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Experimental investigation of adhesive fillet size on barely visible impact damage in metallic honeycomb sandwich panels
Composites Part B 184 (2020) 107723
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Experimental investigation of adhesive fillet size on barely visible impact damage in metallic honeycomb sandwich panels Patrick Kendall a, Mengqian Sun a, Diane Wowk b, Christopher Mechefske a, Il Yong Kim a, *, 1 a b
Queen’s University, Canada Royal Military College, Canada
A B S T R A C T
Aluminum hexagonal honeycomb panels are commonly used in the aerospace industry to reduce weight due to their high stiffness to mass ratio. The panels are commonly involved in incidents where they are dented in the out-of-plane direction which causes plastic deformation in the face-sheet and buckling collapse of the thin repeating cell-walls in the core. This paper investigates the responses to barely-visible-impact-damage (BVID) in aluminum honeycomb sandwich panels in the out-of-plane direction with attention to the structural adhesive. The structural adhesive forms a fillet shape between the face-sheet and the aluminum core during the curing process and in some cases can encompass over 50% of the honeycomb core thickness. The adhesive fillets become stiff after curing and are able to brace the thin metallic cell-walls and prevent buckling in sections of the core enclosed in adhesive. It was shown that larger fillets cause the damage to occur deeper in the core. Force-displacement data collected from quasi-static experiments showed that as the amount of adhesive used in honeycomb panels was increased, the peak force required to produce a specified maximum dent depth increased as well. Absorbed energy positively correlated with an increasing quantity of adhesive; showing improvements of up to 50% when comparing panels with the largest amount of adhesive and no adhesive. This paper provides relationships between the quantity of adhesive used to fabricate metallic honeycomb sandwich panels and the damage resistance and energy absorption under BVID conditions.
1. Introduction Honeycomb sandwich panels are commonly used in the aerospace industry for light-weighting due to their high stiffness to mass ratio [1, 2]. With growing pressure to reduce fuel consumption in the commercial aircraft industry due to environmental concerns, airline companies have evolved to use advanced composite structures like honeycomb panels to reduce mass and thus reduce carbon emissions and operating costs [3]. Honeycomb panels are made up of two face-sheets (typically made of aluminum, carbon fibre or fiberglass) adhered to a hexagonal core (typically constructed using aluminum or Nomex) with structural ad hesive. The structural adhesive is applied as a sheet, but forms a fillet between the core and face-sheet as it is heated during the curing process. The fillet shape is recommended to be large in the aerospace industry for promoting a stronger bond between the face-sheet and core [1,2]. Honeycomb sandwich structures are stiff in bending but are sus ceptible to out-of-plane indentations caused by such events as dropped tools, hail storms or runway debris. These indentations and the under lying damage to the core can reduce the structural integrity of the panels [4]. This has resulted in a large amount of research focused on the pa rameters that affect the out-of-plane stiffness of honeycomb panels
during impact such as the impactor diameter, cell size, cell-wall thick ness, face-sheet thickness, material and core density [5–9]. It has also been shown that the presence of the adhesive fillet can influence the response [10–13], but an experimental investigation specifically considering the size of the adhesive fillets has not yet been reported. The current paper considers the amount of adhesive used during panel fabrication and the effect of the resulting adhesive fillet size on the out-of-plane response during low-velocity impacts. This will be assessed by performing quasi-static indentation on sandwich panels with varying sizes of adhesive fillets, and examining the resulting load curves, surface damage measurements and internal core damage. In the aerospace in dustry, quality control during the manufacturing process is of primary importance, and the size of the adhesive fillet is a feature of the panel that varies, but can also be controlled to some extent. For example, a survey of 58 fillets from a retired aircraft sandwich panel performed by the author revealed overall fillet heights between 0.48 mm and 1.45 mm. The fillet size was also observed to vary between panels, regions within a panel as well as between adjacent cells. It is therefore important to understand the effect that variations in fillet size have on the per formance of the panel during operational loading as well as during impact events.
* Corresponding author. Queen’s University, 99 University Ave, Kingston, ON, K7L 3N6, Canada. E-mail addresses: [email protected] (P. Kendall), [email protected] (I.Y. Kim). 1 Royal Military College, 13 General Crerar Crescent, Kingston, ON, K7K7B4, Canada. https://doi.org/10.1016/j.compositesb.2019.107723 Received 25 October 2019; Received in revised form 11 December 2019; Accepted 16 December 2019 Available online 17 December 2019 1359-8368/Crown Copyright © 2019 Published by Elsevier Ltd. All rights reserved.
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The damage that is of interest in this study is barely-visible-impactdamage (BVID), which refers to dent depths smaller than 1.0 mm [14]. BVID is believed to lower the structural integrity of the honeycomb panel, but commonly goes unnoticed during inspection given the small nature of the dents. A review of low-velocity impact on sandwich structures was conducted by Chai and Zhu [15] in 2011 which outlined many factors such as cell density, cell-wall thickness, face-sheet thick ness, the material used, and the out-of-plane stiffness of the core and how they affect the low-velocity impact response. Researchers such as Clarke [16] used finite element analysis (FEA) to explore the internal buckling damage as well as the energy absorbed in aluminum honey comb panels as well as a parametric study on the factors affecting their response to low-velocity impacts. He found that as the cell size was decreased and as the cell-wall thickness was increased, the dent depth decreased. As the face-sheet thickness increased, the kinetic energy absorption decreased as well as the dent depth. Other forms of research that are related to low-velocity impacts are parametric studies that explore the uniform out-of-plane compression response of honeycomb core. Ashab et al. [17] performed experiments on aluminum honeycomb core and altered the core density between tests by changing the cell-wall thickness and the cell size. By representing the parameter as a ratio of thickness of the cell-wall over cell size, a result of higher energy ab sorption was concluded as the ratio increased. Chai and Zhu [15] identified in their review that primary factors that affect low-velocity impact response are the face-sheet thickness, the core’s out-of-plane compression response and the shear properties of the honeycomb core [18]. The review of the current literature has highlighted an interest in the many parameters that can play a role in the resistance to out-of-plane damage from spherical indenters or uniform compression. The scientific community has identified the effects of basic aspects like cell-wall thickness, cell size or face-sheet thickness, however, the ad hesive fillets that join the face-sheet and core can also have an effect and they have not been widely considered in literature. While panel characteristics such as the face-sheet thickness and core dimensions have been extensively studied, the effects of the adhesive joining the face-sheet to the core has received less attention. In many cases, numerical simulations ignore the adhesive all together [19–23]. Those that include the adhesive typically represent it as a layer and assume that the fillet geometry itself has no effect on the out-of-plane response of the panel to low-velocity impact [11,13]. Zhang et al. [11] performed experimental testing as well as computational simulations to highlight the key parameters that affect energy absorption given a low-velocity impact with a metallic sphere on an aluminum honeycomb panel. This paper included a study of the adhesive layer between the honeycomb core and the aluminum face-sheets. It was found that the presence of adhesive increased the energy absorption by 6.9%. Mah moudabadi et al. [13] performed a parameter study on the energy ab sorption of honeycomb panels subject to quasi-static punch loading by altering the cell-wall thickness, core thickness, face-sheet thickness and the presence of an adhesive layer. The conclusions were that increasing the face-sheet thickness by 0.4 mm increased the absorbed energy by 45%, and that the presence of adhesive increased the energy absorption by 12%. When the core thickness was doubled so was the absorbed energy and when the cell-wall thickness was increased, the absorbed energy increased by 10%. This study included testing with and without adhesive, but did not vary the amount of adhesive or the fillet size be tween tests. The most notable research involving structural adhesive in low-velocity impacts was by Benotto et al. [4] where the adhesive fillets were identified and an FE model was created to show how the core damage depth occurred deeper within the panel when the adhesive was included. Heimbs et al. [12] studied the effects of the presence of ad hesive fillets on the stress-strain response in various directions while building a homogeneous material model and claimed that the adhesive fillets had a negligible effect with the exception of the in-plane direction which increased by a factor of ten. However, this paper did not give details on the procedure used to draw the conclusion on the out-of-plane
effects. The finding conflicts with Umeda et al. [24] as well as Castanie et al. [25] who found that adhesive fillets alter the boundary condition at the top and bottom of the cells which results in significantly higher peak stresses in uniform out-of-plane compressions when they are included. Kashavanarayana et al. [26] modeled the geometry of the nodal adhesive fillets found between double cell-walls in honeycomb core from the expansion process and noted that the in-plane compres sion response became more stiff and corresponded better with experi mental results as the fillet size was increased towards its measured dimension. Kendall et al. [27] found that the presence of adhesive in creases the peak stress and introduces a drop in load carrying capacity between the peak and plateau stress for uniformly loaded panels. They also found that as the quantity of adhesive was increased, more of the cell-walls were braced when embedded in adhesive, thus lowering the densification strain. The commonality in the literature review showed that while adhesive may play an important role in the out-of-plane behavior of honeycomb panels, specific studies dealing with the size of the adhesive fillets during low-velocity impact and BVID scenarios have not been specifically considered. This paper aims to present the effects of structural adhesive fillets on the damage resistance and energy absorption of aluminum honeycomb panels through experimental testing. The study begins by identifying the key difference in the out-of-plane denting response given no adhesive compared with a thin layer of adhesive in an aluminum honeycomb panel. The focus will then shift to the size of the adhesive fillet, to observe differences between panels manufactured with three different amounts of adhesive. The comparisons will be presented in the form of force and displacement curves as well as surface measurements via 3D scanner and internal damage measurements from destructive crosssectioning. 2. Materials and methods The study considering adhesive fillet size was conducted by quasistatic indentation of panel coupons with four different adhesive di mensions at three different dent depths. Each test was repeated three times resulting in a total of 36 manufactured panels being tested. The panels were made with zero, one, two, and three layers of adhesive to alter the adhesive fillet sizes between the core and face-sheets. A single adhesive layer was 0.23 mm thick resulting in panels with uncured adhesive thicknesses of 0 mm, 0.23 mm, 0.46 mm and 0.69 mm. These panels are referred to as Manufactured Panels zero through three (MP0, MP1, MP2 and MP3) respectively as shown in Fig. 1. The MP0 panels were made with adhesive around the perimeter of the coupon only and not in the central contact area where the dents would be created, in order to maintain similar boundary conditions between tests. Force and displacement data were recorded for each test, as well as surface profile dimensions such as dent diameter, dent area, dent volume and internal core damage metrics such as depth of damage, and diameter of damage from destructive cross-sectioning. 2.1. Specimens The honeycomb panels were manufactured by adhering two metallic face-sheets to either side of a metallic honeycomb core. The material properties for the honeycomb face-sheet (7075-T6) and core (5052-H34) used in the experiments are summarized in Table 1. The honeycomb core is 12 mm thick (L) with a cell-wall length (l) of 1.83 mm and a cell-wall thickness (t) of 0.02 mm (Fig. 2 (c)). Each specimen was cut to di mensions of 80 mm length by 80 mm width based on studies conducted by Benotto et al. [4] on the minimum coupon dimensions to negate boundary condition effects. When the adhesive undergoes curing to secure the face-sheets to the honeycomb core, an adhesive fillet is formed due to the drop in viscosity caused from the 120 � C curing temperature and surface tension of the adhesive. The side profile of the resulting fillet can be fit to an elliptical 2
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Fig. 1. Cross-section of a manufactured panel coupon from each sample set prior to BVID, where MP0 has no adhesive and MPX represents manufactured panels with X layers of adhesive.
