Investigating the effects of fluid intrusion on Nomex® honeycomb sandwich structures with carbon fiber facesheets

Composite Structures 206 (2018) 535–549

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Composite Structures journal homepage: www.elsevier.com/locate/compstruct

Investigating the effects of fluid intrusion on Nomex® honeycomb sandwich structures with carbon fiber facesheets

T

⁎

Garam Kima, , Ronald Sterkenburga, Waterloo Tsutsuib a b

School of Aviation and Transportation Technology, Purdue University, West Lafayette, IN 47907, USA School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Honeycomb structures Moisture ingression Strength Failure and mechanical properties

Honeycomb sandwich construction is commonly used in aircraft structures to make parts with a superior strength to weight ratio. Nomex® honeycomb core is used extensively for flooring, skin panels, fairings, engine cowlings, and flight controls. Honeycomb sandwich structures are prone to fluid ingression due to their thin facesheets which get damaged easily by impact or erosion. The purpose of this research was to determine how the mechanical properties of honeycomb sandwich structures were affected if the structure was saturated by a fluid such as water, fuel, hydraulic fluid, or engine oil. The authors focused on the lasting effect on the sandwich structure when aircraft fluids ingress into the structure. The test panels were made of carbon fiber prepreg, and they were bonded to a 12.7 mm thick Nomex® honeycomb core material. The specimens were soaked in water, fuel, hydraulic fluid, or engine oil for 45 days. After the soak period, the specimens were removed from the fluids and left to drain for 30 days. The specimens including the control group were tested with a four-point loading test and impact test in accordance with ASTM standards.

1. Introduction Due to the wide range of applications of composite sandwich structures in the aerospace, automobile, shipbuilding, construction, and transportation industries, there has been a significant increase in their usage [1–6]. Composite sandwich structures consist of multi-layered materials that are made by bonding rigid, high strength skin facings to low density core materials [1–3,5,7,8]. The high strength and low weight of the sandwich concept are the primary advantages of using it in structural components [1–3,5,7–17]. The facesheet of sandwich structures provides tensile, compression, and bending strength, along with the stiffness for the surface of the structure. The core provides rigidity and helps to distribute applied loads over a larger area [5,7,10]. The use of different material types for the facesheets and cores, along with varying the part geometry of the sandwich structure, allows the structure to have different mechanical properties, which enables the structure to be used in various areas with diverse purposes [5–7,18]. In order for the sandwich structure to be used in these varying applications, it is a requirement to understand their static and fatigue characteristics, as well as the failure mechanisms under these same loading conditions [1,7,19,20]. Facesheets are usually made of carbon fiber, fiberglass, or aluminum, and are designed to withstand damage and high in-plane stresses [6,15,21]. The structure gains flexural rigidity ⁎

and strength using the core between two facesheets without a large increase in weight, which is one of the primary advantages of composite sandwich structures. The core is generally made of materials with lower stiffness and strength than the facesheet’s materials. Aluminum and Nomex® honeycombs, synthetic PVC, aluminum foams, balsa wood, and corrugated materials are examples of materials that can be used for the core [6,21]. Nomex® honeycomb core, which is made out of aramid fiber with phenolic resin, was used in this research. Nomex® honeycomb is one of the most widely used honeycomb cores in industry [6,8,17,18]. Composite sandwich structures have a wide range of applications on aircraft [6,8,12,13,15,18,19,22]. On one of the most recently designed commercial aircraft, the Boeing 787, about 50% of the material by weight is comprised of composite materials. Carbon fiber laminate and sandwich structure with carbon fiber skin panels are predominantly used in the airplane [19]. Many of their applications are in the critical structural sections of the aircraft, such as the fuselage, engine cowl, fairing, door, radome, leading edge, primary flight control, secondary flight control, and other non-structural part applications [15,18,19]. The Airbus A340, for example, has its entire vertical tail plane made of composite sandwich structures, composed of glass fiber reinforced prepreg and Nomex® honeycomb core [19]. Because of the location of composite structures in aerospace applications, composite structures often times experience multiple environmental conditions by

Corresponding author. E-mail address: [email protected] (G. Kim).

https://doi.org/10.1016/j.compstruct.2018.08.054 Received 7 May 2018; Received in revised form 7 July 2018; Accepted 27 August 2018 Available online 30 August 2018 0263-8223/ © 2018 Elsevier Ltd. All rights reserved.

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important to understand the impact behavior of the structure [22]. Impact damage causes the following failure mechanism; matrix cracking, tensile and compressive fiber breakage, delamination, etc., [17,22]. JH Park et al. [4] claimed that the damage resistance property of honeycomb structure was influenced by both the facesheets and core. The interface between them also affected the damage resistance property of the structure [2]. Khan et al. [7] reported both the low velocity impact test and high velocity impact, where the latter resulted in the stress wave propagation inside the honeycomb sandwich structure. Hill [21] investigated the damage resistance and tolerance of honeycomb sandwich structures, where he reported damage mechanisms and energy absorption when an impact force of low velocity is applied to the structure. The core of the structure plays a critical role in impact resistance mechanical properties [21]. Buitrago et al. [22] studied the role of core and skin in a honeycomb sandwich structure with regards to energy absorption during impact through finite element analysis. Zinno et al. [17] investigated impact damage mechanisms of the composite structures that depends on their skin thickness, core and skin materials, geometry of the impactor, and impactor velocity. Most of impact energy was absorbed by the skin [17,22]. The main failure mechanisms of the impact test were fiber breakage on the skin and wall deformation of the core [22].

