In the aerospace industry, damage visibility and damage tolerance are key parameters in the design of structural sandwich components. Sandwich core materials need to deliver outstanding structural integrity to withstand impact events and excellent damage visibility during visual inspection. Evonik Industries’ answer to these requirements is its newly developed product, ROHACELL HERO polymethacrylimide foam.
ROHACELL HERO Polymethacrylimide Foam
Besides providing improved damage tolerance and visibility, ROHACELL HERO polymethacrylimide foam also helps achieving weight reduction as it is lightweight but robust. The foam exhibits better mechanical performance even at low densities. When utilized in integral design, ROHACELL HERO does not require potting or secondary bonding, thereby ensuring high process safety.
Impact events or impact damage vary and gain significance based on the class or function of the aircraft component. Impact events can include hail, bird strikes, runway debris that becomes airborne, and tool drops during the maintenance of aircraft. Minor damage caused by certain impact energy is often acceptable as long as it is detectable using standard inspection techniques. Undetected damage needs to survive the Ultimate Load (UL) (Figure 1).
Figure 1. Undetected damage must withstand the Ultimate Load (UL) (source: Airbus)
ROHACELL HERO - An Alternative Solution to Honeycomb
In aircraft applications, sandwich structures, including leading edges, are generally fabricated from honeycomb cores. Nevertheless, a major drawback of honeycomb sandwich components is the tendency of both damaged and undamaged areas of a honeycomb core to collect water inside the open cavities. When the airplane is in service, this unwanted water freezes when temperature fall below 0°C (32°F) and rapidly transforms into steam when temperatures reach 80°C (176°F). This causes skin debonding from the sandwich core. This issue can be addressed with the closed-cell ROHACELL HERO as it significantly reduces the risk of ice or vapor formation in a sandwich component by avoiding water collection.
Demonstration of Impact Damage Tolerance and Damage Visibility of ROHACELL HERO
An analysis performed at the Fraunhofer IWM Institute in 2013 corroborated that the ratio of impact damage size/visibility of ROHACELL HERO is the same of a honeycomb core. The analysis involved testing at an impact energy range of 3-35J at the most critical temperature of -55°C (131°F) (Figures 2 and 3).
Figure 2. Comparison of impact depth of ROHACELL HERO and honeycomb sandwich after test and after 48 hours (source: Fraunhofer Institute – Halle - Germany)
Figure 3. Comparison of impact depth of ROHACELL HERO and honeycomb sandwich at different impact energies (source: Fraunhofer Institute – Halle – Germany)
Cost-Effective Damage Tolerant Structures with ROHACELL HERO
In the fabrication of structural components, ROHACELL HERO core provides more design options that result in a cost-effective process producing high-quality finished components. Production time is greatly reduced owing to the possibility of one-shot curing, convenient machining and thermoforming, and drastic reduction in waste. Integral design with better surface quality is possible with ROHACELL HERO.
All of these benefits together can achieve a 30% reduction in finished component costs, a key reason to consider the new ROHACELL HERO as a substitute to honeycomb. The ROHACELL Shapes service simplifies sandwich component fabrication by providing with cores that have been pre-shaped into turnkey components.
About Evonik Industries - ROHACELL
For more than 40 years, Evonik's ROHACELL structural foam has been offering the aerospace and automotive industries, medical technology and other markets boundless possibilities for creating high performance composite parts and products.
Considering ROHACELL's excellent mechanical properties over a wide temperature and density range, impressive dynamic strength and fatigue behavior, and a temperature resistance of up to 220C (428F), it is no coincidence that Evonik remains a leader in supplying foam cores sheets and shapes for lightweight sandwich construction worldwide.
This information has been sourced, reviewed and adapted from materials provided by Evonik Industries - ROHACELL.
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