are highlighted on an illustration in Fig. 2 (b) which can be compared to the real world fillet shown in Fig. 2 (a). The panels were manufactured following the procedure described by Rion et al. [28] to cure panels with large fillet sizes. The adhesive used was an aircraft grade adhesive film with reference code AF 163-2 K. This � adhesive requires a ramp-up temperature of 1 C/min to a final curing � temperature of 120 C at which it is maintained for 60 min. The curing process was carried out under vacuum on a heat bed where the face-sheet was placed on the bed first, and the adhesive and core were placed on top. The process outlined by Rion et al. [28] describes a preheat phase that allows the adhesive to experience a drop in viscosity and migrate up the cell-walls (due to reduced viscosity and the effect of
Table 1 Material Properties for the honeycomb core and honeycomb face-sheets. Elastic Modulus Yield Strength Poisson’s Ratio
Aluminum 7075-T6 (Face-sheet)
Aluminum 5052-H34 (Core)
71 GPa 440 MPa 0.33
71 GPa 345 MPa 0.33
shape between the cell-walls and face-sheet. The dimensions needed to fit the elliptical fillet include the fillet height (hf ), width (wf ), adhesive thickness (tA ) and overall height of the adhesive (hA ). These dimensions
Fig. 2. (a) Manufactured honeycomb panel with an elliptical adhesive fillet (shown in red) (b) Idealized adhesive fillet geometry, (c) Honeycomb core dimensions with double cell-walls highlighted in blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. Average adhesive dimensions for every coupon tested. (a) adhesive thickness ðtA ), (b) adhesive fillet width ðwf ) and (c) overall adhesive height (hA ). 3
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surface tension) to form a fillet shape during the curing phase. A fillet shape is desired in honeycomb panel manufacturing to strengthen the bond between core and face-sheet and is sought after in industry [1,2]. The curing procedure described above and the progressively larger amounts of adhesive used, resulted in the panels found in Fig. 1. Fig. 3 shows the measured adhesive fillet dimensions for 27 manufactured coupons while Table 2 presents the averaged values for each set (MP1, MP2, MP3). The resulting dimensions from Fig. 3 were recorded as an average of 10 fillets per coupon, while Table 2 is the average of over 90 measurements for each set of coupons. Fig. 4 indicated that as more uncured adhesive layers were used, the thickness of cured adhesive (tA ) and width of the fillets (wf ) increase linearly, while the overall adhesive height (hA ) increases at a higher rate. The mass and volume were measured between panels to calculate the density and is shown to in crease linearly as more adhesive is used in Fig. 4 on the secondary axis. All of the fillet dimensions increased when more adhesive was used and it was not possible to independently control the height, width or thickness. As a result, the study focused on the overall changes in fillet size that result from using more adhesive.
Fig. 4. Average adhesive dimensions for each data set on the left vertical axis and the average density of each data set on the right vertical axis.
impact [29]. After the insert reached the specified maximum displace ment it was retracted from the panel in the opposite direction while still measuring the contact force. This recorded the spring-back phase, which can be compared between tests. Fig. 7 shows the experimental set-up used to measure force and displacement.
2.2. Damage measurement Surface profiles of the dents were obtained by using 3D laserscanning. The resulting point-cloud of the deformed surface was used to measure dimensions such as dent depth, dent diameter, dent area, and dent volume. Fig. 5 provides a visual representation of these dimensions. The profile was captured with a Creaform Handy Scan 700 3D-scanner that produced 3D representations of the surface to within 0.03 mm error. The 3D representation of the deformed surface was compared to a reference plane to obtain a colour contour of the dent, which is shown in Fig. 5 (a). This method was evaluated and tested by Reyno et al. [14] as an effective process for quantifying surface damage in aerospace sand wich panels. The internal damage was measured from cross-sectional images created through destructive sectioning. This involved the cross-sectioning of the coupons using a CNC (computer numerical con trol) diamond blade saw. Fig. 6 shows a cross-sectioned panel with the key dimensions of core damage diameter, core damage depth and core damage thickness highlighted. The depth of core damage was measured on the cell-wall that had the deepest buckling folds. This was typically found at the cell-walls directly below the centre of the indentation. The thickness of core damage was measured between the deepest buckling fold and the tip of the fillet.
2.4. Experimental procedure The 36 coupons underwent displacement controlled compressions using the spherical insert on the Instron machine at three different maximum dent depths. The three dent depths fall within the lower and upper bounds of BVID criteria. The maximum dent depths used were 1.0 mm, 1.5 mm, and 2.0 mm which resulted in residual dent depths of approximately 0.5 mm, 0.8 mm and 1.0 mm. The resulting force versus displacement curves will be compared between tests to highlight the effects of adhesive fillet size on the out-of-plane stiffness of the panels. The dent diameter, dent area, dent volume, and internal damage will also be assessed and compared to draw conclusions on the effects of adhesive with respect to BVID in aluminum honeycomb panels. 3. Results and discussion This study presents the force-displacement relationships between panels with different adhesive fillet sizes. The damaged panels were also scanned to collect measurements for dent area, dent volume, and dent diameter. They were then cross-sectioned to inspect the internal damage and measure the adhesive dimensions in and around the dent based on Fig. 1 (b). Each unique test was repeated three times to ensure repeat ability and a representative curve from each data set was chosen to present the results. The factors will be discussed that affect low-velocity impacts or, in this case, quasi-static denting, are the out-of-plane stiff ness of the core, the flexural stiffness of the face-sheet and the shear properties of the core.