which, through diffusion, they can absorb moisture, oil, aviation hydraulic fluid, anti-icing additive, or jet fuel [11,16,23,24]. Diffusion occurs when there is enough porosity, voids, and vacancies within structures to allow the liquid to permeate the facesheets or honeycomb [11,23,24]. This absorption is largely permanent and can lead to chemical and/or physical aging of the materials used in the sandwich structures. The liquid absorption can also result in the part failure at loads lower than its designed strength [11,12,15,23,24]. If a permeated liquid inside of honeycomb experiences thermal cycles, which is repeated liquid freezing and thawing, the bond between the skin panel and core can be destroyed [11]. The liquid can degrade the strength of the metallic honeycomb core and facesheet by causing corrosion. When the temperature increases, the trapped liquid inside of the honeycomb cell increases the pressure of the cell, and it may degrade the bond between facesheet and core [15]. Kececi and Asmatulu [23] used barrier films to reduce the rate of absorption of fluids, and it considerably improved the mechanical properties of the sandwich structures being tested. Semple et al. [12] performed strength tests on the Nomex® honeycomb sandwich panels in different humidity levels (RH). They found that the strength of the honeycomb significantly decreased in high humidity environments. Zinno et al. [17] analyzed structural performance of Nomex® honeycomb sandwich structure in various conditions, such as: different temperatures, humidity levels, and exposure to chemical or UV radiation. Yeo et al. [24] studied how water ingression and thermal cycle affected the Nomex® honeycomb sandwich structure. Hot and wet conditions degrade the structure significantly, but it recovers some of the strength when it dries. Fogarty [16] reported a different result, where if the honeycomb sandwich structure is designed and manufactured correctly, water ingression can be prevented, even when exposed to moisture. Also, even after water penetrates locally inside of the core, further ingression can be prevented. Due to composite structures being placed under complex dynamic loads, the application is limited to high reliability fields, such as aviation, in order to compensate for the knowledge gap in literature [2,7]. In order for composite sandwich structures to be utilized in primary structures on a large scale, it has to overcome obstacles [2,19,7,13]. The sandwich structure has to be assessed, so that it will not fail or experience excessive structural deformation from the mechanical stresses exerted to the structure during the service life [2,7,14]. The honeycomb sandwich structure is often used for parts which are exposed to quasi-static loads (e.g., flexural and compressive), and dynamic loads [7,13]. Manufacturing defects, damages during operation, and fatigue history on the honeycomb sandwich structure affect its mechanical properties and performance [7,10,13]. Composite sandwich structures have various failure mechanisms from flexural stress from applied bending moment and shear stress from applied shear force. For instance, core shear failure, skin failure, local wrinkling, and delamination are some of the problems [7,10,13]. Jan et al. [25] performed a three-point bending test using honeycomb sandwich specimens. They found that specimen failure was caused by the core buckling, which resulted in a disbond between the facesheet and core. Jen and Chang [9] argued that the bending fatigue strength in the sandwich structure increased when the density of the core also increased. In fact, the major failure mode of the flexural test of the sandwich structure is debonding between the core and the facesheets [9,10,20]. Belouettar et al. [2] investigated the damage formation process of the honeycomb sandwich structure by using a four-point bending test and characterized the fatigue behavior of the structure. Lister [26] performed a three-point bending test on honeycomb sandwich structures that have different orientations of the core and thickness of the facesheets to characterize their mechanical properties. Bey et al. [10] studied cyclic fatigue behavior during a three-point bending test and characterized the behavior based on the specimens’ loading conditions and nature of the skin. Composite honeycomb sandwich structures in aircraft are subjected to high velocity impacts from bird strike or debris during flight. It is

2. Proposed methodology To determine how fluid ingression affects the strength of honeycomb structures, two kinds of experiments were conducted: a four-point bending test and an impact test. The four-point bending test was conducted to investigate the flexural stiffness of the honeycomb specimen under quasi-static loads. More specifically, the objective of the fourpoint bending test was to observe flexural stiffness of the honeycomb structures based on various fluids with which the structure was saturated and to determine how different types of fluids affected the core of the specimen [27]. The impact test was used for dynamic analysis of the honeycomb structure, where an impact loading was applied to the specimen by dropping a striker/weight from a specified height above the specimen. The objective of this dynamic test was to compare the damage resistance properties of each sample with the control group and to observe any negative effects that the fluids may have caused [28]. 2.1. Specimen preparation Specimen preparation can be divided into three processes: 1) preparing facesheets, 2) bonding facesheets to the core, and 3) cutting specimens into the size. The facesheets were made out of carbon fiber prepreg (SGP196P/8552). Each facesheet was 4 plies thick (0.762 mm) and the ply orientation of the facesheets was [-45/90/0/45]. Aluminum caul plates were placed on top of the facesheets to ensure that they would have a uniform flatness on both sides. The facesheets were vacuum bagged and cured with the appropriate cure cycle. After curing, the parts were trimmed to remove rough edges and make them easier to handle. Nomex® honeycomb, 12.7-mm thick, was used as the core material for the specimen. Core density was calculated by using the following equation:

dSI =

1000000W (lwc )

where dSI is the density of the core, W is the mass of the core. l, w, and c are length, width, and thickness of the core, respectively [29]. The density of the core which was used in this research was 61.590 kg/m3. After the facesheets were manufactured, they were bonded to the core on both sides with film adhesive, HCS2404-050. The specification of the film adhesive is attached in Appendix 1. The assembled honeycomb sandwich panel was vacuum bagged and cured in an oven. The cured part was cut into correct specimen size. The dimensions of the 4-point loading test sample were 203.2 mm × 76.2 mm. The dimensions of the 536

