sensitivity to the material type. On the other hand, little difference of damage area can be seen for both of the stacking sequenced specimen under 22 kA strike. This observation reasonably suggests that an energy threshold may exist for the massive destruction of the composite laminates.
tensile strength and modulus of virgin and post-lightning specimens are determined in accordance with ASTM D3039. The mechanical testing is performed on a WDW-100E universal testing machine with a 100 kN load cell and a crosshead speed of 2 mm/min under room temperature. During the in-service time, current discharge often happens more than once between the overhead conductor and composite pole when the humidity of the local environment increases. To make sure the poles can survive such environmental exposure, strike with amplitude current 10 kA, which is the current delivered in the overhead conductor, is applied to the tested samples for the safety operation of the composite poles. Damage forms of one-strike and two-strikes are inflicted on the 45 specimen, while one-strike and three-strikes are made on the 30 specimen, as plotted in Fig. 13, to further study the relationship between strike number and residual strength of tested laminates. The cross sectional view of the two stacking sequenced specimens after lightning infliction is plotted in Fig. 14. The 45 specimen case shown in Fig. 14(a)–(c) demonstrates that the major damage form is interlaminar delamination. However, due to the lightning strike damage, the delamination area initials from both of the top and bottom 45 =0 interface (Fig. 14(a)) to just the top 45 =0 interface (Fig. 14(b)) where lightning strike is applied. For the 30 specimen, the influence of lightning strike on the damage pattern is less noticeable as seen in Fig. 14(d)–(f), despite of the increased number of strikes on the specimen. The cause for the position change of delamination zone on the 45 specimen is majorly due to the increase of matrix cracks, originated from the impact effect of lightning infliction. When lightning strike is applied on the specimen surface, its impact effect induces high stresses near the impact point and initials local matrix cracks [8,28], which in turn initials delaminations at interfaces between plies with different orientation (45 =0 interface in the current work) [28]. As the post-lightning specimen is subjected to external force, high interlaminar stresses drive it to propagate to form a delamination failure [28,29]. For the 30 specimen, however, its failure mode is governed not only by interlaminar delamination, but also intralaminar tensile fracture due to a 0 directionprone stacking sequence arrangement. Typical stress–strain curves for the two stacking sequenced post-lightning specimen is shown in Fig. 15. A linear relationship is observed for the 45 specimen while a significant two-stepped non-linear trend is presented for the 30 one. The linear curve can be explained by the single interlaminar delamination failure of the 45 specimen, while for the 30 sample curve, the first step is governed by interlaminar shear and the second stage changes to intralaminar tensile. It is noted that some springback occurs on the second phase which can be explained by the fracture of laminas on different sub-layers. Fig. 16 shows that the strength and modulus decreases as the lightning strike is applies, and with the number of strike increases, the degradation is more remarkable. This phe-
20mm
3.3. Effect of lightning strike damage on residual mechanical properties To investigate the influence of lightning strike damage on the mechanical behavior of the two stacking sequenced specimen, the
Fig. 13. Post-lightning specimens ready for mechanical testing. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 14. Cross sectional view for post-lightning specimens after mechanical testing.
Fig. 15. Typical stress–strain curves for post-lightning 45 specimen (a) and 30 specimen (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 16. Tensile strength (a) and tensile modulus (b) for post-lightning 45 specimen and 30 specimen.
nomenon can be attributed to the greater acoustic shock effect on the massive increase of crack density in the sub-layer interface when more number of lightning strikes is inflicted on the tested panel. Table 4 summarizes values of lightning strike influenced tensile strength and modulus. It shows that, for both of the two
stacking sequenced specimens, the degradation percentage of modulus and strength exhibit a similar extent as the intensity of applied lightning strike increases. However, for the one-strike damage type, the mechanical properties of the 30 sample are more sensitive to strike damage than the 45 one (13% vs. 5% loss
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Y. Li et al. / Composites: Part A 79 (2015) 164–175 Table 4 Effect of lightning strike on tensile modulus and strength of 45 and 30 specimens. Specimen conditions
Modulus (GPa)
Retention (%)
Strength (MPa)
Retention (%)
45 type
No-strike One-strike Two-strikes
43.61 (0.52) 43.27 (2.56) 38.58 (4.21)
100 99 88
575.98 (28.27) 541.95 (21.23) 496.73 (14.13)
100 94 86
30 type
No-strike One-strike Three-strikes
67.46 (2.41) 59.20 (1.67) 52.61 (3.40)
100 87 78
580.04 (42.57) 488.75 (7.97) 449.27 (13.23)
100 84 77
Values given in parentheses represent standard deviations of three repetitive specimens.
in modulus and strength). Here, a literature [6] is reported for comparison with the current results. Although it has a different material type (CYCOM/FIBERITE G30-500 12K HTA/7714A, 0.16 mm), stacking sequence ([45/02/ 45/03/90]s), manufacturing technique (press molding), dimension (304.8 mm 38.1 mm) and strike damage form (only one-strike), its results show a slight decrease on both of the residual strength and modulus, which has a similar trend to the present work (45 specimen). It is believed that the quasi-isotropic stacking sequence of the 45 specimen has a more resilient capability than the 30 one on diminishing impact from lightning strike and maintaining mechanical performance. 4. Conclusion In this paper, simulated lightning strike with different current levels is exerted on two stacking sequenced UD carbon woven fabric/epoxy laminates to study the damage behavior on the material experimentally. Various inspection techniques are used to assess the damage characteristics of the post-lightning composite specimens and mechanical testing is subsequently conducted to study the effect of lightning strike on the degradation of tensile strength and modulus. For the 45 specimen, a diamond shaped resin pyrolysis zone with a circular fiber damage area and evenly expanded delamination zone is detected, and the relationship between various lightning parameters and damage modes shows a good linear correlation. While for the 30 specimen, the fiber damage area changes to a larger sized diamond shape and the delamination area extends notably along the fiber orientation. Microscopic investigation further reveals that lightning strike greatly damages bonding strength of resin/fiber interface by a coupled thermal–mechanical effect. The residual strength and modulus of post-lightning specimens are reduced as lightning strike is applied on but the sensitivity of failure position is different for the two types. Unlike those in graphite/epoxy laminated composites, the damage areas in the current specimens are constrained in certain sizes due to the different stacking sequence, lamina thickness, and the nylon binder yarns in the weft direction. This discovery suggests that the woven fabric/epoxy structure is a good choice for the fabrication of laminated composites which is likely to be subjected to lightning strikes. Acknowledgements This research is sponsored in part by the Ministry of Science and Technology Fund Project (Contract No. 2015DFA81640) and Aeronautical Science Foundation of China (Contract No. 20130179002) at the Huazhong University of Science and Technology. References [1] Alawar A, Bosze EJ, Nutt S. A composite core conductor for low sag at high temperatures. IEEE Trans Power Del 2005;20(3):2193–9. [2] Tsai YI, Bosze EJ, Barjasteh E, Nutt SR. Influence of hygrothermal environment on thermal and mechanical properties of carbon fiber/fiberglass hybrid composites. Compos Sci Technol 2009;69(3–4):432–7.