2.3. Experimental apparatus A spherical indenter insert was designed and manufactured in-house to be used with the Instron Mechanical Testing Machine. The panel was supported by a two-inch-thick steel base while a 63.25 mm diameter spherical impactor, made of steel, contacted the top face-sheet to create a dent. The steel sphere was displaced into the top face-sheet of the panel at a rate of 0.5 mm/min to maintain a quasi-static condition. Quasi-static conditions have been known to produce the same damage state in metallic honeycomb panels as low-velocity impacts with large mass impactors [29], while producing a more repeatable resulting dent. The indenter used was 1 kg, which meant the ratio of impactor mass to coupon mass was well over 10, which is the criteria for a large mass
3.1. Force and displacement In order to first highlight the effect that the presence of adhesive has on impact resistance, comparisons of results from panels that were manufactured with and without adhesive (MP0 and MP1) were studied. The average overall fillet height of 0.87 mm from set MP1 with one layer of adhesive is representative of the fillets that are typical in aircraft, based on cross-sectioning retired panels. The force versus displacement results are shown in Fig. 8. The slope of the MP0 panels was linear for the duration of the test, while the MP1 panels show a bilinear response. The initial slope in the MP1 coupons between 0 and 0.2 mm was six times larger than the MP0 coupons without adhesive. The peak force values
Table 2 Averaged adhesive dimensions measured from cross-sectioned panels. MP1
MP2
MP3
tA ðmmÞ
0.12
0.24
0.47
hf ðmmÞ
0.75
1.15
2.11
wf ðmmÞ
0.49
0.85
1.09
hA ðmmÞ
0.87
1.39
2.59
4
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Fig. 5. Out-of-plane deviations (mm). (a) Scan ner results used for measuring surface damage where green indicates that the deviations from the reference plane fall within the 0.03 mm threshold and the progressively larger blue col ours indicate larger out-of-plane deviations. (b) Dent depth illustration, (c) dent diameter illus tration, (d) dent area illustration and (e) dent volume illustration. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Destructive inspection measurements of core damage for cross-sectioned honeycomb panel.
spring-back in the MP1 coupons with adhesive remained constant at approximately 60%. This was expected because the honeycomb core and the aluminum face-sheet were not attached in the MP0 coupons and will each spring-back at different magnitudes. The MP1 coupon face-sheets were prevented from fully springing back as they were attached to the crushed core and do not spring-back as much as the face-sheet. Past research has consistently shown that the face-sheet springs-back more than the crumpled core and this study supports that statement [16], [30–32.] It was observed in this study that for a given maximum dent depth, coupons with adhesive result in deeper residual dents than the coupons without adhesive. Fig. 9 shows a comparison between the cross-sectioned panels of an MP0 coupon and an MP1 coupon. The buckled core in the MP0 sample begins at the contact point between the face-sheet and core (top of the cell-walls), while the MP1 sample begins buckling below the adhesive fillet. These findings correlate well with Kendall et al. [27] who performed uniform out-of-plane compression testing on both honeycomb core in isolation and honeycomb panels with adhesive. The referenced study concluded that honeycomb panels with adhesive require a higher force to initiate plastic collapse, which occurs at random locations along the height of the panel’s core, while buckling always began at the top or bottom of the cell-walls when honeycomb core was compressed in isolation. The presence of adhesive in the panels introduces three key differ ences as compared to the panels with no adhesive. The ends of the cellwalls that are embedded in the adhesive are constrained from rotating as
Fig. 7. Experimental set up for creating out-of-plane BVID on the panels.
from the 1 mm, 1.5 mm and the 2 mm maximum dent depths for the MP1 coupons were on average, 6.1% higher than the MP0 samples. The percentage of spring-back measured from the MP0 coupons without adhesive increased as peak force increased while the percentage of 5
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Fig. 8. (a) Force versus displacement results collected from the Instron machine given a quasi-static spherical dent. The figures show a representative curve for each maximum dent depth. (b) A plot of the force and displacement data of the MP0 and MP1 panels with a focus on the initial slopes in the data at the beginning of the test.
Fig. 9. Internal damage for a 1.5 mm maximum dent depth comparing (a) MP0 with no adhesive and (b) MP1 with adhesive.
opposed to being free to rotate, and a higher load is required to initiate buckling in the core. Kendall et al. [27] showed that when comparing panels made with adhesive to isolated honeycomb core, they had higher peak stresses when uniformly compressed. The second difference is that the adhesive effectively thickens the face-sheet resulting in a higher load requirement to displace the surface to the maximum prescribed depth. The third difference is that the adhesive connection between the core and the face-sheet enables a shear load-transfer which occurs as the face-sheet is dented [18]. All three of these conditions cause the panels with adhesive to have a stiffer response to out-of-plane deformation during the first 0–0.2 mm of displacement. Kendall et al. [27] deter mined that the peak stress for a panel with adhesive under uniform compression occurred at strains on the order of 0.2 mm of out-of-plane displacement. Since peak stress is an indication of the onset of insta bility, the change of slope between 0.1 and 0.2 mm can be attributed to the initiation of cell-wall buckling in the first cells below the indenter.
To highlight the effects of adhesive fillet size on the out-of-plane stiffness, panels with increasing thicknesses of uncured adhesive (MP0 – 0 mm, MP1 - 0.23 mm, MP2 - 0.46 mm and MP3 - 0.69 mm) were indented and the force and displacement data for one representative curve was plotted for each maximum dent depth. Fig. 10 shows that the peak force increased by an average of 11.4% with increasing volumes of adhesive from MP1 to MP3. The adhesive effectively stiffens the panel as more force is required to indent it to the same depth when more layers are used. Each panel with adhesive shows a bilinear response, with a higher slope in the first 0–0.2 mm of displacement as more adhesive is used. The force at the transition from the initial linear response was measured for each coupon and there was an increase of 56% between MP1 and MP3. This portion of the curve can be seen in greater detail in Fig. 11 (b). The spring-back displacement percentage was constant at approximately 60% between all coupons with adhesive regardless of the peak force and adhesive amounts. The springback was affected by
Fig. 10. Force and displacement results from quasi-static indentation at three different maximum dent depths, (a) 1 mm, (b) 1.5 mm and (c) 2 mm. The three plots include all four coupons sets.
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Fig. 11. (a) All data sets plotted together for each maximum dent depth. (b) All data sets plotted together highlighting the first 0.5 mm of displacements.
whether or not adhesive was present, but was independent of the size of the adhesive fillets. Fig. 11 (a) provides a plot of all the data sets pre sented together. From this study on how the size of the adhesive fillets alters the damage resistance in honeycomb panels, some explanations are pro posed. A key parameter that affects the behaviour of honeycomb panels with respect to impact resistance is the face-sheet stiffness. Considering the stiffness of the structural aerospace adhesive used, adding more adhesive to the bottom side of the face-sheet increases the flexural ri gidity. This in turn, increases the force needed to deform the face-sheet and cause buckling in the core, as evident at 0.2 mm. By increasing the amount of adhesive used in fabrication, the initial out-of-plane stiffness can be increased, meaning that the honeycomb panel becomes more resistant to BVID, which has not been highlighted in past research ef forts. All the panels showed the same stiffness once past 0.2 mm of displacement, which is when more cell-walls start to buckle as the dent diameter increases. Buckling is initiated more easily as the loading through the cell-wall is not strictly axial due to the rotation of the facesheet as it is depressed. In addition, the plasticity in the face-sheet is spreading over a larger area leading to the out-of-plane panel stiffness being independent of the amount of adhesive used.
Fig. 12. Work done on honeycomb panels with different fillet sizes dented at different maximum dent depths.
3.2. Absorbed energy
uncured adhesive, the only conclusive statement that can be made is that when more adhesive is used to manufacture honeycomb panels, the panel can absorb more energy given BVID. One should note that these improvements in energy absorption come at the cost of a higher density panel and therefore panels with a larger mass. A calculation of ratio of energy absorbed to panels density was done for the 1 mm dent depth results to show that each panel shares the same value of 0.85 when it comes to energy absorption per unit density. Future efforts should focus on simulations including the adhesive geometry and isolating an in crease in height, width and thickness of the fillet to identify the sensi tivity of the absorbed energy to individual dimensions.
The force and displacement results provided a clear picture of damage resistance and out-of-plane stiffness in honeycomb panels with different adhesive geometries. Another commonly studied metric is the energy absorbed during impact, which can be determined by calculating the area under the loading curve to assess both the plastic and elastic energy in the system, and subtracting the area under the unloading curve where the indenter is retracted from the panel. This represents the majority of the elastic energy leaving the system. In the coupons where adhesive was used, not all of the elastic energy was released following unloading. This was evident from Fig. 9 (a) where the face-sheet springsback farther than the crumpled core in cases where no adhesive was used. This indicates that the panels with face-sheets that are connected to the core via adhesive will find an equilibrium between the core and face-sheet where the core will resist the face-sheet from rebounding to its desired shape and the face-sheet will apply tension to the core. The amount of absorbed energy is shown to increase when more adhesive is present as illustrated in Fig. 12. Absorbed energy values increased by 34% between MP1 and MP3 due to the increase in face-sheet stiffness when larger volumes of adhesive are used in manufacturing. By adding additional layers of adhesive film to create larger adhesive fillets, each of the fillet dimensions were gradually increased from MP1 to MP3. Since each fillet dimension increased with a thicker layer of
3.3. Surface damage The surface profile from the 3D scanner results provided measure ments for dent diameter, dent area, and dent volume. The results do not show measurements for the MP0 panels due to the disconnect between the face-sheet and the core, which resulted in surface profiles that did not provide a clear image of the overall damage in the panel. Although higher peak forces were required to reach the maximum dent depth for the panels with more adhesive, the surface profile remained the same between all panels. This means that the residual dent depth, dent diameter, dent area, and dent volume are independent of the amount of 7
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Fig. 13. Surface damage measurements as a result of the 3D scanner. (a) Dent diameter, (b) dent area, and (c) dent volume.
adhesive for a given maximum dent depth, even though more adhesive results in a higher resistance to BVID. Fig. 13 shows the measurements of 27 panels with adhesive and are grouped according to the maximum dent depth prescribed.
than when it is attached to the face-sheet. The size of the adhesive fillet had no effect on the diameter of the damage. It was shown that the surface profile of the dent was not affected by the amount of adhesive and since the core and the face-sheet are tied together, the diameter of the core damage is also not affected. The internal damage measurements highlighted a key finding from this study. The core damage depth is dependant on the overall adhesive height and increases as more adhesive was used. This is because the adhesive fillets enclosing the thin cell-walls are relatively stiff and can prevent the cell-walls from buckling. This causes the initial buckling to begin directly below the tip of the adhesive fillet. Therefore, if the overall adhesive height is increased, the first buckle is formed further into the core and propagates deeper into the core. The effect of the core damage depth and the location of buckling initiation on the residual strength of honeycomb sandwich panels is currently unknown and re quires further study.