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2.3.2. Impact test The impact test was designed and performed based on ASTM D7136/D7136M-15. The impact test was performed using an Instron 9250HV drop tower. A 16 mm diameter hemispheric striker was used for the tip of the dropping object. Mass of the dropping object was 5.5506 kg. ASTM D7136/D7136M-15 recommends two different equations to calculate the appropriate drop height for the test. First, the potential energy of the impactor before testing was calculated by the impact energy calculation equation as follows:

E = CE h where E is potential energy of impactor before beginning the test. CE is the specified ratio of impact energy to specimen thickness, 6.7 J/mm, and h is nominal thickness of specimen [28]. Using calculated potential energy of the impactor, the drop height was calculated using the following equation:

Fig. 1. Honeycomb panel assembly process.

impact test sample were 152.4 mm × 101.6 mm. Specimens were cut using a surface grinder fitted with a diamond coated saw blade. After the specimens were cut, the edges were sanded very lightly to create a smooth edge. The specimens were then lightly blown off by compressed air to remove any contaminants created during cutting and sanding processes. Dimensions (length, width, and thickness) and weight were measured before the specimens were soaked into the fluids. Fig. 1 shows the drawing of Nomex® honeycomb sandwich panel assembly.

H=

E mg

where H is the drop-height of the impactor, m is mass of the impactor, and g is the gravitational acceleration, 9.81 m/s2 [28]. Based on the two equations, the drop height for testing all the specimens was set to 1.8839 m which has an extra 6.75% of height to assist the striker to penetrate the specimen all the way through. According to the law of conservation of energy, the initial potential energy of the impactor is equal to the kinetic energy of the impactor when the impactor contacts the surface of the specimen. This can be described by the following equation:

2.2. Soaking The specimens were soaked in a different type of fluid. Four types of fluid were prepared: JET-A (aircraft jet fuel), aircraft turbine engine oil, Skydrol (hydraulic fluid), and distilled water. These four fluids were chosen because these fluids are commonly used in aircrafts and there is a possibility that these fluid could come into contact with the honeycomb structure of the aircraft. The specification of each fluid is attached in Appendix 2. The specimens were placed in glass containers filled with the fluids. Specimens were fully submerged in their respective fluids. The containers were sealed to make sure any dust or moisture could not enter the containers. Specimens were removed from the containers after 45 days of soaking. The liquid in the specimens were drained for 30 days before testing.

mgH =

1 mv0 2 2

where v0 is velocity of the impactor when it contacts the top surface of the specimen (t = 0) [28]. By solving the above equation for v0 , the initial velocity is calculated as 6.08 m/s. During the test, the force acting on the impactor and time were measured and recorded. Each specimen was tested in the same way. Fig. 3 shows the impact test machine and test fixture with the specimen. After the four-point bending and impact tests were completed, the specimens were cut and opened to check the liquid ingression inside of the specimen. The affected area was discolored inside of the honeycomb cell and cell wall. All specimens were fully ingressed by the fluids during the soaking period. Fig. 4 shows examples of a cut-open specimen (fuel group test specimen (left) and control group test specimen (right)).

2.3. Test 2.3.1. Four-point bending test A four-point bending test was designed and performed based on ASTM C393/C393M-16, Quarter Point Loading test procedure. A servohydraulic loading machine (MTS 810) was used for testing. An ASTM standard fixture was prepared and installed on the machine. The test fixture had 152.4 mm wide bottom span and the 76.2 mm wide top span. 25.4 mm diameter aluminum rods were placed on the top of the spans. The loading rate was 6 mm/min. Fig. 2 displays four-point bending test equipment with the specimen.

3. Results and discussion 3.1. Result of four-point bending test For the four-point bending test, five specimens were prepared for each fluid group. In addition, four specimens were prepared for the

Fig. 2. Four-point bending test machine with specimen. 537

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Fig. 3. Impact test machine and test fixture with specimen.

Fig. 4. Open cut of the specimens (fuel group test specimen (left) and control group test specimen (right)). Fig. 5. Mean Load vs displacement data for each group.

control group, which were not soaked in any liquid. In total, 24 specimens were tested, and test data was collected. Right before the test was performed, the weight of the test specimens was measured to check any weight change of the specimens. It showed that even after a draining period of one week, the specimens still contained some liquid inside. Moisture content of each test specimen was calculated using following equation;

MC =

Table 1 Ultimate failure load for each specimen (N).

Specimen Specimen Specimen Specimen Specimen Mean

WWet −WDry WDry

where MC represents moisture content of test specimen. WWet and WDry are the weight of test specimens when they were wet and dry. The weight change and moisture content of test specimens is described in Appendix 3. In accordance with ASTM C393/C393M-16, four-point bending test results were analyzed by calculating the core shear ultimate stress and facing bending stress for each specimen [27]. Fig. 5 shows the load vs displacement plot for each group. The load value for each group was calculated by averaging the load value for each specimen in each group. Table 1 shows the ultimate failure load for each specimen. Using the load data which was collected from the test, the core shear ultimate stress was calculated by using the following equation:

Fsult =

σ=

1 2 3 4 5

Fuel

Oil

Hydraulic

Water

Control

2092 2275 2269 2326 2133 2219

2188 2192 2306 2176 2395 2252

2221 2218 2408 2165 2261 2255

2021 2025 2009 1968 2101 2025

2436 2047 2065 1991 – 2135

Pmax S 4T (d + c ) b

where σ is facing bending stress, S is span length, and T is facing thickness [27]. Fig. 6 shows the bar graph of mean value of ultimate failure load, core shear ultimate stress, and facing bending stress for each group. Dots on the bars represent the values of each specimen in the group. All of the specimens had the same failure type, which was the core shear failure during testing. Fig. 7 shows the core shear failure of the specimen.

