What is an Energy Absorber?
An energy absorber is a product designed to absorb kinetic energy by compressing at a relatively constant stress. They can be thought of as similar to springs, but unlike a spring that rebounds creating temporary energy storage, an energy absorber does not rebound. A good example of an energy absorber is the bumper on a car, which is intended to absorb a portion of the impact energy from another object, therefore reducing the energy transferred to the occupants.
Why use Duocel® foam as an Energy Absorber?
Duocel foams make excellent energy absorbers because their crush strength can be controlled with varying relative densities. Additionally, the structure of the foam allows it to buckle and compress at a relatively constant stress up to about 50%-70% strain, or displacement, of the foam. This constant compression is referred to as the Crush Plateau (see below), which can be designed to achieve optimal performance in a repeatable manner based on the foam’s controllable material properties.
The area under the curve on the graphic above represents the kinetic impact energy absorbed by the block of foam crushed with specific properties. This is also referred to the work done, and can be calculated with the equation below:
$latex W = F\dot\Delta X$
If properly designed with an appropriate thickness and crush strength, a Duocel foam part can absorb all the energy of an impacting mass, therefore protecting key components behind the foam from large or concentrated impacts.
Besides optimizable crush performance, Duocel foam should be considered as an energy absorber for a few other, unique reasons. Unlike traditional honeycomb materials which are used to absorb energy in a uniaxial direction, Duocel foam’s cellular structure allows it to be impacted at any angle, which allows for unpredictable impact situations. Additionally, the open cells of Duocel foam allow air and gasses to escape easily when the foam is compressed rapidly. This reduces the potential for a “spring-back” effect, allowing the foam to absorb the entire energy of the impact without rebounding.
- Bolt Capture
- Emergency Landing systems
- Blast Mitigataion
- Micrometeoroid Shield
There are a number of foam mechanical characteristics, but next to modulus, the most frequently used is the crush strength or plastic yield strength. When a load is applied to a foam structure, it will initially yield elastically in accord with the Young‘s modulus equation.
However, at approximately 4-6% of strain, depending on the sample size, the foam structure will begin to buckle and collapse continuously at a relatively constant stress. Depending upon the initial relative density of the foam, this constant collapse will proceed to approximately 50-70% of strain.
At that point, the stress / strain curve will begin to rise as the compressed foam enters the “densification” phase. The point in the stress / strain curve where it transitions from the elastic to plastic deformation phase defines the “crush strength” of the foam. This is an important mechanical parameter as it is obviously essential to remain below that level for any structure that is being designed to maintain its shape under design load.
For a more in depth explanation of how Duocel® performs in energy absorber application visit our technical data page on energy absorption.