3.4. Internal damage Finite element simulations have shown that the depth of the damage in the core remains constant as the dent depth increases within the BVID criteria [16], however this was for simulations of idealized panels without adhesive. Fig. 14 shows that by increasing the size of the ad hesive fillet, and as a result the overall height of the adhesive, the initialization of cell-wall buckling is shifted deeper into the core, which increases the depth of core damage. This is shown in Fig. 14 (a) where the MP0 coupons without adhesive have core damage occurring closer to the panel surface. As the overall height of the adhesive increases from MP1 to MP3, the core damage occurs deeper in the panel (Figs. 15 and 16). Fig. 15 shows the average of each sample to highlight the increasing trend in the depth of core damage and the height of adhesive. The thickness of damage was found to have a slight upward trend as the adhesive size was increased, but only in the 1 mm and 1.5 mm cases (Fig. 14 (b)). The diameter of damage (Fig. 14 (c)) was found to be constant amongst all tests given the same maximum dent depth, with the exception of MP0 measurements, which were lower than the panels made with adhesive. Without adhesive, the core and the face-sheet can deform independently so the deformation in the core is more localized
4. Summary and conclusions Three key findings emerged from the experimental measurements for panels with different adhesive fillet sizes. It was shown that panels fabricated with more adhesive required higher peak forces to produce the same maximum dent depth. The increased stiffness also resulted in the absorbed energy increasing with larger adhesive fillet sizes. When the damaged panels were cross-sectioned, it was found that as more adhesive was used to fabricate panels and the overall height of the
Fig. 14. Measurements of internal damage from cross-sectioning panels. (a) the depth of core damage, (b) the thickness of core damage, and (c) the diameter of core damage.
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evidence that the depth of core damage is however, directly related to the overall height of adhesive fillet. It was shown in the resulting cross-sections that the initiation of buckling damage in the core occurred directly below the adhesive fillets due to a bracing effect from the ad hesive on the thin metallic cell-walls that resisted buckling collapse. This shows that a realistic representation of the internal adhesive fillets is required in order to predict the extent and depth of core damage in honeycomb sandwich panels. There are no effects of the adhesive on surface damage given a prescribed displacement and the surface damage will relax at the same residual dent given any amount of adhesive. Although the adhesive fil lets can have a stiffening effect on the face sheet, it does not affect the final, sprung-back surface given a maximum prescribed dent. This finding is important because the panels in service are only measured based on their surface damage, which we now know cannot be directly compared to the internal damage without information on the adhesive fillets. Analytical models and numerical simulations are commonly used to predict the response of honeycomb panels to low-velocity impact. In many cases, the adhesive is not represented to maintain simplicity and reduce computational efforts. It has been shown however, that the size of the adhesive fillet does have an effect on the out-of-plane stiffness, the energy absorption and the core damage. It is therefore required to incorporate the adhesive fillet geometry particularly when considering durability and residual strength of dented aerospace panels. Quality control is a high priority in the manufacturing of aerospace components and the adhesive fillets found in aircraft panels can be manufactured in many different heights, widths and thicknesses. The results from the current study provide a further understanding of the effects of variations in adhesive fillet size on the out-of-plane stiffness of honeycomb sand wich panels as well as their reaction to low-velocity impact events producing BVID.
Fig. 15. Average core damage depth given all three maximum dent depths compared with the increasing adhesive dimensions from MP0 to MP3.
adhesive fillet increased, the core damage propagated deeper into the honeycomb core. As the damage resistance and buckling failures are changed between tests, the surface profile of the residual dent remained constant, therefore it was concluded that the dent depth, diameter, area and volume is independent of the size of adhesive fillets. This finding also means that the percentage of spring-back displacement was con stant between all tests with adhesive at 60% meaning it is independent of the amount of adhesive used as well. When more layers of adhesive were used to manufacture the panels, the thickness, the width, and the height of adhesive fillets increased as well. This in turn increased the out-of-plane stiffness during the initial 0.2 mm of displacement and increased the peak force as evident in the force versus displacement data (Fig. 11 (a), (b)). The increased amount of adhesive effectively increases the face-sheet thickness and also sta bilizes the top of the cell-walls from buckling. The peak force was increased by 17% between no adhesive and three sheets of adhesive film. The amount of absorbed energy was also found to increase with the adhesive fillet size. The amount of absorbed energy increased by 34% when comparing the MP1 and MP3 panels. This is caused by an increase in the face-sheet stiffness given more adhesive, which can be seen in the first 0.2 mm of displacements. The results of the current study show that the size of the adhesive fillet must be considered for accurate predictions of energy absorption during low-velocity impacts of honeycomb sand wich panels. Past simulations of honeycomb panels without adhesive have shown that the depth of core damage is constant for all dent depths falling within the BVID category [16,33]. This paper provides conclusive
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Patrick Kendall: Methodology, Validation, Formal analysis, Inves tigation, Writing - original draft, Visualization. Mengqian Sun: Conceptualization, Methodology, Formal analysis, Writing - review & editing. Diane Wowk: Conceptualization, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Christopher Mechefske: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition. Il Yong Kim: Conceptualization, Resources, Writing - review & editing, Supervision.
Fig. 16. Cross-sections from each coupon set showing an increase in the internal depth of core damage, indicated by the black lines, given a maximum dent depth of 1 mm.
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Acknowledgements
[13] Zarei Mahmoudabadi M, Sadighi M. Experimental investigation on the energy absorption characteristics of honeycomb sandwich panels under quasi-static punch loading. Aero Sci Technol 2019;88:273–86. [14] Reyno T, Marsden C, Wowk D. Surface damage evaluation of honeycomb sandwich aircraft panels using 3D scanning technology. NDT E Int 2018;97:11–9. March. [15] Chai GB, Zhu S. A review of low-velocity impact on sandwich structures. Proc Inst Mech Eng L J Mater Des Appl 2011;225(4):207–30. [16] Clarke LGC. Characterization of low velocity impact damage in metallic honeycomb sandwich aircraft panels using finite element analysis. M.S. Thesis. Royal Military College of Canada; 2017. [17] Ashab ASM, Ruan D, Lu G, Xu S, Wen C. Experimental investigation of the mechanical behavior of aluminum honeycombs under quasi-static and dynamic indentation. Mater Des 2015;74:138–49. [18] Hoo Fatt MS, Park KS. Dynamic models for low-velocity impact damage of composite sandwich panels -Part B: damage initiation. Compos Struct 2001;52 (3–4):353–64. [19] Aminanda Y, Geter A, Sutjipto E, Adesta EYT, Castanie B. Simulation of compression and spring-back phenomena of sandwich structure with honeycomb core subjected to low energy and low velocity impact. 2011. p. 1296–301. [20] Chen Y, Hou S, Fu K, Han X, Ye L. Low-velocity impact response of composite sandwich structures: modelling and experiment. Compos Struct 2017;168:322–34. [21] Foo CC, Seah LK, Chai GB. Low-velocity impact failure of aluminium honeycomb sandwich panels. Compos Struct 2008;85:20–8. [22] Liu L, Feng H, Tang H, Guan Z. Impact resistance of Nomex honeycomb sandwich structures with thin fibre reinforced polymer facesheets. 2018. [23] Xie Z, Zhao W, Wang X, Hang J, Yue X, Zhou X. Low-velocity impact behaviour of titanium honeycomb sandwich structures. 2018. [24] Umeda T, Mimura K. Effects of boundary condition and cell structure on dynamic axial crushing honeycomb. Mechanical Engineering Journal 2018;5(2):1–11. [25] Castani�e B, Bouvet C, Aminanda Y, Barrau JJ, Thevenet P. Modelling of lowenergy/low-velocity impact on Nomex honeycomb sandwich structures with metallic skins. Int J Impact Eng 2008;35(7):620–34. [26] Keshavanarayana SR, Shahverdi H, Kothare A, Yang C, Bingenheimer J. The effect of node bond adhesive fillet on uniaxial in-plane responses of hexagonal honeycomb core. Compos Struct 2017;175:111–22. [27] Kendall P, Sun M, Wowk D, Mechefske C, Kim IY. Experimental and computational studies of the effects of adhesive geometry on the out-of-plane properties of metallic honeycomb sandwich panels. Compos Part B-UNDER Rev 2019. [28] Rion J, Leterrier Y, Månson JAE. Prediction of the adhesive fillet size for skin to honeycomb core bonding in ultra-light sandwich structures. Compos Part A Appl Sci Manuf 2008;39(9):1547–55. [29] Olsson R. Mass criterion for wave controlled impact response of composite plates. Compos Part A Appl Sci Manuf 2000;31(8):879–87. [30] Sun M, Wowk D, Mechefske C, Kim IY. An analytical study of the plasticity of sandwich honeycomb panels subjected to low-velocity impact. Compos B Eng 2019;168:121–8. July 2018. [31] Sun M, Kendall P, Wowk D, Kim IY, Mechefske C. Damage assessment on the surface and honeycomb core of the aluminum sandwich panel subjected to lowvelocity impact. ASME; 2018. V006T09A007. [32] Rellinger T. Detection of low-velocity impact damage in carbon fiber sandwich panels using infrared thermography detection. M.S. Thesis. Royal Military College; 2019. [33] Tyler Reyno. “Optical 3D Scanning , Eddy Current Testing, and Destructive Methods for Assessing Surface and Core Damage in Honeycomb Sandwich Aircraft Panels L ’ application des m� ethodes de lecture optique 3D , essai par courants de Foucault et moyens destructives pou,”. M.S. Thesis. Royal Military College; 2017.