[3] Bosze EJ, Alawar A, Bertschger O, Tsai YI, Nutt SR. High-temperature strength and storage modulus in unidirectional hybrid composites. Compos Sci Technol 2006;66(13):1963–9. [4] Gagne M, Therriault D. Lightning strike protection of composites. Prog Aerosp Sci 2014;64:1–16. [5] Chemartin L, Lalande P, Peyrou B, Chazottes A, Elias PQ. Direct effects of lightning on aircraft structure: analysis of the thermal, electrical and mechanical constrains. J Aerospace Lab 2012;5(AL05-09):1–15. [6] Feraboli P, Miller M. Damage resistance and tolerance of carbon/epoxy composite coupons subjected to simulated lightning strike. Composites: Part A 2009;40(6):954–67. [7] Kawakami H, Feraboli P. Damage of carbon/epoxy composite plates subjected to mechanical impact and simulated lightning. J Aircraft 2010;47 (3):999–1012. [8] Hirano Y, Katsumata S, Iwahor Y, Todoroki A. Artificial lightning testing on graphite/epoxy composite laminate. Composites: Part A 2010;41(10):1461–70. [9] Ogasawara T, Hirano Y, Yoshimura A. Coupled thermal–electrical analysis for carbon fiber/epoxy composites exposed to simulated lightning current. Composites: Part A 2010;41(8):973–81. [10] Abdelal G, Murphy A. Nonlinear numerical modelling of lightning strike effect on composite panels with temperature dependent material properties. Compos Struct 2014;109:268–78. [11] Wang FS, Ding N, Liu ZQ, Ji YY, Yue ZF. Ablation damage characteristic and residual strength prediction of carbon fiber/epoxy composite suffered from lightning strike. Compos Struct 2014;117:222–33. [12] Munoz R, Delgado S, Gonzalez C, Lopez-Romano B, Wang DY, LLorca J. Modeling lightning impact thermo-mechanical damage on composite materials. Appl Compos Mater 2014;21(1):149–64. [13] Liu ZQ, Yue ZF, Wang FS, Ji YY. Combining analysis of coupled electrical– thermal and blow-off impulse effects on composite laminate induced by lightning strike. Appl Compos Mater 2015;22(2):189–207. [14] Potluri P, Young RJ, Rashed K, Manan A, Shyng YT. Meso-scale strain mapping in ud woven composites. Composites: Part A 2009;40(12):1838–45. [15] Barbero EJ. Introduction to composite materials design. 2nd ed. Boca Raton (USA): CRC Press; 2011. p. 44 [chapter 2.4.3]. [16] Subagia AIDG, Kim Y, Tijing LD, Kim CS, Shon HK. Effect of stacking sequence on the flexural properties of hybrid composites reinforced with carbon and basalt fibers. Composites: Part B 2014;58:251–8. [17] Aerospace Recommended Practice ARP 5412. Aircraft lightning environment and related test waveforms. SAE 1999. [18] Niemeyer L, Pietronero L, Wiesmann HJ. Fractal dimension of dielectric breakdown. Phys Rev Lett 1984;52(12):1033–6. [19] Kawakami H. Lightning strike induced damage mechanisms of carbon fiber composites. Ph.D. thesis; University of Washington; 2011. [20] Han JH, Zhang H, Chen MJ, Wang D, Liu Q, Wu QL, et al. The combination of carbon nanotube buckypaper and insulating adhesive for lightning strike protection of the carbon fiber/epoxy laminates. Carbon 2015;94:101–13. [21] Fitaire M, Mantei TD. Some experimental results on acoustic wave propagation in a plasma. Phys Fluids 1972;15(3):464–9. [22] Peruani F, Solovey G, Irurzun M, Mola EE, Marzocca A, Vicente JL. Dielectric breakdown model for composite materials. Phys Rev E 2003;67(6):066121. [23] Pillai AS, Hackam R. Surface flashover of solid insulators in atmospheric air and in vacuum. J Appl Phys 1986;58(1):146–53. [24] Sudarshan TS, Dougal RA. Mechanisms of surface flashover along solid dielectrics in compressed gases: a review. IEEE Trans Dielect Electr Insul 1986;EI-21(5):727–46. [25] Li R, Kardomateas GA. Thermo-elastic crack branching in general anisotropic media. Int J Solids Struct 2005;42(3–4):1091–109. [26] Campbell FC. Structural composite materials. Ohio (USA): ASM International; 2010. p. 336 [chapter 12.2]. [27] George NS, Kenneth LD, Sandra VK, Jay JE, Truong XN. Common practice lightning strike protection characterization technique to quantify damage mechanisms on composite substrates. In: International conference on lightning and static electricity, Seattle (WA); 2013. p. 13–51. [28] Abrate S. Impact on composite structures. New York (USA): Cambridge University Press; 2005. p. 159 [chapter 4.5]. [29] Lesser AJ, Filippov AG. Kinetics of damage mechanisms in laminated composites. In: Int SAMPE symp and exhibition, vol. 36(Part 1); 1991. p. 886–900.
