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Artificial lightning strike tests on PRSEUS panels
Accepted Manuscript Artificial lightning strike tests on PRSEUS panels Juhyeong Lee, Pedram Gharghabi, Dounia Boushab, Trenton M. Ricks, Thomas E. Lacy, Jr., Charles U. Pittman, Jr., Michael S. Mazzola, Alex Velicki PII:
S1359-8368(18)32273-X
DOI:
10.1016/j.compositesb.2018.09.016
Reference:
JCOMB 5986
To appear in:
Composites Part B
Received Date: 21 July 2018 Revised Date:
10 August 2018
Accepted Date: 10 September 2018
Please cite this article as: Lee J, Gharghabi P, Boushab D, Ricks TM, Lacy Jr. TE, Pittman Jr. CU, Mazzola MS, Velicki A, Artificial lightning strike tests on PRSEUS panels, Composites Part B (2018), doi: https://doi.org/10.1016/j.compositesb.2018.09.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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– Cover Page – Manuscript entitled:
Submitted to:
Authored by: Juhyeong Lee,
1, 2*
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Composites Part B: Engineering
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Artificial Lightning Strike Tests on PRSEUS Panels
Pedram Gharghabi, 3 Dounia Boushab, 1 Trenton M. Ricks, 1, 4
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Thomas E. Lacy Jr., 5 Charles U. Pittman Jr., 6 Michael S. Mazzola 7, Alex Velicki 8
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1. Department of Aerospace Engineering, Mississippi State University, Mississippi State, MS 39762, USA 2. Advanced Composites Collaboration for Innovation & Science, Queens Building, University Walk, Bristol BS8 1TR, UK 3. Department of Electrical and Computer Engineering, Mississippi State University, Mississippi State, MS 39762, USA 4. Multiscale and Multiphysics Modeling Branch, Materials and Structures Division, NASA Glenn Research Center, Cleveland, OH 44135, USA 5. Department of Mechanical Engineering, Texas A&M University, MEOB 429, College Station, TX 77843-3123, USA 6. Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA 7. Department of Electrical and Computer Engineering, University of North Carolina at Charlotte, Charlotte, NC 28223, USA 8. The Boeing Company, Huntington Beach, CA 92647-2099, USA
Keywords: lightning strike tests, PRSEUS, stitched laminated composites.
* Corresponding author Juhyeong Lee PhD
Advanced Composites Collaboration for Innovation & Science University of Bristol Queens Building, University Walk, Bristol BS8 1TR, UK
Email: [email protected] and [email protected]
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Abstract The lightning damage resistance of Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) panels was characterized experimentally. Two unprotected PRSEUS panels were
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subjected to standard impulse current waveforms (consistent with actual lightning strikes) with 50, 125, and 200 kA nominal peak currents at a variety of panel locations. Lightning-induced damage to the PRSEUS panels was a strong function of (1) the peak current, (2) the lightning
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attachment location (mid-bay, stringer, frame, etc.) that involved different through-thickness Vectran™ stitching, and (3) the presence of a surface finish paint.
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The sizes of the damaged regions increased as the peak current increased, since greater peak current leads to more Joule heating. The lightning-damaged PRSEUS panels exhibited unique damage features due to the presence of through-thickness Vectran™ stitches and warp-knitted fabrics. Through-thickness Vectran™ stitches constrained the development and spread of intense
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local damage in the vicinity of the lightning attachment point. The polyester warp-knit threads used to stitch the warp-knitted laminates together appeared to influence the development of widespread small-scale fiber damage in the region surrounding the strike. Consequently, the
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Vectran™ structural stitches, as well as the polyester knit threads holding the tows together, have a significant beneficial effect on both the size of the impingement region and the subsequent
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damage propagation within the laminate. In addition, damage to the painted panel was greater for each current level than for the sanded (unpainted) panel.
Introduction
Lightning strike is a potential threat to composite aircraft. Since traditional carbon/epoxy composites have lower thermal and electrical conductivities than metallic materials, lightning
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strikes can cause severe damage to composite structures [1-4]. Matrix thermal decomposition, carbon fiber breakage/ablation, and delamination have been typically observed in laminated composites that have sustained lightning strikes.
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The Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept [5-8] refers to an integrated composite structural design developed by the Boeing Company under funding from and in collaboration with the NASA Langley Research Center (LaRC) (Fig. 1). The PRSEUS
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concept uses through-thickness Vectran™ stitches to improve the composite structural integrity over traditional laminated composites. Resisting out-of-plane delamination is enhanced by the
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formation of stitch bridging zones [9, 10]. The damage arresting capabilities were validated by subjecting both flat and curved PRSEUS panels to axial tension, axial compression, internal pressure, and combined axial and internal pressure loadings [11-14]. Furthermore, the PRSEUS concept offers structural weight savings, while ensuring all relevant performance requirements
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and safety criteria are satisfied. Lovejoy [15] reported that a PRSEUS wing design exhibited a 9% weight savings compared to a stiffened composite wing. These results suggest that the PRSEUS concept can be used for future structural applications.
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Mississippi State University (MSU) received three PRSEUS panels from the NASA LaRC and one PRSEUS panel from the Boeing Company to be used for simulated lightning strikes.
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The NASA and Boeing PRSEUS panels have different geometries, configurations, and surface treatments. While the stiffener spacing, geometries, layups, and a type of surface paint for the NASA panels are available in the open literature [5-8], those for the Boeing panel are proprietary. In this study, two LaRC PRSEUS panels were subjected to standard Society of Automotive Engineers (SAE) impulse current waveforms [16] (consistent with actual lightning strikes) with 50, 125, and 200 kA nominal peak currents at a variety of panel locations. The type
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and extent of damage were characterized at each attachment location. One of the LaRC panel outer mold line (OML) skins was lightly sanded using 100, 200, and then 400-grit sandpapers prior to lightning strike tests to help assess the influence of exterior surface paint on lightning
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damage. Lightning damage on the sanded LaRC PRSEUS panel was compared with those of the painted panel.
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PRSEUS Panel Construction
A fully integrated PRSEUS panel has an outer skin and interior stringers, frames, and tear
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straps. Each PRSEUS subcomponent is produced from stitched preforms constructed of multi-axial warp-knitted fabrics (Fig. 2a; [17]). Warp-knitted fabrics and their carbon fiber tows are held together by thermoplastic threads (preferably polyester [6, 7, 18]) in order to (1) prevent crimping or tow undulations and (2) enable easier handling [19]. The white lines in Fig. 2b
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correspond to polyester knit threads [18]. PRSEUS panel warp-knitted fabrics (Fig. 2b) consist of multiple layers of oriented unidirectional Hexcel standard modulus AS4 carbon fiber tows. High-performance thermoplastic threads made of Vectran™, are used to stitch the pre-
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assembled dry material stacks for the skins, stringers, frames, and tear straps. Vectran™ stitching threads are typically made of three or four 400-denier Vectran™ threads twisted together for
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better strength and coated with nylon for thermal stability [6, 7]. A Hexcel VRM-34 toughened epoxy resin is then infused into a stitched pre-assembly and then oven-cured using the Controlled Atmospheric Pressure Resin Infusion (CAPRI) process [20]. The materials, layups, and total stack thicknesses for a PRSEUS panel (including all subcomponents) are listed in Table 1. The Stack A layup has a nominal thickness of 1.32 mm with a
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balanced symmetric stacking sequence, [+45/-45/0/0/90/0/0/-45/+45]. A detailed description of the ply stacking sequences in the PRSEUS panel is available in references [6, 7]. Two-needle single-sided stitching technologies are used to sew a stitching thread through the
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built-up preforms from the PRSEUS panel OML side [6-8]. Through-thickness Vectran™ stitching increases the laminate’s interlaminar fracture toughness. A locking thread holds the vertical stitches in place. Figures 3a-3b show a typical PRSEUS frame/stringer intersection with
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flange-to-skin stitching. In Fig. 3b, two parameters define a given stitch pattern: stitching spacing (Ss) and stitching pitch (Sp). For a prototype PRSEUS panel, Ss = 25.4 mm and Sp = 5.1 mm [21].
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A single-sided chain-stitch seam introduces two rows of continuous stitching (Fig. 3c). It attaches stringer/frame flanges to the PRSEUS skin layers. Figure 3d shows a flat PRSEUS panel after single-sided 3D-seam stitching and prior to resin infusion. A continuous stitching line is used to attach a given stringer/tear strap or frame/tear strap combination to the overlapping
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PRSEUS skins. In regions where the upstanding web of a given frame intersects the upstanding web on an orthogonal stringer, no through-thickness stitching is possible. A detailed description
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of the single-sided stitching implemented in the PRSEUS concept is available in references [6-8].
