中 華 大 學 碩 士 論 文
Effect of Pre-Soaking in Water on the Static and Fatigue Strengths of Aluminum Honeycomb
系 所 別：機 械 工 程 學 系 碩 士 班
學號姓名：M09808045 Ikramullah Zein (伊廣) 指導教授：陳 俊 宏
任 貽 明 博 士
中 華 民 國 101 年 1 月
Honeycomb structure is an array of open cells, shaped from thin sheets material and attached to each other. These structures are widely applied in aerospace, civil and mechanical industries due to their excellent stiffness/weight ratio, heat and acoustic insulation properties. Due to the honeycomb structure has high rigidity to withstand the weight, strength characteristics, high fatigue resistance and its buoyancy, honeycomb structure were applied for aircraft, satellite, aerospace and also used for the devices that work in the water and at the place with high humidity. This study aims to find the static and fatigue strength of aluminum honeycomb structures after immersed into the water. The experiments were conducted in room temperature and before conducting the experiment; all specimens (aluminum honeycomb sandwich structure) were immersed in water with different time of immersion. Those are 10 weeks, 12 weeks and 14 weeks.
The specimens for the core of honeycomb structure and for the face sheets are made from aluminum 3104 and aluminum 5052, respectively. In static tests, most of the specimens fracture due to face wrinkling and acquired mean ultimate strength for 10 weeks, 12 weeks and 14 weeks of immersion are 2166.71 N, 2117.64 N and 2092.99 N, respectively. Five load levels were chosen for each conditions, those are 50%, 55%, 60%, 65% and 70% for condition 10 weeks of immersion and 55%, 60%, 65% 70% and 75% for conditions 12 weeks and 14 weeks of immersion. The results of fatigue tests, most of the specimens fracture due to core buckling and few specimens fracture due to adhesive debonding.
Keywords: Honeycomb Sandwich Structure; Humidity; Immersion Time;
Ultimate Strength; Static and Fatigue Tests
蜂窩結構是一系列的開放的細胞，由薄片材料和附屬的陣列形成。這 些結構被廣泛應用於航空，民用和機械行業，由於其優異的剛度/重量比 例，具有隔熱和隔音的屬性。由於蜂窩狀結構，具有高剛度，以承受重量，
強度大的特點，有耐疲勞性和浮力，蜂窩結構應用於飛機，衛星，航空，也 使用這個設備在水裡和濕度高的地方。這項研究的目的是找到沉浸入水後的 鋁蜂窩結構的靜態和疲勞強度。實驗在實驗進行前的室溫下進行;所有標本
（鋁蜂窩夾層結構）浸泡在水中有不同的浸泡時間。這些是 10 週，12 週和 14 週。蜂窩結構的核心標本和面孔板料標本，分別由鋁 3104 和鋁 5052 構 成。在靜態測試中，大部分的標本斷裂是由於面對起皺和獲取極限強度的方 法，分別為 2166.71 N，2117.64 N 和 2092.99 N，分別為 10 週，12 週和 14 週的浸泡。五負荷水平有個別的條件選擇，這些是 50％，55％，60％，65
％和 70％的條件是浸泡 10 週和 55％，60％，65％～70％的條件是浸泡 12 週和 75％的條件是浸泡 14 週。疲勞試驗結果，大多數的標本斷裂是由於核 心屈曲，幾個標本骨折是由於膠粘劑剝離。
First of all I would like to say “Alhamdulillahi Rabbil ‘alamin” and my gratitude to Allah SWT for giving me a chance to continue my study abroad, especially in Chung Hua University, Taiwan. Also, I would like to thank you to the following people who supported, assisted and advised me to finish this thesis.
In the beginning, I would like to acknowledge my thank you to my advisors Dr. Juhn-Horng, Chen and Dr. Yi-Ming, Jen, who willing to contribute their time to advise me and provide guideline and materials for me to finish this thesis. They also have been guided me in earnest and gave me support during my study. I would like to acknowledge to Dr. Yung-Chuan, Chiou and Dr. Ti-Kuang, Hou, who spent their time to be the committee members in my oral defense and also gave suggestions and inputs for my thesis. I also appreciate my grateful to all my lecturers in Mechanical Engineering Department, for their expertise, that had concern regarding my academic requirements. In addition, I would like to acknowledge the academic and technical support of the Administrators in Mechanical Engineering Department, Chung Hua University, for their efforts.
My great grateful to my late father, Muhammad Zein and my mother Salmiah, my brother, Irwansyah Zein and my sisters, Intan Sahara Zein and Keumala Citra Sarina Zein, who give me an encouragement, support and care about me, so far, without their support, I won’t be able to study here in Taiwan.
Futhermore, I would like to thank you to my Acehnese student friends in Chung Hua University, Bang Deni, Bang Iqbal, Hendri, Joni, Bang agus, Bang Ilham, Iqmal, Kak Munira, Nadya, Okta, Reza, Astrid and Desi and all of Acehnese students in Taiwan. And I also appreciate my mechanical engineering’s classmates and my M111 laboratory colleagues, who help and care about me.
Finally, I will dedicate this thesis to everyone, who gave me support and assisted me. I am pleased to study in Chung Hua University and to be one of its alumni. Thank you CHU and Mechanical Engineering Department.