Contents lists available at ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Experimental study of damage characteristics of carbon woven fabric/epoxy laminates subjected to lightning strike Yichao Li a,b, Renfu Li a,b,⇑, Luyi Lu a,b,*, Xianrong Huang a a b
Department of Aerospace Engineering, Huazhong University of Science and Technology, 430074 Wuhan, China Digital Manufacturing Equipment and Technology National Key Laboratory, Huazhong University of Science and Technology, 430074 Wuhan, China
a r t i c l e
i n f o
Article history: Received 12 June 2015 Received in revised form 21 September 2015 Accepted 24 September 2015
Keywords: A. Fabrics/textiles B. Delamination C. Damage mechanics D. Electron microscopy
a b s t r a c t This paper experimentally investigates the damage characteristics of two stacking sequenced ([452/02/ 452/902]s, [302/02/ 302/902]s) carbon woven fabric/epoxy laminates subjected to simulated lightning strike. Characteristics of the damage are analyzed using visual inspection, image processing, ultrasonic scanning and scanning electron microscope. The mechanical properties of post-lightning specimens are then studied. Observations show that as the lightning strike is intensified, an enlarged resin pyrolized area appears majorly along the weft orientation while the delamination region extends equally to both of the warp and the weft direction. The resin/fiber interfacial bonding is severely damaged by a thermal–mechanical effect due to lightning strike infliction. Mechanical testing further shows that the stacking sequence can influence the failure significantly. Compared with prepreg taped material, the restrained damage area due to special designed stacking sequence, lamina thickness and the weft nylon binder make the woven fabric reinforcement a good choice for the fabrication of lightning protection structures. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Carbon fiber reinforced polymer (CFRP) composites have been widely used in aerospace industry for decades due to their high specific stiffness and strength. With the emergence of various affordable carbon fibers and cost-effective manufacturing process, the applications of CFRP composites have been also widely used in automotive, sports and civil infrastructures. In recent years, the support of smart grid and green, sustained energy utility in China and worldwide has attracted an increase interest in the potential replacement of large amount of electrical products by composite structures in the electric power industry [1–3]. As one may easily see most of the structures in electric power industry often work outdoor all year round, damage from lightning strike is one of the primary issues which needs to be addressed before the use of composite materials broadly spreads into the power industry. The phenomenon of lightning strikes on CFRP composites has been investigated in aerospace industry for decades. A large number of studies have been conducted and various protection methods ⇑ Corresponding authors at: Department of Aerospace Engineering, Huazhong University of Science and Technology, 430074 Wuhan, China. E-mail address: [email protected] (R. Li). http://dx.doi.org/10.1016/j.compositesa.2015.09.019 1359-835X/Ó 2015 Elsevier Ltd. All rights reserved.
have been proposed as summarized in [4]. Some representative papers are briefly surveyed in this section. Chemartin et al. [5] proposed a comprehensive study on the direct effects of lightning on composite structures from the formation of lightning arc, the strike to the aircraft structure and the thermal and mechanical constraints during the damage process. They discovered that Joule heating and overpressure are the two main factors for the various damage forms of composites. Feraboli et al. [6,7] investigated the damage response of carbon fiber/epoxy specimens with and without the influence of fastener under lightning strikes. The study showed the damage pattern was dramatically influenced by the fastener with a through-thickness delamination mode. But the damage area cannot be fully demonstrated due to the limited geometry of the specimens. Hirano et al. [8] examined a series of post-lightning graphite/epoxy composite laminates by means of visual inspection, ultrasonic testing and X-ray inspection technique and categorized the damage pattern into fiber damage, resin deterioration and internal delamination modes, which have strong relationships to certain lightning parameters. In the aspect of numerical simulation, Ogasawara et al. [9] proposed a coupled thermal–electrical method to simulate the lightning strike damage of CFRP. Their results suggested that both dielectric breakdown at the interlayer gaps and surface recession govern the damage behavior in the thickness direction. Abdelal and Murphy [10]
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presented a temperature dependent material modeling procedure to simulate the thermal damage of composite panel with the influence of embedding copper mesh and numerically validated the protection mechanism which is to consume lightning energy on damaging copper mesh itself. A coupled thermal–electrical–structural with element deletion method was reported by Wang et al. [11] to study the ablation damage behavior of carbon fiber/epoxy laminates. It showed that the temperature could be an adequate indicator to describe the damage condition of composites after lightning infliction. Other simulation studies that focused on both thermal–electrical and mechanical damage from lightning can be referred to [12,13]. The current work is part of a funded project which aims to design a series of composite poles subject to various levels of voltages in the remote central and western regions of China. Due to that the poles have to work in harsh environments, the convenience in transportation and the resistance to the local environmental attacks such as lightning strike, moisture and corrosion become the major concerns for the choice of material and design process. As such, carbon woven fabric has been selected as the material candidate for its excellent mechanical property and relative inexpensive cost compared to other materials, for example the prepreg taped ones. However, literature search shows that little attention has been paid to the study of lightning strike on woven fabric materials, which is an essential issue that could determine the acceptance or failure of the CFRP composite pole. For such composites, one primary concern is that the influences from the stacking sequence of the laminas on the damage patterns and the integrity of the composite structures after a lightning strike is inflicted on. Therefore, the present work focuses on the investigation of the basic damage mechanics of two stacking sequenced ([452/02/ 452/902]s, [302/02/ 302/902]s) carbon woven fabric/epoxy laminates. The reason to choose the stacking sequence of [452/02/ 452/902]s is because of its quasi-isotropic property which is required by the components on the pole such as main body and cross arm. In the meanwhile, the stacking sequence of [302/02/ 302/902]s is chosen because it can be used for the design of connections, attachment or fixing components which subject to a major longitudinal loading. The responses of these stacking sequences to lightning strike, as well as its postlightning residual strength are essential for the safety concern of the composite poles. Three types of lightning strikes with amplitude current 38 kA, 32 kA and 22 kA are applied to the two stacking sequenced specimen discussed above. Damage types and properties are analyzed by visual inspection, image processing, ultrasonic testing and field emission scanning electron microscope. The effect of stacking sequence on the formation of various damage forms is discussed and mechanical properties of post-lightning specimens are subsequently studied. The lightning damage mechanism in microscopic level is also explored. 2. Experimental procedure 2.1. Constituent materials UD woven fabrics [14] are materials with a majority of fiber yarns in the warp direction (98%) and a minority of thread yarns in the weft direction. Fiber yarns are designed to withstand unidirectional loadings while the thread yarns provide handling and drape properties for the woven fabric [15]. In this work, TORAY T700SC-12K UD woven fabric is used with single layer density of 300 g/m2 in the warp direction. Nylon binder yarns with single
Table 1 Properties of the carbon woven fabrics. T700SC-12K Longitudinal tensile modulus (GPa) Longitudinal tensile strength (MPa) Transverse tensile modulus (GPa) Transverse tensile strength (MPa) Shear modulus (GPa) Shear strength (MPa) Elongation (%) Volume density (g/cm3 ) Weight per unit length (tex) Width of single fiber yarn (mm) Thickness of single fiber yarn (mm)
250 4900 18 23 7 59 2.1 1.8 800 2.4 0.25
Table 2 Properties of the epoxy resin. 760E/766H Density (g/cm3 ) Viscosity (mPa s, 25 C) Tensile modulus (MPa) Tensile strength (MPa) Flexural modulus (MPa) Flexural strength (MPa) Elongation (%) Heat resistance (°C) Water absorption (7 d at 23 C, %)
1.2 1400 3000 67 3000 105 4.0 71 0.25–0.45
layer density of 30 g/m2 in the weft direction hold the warp yarns from fall-apart. AIRSTONE 760E/766H of Dow Chemical Company is chosen as epoxy resin/hardener. Parameters of the UD carbon woven fabric and resin matrix are presented in Tables 1 and 2, respectively.
