### Solid Metal Versus Sandwich Panels

In this post we will discuss the strength to weight ratio benefits of solid metal versus sandwich panels.

Historically, solid Aluminum alloy materials have been used for many many decades in the aerospace industry. They have a solid proven track record of flight hours, they are quite durable, strong and fairly light weight. However, there are many applications where the use of solid aluminum is too heavy for the weight to stiffness ratio it offers, thus leading to the study of solid metal versus sandwich panels.

#### Some good sandwich panel application examples are:

• Cabin interiors structures
• Cabin floor panels
• Engine nacelle components
• Cargo panels
• Aileron balance panels
• Etc.

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#### So what are the drawbacks of solid metal versus sandwich panels and why did these new materials become so popular?

As mentioned in this FAA aircraft structures document:

In the 1960s, ever larger aircraft were needed and developed to carry passengers. As engine technology improved, the jumbo jet was engineered and built. Still primarily aluminum with a semi-monocoque fuselage, the sheer size of the airliners of the day initiated a search for lighter and stronger materials from which to build them, thus researching solid metal versus sandwich panels. The use of honeycomb constructed panels in Boeing’s airline series saved weight while not compromising strength. Initially, aluminum core with aluminum or fiberglass skin sandwich panels were used on wing panels, flight control surfaces, cabin floor boards, and other applications.

A steady increase in the use of honeycomb and foam core sandwich components and a wide variety of composite materials characterizes the state of aviation structures from the 1970s to the present. Advanced techniques and material combinations have resulted in a gradual shift from solid metal versus sandwich panels and aluminum to carbon fiber and other strong, lightweight materials.

These new materials are engineered to meet specific performance requirements for various components on the aircraft. Many airframe structures are now made of more than 50 percent advanced composites, with some airframes approaching 100 percent. The term “Very Light Jet” (VLJ) has come to describe a new generation of jet aircraft made almost entirely of advanced composite materials.

#### Benefits of Honeycomb Sandwich Structure:

It all boils down to efficiency for a particular application of solid metal versus sandwich panels. The least efficient on the list is obviously solid metal. Next would be a shape of the metal that is separated with a web, an example would be an I-Beam. The I-beam presents a significant improvement over a solid metal for the same application and presents much higher efficiency in terms of the strength to weight ratio of the member.

But what if we can substitute this I-Beam solid metal versus sandwich panels? Sandwich panel construction includes a porous (mainly honeycomb) core and facing materials structurally bonded together using a strong adhesive material. This is a highly efficient construction, bees know what they are doing (beehives are all honeycomb core).

Looking at the structure of solid metal versus sandwich panels, the flanges of the metal I-Beam can be compared to the facings of the sandwich panel. While the honeycomb core can be compared to the web of the I-Beam. The skins of the sandwich panel, similar to the flanges of the I-Beam, carry the bending stresses with one skin in tension and the other in compression.

The core of the sandwich panel, similar to the I-Beam’s vertical web, resists the out of plane shear loads and keeps the panel skins apart. This provides the high moment of inertia and thus high stiffness. Even better than the I-Beam the sandwich panel core provides a continuous support to the facings as opposed to the centrally located web of the I-Beam.

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At the same time the sandwich panel is much lighter and stiffer, thus giving it the high strength to weight ratio. Figure 2 below demonstrates the strength to weight ratio benefits of solid metal versus sandwich panels.

#### Bending Stiffness Comparison:

Solid Metal:

In simple terms, the flexural or bending stiffness of a solid rectangular Al plate with a width ‘b’, thickness ‘t’, and a material Young’s modulus E is:

Stiffness E_solid          = $E*I src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-=E*b*t^3/12$ —————————> (1)

Aluminum Sandwich Panel:

As shown in Figure 2 above, the sandwich panel is assumed to have the same skin material as the solid, the same width and Young’s modulus. Thus the flexure stiffness of a sandwich panel is calculated as follows:

Stiffness E2_sandwich = $E*(t/2)*h^2*b/2$ —————————> (2)

Assumptions:

• Young’s modulus = 1.05 E7 psi
• Total solid metal thickness t = 1.0 in
• Sandwich panel skin thickness tf = t/2 = 0.5 in
• Sandwich panel core thickness tc = t = 1.0 in
• Total sandwich panel nominal thickness = 2.0 in
• Width b = 1.0 in
• Sandwich Panel h = Distance between the mid planes of the face sheets = $tc+ tf/2 + tf/2 src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" data-=1.5 in$

Substituting all the values into (1) and (2) and taking the ratio of (2)/(1):

The approximate stiffness ratio rounded up to the closest integer E2/E1 = 7.0

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Benefits:

So clearly we can see from the Figure 2 that the weight increase is really small, while the stiffness and ultimate strength go up significantly.

There are some limitations of sandwich panels, for example:

• They generally have 2D orthotropic mechanical properties, which do not allow them to be used in all applications
• They are also not the best choice where a lot of holes are required
• They are required to be certified for their fire resistance properties due to the materials used in the construction such as the skins, core and adhesives. But this aspect is fairly mature now and highly developed in today's structures

Even with all their limitations, sandwich panels, especially non metallic options offer huge weight savings with much higher stiffness compared to their all metal options. Non metallic panels are also highly durable, almost immune to fatigue, and corrosion resistant unlike their solid metal counterparts. This makes them highly attractive materials for many structural applications especially in the aerospace industry.

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Surya Batchu

Surya Batchu is the founder of Stress Ebook LLC. A senior stress engineer specializing in aerospace stress analysis and finite element analysis, Surya has close to a decade and a half of real world industry experience. He shares his expertise with you on this blog and the website via paid courses, so you can benefit from it and get ahead in your own career.