investigating the effects of fluid intrusion on nomex ...ciec/proceedings_2017/ciec/ciec... ·...

Session CIEC 361

Proceedings of the 2017 Conference for Industry and Education Collaboration Copyright ©2017 American Society for Engineering Education

Investigating The Effects of Fluid Intrusion on Nomex® Honeycomb

Structures with Carbon Fiber Face Sheets

Garam Kim, Tyler Futch, Ronald Sterkenburg, Sadat Ahsan,

Gozdem Kilaz, Brian Kozak

Purdue University

Abstract

Honeycomb sandwich construction is often used in aircraft structures to make aircraft parts

with a good strength to weight ratio. Nomex® honeycomb core is used extensively for

flooring, skin panels, fairings, engine cowlings, and flight controls. Honeycomb core

structures are prone to fluid intrusion due to their thin face sheets which get damaged easily

by impact or erosion. The purpose of this research was to determine how the mechanical

properties of honeycomb core structures were affected if the honeycomb core was saturated

by a fluid such as water, fuel, hydraulic fluid or engine oil. Especially honeycomb core

structures located near the bilge areas of the aircraft or bottom of engine nacelles or

cowlings, where standing fluids are common will be prone to fluid intrusion. The test panels

were made of carbon fiber pre-preg (IM7 5774) face sheets bonded to a 0.5 inch Nomex®

core. Face sheets and core were bonded with a film adhesive. The test specimens were cut to

size from the cured panels. The test specimens were soaked in either water, fuel, hydraulic

fluid, or engine oil for 45 days. After the soak period the test specimens and the control

group were tested with a four-point loading test and impact test in accordance with ASTM

standards. The collected data was analyzed with an ANOVA and Bonferroni statistical

significance test.

literature review

Due to the wide range of applications of composite sandwich structures in the aerospace,

automobile, shipbuilding, construction, and rail transportation industries, there has been a

significant increase in their usage. Composite sandwich structures consist of multi-layered

materials that are made by bonding rigid, high strength skin facings to low density core

materials. The high strength and low weight ratios of the sandwich concept are the primary

advantages of using it in structural components, such as fuselage, wing flaps, and fairings.

Primary advantages of using bonded joint composite structures include: reduction of local

delamination because no holes are required, significant reduction in the weight of the joints,

the ability to assemble dissimilar materials, prevent galvanic corrosion of conductor metals,

and design a smooth external surface. Before the sandwich structure is to be used for

different applications, there is a great need to better understand their static and fatigue

behavior, as well as the various failure mechanisms under static and fatigue loading

conditions.[1]

Because of the location of composite structures in aerospace applications, they experience

multiple environmental conditions by which, through diffusion, they can absorb moisture,

Session CIEC 361


oil, aviation hydraulic fluid, anti-icing additive, or jet fuel. Diffusion occurs when there is

enough porosity, voids and vacancies within structures to allow the liquid to permeate the

face sheets or honeycomb. This absorption is largely permanent and can lead to chemical

and/or physical aging of the material, and can result in the part failing at loads lower than its

designed strength. In one study, a researcher used barrier films to reduce absorption of fluids,

and it considerably improved the mechanical properties of the sandwich structures.[2]

Sandwich structures are made of two face sheets and a core. The face sheets are co-cured to

the low density lightweight core. These sheets are manufactured from thin, high strength

materials. Face sheets are usually made of; carbon fiber, fiberglass, or aluminum and are

designed to withstand damage and high in plane stresses.[3] The structure gains it flexural

rigidity and strength in the transverse direction from the core. The core separates the two face

sheets and greatly increases the structure’s strength without a large increase in weight: one of

the primary advantages of composite sandwich structures. The core is generally made of

materials with lower stiffness and strength than the face sheet’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.[3]

Because of the lack of knowledge of composite structures under complex dynamic loads, its

expansion to fields in which high reliability is demanded, such as aviation, has been

constrained. There are numerous challenges that must be met before composites sandwich

structures can be used in primary structures and before there can a widespread application of

it. Before this can be achieved, the sandwich structure needs to be assessed in order to verify

that any stresses experienced during the service life will not result in failure, or excessive

structural deformation before the damage is detected.[4]

The major failure mode of the structure is the de-bonding of the adhesive between the face

sheet and the core. A finite element analysis approach was utilized by Jen, Y. M., & Chang,