Pmax (d + c ) b

3.2. Result of impact test

where Fsult is core shear ultimate strength, Pmax is maximum force prior to failure, d is sandwich thickness, c is core thickness, and b is sandwich width [27]. The facing bending stress was calculated by using the following equation:

For impact testing, five specimens were prepared for each fluid group and four specimens were taken for the control group, which were not soaked in any fluid. In total 24 specimens were tested and the data 538

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Fig. 6. Ultimate failure load (a), core shear ultimate stress (b), and facing bending stress (c) for each specimen group.

was collected. The weight of the impact test specimens were also measured right before the test. The weight change and moisture content of test specimens is described in Appendix 3. During the test, all of the test specimens were fully penetrated by the impactor. Fig. 8 shows a representative example of the impact test data plot with schematic representations of event history during the test. In Fig. 8, Fast Fourier Transform (FFT) with 2048 cutoff frequency was used as a smoothing function. At Event 1, the impactor meets the top facesheet of specimen. At Event 2, the impactor penetrates through the top facesheet of specimen and meets the honeycomb core. At Event 3, the impactor passes through the honeycomb core and meets the bottom facesheet of specimen. At Event 4, the impactor penetrates the bottom facesheet of specimen. Finally, at Event 5, the impactor penetrates all the way through the specimen. Using a measured load on the impactor, the acceleration of the impactor for each specimen was calculated by Newton’s second law of motion:

a (t ) = F (t )/ m where a is acceleration of the impactor at time t, and F (t ) is the measured force at time (t). Velocity, displacement, and absorbed energy of the impactor was calculated using integral function provided in a data analysis software, Origin. The integral function in Origin utilize trapezoidal numerical approximation as follows:

∫x

xn

1

n−1

f (x ) dx ≈

Fig. 8. Impact test data plot (raw and smoothed) and schematic representations of event history during the test.

calculated by subtracting the acceleration integration from v0 , which is the velocity of the impactor at time t = 0, which is when the impactor contacts the top surface of the specimen as shown in the following equation:

1

∑ (xi +1−xi ) 2 [f (xi +1) + f (xi)] i=1

As the impactor contacts the top facesheet of the specimen, velocity of the impactor starts to decrease due to the deceleration motion of the impactor. (i.e., the acceleration acts on the opposite direction to the impactor’s downward movement). Therefore, the impactor velocity was

v (t ) = v0−

∫0

t

a (t ') dt '

where v (t ) is velocity of the impactor at time t. Displacement of the impactor was calculated by integrating the velocity history of the

Fig. 7. Specimen failure type (core shear failure) during 4-point bending test. 539

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Fig. 9. Mean acceleration vs time plot (a), mean velocity vs time plot (b), mean displacement vs time plot (c), mean force vs displacement plot (d), and mean absorbed energy vs time plots (e) of each group.

impactor as shown in the following equation:

δ (t ) =

∫0

t

v (t ') dt '

where δ (t ) is displacement of the impactor at time t. Using calculated displacement, a force vs. time plot was generated. The energy absorbed by the specimen was calculated by computing the bounded area below the force vs displacement curve as follows:

U (δ ) =

∫0

δ

F (δ ') dδ '

where U (δ ) is the absorbed energy of the impactor at displacement δ . Mean acceleration, velocity, displacement, and absorbed energy within each group were calculated. Fig. 9 shows the mean acceleration, velocity, absorbed energy, and displacement vs time graph of each groups. In accordance with ASTM D7136/D7136M-15, the impact test results were analyzed by calculating F1, FMax , E1, and EMax for each specimen [28]. F1 and E1 represent load and energy value when initial change in the stiffness characteristic occurs. FMax represents ultimate failure load and EMax represent absorbed energy when the force reaches the ultimate failure load [28]. In this research two different FMax were used for the analysis: FMax1 and FMax2 . FMax1 represents the ultimate failure load when the impactor penetrates the top facesheet, and FMax2 represents the ultimate failure load when the impactor penetrates the bottom facesheet. The collected load data had significant noise. Therefore, two different cases were used to analyze FMax and EMax . One with raw data, and one with smoothed data [28]. Smoothing the data was performed using Fast Fourier Transform (FFT) with 2048 cutoff frequency. In the plot of impact load, P, vs displacement, δ , the slope, s = ΔP /Δδ , represents the stiffness of the structure [30]. Using smoothed curve in load vs displacement plot, the slope at each point (with a window width of 5) was calculated for each specimen. The maximum slope while the impactor penetrates top and bottom facesheet was calculated respectively, and analyzed. Fig. 10 indicates the attempted approach to characterize the impact behavior of the sandwich structure (specimen 3). Table 2 shows the specific characterized values for the average of each group. Fig. 11 shows the bar graph of the

FMax2−Rawdata , FMax1−Smootheddata FMax1−Rawdata , Fig. 10. F1, FMax2−Smootheddata values in raw and smoothed load vs time plot (specimen 3).

specific characterized values for the average of each group. Dots on the bars represent the values of each specimen in the group. Impact damage analysis for each test specimen was performed. The impact damage was analyzed using the size of the impact damage. Length, width, 45° and −45° direction of the impact damage were measured as shown in Fig. 12. Two different values were used to compare: the mean and the maximum damage size of the four measured dimensions. The measured size of impact damage for each test specimen is described in Appendix 4. Fig. 13 shows the mean damage size and maximum damage size of each group. The result of the four-point bending test showed that the mean ultimate failure load of the water group was lower than other groups (5.15% lower than control group). The core shear ultimate stress graph and face bending stress graph of each group showed the stresses of the 540

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Table 2 Mean value of F1, FMax1−Rawdata , FMax2−Rawdata , FMax1−Smootheddata , FMax2−Smootheddata , E1, EMax1−Rawdata , EMax2−Rawdata , EMax1−Smootheddata , EMax2−Smootheddata , max. slope (top facesheet), and max. slope (bottom facesheet) for each group.

