Honeycomb Crush Test Report

David Ashby - 120092684

MEC3019 - Honeycomb Crush Test Report

Contents

PageIntroduction 1Experimental Procedure 2Results 3Discussion 10Conclusion 11

Introduction

The aim of this report is document the experimental procedure for crushing a variety of samples of honeycomb material. As part of the experiment the deformation and failure modes will be observed. Then the data collected along with before and after photos will be used to determine which sample is the strongest, based on their mean crush stress, and why. The data collected will be compared to that from manufacturers of similar products to try to determine the materials and manufacturing technique used for each type of honeycomb.

Honeycomb is used in sandwich panels to increase their stiffness by increasing the second moment of area whilst adding little mass, for example in aeroplane floors. They can also be used in energy absorbing applications such as buffers on trains. For both these types of applications it is important to know the compressive strength. The advantages of using honeycomb are its low density and relatively high strength to weight ratio. The common materials for constructing honeycomb include aluminium, stainless steel and Nomex.

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Experimental Procedure

First the key dimensions of each sample need to be measured, these include sample height, width and thickness. As well as material thickness and the locations where this is doubled. Also, the number of hexagons and their geometry needs to be recorded. This is done using a set of digital Vernier callipers. The measurements taken are used to calculate the whole area and true area. Whole area is calculated by multiplying the width by the thickness of the sample. To get the true area the area inside the hexagons is calculated then multiplied by the number of hexagons and this value is subtracted from the whole area. With this data the force applied per unit area and the corresponding real stresses can be calculated.

Three different types of honeycomb are to be used and each type is tested 4 times. Twice in crushing at the faster speed of 20mm/min and then twice in crushing at a slower speed of 50mm/min. The machine used to conduct the tests is a Shimadzu AG-Xplus universal test machine. The set up is shown to the right in figure 1. Recordings of the force applied and stroke are automatically recorded every 0.01 seconds. To apply the force evenly a steel disc, with a mass of 2.64kg, is placed on top of the sample between it and the surface applying the load. Therefore the force of this acting under gravity (2.64x9.805=25.885N) must be added to the force applied by the machine to get the total force applied.

To calculate the average force applied the data between displacements of 10mm and 30mm will be used. This will therefore ignore the large peak at the start of the test and measure the force once the honeycomb has buckled and is crushing at a constant rate. The corresponding force per unit area will be calculated by dividing the average force by the whole area. The Corresponding real stresses will be calculated by dividing the average force by the true area.

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Figure 1 – Photo of the compressive test machine set up

Fixed support

Honeycomb Sample

Plate with mass of 2.64kg

Load Applied


Results

Sample Before After

A1

A2

A3

A4

3


Sample Before After

B1

B2

B3

B4

4


Sample Before After

C1

C2

C3

C4

5


Test Sample Measurements

SampleMaterial

Thickness (mm)

Number of Hexagons

Area per Hexagon

(m2)

Whole area(m2)

True area (m2)

Sample height (mm)

A1 0.2 14 9.100 x10-5 47mmx35mm=1.645 x10-3 3.710x10-4 78

A2 0.2 14 9.241 x10-5 48.71mmx37.07mm=1.806 x10-3 5.119 x10-4 75.7

A3 0.2 14 9.060 x10-5 46.73mmx37.24mm=1.740 x10-3 4.718 x10-4 76.13

A4 0.2 14 8.922 x10-5 45.55mmx36.64mm=1.669 x10-3 4.199x10-4 76

Average 0.2 14 9.081 x10-5 1.715 x10-3 4.437 x10-4 76.46

B1 0.2 20 8.720x10-5 49mmx47mm= 2.303x10-3 5.590 x10-4 79

B2 0.2 20 8.707x10-5 50.40mmx42.75mm=2.139 x10-3 3.978 x10-4 75

B3 0.2 20 8.520 x10-5 46mmx51mm=2.295 x10-3 5.910 x10-4 74.5

B4 0.2 20 8.533 x10-5 55.44mmx43.77mm=2.427 x10-3 7.200x10-4 74

Average 0.2 20 8.620 x10-5 2.291 x10-3 5.670x10-4 75.63

C1 0.2 18 8.790 x10-5 49mmx49mm=2.401 x10-3 9.168 x10-4 75

C2 0.2 18 8.261x10-5 47.67mmx48.86mm=2.329 x10-3 9.146x10-4 75

C3 0.2 22.5 8.730 x10-5 51.45mmx49.47mm=2.545 x10-3 6.799x10-4 74.85

C4 0.2 22.5 9.260x10-5 50.02mmx50.47mm=2.525 x10-3 5.351x10-4 76

Average 0.2 20.25 8.760 x10-5 2.450 x10-3 7.616x10-4 75.21

A and B samples have hexagons with two layers where the top and bottom of each row of hexagons join as shown by the black lines in figure 2 below. C has three layers where the black lines are shown below in figure 3.