This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and National Defense Canada. Technical advice was gratefully received from Dr. Ross Underhill, as well as Mr. Tanner Rellinger and Mr. Tyler Reyno of the Royal Military College of Canada. Test samples were prepared by Andrew Hardman, Mark Boctor and Tony Badea. This research could not have been possible without the support and collaboration of the Structural and Multidisci plinary Systems Design Group at Queen’s University. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107723. References [1] Keith RTB, Armstrong B. Care and repair of advanced composites. warrendale, PA: Society of Automotive Engineers, Inc; 1998. [2] Bitzer T. Honeycomb technology; materials, Design, manufacturing, applications and testing. First. London, UK: Chapman & Hall; 1997. [3] United States of America. Aviation greenhouse gas emissions reduction plan. 2012. [4] Benotto JD. Predicting the effects of dent size on the stress. In: Aluminum Honeycomb Aircraft Panels Subject to Low-Velocity Impact Damage Pr�ediction des effets de la taille d ’ indentation sur les contraintes dans les panneaux d ’ a� eronef en aluminum en nid d ’ abeil. Royal Military College; 2018. M.S. Thesis. [5] Hou B, Wang Y, Sun TF, Liu JG, Zhao H. On the quasi-static and impact responses of aluminum honeycomb under combined shear-compression. Int J Impact Eng 2019;131:190–9. April. [6] Zhang D, Lu G, Ruan D, Fei Q, Duan W. Quasi-static combined compression-shear crushing of honeycombs: an experimental study. Mater Des 2019;167:107632. [7] He W, Yao L, Meng X, Sun G, Xie D, Liu J. Effect of structural parameters on lowvelocity impact behavior of aluminum honeycomb sandwich structures with CFRP face-sheets. Thin-Walled Struct 2019;137:411–32. January. [8] Liu J, Wang Z, Hui D. Blast resistance and parametric study of sandwich structure consisting of honeycomb core filled with circular metallic tubes. Compos B Eng 2018;145:261–9. March. [9] Raju KS, Smith BL, Tomblin JS, Liew KH, Guarddon JC. Impact damage resistance and tolerance of honeycomb core sandwich panels. J Compos Mater 2008;42(4): 385–412. [10] Kendall P, Sun M, Wowk D, Mechefske C, Kim IY. Improved out-of-plane material model for metallic honeycomb panels to account for adhesive boundary conditions. CASI 2019:2–11. [11] Zhang D, Jiang D, Fei Q, Wu S. Experimental and numerical investigation on indentation and energy absorption of a honeycomb sandwich panel under lowvelocity impact. Finite Elem Anal Des 2016;117–118:21–30. [12] Heimbs S, Middendorf P, Maier M. Honeycomb sandwich material modeling for dynamic simulations of aircraft interior components. In: 9th international LSDYNA users conference; 2006. p. 1–13.
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Experimental investigation of adhesive fillet size on barely visible impact damage in metallic honeycomb sandwich panels Patrick Kendall a, Mengqian Sun a, Diane Wowk b, Christopher Mechefske a, Il Yong Kim a, *, 1 a b
Queen’s University, Canada Royal Military College, Canada
A B S T R A C T
Aluminum hexagonal honeycomb panels are commonly used in the aerospace industry to reduce weight due to their high stiffness to mass ratio. The panels are commonly involved in incidents where they are dented in the out-of-plane direction which causes plastic deformation in the face-sheet and buckling collapse of the thin repeating cell-walls in the core. This paper investigates the responses to barely-visible-impact-damage (BVID) in aluminum honeycomb sandwich panels in the out-of-plane direction with attention to the structural adhesive. The structural adhesive forms a fillet shape between the face-sheet and the aluminum core during the curing process and in some cases can encompass over 50% of the honeycomb core thickness. The adhesive fillets become stiff after curing and are able to brace the thin metallic cell-walls and prevent buckling in sections of the core enclosed in adhesive. It was shown that larger fillets cause the damage to occur deeper in the core. Force-displacement data collected from quasi-static experiments showed that as the amount of adhesive used in honeycomb panels was increased, the peak force required to produce a specified maximum dent depth increased as well. Absorbed energy positively correlated with an increasing quantity of adhesive; showing improvements of up to 50% when comparing panels with the largest amount of adhesive and no adhesive. This paper provides relationships between the quantity of adhesive used to fabricate metallic honeycomb sandwich panels and the damage resistance and energy absorption under BVID conditions.
1. Introduction Honeycomb sandwich panels are commonly used in the aerospace industry for light-weighting due to their high stiffness to mass ratio [1, 2]. With growing pressure to reduce fuel consumption in the commercial aircraft industry due to environmental concerns, airline companies have evolved to use advanced composite structures like honeycomb panels to reduce mass and thus reduce carbon emissions and operating costs [3]. Honeycomb panels are made up of two face-sheets (typically made of aluminum, carbon fibre or fiberglass) adhered to a hexagonal core (typically constructed using aluminum or Nomex) with structural ad hesive. The structural adhesive is applied as a sheet, but forms a fillet between the core and face-sheet as it is heated during the curing process. The fillet shape is recommended to be large in the aerospace industry for promoting a stronger bond between the face-sheet and core [1,2]. Honeycomb sandwich structures are stiff in bending but are sus ceptible to out-of-plane indentations caused by such events as dropped tools, hail storms or runway debris. These indentations and the under lying damage to the core can reduce the structural integrity of the panels [4]. This has resulted in a large amount of research focused on the pa rameters that affect the out-of-plane stiffness of honeycomb panels
during impact such as the impactor diameter, cell size, cell-wall thick ness, face-sheet thickness, material and core density [5–9]. It has also been shown that the presence of the adhesive fillet can influence the response [10–13], but an experimental investigation specifically considering the size of the adhesive fillets has not yet been reported. The current paper considers the amount of adhesive used during panel fabrication and the effect of the resulting adhesive fillet size on the out-of-plane response during low-velocity impacts. This will be assessed by performing quasi-static indentation on sandwich panels with varying sizes of adhesive fillets, and examining the resulting load curves, surface damage measurements and internal core damage. In the aerospace in dustry, quality control during the manufacturing process is of primary importance, and the size of the adhesive fillet is a feature of the panel that varies, but can also be controlled to some extent. For example, a survey of 58 fillets from a retired aircraft sandwich panel performed by the author revealed overall fillet heights between 0.48 mm and 1.45 mm. The fillet size was also observed to vary between panels, regions within a panel as well as between adjacent cells. It is therefore important to understand the effect that variations in fillet size have on the per formance of the panel during operational loading as well as during impact events.
* Corresponding author. Queen’s University, 99 University Ave, Kingston, ON, K7L 3N6, Canada. E-mail addresses: [email protected] (P. Kendall), [email protected] (I.Y. Kim). 1 Royal Military College, 13 General Crerar Crescent, Kingston, ON, K7K7B4, Canada. https://doi.org/10.1016/j.compositesb.2019.107723 Received 25 October 2019; Received in revised form 11 December 2019; Accepted 16 December 2019 Available online 17 December 2019 1359-8368/Crown Copyright © 2019 Published by Elsevier Ltd. All rights reserved.