Mean F1 (N) Mean FMax1 (N) – Raw data Mean FMax2 (N) – Raw data Mean FMax1 (N) – Smoothed data Mean FMax2 (N) – Smoothed data Mean E1 (J) Mean EMax1 (J) – Raw data Mean EMax2 (J) – Raw data Mean EMax1 (J) – Smoothed data Mean EMax2 (J) – Smoothed data Max Slope (Top) (N/m) Max Slope (Bottom) (N/m)

Fuel

Oil

Hydraulic

Water

Control

694 1649 1694 1331

705 1581 1617 1294

679 1579 1701 1266

682 1611 1702 1286

701 1660 1726 1272

1412

1376

1430

1413

1389

0.351 5.689 16.090 5.696

0.348 5.363 15.124 5.147

0.353 5.517 15.818 5.313

0.352 4.841 15.873 5.435

0.369 5.265 15.509 5.163

16.053

15.272

16.039

15.922

15.646

330,744 432,586

335,616 427,707

326,974 413,802

323,810 419,229

344,002 414,725

Fig. 12. Impact damage size analysis on Impact test specimen.

performed, which does not require normal distribution of data. The analysis on ultimate failure load, core shear ultimate stress, and face bending stress showed that there was no significant degradation in any of the fluid groups compared to the control group. The results of the statistical analysis are attached in Appendix 5. Impact test results were also analyzed using statistical analysis. A Mann-Whitney U test with Bonferroni correction (m = 4 which makes significant level = α/ m = 0.05/4 = 0.0125) was perform on F1, FMax1−Rawdata, FMax2−Rawdata , FMax1−Smootheddata , FMax2−Smootheddata , E1, EMax2− EMax1−Rawdata , EMax2−Rawdata , EMax1−Smootheddata , Smootheddata , max. slope (top facesheet), and max. slope (bottom facesheet) among the groups. The result of statistical analysis indicated that none of these values among groups showed any significant degradation. The result of the statistical analysis is attached in Appendix 5. The measured impact damage size for each test specimen was

water group was concentrated on the lower bottom half of the spread when compared to the other groups. On the other hand, the mean ultimate failure load of other groups (i.e., fuel, oil, and hydraulic fluid) was higher than the control group. A statistical analysis was performed to determine if the difference between the groups was significant. Since the sample size was small (5 per group), it was difficult to assume that the data was normally distributed. Also, Shapiro-Wilk test indicated that some data was not significantly drawn from a normally distributed population. Therefore, Mann-Whitney U test with Bonferroni correction (m = 4 which makes significant level = α/m = 0.05/4 = 0.0125) was

Fig. 11. Mean F1 (a), FMax1−Rawdata (b), FMax2−Rawdata (c), FMax1−Smootheddata (d), FMax2−Smootheddata (e), E1 (f), EMax1−Rawdata (g), EMax2−Raw (h), EMax1−Smootheddata (i), EMax2−Smootheddata (j), max. slope (top facesheet) (k), and max. slope (bottom facesheet) (l) for each group. 541

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Fig. 13. Mean damage size (a) and maximum damage size (b) for each fluid group.

analyzed using a statistical tool. Mann-Whitney test with Bonferroni correction (m = 4 which makes significant level = α/m = 0.05/ 4 = 0.0125) on mean damage size and maximum damage size was performed between each fluid group and the control group. As a result, none of the analyses showed that none of the fluid groups had a bigger mean damage size and maximum damage size than the control group. The result of the statistical analysis is attached in Appendix 5. After the tests, one sample from each group; fuel, oil, hydraulic, water, and control group was cut open and polished for micro-scale observation. The observation was performed to determine how Nomex® honeycomb core is affected by contacting with aircraft liquid, such as an erosion on phenolic resin coat or aramid fiber of the core. Since the thickness of the wall and amount of phenolic resin on the corner are vary, it was unable to perform statistical analysis on the microscopic data. However, the microscopic picture of each fluid group did not show any significant erosion on the core when comparing to the control group. Fig. 14 shows the example of microscopic picture of the sample and Fig. 15 shows the honeycomb cell wall and corner for each samples. 4. Conclusions In this paper, the effects of fluid ingress in a composite honeycomb structure made with a Nomex® honeycomb core were studied. The authors focused on the lasting effect (i.e., change in mechanical properties) on the sandwich structure when the structure was exposed to aircraft fluids. Two different tests were conducted. A four-point bending test was conducted to study the flexural stiffness of the specimen (quasistatic study), and an impact test was conducted to study the damage resistance characteristic of the specimen (dynamic study).

Fig. 14. Cross sectional microscopic picture of honeycomb cell wall and corner (control group).

Fig. 15. Cross sectional microscopic picture of honeycomb cell wall and corner for each group; fuel (a), oil (b), Hydraulic (c), water (d), and control (e). 542

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soaking period. Each specimen was tested to determine if there was any permanent effect once the specimens were soaked in liquids. Although the results of this research showed that there was no permanent effect on the structure, further research is needed to investigate how the honeycomb structure is affected when it is still wet.