Laboratory-scale Lightning Strike Testing Conditions
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High Impulse Current Generator
A one-stage impulse current generator was designed to produce standard impulse current waveforms consistent with actual lightning strikes [22, 23]. The impulse current generator built at the MSU-High-Voltage Lab (MSU-HVL) is able to produce double exponential current waveforms with up to 200 kA peak currents. The trigatron spark gap switch, which triggers an impulse current discharge, consists of two electrodes operating in air at atmospheric pressure: the
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upper electrode was connected to a set of capacitors, and the lower electrode was grounded. Electric current was discharged across the gap between the two electrodes. Electrical Grounding Conditions
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Once lightning strikes a PRSEUS panel, a substantial electrical current flows through the panel. Most composite aircraft panels are designed to distribute electric current over their outer surfaces. This condition was ensured by connecting electrical grounding terminals along the four
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edges of the laboratory panels. Figure 4 shows the grounding connections on a PRSEUS panel to the grounded current-return structure of the lightning simulator prior to lightning strike tests. The
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four PRSEUS panel edges were smoothly sanded to provide uniform electrical contact surfaces. Flexible braided copper straps were then inserted between aluminum brackets and the PRSEUS panel to return the current to the energy storage capacitors. C-clamps were used to secure the
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aluminum angle brackets to the panel’s OML side (Fig. 4b).
Artificial Lightning Current Waveform
Figure 5a compares the standard SAE impulse current waveform component A [16] with a
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measured 200 kA peak current impulse waveform generated at the MSU-HVL. The standard SAE waveform A [16] has a 200 kA peak current, rise time (T1) of 6.4 µs, and decay time (T2) of
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69 µs with a ±20% tolerance level. The MSU-HVL 200 kA peak current waveform exhibited similar temporal characteristics (i.e., T1 and T2). The difference in the rise time between the MSU-HVL lightning waveform and the SAE waveform A is not significant because the rise time is typically less important than decay time in determining the time response of an impulse current waveform [24]. Lightning damage is governed by the amount of electrical energy, defined as the action integral (integral of the square of the time-varying current over its time
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duration [25]). Figure 5b shows the MSU-HVL impulse current waveforms with 50, 125, and 200 kA nominal peak currents. Note that rise and decay times of a current waveform strongly depend on the circuit parameters for the system (i.e., resistance and inductance). These are
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independent of the charging voltage, peak current, and test specimen. An impulse current generator typically has a constant resistance and inductance. Thus, the resulting rise and decay times of a current waveform are relatively constant, regardless of the magnitude of the peak
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current. Thus, all MSU-HVL lightning waveforms showed identical rise and decay times. However, the action integrals of MSU-HVL lightning waveforms varied depending on the peak
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current.
Lightning Strike Locations on the PRSEUS Panels
Simulated impulse current waveforms were applied at four representative lightning
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attachment locations on the OML skins of two LaRC PRSEUS panels: (1) mid-bay, (2) stringer, (3) frame, and (4) frame/stringer intersection. Three peak current levels were considered: (1) 50 kA (commonly used for laboratory-scale lightning strike tests [1, 2, 26-33]), (2) 125 kA
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(slightly higher than the subsequent return stroke defined in [16]), and (3) 200 kA (consistent with the first return stroke defined in [16]).
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A total of forty simulated lightning strike tests were conducted on the two PRSEUS panels (Table 2). The number and location of tests on each panel was determined to avoid lightning damage overlap between adjacent strike locations. Twenty-four tests (including two calibration tests) on the sanded panel and sixteen tests on the painted panel were performed at a variety of key locations including the mid-bay, stringer, and frame locations, in addition to the frame/stringer intersection.
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For both the sanded and painted panels, all measured peak currents were within 10% of the target peak currents with the exception of one 200 kA mid-bay strike due to a prefire of the main gap during charging of the energy storage capacitors. This proves the reliability of the MSU-
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HVL impulse current generator. While the experimental work reported here is not intended to be a certification test under the SAE standard, the standard SAE waveform [16] has a ±10% peak
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current tolerance.
Lightning Damage Characterization
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Results and Discussion
Two primary damage types can be identified for each PRSEUS panel based on surface observations (cf., Fig. 6): (1) intense local damage near the attachment location and (2) surrounding widespread surface damage. The intense local damage includes severe fiber
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rupture, tow splitting, matrix decomposition, and underlying delamination. The regions with severe fiber rupture/tow splitting and underlying delamination were fairly elongated along the outermost lamina’s fiber direction. Carbon fiber damage is associated with Joule heating
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resulting from an applied electrical current. Rapid heating of the carbon fibers results in a severe
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contraction of the fiber/tows due to carbon fiber’s negative coefficient of thermal expansion. This contraction may lead to large-scale fiber rupture/tow splitting in the high thermal gradient inner
region
near
the
lightning
attachment
point.
The
regions
of
severe
fiber
rupture/delamination are surrounded by an outer region with a large number of small clusters (“tufts”) of broken fibers with a periodic arrangement consistent with the spacing between warpknit polyester threads (Fig. 2b). Localized tufts of broken fibers arguably occur in the surrounding region where the fibers are relatively unconstrained between polyester warp-knit
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threads. There is also evidence of mild scorching (or burning) of the OML surface in this region. The distribution of the damaged tufts is not likely a result of galvanic currents conducted from the arc attachment point due to their distances from the attachment point and amount of
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intervening matrix. The authors are pursuing a theory of secondary currents in this ‘outer’ region induced by transient magnetic fields from the lightning currents in the arc and the panel. Such surface damage may also result from a combination of Joule heating and direct lightning heat
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fluxes (electronic or ionic recombination, convection flux, radiation flux, etc. [34]). Lightning strikes create a narrow cylindrical plasma arc channel accompanied with radial heat fluxes [34-
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36]. Direct lightning heat fluxes are independent of the electrical conduction path (i.e., outer ply fiber direction), but strongly depend on thermal boundary conditions (i.e., convective/radiative heat transfer coefficients and ambient temperature). Hence, the radial heat fluxes often create surface damage emanating from the center of the lightning attachment point [34].
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The periodic distribution of small broken fiber clusters in the outer region surrounding the intense local damage region near the lightning attachment location appears consistent with the spacing of warp-knitted fabric polyester threads. After lightning strike tests, the polyester knit
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threads in regions away from the severe damage zone appeared to remain intact. Knitting threads produce through-thickness changes that affect the electrical and mechanical wave propagation
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through the laminate. Local contraction/densification of the fibers due to the presence of the periodically distributed warp-knit threads may effectively constrain (“pin”) fiber tows or may change the local thermal/electrical conductivities of the laminate. Such influences may explain the formation of small, periodically distributed clusters of broken fibers (Fig. 6). This unique lightning damage feature has not been observed in laminated composites [1, 2, 26-33].
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Effect of Peak Current and Attachment Point on Lightning Damage Formation Figure 7 shows typical lightning damage resulting at the mid-bay locations of both sanded and painted PRSEUS panels subjected to 50, 125, and 200 kA nominal peak currents. The A* in
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the figure indicates an intense local damage area normalized by the maximum damaged area found for this group (200 kA mid-bay strike on the painted panel). The horizontal solid lines shown in the figures correspond to through-thickness Vectran™ structural stitch lines where the
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skin stacks are sewn to the adjacent tear straps/stringers. The sizes of the regions with both intense local damage (solid ellipses) and widespread outer surface damage (dotted circles)
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increased as the peak current increased since greater galvanic and induced peak currents lead to more Joule heating. The highly damaged areas with intense local damage increased dramatically with increasing current. Close inspection of these areas revealed substantial fiber ruptures in the outermost +45˚ ply, large-scale matrix decomposition, some fiber breakage and tow splitting in
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the underlying -45˚ ply, as well as visible delamination between the top two plies. The discrete lightning damage was most pronounced in the outermost ply. Less electrical current penetrates into the underlying plies; the amount decreases with depth [37, 38]. Thus, more instantaneous
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Joule heating occurs in the outermost ply and decreases for each successive inner ply. The observed damage seems consistent with this assumption. When assessing the sizes of the regions
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with both intense local damage and widespread surface damage, the painted panel exhibited greater lightning surface damage than the sanded panel for a given peak current. The white paint applied to the LaRC panels is an aerospace-grade waterborne dielectric primer/paint. Nonconducting surface paints have been observed to decrease the arc diameter at the attachment point on painted aluminum [39] and prevent the reattachment of lightning arcs leading to a higher current density at the attachment location [40, 41]. This exacerbates lightning damage to
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underlying composite structures [39]. Therefore, the presence of the dielectric paint results in more surface damage than for similar strikes to the more conducting surface of the sanded PRSEUS panel.