L. Y. to determine the local elastic modulus by taking account of the adhesive’s geometry

and dimensions. In order to properly evaluate the fatigue life of the studied sandwich

structures, several global and local parameters were utilized.[5] Jen and Chang utilized a

circular shaped local parameter in their experiment, this combined the peeling stress and the

shear stress of the adhesive on the de-bonding plane, which correlates with the fatigue life of

the sandwich beams with various core densities effectively. The analysis proved to be

accurate when compared to the de-bonding locations observed in fatigue tests. These are key

areas to examine during testing of composite sandwich structures.[5]

One of the biggest challenges in aircraft design is weight to strength ratio. It is critical to

retain a strong, stable structure that can withstand the various loads aircraft experience in

flight, and to ensure that it is still light enough to fly from point A to point B.[6] The face

sheets are mainly manufactured from fiber glass and carbon fiber reinforced prepregs with

epoxy resin matrices. The core material widely used in sandwich structures is Nomex®

honeycomb.[6] Composite sandwich structures have a wide range of applications on aircraft.

Many of their applications are on fairings, doors, radomes, leading edges, primary flight

controls, secondary flight controls, and non-structural part applications. The Airbus A340,

for example, has its entire vertical tail plane made of composite sandwich structures.[6] In

order for composite sandwich structures to be more widely used in commercial aviation,

many of the challenges with these structures will need to be addressed in the future.[6]

Problem statement

Usage of honeycomb materials in various industries is increasing. In the case of the

aerospace industry, lightweight and high strength aircraft structures are very important.

Therefore, honeycomb structures occupy a large portion of airplane. There are many types of

Session CIEC 361


fluids that come into contact with the aircraft such as moisture, fuel, hydraulic fluids, and oil.

Aircraft fluids can be a possible factor that causes defects affecting the strength of

honeycomb structures. This is especially true for the areas where liquids can be collected

such as at the bottom sides of engine cowlings where there is a higher chance to be exposed

to aviation liquids. Therefore, it is important to find out how various aircraft fluids affect the

mechanical properties of honeycomb structures that have been saturated.

Proposed methodology

To determine how aircraft fluids affect honeycomb structures two kinds of experiments were

prepared; four-point loading test and impact test.[7] The four-point loading test is used for

quasi-static analysis and it is utilized to investigate the flexural stiffness of the specimen. The

objective of a four-point loading test is to observe flexural stiffness of honeycomb core for

different groups and compare each other to determine how fluids affect the core of the

specimen.[8] The damage resistance test is used for dynamic analysis and it is conducted by

causing an impact to the specimen by dropping a weight from a specified height above the

specimen. The objective of this test is to compare the damage resistance properties of each

sample with the control group and to observe any negative effects.

Manufacturing

Sample manufacturing can be divided into 3 processes; manufacturing face sheets, bonding

face sheets to the core, and cutting specimen into appropriate size. The face sheets were

made out of carbon fiber prepreg (IM7 5442). They were cut into 10-inch x 14-inch plies

with a 0.03-inch thickness. Each face sheet was 4 plies thick and the orientation of the face

sheets were [-45/90/0/45]. Aluminum caul plates were placed on top of the face sheets to

insure that they would have a uniform flatness on both sides. The sheets were vacuum

bagged and cured with the appropriate cure cycle. Figure 1 illustrated the complete face sheet

of the honeycomb panel. After curing, the parts were trimmed to remove rough edges and

make them easier to handle. Nomex® honeycomb, 0.5-inch thick, was used as the core

material for the specimen. They were cut to 10-inch x 14-inches and prepared for bonding.

After the face sheets were made, they were bonded to the core on both sides with film

adhesive, HCS2404-050. The assembled honeycomb sandwich panel was vacuum bagged

and cured in an oven. Figure 2 showed the bagging process of the honeycomb panel. The

cured part was cut into correct specimen size. The dimensions of the 4-point loading test

sample were 8-inch x 3-inch. The dimensions of the impact test sample were 6-inch x 4 inch.

Samples were cut using a surface grinder fitted with a diamond coated saw blade. After the

specimens were cut, the edges were sanded lightly to create a smooth edge. The samples

were then lightly blown off by compressed air to remove any contaminants created during

cutting and sanding processes. The top and bottom surface of samples were wiped with clean

rag and acetone. Dimensions (Length, width and thickness) and weight were measured before

the samples were soaked into the fluids. Figure 3 showed the complete set of impact test

specimens

Session CIEC 361


Figure 1. Face sheet of the honeycomb panel

Figure 2. Honeycomb panel assembly process

Figure 3. Manufactured honeycomb panel specimen (impact test)

Soaking

All of the samples were soaked into a different kind of aircraft fluid. Four kinds of aircraft

fluids were prepared; distilled water, aircraft turbine engine oil, JET-A (Aircraft Jet Fuel) and

Skydrol (Hydraulic Fluid). The reason of choosing these four kinds of liquid was because

these fluids were used on aircraft and there was an opportunity that these fluid could come in

contact with the honeycomb structure of the aircraft. The specimens were soaked in glass

containers that were big enough to soak all the specimens that were prepared. Glass

containers were used to reduce any reaction between the fluid and container. Samples were

soaked all the way from the bottom to the top, so that there was no area of the specimens that

did not touch the fluid. After the samples were soaked into the fluid, containers were sealed

Session CIEC 361


to make sure any dust or moisture could not enter the containers. Samples were removed

from the containers after 45 days of soaking. The samples were dried before testing.