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Figure 2 – Structure of A & B Samples Figure 3 – Structure of C Samples


Graphs of Test Results

0 5 10 15 20 25 30 35 400

1

2

3

4

5

6

7

8

9

10

A Test Results - Force v Displacement

A1A2A3A4

Displacement (mm)

Forc

e (k

N)

0 5 10 15 20 25 30 350

5

10

15

20

25

B Test Results - Force v Displacement

B1B2B3B4

Displacement (mm)

Forc

e (k

N)

7


0 5 10 15 20 25 30 35 400

2

4

6

8

10

12

14

16

C Test Results - Force v Displacement

C1C2C3C4

Displacement (mm)

Forc

e (k

N)

8


Test Results

Sample Crush Speed (mm/min)

Peak Force (kN)

Average Force (kN)

Corresponding Force per Unit Area (MN/m2)

Corresponding Real Stresses

(MPa)

A1 20 8.96 5.49 3.340 14.811

A2 20 7.07 5.51 3.050 10.759

A3 50 7.70 5.43 3.122 11.516

A4 50 8.20 5.27 3.160 12.560

Average - 7.98 5.43 3.168 12.411

B1 20 16.10 10.71 4.651 19.161

B2 20 17.70 10.79 5.046 27.131

B3 50 21.30 11.52 5.020 19.492

B4 50 19.20 11.59 4.776 16.098

Average - 18.58 4.87 4.873 20.471

C1 20 14.10 9.21 3.834 10.040

C2 20 13.80 9.68 4.154 10.580

C3 50 14.30 10.17 3.994 14.951

C4 50 12.60 10.45 4.139 19.527

Average - 13.70 9.87 4.030 13.775

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Discussion

From the test result it can be seen that on average the B samples required almost twice the stress, 20.5 MPa v 12.41Mpa, of the A samples to be deformed by the same amount. This is despite both samples having a very similar hexagon structure. The A samples had fewer hexagons in each sample which makes sense given the lower average force needed to be applied to crush the samples. However, as the hexagons were roughly the same size and the material thickness, the same 0.2mm, then it can be assumed that the material of the honeycombs differs between the A samples and B samples. From the test result and the appearance of the samples I would predict that A were made of aluminium and B were stainless steel. Both appeared to be expanded honeycomb created by placing adhesive in strips on flat sheets of material then pulling the material apart once the adhesive has set to form the honeycomb shape. To confirm the materials of these samples further tests would have to be done. I would recommend measuring the mass of each sample to calculate the density and see if this corresponds to either aluminium or steel. As well as this a magnet should be applied to the samples to determine if either sample is magnetic, if so then the material is probably a form of steel.

The C samples have a slightly higher average value for real stress than the A samples of 13.8MPa verses 12.4MPa. But the C samples are on average much less than the B samples. This suggests that the material of C is, like A, aluminium. Again the tests mentioned earlier should be repeated for C to try and confirm its material. However, C has a different hexagon structure to A and B with each hexagon being spilt into two. This would help to increase the force required to crush the samples, reflected in C having an average crush force approximately double that of the other two samples, but its stress is similar to A. This is probably because it is the same material but has a larger true area of 7.616x10-4 m2 compared to 4.437 x10-4 m2 for A. The structure of the hexagons implies the honeycomb was created using a corrugated process. Where sheets are pressed into a corrugated shape then bonded to each other with adhesive, in the case of this honeycomb adding a flat sheet between the two corrugated sheets.

From the photos it can be seen that both A and B samples deformed in a very similar way with the hexagons staying bonded together and the individual columns that make up the hexagons buckling. The A samples have deformed at both ends whereas the B samples have only deformed at the end where the load was applied. This implies that the B samples were made of a material with a higher young’s modulus again implying they were made of steel whereas A was probably aluminium. The C samples deformed differently to the other two. The columns have also buckled but as part of the crushing the outside columns have debonded and been pushed away from the rest of the honeycomb.

Testing at different crush speeds doesn’t appear to have made any real difference in the results, the only difference is at the slower speed of 20mm/min more data is collected. This is because the force and displacement is recorded every 0.01 seconds and the lower speed tests last longer to deform the samples by approximately the same displacement.

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The A samples are comparable to HexWeb® CR III a product from Hexcel (http://www.hexcel.com/Resources/DataSheets/Honeycomb-Data-Sheets/CR3_us.pdf) the cells of A were approximately 10mm wide which is roughly equivalent to 3/8” cell size of the Hexcel product. With a nominal density pcf of 6.5 the minimum average crush strength is given as 505psi which is 3.48MPa. This is close to the 3.2MPa for the corresponding force per unit area found during the experiment. This implies a 5052 grade of aluminium that is used for the Hexcel product is probably used for the A sample of honeycomb.

Conclusions

The B samples are able to withstand the greatest mean crush stress (average force per unit area of honeycomb material) with an average of 4.8MN/m2. This is probably due to this honeycomb being made of stainless steel whereas the other two are probably aluminium. The A samples have the lowest mean crush stress at 3. 2MN/m2. This is due to having less material per unit area as it has the fewest hexagons. This is probably due to the process used to create the honeycomb as I expect it is expanded honeycomb. On the other hand, the C samples withstand a similar real stress, within the material, to the A samples of 13.8MPa and 12.4MPa respectively. Leading to the assumption that both A and C are made of the same materials, probably aluminium. However, as the C samples have their hexagons split in half, there is more material per unit area, effectively creating more columns in the material that need to be buckled, making it harder to crush. This means that the C samples of honeycomb require a greater mean crush stress of 4.0MN/m2 compared to 3.2MN/m2 for the A samples.

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http://www.hexcel.com/Resources/DataSheets/Honeycomb-Data-Sheets/CR3_us.pdf

Documents

Honeycomb Crush Test Report