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Typical lightning damage at the stitched stringer locations of the sanded and painted PRSEUS panels subjected to 50, 125, and 200 kA nominal peak currents is shown in Fig. 8. The horizontal black lines indicate the Vectran™ stitching lines. The intense local damage areas are
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normalized as they were in the mid-bay locations in Fig. 7. Again, the lightning-damaged region increased as the peak current increased. In contrast to Fig. 7, lightning damage development was
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clearly constrained by the through-thickness Vectran™ stitches. For example, the regions with intense local damage at the stringer locations involved large-scale fiber tow ruptures in the outermost +45˚ ply. The shape of the intense damage domain, however, was constrained between the Vectran™ stitch lines (i.e., the damaged zones tended to elongate parallel to the stitching
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lines rather than along the major fiber axis in the outermost ply) (Fig. 8). Despite intense Joule heating and catastrophic damage to the warp-knit skin stacks, the Vectran™ stitches remained essentially intact in the vicinity of the lightning attachment location. Vectran™ stiches, therefore,
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provide mechanical constraints that inhibit damage propagation across stitch lines. This seems remarkable, since Vectran™ stitches completely decompose at 400˚C [42], whereas the local
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lightning attachment temperature in the PRSEUS panel may drastically exceed this value. There appeared to be less visible delamination between plies in these locations relative to mid-bay strikes. Similar to the mid-bay strikes, a periodic distribution of small carbon fiber tufts was observed in an outer region surrounding the heavily damaged elliptical region close to the attachment point. The tufts were due to polyester warp-knit thread confinement. The sizes of the
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regions with both intense local damage and widespread surface damage to the painted panel (Figs. 8d-8f) were much greater than those for the sanded panel (Figs. 8a-8c). Typical lightning damage resulting from 125 kA nominal peak currents at the mid-bay,
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stringer, frame, and frame/stringer interconnection locations of the painted panel is shown in Fig. 9. The intense local damage area was normalized by that of a 200 kA stringer strike on the painted panel. The polyester warp-knit threads outside the lightning attachment location
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appeared to remain intact after lightning strike tests. Such polyester knitting threads (or their effects on carbon fiber spacing/alignment at the thread knit locations after resin curing) might
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provide mechanical constraints to the fiber tows leading to formation of a distribution of broken carbon fiber clusters (tufts) in the outermost lightly damaged regions. The regions of intense local damage at the stringer and frame locations were mostly confined between adjacent throughthickness Vectran™ stitches.
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While the through-thickness Vectran™ stitching clearly plays a major role in mitigating lightning damage development, other factors contribute to the damage resistance of PRSEUS panels. For example, the total composite thickness is different at the mid-bay (2.64 mm), stringer
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(5.28 mm), frame (6.60 mm), and frame/stringer interconnection (7.92 mm) locations. The local composite geometry and layup undoubtedly affect the dynamic behavior, mechanical and
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thermal strains, and other responses that influence lightning damage. The effect of panel thickness on damage remains to be fully explored. The areas of intense local damage for both sanded and painted PRSEUS panels are plotted as a function of the measured peak lightning current in Fig. 10. These regions were determined by visual inspection and are considered approximate. The sizes of the regions with intense local damage increased as the peak current increased. The painted panel exhibited larger damage sizes
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than the sanded panel at equivalent peak currents. Smaller differences between the painted and sanded panels were observed at 50 kA peak currents due to lower damage levels. Such differences were more pronounced for higher peak currents. For all given peak currents, the
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sanded panel showed small variations of the intense local damage between strike locations. At 50 and 125 kA peak currents, the intense local damage in the sanded panel was insensitive to lightning attachment locations. Although the 200 kA stringer strike showed a greater degree of
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intense damage compared to the 200 kA mid-bay strike, this difference was relatively small compared that in the painted panel. In contrast, the painted panel showed large variations of the
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intense local damage between each strike locations for higher peak currents. At 125 kA peak currents, the greatest degree of visible surface damage occurred at the frame/stringer interconnections or at the mid-bay; both of these locations do not contain through-thickness stitching. The minimum visible lightning damage occurred at the stringer locations where the
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damage development was significantly arrested/constrained by the presence of Vectran™ stitching. At 200 kA peak currents, the intense local damage was greater in the mid-bay location than in the stringer location. This is clearly shows that the stitching has a profoundly positive
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effect on the lightning damage resistance of the PRSEUS panels.” Of course, the panel surface
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treatment (painting) plays a major role in damage development..
Non-Destructive Evaluation of the Damaged PRSEUS Panels Preliminary through-thickness ultrasonic (TTU) C-scan imaging was performed by Aurora Flight Sciences in Columbus, MS to assess the internal damage (i.e., delamination, etc.) in the lightning-damaged PRSEUS panels. These panels were inspected using a 5 MHz transducer with a 50 mm water path and 2.5 dB baseline scanning at 2 mm index. High frequency ultrasonic
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waves were transmitted by a transducer from the OML side and captured by a receiver located on the inner mold line (IML) side of the PRSEUS panel. In this study, TTU C-scan results were obtained at the mid-bay locations. Since the PRSEUS panels were delivered to MSU without
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establishment of viable C-scan standards, the following C-scan data are provided for reference purposes only; only a few reference standards [43, 44] exist for assessing internal mechanical damage for PRSEUS panels.
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The extent of lightning-induced internal damage at the mid-bay locations increased as the peak current increased. Figure 11 contains identically scaled photographs and the corresponding
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TTU C-scan images of lightning damage at the mid-bay regions of the painted panel subjected to 50, 125, and 200 kA nominal peak currents. The blue regions in the upper C-scan images correspond to significant signal attenuation indicative of internal damage; the sizes of these domains are approximated by the dashed blue lines in the C-scan images. The boundaries of
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these domains (dashed lines) are superimposed on the corresponding photographs of the visible lightning damage. The results show that the intense fiber damage detected by visual inspection clearly underestimate the internal damage detected by TTU C-scan; the regions of the internal
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damage detected by TTU C-scan (represented by blue dotted ellipses) were somewhat larger than those of the intense local damage (illustrated by red ellipses) determined by visual inspection. In
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the figure, R denotes the ratio of the intense local damage area to the internal damage area. R gradually increased as the peak current increased. For instance, R increased from 0.29 at 50 kA nominal peak current to 0.33 at 125 kA nominal peak current, to 0.47 at 200 kA nominal peak current. This suggests possible delamination between skin stacks (or other internal damage) not detectable by visual inspection was present. More clearly, the intense fiber damage by visual
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inspection and the internal damage from C-scan images (with a larger degree of signal attenuation) appear to be more closely aligned in the outermost +45 ̊ ply.
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Concluding Remarks & Future Work
The lightning damage resistance of two NASA Langley Research Center (LaRC) Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) panels was characterized. A series of
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lightning strike tests with nominal 50, 125, and 200 kA peak currents were performed at four representative locations (i.e., the mid-bay, stringer, frame, and frame/stringer intersection).
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Lightning-induced damage to the PRSEUS panels was a strong function of (1) the peak current, (2) the lightning attachment location (mid-bay, stringer, frame, etc.) that involved different Vectran™ structural stitching, and (3) the presence of a surface finish. Lightning damage was grouped into two distinct types: (1) intense local damage occurring in the vicinity of the
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lightning attachment point, where severe fiber rupture, tow splitting, matrix decomposition, delamination, etc. all occur, and (2) mild surface damage occurring in the surrounding region, characterized by smaller scale fiber damage in the form of periodically dispersed tufts of broken
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fibers and some scorching/burning.
The sizes of the regions with both intense local damage and widespread surface damage
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increased as the peak current increased. Greater peak current leads to more Joule heating causing severe damage. The damaged PRSEUS panels exhibited unique lightning damage features due to use of through-thickness Vectran™ stitching and warp-knitted fabrics. Through-thickness Vectran™ stitching was highly effective in mitigating and constraining regions of intense lightning damage. The Vectran™ stitching appeared relatively intact after all tests, even those conducted at high nominal peak currents. In contrast, for attachment locations with no
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underlying stitching (i.e., mid-bay and frame/stringer interconnections), both the size and severity of the resulting damage were greater than for analogous cases where through-thickness Vectran™ stitching was present. The polyester knit threads used in the warp-knitted skin stacks
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appear to influence lightning damage formation in surrounding regions just outside of the lightning attachment point (the area with severe fiber damage, matrix decomposition, etc.). Small clusters of broken fibers with a size and periodic spacing consistent with the polyester warp-knit
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thread architecture were formed in this outer region. Both the through-thickness Vectran™ stitching and polyester warp-knit threads play a beneficial role in reducing the initial strike
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damage size and arresting delamination between the layers. Overall, use of through-thickness Vectran™ stitching, in combination with warp-knitted skin stacks, may profoundly improve the lightning damage resistance of PRSEUS components relative to more traditional (nonreinforced) laminated composite structures. In general, the painted panel exhibited a higher
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degree of visible damage for a given peak current level.
Additional phased-array ultrasonic inspection of the painted and sanded PRSEUS panels is being currently performed at Mississippi State University (MSU) using newly acquired non-
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destructive inspection equipment. Such measurements are coupled with destructive sectioning of both PRSEUS panels at each lightning strike zone to better characterize (1) the through-thickness
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integrity of Vectran™ stitching and polyester warp-knit threads, (2) internal damage morphologies, such as delamination between skin stacks and within plies in a single stack, and (3) the through-thickness thermal matrix damage distribution. Lastly, lightweight lightning protection layers (i.e., copper mesh, carbon fiber paper, graphene paper) are being integrated in the two remaining PRSEUS panels to mitigate damage development.
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Acknowledgements This work was performed in the Marvin B. Dow Advanced Composites Institute at
The Boeing Company (contract # 1188469).