4 points loading test

A 4-point loading test was designed and performed based on ASTM C393/C393M-16. MTS

810 test machine was used for testing. Figure 4 depicted the 4-point loading test specimen

that was ready to be tested. Figure 5 showed the test machine (MTS 810) that was used for 4-

point loading test. ASTM standard fixture was prepared and installed appropriately on the

machine. Bottom span was 6 inches wide and top span was 3 inch wide. 1-inch diameter

aluminum rods were placed on the top of the spans. The test speed was 6 mm/min. Figure 6

displayed 4-point loading test process with the specimen.

Figure 4. 4-point loading test specimen

Figure 5. 4-Point loading test machine

Session CIEC 361


Figure 6. 4-point loading test machine with specimen

Impact test

The impact test was designed and performed based on ASTM D7136/D7136M-15. Figure 7

showed the impact test specimen that was ready to be tested. The impact test was performed

using Instron 9250HV drop tower. Figure 8 showed the Impact test machine and test fixture

with specimen. A 16mm hemispheric striker was used for tip of dropping object. Mass of

dropping object was 5.5506kg (ASTM 5.5±0.25 kg). Drop height was 1.7647m, and impact

velocity was 5.88409. Extra 6.75% of height was added to help the striker to penetrate the

specimen all the way through. Each specimen was tested in the same way. Figure 9

illustrated the damaged impact test specimen.

Figure 7. Impact test specimen

Figure 8. Impact test machine and test fixture with specimen

Session CIEC 361


Figure 9. Damaged impact test specimen

Result of 4-point bending test

For 4-point loading test five specimens were prepared for each liquid group and four

specimens were prepared for the control group which were not soaked into any liquid. In

total, 24 specimens were tested and test data was collected. Data analysis was performed with

an advanced analysis tool, OriginPro.

There was a lot of noise on the raw data, therefore adjacent-averaging smooth method with 5

points of window was used to smooth the raw data. Displacement was used as the

independent variable and load was used as the dependent variable. Graph 1 illustrated the

results of the 4-point loading test. Specimen 1 to 5 were in fuel group, specimen 6 to 10 were

in oil group, specimen 11 to 15 were in Skydrol group, specimen 16 to 20 were in water

group and specimen 21 to 24 were in control group. During testing, the honeycomb core of

the sample started to break at the ultimate load and load started to decrease after that.

Therefore, ultimate load of each smoothed sample data was used for data analysis. First,

ultimate load for each smoothed sample data was collected. Grubb’s test was used for

samples in each liquid group to identify any outliers in the group. A significance level of 0.05

was selected. If there were any outliers, those values needed to be excluded when group

comparison analysis was performed. Grubb’s test indicated that the ultimate load of sample

21 was isolated from the ultimate load of other samples in the group, which meant it was an

outlier. One-way ANOVA test and Bonferroni test were performed among each group to find

there was any significant difference between groups. A significant level of 0.05 was selected

and sample 21 was not included in the tests. Table 1 showed the result of one-way ANOVA

test for 4-point loading test results. As a result, ANOVA test indicated that there was a

significant difference among the groups, and Bonferroni test indicated which group had a

significant difference with other groups. The results showed that there was a significant

difference between the fuel group and the water group (fuel > water), oil group and water

group (oil > water), hydraulic group and water (hydraulic > water), oil group and control

group (oil > control), and hydraulic group and control group (hydraulic > control). Table 2

showed the result of Bonferroni test for 4-point loading test results.

Session CIEC 361


Graph 1. 4-point loading test result of 24 specimens

Table 1. ANOVA test result for 4-point loading test results

Table 2. Bonferroni Test for 4-point loading test results

Session CIEC 361


Result of impact test

For impact testing, five specimens were prepared for each liquid group and four specimens of

the control group specimens. In total 24 specimens were tested and the data was collected. A

load vs. time graph was made for each specimen and Fast Fourier Transform smoothing

function with 20 points of window was used to smooth the graph and remove the noise.