January 6, 2012, Hsinchu-Taiwan
TABLE OF CONTENTS
Abstract... I Acknowledgement ... III Table of Contents ... IV List of Table ... VI List of Figure ...VII
Chapter I Introduction ...1
1.1 Background ...1
1.2 Aluminum Honeycomb Sandwich Structure Manufacturing ...2
1.3 Experimental Purposes ...4
1.4 Experimental Procedure ...4
1.5 Limitation of The Experiment...4
1.6 Chapter Summary ...5
Chapter II Literature Review ...9
2.1 Analysis of Mechanical Behavior of The Honeycomb Structure ...9
2.2 Analysis of The Structural Strength of Honeycomb Parts ... 2.3 Fatigue Properties of The Honeycomb Structural Panels ...11
2.4 Temperature and Humidity Environment as The Fatigue Properties of 2.5 The Honeycomb Structure ...12
Temperature and Environment effects on Fatigue Strength ...14
2.5.1 Temperature ...14
2.5.2 Environment ...14
Chapter III Experimental Program ...15
3.1 3.2 Specimen for Experimental ...15
Exper 3.2.1 imental Instrument ...15
Measurement Instrument (Instron 8872 Series)...16
188.8.131.52 Measurement Parameters for Static ...16
184.108.40.206 Measurement Parameters for Fatigue ...16
3.2.2 Experimental Condition for Static Experiment ...17
3.2.3 Experimental Condition for Fatigue Experiment ...17
3.2.4 Four‐point Bending Test ...18
3.3 Experimental Procedures ...18
Chapter IV Results and Discussion ...24
4.1 Aluminum Honeycomb Structure Static Test Result ...24
4.2 Aluminum Honeycomb Structure Fatigue Test Result ...25
4.2.1 Aluminum Honeycomb Structure Static Test Result for 100%RH and 10 Weeks of Immersion ...25
4.2.2 Aluminum Honeycomb Structure Static Test Result for 100%RH and 12 Weeks of Immersion ...26
4.2.3 Aluminum Honeycomb Structure Static Test Result for 100%RH and 14 Weeks of Immersion ...27
Chapter V Conclusions, Contributions and Suggestions ...51
5.1 Conclusions ...51
5.2 Contributions ...52
5.3 Suggestions ...52
LIST OF TABLES
Table 3.1 Honeycomb Structure and The Material Properties of Aluminum alloy. ………... 19
Table 4.1 Data of Static strength for Aluminum Honeycomb Specimen in 50% of Humidity ………... 28
Table 4.2 Data of Static Strength of Aluminum Honeycomb Structure in 100% of Humidity………...…. 29
Table 4.3 Fatigue Data of Aluminum Honeycomb structure in Conditions 100%RH and 10 Weeks Immersion Time ……... 30
Table 4.4 Fatigue Data of Aluminum Honeycomb structure in Conditions 100%RH and 12 Weeks Immersion Time ……... 31
Table 4.5 Fatigue Data of Aluminum Honeycomb structure in Conditions 100%RH and 14 Weeks Immersion Time ……... 32
LIST OF FIGURES
Figure 1.1 Expansion Manufacturing Process ………... 6
Figure 1.2 Corrugation Manufacturing Process ……… 7
Figure 1.3 Honeycomb Cell Configurations. ………... 8
Figure 3.1 Honeycomb Structure Specimen Model ……….. 20
Figure 3.2 Aluminum Honeycomb Geometric Dimensions and Force Conditions………. 21
Figure 3.3 Instron 8872 Uniaxial Material Testing System Type……... 22
Figure 3.4 Four-Point Bending Fixture ………... 23
Figure 4.1 Graphs of The Different of Mean Ultimate Static Strength …... 33
Figure 4.2 Aluminum Honeycomb Structure in Different Condition ... 34
Figure 4.3 Damage Aluminum Honeycomb Structure in Conditions 100%RH and 10 Weeks of Immersion Time After Static Test... 35
Figure 4.4 Damage Aluminum Honeycomb Structure in Conditions 100%RH and 12 Weeks of Immersion Time After Static Test... 36
Figure 4.5 Damage Aluminum Honeycomb Structure in Conditions 100%RH and 14 Weeks of Immersion Time After Static Test... 37
Figure 4.6 Graph of Static Strength of Aluminum Honeycomb Structure in 100%RH and 10 Weeks Immersion Time …... 38
Figure 4.7 Graph of Static Strength of Aluminum Honeycomb Structure in 100%RH and 12 Weeks Immersion Time …... 39
Figure 4.8 Graph of Static Strength of Aluminum Honeycomb Structure in 100%RH and 14 Weeks Immersion Time …... 40
Figure 4.9 Graph of Static Strength of Aluminum Honeycomb structure in 100%RH in 10 Weeks, 12 Weeks and 14 Weeks Immersion
Time ………. 41
Figure 4.10 S-N Curve of Aluminum Honeycomb Structure in 100%RH and 10 Weeks Immersion Time ………. 42
Figure 4.11 Damage Aluminum Honeycomb Structure in Conditions 100%RH and 10 Weeks of Immersion Time after Fatigue Test. 43
Figure 4.12 S-N Curve of Aluminum Honeycomb Structure in 100%RH and 12 Weeks Immersion Time ………. 44
Figure 4.13 Damage Aluminum Honeycomb Structure in Conditions 100%RH and 12 Weeks of Immersion Time after Fatigue Test. 45
Figure 4.14 S-N Curve of Aluminum Honeycomb Structure in 100%RH and 14 Weeks Immersion Time ………. 46
Figure 4.15 Damage Aluminum Honeycomb Structure in Conditions 100%RH and 14 Weeks of Immersion Time after Fatigue Test. 47
Figure 4.16 Combinations of S-N Curve of Aluminum Honeycomb Structure in 100%RH and in 10, 12 and 14 Weeks Immersion Time ………... 48
Figure 4.17 Graph of The Influence of Load Level (%) on the Cycles to Failure (cycles) of Aluminum Honeycomb Structure with Different immersion times ………. 49
Figure 4.18 Graph of Relationships Between the Stroke Range () and Cycles to Failure (Nf) of Aluminum Honeycomb Structure
with Different Immersion 50
CHAPTER I INTRODUCTION
Honeycomb structure is an array of open cells, formed from very thin sheets of material attached to each other . This structure seems like the bee’s honeycomb in nature. It can be made from any thin flat material, and more than 500 different kinds of honeycomb have been made or manufactured.
Paper honeycomb was the first made by Chinese over 2000 years ago and used it for ornaments and still used it for today. In 1845, tubular railroad bridge in Wales was the earliest man-made sandwich structure had been record. It consisted of a large rectangular tube, the floor of which supported railroad track, and through which trains ran. At the year 1905 in Germany, the first honeycomb core patent was issued, it is covering a manufacturing method for the production of Kraft paper honeycomb and it is Budwig Patent . Later, between World War I and World War II, the Italian seaplanes used plywood surfaces that attached to a balsa wood core as the primary structure of the plane, but, The first aircraft sandwich panel was manufactured using thin mahogany and bonded to an end- grain balsa wood core, used as the primary structure of the pontoons of a seaplane.
In 1945 the first all-aluminum honeycomb sandwich was made. During this period, the development of better adhesives for attachment of facings to the cores also found .