2.2. Fabrication of specimens The UD carbon woven fabric/epoxy laminates are prepared by using vacuum assisted resin transfer molding (VARTM) method. A schematic layout is shown in Fig. 1(a) and the preparation of specimen fabrication is presented in Fig. 1(b). VARTM [16] is a highly efficient and cost effective molding technique for composite part made from fabrics. A vacuum bag is covered on the mold, and resin is transported through an inlet port on the bag and impregnated into carbon fabrics with the help of a well-permeated resin distribution medium (bleeder), meanwhile, the extra air is evacuated from the vacuum outlet port. The UD carbon woven fabrics are placed on a 500 mm 500 mm sized mold with a stacking sequence of [452/02/ 452/902]s and [302/02/ 302/902]s. The 400 mm 400 mm sized laminate with a total of 16 plies carbon woven fabrics is then fabricated. The weight fraction of carbon fibers is approximately 75%. Epoxy resin and hardener with a mixture ratio of 3:1 by weight is injected into the bag under a pressure of 1 bar. The impregnated carbon fiber is pre-cured for 24 h at a temperature of 30 C, and then the laminate is taken to an autoclave for 8 h post-curing at a temperature of 70 C. The test specimens are cut with a diamond-coated saw to the dimension of 150 mm 100 mm 4.0 mm for lightning strike testing and 250 mm 25 mm 4.0 mm for mechanical testing.
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Fig. 1. Schematic of VARTM (a) and preparation of specimen fabrication (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.3. Lightning strike generator and experimental setup The lightning strike is simulated by a lightning impact highcurrent generator (HRHG(A)-100 kA) produced by WuGao HuaRui Voltage Technology Co. Ltd. (Fig. 2(a)). It consists of 24 high voltage capacitors (100 kV and 4 lF), an adjustable resistor, a charge supply (100 kV and 80 kJ), a control board, a protection system and a test platform. Capacitors can be charged on the control board to generate lightning strike with specific current amplitude. If emergent situations happen, the protection system can force the device to discharge. Resistors are used for generation of different
waveforms, and the amplitude of the lightning current can be achieved by adjusting the voltage charge of the capacitors. The whole device can produce current amplitude ranges from 1 kA to 100 kA. The setup of the test platform is demonstrated in Fig. 2(b). Test specimen is placed on a rectangular copper panel fixed by two red colored columns. A C-shaped aluminum component is installed on the bottom surface of a circular copper plate, and the other end attached to a copper striker by screws. The distance between the striker tip and the specimen is 1.5 mm, which is slightly shorter than the setting in [6,8] (2–3 mm). The reason is to guarantee
Fig. 2. Configuration of lightning impact high-current generator (a) and magnified setup of the test platform (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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the breakthrough of specimen and expect less energy dissipation during the striking process. In order to record the impulse current of the lightning strike, a current transformer (Rogowski coil) is installed on the C-shaped component and an oscilloscope is used to measure the waveform parameters. A copper sheet is connected to the bottom of the specimen to the earth ground. It should be noted that the data obtained from oscilloscope is in voltage form, so data transformation should be applied to get the current waveform. 2.4. Test setup Two types of laminates with stacking sequence of [452/02/ 452/902]s (45 specimen for short) and [302/02/ 302/902] s (30 specimen for short) are prepared for the experiment. The lightning strike current waveform is set as component A, which represents the first return stroke of the lightning infliction according to SAE ARP 5412 [17]. Three types of strike with amplitude current 22 kA, 32 kA and 38 kA are applied to investigate the damage characteristics of the composite specimens. The action R R 2 integral i dt and the charge transfer i dt , which are defined as the temporal integration of the first and second order of the electric current, are two proper parameters for the evaluation of lightning strike effects. The action integral is considered as a representative value to describe the amount of energy transferring into resistive heating in the inflicted material, while the charge transfer is used to measure the damage severity by the heat of lightning current to the surface of the specimens. Another parame_ _ ter, I0:2 peak I10—90% (Ipeak and I 10—90% refers to peak current and rise rate of current from 10% to 90% of peak, respectively) is applied to evaluate the influence of lightning arc acoustic shock [18,19] on the damage of tested specimens. The related experimental parameters are summarized in Table 3. 2.5. Field emission scanning electron microscopy
3. Results and discussions 3.1. Post-lightning damage inspection 3.1.1. Visual damage inspection Tests of the simulated lightning strike in compliance with the conditions in Table 3 have been conducted on 12 specimens, 6 for [452/02/ 452/902]s and 6 for [302/02/ 302/902]s. A violent explosive sound is heard with noticeable smoke seen on the surface of the specimen near the lightning attachment area. It is noted that a significant fire flame appeared on both of the 45 and 30 specimen under 38 kA and 32 kA current level, while no distinct fire and spark is spotted for the 22 kA case. Fig. 3 is a snapshot of the surface burning from the 30 specimen under the 38 kA lightning strike. It can be clearly seen that the fiber yarns in the lightning zone are broken and lifted up. The damaged fiber tip is ignited with the fire expanding along the fiber direction. A large area of resin is degraded on the specimen surface, leaving the bare UD fiber yarns to the outside. Fig. 4(a)–(f) is a top view of specimen surfaces after inflicted by three types of lightning strikes, and a magnified view on the surface of 45 and 30 specimen after 38 kA is presented in Fig. 5. Using the image processing technique, typical damage profiles for the two stacking sequenced specimens are demonstrated in Fig. 6. From the visual inspection of the post-lightning specimens, the damage forms can be classified as fiber damage and resin pyrolized damage. To comprehensively evaluate the damage response inflicted from lightning strike, visual damage forms of the present specimens (45 specimen) are compared with two other lightning strike literatures [8,20] which conducted in unprotected and protected carbon fiber/epoxy laminates, respectively. For [8], lightning strikes are applied to prepreg taped (IM600/133, 0.15 mm) graphite/epoxy laminate manufactured by autoclave molding with a stacking sequence of [45/0/ 45/90]4s. Paper in [20] proposes a ‘‘CFRP laminate – epoxy adhesive – buckypaper (MWCNTs, 70 lm)”
Cross sections are cut off from the damaged specimens and dried for 24 h in an oven with a temperature of 50 C. Then, their surfaces are coated with gold and examined by a field emission scanning electron microscope (SEM) Nova NanoSEM 450. The device has a magnification range of 40–400,000 and an imaging resolution of 1.6 nm (1 kV) in high vacuum mode. 2.6. Ultrasonic inspection All post-lightning specimens are inspected by throughtransmission method on a C-scan ultrasonic flaw detector device. A transmitter and a receiver controlled by a three-axis bridge are placed on the opposite side of the specimen and a jet of water is accompanied when the ultrasound is emitted. Considering the significant attenuation of ultrasound signal in woven fabric composites, a 0.5 MHz transducer is used as the transmitter. The signal data is acquired and processed by NDTS-90 (NDT Systems).
