Sandwich construction is a composite material structure combining low weight, high strength and good dynamic properties. Typically a sandwich composite consists of three main parts: two thin, stiff and strong facing layers separated by a thick, light and weaker inner core. The faces are adhesively bonded to the core to obtain a load transfer between the components. This way the properties of each separate component is utilized to the structural advantage of the whole assembly leading to a very high stiffness-to-weight and high bending strength-to-weight ratio. As a result sandwich components achieve the same structural performance as conventional materials with less weight.

Figure 1: Honeycomb sandwich composite

Source: http://www.eng.uab.edu/compositesLab/F_sandwch3.htm (University of Alabama)

Sandwich constructions can be realised with a great variety of materials both for facing layers and inner core.

The facing layers are typically realised by aluminium plates, high presure laminates, glass fibre reinforced plastics etc.

For the core material quite different realisations exist. The two most common ones are honeycombs (cf. Figure 1) and foams. Honeycombs have been used successfully for decades in airplanes. They are made from aluminium and more recently from glass or aramid fibre reinforced plastics. Typical foams used for sandwich cores are: chlorofluorocarbons (CFCs), halonfluorocarbons (HFCs) and CO₂-foamed materials (the latter being the present lane of development).

Manufacurers

• M/s. Hexcel Corporation Inc., USA (leading international composites manufacturer for railways),
• Kansas Structural Composites Inc., USA,
• Nida Core, Finland
• Fiberline Composites A/S, Denmark
• Concargo Composites, UK

While sandwich composites (especially aluminium honeycomb structures) have been used in aerospace technology for several decades, only recent technological advances and price decline have made applications to railway sector feasible and attractive.

Sandwich structures are presently used in the rail industry for a number of purposes, e.g.

• as energy absorbing material (for buffers, fenders and driver protection)
• for carriage interior panels
• as structural panels for the separating floor of coaches (e.g. for the IC2000)

According to TIFAC (http://www.tifac.org.in) the main advantages of sandwich construction are:

• high rigidity combined with higher strength to weight ratio
• smoother exterior
• better stability
• high load carrying capacity
• increased fatigue life
• crack growth and fracture toughness characteristics are better compared to solid laminates
• thermal and acoustical insulation
• high bi-axial compression load bearing ability

The transfer of sandwich composites from aerospace to ground transportation is rather slow to take off, for several reasons:

• Costs: sandwich composites are only starting to become cheap enough to be applicable outside (government sponsored) aerospace programs.
• Processing: high investment in the specialist tooling and equipment required to process composites.
• Training of work force: work force has to be instructed how to handle these new materials

There are also some specific problems about individual sandwich types, e.g. the danger of water take-up in sandwich composites with honeycomb core.

Generally, composites such as sandwich elements can be used for structural and for non-structural applications (e.g. interior coach panels). According to TIFAC, weight savings of up to 75 % can be reached in non-structural and up to 50 % in structural components. These are very optimistic estimates. The following examples describes two applications already realised:

Application of sandwich composites for structural elements

Hexcel claims that the separating floor structural panels produced for Schindler Waggon AG (to be used in IC2000 double-deck train carriage) are 20% lighter than the conventional welded aluminum extrusion profile system.

Application of sandwich composites for non-structural elements

For non-structural elements (such as interior panels) the weight savings achievable are even higher. According to Schuon 1998, Interior panels for passenger coaches are approx. 35% lighter than conventional panels, which reduces the total coach weight by some 350 kg. Given typical coach weights of 40 tons this is a weight reduction of ~ 1 %.

Overall effect

An overall effect on energy consumption strongly depends on the specific application and cannot be assessed in a general manner.

The following rough estimate may give a general idea of the potential:

Currently, applications of sandwich materials are mainly limited to interior equipment (floor panels, interior wall and roof panels etc).

• Interior equipment usually accounts for 10 - 20 % of the total coach weight
• It is assumed that about one half of the corresponding components may be substituted by sandwich components (panels, floor etc)
• The relevant components may be reduced by up to 50 %.

This yields the maximum weight reduction through sandwich materials will be around 5 %.

The following elasticity table gives estimates for the overall effect on energy consumption.

	Traction	Brake energy recovery	Effect on train mass	Elasticity with regard to train mass	Effect on total energy consumption for traction
High speed train	electric	no	5 %	0,17	1 %
		yes		0,12	1 %
Intercity train	electric	no		0,19	1 %
		yes		0,14	1 %
	diesel	-		0,19	1 %
Regional train	electric	no		0,52	3 %
		yes		0,44	2 %
	diesel	-		0,52	3 %
Suburban train	electric	no		0,64	3 %
		yes		0,57	3 %
	diesel	-		0,64	3 %
Range:					1 – 3 %

The environmental impact of composite materials cannot be assess in a general fashion but rather depends on the concrete materials used. The following aspects play a role:

• Composite materials always pose recycling problems.
• Possible environmental impacts of the substances used for honeycomb structures have to be carefully assessed, e.g. foam cores (such as CFCs) that contribute to atmospheric ozone depletion.

A target conflict can therefore arise between energy efficiency gains through sandwich materials and newly induced recycling problems. However, given the long life-time of rail vehicles, the environmental benefit of mass reduction will presumably compensate slight drawbacks in other fields.