Aluminum honeycomb structures are widely apply for aircraft such as uses in cabin decks, bulkheads, wing and tail trailing edges, control surface, doors and access panels, leading edges, trailing edges, flaps, spoilers, elevators, rudders, cowlings, floors, side panels and ceilings also uses for helicopter’s blades. For aerospace this structures uses in the bodies of the aerospace and also uses in space telescope the Jet Propulsion Laboratory (JPL). In civil application, the honeycomb uses for floating roofs, floating docks, bridge and so on. For mechanical application, this structure uses for energy absorption, air directionalization and
thermal versatility. In electrical application use in LED (Light Emitting Diode) technology and also in loudspeaker technology and in architectural industries are use for acoustic also in marble curtain wall and so on. Also in sport structures, many applications use aluminum honeycomb structures such as in snow skies, canoe and so on and also for marine stuff and for all structures that require stiffness of the structures, due to their excellent stiffness, weight ratio, heat resistance, energy-absorbing capacity and acoustic insulation properties.
As explained at passage before that honeycomb structure is not only used for aircraft, satellite, aerospace, and other equipment that work in open air or in place with low level of humidity, but also the honeycomb structure applied for the devices that work in the water and at the place with high humidity. Due to the honeycomb structure has high rigidity to withstand the weight, strength characteristics, high fatigue resistance and its buoyancy.
Because of so many utilization of aluminum honeycomb sandwich structure in area with high humidity, it is necessary to conduct an experiment to obtain or to discover the strength of the aluminum honeycomb sandwich structure by providing static and fatigue test.
1.2 Aluminum Honeycomb Sandwich Structure Manufacturing
There are five methods to manufacture metal honeycomb, based on the way in which the nodes are attached. These methods are resistance adhesive bonding, welding, brazing, diffusion bonding, and thermal diffusion. Probably more than 95% of honeycombs are produced through adhesive bonding. The limitation of adhesive bonded honeycomb is represented by the maximum temperature that it can withstand, usually 399°C with polyimide or 204°C with nylon epoxy and nitrile phenolic adhesives. When higher operational temperatures or severe environmental conditions are required, cores can be manufactured using resistance welding, brazing, or diffusion bonding, even though these processes are more expensive than adhesive bonding .
There are two basic techniques to convert a sheet of metal into honeycomb:
the expansion process and the corrugation process 1.2.1 Expansion process
The majority of the adhesive bonded core made by the expansion process shown in Figure 1.1. At first, a corrosion resistant coating is applied to the foil sheets and adhesive lines are printed. After the sheets are cut and stacked one upon the other. The HOBE is brought to the autoclave, where the adhesive is cured under high temperature and pressure. Then the HOBE is cut into slices of the required thickness and expanded: when the metallic cores are expanded, the sheets yield plastically at the node-free wall joints and thereby retain their expanded geometric shape . The method described is working for metallic honeycomb; for non-metallic honeycomb, like the Nomex TM., the process is a little different. Here is enough to say that the honeycomb after the expansion has to be held in a rack. Then the honeycomb block is dipped in liquid resin and oven cured .
1.2.2 Corrugation process
The corrugation method, invented by Hexcel, was the first used to produce aluminum honeycomb. This system is still used today to produce high-density metallic cores, even though is expensive . The method is shown in Figure 1.2.
In the process, the sheets are first corrugated and adhesive is placed at the nodes (now this is done automatically). Then the sheet are piled up and cured in the oven.
As the sheets cannot be compressed much during the curing, a thick layer of adhesive is required. Generally the quantity of adhesive needed in the corrugation process can be ten times that needed in the expansion manufacturing process.
Brazing, diffusion bonding or spot-welding are just different variants of the corrugation process, in which the nodes are welded instead to being adhesively bonded. Corrugated aluminum honeycomb is made because above 12 pcf it becomes impossible to expand the HOBE. In Figure 1.3 are represented the most common cell configurations manufactured .
1.3 Experimental Purposes
The purpose of this study is an experimental approach to understanding the effects of pre-soaking in water of the aluminum honeycomb sandwich structures to the static and fatigue strength.
1.4 Experimental Procedure
This study is using Instron-8872 type uniaxial dynamic materials testing systems, distilled water and aluminum honeycomb specimens in four-point bending static and fatigue strength analysis, to find the effects of pre-soaking in water of aluminum honeycomb structure strength. The main contents include:
four-point bending static and fatigue strength test and analysis. All of the tests were carried out at room temperature and compared with the differences of humidity in aluminum honeycomb sandwich structures.
1.5 Limitation of the experiment
This experiment needs to make the limitation of the experiment, to determine which material, method and standard that use in this experiment. There is some information about the experiment:
1. The material used in this experiment is aluminum 3104 and aluminum 5052 and this aluminum formed into honeycomb sandwich panel (see detail in chapter 3).
2. Distilled water used for this experiment, and all specimens immersed in the water.
3. 14 weeks, 12 weeks and 10 weeks are the different set-up time of the duration of immersion.
4. Four-bending static and fatigue tests as reference to the ASTM C393-06 standard.
1.6 Chapter Summary
The study divided into five chapters: The first chapter is introduction, which include: background, experimental purposes, experimental procedure, limitation of the experiment and chapter summary. The second chapter is literature review and basic theory of the honeycomb sandwich structure. Chapter three contains a complete explanation of the experimental, which consist of procedures for the experiment, samples for experimental production process and equipment. Chapter four discusses about results, analysis and discussion of the experiment. Finally, chapter five consist the conclusions of the experiment and discussion about aluminum honeycomb sandwich structures.
Figure 1.1. Expansion manufacturing process .
Figure 1.2. Corrugation manufacturing process .
Figure 1.3. Honeycomb cell configurations .
CHAPTER II LITERATURE REVIEW
This chapter introduce about the study that related with the research that will discuss in this experiment. There are several difference methods outfit with the study. Previous researches have been conducted for many purposes, such as to find the mechanical behavior of honeycomb structure, the structural strength of honeycomb parts and the fatigue properties of the honeycomb. There are some literature reviews included studies at room temperature and to consider the temperature, humidity and mixing effect of temperature and humidity.
