School of Mechanical and Aeronautical Engineering MM3315 Aerospace Materials Assignment
Sandwich Construction Session: AY 12/13 Semester: 2 11-01-2013 Class: DARE/FT/3A/24 DARE/FT/3A/24
Members: Seran Karikalan (leader) Ong Jovi Ng Aiting Jonathan Joseph Wee Sivaguru S/O Sivagnanam
1002764 1019247 1067138 1019052 1003260
I.
Declaration Page
Plagiarism Declaration:
We declare that this Aerospace Materials assignment submitted for assessment is our own work and does not involve plagiarism. The sources had been cited and refer enced in the References Chapter.
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I.
Declaration Page
Plagiarism Declaration:
We declare that this Aerospace Materials assignment submitted for assessment is our own work and does not involve plagiarism. The sources had been cited and refer enced in the References Chapter.
2992
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II.
Acknowledgements
The team would like to express their gratitude gratit ude to the following individuals who helped up in the success of our task.
Mr Bang Toong Kiang, BSc (Met/Mech)(Hons)(Manchester,UK), MSc ( AeroMaterials) AeroMaterials) (Cranfield,UK) For being the source of inspiration for this module and project, Mr Bang had allowed us to understand the concepts that were vital to the completion of our project. We thank him for his unending patience and guidance.
Mr Pang (TSO) He had helped our team in providing access to the necessary laboratory tools and assistance that were crucial to the project.
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III.
Contribution of Each Members
Seran, being the team-leader, delegated the task among the group and supervised the members on every segment of the project. Seran was involved in literature survey, research of the theoretical fundamentals, experimental testing and the construction of the jig. Seran was also involved in compiling the report to ensure consistency and fluency of the content. Jovi’s contribution took place in planning and testing of the Experiment 1; after determining the fiber-orientation through the microscope. Jovi had also helped with the Experiment 2, construction of the SS (refer to Section V: Nomenclature), obtaining E and lastly the portion of the report on the tensile strength for different fiber orientation. Jonathan’s contribution began with the planning and brainstorming on the approach the task. Jonathan also assisted in the cutting of the various dog-bone pieces to be used for the various tests, as well as the assembly of the SS. He was also involved in the portion of report on glue testing. Aiting was responsible in cutting of the vanguard sheet for all 3 ex periments, determining the dimensions of the vanguard sheet used to construct the SS, and also in-charge of building the SS. Aiting contributed in report on the assignment flow chart, Experiment 3, and references part. Siva’s contribution includes the assistance in planning and painting of the jig. He also provided assistance in the Experiment 1 and 2, as well as the assembly of the SS. Lastly his part of the report was to prepare abstract and nomenclature.
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IV.
Abstract
This report focusses and provides a detailed anal ysis of the findings on which fibreorientation (transverse/longitudinal/diagonal) has highest tensile properties, best glue to be used for SS, designs selected to build the core to construct the SS, identify the best design of the core in terms of bearing heaviest load with minimal deflection with comparison from theoretical values. This report also focuses on the errors made by the team, the appropriate corrective actions taken and future development/recommendation could be used for this assignment.
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Contents
I.
Declaration Page ................................................................................................................ 1
II.
Acknowledgements ............................................................................................................ 2
III.
Contribution of Each Members ....................................................................................... 3
IV.
Abstract ........................................................................................................................... 4
V. Nomenclature ..................................................................................................................... 7 VI.
List of Equipment ............................................................................................................ 8
VII.
Assignment Flowchart ..................................................................................................... 9
1.
Introduction: SS................................................................................................................ 10 1.1
2.
History ....................................................................................................................... 10
Literature Survey .............................................................................................................. 10 2.2 Types of SS .................................................................................................................... 11
3.
Experiment 1: Fibre-Orientation Testing ......................................................................... 12 3.1 Aim................................................................................................................................. 12 3.2 Process............................................................................................................................ 12
4.
Experiment 2: Choosing Glue .......................................................................................... 16 4.1 Adhesive Quality ............................................................................................................ 16 4.1.1 Procedure for testing of glue ................................................................................... 17 4.2 Cost of glue .................................................................................................................... 18 4.3 Ease of application ......................................................................................................... 19 4.4
Glue Selection............................................................................................................ 19 Page | 5
5.