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The damage that is of interest in this study is barely-visible-impactdamage (BVID), which refers to dent depths smaller than 1.0 mm [14]. BVID is believed to lower the structural integrity of the honeycomb panel, but commonly goes unnoticed during inspection given the small nature of the dents. A review of low-velocity impact on sandwich structures was conducted by Chai and Zhu [15] in 2011 which outlined many factors such as cell density, cell-wall thickness, face-sheet thick ness, the material used, and the out-of-plane stiffness of the core and how they affect the low-velocity impact response. Researchers such as Clarke [16] used finite element analysis (FEA) to explore the internal buckling damage as well as the energy absorbed in aluminum honey comb panels as well as a parametric study on the factors affecting their response to low-velocity impacts. He found that as the cell size was decreased and as the cell-wall thickness was increased, the dent depth decreased. As the face-sheet thickness increased, the kinetic energy absorption decreased as well as the dent depth. Other forms of research that are related to low-velocity impacts are parametric studies that explore the uniform out-of-plane compression response of honeycomb core. Ashab et al. [17] performed experiments on aluminum honeycomb core and altered the core density between tests by changing the cell-wall thickness and the cell size. By representing the parameter as a ratio of thickness of the cell-wall over cell size, a result of higher energy ab sorption was concluded as the ratio increased. Chai and Zhu [15] identified in their review that primary factors that affect low-velocity impact response are the face-sheet thickness, the core’s out-of-plane compression response and the shear properties of the honeycomb core [18]. The review of the current literature has highlighted an interest in the many parameters that can play a role in the resistance to out-of-plane damage from spherical indenters or uniform compression. The scientific community has identified the effects of basic aspects like cell-wall thickness, cell size or face-sheet thickness, however, the ad hesive fillets that join the face-sheet and core can also have an effect and they have not been widely considered in literature. While panel characteristics such as the face-sheet thickness and core dimensions have been extensively studied, the effects of the adhesive joining the face-sheet to the core has received less attention. In many cases, numerical simulations ignore the adhesive all together [19–23]. Those that include the adhesive typically represent it as a layer and assume that the fillet geometry itself has no effect on the out-of-plane response of the panel to low-velocity impact [11,13]. Zhang et al. [11] performed experimental testing as well as computational simulations to highlight the key parameters that affect energy absorption given a low-velocity impact with a metallic sphere on an aluminum honeycomb panel. This paper included a study of the adhesive layer between the honeycomb core and the aluminum face-sheets. It was found that the presence of adhesive increased the energy absorption by 6.9%. Mah moudabadi et al. [13] performed a parameter study on the energy ab sorption of honeycomb panels subject to quasi-static punch loading by altering the cell-wall thickness, core thickness, face-sheet thickness and the presence of an adhesive layer. The conclusions were that increasing the face-sheet thickness by 0.4 mm increased the absorbed energy by 45%, and that the presence of adhesive increased the energy absorption by 12%. When the core thickness was doubled so was the absorbed energy and when the cell-wall thickness was increased, the absorbed energy increased by 10%. This study included testing with and without adhesive, but did not vary the amount of adhesive or the fillet size be tween tests. The most notable research involving structural adhesive in low-velocity impacts was by Benotto et al. [4] where the adhesive fillets were identified and an FE model was created to show how the core damage depth occurred deeper within the panel when the adhesive was included. Heimbs et al. [12] studied the effects of the presence of ad hesive fillets on the stress-strain response in various directions while building a homogeneous material model and claimed that the adhesive fillets had a negligible effect with the exception of the in-plane direction which increased by a factor of ten. However, this paper did not give details on the procedure used to draw the conclusion on the out-of-plane
effects. The finding conflicts with Umeda et al. [24] as well as Castanie et al. [25] who found that adhesive fillets alter the boundary condition at the top and bottom of the cells which results in significantly higher peak stresses in uniform out-of-plane compressions when they are included. Kashavanarayana et al. [26] modeled the geometry of the nodal adhesive fillets found between double cell-walls in honeycomb core from the expansion process and noted that the in-plane compres sion response became more stiff and corresponded better with experi mental results as the fillet size was increased towards its measured dimension. Kendall et al. [27] found that the presence of adhesive in creases the peak stress and introduces a drop in load carrying capacity between the peak and plateau stress for uniformly loaded panels. They also found that as the quantity of adhesive was increased, more of the cell-walls were braced when embedded in adhesive, thus lowering the densification strain. The commonality in the literature review showed that while adhesive may play an important role in the out-of-plane behavior of honeycomb panels, specific studies dealing with the size of the adhesive fillets during low-velocity impact and BVID scenarios have not been specifically considered. This paper aims to present the effects of structural adhesive fillets on the damage resistance and energy absorption of aluminum honeycomb panels through experimental testing. The study begins by identifying the key difference in the out-of-plane denting response given no adhesive compared with a thin layer of adhesive in an aluminum honeycomb panel. The focus will then shift to the size of the adhesive fillet, to observe differences between panels manufactured with three different amounts of adhesive. The comparisons will be presented in the form of force and displacement curves as well as surface measurements via 3D scanner and internal damage measurements from destructive crosssectioning. 2. Materials and methods The study considering adhesive fillet size was conducted by quasistatic indentation of panel coupons with four different adhesive di mensions at three different dent depths. Each test was repeated three times resulting in a total of 36 manufactured panels being tested. The panels were made with zero, one, two, and three layers of adhesive to alter the adhesive fillet sizes between the core and face-sheets. A single adhesive layer was 0.23 mm thick resulting in panels with uncured adhesive thicknesses of 0 mm, 0.23 mm, 0.46 mm and 0.69 mm. These panels are referred to as Manufactured Panels zero through three (MP0, MP1, MP2 and MP3) respectively as shown in Fig. 1. The MP0 panels were made with adhesive around the perimeter of the coupon only and not in the central contact area where the dents would be created, in order to maintain similar boundary conditions between tests. Force and displacement data were recorded for each test, as well as surface profile dimensions such as dent diameter, dent area, dent volume and internal core damage metrics such as depth of damage, and diameter of damage from destructive cross-sectioning. 2.1. Specimens The honeycomb panels were manufactured by adhering two metallic face-sheets to either side of a metallic honeycomb core. The material properties for the honeycomb face-sheet (7075-T6) and core (5052-H34) used in the experiments are summarized in Table 1. The honeycomb core is 12 mm thick (L) with a cell-wall length (l) of 1.83 mm and a cell-wall thickness (t) of 0.02 mm (Fig. 2 (c)). Each specimen was cut to di mensions of 80 mm length by 80 mm width based on studies conducted by Benotto et al. [4] on the minimum coupon dimensions to negate boundary condition effects. When the adhesive undergoes curing to secure the face-sheets to the honeycomb core, an adhesive fillet is formed due to the drop in viscosity caused from the 120 � C curing temperature and surface tension of the adhesive. The side profile of the resulting fillet can be fit to an elliptical 2
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Fig. 1. Cross-section of a manufactured panel coupon from each sample set prior to BVID, where MP0 has no adhesive and MPX represents manufactured panels with X layers of adhesive.
are highlighted on an illustration in Fig. 2 (b) which can be compared to the real world fillet shown in Fig. 2 (a). The panels were manufactured following the procedure described by Rion et al. [28] to cure panels with large fillet sizes. The adhesive used was an aircraft grade adhesive film with reference code AF 163-2 K. This � adhesive requires a ramp-up temperature of 1 C/min to a final curing � temperature of 120 C at which it is maintained for 60 min. The curing process was carried out under vacuum on a heat bed where the face-sheet was placed on the bed first, and the adhesive and core were placed on top. The process outlined by Rion et al. [28] describes a preheat phase that allows the adhesive to experience a drop in viscosity and migrate up the cell-walls (due to reduced viscosity and the effect of
Table 1 Material Properties for the honeycomb core and honeycomb face-sheets. Elastic Modulus Yield Strength Poisson’s Ratio
Aluminum 7075-T6 (Face-sheet)
Aluminum 5052-H34 (Core)
71 GPa 440 MPa 0.33
71 GPa 345 MPa 0.33
shape between the cell-walls and face-sheet. The dimensions needed to fit the elliptical fillet include the fillet height (hf ), width (wf ), adhesive thickness (tA ) and overall height of the adhesive (hA ). These dimensions
Fig. 2. (a) Manufactured honeycomb panel with an elliptical adhesive fillet (shown in red) (b) Idealized adhesive fillet geometry, (c) Honeycomb core dimensions with double cell-walls highlighted in blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. Average adhesive dimensions for every coupon tested. (a) adhesive thickness ðtA ), (b) adhesive fillet width ðwf ) and (c) overall adhesive height (hA ). 3
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surface tension) to form a fillet shape during the curing phase. A fillet shape is desired in honeycomb panel manufacturing to strengthen the bond between core and face-sheet and is sought after in industry [1,2]. The curing procedure described above and the progressively larger amounts of adhesive used, resulted in the panels found in Fig. 1. Fig. 3 shows the measured adhesive fillet dimensions for 27 manufactured coupons while Table 2 presents the averaged values for each set (MP1, MP2, MP3). The resulting dimensions from Fig. 3 were recorded as an average of 10 fillets per coupon, while Table 2 is the average of over 90 measurements for each set of coupons. Fig. 4 indicated that as more uncured adhesive layers were used, the thickness of cured adhesive (tA ) and width of the fillets (wf ) increase linearly, while the overall adhesive height (hA ) increases at a higher rate. The mass and volume were measured between panels to calculate the density and is shown to in crease linearly as more adhesive is used in Fig. 4 on the secondary axis. All of the fillet dimensions increased when more adhesive was used and it was not possible to independently control the height, width or thickness. As a result, the study focused on the overall changes in fillet size that result from using more adhesive.
Fig. 4. Average adhesive dimensions for each data set on the left vertical axis and the average density of each data set on the right vertical axis.
impact [29]. After the insert reached the specified maximum displace ment it was retracted from the panel in the opposite direction while still measuring the contact force. This recorded the spring-back phase, which can be compared between tests. Fig. 7 shows the experimental set-up used to measure force and displacement.