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[2]
Hirano Y, Katsumata S, Iwahori Y, Todoroki A. Artificial Lightning Testing on Graphite/Epoxy Composite Laminate. Composites Part A: Applied Science and Manufacturing. 2010;41(10):1461-70. Feraboli P, Kawakami H. Damage of Carbon/Epoxy Composite Plates Subjected to Mechanical Impact and Simulated Lightning. Journal of Aircraft. 2010;47(3):999-1012. Kawakami H, Feraboli P. Lightning Strike Damage Resistance and Tolerance of Scarf-Repaired Mesh-protected Carbon Fiber Composites. Composites Part A: Applied Science and Manufacturing. 2011;42(9):1247-62. Wang F, Ji Y, Yu X, Chen H, Yue Z. Ablation Damage Assessment of Aircraft Carbon Fiber/Epoxy Composite and its Protection Structures Suffered from Lightning Strike. Composite Structures. 2016;145:226-41. Velicki A, Thrash P. Advanced Structural Concept Development Using Stitched Composites. Proceedings of the 49th AIAA/ASME/ASCE/ASC Structures, Structural Dynamics, and Materials Conference. Schaumburg, IL2008. p. 7-10. Velicki A, Yovanof N, Baraja J, Linton K, Li V, Hawley A, et al. Damage Arresting Composites for Shaped Vehicles-Phase I Final Report. NASA/CR-2009-215932, NASA; 2009. Velicki A, Yovanof N, Baraja J, Linton K, Li V, Hawley A, et al. Damage Arresting Composites for Shaped Vehicles-Phase II Final Report. NASA/CR-2011-216880, NASA; 2011. Velicki A, Jegley D. PRSEUS Development for the Hybrid Wing Body Aircraft. Proceedings of the 11th AIAA Aviation Technology, Integration, and Operation. Virginia Beach, VA2011. Jain LK, Mai Y-W. Mode I delamination toughness of laminated composites with throughthickness reinforcement. Applied Composite Materials. 1994;1(1):1-17. Dransfield KA, Jain LK, Mai Y-W. On the Effects of Stitching in CFRPs - I. Mode I Delamination Toughness. Composites Science and Technology. 1998;58(6):815-27. Bergan A, Bakuckas J, Awerbuch J, Tan T-M. Assessment of damage containment features of a full-scale PRSEUS fuselage panel. Composite Structures. 2014;113:174-85. Lovejoy AE, Rouse M, Linton KA, Li VP. Pressure Testing of a Minimum Gauge PRSEUS Panel. Proceedings of the 52nd AIAA Structures Dynamics and Materials Conference. National Harbor, MD2011. Yovanof N, Lovejoy AE, Baraja J, Gould K. Design, Analysis and Testing of a PRSEUS Pressure Cube to Investigate Assembly Joints. Proceedings of the Aircraft Airworthiness and Sustainment Conference. Baltimore, MD2012. Leone Jr FA, Jegley DC. Compressive Loading and Modeling of Stitched Composite Stiffeners. Proceedings of the 57th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference. San Diego, CA2016. p. 2179. Lovejoy AE. Preliminary Weight Savings Estimate for a Commercial Transport Wing Using Rod-stiffened Stitched Composite Technology. Proceedings of the 56th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference. Kissimmee, FL2015. p. 1873.
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[1]
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Mississippi State University. The testing and reporting tasks of this project were fully funded by
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[22] [23] [24] [25]
[26]
[27] [28] [29]
[30]
[31]
[32]
[33]
[34]
[35]
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[21]
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[20]
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[19]
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[18]
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[17]
Society of Automotive Engineers (SAE). Aircraft Lightning Environment and Related Test Waveforms. Aerospace Recommended Practice ARP 5412. Warrendale, PA: SAE International; 1999. Sugie T, Nakai A, Hamada H. Effect of CF/GF fibre hybrid on impact properties of multi-axial warp knitted fabric composite materials. Composites Part A: Applied Science and Manufacturing. 2009;40(12):1982-90. SAERTEX. SAERTEX Thermoplastic Non-Crimp Carbon Fiber Fabrics. SAERTEX GmbH&Co; 2016. Karahan M, Lomov SV, Bogdanovich AE, Mungalov D, Verpoest I. Internal geometry evaluation of non-crimp 3D orthogonal woven carbon fabric composite. Composites Part A: Applied Science and Manufacturing. 2010;41(9):1301-11. Woods J, Modin A, Hawkins R, Hanks D. Controlled atmospheric pressure infusion process. International Patent WO. 2003;3(101708):A1. Bergan AC. Test and Analysis of Stitched Composite Structures to Assess Damage Containment Capability: PhD Dissertation, Drexel University; 2014. Lacy TE, Mazzola, M.S., Lee, J., Gharghabi, P., Boushab, D., and Ricks, T.M. Lightning Strike Testing on PRSEUS Panels. Boeing Technical Report (Yr 2016-2017)2017. Lacy TE, Mazzola, M.S., Kluss, J., Boushab, D., Gharghabi, P., Lee, J., and Ricks, T.M. Lightning Strike Testing on PRSEUS Panels. Boeing Technical Report (Yr 2017-2018)2018. Schon K. Characterisation and Generation of High Impulse Voltages and Currents. High Impulse Voltage and Current Measurement Techniques: Springer; 2013. p. 5-38. Heidler F, Zischank W, Flisowski Z, Bouquegneau C, Mazzetti C. Parameters of Lightning Current Given in IEC 62305-Background, Experience and Outlook. Proceedings of the 29th International Conference on Lightning Protection2008. p. 6. Feraboli P, Miller M. Damage Resistance and Tolerance of Carbon/Epoxy Composite Coupons Subjected to Simulated Lightning Strike. Composites Part A: Applied Science and Manufacturing. 2009;40(6):954-67. Gou J, Tang Y, Liang F, Zhao Z, Firsich D, Fielding J. Carbon Nanofiber Paper for Lightning Strike Protection of Composite Materials. Composites Part B: Engineering. 2010;41(2):192-8. Kawakami H. Lightning Strike Induced Damage Mechanisms of Carbon Fiber Composites: Dissertation, University of Washington; 2011. Chen X, Liu G, Wang H. The Residual Strength Test and Analysis of Composite Rudder After Lightning Strike. Proceedings of the 18th International Conference on Composite Materials. Jeju, South Korea2011. Szatkowski GN, Dudley KL, Koppen SV, Ely JJ, Nguyen TX, Ticatch LA, et al. Common Practice Lightning Strike Protection Characterization Technique to Quantify Damage Mechanisms on Composite Substrates. Proceedings of the International Conference on Lightning and Static Electricity. Seattle, WA2013. Yamashta S, Ohsawa I, Takahashi J. Structural Integrity of Carbon Fiber Reinforced Polyprophylene After Lightning Strike. Proceedings of SAMPE Europe's 35th International Conference. Paris, France2014. Yin J, Chang F, Li S, Yao X, Sun J, Xiao Y. Lightning Strike Ablation Damage Influence Factors Analysis of Carbon Fiber/Epoxy Composite Based on Coupled Electrical-Thermal Simulation. Applied Composite Materials. 2016:1-18. Hirano Y, Yokozeki T, Ishida Y, Goto T, Takahashi T, Qian D, et al. Lightning damage suppression in a carbon fiber-reinforced polymer with a polyaniline-based conductive thermoset matrix. Composites Science and Technology. 2016;127:1-7. Chemartin L, Lalande P, Peyrou B, Chazottes A, Elias P, Delalondre C, et al. Direct Effects of Lightning on Aircraft Structure: Analysis of the Thermal, Electrical and Mechanical Constraints. AerospaceLab. 2012(5):1-15. Bazelyan EM, Raizer YP. Lightning Physics and Lightning Protection: CRC Press; 2000.
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[16]
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[42] [43] [44]
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[39]
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Rakov VA, Uman MA. Lightning: Physics and Effects. Cambridge, United Kingdom: Cambridge University Press; 2003. Lee J, Lacy, T.E., Pittman Jr., C.U. & Mazzola, M.S. Thermal Response of Carbon Fiber Epoxy Composites with Metallic and Nonmetallic Protection Layers to Simulated Lightning Currents. Polymer Composites. DOI: 10.1002/pc.24502, 2017. Lee J, Lacy, T.E., Pittman Jr., C.U. & Mazzola, M.S. Temperature-Dependent Thermal Decomposition of Carbon/Epoxy Laminates Subjected to Simulated Lightning Currents. Polymer Composites. DOI: 10.1002/pc.24535, 2017. Martins RAS. Experimental and Theoretical Studies of Lightning Arcs and their Interaction with Aeronautical Materials: PhD. dissertation, L’UNIVERSITE PARIS-SACLAY; 2016. Clark H. Advanced Development on Vulnerability/Survivability of Advanced Composite Structures. DTIC Document #ADB955855; 1972. Drumm F, Bäuml G, Zischank W, Brocke R, Schönau J. Behaviour of Protected Composite Materials Exposed to Lightning Impulse and Continuing Currents. Proceedings of the 24th International Conference on Lightning Protection, Birmingham, UK1998. MatWeb. Kuraray Vectran® HT 1500/300 LCP Fiber Material Property Data. 2014. Johnston PH. Ultrasonic Nondestructive Evaluation of PRSEUS Pressure Cube Article in Support of Load Test to Failure. NASA/TM-2013-217799, NASA; 2013. Johnston PH, Juarez PD. Ultrasonic Nondestructive Evaluation of Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) During Large-Scale Load Testing and Rod Push-Out Testing. NASA/TM-2016-218978, NASA; 2016.