Fig. 3. Snapshot of surface burning for 30 specimen under 38 kA strike. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 3 Test conditions and lightning strike parameters for the current experiment. Stacking sequence
Waveform (ls)
Current amplitude (kA)
Action integral (A2 s)
Charge transfer (C)
0:2 _ I10—90% Ipeak
[452/02/ 452/902]s [302/02/ 302/902]s
6.8/24 6.8/24
22 22
13230 13230
0.58 0.58
2.1e10 2.1e10
[452/02/ 452/902]s [302/02/ 302/902]s
6/22.5 6/22.5
32 32
18830 18830
0.71 0.71
3.4e10 3.4e10
[452/02/ 452/902]s [302/02/ 302/902]s
6.4/22.8 6.4/22.8
38 38
22660 22660
0.82 0.82
4.2e10 4.2e10
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(a) [452/02/-452/902]s 38 kA
(b) [302/02/-302/902]s 38 kA
(c) [452/02/-452/902]s 32 kA
(d) [302/02/-302/902]s 32 kA
(e) [452/02/-452 /902]s 22 kA
(f) [302/02/-302 /902 ]s 22 kA
Fig. 4. Top view of the specimens after lightning strike damage. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Close-up view of the surface damage on 45 specimen (a) and 30 specimen (b) after lightning strike. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
configuration for lightning protection. The adhesive contains three types which are: pure epoxy resin (neat adhesive), CNTs (1.0 wt%) embedded epoxy resin (conductive adhesive) and boron nitride (20 wt%) embedded epoxy resin (insulating adhesive). The laminate has the same constituents and manufacturing process with [8] and its lay-up sequence is [45/0/ 45/90]2s.
Comparing the present 38 kA strike result with the 40 kA case in [8,20], one can observe that the break and lift up of carbon fiber is all initiated from the lightning attachment area for the three studies. However, the fiber damage pattern presents differently. Both Fig. 8(a) of [8] and Fig. 6 (a1 ) of [20] show that carbon fiber dissipated along the fiber orientation to the upper and bottom
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Fig. 6. Damage profile for post-lightning 45 specimen (a) and 30 specimen (b) under 38 kA strike. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
boundary of their specimens, while in the present test (45 specimen), the fiber damage region is restricted to a circle with diameter of about 22 mm (Fig. 6(a)). Extensive fiber lift up along the ply direction is shown in Fig. 8(b) of [8], while only a few fibers around the lightning attachment area are seen to break up in the present work (Fig. 5(a)). This big disparity is due to the different stacking sequence, lamina thickness and reinforcement structure of these three works. For the present test, the [452/02/ 452/902]s stacking sequence can be seen as a [45/0/ 45/90]s type with each layer consists of two original carbon woven fabric, resulting in a lamina thickness of 0.5 mm. This ply thickness is more than three times of the value in [8,20] (0.15 mm), which greatly strengthens its lightning resistance capability. Besides, for the present material, the existence of nylon binder yarns in the weft direction can restrain the out-of-plane freedom of carbon fiber. This also attributes to the decreased damage area comparing to the unprotected specimens in [8,20]. When carbon laminate is equipped with insulating adhesive and buckypaper as shown in Fig. 8 (c1 ) of [8], its visual damage is significantly minimized to the top protection medium, outperforming the present work. However, the cost for the addition of special designed conductive layers and adhesive, as well as the more complicated fabrication technique will potentially prevent its widespread promotion. Compared to the 45 specimen, the fiber damage area in the 30 specimen of the present study exhibits a diamond shaped appearance with the longer diagonal axis lies along the ply orientation (Fig. 6(b)). Large bundle of fiber break up and sublimation can be observed several layers deep around the lightning attachment area (Fig. 5(b)). One may further observe that although the fiber damage zone of the 30 specimen is larger than the 45 one, it does not extend to the specimen boundary which is shown in Fig. 8(a) of [8], instead, the damage size has a limited length along the fiber direction (L2 in Fig. 6(b)). These observations demonstrate that the different stacking sequence will have different responses to the lightning strike. Around fiber damage region, the white area with bare fiber yarns unprotected from resin matrix is resin pyrolysis area (Fig. 4). It can be seen that the area of the resin damage exhibits a diamond shape for both of the 45 and 30 specimens (Fig. 6). If L1 and L2 are defined as the length of the longer and shorter diagonals of the resin pyrolized zone, respectively, L1 is found to be 2 times of L2 for both of the two type specimens under 22 kA, 32 kA and 38 kA strike. Besides, the longer diagonal axis of the diamond shape just lies in the vertical direction of the fiber orientation and extends a large distance from the lightning attachment area. This observation is different to the results in [8,20], in which resin deterioration extended primarily along the fiber direction. This disparity of resin pyrolysis pattern comes from the differences in material types used in the corresponding experiments.