Statistical analysis, Mann-Whitney U test with Bonferroni correction (m = 4 which makes significant level = α/m = 0.05/4 = 0.0125), indicated that there is no significant difference between any of fluid groups to the control group on the characterized values which was used to analyze structural behavior of the test specimens. Based on the result of the statistical analysis, it is possible to conclude that traditional aircraft fluids (i.e., fuel, oil, hydraulic, and water) did not have a significant effect on the lasting flexural stiffness and damage resistant characteristic of Nomex® honeycomb sandwich structure with 45 days of fluid exposure. This research focused on the lasting effect on a Nomex honeycomb structure when the structure contacts with aircraft fluids. Therefore, the specimens were drained and dried after the

Acknowledgement The authors acknowledge Ben Denos in Composites Manufacturing & Simulation Center (CMSC) and Hanxi Sun in Purdue Statistical Consulting Service (CSC) for their support of the test conducting and the test data analysis.

Appendix 1 HCS2404-050 CYTEC Adhesive Film Meltbond 1515-4 is modified epoxy adhesive film. Meltbond 1515-4 is designed to be used for metal to metal, composite bonding, and cosmetic surfacing. Product description (Safety data sheet from the manufacturer). 0.030 – 0.100 (146 – 486) 338 °F (170 °C) G’ knee by dynamic mechanical analysis −65 to 320 °F (−54 to 160 °C) 250 to 350°F (121 to 177 °C)

Nominal Weight, lb/ft2 (g/m2) Tg Dry Service Temp. Range Cure Temp. Range Mechanical property (Technical data sheet from the manufacturer).

Double Lap Shear (MPa) Test Temperature 65 °F (−54 °C) Test Temperature 75 °F (24 °C) Test Temperature 160 °F (71 °C) Test Temperature 270 °F (132 °C) Double Lap Shear psi (MPa) 14 days at 71 °C, 100% R.H. Honeycomb Flatwise Tensile psi (MPa) 14 days at 71 °C, 100% R.H. Double Cantilever Beam in-lb/in2

3495 (24.1) 4708 (32.5) 4141 (28.6) 2023 (13.9) 3208 (22.1) 750 (5.2) 4.9

Sandwich Beam Shear psi (MPa) Test Temperature 65 °F (−54 °C) 776 (5.4) Test Temperature 75 °F (24 °C) 704 (4.9) Test Temperature 160 °F (71 °C) 651 (4.5) Honeycomb Flatwise Tensile psi (MPa) Test Temperature 65 °F (−54 °C) 1021 (7.0) Test Temperature 75 °F (24 °C) 1091 (7.5) Test Temperature 160 °F (71 °C) 1002 (6.9)

Appendix 2 Fuel: Jet-A Jet-A is kerosene-type fuel. Freeze point of −40 °C or below. It does not typically contain static dissipator additive Ingredients (Safety data sheet from the manufacturer).

Name

CAS Number

% Concentration

Kerosine (petroleum) Naphthalene

8008-20-6 91-20-3

100 0–3

Property (Technical data sheet from the manufacturer).

Acidity, mg KOH/g Aromatics, Vol.(%) Sulphur, mercaptan, Wt% Sulphur, total, Wt% 10% Distillation, °C Final Boiling Point, °C Distillation Residue, % Distillation Loss, % Flash Point, °C Density @ 15 °C, kg/m3 Freeze Point, °C Viscosity @ −20 °C, mm/s Net Heat of Combustion, MJ/kg

0.10 Max. 25 Max. 0.003 Max. 0.30 205 Max. 300 Max 1.5 Max. 1.5 Max. 38.0 Min. 775/840.0 −40 Max 8.0 Max. 42.8 Min.

One of the following shall be met 1) Smoke Point, mm, or 2) Smoke Point, mm, and Naphthalenes, Vol. % Copper Strip Corrosion, 2 h % 100 °C Thermal Stability Filter pressure drop, mm Hg Tube Deposits Existent Gum, mg/100 mL. Water Reaction, Interface Rating MSEP Rating Without electrical conductivity additive With electrical conductivity additive

Oil - Mobil Jet Oil II 543

25 Min. 18 Min. 3.0 Max No. 1 25 Max. < 3 Max. 7 Max. 1b Max. 85 70

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Mobil Jet Oil II is a high performance aircraft-type gas turbine lubricant formulated with a combination of a highly stable synthetic base fluid and a unique chemical additive package. The effective operating range of Mobil Jet Oil II is between −40 °C (−40 °F) and 204 °C (400 °F) Ingredient (Safety data sheet from the manufacturer).

Name

CAS Number

% Concentration

N-PHENYL-1-NAPHTHYLAMINE 9,10-ANTHRACENEDIONE, 1,4-DIHYDROXYALKYLATED DIPHENYL AMINES TRICRESYL PHOSPHATE

90-30-2 81-64-1 68411-46-1 1330-78-5

1% < 0.1% 1– < 5% 1– < 3%

Property (Technical data sheet from the manufacturer).