2.1 Analysis of Mechanical Behavior of the Honeycomb Structure
According to Gibson and Ashby in 1988 , researched about the honeycomb structure in the plane and on the plane (Out-of-Plane). The research about mechanical properties of the two directions of the damage of cellular material into the type of detailed analysis had been conducted and also the research for the subsequent establishment of a honeycomb structure of the basic theoretical framework. In 1995, Shi, et al  for the hexagonal honeycombs structure; they found that the geometric structure of the corresponding numerically obtained shear strength. Albuquerque, et al in 1999  they research about the defect in cellular wall strength of the honeycomb structure when the manufacturing process of the honeycomb, they found that the number of defects in the cellular wall of honeycomb structures cause by buckling stress reduction. In 2000, Becker  using a nonlinear equation for the performance of cellular honeycombs structure in the thickness direction rigidity of plane. In 2003, Doyoyo, et al  researched on the aluminum honeycomb panels using both forward load and shear load under the mechanical behavior analysis. They found that among the shear load and the forward load the honeycomb structures showed two different types of damage, the forward load generated bending damage of cellular, and shear load cause plastic fracture of the cellular. In 2005, Yang, et al
 researched about the difference of wall edges at the honeycomb structure core, using biaxial compression load in the elastic buckling behavior analysis. In 2006, Pan, et al  compared the theory with experimental for the longitudinal shear strength of honeycomb structure and honeycomb structure to withstand the damage when the vertical shear strength applied.
2.2. Analysis of the Structural Strength of Honeycomb Parts
Plantema  and Allen  were the first researchers who researched of the mechanical behavior under static load on the sandwich structure and for the static mechanical analysis of honeycomb panels, in 1966 and 1969. In 1997, Saito, et al  using numerical analysis and experiments for the honeycomb structure with the panel, found the initial of the Young's modulus and damping coefficient of the honeycomb structure. In 1998, Zhao, et al  did an experiment of the different rates of the impact load to the honeycomb structure for the aluminum honeycomb structure and analysis mutual compression behavior of the honeycomb structure. The results indicated that the honeycomb structure could bear with high impact. In 1999, Meraghni, et al  on the honeycomb structure panels, used numerical analysis and finite element simulation to the experiment, drive the Young's modulus and shear modulus equivalent of the honeycomb structure panels. In the same year, Petras, et al  using three-point bending test, classification types of various damaged of the honeycomb panels. The type of damage the panel is divided into panels yield (Face Yielding), internal cellular depression (Intra-Cell Dimpling), panel folds (Face Wrinkling) and other three different types. In the same year (1999), Paik, et al  researched about the effect of the parameters to the strength of the honeycomb structure panels. The results showed that the thickness and height of the honeycomb cellular wall increase will increase the honeycomb structure panel strength. In 2000, Lee, et al  for the glass fiber/epoxy composite material of the panel, to consider it to be a single point of load damage caused by depression, study with numerical and finite element analysis. The panel found that the greater of the thickness and the
relative density of honeycomb structure is high; the intensity of the panel will be higher. In 2003, Zehnder, et al  researched for debonding fracture between cellular and plywood, using fracture mechanics theory and experiments, found the strain energy release rate and the relationship between bonding strength. In 2005, Nguyen, et al  using the finite element method, predicted the cellular structure to withstand the impact caused by the low rate of the fracture. Experimental results showed that the ratio of the finite element model could accurately predict under low velocity impact.
2.3 Fatigue Properties of the Honeycomb Structure Panels
There are many related experiments to honeycomb structure in the stress or strain cycles, also the cases of fatigue failure of the honeycomb and fatigue strength of honeycomb structure in room temperature behavior. Huang, et al 
in 1996, researched about the cellular material in the plane direction, using Paris law, Coffin-Manson law and Basquin law to conduct the fatigue life prediction.
The results showed that the intensity of stress cycles, honeycomb cellular geometry and relative density are affected the honeycomb fatigue life. Burman and Zenkert  in 1997, analyzed for no defects of the foam honeycomb plate fatigue strength using four point bending test, the results showed that the stress amplitude on fatigue life increase significantly than the average stress, and when the stress is almost zero, fatigue life significantly decreasing the trend. In the same year, Burman and Zenkert  conduct the same fatigue strength of the analysis, but for a defect of the foam honeycomb panel, the result successfully predict the crack of the cellular foam structure life by application of the local stress.
Schaffner, et al  in 2000 using cumulative damage fatigue the concept of the Voronoi type of honeycomb structure to conduct fatigue damage analysis showed that random number distribution of the Voronoi honeycomb than the general hexagonal-type honeycomb structure for fatigue damage is more sensitive. Harte, et al  in 2001, aluminum alloy foam honeycomb panels using four-point bending test method to conduct fatigue strength analysis, which showed cellular intensity will be decreased along with decreasing the cycles of load stress ratio. In
, Huang, et al  conducted research and discussion on the honeycomb structure to withstand plane multi-axis load the fatigue behavior, the results indicated that the honeycomb of the fatigue strength and the cycles of stress range, the structure itself has the defect, crack length, cellular size and on the relative density honeycomb. In 2003, Kulkarni, et al  bear the load of the foam-type moment honeycomb panels, using crack growth rate to predict their fatigue life. The research will undermine the program is divided into three stages, first by the imposed load of crack began to form around, then cracks will run through the middle part of the honeycomb, and finally crack and below the honeycomb panel along the border continue to grow. The study concluded that there would be 85% fracture at the first phase of the life.
2.4 Temperatures and Humidity Environment as the Fatigue Properties of the Honeycomb Structures
The previous literature of the experimental on the effects of temperature and humidity for the simple structure of honeycomb structure or honeycomb sandwich mechanical properties were seldom. In 1999, Andrews, et al [26, 27] for aluminum alloy honeycomb structure of the creep behavior conducted a research, including theoretical analysis and experimental verification was described in detail. In 2004, Veazie, et al , of E-glass/vinyl ester resin and PVC panel, the core material of the composite material in simulated sea water at 80°C, the condition is at the temperature 80°C and the humidity at 90% R.H. The result showed that the strain energy release rate (Gc) because the specimen after immersion in seawater more than 50% decline, the cracks of the specimen of the presence or absence of pre-soaked in water for a long time, its strain energy release rate was not much difference. Gates, et al  was used in 2006, and others with pre-crack doing the experiment to the honeycomb specimen at room temperature, -196°C, -269°C and three-point bending test conducted, the experimental results showed that the radial degree of panel will increase when the temperature is low, and the finite element method simulation of displacement of the control analysis, the results showed that the lower the temperature, the
material can withstand the higher load, also the higher deformation. In 2009, Soni, et al , for the carbon fiber or glass fiber panels and PMI foam material made of composite sandwich structures at room temperature, 0°C, -30°C and -60°C, doing the research at Four-point bending test condition, the results showed that under various conditions of carbon fiber/PMI folders are better than the static strength of glass fiber/PMI. The experimental temperatures of their fatigue test are at -60°C and at room temperature. Fatigue life is one hundred times of the cycle life; the conditions in the main shear failure modes are all PMI formed shear failure. In the same year, Allaoui, et al , of corrugated cardboard laminated structure made by composite materials, often wet and humid environment for static pulled up the experiment. The results showed that the increasing of the humidity will decrease the shear modulus of this composite, but the strain is rising with increasing the humidity. Siriruk, et al , the polymer foam material and glass fiber reinforced polymer composite panels for cracks with and without pre- conditions for water immersion study. The results showed there was water soaked, pre-cracks with the panel and the interface of foam material growth, the strain energy release rate than those not soaked in seawater reduced by 30%.
