Properties
Duocel® Foam for Impact Absorption Applications
Energy absorbers are a class of products that generally absorb kinetic mechanical energy by compressing or deflecting at a relatively constant stress over an extended distance, and not rebounding. Springs perform a somewhat similar function, but they rebound, hence they are energy storage devices, not energy absorbers.
Foam materials are porous structures, and as discussed in the crush strength section of foam properties, they have a unique stress strain curve as reproduced below.
Once an applied stress exceeds the crush plateau, foam will begin to compress at a fairly constant stress out to about 50-70% of strain. This extended section of the stress / strain curve defines the behavior of an ideal energy absorber. In this zone, the area under the curve represents the product of stress × strain, or “work”. In an actual foam block of finite size this would be represented as:
Force × Displacement
Recognizing that
Force (pounds) × Displacement (feet) = Work (foot • pounds)
and
Work (foot • pounds) = kinetic energy (foot • pounds)
it can be seen that the work that is done by compressing a foam block is equivalent to the kinetic energy of a mass that might impact that block. If properly designed with appropriate thickness and compression strength, a foam block could absorb all of the energy of an impacting mass. Most importantly, the structure the foam block was attached to (and protecting) would never see a force higher than the foam crush strength. Thus, by absorbing the energy of the impacting mass over a controlled distance with a constant force, the protected structure would not have to endure a concentrated high-energy / high force impact that would occur if the mass impacted the structure directly, with potentially catastrophic results.
This is the theory behind extended automobile bumpers that stroke with a fixed force under impact load to eliminate or minimized damage to the vehicle and its occupants.
While the first half of the foam stress / strain curve is the section generally used in the design of any foam energy absorber, the second half, or safety “back-up” zone section represents a special energy absorption reserve. When designing energy absorbers, you have to proceed with the best data available. If the situation is critical enough to need an energy absorber in the first place, it is also prudent to provide some form of reserve capacity in the event the impact loads are not fully known, and could be significantly exceeded. By using the increasing stress / strain curve in the densification section, this allows unexpected energy to be absorbed with an increasing resistance. The payload being protected might experience higher than normal loading and minor damage in this case. However, it is less than would be experienced if the energy absorber had a completely flat stress / strain curve and it fully “bottomed out” in a hard, catastrophic impact at the strain limit.
There are a number of porous structures that are used as energy absorbers. Polystyrene shipping “peanuts”, honeycomb, and polyurethane foam seat cushions are just a few typical examples. All of these materials have perfectly justifiable uses but they do have limitations as noted below:
1. Polymer –based foams and honeycombs have stress / strain curves that are significantly affected by temperature, age, exposure to ultraviolet light, solvents, and other environmental factors.
2. Honeycombs exhibit their characteristic stress / strain curve only if the compression direction is within a few degrees of their orthogonal cell axis, or if they are constrained in a tube to prevent lateral buckling.
3. Honeycombs and closed-cell foams trap gasses within their cells, thus adding a pneumatic “spring” effect that not only disrupts the flat crush plateau, but can add a “rebound” behavior that partially negates the original energy absorption curve.
ERG Duocel foams, being made of metals, carbon, and ceramics, are comparatively insensitive to these environmental factors, and thus they can reliably provide predictable energy absorption performance after even years of storage or exposure in extreme military standard environmental conditions.
Unlike honeycombs, which are considered one-directional, or “orthotropic” foams, Duocel has the same structure in all three dimensions and is defined as an “isotropic” foam. Accordingly, Duocel has the same stress / strain curve regardless of impact direction. This makes it particularly useful in situations where there could be unpredictable variations in the direction and magnitude of an impact.
Duocel foams are also open-celled, so they do not hermetically trap gasses that can create a pneumatic “spring “effect at low impact velocities or create rebound. At very high velocities, there is a fluid flow friction effect as the entrained gas is rapidly squeezed out of the open-celled foam structure. In this case, there is still no effective spring-back, but the increased initial resistance can provide a convenient “crush plateau enhancement” if the impact is at a much higher velocity than anticipated. If this feature is intentionally incorporated into the design, it can be controlled by selection of the foam pore size and airflow resistance.
Based on these characteristics, Duocel energy absorbers are commonly used in aerospace and military applications where operational performance is critical, and the storage, use, and operational environmental conditions are likely to be severe.