Experiment 3: SS Construction and Deflection Testing .................................................. 20 5.1 Aim................................................................................................................................. 20 5.2 Choosing Shapes for the Core ........................................................................................ 20 5.3 Processes ........................................................................................................................ 21 5.3.1 Construction of SS .................................................................................................. 21 5.3.2 Construction of testing apparatus-Jig ...................................................................... 24 5.4
Testing of the SS........................................................................................................ 25
5.5
Results and Analysis .................................................................................................. 26
6.
Conclusion........................................................................................................................ 30
7.
Errors and Recommendations .......................................................................................... 31
8.
References ........................................................................................................................ 33
9.
Appendix .......................................................................................................................... 34
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V.
Nomenclature
SS=Sandwich Structure
E=Young’s Modulus of Elasticity σ=Direct Stress ε=Direct Strain A=Cross-Sectional Area L =Gauge Length x=Elongation g=Acceleration due to Gravity = 9.81m/s 2 P=Applied Force D=Bending Stiffness δ=Deflection K b=Bending Deflection Coefficient I=Second Moment of Inertia tf =Thickness of faces tc=Thickness of core b=Breadth of SS l=Length of SS Page | 7
VI.
List of Equipment
1. Hounsfield H5K-W Machine 2. Microscope 3. Ruler 4. Vernier-Caliper 5. Digital Weighing Scale 6. Pair of Shear/Scissors
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VII.
Assignment Flowchart
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1. Introduction: SS
According to the American Society for Testing and Materials (ASTM), the definiti on of a SS is: a special form of a laminated composite comprising of a combination of dissimilar materials that are bonded together to each other so as to make use of the properties of each distinct component to the structural improvement of the whole assembl y
1.1 History
The first use of SS on man-made structure traces all the way back to 1652; where an engineer named, Wendelin Schildknecht built a bridge using sandwiched beam. However it was only after the World War II where the SS became popular.
2. Literature Survey
SS are made up of lightweight thick core situated between thin stiff skins. Suitable strong adhesives are used ensure sufficient bonding, thus uninterrupted transfer of forces from the core to the skin and from core to core. By separating the skins this way, majority of the material; thus mass of the structure are placed further away from the neutral axis (the imaginary line of a structure that does not have any bending moments). This improves the second moment of inertia of the structure which in turn increases the E of the material. Result – structure being much stiffer and the ability to carry more loads without failure. Due to t heir Page | 10
high strength-to-weight ratio and high stiffness, SS are used in high performance applicat ions that demand high strength-to-weight ratio such as aeronautical st ructures and high speed automotive and marine structures.
2.2 Types of SS
The 3 basic types of SS are corrugated, foam and honeycomb core.
Figure 2.1: Corrugated core
Figure 2.2: Foam core
Figure 2.3: Honeycomb core Page | 11
3. Experiment 1: Fibre-Orientation Testing
Vanguard sheet is an anisotropic material; therefore, the testing of tensile property of the vanguard sheet in different fibre-orientation was c onducted (refer to Figure 3.2). It was believed that to have a best high strength SS, vanguard sheet best orientation was taken in consideration.
3.1 Aim
- Test for the tensile property of the vanguard sheet in different fibre-orientations - Determine the strongest fibre-orientation - Calculate E of the vanguard sheet
3.2 Process
The fibre-orientation of the vanguard sheet was determined by placing the sheet under the microscope. The camera and software allowed the fi bre-orientation of the vanguard sheet to be captured on the computer.
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Figure 3.1: Vanguard sheet fibre-orientation captured by microscope
With the fibre-orientation of the vanguard sheet determined, 2 test pieces in dog-bone shape was prepared for the three different orientations each.
Figure 3.2: Fibre-orientation for dog-bone shape test pieces
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Figure 3.3: Dog-bone shape plate with measurements (mm)
With the dog-bone pieces, tensile testing using Hounsfield H5K-W machine was conducted, measuring tensile strength it (paper) could withstand before it fracture. In order for the testing to be conducted properly, clamping of the vanguard bone was to be tightly in place. Folds or tear forms weak point or area of stress concentration, would affect the test result. With the fracture test-pieces and the printed report, observation, analysis and discussion on the experiment with regards to the fibre-orientation was conducted. Factors included during discussion were the fracture characteristics, the direction of fracture and the report which contains load-elongation graph of the testing. For full set of data on the testing, refer to appendix section. With the values obtained from the test report, calculation of E of the vanguard sheet was done according to ISO 527 Plastics – Determination of Tensile Properties, Part 1 and Part 3. This specifies to take strain at 25% and 5% of the total elongation and the corresponding stress (from stress – strain graph). Load – elongation graph was converted into stress – strain graph.