Based on the literature reviews above, the fatigue properties of the cellular aspects of the honeycomb structure of multi-plane load to bear the fatigue behavior of the main research object, and the study of cellular types are mostly non-foam type cellular structure or honeycomb panel structures. As for the honeycomb sandwich structure or a foam material are composite materials, most of the previous literature to withstand static load under different conditions or in the general environment and studies in stress analysis, to withstand the dynamic load conditions. The relevance of researches is seldom, the study to the effect on temperature and humidity of honeycomb structure for static and fatigue strength of are rarer. Therefore, this study will be conducted under different humidity conditions, aluminum honeycomb plate under four point bending static and fatigue strength of the experiments and analysis of its mechanical properties for a
series of assessments to be more complete set of aluminum honeycomb plate temperature humidity of the properties.
2.5 Temperature and Environment Effects on Fatigue Strength
There are more considerations to design materials; such as temperature and environment, which can influence the strength of the material.
At temperature beyond approximately one-half of the melting point of the material, creep becomes important. Many materials experience a significant reduction in fracture toughness at low temperatures. At high temperatures can cause annealing, which may remove beneficial residual compressive stresses .
The effect of fatigue strength, when fatigue loading takes place in a corrosive environment, the results are more significant than predicted by considering fatigue and corrosion separately. This corrosive environment attacks the surface of a metal and produces an oxide film. This oxide film would serve as a protective layer and prevent further corrosion of the metal. Cyclic loading cause localized cracking of the thin film layer, which exposes fresh metal surface to the corrosive environment. At the same time corrosion causes localized pitting of the surface, and these pits serve as stress concentrations. This interaction of fatigue and corrosion usually called corrosion-fatigue. To obtain corrosion-fatigue data are greatly influenced by loading frequency, low frequency tests allow more time for corrosion to take place, and resulting fatigue lives are shorter .
Based on the description of previous explanation, this study will find the influence of aluminum honeycomb sandwich structures strength on the environment, whether the environment could cause changes or reduces the strength of aluminum honeycomb sandwich structures.
There are some requirements to conduct the static and fatigue experiments, these requirements need to be prepared firstly, which are included: the experimental specimen, the instrument and the experimental method.
3.1 Specimen for Experimental
The specimen that is used in this experiment is aluminum honeycomb sandwich structures. It is made from aluminum 3104 for the incline of honeycomb and for the face sheet of the honeycomb is aluminum 5052. The core of the specimen shaped hexagon closely resembles as bee’s honeycomb, formed layer by layer of the aluminum sheets that bonded together with range of the glue, after that the sheets pulled out to form the honeycomb shape and generate the honeycomb structure. At the top and bottom of the core is covered with aluminum sheets are called face sheet of the honeycomb sandwich structures, as seen in Figure 3.1. The dimension of the honeycomb core structures for this experiment are: the thickness of the incline wall (t) is 0.07 mm, the length of the incline wall (h) is 1 mm, length of the central wall (l) is 3 mm, the diameter of the incline honeycomb is 3 mm and the height of the honeycomb core (hc) is 20 mm. The dimension of this honeycomb structures are 200 mm for the length of the specimen, height and width are 20 mm, and 60 mm, respectively, as seen in Figure 3.2.
3.2 Experimental Instruments
The experiment required some of tests and instrument to conduct the static and fatigue tests.
3.2.1 Measurement Instrument (Instron 8872 Series)
The instrument used for this experiment was Instron-8872; this machine used DAX (Data Acquisition) and SAX (Single Axis Max) computer interface.
This software suits to Instron-8800 series as a control system, which is design to fatigue and static testing requirements on servo-hydraulic systems. A controller contains an Integrated Axis Controller card (IAC) for each axis and for 8872 models has only 1 IAC. The 8872 model requires 2 SCMs (Sensor Condition Module), one for the hydraulic actuator LVDT (Linear Variable Differential Transformer) and one for the load cell .
The hydraulic power supply for 8872 model is designed to operate with 207 bars. The load cell pre-installed onto the hydraulic actuator piston rod, leaving the base free. The load cell is dynamically compensated to cope with inertia forces arising from the moving piston. This model also has load frame that has a single-piece base, with T-slot to accommodate a range of grips and other testing accessories .
There are difference computer interfaces to conduct static and fatigue test.
For static test used DAX interface and for fatigue test used SAX interface.
220.127.116.11 Measurement Parameters for Static
As mention before, static test was used DAX interface to conduct the experiment. For DAX interface needs to setup the waveform generator. For frequency, it must set to be 0.001 Hz, or period to 1000 second, stroke rate at 0.01 mm/sec and the cycle count to be 1 cycle.
18.104.22.168 Measurement Parameters for Fatigue
This test used 7 Hz for the frequency of the cycle, and for fatigue test it is using SAX interface to conduct the experiment. The load ratio of this experiment is 10; load cycle is counted until 1,000,000 cycles. Then it is needed to choose load as control mode amplitude and mean level for control parameters.
3.2.2 Experimental Conditions for Static Experiment
Preparing the variables to accomplished the static experiment to acquire static data. Those are choosing such as the condition of the measurement and time of the experiment. Before doing the experiment, all the specimens were immersed in water, and all the experiments were conducted in room temperatures. For this experiment, the condition of the experiment is differentiated on the time of immersion; those are 10 weeks, 12 weeks and 14 weeks of immersions.
3.2.3 Experimental Conditions for Fatigue Experiment
Fatigue tests were carried out after static test. After completing static test and acquiring static data namely yield point or ultimate stress strength of the aluminum honeycomb sandwich structures, subsequently prepare to conduct the fatigue experiment. It is necessary to get stress amplitude and mean stress to setup experimental condition in the machine. Stress amplitude and mean stress value depend on the experimental condition, which each condition has different value.
