5

“cross-linked” foams to “linear” foams. Of the foam energy in a given frequency range. It also is aided types commonly used, SAN (Styrene-AcryloNitrile) by the shape of the honeycomb cell, where sound Foams are the most forgiving, but cost is at a pre- waves bounce from cell wall to cell wall and get mium. It also is common in foams for the tem- further absorbed by the visco-elastic nature of the perature resistance to decrease when their elastic- plastic. It should be understood that not all ther- ity is increased. This is RIGID-ELASTIC TECHNOL- moplastics are viscoelastic. Polypropylene, which OGY . This is Nida-Core. By comparison, Nida-Core is used in Nida-Core H8PP, is visco-elastic and thermoplastic honeycombs (or RIGID-ELASTIC gives H8PP its unique properties of impact resis- TECHNOLOGY ) have elasticity in the 200 percent tance, resilience and sound damping. The hex- range! In real-life terms, the better the elasticity, agonal cell form provides the compressive the greater the IMPACT STRENGTH and derived strength that separates the two skins to maintain TOUGHNESS. Or, in reverse comparison, the stiffer panel stiffness. Impact loads are dissipated by the the core material, the better it transfers impact elastic and damped response of the core under and vibration energy from the side of the impact the skin — a controlled deflection with recovery. (or outside skin) to the inside skin, thus subjecting This equates to the spring-and-shock-absorber sys- the inside skin to face buckling, delamination or tem used in automotive suspension. Without the catastrophic failure. The basic design criterion for damping component, the structure would respond RIGID-ELASTIC TECHNOLOGY is damage tolerance like a spring and have resonance. Damping indi- — a measure of the panel’s retention of its struc- cates an energy conversion, or hysteresis. The “Law tural properties after damage compared with its of the Conservation of Energy” states that energy undamaged properties. It is considered desirable cannot be created or destroyed; however, you for core to deform elastically yet remain intact with can convert the energy to another form. In this the facings. This enables a panel to support a case the kinetic energy of the impact is converted considerable percentage of its designed dynamic into small amounts of heat as the viscous nature of loads, despite the damage. In theory, this property the polypropylene provides resistance to deflec- can be advantageous when parts are designed to tion, as well as to recovery. The damped resilience be “under-built,” that is, they have the damage permits the use of lower safety factors in designing tolerance calculated into the part itself, thus sav- structures because they are less prone to catastro- ing weight and cost. Another important design as- phic failure. Other core materials, such as balsa pect of RIGID-ELASTIC TECHNOLOGY is its ability to and rigid foams, will be initially stiffer, stiff enough dampen sound and aid in quieting the structure. to tempt a designer to use thinner laminates. While One must not confuse the two acoustical phe- they may be more rigid, that very rigidity makes nomena: sound transmission loss and sound ab- them prone to catastrophic failure under impact sorption. Sound transmission loss relates to the use because there is no damping or shock absorption. of sandwich panel as a sound barrier, in which Failure modes in balsa-cored panels include con- case elastic honeycomb core is not very effective tra-coup de-lamination where a plug of end-grain in higher frequencies, although it is extremely ef- balsa is dislodged under the impact point, which fective in lower frequencies. (In the 125 to 150 Hz pushes the opposite skin from the core. Rigid range of structure-borne vibrations.) Another great foams will demonstrate different failure modes, plus for all honeycomb sandwiches is great fatigue such as diagonal core ruptures or delaminations resistance and toughness. By nature of its design, starting in the zone under the point of impact, a honeycomb’s cells form thousands of small where the core is crushed but the skin recovers. webs inside the panel, which means that failure These are all forms of brittle failure. Since balsa- of a web (or even a series of webs) does not inevi- and rigid foam-cored sandwiches are very reso- tably lead to catastrophic failure of the whole nant, they have, in some cases, demonstrated panel. catastrophic failure when subjected to operating T he criteria for sound transmission loss is high conditions at their natural harmonic. weight and low flexural stiffness (just the opposite Sandwich core structures made with thin, high- of RIGID-ELASTIC TECHNOLOGY), which is why lead strength skins and H8PP polypropylene honey- is an effective sound barrier. The visco-elastic na- comb core also demonstrate the desirable ture of the plastic honeycomb technology e f f e c t i v e l y c a n c e l s o u t t h e s o u n d a n d v i b r a t i o
n n o f

WWW.NIDA-CORE.COM

118943_NidaCore022607.pu... Friday, March 02, 2007 02:12 page 5 Magenta Magenta Yellow Yellow Blac Blac Cyan Cyan k k