2.2. Damage measurement Surface profiles of the dents were obtained by using 3D laserscanning. The resulting point-cloud of the deformed surface was used to measure dimensions such as dent depth, dent diameter, dent area, and dent volume. Fig. 5 provides a visual representation of these dimensions. The profile was captured with a Creaform Handy Scan 700 3D-scanner that produced 3D representations of the surface to within 0.03 mm error. The 3D representation of the deformed surface was compared to a reference plane to obtain a colour contour of the dent, which is shown in Fig. 5 (a). This method was evaluated and tested by Reyno et al. [14] as an effective process for quantifying surface damage in aerospace sand wich panels. The internal damage was measured from cross-sectional images created through destructive sectioning. This involved the cross-sectioning of the coupons using a CNC (computer numerical con trol) diamond blade saw. Fig. 6 shows a cross-sectioned panel with the key dimensions of core damage diameter, core damage depth and core damage thickness highlighted. The depth of core damage was measured on the cell-wall that had the deepest buckling folds. This was typically found at the cell-walls directly below the centre of the indentation. The thickness of core damage was measured between the deepest buckling fold and the tip of the fillet.
2.4. Experimental procedure The 36 coupons underwent displacement controlled compressions using the spherical insert on the Instron machine at three different maximum dent depths. The three dent depths fall within the lower and upper bounds of BVID criteria. The maximum dent depths used were 1.0 mm, 1.5 mm, and 2.0 mm which resulted in residual dent depths of approximately 0.5 mm, 0.8 mm and 1.0 mm. The resulting force versus displacement curves will be compared between tests to highlight the effects of adhesive fillet size on the out-of-plane stiffness of the panels. The dent diameter, dent area, dent volume, and internal damage will also be assessed and compared to draw conclusions on the effects of adhesive with respect to BVID in aluminum honeycomb panels. 3. Results and discussion This study presents the force-displacement relationships between panels with different adhesive fillet sizes. The damaged panels were also scanned to collect measurements for dent area, dent volume, and dent diameter. They were then cross-sectioned to inspect the internal damage and measure the adhesive dimensions in and around the dent based on Fig. 1 (b). Each unique test was repeated three times to ensure repeat ability and a representative curve from each data set was chosen to present the results. The factors will be discussed that affect low-velocity impacts or, in this case, quasi-static denting, are the out-of-plane stiff ness of the core, the flexural stiffness of the face-sheet and the shear properties of the core.
2.3. Experimental apparatus A spherical indenter insert was designed and manufactured in-house to be used with the Instron Mechanical Testing Machine. The panel was supported by a two-inch-thick steel base while a 63.25 mm diameter spherical impactor, made of steel, contacted the top face-sheet to create a dent. The steel sphere was displaced into the top face-sheet of the panel at a rate of 0.5 mm/min to maintain a quasi-static condition. Quasi-static conditions have been known to produce the same damage state in metallic honeycomb panels as low-velocity impacts with large mass impactors [29], while producing a more repeatable resulting dent. The indenter used was 1 kg, which meant the ratio of impactor mass to coupon mass was well over 10, which is the criteria for a large mass
3.1. Force and displacement In order to first highlight the effect that the presence of adhesive has on impact resistance, comparisons of results from panels that were manufactured with and without adhesive (MP0 and MP1) were studied. The average overall fillet height of 0.87 mm from set MP1 with one layer of adhesive is representative of the fillets that are typical in aircraft, based on cross-sectioning retired panels. The force versus displacement results are shown in Fig. 8. The slope of the MP0 panels was linear for the duration of the test, while the MP1 panels show a bilinear response. The initial slope in the MP1 coupons between 0 and 0.2 mm was six times larger than the MP0 coupons without adhesive. The peak force values
Table 2 Averaged adhesive dimensions measured from cross-sectioned panels. MP1
MP2
MP3
tA ðmmÞ
0.12
0.24
0.47
hf ðmmÞ
0.75
1.15
2.11
wf ðmmÞ
0.49
0.85
1.09
hA ðmmÞ
0.87
1.39
2.59
4
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Fig. 5. Out-of-plane deviations (mm). (a) Scan ner results used for measuring surface damage where green indicates that the deviations from the reference plane fall within the 0.03 mm threshold and the progressively larger blue col ours indicate larger out-of-plane deviations. (b) Dent depth illustration, (c) dent diameter illus tration, (d) dent area illustration and (e) dent volume illustration. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Destructive inspection measurements of core damage for cross-sectioned honeycomb panel.
spring-back in the MP1 coupons with adhesive remained constant at approximately 60%. This was expected because the honeycomb core and the aluminum face-sheet were not attached in the MP0 coupons and will each spring-back at different magnitudes. The MP1 coupon face-sheets were prevented from fully springing back as they were attached to the crushed core and do not spring-back as much as the face-sheet. Past research has consistently shown that the face-sheet springs-back more than the crumpled core and this study supports that statement [16], [30–32.] It was observed in this study that for a given maximum dent depth, coupons with adhesive result in deeper residual dents than the coupons without adhesive. Fig. 9 shows a comparison between the cross-sectioned panels of an MP0 coupon and an MP1 coupon. The buckled core in the MP0 sample begins at the contact point between the face-sheet and core (top of the cell-walls), while the MP1 sample begins buckling below the adhesive fillet. These findings correlate well with Kendall et al. [27] who performed uniform out-of-plane compression testing on both honeycomb core in isolation and honeycomb panels with adhesive. The referenced study concluded that honeycomb panels with adhesive require a higher force to initiate plastic collapse, which occurs at random locations along the height of the panel’s core, while buckling always began at the top or bottom of the cell-walls when honeycomb core was compressed in isolation. The presence of adhesive in the panels introduces three key differ ences as compared to the panels with no adhesive. The ends of the cellwalls that are embedded in the adhesive are constrained from rotating as
Fig. 7. Experimental set up for creating out-of-plane BVID on the panels.
from the 1 mm, 1.5 mm and the 2 mm maximum dent depths for the MP1 coupons were on average, 6.1% higher than the MP0 samples. The percentage of spring-back measured from the MP0 coupons without adhesive increased as peak force increased while the percentage of 5
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Fig. 8. (a) Force versus displacement results collected from the Instron machine given a quasi-static spherical dent. The figures show a representative curve for each maximum dent depth. (b) A plot of the force and displacement data of the MP0 and MP1 panels with a focus on the initial slopes in the data at the beginning of the test.
Fig. 9. Internal damage for a 1.5 mm maximum dent depth comparing (a) MP0 with no adhesive and (b) MP1 with adhesive.
opposed to being free to rotate, and a higher load is required to initiate buckling in the core. Kendall et al. [27] showed that when comparing panels made with adhesive to isolated honeycomb core, they had higher peak stresses when uniformly compressed. The second difference is that the adhesive effectively thickens the face-sheet resulting in a higher load requirement to displace the surface to the maximum prescribed depth. The third difference is that the adhesive connection between the core and the face-sheet enables a shear load-transfer which occurs as the face-sheet is dented [18]. All three of these conditions cause the panels with adhesive to have a stiffer response to out-of-plane deformation during the first 0–0.2 mm of displacement. Kendall et al. [27] deter mined that the peak stress for a panel with adhesive under uniform compression occurred at strains on the order of 0.2 mm of out-of-plane displacement. Since peak stress is an indication of the onset of insta bility, the change of slope between 0.1 and 0.2 mm can be attributed to the initiation of cell-wall buckling in the first cells below the indenter.
To highlight the effects of adhesive fillet size on the out-of-plane stiffness, panels with increasing thicknesses of uncured adhesive (MP0 – 0 mm, MP1 - 0.23 mm, MP2 - 0.46 mm and MP3 - 0.69 mm) were indented and the force and displacement data for one representative curve was plotted for each maximum dent depth. Fig. 10 shows that the peak force increased by an average of 11.4% with increasing volumes of adhesive from MP1 to MP3. The adhesive effectively stiffens the panel as more force is required to indent it to the same depth when more layers are used. Each panel with adhesive shows a bilinear response, with a higher slope in the first 0–0.2 mm of displacement as more adhesive is used. The force at the transition from the initial linear response was measured for each coupon and there was an increase of 56% between MP1 and MP3. This portion of the curve can be seen in greater detail in Fig. 11 (b). The spring-back displacement percentage was constant at approximately 60% between all coupons with adhesive regardless of the peak force and adhesive amounts. The springback was affected by
Fig. 10. Force and displacement results from quasi-static indentation at three different maximum dent depths, (a) 1 mm, (b) 1.5 mm and (c) 2 mm. The three plots include all four coupons sets.
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Fig. 11. (a) All data sets plotted together for each maximum dent depth. (b) All data sets plotted together highlighting the first 0.5 mm of displacements.
whether or not adhesive was present, but was independent of the size of the adhesive fillets. Fig. 11 (a) provides a plot of all the data sets pre sented together. From this study on how the size of the adhesive fillets alters the damage resistance in honeycomb panels, some explanations are pro posed. A key parameter that affects the behaviour of honeycomb panels with respect to impact resistance is the face-sheet stiffness. Considering the stiffness of the structural aerospace adhesive used, adding more adhesive to the bottom side of the face-sheet increases the flexural ri gidity. This in turn, increases the force needed to deform the face-sheet and cause buckling in the core, as evident at 0.2 mm. By increasing the amount of adhesive used in fabrication, the initial out-of-plane stiffness can be increased, meaning that the honeycomb panel becomes more resistant to BVID, which has not been highlighted in past research ef forts. All the panels showed the same stiffness once past 0.2 mm of displacement, which is when more cell-walls start to buckle as the dent diameter increases. Buckling is initiated more easily as the loading through the cell-wall is not strictly axial due to the rotation of the facesheet as it is depressed. In addition, the plasticity in the face-sheet is spreading over a larger area leading to the out-of-plane panel stiffness being independent of the amount of adhesive used.