For condition 100%RH in 10 weeks immersion time, the stress amplitude and mean stress for load levels are 50%, 55%, 60%, 65% and 70% of experimental loads are 98.7021 kgf, 108.5724 kgf, 118.4426 kgf, 128.3128 kgf, 138.1830 kgf and -120.6360 kgf, -132.6996 kgf, -144.7632 kgf, -156.8268 kgf and -168.8903 kgf, respectively. For condition 100%RH in 12 weeks immersion time, the stress amplitude and mean stress in 55%, 60%, 65%, 70% and 75% of experimental loads are 106.8544 kgf, 116.5685 kgf, 126.2825 kgf, 135.9965 kgf, 145.7106 kgf and -130.5998 kgf, -142.4725 kgf, -154.3453 kgf, -166.2180 and -178.0907 kgf, respectively. For condition 100%RH 14 weeks immersion time, the stress amplitude and stress mean for this condition for 55%, 60%, 65%, 70% and 75%
are 105.6101 kgf, 115.2109 kgf, 124.8119 kgf, 134.4127 kgf, 144.0136 kgf and - 129.0790 kgf, -140.8133 kgf, -152.5478 kgf, -164.2822 kgf and -176.0166 kgf, respectively.
3.2.4 Four-Point Bending Tests
All experiments operate on four-point jig. This jig has two upper rollers with 50 mm inner span to apply the loading, and two lower rollers with 150 mm outer span as supports. This four-point bending tests specifications in compliance with ASTM C393-06  standards, as shown in Figure 3.4.
Firstly, four-points bending static ultimate strength test is conducted to find out honeycomb panels in variety of humidity environment of the static ultimate strength. Experiment will be under the control of stroke ratio rate of 0.01 mm/sec and the displacement is 5 mm, the static conditions of the experiment is set to make the specimen fracture to obtain the ultimate strength.
For fatigue strength test, the four-points bending test specifications are the same with static strength test; it is accordance to ASTM C393-06 . In the fatigue test the load ratio (R) is set in 10 (load ratio is defined as the minimum load/maximum load) and the frequency is set to 7 Hz.
3.3 Experimental Procedures
There are two parts to accomplish the experiment as listed below:
First, the static experiment is conducted to the effect of time and humidity to honeycomb sandwich structure with four bending test, in static conditions.
And then, the fatigue experiment is conducted to the effect of time and humidity to honeycomb sandwich structure with four-points bending test, in fatigue conditions.
Table 3.1. Honeycomb Structure and the Material Properties of Aluminum Alloy.
Modulus of Elasticity, E
Poisson's Ratio, n
Density, D (g/cc)
Shear Modulus, G
Yield Strength, σys
3104 69 0.34 2.72 26 260
5052 70.3 0.33 2.68 25.9 193
Figure 3.1. Honeycomb structure specimen model.
F 2 F 2
F 2 F 2
Figure 3.2. Aluminum honeycomb geometric dimensions and force conditions.
Figure 3.3. Instron 8872 uniaxial material testing system type.
Figure 3.4. Four-points bending fixture.
RESULTS AND DISCUSSION
After conducting static and fatigue experimental in all of the experimental procedures with immersion times are 10 weeks, 12 weeks and 14 weeks for all conditions in 100%RH, then the results obtained from both types of experiments which are described in this chapter. The results for the static experiments are ultimate stress strength of aluminum honeycomb sandwich structures and for the fatigue experiments were obtained the endurance of the aluminum honeycomb sandwich structures fit on S-N curve. All the results will be explained in detail as the following below.
4.1. Aluminum Honeycomb Structure Static Test Result
Static experiments were conducted in 100% humidity (RH) and in room temperature. The mean ultimate stress strength for 50% humidity is 2172.92 N, as shown in Table 4.1.
The ultimate strength for 100%RH depends on immersion time. For 10 weeks of immersion, the ultimate strengths are 2164.82 N, 2142.44 N and 2192.87 N, and the mean ultimate strength is 2166.71 N. The ultimate strengths for 12 weeks of immersion are 2013.65 N, 2110.89 N and 2228.45 N; mean of the ultimate strength is 2117.64 N. While the ultimate strengths for 14 weeks immersion are 2013.54 N, 2194.55 N and 2070.91 N and the mean of the ultimate strength is 2092.99 N.
In Table 4.2 shows the result for the ultimate strength of 100%RH in difference of immersion time. In Figure 4.1, shows the graph of difference mean of aluminum ultimate strength at difference conditions. After immersion in water, the colors of outer surface of aluminum honeycomb structures, which directly exposed to water, the surface becomes not shining anymore. It caused by corrosion layer of the aluminum honeycomb and covers the surface of the
structures. This layer changes its color according to time of immersion, as shown in Figure 4.2.
In static test, most of specimens damaged caused by core shear failure, as shown in Figure 4.3. In this figure shows photograph of damaged specimen at condition 100%RH and 10 weeks of immersion time. Moreover, in this figure shows core shear failure damaged of the honeycomb structure.
In Figure 4.4 shows the damaged of the aluminum honeycomb structure in 100%RH condition and 12 weeks of immersion time. In this condition, the damage occurs also due to core shear failure. Figure of the aluminum honeycomb structure in condition 100%RH and 14 weeks immersion time is showed in Figure 4.5. For this condition, aluminum honeycomb structure damage also caused by core shear failure.
The graph of the static test result is shown in Figure 4.6 until Figure 4.8, which is showed the ultimate static strength for each condition. Figure 4.6 shows the result of the ultimate static strength for 100%RH and 10 weeks of immersion with 3 difference specimens, the value of the ultimate static strength also can be seen in Table 4.2. Figure 4.9 shows the compare of static tests result of the aluminum honeycomb in conditions 100%RH with 10 weeks, 12 weeks and 14 weeks of immersion time.
4.2 Aluminum Honeycomb Structure Fatigue Test Result
Fatigue test conducted after all static tests finished. Fatigue test also conducted with the same condition as static test conditions.
