Contents[Show]

Lightweight sandwich panels

For the production of PV panels for the solar bike, the Maxun One, I need ultralight sandwich panels. Because suitable panels are not produced, I wanted to find out if it was possible to make them myself. But first I wanted to calculate the theoretical minimum weight of a sandwich panel.

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 experience was that 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. So, this is a reduction in weight of 56%.

Lightweight sandwich test panels
Lightweight sandwich test panels

Preface

Nog doen....

The most lightweight skin fabric

For skins with minimum weight, TeXtreme carbon fiber is used. 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

To find out the required skin thickness, I have built and tested several sandwich panels. The bending strength is measured with a four-point bending test (see below). 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 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. Made with vacuum bagging (see below).
  • Skin to core bonding, an abundant amount of 100g/m2 epoxy has been used per skin.

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

Step 1 Calculation of the minimum panel weight without the skin to core bonding

The test panel 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. The laminate weight then becomes 130g/m2, calculated with this online tool: https://www.r-g.de/en/laminatecalculator.html

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

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

Step 2 Calculation of the minimum skin to honeycomb core bonding weight

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. For minimum weight, only glue has to be applied between the honeycomb cell walls and the skin, in the form of adhesive fillets.

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 

bonding weight = 0,858 r² σ / s

The bonding weight for a honeycomb with cell size (s) = 3,2mm and with epoxy bonding (σ) = 1150kg/m³:

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

The final weight of the sandwich panel

Fillet radius = 0,5mm (glue width is 2 * 0.5mm = 1mm):

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

Compressive strength

Nog doen...

Stiffness

Nog doen...

Peel strength

Nog doen...

Epilogue

  • Some other studies on this subject say that the optimal core-skin adhesive weight is 40g/m2. This corresponds to a fillet radius of 0,35mm in our example.
  • 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. 

Which manufacturer can supply these panels?

I am currently looking for manufacturers that can supply strong sandwich panels of about 700g/m2 as described above.
Who can help me with this?

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

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 bending is 2mm +/- 0,1mm with a load of 13kg.

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

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.
Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.