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Table 3.1: Values of calculated stress, strain and E for different fibre-orientation Based on the result, the fibre-orientation that gave the best E is the longitudinal fibre direction. Page | 15
4. Experiment 2: Choosing Glue
The type of glue to be used would affect the properties of t he SS that was to be constructed. The criteria used for glue selection as follows: 1. Adhesive Quality of glue using 2. Cost of glue 3. Ease of application of glue The best glue would be then chosen based on the glue’s ranking shown in Table 4.2. Three types of glue were chosen to represent the var ious types of glue available. 1. Water-Based Glue (WBG) 2. Epoxy 2-Part Resin 3. UHU Glue
4.1 Adhesive Quality
This was conducted as the procedures stated below and the best glue for this criterion was chosen by observing the mode of failure. Refer to appendix section for the complete results analysis of this portion.
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4.1.1 Procedure for testing of glue
1: The dog-bone was first cut into two pieces. 2: The two halves were joined back by gluing the overlapping area of 1 cm 2 (refer to Figure 4.1).
Figure 4.1: Specimens used for glue testing
It was found that UHU glue was stronger than the paper ( Figure 4.2). Main objective of this test was to determine which glue was stronger than the paper, which was an important factor in eliminating failure of glue shearing in the SS. Choosing the glue stronger than the paper would simulate a situation where the face and core of the SS as a single part.
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Figure 4.2: Modes of failure (from left: Epoxy, WBG and UHU glue)
4.2 Cost of glue
Table 4.1: Comparing the cost of individual glue
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4.3 Ease of application
Both the WBG and UHU glue was convenient as they could be directly applied t o surface. For the epoxy, both parts had to be mixed prior to application which made it difficult. Curing time was another factor. The WBG took a slightly longer time to c ure than UHU glue. The epoxy took relatively longer curing time. The longer waiting time made it dif ficult to align the test-piece correctly as the glue cures.
4.4 Glue Selection
Table 4.2: Showing the individual glue’s ranking
Since UHU glue ranked number 1, it was the chosen glue for the use with construction of the SS.
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5. Experiment 3: SS Construction and Deflection Testing
5.1 Aim
1) Test the strength of different shape of the core of SS. 2) Determine the strongest core of SS. 3) Compare the strength of SS with equivalent mass of piles of plain vanguard sheets pasted together. Since vanguard sheets were dealt with for this assignment, honeycomb core SS (refer to section 2.2) was concentrated.
5.2 Choosing Shapes for the Core
Honeycomb SS come in many different kinds of core design. Since human hands were to be used to construct the structure, the shapes of the core were chosen that was easier to fold and with higher tessellating characteristics. The 4 chosen shapes were: 1. Hexagon 2. Elongated hexagon 3. Equilateral triangle 4. Square
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The possible failure modes that the face of SS might have to withstand are the face yield, dimpling and wrinkling and shearing between the cores. This was why the choice of adhesive was important so as to hold the cores in place when applying the loads. The orientation of the vanguard sheet used as the face of the structure also affects the strength of the overall structure.
5.3 Processes
Results and conclusions from previous experiments are used in this experiment to build the SS.
5.3.1 Construction of SS
As one of the ultimate aims was to determine which shape (of core) that has the best contribution to overall strength of the SS, the area enclosed b y each core to be constant. That will allow the experiment to have only one variable and thus more accurate conclusion to be made at the end of this assignment. The vanguard sheet strip was folded according to the measurement shown in Figure 5.1 and the ends were glued together.
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Figure 5.1: Showing dimensions of different cores Then the sides of individual core were glued together and dried. Made sure there are no gaps in between the cores.
Figure 5.2: Elongated hexagonal (top) and hexagonal cores (bottom)
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Figure 5.3: Equilateral triangle cores
Figure 5.4: Completed SS with square cores
After the cores were ready, the vanguard sheets in longitudinal fiber-orientation (perpendicular to applied load) were used as the fa ces. The completed SS is weighted and average weight is calculated. Plain vanguard sheet were cut in 40mm x 165mm and the sheets were glued in plies until the average weight was reached.
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5.3.2 Construction of testing apparatus-Jig
The jig was used to measure the deflection of the SS when loads are applied to the structure. To make sure the results were accurate, a level and solid jig was built. Firstly, basswood was cut according to the dimension shown in Figure 5.5.