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[36]
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Table 1. Material, thickness, and layup of LaRC PRSEUS sub-components [6, 7]. Material AS4/VRM-34a
Exterior Skin Tear Straps Frame Stringer Foam Core
Thickness (mm)
Stack A/Stack A
2.64
AS4/VRM-34
a
Stack A
1.32
AS4/VRM-34
a
Stack A/Stack A
2.64
AS4/VRM-34
a
Stack A
1.32
-
12.70
Rohacell 110WF
Rod
Layupc
T800/3900-2B
b
-
a
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Part
9.53 (dia.)
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Warp-knitted AS4 carbon fiber fabric and VRM-34 epoxy resin processed via resin infusion. b Pre-cured unidirectional T800 fibers with a 3900-2B epoxy resin. c Stack A = [+45/-45/0/0/90/0/0/-45/+45].
Table 2. Lightning strike tests on PRSEUS panels. Target Peak Current (kA)
Stringer
1
1
3
3
3
3
12
3
3
-
-
6
2
2
-
-
4
50
3
2
1
1
7
125
2
2
1
2
7
200
1
1
-
-
2
50a Sanded Panel
50 125
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200
Painted Panel
Number of Tests Frame/Stringer Frame Intersection -
Mid-bay
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Panel Type
Total Number of Lightning Strike Tests
a
Initial calibration tests.
Total 2
40
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Figure 1. Schematic of PRSEUS concept [7].
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Figure 2. (a) Schematic of multiaxial warp-knitted fabric [17] and (b) photograph of SAERTEX multiaxial warp-knitted dry carbon fiber fabric [18].
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Figure 3. Single-sided stitching for the PRSEUS concept: (a) frame/stringer intersection (adapted from [21]), (b) flange-to-skin stiches [21], (c) single-sided stitch seam (adapted from [8]), and (d) a complete flat PRSEUS panel preform after stitching (adapted from [8]).
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Figure 4. Electrical grounding condition along the edges of the sanded PRSEUS panel: (a) IML view and (b) OML view. The OML skin is slightly sanded to minimize possible fiber damage, while all four edges are fully sanded for better contact with the aluminum brackets.
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Figure 5. SAE waveform component A [16] and MSU-HVL impulse current waveform (200 kA peak current)
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Figure 6. Two lightning damage categories observed at the sanded PRSEUS mid-bay location subjected to 200 kA nominal peak current.
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Figure 7. Typical lightning damage at the unstitched mid-bay locations induced in sanded (left) and painted (right) PRSEUS panels subjected to (a, b) 50 kA, (c, d) 125 kA, and (e, f) 200 kA nominal peak currents. Measured peak currents are included for clarity. A* represents the area of intense local damage normalized by that for the painted panel subjected to 200 kA mid-bay strike.
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Figure 8. Typical lightning damage at the stitched stringer locations induced in sanded (left) and painted (right) PRSEUS panels subjected to (a, b) 50 kA, (c, d) 125 kA, and (e, f) 200 kA nominal peak currents. Measured peak currents are included for clarity. A* defines the area of intense local damage normalized by that for the painted panel subjected to 200 kA mid-bay strike.
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Figure 9. Typical lightning strike damage to the painted panel subjected to 125 kA nominal peak currents at (a) mid-bay, (b) stringer, (c) frame, and (d) frame/stringer intersection locations. Measured peak currents are included for clarity. A* corresponds to the area of intense local damage normalized by that for the painted panel subjected to 200 kA mid-bay strike.
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Figure 10. Normalized intense local damage area, A*, versus the measured peak current.
Figure 11. TTU C-scan images (top) and photographs (bottom) of lightning damage observed at painted panel mid-bay locations subjected to: (a) 50 kA, (b) 125 kA, and (c) 200 kA. R denotes the ratio of the intense local damage area to the internal damage are from C-scan signal attenuation.
S1359-8368(18)32273-X
DOI:
10.1016/j.compositesb.2018.09.016
Reference:
JCOMB 5986
To appear in:
Composites Part B
Received Date: 21 July 2018 Revised Date:
10 August 2018
Accepted Date: 10 September 2018
Please cite this article as: Lee J, Gharghabi P, Boushab D, Ricks TM, Lacy Jr. TE, Pittman Jr. CU, Mazzola MS, Velicki A, Artificial lightning strike tests on PRSEUS panels, Composites Part B (2018), doi: https://doi.org/10.1016/j.compositesb.2018.09.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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– Cover Page – Manuscript entitled:
Submitted to:
Authored by: Juhyeong Lee,
1, 2*
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Composites Part B: Engineering
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Artificial Lightning Strike Tests on PRSEUS Panels
Pedram Gharghabi, 3 Dounia Boushab, 1 Trenton M. Ricks, 1, 4
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Thomas E. Lacy Jr., 5 Charles U. Pittman Jr., 6 Michael S. Mazzola 7, Alex Velicki 8
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1. Department of Aerospace Engineering, Mississippi State University, Mississippi State, MS 39762, USA 2. Advanced Composites Collaboration for Innovation & Science, Queens Building, University Walk, Bristol BS8 1TR, UK 3. Department of Electrical and Computer Engineering, Mississippi State University, Mississippi State, MS 39762, USA 4. Multiscale and Multiphysics Modeling Branch, Materials and Structures Division, NASA Glenn Research Center, Cleveland, OH 44135, USA 5. Department of Mechanical Engineering, Texas A&M University, MEOB 429, College Station, TX 77843-3123, USA 6. Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA 7. Department of Electrical and Computer Engineering, University of North Carolina at Charlotte, Charlotte, NC 28223, USA 8. The Boeing Company, Huntington Beach, CA 92647-2099, USA
Keywords: lightning strike tests, PRSEUS, stitched laminated composites.
* Corresponding author Juhyeong Lee PhD
Advanced Composites Collaboration for Innovation & Science University of Bristol Queens Building, University Walk, Bristol BS8 1TR, UK
Email: [email protected] and [email protected]
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Abstract The lightning damage resistance of Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) panels was characterized experimentally. Two unprotected PRSEUS panels were
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subjected to standard impulse current waveforms (consistent with actual lightning strikes) with 50, 125, and 200 kA nominal peak currents at a variety of panel locations. Lightning-induced damage to the PRSEUS panels was a strong function of (1) the peak current, (2) the lightning
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attachment location (mid-bay, stringer, frame, etc.) that involved different through-thickness Vectran™ stitching, and (3) the presence of a surface finish paint.
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The sizes of the damaged regions increased as the peak current increased, since greater peak current leads to more Joule heating. The lightning-damaged PRSEUS panels exhibited unique damage features due to the presence of through-thickness Vectran™ stitches and warp-knitted fabrics. Through-thickness Vectran™ stitches constrained the development and spread of intense
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local damage in the vicinity of the lightning attachment point. The polyester warp-knit threads used to stitch the warp-knitted laminates together appeared to influence the development of widespread small-scale fiber damage in the region surrounding the strike. Consequently, the
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Vectran™ structural stitches, as well as the polyester knit threads holding the tows together, have a significant beneficial effect on both the size of the impingement region and the subsequent
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damage propagation within the laminate. In addition, damage to the painted panel was greater for each current level than for the sanded (unpainted) panel.
Introduction
Lightning strike is a potential threat to composite aircraft. Since traditional carbon/epoxy composites have lower thermal and electrical conductivities than metallic materials, lightning
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strikes can cause severe damage to composite structures [1-4]. Matrix thermal decomposition, carbon fiber breakage/ablation, and delamination have been typically observed in laminated composites that have sustained lightning strikes.
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The Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept [5-8] refers to an integrated composite structural design developed by the Boeing Company under funding from and in collaboration with the NASA Langley Research Center (LaRC) (Fig. 1). The PRSEUS
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concept uses through-thickness Vectran™ stitches to improve the composite structural integrity over traditional laminated composites. Resisting out-of-plane delamination is enhanced by the
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formation of stitch bridging zones [9, 10]. The damage arresting capabilities were validated by subjecting both flat and curved PRSEUS panels to axial tension, axial compression, internal pressure, and combined axial and internal pressure loadings [11-14]. Furthermore, the PRSEUS concept offers structural weight savings, while ensuring all relevant performance requirements
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and safety criteria are satisfied. Lovejoy [15] reported that a PRSEUS wing design exhibited a 9% weight savings compared to a stiffened composite wing. These results suggest that the PRSEUS concept can be used for future structural applications.
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Mississippi State University (MSU) received three PRSEUS panels from the NASA LaRC and one PRSEUS panel from the Boeing Company to be used for simulated lightning strikes.