Graph 2 illustrates the results of the impact test. Specimen 1 to 5 were in fuel group,

specimen 6 to 10 were in oil group, specimen 11 to 15 were in Skydrol group, specimen 16 to

20 were in water group and specimen 21 to 24 were in control group. Three different

methods were used for the drop test data analysis. The first method was to compare the

ultimate load of the sample when the tip of the striker met the top face sheet. As the striker

met the face sheet of the sample, load increased and reached ultimate load. If the honeycomb

core that supported the face sheet was affected by the fluid and the mechanical property

changed, the ultimate load changed as well. The ultimate load was collected from smoothed

graph of each specimen. Grubb’s test was performed to determine ultimate load in each

liquid group to find if there were any outliers in the groups. The test showed that there were

no outliers. The ANOVA test and Bonferroni test were performed to check for any

significant difference among the group. 0.05 was used for significant level, ɑ. Neither test

showed significant difference among the groups. Table 3 and table 4 showed the result of

each test.

Graph 2. Impact test result of 24 specimens

Session CIEC 361


Table 3. ANOVA test result for the ultimate load comparison

Table 4. Bonferroni test result for the ultimate load coparison

The second method used compared the max slope value in load vs. time graph when the top

of the striker hit the top of the face sheet. The slope of the graph indicated the stiffness of the

carbon fiber face sheet material and honeycomb core. OriginPro “Move Slope” function was

used to find the maximum slope of the load vs. time graph. After the slope value was

collected at each point, slope vs. time graph was generated and the maximum value of the

graph when the striker hit the face sheet was recorded. Grubb’s test was used to find any

outliers in the groups (ɑ=0.05). Specimen 2 and specimen 20 were indicated as outliers,

therefore, specimen 2 and specimen 20 were excluded for the ANOVA test and Bonferroni

test. The ANOVA test and Bonferroni test were used to check if there was any significant

difference among the groups (ɑ=0.05). The tests indicated that there was no significant

difference among the groups. Table 5 and table 6 showed the result of each test.

Table 5. Anova test result for the 1st slope comparison

Session CIEC 361


Table 6. Bonferroni test result for the 1st slope comparison

The third method used compared the slope of the graph when the tip of the striker met the

honeycomb core after it passes through the top face sheet. The slope of the graph indicated

the stiffness of the material and the stiffness of the honeycomb core was found by recording

the slope when the tip hit the core. The face sheet was stiffer than the honeycomb core,

therefore, the load started to decrease after the tip passed the face sheet. The slope of the

decreasing load was affected by the stiffness of the honeycomb core. The slope value of the

graph when the tip met the core showed how honeycomb core mechanical properties got

affected by the different types of liquids. The “Move Slope” function of OriginPro was used

for smoothed load vs. time graph. After the slope value was collected at each point, a slope

vs. time graph was generated and the minimum value of the graph when the striker hit the

honeycomb core was collected. Grubb’s test was used to find any outliers in the groups

(ɑ=0.05) and it indicated specimen 14 as the only outlier. Specimen 14 was excluded from

ANOVA test and Bonferroni test. The ANOVA test and Bonferroni test were performed to

find any significant difference among the groups (ɑ=0.05). The analysis showed that the

minimum slope of the Skydrol group was significantly lower than fuel group and oil group.

Table 7 and table 8 showed the result of each test.

Table 7. Anova test result for the 2nd slope comparison

Table 8. Bonferroni test result for the 2nd slope comparison

Session CIEC 361


Recommendation

It was difficult to determine if the fluids fully penetrated the specimen during the soaking

period. Therefore, after the testing was completed the specimen were cut and it was

determined that all specimens were fully penetrated with fluid during the soaking period.

Figure 10 showed examples of a cut open specimen (fuel group specimen and control group

specimen). The data analysis indicated a mixed result. The first and second data analysis of

the impact test showed that there was no significant difference between specimens that were

soaked in a fluid compared to the control group. Which meant that during a short period of

time (45 days) the structural strength of a honeycomb aircraft part was not affected. Third

data analysis of the impact test indicated that there was a significant difference between some

of the groups, but there was not any group which had a significant difference with the control

group. The 4-point bending test showed that there was a significant difference between

groups, however, the results of the specimens that were subjected to a fluid were not lower

than the control group. Additional testing over longer periods of time are necessary to

determine if fluid intrusions will affect the airworthiness condition of aircraft with

honeycomb core structures.