Figure 5.5: Dimensions of basswood for jig Then pieces of basswood were assembled as in Figure 5.6 with L-brackets. Loading was achieved as shown in Figure5.7.
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Figure 5.6: Jig making in process
Figure 5.7: Completed jig
5.4 Testing of the SS
A dial gauge was used to measure deflection. Approach loading was used to do the testing of the SS; starting with 0.5kg to 2.5kg with increments of 0.5kg. (Refer to the Figure 5.8 for the test set-up). All the readings for all 4 structures were recorded in the Table 5.4.
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Figure 5.8: Initial test set-up procedure
Figure 5.9: Face wrinkle failure after testing
5.5 Results and Analysis
Table 5.1: Average weight of SS
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The theoretical values of the deflection were determined to see how accurate the test was done. Since the jig was designed to have a central loading with both ends fixed, the k b was
.
The formula used for this part of the section:
Assumption: Glue being stronger than the vanguard sheet, the whole structure was considered as a single structure. There would not be any glue s hearing taking place within cores or at face-core bonding. This ensured the validity of the altered deflection formula.
Figure 5.10: Cross-sectional view of all four SS (used to calc ulate I) The second moment of inertia, I of the structures was calculated by:
Refer to appendix section for full calculations
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Table 5.2: Various measurements of SS
Showing calculation for Hexagonal core SS using 4.9N load:
This calculation was repeated for other structures. All the calculation was recorded in the table below.
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Table 5.3: Theoretical deflection of the structure
Table 5.4: Actual deflection of the structure
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Table 5.5: Percentage of error between actual and theoretical deflection
6. Conclusion
The interest of having theoretical and actual values was to do a relative comparison. The percentage error was not the upmost concern. Based on the physical testing, elongated hexagonal core was the strongest. This had been proved by the theoretical data, followed by triangle and square which had about the same strength. Again, this had been agreed by the theoretical data. The we akest SS from the physical testing would be the hexagonal core. This was contradicting with the theoretical calculations. The possible reason would be discussed in t he next section. Apart from that, the only mode of failure witnessed was face wrinkling. The other SS were too rigid and failure was not observed. The non-SS was a lot weaker than the SS. The core in the SS helped to minimize the materials used and provide many I-beam structures within the Page | 30
whole SS. Lowering the weight and producing a high strength to weight ratio was why SS were a lot stronger than plain vanguard sheet glued together i n plies.
7. Errors and Recommendations
The team possessed very minimal knowledge on sandwich theory and its construction. Starting with the grain orientation testing, the amount of fibre in a direction parallel to the load varied in the initial testing itself. This could be clearly seen by the variations in E value for longitudinal specimen 1 and 2. Thus when making the cores, it was highly possible for variations in fibre-orientation, causing error in the 1st assumption of declaring a fixed value for E. Since the core was relatively thick, it had high D which prevented it from failing even at highest applied load. Hence failure modes were not clearl y observed. While theoretical calculations show that hexagonal core supposed to be the second strongest structure, physical testing showed it was the least strong with an error over 700%. Reason could be since it was the first structure to be made, there involved high chances of imperfections in the construction and unintentional mishandling. Finding the theoretical deflection was the biggest challenge for the group. Of all sources, most were considered too high-level for the group. The basic formula, which was understandable by the team, had too much assumption, adding to the total error. With unconditional guidance from Mr Bang, second moment of inertia was calculated individually by measuring the crosssectional webs’ and flanges’ dimensions for each structure instead of assuming, reducing
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percentage error. Given more time, the team could refine the formula for the SS and also use explore into FEM analysis to simulate the deflections and reduce the error. Through this assignment, the team had ventured into a completely new field of engineering and research. The team would like to bring this ass ignment beyond its scope by analysing more into the research and development of SS in future.
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8. References
R EFERENCE FROM WEBSITE
‘Test conditions for films and sheets’, Plastics-Determination of tensile properties, (1995) , accessed 27 December 2012.
Dr Uday K. Vaidya, ‘Sandwich Composites’, The Future is Advanced Plastics and Composites, , accessed 29 December 2012.
‘Basic Design Principles’, about.com Compasites/Plastics, (1968) , accessed 29 December 2012.
R EFERENCE FROM SP LIBRARY
Archilles Petras, ‘Design of SS’, PhD Thesis (Cambridge University, 1998)
Dan Zenkert, ‘An introduction to SS’, A. Olsson and R. Reichard (eds), The Chameleon Press LTD., London, 1995.
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