The process of resin damage can be regarded as dielectric surface discharge due to the insulation property of resin matrix [19,21,22,18,23,24]. The dielectric breakdown model [18], which assumes the local discharge direction is determined by the potential gradient at that point, can be used to explain the resin damage shape in the present study. One can figure out that the potential gradient perpendicular to the fiber orientation is tremendously larger than that along with the fiber direction due to the coupled interaction of strong orthotropic electrical behavior of carbon/ epoxy laminate and dielectric property of nylon binder yarns in the weft direction. Thus, the great potential gradient leads to the relatively bigger resin pyrolized damage in the direction perpendicular to the fiber orientation. One benefit for the large resin damage area is that it consumes a large amount of energy from lightning infliction, leaving small portion of energy to make damages to the fiber yarns embedded in the matrix. 3.1.2. Microscopic damage inspection The microscopic inspection is conducted to unveil the damage mechanisms on the microstructural point of view which is helpful for the understanding of the internal damage induced from lightning strike. Three samples, with the locations showing in Fig. 7, are cut off from the 45 specimen (38 kA strike damage) for microscopic inspection. Fig. 8 shows a SEM image of a delamination area at cross section B. It can be seen that a major delamination happens on the first two sub-layers with its propagation along the 45 /45 interface (Fig. 8(a)). The resin matrix between the two layers is completely pyrolized with scattered carbon filaments hanging on
C 5
75 B A
20
50
10
45 90 0
10mm
Fig. 7. Location of the cross sections for microscopic inspection (45 specimen, 38 kA). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. SEM micrograph of the delamination area (a) and a single carbon filament (b) at cross section B (45 specimen, 38 kA).
the edge of the fiber bundles. A secondary branch [25] can also be observed between the second and third sub-layer (45 /0 ). It propagates along the resin/fiber interface and branches into two paths with that one goes into the interval space between two adjacent fiber bundles. Fig. 8(b) plots a magnified view on the surface of a single carbon filament after lightning strike. It is noted that the carbon fiber is intact but its surface is wrapped with spherical shaped residues. These substances are the pyrolized resin remnants and most of which agglomerated into large particles under the high temperature environment. Presented in Fig. 9 is the configuration of fiber bundles before and after lightning strike, located at cross section C and A, respectively. Fibers are shown closely bonded by resin matrix before lightning infliction (Fig. 9(a)), while significant detachment is discovered on the interface between fiber filaments and resin matrix (Fig. 9(b)) after strike occurs. Fig. 10 shows the damage pattern of a fiber bundle near a delamination area (cross section A). Most of the resin matrix has been pyrolized and vaporized, leaving the individual fiber filaments un-bonded. After magnifying a certain area of the fiber bundles, one can clearly see that there is a big gap between two carbon fibers and the resin matrix in between is cracked, which is possibly due to a compression force generated from the thermal stress inside resin matrix. This observation indicates that the internal resin damage during the lightning strike process not only is induced by the pyrolysis, but also by the combined thermal and mechanical interaction.
3.1.3. Internal damage inspection Besides visual and microscopic inspection of damage forms, the internal damages caused by lightning strike will be studied next by ultrasonic scanning technique. Fig. 11(a)–(f) plots the C-scan results for the two stacking sequenced specimens under the three current amplitudes. The dark area on each of the sub-figures demonstrates the delamination zone. The profile of the resin pyrolysis area in the corresponding panel is also overlaid on each sub-figure for comparison. The two dark columns in the vertical direction are iron wires, which support the specimen when the scanning is under going. The adoption of through-transmission method guarantees the accuracy of the results with a less strict requirement on the transducer alignment to the surface of the test specimen [26]. It can be seen from Fig. 10(a), (c), and (e), a gradual enlarged delamination area appears as the lightning strike level is intensified for the 45 specimen. The damage shape extends simultaneously along the fiber and the vertical direction within the resin damage profile. This result is different from the C-scan result in [8] (Fig. 10) which exhibits the shape of a pair of fans along the fiber direction and the quadrilateral shape presented in [20] (Fig. 6 (a4 )). The reason for this difference is also due to the stacking sequence and lamina thickness. From the B-scan results in [8,20], it can be seen that the damage depth is about 1.2 mm and 0.9 mm, respectively, indicating that the damage extends throughout the first [45/0/ 45/90] unit for the two studies. Therefore, delamination propagation along diverse ply direction in each layer
Fig. 9. SEM micrograph of undamaged fiber bundles at cross section C (a) and after 38 kA lightning strike damage at cross section A (b).
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Fig. 10. SEM micrograph of resin/fiber interfacial damage after 38 kA lightning strike damage at cross section A (a) and the magnified view of a resin/fiber interface (b).
(a) [452/02/-452 /902]s 38 kA
(b) [302/02/-302/902]s 38 kA
(c) [452/02/-452/902] s 32 kA
(d) [302/02/-302/902 ] s 32 kA
(e) [452/02 /-452 /902] s 22 kA
(f) [302/02/-302 /902]s 22 kA
Fig. 11. Overview of the C-scan image combined with resin pyrolysis area.
contributes to the fan shaped and quadrilateral shaped profile. However, damage only penetrates throughout the first two 45 layers for the present work. As such, the damage profile extends regularly along the fiber direction. The reason for the vertical extension is unknown. It may also attribute to the nylon yarns in the weft direction which deserves further study. Under the protec-
tion configuration, the CFRP laminate is almost intact from Fig. 8 (c4 ) of [20], showing the best lightning strike prevention ability. These observations suggest that although the present specimens can restrain the strike damage to some extent, its effect cannot be comparable to the laminates which have special designed protection constituent.
