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Lightweight sandwich panels

Here we will calculate the theoretical minimum weight of a sandwich panel. This is used for my solar bike, the Maxun One, which requires ultralight PV panels. The sandwich panels that I need are so low-weight, lighter than panels used in aerospace, that these aren't manufactured anywhere to my knowledge. Therefore I wanted to find out if it was possible to build these myself.

Ultralight sandwich panels
Ultralight sandwich panels

The solar panel size is 900 x 650 x 10mm. For the first solar bike, I used Alustep 300 Light sandwich panels. These have glass fiber skins and an aluminum honeycomb core, the weight is 1,6kg/m2. A lot of experience has been gained with these panels, since they have been used more than 5000km. The panels are strong enough but too heavy. I want to use much lighter panels for the next solar bikes because a low weight will burden the panel holder construction much less on bad roads.

As we will see, it is possible to build sandwich panels with the same strength as the Alustep 300 Light panels but with a weight of about 700g/m2 instead of 1600g/m2. 

Lightweight sandwich test panels
Lightweight sandwich test panels

The most lightweight skin fabric

For the skins I will use TeXtreme carbon fiber. TeXtreme uses spread tow fabrics which increases the performance as well as reducing the laminate weight up to 20%:

TeXtreme carbon fiber
TeXtreme carbon fiber

Step 1 Determination of the required skin thickness

To find out the required skin thickness, I have built and tested several sandwich panels. The bending strength is measured with a simple four-point bending test:

Four point bending test
Four point bending test

The panels are built up with TeXtreme carbon fabric and Aramid (Nomex) honeycomb. Because of the wind drag, the maximum thickness is 10mm.

It turned out that the sandwich panel has same strength as the Alustep if it is built up as follow:

  • Core: Aramid honeycomb Cormaster C2 C2-3,2-29 290g/m2, thickness 10mm, cell size 3,2mm
  • Skin: TeXtreme 80g/m2, laminate weight 180g/m2, thickness 0,2mm. I made this with vacuum bagging (see below).
  • Skin to core bonding: an abundant amount of 100g/m2 epoxy has been used per skin since weight plays no role in this test.

The weight of this test panel is 850g/m2.

Step 2 Calculation of the minimum panel weight without the skin to core bonding

The test panel (see step 1) was manufactured just to determine the strength, but no attention was paid to low weight. The weight of the skins can be reduced by using resin infusion instead of vacuum bagging. The laminate weight then becomes 130g/m2, calculated with this online tool: https://www.r-g.de/en/laminatecalculator.html

So, without the skin to core bonding, the minimum weight up to now is 550g/m2:

  • Core: Aramid honeycomb 290g/m2
  • Skins: TeXtreme 130g/m2

Step 3 Calculation of the minimum skin to core bonding weight

The weight of the skin to core bonding plays a serious role in the total panel weight. Therefore only glue has to be applied between the honeycomb cell walls and the skin in the form of adhesive fillets. Note that epoxy is used for laminating but it is not the best glue for skin to core bonding. It is better to use glue that has been developed specially for this purpose. 

Now I will draw up a formula for the skin to core bonding weight in relation to the fillet radius and the honeycomb cell size:

bonding weight / m² = density * adhesive fillet area * fillet length per cell * number of cells

cell size = s
adhesive fillet radius = r
glue density = σ

Honeycomb core cell geometry
Honeycomb core cell geometry

 

Adhesive fillet area
Adhesive fillet area

adhesive fillet area (if the fillet shape is circular) = r² -  1/4 π r² = r² (1 - π/4) 
fillet length per cell = 6L = 6 * s / √3
number of cells / m² = 1 / cellarea = 1 / (1,5L²√3) = 2 / (s² √3)
bonding weight = σ r² (1 - π/4) 6 * s / √3 * 2 / (s² √3) = σ r² (1 - π/4) 12 * s / (√3 s² √3) = σ r² (1 - π/4) 4 / s 

The bonding weight = 0,858 r² σ / s

This is the bonding weight for a honeycomb with cell size (s) = 3,2mm and with epoxy bonding (σ = 1150kg/m³) in relation to the fillet radius:

adhesive fillet radius [mm] bonding weight [g/m²]
0,3 28
0,4 49
0,5 77
0,6 111

Adhesive fillet: radius=0,5mm, cell size=3,2mm

Now we must determine the required fillet radius. According to this study, the optimal bonding weight can be 40g/m2. This corresponds to a fillet radius of 0,35mm in our example. To be on the safe side, I take a fillet radius of 0.5mm, that will have still a low weight of 77g/m2.

The final weight of the sandwich panel

If we take a fillet radius of 0,5mm, the total panel weight is 700g/m²:

core + 2(skin + bonding) = 290 + 2(130 +77) = 700g/m²

Sandwich panel skin to core bonding with heat activated adhesive film

To save weight, it is necessary to use a method where we can apply exactly the minimum required amount of glue in the form of fillets concentrated around the edges of the honeycomb. One method we could use is dipping the honeycomb core in glue, but this will soon become a big mess. It is more convenient to use a heat-activated adhesive film, which is dry during processing. Due to weight savings, we cannot use the adhesive film in the normal way. We only want to use the glue and not the film itself. This can be achieved with the following method. This is a so-called multi-step procedure, first one face will be glued and then the other.

The film is applied to the top of the honeycomb, pressed and heated in an oven until the glue becomes soft and sticky. Then the film is removed and a certain amount of glue will remain around the edges of the honeycomb. This must be done quickly to prevent the glue from hardening. When the glue has cooled down and is dry, the skin is applied to the core, pressed and then glued in an oven. The amount of glue must be precisely dosed by setting the time in the oven and the temperature.

Epilogue

  • A superfluous adhesive layer of 0,1mm thickness causes an extra weight of about 110g/m2.
  • The core-skin bonding is very crucial for the final sandwich strength and it is quite a challenge to apply just enough glue for getting a nice adhesive fillet. 

I'm currently looking for manufacturers that can supply strong sandwich panels of about 700g/m2 as described above.

Links

Extensive studies have been carried out by other people on this subject, the following links are particularly interesting:

Ultra-Light Photovoltaic Composite Sandwich Structures
Fabrication and Structural Equivalency Analysis of CFRP Nomex Core Sandwiched Panels for FSAE Race Car Chassis
Best Practice Guide forSandwich Structures in Marine Applications

Heat activated adhesive films

VTA 260
Everad adhesive films and webs
HexPly® ES/EH 84
Masterbond
Redux 312

The used sandwich panel four-point bending test

A four-point bending test is more convenient to use than a three-point test because the weight has automatically two supports points. The alustep 300 light panel serves as reference because this panel proved to be sufficiently strong in practice. Its bending is 2mm +/- 0,1mm with a load of 13kg in this setup:

Sandwich panel four-point bending test
Sandwich panel four-point bending test

The used vacuum bagging method

  • Vacuum bag
  • Breather
  • Impervious release film
  • Bleeder
  • Perforated release film
  • Peel play
  • TeXtreme fabric with 140g/m2 epoxy
  • PVA
  • Glass panel
  • Vacuum bag

Do you have any comments about the website or problems with shopping? Please let me know.
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