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The NASA and Boeing PRSEUS panels have different geometries, configurations, and surface treatments. While the stiffener spacing, geometries, layups, and a type of surface paint for the NASA panels are available in the open literature [5-8], those for the Boeing panel are proprietary. In this study, two LaRC PRSEUS panels were subjected to standard Society of Automotive Engineers (SAE) impulse current waveforms [16] (consistent with actual lightning strikes) with 50, 125, and 200 kA nominal peak currents at a variety of panel locations. The type
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and extent of damage were characterized at each attachment location. One of the LaRC panel outer mold line (OML) skins was lightly sanded using 100, 200, and then 400-grit sandpapers prior to lightning strike tests to help assess the influence of exterior surface paint on lightning
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damage. Lightning damage on the sanded LaRC PRSEUS panel was compared with those of the painted panel.
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PRSEUS Panel Construction
A fully integrated PRSEUS panel has an outer skin and interior stringers, frames, and tear
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straps. Each PRSEUS subcomponent is produced from stitched preforms constructed of multi-axial warp-knitted fabrics (Fig. 2a; [17]). Warp-knitted fabrics and their carbon fiber tows are held together by thermoplastic threads (preferably polyester [6, 7, 18]) in order to (1) prevent crimping or tow undulations and (2) enable easier handling [19]. The white lines in Fig. 2b
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correspond to polyester knit threads [18]. PRSEUS panel warp-knitted fabrics (Fig. 2b) consist of multiple layers of oriented unidirectional Hexcel standard modulus AS4 carbon fiber tows. High-performance thermoplastic threads made of Vectran™, are used to stitch the pre-
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assembled dry material stacks for the skins, stringers, frames, and tear straps. Vectran™ stitching threads are typically made of three or four 400-denier Vectran™ threads twisted together for
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better strength and coated with nylon for thermal stability [6, 7]. A Hexcel VRM-34 toughened epoxy resin is then infused into a stitched pre-assembly and then oven-cured using the Controlled Atmospheric Pressure Resin Infusion (CAPRI) process [20]. The materials, layups, and total stack thicknesses for a PRSEUS panel (including all subcomponents) are listed in Table 1. The Stack A layup has a nominal thickness of 1.32 mm with a
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balanced symmetric stacking sequence, [+45/-45/0/0/90/0/0/-45/+45]. A detailed description of the ply stacking sequences in the PRSEUS panel is available in references [6, 7]. Two-needle single-sided stitching technologies are used to sew a stitching thread through the
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built-up preforms from the PRSEUS panel OML side [6-8]. Through-thickness Vectran™ stitching increases the laminate’s interlaminar fracture toughness. A locking thread holds the vertical stitches in place. Figures 3a-3b show a typical PRSEUS frame/stringer intersection with
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flange-to-skin stitching. In Fig. 3b, two parameters define a given stitch pattern: stitching spacing (Ss) and stitching pitch (Sp). For a prototype PRSEUS panel, Ss = 25.4 mm and Sp = 5.1 mm [21].
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A single-sided chain-stitch seam introduces two rows of continuous stitching (Fig. 3c). It attaches stringer/frame flanges to the PRSEUS skin layers. Figure 3d shows a flat PRSEUS panel after single-sided 3D-seam stitching and prior to resin infusion. A continuous stitching line is used to attach a given stringer/tear strap or frame/tear strap combination to the overlapping
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PRSEUS skins. In regions where the upstanding web of a given frame intersects the upstanding web on an orthogonal stringer, no through-thickness stitching is possible. A detailed description
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of the single-sided stitching implemented in the PRSEUS concept is available in references [6-8].
Laboratory-scale Lightning Strike Testing Conditions
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High Impulse Current Generator
A one-stage impulse current generator was designed to produce standard impulse current waveforms consistent with actual lightning strikes [22, 23]. The impulse current generator built at the MSU-High-Voltage Lab (MSU-HVL) is able to produce double exponential current waveforms with up to 200 kA peak currents. The trigatron spark gap switch, which triggers an impulse current discharge, consists of two electrodes operating in air at atmospheric pressure: the
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upper electrode was connected to a set of capacitors, and the lower electrode was grounded. Electric current was discharged across the gap between the two electrodes. Electrical Grounding Conditions
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Once lightning strikes a PRSEUS panel, a substantial electrical current flows through the panel. Most composite aircraft panels are designed to distribute electric current over their outer surfaces. This condition was ensured by connecting electrical grounding terminals along the four
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edges of the laboratory panels. Figure 4 shows the grounding connections on a PRSEUS panel to the grounded current-return structure of the lightning simulator prior to lightning strike tests. The
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four PRSEUS panel edges were smoothly sanded to provide uniform electrical contact surfaces. Flexible braided copper straps were then inserted between aluminum brackets and the PRSEUS panel to return the current to the energy storage capacitors. C-clamps were used to secure the
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aluminum angle brackets to the panel’s OML side (Fig. 4b).
Artificial Lightning Current Waveform
Figure 5a compares the standard SAE impulse current waveform component A [16] with a
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measured 200 kA peak current impulse waveform generated at the MSU-HVL. The standard SAE waveform A [16] has a 200 kA peak current, rise time (T1) of 6.4 µs, and decay time (T2) of
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69 µs with a ±20% tolerance level. The MSU-HVL 200 kA peak current waveform exhibited similar temporal characteristics (i.e., T1 and T2). The difference in the rise time between the MSU-HVL lightning waveform and the SAE waveform A is not significant because the rise time is typically less important than decay time in determining the time response of an impulse current waveform [24]. Lightning damage is governed by the amount of electrical energy, defined as the action integral (integral of the square of the time-varying current over its time
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duration [25]). Figure 5b shows the MSU-HVL impulse current waveforms with 50, 125, and 200 kA nominal peak currents. Note that rise and decay times of a current waveform strongly depend on the circuit parameters for the system (i.e., resistance and inductance). These are
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independent of the charging voltage, peak current, and test specimen. An impulse current generator typically has a constant resistance and inductance. Thus, the resulting rise and decay times of a current waveform are relatively constant, regardless of the magnitude of the peak
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current. Thus, all MSU-HVL lightning waveforms showed identical rise and decay times. However, the action integrals of MSU-HVL lightning waveforms varied depending on the peak
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current.
Lightning Strike Locations on the PRSEUS Panels
Simulated impulse current waveforms were applied at four representative lightning
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attachment locations on the OML skins of two LaRC PRSEUS panels: (1) mid-bay, (2) stringer, (3) frame, and (4) frame/stringer intersection. Three peak current levels were considered: (1) 50 kA (commonly used for laboratory-scale lightning strike tests [1, 2, 26-33]), (2) 125 kA
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(slightly higher than the subsequent return stroke defined in [16]), and (3) 200 kA (consistent with the first return stroke defined in [16]).
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A total of forty simulated lightning strike tests were conducted on the two PRSEUS panels (Table 2). The number and location of tests on each panel was determined to avoid lightning damage overlap between adjacent strike locations. Twenty-four tests (including two calibration tests) on the sanded panel and sixteen tests on the painted panel were performed at a variety of key locations including the mid-bay, stringer, and frame locations, in addition to the frame/stringer intersection.
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For both the sanded and painted panels, all measured peak currents were within 10% of the target peak currents with the exception of one 200 kA mid-bay strike due to a prefire of the main gap during charging of the energy storage capacitors. This proves the reliability of the MSU-
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HVL impulse current generator. While the experimental work reported here is not intended to be a certification test under the SAE standard, the standard SAE waveform [16] has a ±10% peak
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current tolerance.
Lightning Damage Characterization
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Results and Discussion
Two primary damage types can be identified for each PRSEUS panel based on surface observations (cf., Fig. 6): (1) intense local damage near the attachment location and (2) surrounding widespread surface damage. The intense local damage includes severe fiber
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rupture, tow splitting, matrix decomposition, and underlying delamination. The regions with severe fiber rupture/tow splitting and underlying delamination were fairly elongated along the outermost lamina’s fiber direction. Carbon fiber damage is associated with Joule heating
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resulting from an applied electrical current. Rapid heating of the carbon fibers results in a severe
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contraction of the fiber/tows due to carbon fiber’s negative coefficient of thermal expansion. This contraction may lead to large-scale fiber rupture/tow splitting in the high thermal gradient inner
region
near
the
lightning
attachment
point.
The
regions
of
severe
fiber
rupture/delamination are surrounded by an outer region with a large number of small clusters (“tufts”) of broken fibers with a periodic arrangement consistent with the spacing between warpknit polyester threads (Fig. 2b). Localized tufts of broken fibers arguably occur in the surrounding region where the fibers are relatively unconstrained between polyester warp-knit
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threads. There is also evidence of mild scorching (or burning) of the OML surface in this region. The distribution of the damaged tufts is not likely a result of galvanic currents conducted from the arc attachment point due to their distances from the attachment point and amount of
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intervening matrix. The authors are pursuing a theory of secondary currents in this ‘outer’ region induced by transient magnetic fields from the lightning currents in the arc and the panel. Such surface damage may also result from a combination of Joule heating and direct lightning heat
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fluxes (electronic or ionic recombination, convection flux, radiation flux, etc. [34]). Lightning strikes create a narrow cylindrical plasma arc channel accompanied with radial heat fluxes [34-
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36]. Direct lightning heat fluxes are independent of the electrical conduction path (i.e., outer ply fiber direction), but strongly depend on thermal boundary conditions (i.e., convective/radiative heat transfer coefficients and ambient temperature). Hence, the radial heat fluxes often create surface damage emanating from the center of the lightning attachment point [34].