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Apart from Fig. 11(a), (c), and (e), results in Fig. 11(b), (d), and (f) show that, under the same level of lightning strike, a notable larger delamination area exists for each of the 30 specimens than that of the 45 ones. The delamination shape is also observably different for the two types. The cause for the delamination damage is majorly due to Joule heating effect. When lightning strike inflicts, the temperature of the lightning attachment area is up to 3500 K [9] which is high enough to pyrolize the surface resin matrix and sublimate the internal fiber yarns. Subsequent lightning current is delivered inside and generates Joule heating along the fiber direction due to the good conductivity of carbon fiber. In this process, the interfacial bonding of the matrix with its adjacent fibers is severely damaged and the resin between sub-layers is broken up into small pieces under the influence of thermal–mechanical interacting stresses (Figs. 8–10). It agglomerates and pyrolizes to the gaseous substances which are entrapped and accumulated in the interlaminar zone. Finally, under the high temperature, high pressure and limited volume space environment, the entrapped gas explodes and causes massive delamination damage. 3.1.4. Discussion The different delamination and fiber damage pattern for the two kind specimens is largely due to the stacking sequence arrangement. From the above analysis, it can be seen that the gases produced from Joule heating is the essential factor to induce the extension of internal damage. At the same level of lightning strike, the energy transferred into Joule heating (action integral) is considered to be the same, which generates equal amount of gases in the two stacking sequenced specimens. For the 30 specimen, fibers are distributed to form small cells around the 0 direction so that the gases produced from Joule heating effect are trapped in a small and narrow area. Therefore, due to the quick elevation of local temperature and pressure, damage is seriously increased. For the 45 one, however, the larger angle between sub-layers disperses the Joule heating to a greater region so that the damage produced by the pyrolized gases is restrained. Fiber damage can be seen as an external demonstration of gas explosion inside specimen. It is controlled by a coupled effect of surface recession [9] from fiber sublimation, shockwave and resistive heating. When lightning strike intensity is low, the impact of gas explosion is not enough to break up fiber yarns, thus only delamination damage is created. As the strike level increases, the energy of gas explosion not only produces delamination, but also extends its impact to the surface and makes the fiber damage. As such, the different appearances of fiber damage for the two typed specimens can also attribute to the influence of stacking sequence. The restricted effect of pyrolysis gases on the quasi-isotropic ply orientation determines the fiber damage area to be a circular shape for the 45 specimen, while the narrowly distributed gases prone to 0 direction intensifies fiber breakage along its ply orientation significantly for the 30 one. It should be noted that the unique woven fabric structure, the nylon binder yarns in the weft direction, greatly influences the fiber damage pattern. Although the temperature at the lightning attachment area is up to 3500 K, the temperature for the remaining zone is below 1000 K, which leaves the majority of nylon yarns undamaged. When lightning strike inflicts on the specimens, they bind and protect fiber yarns from massive break up, thus restrain the fiber damage area to a limited region. 3.2. Effect of waveform parameters on damage forms The results of the relationship between waveform parameters and the damage patterns for the two stacking sequenced specimen can be revealed in Fig. 12. Fig. 12(a) shows that a linear relationship between the charge transfer and the resin damage area exists
(a) Charge transfer vs. Resin pyrolysis area
(b) Action integral vs. Internal damage area
(c) Acoustic shock vs. Fiber damage area Fig. 12. Relationship between lightning parameters and areas of different damage patterns for stacking sequence [452/02/ 452/902]s and [302/02/ 302/902]s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
for both of the 45 and 30 specimen. One may also see that the influence from stacking sequence is relatively small on the resin
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damage area under the same lightning strike level. For resin damage is largely a process of surface discharge interaction with specimen, the damage region is mainly determine by the intensity (charge transfer) of the lightning strike. Comparing with the results in [8] (Fig. 21), it can be seen that the present area is larger (2800 mm2 vs. 2300 mm2 ) under the same electrical charge (0.82 C), suggesting that more energy is spent on the formation of surface resin damage for the present specimen, so that less energy is left to make damages to the fiber yarns embedded in the matrix. Plotted in Fig. 12(b) is the relationship between the action integral and the internal damage area. For the 45 specimen, the delamination area linearly increases with the action integral, while a nonlinearly increase pattern is shown for the 30 sample. The main trigger of the internal damage is the Joule heating generated from lightning current and high electrical resistance of composite laminates. As such, different stacking sequence may produce inconsistent distribution of Joule heating that subsequently pyrolizes resin matrix around fiber yarns and causes delamination damage in the test specimens. Here, the present delamination area is not only compared with results in [8,20], but also with another literature [27] which is a traditional copper embedded aerospace composite laminate fabricated from tape (Hexcel HexPly 8552/ AS4, 0.15 mm), fabric (Hexcel HexPly 8552/A193-PW, 3K-70-PW, 0.2 mm) and coated with a typical aerospace primer and paint finishing scheme with vacuum bag molding. A layer made up of Cytec Surfacemaster 905C Composite Surfacing Film and Dexmet 3CU7100FA expanded copper foil is attached on the top of the laminate for protection ([(0/90F)(45/90/ 45/0)2]s). First in comparison with the unprotected specimens in [8] (Fig. 22), it shows that the delamination area is smaller (1800 mm2 vs. 3000 mm2 ) for the present work under the same action integral (22,660 A2 s). This result is also smaller than the unprotected case in [20,27], which are 3380 mm2 and 3677 mm2 , respectively. Second, comparing with the protected ones, the present results is also smaller than the copper embedded one in [27] (1800 mm2 vs. 2735 mm2 ), but is much larger than the special protected laminates in [20] (1800 mm2 vs. 0 mm2 ). In summary, the delamination area follows the order: [20] (protected case) < current study < [27] (protected case) < [8] (unprotected case) < [20] (unprotected case) < [27] (unprotected case). It should be noted that the present work is only subjected to 38 kA strike level. Results in [20,27] also tested lightning strike to 100 kA, they found that the effect of their protection medium is more remarkable under 100 kA strike. However, the damage behavior of the present specimens under high level strike is unknown, which deserves further study. The influence of acoustic shock on the fiber damage size is shown in Fig. 12(c) in which a similar trend to that in Fig. 12(b) can be observed. These two observations indicate that the structure of the stacking sequence has great influence on the damage characteristics of the fibers. With increase of the acoustic shock generated from lightning infliction, it deteriorates the fiber–resin matrix bond and facilitates the fiber damage and internal delamination. The area of fiber damage for the present 38 kA results (45 specimen) are comparable to the ones in [8] (Fig. 19), which are 480–500 mm2 , suggesting a less sensitivity to the material type. On the other hand, little difference of damage area can be seen for both of the stacking sequenced specimen under 22 kA strike. This observation reasonably suggests that an energy threshold may exist for the massive destruction of the composite laminates.