Figure 10. Open cut of the specimens (Fuel group specimen vs control group specimen)

Bibliographic information

[1] Abbadi, A., Azari, Z., Belouettar, S., Gilgert, J., & Freres, P. (2010). Modelling the fatigue behaviour of

composites honeycomb materials (aluminium/aramide fibre core) using four-point bending tests.

International Journal of Fatigue, 32(11), 1739-1747.

[2] Kececi, E. & Asmatulu, R. (2016). Effects of moisture ingressions on mechanical properties of honeycomb-

structured fiber composites for aerospace applications. The International Journal of Advanced

Manufacturing Technology, pp 1-12, DOI:10.1007/s00170-016-8744-8.

[3] Hill, M. D. (2007). Damage resistance and tolerance investigation of carbon/epoxy skinned honeycomb

sandwich panels (Doctoral dissertation, Loughborough University)

[4] Belouettar, S., Abbadi, A., Azari, Z., Belouettar, R., & Freres, P. (2009). Experimental investigation of static

and fatigue behaviour of composites honeycomb materials using four point bending tests. Composite

Structures, 87(3), 265-273.

[5] Jen, Y. M., & Chang, L. Y. (2008). Evaluating bending fatigue strength of aluminum honeycomb sandwich

beams using local parameters. International Journal of Fatigue, 30(6), 1103-1114.)

Session CIEC 361


[6] Hermann, A. S., Zahlen, P. C., & Zuardy, I. (2005). Sandwich Structures 7: Advancing with Sandwich

Structures and Materials. In Sandwich Structures Technology in Commercial Aviation (pp. 13-26). Springer

Netherlands. Retrieved October 12, 2016, from http://link.springer.com/chapter/10.1007/1-4020-3848-8_2

[7] ASTM Standard D7250, 2016, “Standard Practice for Determining Sandwich Beam Flexural and Shear

Stiffness,” ASTM International, West Conshohocken, PA, 2016, DOI: 10.1520/D7250_D7250M-16,

www.astm.org.

[8] ASTM Standard D7766, 2016, “Standard Practice for Damage Resistance Testing of Sandwich

Constructions,” ASTM International, West Conshohocken, PA, 2016, DOI: 10.1520/D7766_D7766M-16,

www.astm.org.

Biographical information

Garam Kim is a graduate research assistant in the advanced composite laboratory of Purdue University.

Currently, he is completing his master program at the School of Aviation and Transportation Technology at

Purdue University. He is a certificated Airframe & Powerplant mechanic (A&P) and he has experience as an

aircraft maintenance technician in the Republic of Korea Air force. He is focusing on designing, manufacturing

and testing composite material parts for his study.

Tyler Futch is from the city of Phoenix, Arizona in the United States. After high school, Tyler received his

undergraduate degree from Purdue University majoring in Aeronautical Engineering Technology. While

managing the composites laboratory at the Purdue Airport, Tyler completed a master's degree with the

department of Aviation Technology. Presently, Tyler is working on a doctorate degree in technology at Purdue

University with a concentration in composite manufacturing.

Dr. Ronald Sterkenburg is a professor at the School of Aviation and Transportation Technology at Purdue

University. He is a certificated Airframe & Powerplant mechanic (A&P), holds an Inspector Authorization (IA)

and performs the duties as a Designated Mechanic Examiner. Dr. Sterkenburg's main research interests are in

advanced composite materials for aerospace vehicles. He has published many articles, book chapters and books

on all types of aviation maintenance topics.

Sadat Ahsan is an undergraduate research assistant in the department of Aviation Technology at Purdue

University. He is currently pursuing a Bachelor of Science degree in Aeronautical Engineering with a minor in

Product Lifecycle Management. He is teaching assistant for composite manufacturing courses.

Gozdem Kilaz is an assistant professor in the School of Engineering Technology, at Purdue University, with a

joint appointment in the Laboratory of Renewable Resources Engineering. Dr. Kilaz earned Bachelor’s,

Master’s, and Ph.D. degree in Chemical Engineering. Currently, Dr. Kilaz serves as the Chief Scientist of the

Air Transport Institute for Environmental Sustainability (AirTIES) center, in the Aviation Department of

Purdue University. She is teaching aviation fuels and materials.

Dr. Kozak is an Assistant Professor in the School of Aviation and Transportation Technology. He currently

teaches within the Aeronautical Engineering Technology (AET) and Unmanned Aerial Systems (UAS)

majors. Dr. Kozak holds Private Pilot Airplane Single Engine Land (ASEL), Airframe and Powerplant (A&P),

and Small Unmanned Aerial Systems Remote Pilot (sUAS) certificates from the FAA. He has strong personal

interests in aviation where he enjoys flying aircraft and building a composite airplane.

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