4.2.1 Aluminum Honeycomb Structure Fatigue Test Result for 100%RH and 10 weeks of immersion
In each condition of fatigue test conducted in several load, such as in 100%RH in 10 weeks immersion time, experiment were conducted in five differences load level those are 50%, 55%, 60%, 65% and 70% and the results were obtained from the experiments for this condition: in 50% load level is
2151.707 N of the load value and 1,000,000 number of cycle, in 55% load level the aluminum honeycomb specimen fractured is 2366.877 N at 468,117 cycles. In 60% load level the aluminum honeycomb specimen fractured is 2582.048 N at 100.824 cycles. In 65% load level the aluminum honeycomb specimen fractured is 2797.219 at 44.765 cycles. In 70% load level the aluminum honeycomb specimen fractured is 3012.39 N at 26,150 cycles. These results could be seen in Table 4.3 and as shown in S-N curve in Figure 4.10. In this condition some of the specimen fracture due to adhesive debonding, as shown in Figure 4.11, which shows sample of the photograph of the aluminum honeycomb structures damaged.
4.2.2. Aluminum Honeycomb Structure Fatigue Test Result for 100%RH and 12 weeks of immersion
In condition 100%RH and 12 weeks of immersion, fatigue tests were also conducted in five differences load level, in 55%, 60%, 65%, 70% and 75%. In 55% load level at maximum load 2117.66 N, the specimen did not fracture until 1,000,000 cycles. For 60% load level, aluminum honeycomb structure fractures at load 2541.192 N and at 425,879 cycles. At 65% load level, aluminum honeycomb structure fracture is 2752.958 N at 410,712 cycles. At 70% load level, the aluminum honeycomb structure fracture is 2964.724 N at 122,228 cycles. At 75%
load level, the aluminum honeycomb structure fracture is 3176.49 N at 43,882 cycles. This fatigue data also could be seen in Table 4.4 and also as shown in Figure 4.12. This figure shows graph of S-N curve of the aluminum honeycomb structure in condition 100%RH and 12 weeks immersion. In this condition, most of aluminum honeycomb structure fracture due to core shear failure as shown in Figure 4.13, which shows a sample of the photograph of fractured aluminum honeycomb structure.
4.2.3 Aluminum Honeycomb Structure Fatigue Test Result for 100%RH and 14 weeks of immersion
For conditions 100%RH and 14 weeks immersion time, fatigue tests were also conducted in five differences load level as same as with 100%RH and 12 weeks immersion conditions, those are 55%, 60%, 65%, 70% and 75% of load level. At 55% of load level, aluminum honeycomb structure did not fracture until 1,000,000 cycles at 2302.299 N. At 60% load level, aluminum honeycomb structure fracture at 2511.597 N at 263,982 cycles. At 65% load level, aluminum honeycomb structure fracture at 2720.897 N at 175,085 cycles. At 70% load level, aluminum honeycomb structure fracture at 2930.197 N at 72,139 cycles. At 75%
load level, aluminum honeycomb structure fracture at 3139.494 N at 64,341 cycles. This fatigue results also could be seen in Table 4.5. In Figure 4.14, shows S-N curve of fatigue data of the aluminum honeycomb structure in condition 100%RH and 14 weeks immersion and in Figure 4.15 shows the photograph of an aluminum honeycomb structure specimen, which is show the consequence of the fractured to aluminum honeycomb structure in 100%RH and 14 weeks immersion time conditions. It shows that core shear failure caused fracture to aluminum honeycomb structure in conditions 100%RH and 14 weeks immersion.
In Figure 4.16 shows the combination graphs of the S-N curve for the aluminum honeycomb structure. In these graphs show the differences of the fatigue test results from different conditions 100%RH and 10 weeks, 100%RH and 12 weeks and 100%RH and 14 weeks. It shows significant changes occur on the strength of the aluminum honeycomb structures in the range of conditions 100%RH for 10 weeks and 14 weeks immersion time.
Figure 4.17 shows the combination of the load level and cycles to failure of the aluminum honeycomb structure. The graph looks similar with the graph in figure 4.16, but in this graph shows the differences of the load level at difference conditions. Figure 4.18 shows the differences of the stroke range () against cycles to failure (Nf) of the aluminum honeycomb structure.
Table 4.1 Data of Static Strength for Aluminum Honeycomb Specimen in 50%
Humidity (as-received) Ultimate Static Strength,
Mean Ultimate Static Strength, F (N)
2094.435 2231.775 2172.915 2192.535
Table 4.2 Data of Static Strength of Aluminum Honeycomb Structure in 100%RH Time of
Static Ultimate Strength, F (N)
Mean of Static Ultimate Strength, F (N) 2164.82
Table 4.3 Fatigue Data of Aluminum Honeycomb Structure in Conditions 100%RH and 10 Weeks Immersion Time
Load level Maximum Load, F (N)
Cycle to failure, Nf (cycle)
50% 2151.707 > 1,000,000
55% 2366.877 468,117
60% 2582.048 100,824
65% 2797.219 44,765 70% 3012.39 26,150
Table 4.4 Fatigue Data of Aluminum Honeycomb Structure in Conditions 100%RH and 12 Weeks Immersion Time
Load level Maximum Load, F (N)
Cycle to failure, Nf
55% 2117.66 > 1,000,000
60% 2541.192 425,879
65% 2752.958 410,712
70% 2964.724 122,228
75% 3176.49 43,882
Table 4.5 Fatigue Data of Aluminum Honeycomb Structure in Conditions 100%RH and 14 Weeks Immersion Time
Load level Maximum Load, F (N)
Cycle to failure, Nf (cycle)
55% 2302.299 > 1,000,000
60% 2511.597 263,982
65% 2720.897 175,085
70% 2930.197 72,139 75% 3139.494 64,341
Different Levels of Humidity and Immersion Time
0 500 1000 1500 2000 2500
Maen U ltim a te Stren gt h , F (N )
as-recieved 100%RH, 10 weeks
12 weeks 100%RH, 14 weeks
Figure 4.1 Graphs of the different of mean ultimate static strength.
Figure 4.2 Aluminum Honeycomb Structure in different condition. Front view:
(a) 50%RH 0 week immersion, (b) 100%RH 10 weeks immersion, (c) 100%RH 12 weeks immersion, (d) 100%RH 14 weeks immersion.
Side view: (e) 50%RH 0 week immersion, (f) 100%RH 10 weeks immersion, (g) 100%RH 12 weeks immersion, (h) 100%RH 14 weeks immersion.
a b c
e f g h
Figure 4.3 Damaged Aluminum honeycomb structure in conditions 100%RH and 10 weeks of immersion time after static test.