tensile strength and modulus of virgin and post-lightning specimens are determined in accordance with ASTM D3039. The mechanical testing is performed on a WDW-100E universal testing machine with a 100 kN load cell and a crosshead speed of 2 mm/min under room temperature. During the in-service time, current discharge often happens more than once between the overhead conductor and composite pole when the humidity of the local environment increases. To make sure the poles can survive such environmental exposure, strike with amplitude current 10 kA, which is the current delivered in the overhead conductor, is applied to the tested samples for the safety operation of the composite poles. Damage forms of one-strike and two-strikes are inflicted on the 45 specimen, while one-strike and three-strikes are made on the 30 specimen, as plotted in Fig. 13, to further study the relationship between strike number and residual strength of tested laminates. The cross sectional view of the two stacking sequenced specimens after lightning infliction is plotted in Fig. 14. The 45 specimen case shown in Fig. 14(a)–(c) demonstrates that the major damage form is interlaminar delamination. However, due to the lightning strike damage, the delamination area initials from both of the top and bottom 45 =0 interface (Fig. 14(a)) to just the top 45 =0 interface (Fig. 14(b)) where lightning strike is applied. For the 30 specimen, the influence of lightning strike on the damage pattern is less noticeable as seen in Fig. 14(d)–(f), despite of the increased number of strikes on the specimen. The cause for the position change of delamination zone on the 45 specimen is majorly due to the increase of matrix cracks, originated from the impact effect of lightning infliction. When lightning strike is applied on the specimen surface, its impact effect induces high stresses near the impact point and initials local matrix cracks [8,28], which in turn initials delaminations at interfaces between plies with different orientation (45 =0 interface in the current work) [28]. As the post-lightning specimen is subjected to external force, high interlaminar stresses drive it to propagate to form a delamination failure [28,29]. For the 30 specimen, however, its failure mode is governed not only by interlaminar delamination, but also intralaminar tensile fracture due to a 0 directionprone stacking sequence arrangement. Typical stress–strain curves for the two stacking sequenced post-lightning specimen is shown in Fig. 15. A linear relationship is observed for the 45 specimen while a significant two-stepped non-linear trend is presented for the 30 one. The linear curve can be explained by the single interlaminar delamination failure of the 45 specimen, while for the 30 sample curve, the first step is governed by interlaminar shear and the second stage changes to intralaminar tensile. It is noted that some springback occurs on the second phase which can be explained by the fracture of laminas on different sub-layers. Fig. 16 shows that the strength and modulus decreases as the lightning strike is applies, and with the number of strike increases, the degradation is more remarkable. This phe-
20mm
3.3. Effect of lightning strike damage on residual mechanical properties To investigate the influence of lightning strike damage on the mechanical behavior of the two stacking sequenced specimen, the
Fig. 13. Post-lightning specimens ready for mechanical testing. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 14. Cross sectional view for post-lightning specimens after mechanical testing.
Fig. 15. Typical stress–strain curves for post-lightning 45 specimen (a) and 30 specimen (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 16. Tensile strength (a) and tensile modulus (b) for post-lightning 45 specimen and 30 specimen.
nomenon can be attributed to the greater acoustic shock effect on the massive increase of crack density in the sub-layer interface when more number of lightning strikes is inflicted on the tested panel. Table 4 summarizes values of lightning strike influenced tensile strength and modulus. It shows that, for both of the two
stacking sequenced specimens, the degradation percentage of modulus and strength exhibit a similar extent as the intensity of applied lightning strike increases. However, for the one-strike damage type, the mechanical properties of the 30 sample are more sensitive to strike damage than the 45 one (13% vs. 5% loss
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Y. Li et al. / Composites: Part A 79 (2015) 164–175 Table 4 Effect of lightning strike on tensile modulus and strength of 45 and 30 specimens. Specimen conditions
Modulus (GPa)
Retention (%)
Strength (MPa)
Retention (%)
45 type
No-strike One-strike Two-strikes
43.61 (0.52) 43.27 (2.56) 38.58 (4.21)
100 99 88
575.98 (28.27) 541.95 (21.23) 496.73 (14.13)
100 94 86
30 type
No-strike One-strike Three-strikes
67.46 (2.41) 59.20 (1.67) 52.61 (3.40)
100 87 78
580.04 (42.57) 488.75 (7.97) 449.27 (13.23)
100 84 77
Values given in parentheses represent standard deviations of three repetitive specimens.
in modulus and strength). Here, a literature [6] is reported for comparison with the current results. Although it has a different material type (CYCOM/FIBERITE G30-500 12K HTA/7714A, 0.16 mm), stacking sequence ([45/02/ 45/03/90]s), manufacturing technique (press molding), dimension (304.8 mm 38.1 mm) and strike damage form (only one-strike), its results show a slight decrease on both of the residual strength and modulus, which has a similar trend to the present work (45 specimen). It is believed that the quasi-isotropic stacking sequence of the 45 specimen has a more resilient capability than the 30 one on diminishing impact from lightning strike and maintaining mechanical performance. 4. Conclusion In this paper, simulated lightning strike with different current levels is exerted on two stacking sequenced UD carbon woven fabric/epoxy laminates to study the damage behavior on the material experimentally. Various inspection techniques are used to assess the damage characteristics of the post-lightning composite specimens and mechanical testing is subsequently conducted to study the effect of lightning strike on the degradation of tensile strength and modulus. For the 45 specimen, a diamond shaped resin pyrolysis zone with a circular fiber damage area and evenly expanded delamination zone is detected, and the relationship between various lightning parameters and damage modes shows a good linear correlation. While for the 30 specimen, the fiber damage area changes to a larger sized diamond shape and the delamination area extends notably along the fiber orientation. Microscopic investigation further reveals that lightning strike greatly damages bonding strength of resin/fiber interface by a coupled thermal–mechanical effect. The residual strength and modulus of post-lightning specimens are reduced as lightning strike is applied on but the sensitivity of failure position is different for the two types. Unlike those in graphite/epoxy laminated composites, the damage areas in the current specimens are constrained in certain sizes due to the different stacking sequence, lamina thickness, and the nylon binder yarns in the weft direction. This discovery suggests that the woven fabric/epoxy structure is a good choice for the fabrication of laminated composites which is likely to be subjected to lightning strikes. Acknowledgements This research is sponsored in part by the Ministry of Science and Technology Fund Project (Contract No. 2015DFA81640) and Aeronautical Science Foundation of China (Contract No. 20130179002) at the Huazhong University of Science and Technology. References [1] Alawar A, Bosze EJ, Nutt S. A composite core conductor for low sag at high temperatures. IEEE Trans Power Del 2005;20(3):2193–9. [2] Tsai YI, Bosze EJ, Barjasteh E, Nutt SR. Influence of hygrothermal environment on thermal and mechanical properties of carbon fiber/fiberglass hybrid composites. Compos Sci Technol 2009;69(3–4):432–7.
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