Fig. 12. Work done on honeycomb panels with different fillet sizes dented at different maximum dent depths.
3.2. Absorbed energy
uncured adhesive, the only conclusive statement that can be made is that when more adhesive is used to manufacture honeycomb panels, the panel can absorb more energy given BVID. One should note that these improvements in energy absorption come at the cost of a higher density panel and therefore panels with a larger mass. A calculation of ratio of energy absorbed to panels density was done for the 1 mm dent depth results to show that each panel shares the same value of 0.85 when it comes to energy absorption per unit density. Future efforts should focus on simulations including the adhesive geometry and isolating an in crease in height, width and thickness of the fillet to identify the sensi tivity of the absorbed energy to individual dimensions.
The force and displacement results provided a clear picture of damage resistance and out-of-plane stiffness in honeycomb panels with different adhesive geometries. Another commonly studied metric is the energy absorbed during impact, which can be determined by calculating the area under the loading curve to assess both the plastic and elastic energy in the system, and subtracting the area under the unloading curve where the indenter is retracted from the panel. This represents the majority of the elastic energy leaving the system. In the coupons where adhesive was used, not all of the elastic energy was released following unloading. This was evident from Fig. 9 (a) where the face-sheet springsback farther than the crumpled core in cases where no adhesive was used. This indicates that the panels with face-sheets that are connected to the core via adhesive will find an equilibrium between the core and face-sheet where the core will resist the face-sheet from rebounding to its desired shape and the face-sheet will apply tension to the core. The amount of absorbed energy is shown to increase when more adhesive is present as illustrated in Fig. 12. Absorbed energy values increased by 34% between MP1 and MP3 due to the increase in face-sheet stiffness when larger volumes of adhesive are used in manufacturing. By adding additional layers of adhesive film to create larger adhesive fillets, each of the fillet dimensions were gradually increased from MP1 to MP3. Since each fillet dimension increased with a thicker layer of
3.3. Surface damage The surface profile from the 3D scanner results provided measure ments for dent diameter, dent area, and dent volume. The results do not show measurements for the MP0 panels due to the disconnect between the face-sheet and the core, which resulted in surface profiles that did not provide a clear image of the overall damage in the panel. Although higher peak forces were required to reach the maximum dent depth for the panels with more adhesive, the surface profile remained the same between all panels. This means that the residual dent depth, dent diameter, dent area, and dent volume are independent of the amount of 7
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Fig. 13. Surface damage measurements as a result of the 3D scanner. (a) Dent diameter, (b) dent area, and (c) dent volume.
adhesive for a given maximum dent depth, even though more adhesive results in a higher resistance to BVID. Fig. 13 shows the measurements of 27 panels with adhesive and are grouped according to the maximum dent depth prescribed.
than when it is attached to the face-sheet. The size of the adhesive fillet had no effect on the diameter of the damage. It was shown that the surface profile of the dent was not affected by the amount of adhesive and since the core and the face-sheet are tied together, the diameter of the core damage is also not affected. The internal damage measurements highlighted a key finding from this study. The core damage depth is dependant on the overall adhesive height and increases as more adhesive was used. This is because the adhesive fillets enclosing the thin cell-walls are relatively stiff and can prevent the cell-walls from buckling. This causes the initial buckling to begin directly below the tip of the adhesive fillet. Therefore, if the overall adhesive height is increased, the first buckle is formed further into the core and propagates deeper into the core. The effect of the core damage depth and the location of buckling initiation on the residual strength of honeycomb sandwich panels is currently unknown and re quires further study.
3.4. Internal damage Finite element simulations have shown that the depth of the damage in the core remains constant as the dent depth increases within the BVID criteria [16], however this was for simulations of idealized panels without adhesive. Fig. 14 shows that by increasing the size of the ad hesive fillet, and as a result the overall height of the adhesive, the initialization of cell-wall buckling is shifted deeper into the core, which increases the depth of core damage. This is shown in Fig. 14 (a) where the MP0 coupons without adhesive have core damage occurring closer to the panel surface. As the overall height of the adhesive increases from MP1 to MP3, the core damage occurs deeper in the panel (Figs. 15 and 16). Fig. 15 shows the average of each sample to highlight the increasing trend in the depth of core damage and the height of adhesive. The thickness of damage was found to have a slight upward trend as the adhesive size was increased, but only in the 1 mm and 1.5 mm cases (Fig. 14 (b)). The diameter of damage (Fig. 14 (c)) was found to be constant amongst all tests given the same maximum dent depth, with the exception of MP0 measurements, which were lower than the panels made with adhesive. Without adhesive, the core and the face-sheet can deform independently so the deformation in the core is more localized
4. Summary and conclusions Three key findings emerged from the experimental measurements for panels with different adhesive fillet sizes. It was shown that panels fabricated with more adhesive required higher peak forces to produce the same maximum dent depth. The increased stiffness also resulted in the absorbed energy increasing with larger adhesive fillet sizes. When the damaged panels were cross-sectioned, it was found that as more adhesive was used to fabricate panels and the overall height of the
Fig. 14. Measurements of internal damage from cross-sectioning panels. (a) the depth of core damage, (b) the thickness of core damage, and (c) the diameter of core damage.
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evidence that the depth of core damage is however, directly related to the overall height of adhesive fillet. It was shown in the resulting cross-sections that the initiation of buckling damage in the core occurred directly below the adhesive fillets due to a bracing effect from the ad hesive on the thin metallic cell-walls that resisted buckling collapse. This shows that a realistic representation of the internal adhesive fillets is required in order to predict the extent and depth of core damage in honeycomb sandwich panels. There are no effects of the adhesive on surface damage given a prescribed displacement and the surface damage will relax at the same residual dent given any amount of adhesive. Although the adhesive fil lets can have a stiffening effect on the face sheet, it does not affect the final, sprung-back surface given a maximum prescribed dent. This finding is important because the panels in service are only measured based on their surface damage, which we now know cannot be directly compared to the internal damage without information on the adhesive fillets. Analytical models and numerical simulations are commonly used to predict the response of honeycomb panels to low-velocity impact. In many cases, the adhesive is not represented to maintain simplicity and reduce computational efforts. It has been shown however, that the size of the adhesive fillet does have an effect on the out-of-plane stiffness, the energy absorption and the core damage. It is therefore required to incorporate the adhesive fillet geometry particularly when considering durability and residual strength of dented aerospace panels. Quality control is a high priority in the manufacturing of aerospace components and the adhesive fillets found in aircraft panels can be manufactured in many different heights, widths and thicknesses. The results from the current study provide a further understanding of the effects of variations in adhesive fillet size on the out-of-plane stiffness of honeycomb sand wich panels as well as their reaction to low-velocity impact events producing BVID.
Fig. 15. Average core damage depth given all three maximum dent depths compared with the increasing adhesive dimensions from MP0 to MP3.
adhesive fillet increased, the core damage propagated deeper into the honeycomb core. As the damage resistance and buckling failures are changed between tests, the surface profile of the residual dent remained constant, therefore it was concluded that the dent depth, diameter, area and volume is independent of the size of adhesive fillets. This finding also means that the percentage of spring-back displacement was con stant between all tests with adhesive at 60% meaning it is independent of the amount of adhesive used as well. When more layers of adhesive were used to manufacture the panels, the thickness, the width, and the height of adhesive fillets increased as well. This in turn increased the out-of-plane stiffness during the initial 0.2 mm of displacement and increased the peak force as evident in the force versus displacement data (Fig. 11 (a), (b)). The increased amount of adhesive effectively increases the face-sheet thickness and also sta bilizes the top of the cell-walls from buckling. The peak force was increased by 17% between no adhesive and three sheets of adhesive film. The amount of absorbed energy was also found to increase with the adhesive fillet size. The amount of absorbed energy increased by 34% when comparing the MP1 and MP3 panels. This is caused by an increase in the face-sheet stiffness given more adhesive, which can be seen in the first 0.2 mm of displacements. The results of the current study show that the size of the adhesive fillet must be considered for accurate predictions of energy absorption during low-velocity impacts of honeycomb sand wich panels. Past simulations of honeycomb panels without adhesive have shown that the depth of core damage is constant for all dent depths falling within the BVID category [16,33]. This paper provides conclusive
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Patrick Kendall: Methodology, Validation, Formal analysis, Inves tigation, Writing - original draft, Visualization. Mengqian Sun: Conceptualization, Methodology, Formal analysis, Writing - review & editing. Diane Wowk: Conceptualization, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Christopher Mechefske: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition. Il Yong Kim: Conceptualization, Resources, Writing - review & editing, Supervision.
Fig. 16. Cross-sections from each coupon set showing an increase in the internal depth of core damage, indicated by the black lines, given a maximum dent depth of 1 mm.
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Acknowledgements
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