Foam Core Materials For Aerospace Applications

Blake Juhl, Director of Sales and Marketing at Evonik, gives an overview of the properties and applications of ROHACELL rigid foam and an introduction to the new ROHACELL HERO product.

Could you give a brief overview of the ROHACELL rigid foam and typical applications that can see real benefits from using it?

More and more industries are discovering the benefits of designing structural components using sandwich construction. Strong internal support structures are needed for these applications, but the driver is most often the reduction of weight in the total structure.

As a robust and extremely lightweight core material, ROHACELL foam is increasingly the product of choice for sandwich designs.  ROHACELL is primarily used as the inner supporting "core" sandwiched between the "skins" of the sandwich construction.

These skins are composed of fibers that include glass, carbon, Kelvar and other reinforcements that are impregnated with a number of resin systems, such as epoxy, phenolic, BMI, cyanate ester and others.  Producing the finished sandwich requires the use of typical processes to cure the resin and combine it with the core.  

Most common are infusion, or in the case of using a prepreg, autoclave curing.  Important to polymerization of the resin is applying the precise amount of pressure in the right temperature environment (180°C is most frequently used in the aircraft industry). The end result is an extremely light, but rigid and robust structure.

ROHACELL is widely recognized for its performance in the aerospace sector where light, strong and resilient structures are demanded.  Aircraft, transportation and launch vehicles are typical target platforms.  Flight control surfaces, rotor blades, radomes and satellite containment fairings are other common applications.

Are there any new developments in the world of ROHACELL foam cores?

We recognize the cost and efficiency challenges faced by today's aircraft industry.  Building structures that are very lightweight delivers weight savings that improves aircraft efficiency without sacrificing structural strength.  Today, more and more class A and B parts are being built in sandwich.

A sandwich design using a ROHACELL foam core is one of the best options available and we continue to expand our product offerings to give designers the tools they require.

Our newest product introduction is ROHACELL HERO - available in densities of 51, 71, 110, 150 and 200 kg/m³  (3.2, 4.4, 6.8, 9.4, 12.5 lbs/ft³).  This new addition to our product line is ideal for Class A and B parts in commercial aircraft and the OEMs agree.

What does ROHACELL HERO offer that the historical ROHACELL products do not?

HERO not only meets aircraft damage visibility requirements while offering the same well-known mechanical excellence of all ROHACELL products, but it can boast an elongation at break performance that is 3 times higher than any of our other grades.

Our goal in the development of HERO was to address the industry need for higher elongation structural foam core.

The result was a product that offers between 8 and 10 percent elongation at break, depending on the product density selected.  Fracture toughness (resistance to crack propagation) is also increased as a result of the higher elongation.

Are there specific applications where ROHACELL HERO offers exceptional benefits for an aircraft structure or aircraft?

Absolutely! The requirements for all aircraft surfaces and covers are driven by stiffness and robustness.  This includes landing gear doors, belly fairings, flight control surfaces, radomes and nacelles, to name a few. The surfaces of all of these parts are subject to impact when an aircraft is in flight or even when undergoing scheduled maintenance.

When a surface impact of any type occurs, it is critical that the damage be easily detectable through simple visual inspection.

HERO has excellent fracture toughness to withstand an impact event and also excellent visibility of the damage once the impact has occurred.  Not only does HERO provide this critical damage tolerance and visibility advantage, it can also offer the added advantage of part cost reductions in production.

Why would a company select ROHACELL HERO over other sandwich core options, such as honeycomb structures?

Unfortunately, a sandwich solution with a typical honeycomb core can be limited in service functionality and can also be quite complex to produce.  For example, processing honeycomb cores requires a costly two-step curing cycle and the resulting installed structure is still subject to freeze damage whether it is damaged or not.

A recent independent study conducted by the Fraunhofer Institute concluded that ROHACELL HERO not only survives impact scenarios typical of everyday airplane flight exposure,  but if and when a structure is impacted, it can be easily seen and reviewed for health monitoring and flight assurance purposes.

If the aircraft part's outer skin suffers a breach, the 100% closed cell HERO core will not absorb water, which is not true of structures where Honeycomb type cores are used.

A unique benefit of all ROHACELL foam cores, including HERO, is the homogeneous and isotropic properties that make it mechanically stable from all directions.

HERO does not require expensive core stabilization cure cycles, processing steps or additional materials to close out exposed unstable edges.

ROHACELL HERO is a solution designed for the challenges of the future in aircraft structural design. It starts with a professional consultation between us and the customer - we know our products and are ready to support both designers and manufacturing/production teams to help them realize how they can use ROHACELL to their advantage.

About Blake Juhl

Blake Juhl is the Director of Sales and Marketing for Evonik Industries' ROHACELL® product line in the Americas.

He started his reinforced composites career after graduating from California State University at Northridge in 1987, and has been involved in processing,  selling and marketing of advanced composite materials and related products ever since.

He has been employed with Evonik since 2001, and most recently was appointed Global  Launch Manager for the newly developed ROHACELL® HERO Grade.