Figure 4.4 Damaged Aluminum honeycomb structure in conditions 100%RH and 12 weeks of immersion time after static test.
Figure 4.5 Damaged aluminum honeycomb structure in conditions 100%RH and 14 weeks of immersion time after static test.
Figure 4.6 Graph of static strength of aluminum honeycomb structure in 100%RH and 10 weeks immersion time.
0 500 1000 1500 2000 2500
Lo ad ; F (N )
0 0.4 0.8
1.2 1.6 2
Figure 4.7 Graph of static strength of aluminum honeycomb structure in 100%RH and 12 weeks immersion time.
0 0.4 0.8 1.2 1.6
0 500 1000 1500 2000 2500
Lo ad ; F (N )
Figure 4.8 Graph of static strength of aluminum honeycomb structure in 100%RH and 14 weeks immersion time.
0 0.4 0.8 1.2 1.6
0 500 1000 1500 2000 2500
Lo ad ; F (N )
Figure 4.9 Graph of static strength of aluminum honeycomb structure in 100%RH in10 weeks, 12 weeks and 14 weeks immersion time.
0 0.4 0.8 1.2 1.6
500 1000 1500 2000 2500
Loa d; F (N )
100%RH 10 weeks 100%RH 12 weeks 100%RH 14 weeks
Figure 4.10 S-N curve of aluminum honeycomb structure in 100%RH and 10 weeks immersion time.
Cycles to Failure; Nf (Cycles)
Maximum Load; Fmax (N)
Figure 4.11 Damaged aluminum honeycomb structure in conditions 100%RH and 10 weeks of immersion time after fatigue test.
Figure 4.12 S-N curve of aluminum honeycomb structure in 100%RH and 12 weeks immersion time.
Cycles to Failure; Nf (Cycles)
FMaximum Load; max (N)
Figure 4.13 Damaged aluminum honeycomb structure in conditions 100%RH and 12 weeks of immersion time after fatigue test.
Figure 4.14 S-N curve of aluminum honeycomb structure in 100%RH and 14 weeks immersion time.
Cycles to Failure; Nf (cycles)
Ma x im u m Lo ad; Fmax
100%RH - 14 weeks
Figure 4.15 Damaged aluminum honeycomb structure in conditions 100%RH and 14 weeks of immersion time after fatigue test.
Figure 4.16 S-N curves of aluminum honeycomb structures with various immersion times.
Cycles to Failure; Nf
Ma xim u m Lo ad; Fmax
100%RH - 10 weeks 100%RH - 14 weeks 100%RH - 12 weeks
Cycles to Failure; Nf
L oa d Le v e l (% )
100%RH 10 weeks 100%RH 12 weeks 100%RH 14 weeks
Figure 4.17 Graph of the influence of load level (%) on the cycles to failure (cycles) of aluminum honeycomb structures with different immersion times.
Figure 4.18 Graph of relationships between the stroke range (∆) and cycles to failure (Nf) of aluminum honeycomb structures with different immersion times.
0 0.4 0.8 1.2 1.6
Str o k e Range ; (mm )
05 3x105 4x105 5x105
100%RH 1 100%RH 12 100%RH 1
0 weeks weeks 4 weeks
Load level = 60%
CONCLUSIONS, CONTRIBUTIONS AND SUGGESTIONS
Aluminum honeycomb structure recently widely used in areas with high humidity. In this experiment could be found some of conclusions to the effect of time on the aluminum honeycomb structure with conducted in static and fatigue test in high humidity.
Some of conclusions are found from the experiments as the following:
1. On static test was found that the strength of aluminum honeycomb structure changes accordance with the longest time of immersion. The specimens change color according to the time of immersion due to oxidation.
2. Mean ultimate strength from this test is 2166.71 N, at 100%RH and 10 weeks immersion. In this condition most of the aluminum honeycombs structures fracture due to core shear failure.
3. While at 100%RH and 12 weeks immersion, mean ultimate strength is 2117.64 N. Also as in 100%RH and 10 weeks immersion condition, most of the aluminum honeycomb structure fractured due to core shear failure.
4. Then 100%RH and 14 weeks immersions, mean ultimate strength is 2092.99 N. Also as in 100%RH and 10 weeks immersion and 100%RH and 12 weeks immersion conditions, most of the aluminum honeycomb structure fractured due to core shear failure.
5. On fatigue test at 100%RH and 10 weeks immersion, aluminum honeycomb structure fractured in the loading of 55%, 60%, 65% and 70%.
Most of specimens fractured due to adhesive debonding.
6. On fatigue test at 100%RH and 12 weeks immersion, aluminum honeycomb structure fractured in the load level of 60%, 65%, 70% and 75%. Most of specimens fractured due to core shear failure.
7. On fatigue test at 100%RH and 14 weeks immersion, aluminum honeycomb structure fracture in the loading of 60%, 65%, 70% and 75%, most of specimens fractured due to core shear failure.
8. The comparison to fatigue strength between the specimens with 100%RH and 10 weeks immersion, 100%RH and 12 weeks immersion and 100%RH and 14 weeks immersion conditions show that the specimens with 100%RH and 14 weeks immersion has the lowest of the fatigue strength and the specimens with 100%RH and 10 weeks immersion has the highest of the fatigue strength for this experiment.
The contribution of this study is to help people, workers or researchers who work in area with high humidity condition or in the water, such as researcher who use research submarine or any instruments that often use in that area. So they know the instruments they use have a limit of its strength, and levels of risk accident to human were caused by instrumental error could be reduced.
In this study was found so many deficiencies and weaknesses, consisting of number of sample of fatigue data, it should be more than or equal to 3 sample for each conditions and the longest time of immersion also researched for various humidity. Because of these deficiencies and weaknesses, it could be given some suggestions for other researchers:
1. To obtained valid data for fatigue test, number of sample for fatigue data should be more than or equal to 3 samples.
2. Immersion time in this study is the minimum time. Hopefully, in the next experiment the other researchers could take more time to soak the specimen in water, i.e. 10 weeks, 20 weeks, and 30 weeks and so on.
3. Other researchers also could conduct a research to find the differences of the static and fatigue strengths between specimen that immersed in distilled water and immersed in seawater.
4. Other researcher can conduct a research in various humidity, such as:
50%RH, 75%RH, 80%RH or 90%RH or combination of them.
5. Also, the researcher could conduct a research in various temperature of the water. Such as in high temperature or in low temperature or combination of them.
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