Viscosity cSt @ 40 °C cSt @ 100 °C cSt @ −40 °C % change @ −40 °C after 72 h Pour Point, °C, ASTM D 97 Flash Point, °C, ASTM D 92 Fire Point, °C Autogenous Ignition Temp, °C TAN (mg KOH/g sample)

Density @15 °C, kg/l, ASMT D 4052 Evaporation Loss, % 6.5 h @ 204 °C, 29.5″ Hg 6.5 h @ 232 °C, 29.5″ Hg 6.5 h @ 232 °C, 5.5″ Hg (Equals pressure @ 40,000 Ft. altitude) Foam, ml Sequence I, 24 °C Sequence II, 93.5 °C

27.6 5.1 11,000 0.15 −59 270 285 404 0.03

1.0035 3.0 10.9 33.7

8 10

Hydraulic Fluid – Skydrol LD-4 Fire-resistant hydraulic fluid based on phosphate ester chemistry Ingredient (Safety data sheet from the manufacturer).

Name

CAS Number

% Concentration

Tributyl phosphate Reaction mass of butyl diphenyl phosphate and dibutyl phenyl phosphate and tributyl phosphate 2-Ethylhexyl 7- oxabicyclo[4.1.0]heptane-3- carboxylate butylated hydroxytoluene

126-73-8 EC Number: 907-672-2 62256-00-2 128-37-0

55–65% 20–40% < 10% 1–5%

Property (Technical data sheet from the manufacturer).

Viscosity in cSt −65 °F/−54 °C 100 °F/38 °C 210 °F/99 °C Pour Point (°C) Specific Gravity, 25 °C/25 °C Density at 100 °F/37 °C, g/cm3 Coefficient of Expansion Weight, 75 °F/24 °C Moisture, % Acidity (neutralization number) Bulk Modulus (psi) Surface Tension, 25 °C Heat of Combustion

Foam (ml foam/sec. collapse) Sequence 1 24 °C Sequence 2 93 °C Sequence 3 24 °C Specific Heat 38 °C 93 °C 149 °C Thermal Conductivity 38 °C 93 °C 149 °C Conductivity, microsiemens/cm

1185 cSt 11.42 cSt 3.93 cSt < −62 °C 1.009 0.990 9.2 × 10-4/°C 1008 kg/m3 0.15 0.10 max 15,237 × 105 Pa 28.2 dyn/cm 7.33 kcal/g

Water Ingredient (Safety data sheet from the manufacturer).

Name

CAS Number

% Concentration

Deionized Water

7732-18-5

100

Appendix 3 Weight change and moisture content for each test specimen Four-point bending test specimen.

544

50 cm3/25 s 10 cm3/5 s 40 cm3/20 s 0.437 kcal/(kg. °C) 0.472 kcal/(kg. °C) 0.507 kcal/(kg. °C)

1.89 × 10−3 1.79 × 10−3 1.66 × 10−3 0.43

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Specimen

Weight-dry (g)

Weight-wet (g)

Weight change (g)

Moisture content (%)

Fuel

1 2 3 4 5

55.339 56.034 56.205 56.257 55.583

57.647 61.185 60.388 60.680 60.261

2.308 5.151 4.183 4.423 4.678

4.171 9.193 7.442 7.862 8.416

Oil

1 2 3 4 5

55.872 55.433 55.638 55.417 56.424

59.428 60.668 62.620 60.794 63.053

3.556 5.235 6.982 5.377 6.629

6.365 9.444 12.549 9.703 11.749

Hydraulic

1 2 3 4 5

55.534 55.926 56.104 55.154 56.005

59.595 63.909 60.996 58.686 61.971

4.061 7.983 4.892 3.532 5.966

7.313 14.274 8.720 6.404 10.653

Water

1 2 3 4 5

55.723 56.009 55.946 56.058 55.926

59.638 60.113 62.573 62.816 62.474

3.915 4.104 6.627 6.758 6.548

7.026 7.327 11.845 12.055 11.708

Control

1 2 3 4

56.008 55.086 55.609 55.104

56.413 55.480 55.949 55.508

0.405 0.394 0.340 0.404

0.723 0.715 0.611 0.733

Specimen

Weight-dry (g)

Weight-wet (g)

Weight change (g)

Moisture content (%)

Fuel

1 2 3 4 5

55.491 56.209 55.738 55.223 55.397

60.220 59.902 63.719 62.411 60.205

4.729 3.693 7.981 7.188 4.808

8.522 6.570 14.319 13.016 8.679

Oil

1 2 3 4 5

56.363 55.536 56.301 55.305 55.670

58.409 61.715 58.959 57.527 61.658

2.046 6.179 2.658 2.222 5.988

3.630 11.126 4.721 4.018 10.756

Hydraulic

1 2 3 4 5

55.647 55.548 55.532 56.152 56.116

60.390 60.021 59.635 58.915 60.783

4.743 4.473 4.103 2.763 4.667

8.523 8.052 7.389 4.921 8.317

Water

1 2 3 4 5

55.623 55.584 55.410 55.453 55.154

57.748 61.394 58.114 58.804 61.074

2.125 5.810 2.704 3.351 5.920

3.820 10.453 4.880 6.043 10.734

Control

1 2 3 4

55.694 55.662 55.512 56.105

56.131 56.113 55.975 56.478

0.437 0.451 0.463 0.373

0.785 0.810 0.834 0.665

Impact test specimen.

Appendix 4 Impact damage size for each test specimen.

