The sandwich is analogous to an I-Beam, in the case of an I- Beam the direct compression and tension loads are carried by the flanges and the shear loads are carried by the web. Whereas in the case of the sandwich construction, the direct compression and tension loads are carried by the facings and shear loads by the core. Vacuum bag process ensures a perfect bond between the facings and the core hence, allowing the Sandwich panels to be analyzed using the Bending Moment formulas for beams. Other Methods like In-situ Foaming result in Insulated Panels having negligible structural strength. There is no structural bond between the facings and the core.

During Vacuum bagging, an UDL of one atmospheric pressure is applied for 10 Hours, to bond the facing to the core. Pre-foamed boards of rigid Polyurethane Foam of 70 kg/cubic meter form the core. This ensures uniform void free bonding of the facing to the core. Other Methods like in-situ Foaming, result in voids, air cavities etc, which if gets filled up with water/air results in continuous debonding of the panels as a result of expansion/ contraction of water/air from temperature changes. This also results in water leakage into the Cabin.

Vacuum Bagging ensures a level surface of the panels without any waviness, cavities, de-bonding etc. This is an important requirement for EMI shielding. EMI Shielded Shelters made using Vacuum Bagging, exhibit the same levels of EMI Shielding even after 10 years of deployment.

Shelter constructed with assembling together structural sandwich panels of (thickness 62 +0/+1 mm) in case of roof or floor panels and rest of the vertical panels (51 +0/+1 mm) can carry high payloads. They exhibit high strength to weight ratios. A Test panel of sandwich construction has to withstand 800 Kg of load with out failure as per the shelter ATP (the actual failure loads generally exceed 1500 Kg). The figure below depicts the loading conditions of the test Panel.

Size of the test panel: 400mm x 900mm (b x l)
Length of the span 'L' = 750mm
Facing thickness't' = 1.5 mm
Thickness of the Core 'c' = 60 mm
Thickness of the panel'd' = 63 mm
The load applied 'P' = 800 kg i.e 7848 N

To calculate the Facing Stress at a load of 800 kg, the following formula is used:

FS = [ P x L / 4] / [ t x b x (d + c)]

FS = [ 7848 x 0.75 x 1000 / 4] / [ 1.5 x 0.4 x (0.063+0.060)]

FS = 19.94 N/mm sq = 19.94 MPa = 2.03 kg/mm sq

Facing stress at a bending load of 800 kg, centrally applied = 2.03 Kg / msq.

To find out the equivalent UDL for the same Facing stress the following formula:

FS = [ P x L / 8] / [ t x b x (d+c)] is used.

19.94 MPa = [ P(udl) x 0.75 x 1000 / 8] / [ 1.5 x 0.4 x (0.063 + 0.06)]

by which we find that P(udl) is 15,700.8 N or 1600 kg. and the intensity of load is 4446 kg/m sq.

The sandwich construction consists of two strong thin metal facings spaced far enough by means of a light-weight core which is bonded to the facings through a bonding medium like an adhesive layer adopting State-of-the art Vacuum Bag Bonding process. Depending upon the equipment layout, welded light alloy grid structure will be incorporated in the core portion of the panel for providing mounting points to the equipment.

An Exploded view of the Floor panel is shown below. In the floor panel Marine Grade Plywood of 9mm thickness is incorporated below the inner metal sheet facing. This acts as a pressure pad and it avoids local denting of the Floor panel by personnel walking with field boots or due to the loading/unloading/shifting of equipment inside the shelter.

FLOOR PANEL EXPLODED VIEW

1. Metal sheet facing (1.5 / 1.2 /2.5) mm thk
2. Marine Grade Plywood, 9 mm thk.
3. Welded light alloy grid incorporated in 70Kg/cu m RPUF 50mm thk.
4. Metal Sheet facing 1.5mm thk.