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The periodic distribution of small broken fiber clusters in the outer region surrounding the intense local damage region near the lightning attachment location appears consistent with the spacing of warp-knitted fabric polyester threads. After lightning strike tests, the polyester knit
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threads in regions away from the severe damage zone appeared to remain intact. Knitting threads produce through-thickness changes that affect the electrical and mechanical wave propagation
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through the laminate. Local contraction/densification of the fibers due to the presence of the periodically distributed warp-knit threads may effectively constrain (“pin”) fiber tows or may change the local thermal/electrical conductivities of the laminate. Such influences may explain the formation of small, periodically distributed clusters of broken fibers (Fig. 6). This unique lightning damage feature has not been observed in laminated composites [1, 2, 26-33].
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Effect of Peak Current and Attachment Point on Lightning Damage Formation Figure 7 shows typical lightning damage resulting at the mid-bay locations of both sanded and painted PRSEUS panels subjected to 50, 125, and 200 kA nominal peak currents. The A* in
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the figure indicates an intense local damage area normalized by the maximum damaged area found for this group (200 kA mid-bay strike on the painted panel). The horizontal solid lines shown in the figures correspond to through-thickness Vectran™ structural stitch lines where the
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skin stacks are sewn to the adjacent tear straps/stringers. The sizes of the regions with both intense local damage (solid ellipses) and widespread outer surface damage (dotted circles)
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increased as the peak current increased since greater galvanic and induced peak currents lead to more Joule heating. The highly damaged areas with intense local damage increased dramatically with increasing current. Close inspection of these areas revealed substantial fiber ruptures in the outermost +45˚ ply, large-scale matrix decomposition, some fiber breakage and tow splitting in
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the underlying -45˚ ply, as well as visible delamination between the top two plies. The discrete lightning damage was most pronounced in the outermost ply. Less electrical current penetrates into the underlying plies; the amount decreases with depth [37, 38]. Thus, more instantaneous
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Joule heating occurs in the outermost ply and decreases for each successive inner ply. The observed damage seems consistent with this assumption. When assessing the sizes of the regions
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with both intense local damage and widespread surface damage, the painted panel exhibited greater lightning surface damage than the sanded panel for a given peak current. The white paint applied to the LaRC panels is an aerospace-grade waterborne dielectric primer/paint. Nonconducting surface paints have been observed to decrease the arc diameter at the attachment point on painted aluminum [39] and prevent the reattachment of lightning arcs leading to a higher current density at the attachment location [40, 41]. This exacerbates lightning damage to
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underlying composite structures [39]. Therefore, the presence of the dielectric paint results in more surface damage than for similar strikes to the more conducting surface of the sanded PRSEUS panel.
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Typical lightning damage at the stitched stringer locations of the sanded and painted PRSEUS panels subjected to 50, 125, and 200 kA nominal peak currents is shown in Fig. 8. The horizontal black lines indicate the Vectran™ stitching lines. The intense local damage areas are
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normalized as they were in the mid-bay locations in Fig. 7. Again, the lightning-damaged region increased as the peak current increased. In contrast to Fig. 7, lightning damage development was
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clearly constrained by the through-thickness Vectran™ stitches. For example, the regions with intense local damage at the stringer locations involved large-scale fiber tow ruptures in the outermost +45˚ ply. The shape of the intense damage domain, however, was constrained between the Vectran™ stitch lines (i.e., the damaged zones tended to elongate parallel to the stitching
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lines rather than along the major fiber axis in the outermost ply) (Fig. 8). Despite intense Joule heating and catastrophic damage to the warp-knit skin stacks, the Vectran™ stitches remained essentially intact in the vicinity of the lightning attachment location. Vectran™ stiches, therefore,
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provide mechanical constraints that inhibit damage propagation across stitch lines. This seems remarkable, since Vectran™ stitches completely decompose at 400˚C [42], whereas the local
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lightning attachment temperature in the PRSEUS panel may drastically exceed this value. There appeared to be less visible delamination between plies in these locations relative to mid-bay strikes. Similar to the mid-bay strikes, a periodic distribution of small carbon fiber tufts was observed in an outer region surrounding the heavily damaged elliptical region close to the attachment point. The tufts were due to polyester warp-knit thread confinement. The sizes of the
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regions with both intense local damage and widespread surface damage to the painted panel (Figs. 8d-8f) were much greater than those for the sanded panel (Figs. 8a-8c). Typical lightning damage resulting from 125 kA nominal peak currents at the mid-bay,
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stringer, frame, and frame/stringer interconnection locations of the painted panel is shown in Fig. 9. The intense local damage area was normalized by that of a 200 kA stringer strike on the painted panel. The polyester warp-knit threads outside the lightning attachment location
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appeared to remain intact after lightning strike tests. Such polyester knitting threads (or their effects on carbon fiber spacing/alignment at the thread knit locations after resin curing) might
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provide mechanical constraints to the fiber tows leading to formation of a distribution of broken carbon fiber clusters (tufts) in the outermost lightly damaged regions. The regions of intense local damage at the stringer and frame locations were mostly confined between adjacent throughthickness Vectran™ stitches.
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While the through-thickness Vectran™ stitching clearly plays a major role in mitigating lightning damage development, other factors contribute to the damage resistance of PRSEUS panels. For example, the total composite thickness is different at the mid-bay (2.64 mm), stringer
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(5.28 mm), frame (6.60 mm), and frame/stringer interconnection (7.92 mm) locations. The local composite geometry and layup undoubtedly affect the dynamic behavior, mechanical and
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thermal strains, and other responses that influence lightning damage. The effect of panel thickness on damage remains to be fully explored. The areas of intense local damage for both sanded and painted PRSEUS panels are plotted as a function of the measured peak lightning current in Fig. 10. These regions were determined by visual inspection and are considered approximate. The sizes of the regions with intense local damage increased as the peak current increased. The painted panel exhibited larger damage sizes
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than the sanded panel at equivalent peak currents. Smaller differences between the painted and sanded panels were observed at 50 kA peak currents due to lower damage levels. Such differences were more pronounced for higher peak currents. For all given peak currents, the
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sanded panel showed small variations of the intense local damage between strike locations. At 50 and 125 kA peak currents, the intense local damage in the sanded panel was insensitive to lightning attachment locations. Although the 200 kA stringer strike showed a greater degree of
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intense damage compared to the 200 kA mid-bay strike, this difference was relatively small compared that in the painted panel. In contrast, the painted panel showed large variations of the
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intense local damage between each strike locations for higher peak currents. At 125 kA peak currents, the greatest degree of visible surface damage occurred at the frame/stringer interconnections or at the mid-bay; both of these locations do not contain through-thickness stitching. The minimum visible lightning damage occurred at the stringer locations where the
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damage development was significantly arrested/constrained by the presence of Vectran™ stitching. At 200 kA peak currents, the intense local damage was greater in the mid-bay location than in the stringer location. This is clearly shows that the stitching has a profoundly positive
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effect on the lightning damage resistance of the PRSEUS panels.” Of course, the panel surface
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treatment (painting) plays a major role in damage development..
Non-Destructive Evaluation of the Damaged PRSEUS Panels Preliminary through-thickness ultrasonic (TTU) C-scan imaging was performed by Aurora Flight Sciences in Columbus, MS to assess the internal damage (i.e., delamination, etc.) in the lightning-damaged PRSEUS panels. These panels were inspected using a 5 MHz transducer with a 50 mm water path and 2.5 dB baseline scanning at 2 mm index. High frequency ultrasonic
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waves were transmitted by a transducer from the OML side and captured by a receiver located on the inner mold line (IML) side of the PRSEUS panel. In this study, TTU C-scan results were obtained at the mid-bay locations. Since the PRSEUS panels were delivered to MSU without
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establishment of viable C-scan standards, the following C-scan data are provided for reference purposes only; only a few reference standards [43, 44] exist for assessing internal mechanical damage for PRSEUS panels.
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The extent of lightning-induced internal damage at the mid-bay locations increased as the peak current increased. Figure 11 contains identically scaled photographs and the corresponding
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TTU C-scan images of lightning damage at the mid-bay regions of the painted panel subjected to 50, 125, and 200 kA nominal peak currents. The blue regions in the upper C-scan images correspond to significant signal attenuation indicative of internal damage; the sizes of these domains are approximated by the dashed blue lines in the C-scan images. The boundaries of
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these domains (dashed lines) are superimposed on the corresponding photographs of the visible lightning damage. The results show that the intense fiber damage detected by visual inspection clearly underestimate the internal damage detected by TTU C-scan; the regions of the internal
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damage detected by TTU C-scan (represented by blue dotted ellipses) were somewhat larger than those of the intense local damage (illustrated by red ellipses) determined by visual inspection. In
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the figure, R denotes the ratio of the intense local damage area to the internal damage area. R gradually increased as the peak current increased. For instance, R increased from 0.29 at 50 kA nominal peak current to 0.33 at 125 kA nominal peak current, to 0.47 at 200 kA nominal peak current. This suggests possible delamination between skin stacks (or other internal damage) not detectable by visual inspection was present. More clearly, the intense fiber damage by visual
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inspection and the internal damage from C-scan images (with a larger degree of signal attenuation) appear to be more closely aligned in the outermost +45 ̊ ply.