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Specimen Damage Length (mm)

Damage Width (mm)

Damage 45° (mm)

Damage -45° (mm)

Mean Damage Size (mm)

Max. Damage Size (mm)

Fuel

1 2 3 4 5

41.275 43.637 49.987 49.200 50.648

38.887 33.325 37.287 27.762 32.461

39.675 47.625 37.287 28.575 48.666

36.500 34.112 39.675 33.325 34.315

39.084 39.675 41.059 34.715 41.523

41.275 47.625 49.987 49.200 50.648

Oil

1 2 3 4 5

41.275 47.625 42.062 27.762 49.174

30.937 34.112 31.750 34.112 42.456

36.500 42.062 32.537 42.850 45.555

32.537 30.937 35.712 37.287 42.101

35.312 38.684 35.516 35.503 44.821

41.275 47.625 42.062 42.850 49.174

Hydraulic 1 2 3 4 5

38.100 49.200 36.500 43.637 48.908

37.287 34.112 40.462 30.937 33.807

34.925 46.812 37.287 42.443 42.799

30.937 32.537 36.906 35.712 36.347

35.312 40.665 37.789 38.183 40.465

38.100 49.200 40.462 43.637 48.908

Water

1 2 3 4 5

49.200 49.987 48.412 30.937 47.574

40.462 37.287 40.462 38.887 33.934

52.375 42.062 53.162 53.162 42.697

34.925 35.712 32.537 32.537 36.627

44.240 41.262 43.644 38.881 40.208

52.375 49.987 53.162 53.162 47.574

Control

1 2 3 4

44.450 22.225 45.237 30.937

38.100 30.150 32.537 36.500

50.800 23.012 36.500 31.750

37.287 31.750 33.325 36.500

42.659 26.784 36.900 33.922

50.800 31.750 45.237 36.500

Appendix 5 Statistical analysis result Four-point bending test Mann-Whitney U test on the ultimate failure load (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x ) ).

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Z Statistic

U

P-value

1.10227 1.10227 1.10227 −0.85732

15 15 15 6

0.91105 0.91105 0.91105 0.19563

Z Statistic

U

P-value

1.10227 1.10227 0.61237 −0.85732

15 15 13 6

0.91105 0.91105 0.80437 0.19563

Z Statistic

U

P-value

1.10227 1.10227 0.36742 −1.83712

15 15 12 2

0.91105 0.91105 0.72985 0.03310

Mann-Whitney U test on the core shear ultimate stress (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x ) ).

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Mann-Whitney U test on the face bending stress (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x ) ).

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Impact test Mann-Whitney U test on the F1 (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x ) ).

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Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Z Statistic

U

P-value

−0.24598 0.12247 −0.61237 −0.61237

8.5 11 7 7

0.40285 0.64335 0.27015 0.27015

Mann-Whitney U test on the FMax1−Rawdata (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x )) .

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Z Statistic

U

P-value

−0.61237 −0.85732 −0.85732 −0.85732

7 6 6 6

0.27015 0.19563 0.19563 0.19563

Z Statistic

U

P-value

−0.36742 −1.34722 −0.36742 −0.85732

8 4 8 6

0.35665 0.08895 0.35665 0.19563

Z Statistic

U

P-value

1.83712 −0.12247 −0.12247 −0.12247

18 9 9 9

0.98133 0.45126 0.45126 0.45126

Z Statistic

U

P-value

0.36742 −0.61237 1.10227 0.85732

12 7 15 14

0.72985 0.27015 0.91105 0.86483

Z Statistic

U

P-value

−0.36742 −0.85732 −0.85732 0.12247

8 6 6 11

0.35665 0.19563 0.19563 0.64335

Z Statistic

U

P-value

0.61237 0.61237 −0.12247 −0.85732

13 13 9 6

0.80437 0.80437 0.45126 0.19563

Mann-Whitney U test on the FMax 2−Rawdata (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x )) .

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Mann-Whitney U test on theFMax1−Smootheddata (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x )).

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Mann-Whitney U test on theFMax2−Smootheddata (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x ) ).

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Mann-Whitney U test on the E1 (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x ) ).

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Mann-Whitney U test on the EMax1−Rawdata (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x ))

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Mann-Whitney U test on the EMax 2−Rawdata (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x ))

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Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Z Statistic

U

P-value

0.61237 −0.36742 0.36742 0.85732

13 8 12 14

0.80437 0.35665 0.72985 0.86483

Z Statistic

U

P-value

−0.61237 −1.10227 −1.10227 −0.85732

7 5 5 6

0.27015 0.13517 0.13517 0.19563

Z Statistic

U

P-value

0.12247 −0.12247 0.85732 −0.12247

11 9 14 9

0.64335 0.45126 0.86483 0.45126

Z Statistic

U

P-value

−0.85732 −0.36742 −0.85732 −1.59217

6 8 6 3

0.19563 0.35665 0.19563 0.05567

Mann-Whitney U test on theEMax1−Smootheddata (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x ))

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Mann-Whitney U test on theEMax 2−Smootheddata (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x )).

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Mann-Whitney U test on the max. slope (top facesheet) (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x ) ).

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Mann-Whitney U test on the max. slope (bottom facesheet) (H0: F (x ) ≥ G (x ), Ha: F (x ) < G (x ) ).

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Z Statistic

U

P-value

0.36742 0.36742 −0.12247 0

12 12 9 10

0.72985 0.72985 0.45126 0.54874

Z Statistic

U

P-value

0.85732 0.61237 0.85732 1.59217

14 13 14 17

0.19563 0.27015 0.19563 0.05567

Z Statistic

U

P-value

0.85732 0.36742 0.36742 1.84482

14 12 12 18

0.19563 0.35665 0.35665 0.03253

Impact damage size analysis Mann-Whitney U test on the mean damage size (H0: F (x ) ≤ G (x ), Ha: F (x ) > G (x ) ).

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

Mann-Whitney U test on the max. damage size (H0: F (x ) ≤ G (x ), Ha: F (x ) > G (x ) ).

Fuel vs Control Oil vs Control Hydraulic vs Control Water vs Control

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.compstruct.2018.08.054.

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