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Concluding Remarks & Future Work
The lightning damage resistance of two NASA Langley Research Center (LaRC) Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) panels was characterized. A series of
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lightning strike tests with nominal 50, 125, and 200 kA peak currents were performed at four representative locations (i.e., the mid-bay, stringer, frame, and frame/stringer intersection).
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Lightning-induced damage to the PRSEUS panels was a strong function of (1) the peak current, (2) the lightning attachment location (mid-bay, stringer, frame, etc.) that involved different Vectran™ structural stitching, and (3) the presence of a surface finish. Lightning damage was grouped into two distinct types: (1) intense local damage occurring in the vicinity of the
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lightning attachment point, where severe fiber rupture, tow splitting, matrix decomposition, delamination, etc. all occur, and (2) mild surface damage occurring in the surrounding region, characterized by smaller scale fiber damage in the form of periodically dispersed tufts of broken
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fibers and some scorching/burning.
The sizes of the regions with both intense local damage and widespread surface damage
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increased as the peak current increased. Greater peak current leads to more Joule heating causing severe damage. The damaged PRSEUS panels exhibited unique lightning damage features due to use of through-thickness Vectran™ stitching and warp-knitted fabrics. Through-thickness Vectran™ stitching was highly effective in mitigating and constraining regions of intense lightning damage. The Vectran™ stitching appeared relatively intact after all tests, even those conducted at high nominal peak currents. In contrast, for attachment locations with no
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underlying stitching (i.e., mid-bay and frame/stringer interconnections), both the size and severity of the resulting damage were greater than for analogous cases where through-thickness Vectran™ stitching was present. The polyester knit threads used in the warp-knitted skin stacks
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appear to influence lightning damage formation in surrounding regions just outside of the lightning attachment point (the area with severe fiber damage, matrix decomposition, etc.). Small clusters of broken fibers with a size and periodic spacing consistent with the polyester warp-knit
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thread architecture were formed in this outer region. Both the through-thickness Vectran™ stitching and polyester warp-knit threads play a beneficial role in reducing the initial strike
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damage size and arresting delamination between the layers. Overall, use of through-thickness Vectran™ stitching, in combination with warp-knitted skin stacks, may profoundly improve the lightning damage resistance of PRSEUS components relative to more traditional (nonreinforced) laminated composite structures. In general, the painted panel exhibited a higher
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degree of visible damage for a given peak current level.
Additional phased-array ultrasonic inspection of the painted and sanded PRSEUS panels is being currently performed at Mississippi State University (MSU) using newly acquired non-
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destructive inspection equipment. Such measurements are coupled with destructive sectioning of both PRSEUS panels at each lightning strike zone to better characterize (1) the through-thickness
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integrity of Vectran™ stitching and polyester warp-knit threads, (2) internal damage morphologies, such as delamination between skin stacks and within plies in a single stack, and (3) the through-thickness thermal matrix damage distribution. Lastly, lightweight lightning protection layers (i.e., copper mesh, carbon fiber paper, graphene paper) are being integrated in the two remaining PRSEUS panels to mitigate damage development.
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Acknowledgements This work was performed in the Marvin B. Dow Advanced Composites Institute at
The Boeing Company (contract # 1188469).
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Mississippi State University. The testing and reporting tasks of this project were fully funded by
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Rakov VA, Uman MA. Lightning: Physics and Effects. Cambridge, United Kingdom: Cambridge University Press; 2003. Lee J, Lacy, T.E., Pittman Jr., C.U. & Mazzola, M.S. Thermal Response of Carbon Fiber Epoxy Composites with Metallic and Nonmetallic Protection Layers to Simulated Lightning Currents. Polymer Composites. DOI: 10.1002/pc.24502, 2017. Lee J, Lacy, T.E., Pittman Jr., C.U. & Mazzola, M.S. Temperature-Dependent Thermal Decomposition of Carbon/Epoxy Laminates Subjected to Simulated Lightning Currents. Polymer Composites. DOI: 10.1002/pc.24535, 2017. Martins RAS. Experimental and Theoretical Studies of Lightning Arcs and their Interaction with Aeronautical Materials: PhD. dissertation, L’UNIVERSITE PARIS-SACLAY; 2016. Clark H. Advanced Development on Vulnerability/Survivability of Advanced Composite Structures. DTIC Document #ADB955855; 1972. Drumm F, Bäuml G, Zischank W, Brocke R, Schönau J. Behaviour of Protected Composite Materials Exposed to Lightning Impulse and Continuing Currents. Proceedings of the 24th International Conference on Lightning Protection, Birmingham, UK1998. MatWeb. Kuraray Vectran® HT 1500/300 LCP Fiber Material Property Data. 2014. Johnston PH. Ultrasonic Nondestructive Evaluation of PRSEUS Pressure Cube Article in Support of Load Test to Failure. NASA/TM-2013-217799, NASA; 2013. Johnston PH, Juarez PD. Ultrasonic Nondestructive Evaluation of Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) During Large-Scale Load Testing and Rod Push-Out Testing. NASA/TM-2016-218978, NASA; 2016.
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Table 1. Material, thickness, and layup of LaRC PRSEUS sub-components [6, 7]. Material AS4/VRM-34a
Exterior Skin Tear Straps Frame Stringer Foam Core
Thickness (mm)
Stack A/Stack A
2.64
AS4/VRM-34
a
Stack A
1.32
AS4/VRM-34
a
Stack A/Stack A
2.64
AS4/VRM-34
a
Stack A
1.32
-
12.70
Rohacell 110WF
Rod
Layupc
T800/3900-2B
b
-
a
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Part
9.53 (dia.)
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Warp-knitted AS4 carbon fiber fabric and VRM-34 epoxy resin processed via resin infusion. b Pre-cured unidirectional T800 fibers with a 3900-2B epoxy resin. c Stack A = [+45/-45/0/0/90/0/0/-45/+45].
Table 2. Lightning strike tests on PRSEUS panels. Target Peak Current (kA)
Stringer
1
1
3
3
3
3
12
3
3
-
-
6
2
2
-
-
4
50
3
2
1
1
7
125
2
2
1
2
7
200
1
1
-
-
2
50a Sanded Panel
50 125
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200
Painted Panel
Number of Tests Frame/Stringer Frame Intersection -
Mid-bay
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Panel Type
Total Number of Lightning Strike Tests
a
Initial calibration tests.
Total 2
40
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Figure 1. Schematic of PRSEUS concept [7].
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Figure 2. (a) Schematic of multiaxial warp-knitted fabric [17] and (b) photograph of SAERTEX multiaxial warp-knitted dry carbon fiber fabric [18].
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Figure 3. Single-sided stitching for the PRSEUS concept: (a) frame/stringer intersection (adapted from [21]), (b) flange-to-skin stiches [21], (c) single-sided stitch seam (adapted from [8]), and (d) a complete flat PRSEUS panel preform after stitching (adapted from [8]).
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Figure 4. Electrical grounding condition along the edges of the sanded PRSEUS panel: (a) IML view and (b) OML view. The OML skin is slightly sanded to minimize possible fiber damage, while all four edges are fully sanded for better contact with the aluminum brackets.
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Figure 5. SAE waveform component A [16] and MSU-HVL impulse current waveform (200 kA peak current)
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Figure 6. Two lightning damage categories observed at the sanded PRSEUS mid-bay location subjected to 200 kA nominal peak current.
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Figure 7. Typical lightning damage at the unstitched mid-bay locations induced in sanded (left) and painted (right) PRSEUS panels subjected to (a, b) 50 kA, (c, d) 125 kA, and (e, f) 200 kA nominal peak currents. Measured peak currents are included for clarity. A* represents the area of intense local damage normalized by that for the painted panel subjected to 200 kA mid-bay strike.
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Figure 8. Typical lightning damage at the stitched stringer locations induced in sanded (left) and painted (right) PRSEUS panels subjected to (a, b) 50 kA, (c, d) 125 kA, and (e, f) 200 kA nominal peak currents. Measured peak currents are included for clarity. A* defines the area of intense local damage normalized by that for the painted panel subjected to 200 kA mid-bay strike.
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Figure 9. Typical lightning strike damage to the painted panel subjected to 125 kA nominal peak currents at (a) mid-bay, (b) stringer, (c) frame, and (d) frame/stringer intersection locations. Measured peak currents are included for clarity. A* corresponds to the area of intense local damage normalized by that for the painted panel subjected to 200 kA mid-bay strike.
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Figure 10. Normalized intense local damage area, A*, versus the measured peak current.
Figure 11. TTU C-scan images (top) and photographs (bottom) of lightning damage observed at painted panel mid-bay locations subjected to: (a) 50 kA, (b) 125 kA, and (c) 200 kA. R denotes the ratio of the intense local damage area to the internal damage are from C-scan signal attenuation.