Proper performance data ease the task of specifying cushioning materials.

Proper performance data ease the task of specifying cushioning materials

With the wide range of damping and cushioning materials available today, selecting the one best-suited to a particular shock- or vibration-control application involves examining a broad - and potentially confusing - spectrum of physical and performance properties.

It is a common misconception that the softer a material feels, the better it absorbs shock or cushions an impact. In reality, a product's subjective feel provides little, if any, information needed to select the proper material grade and energy-absorption properties.

A cushioning material performs its fundemental role - reducing the forces created when one surface comes abruptly into contact with another - by compressing or deforming in such a way as to produce a gradual, rather than instantaneous, change in velocity. This reduces potentially high decelerations to more moderate values and thereby minimizes damaging impact forces.

Quick recovery vs. slow

Typical cushioning materials are foams, in various densities and thicknesses, and solid elastomers. These materials differ not only in their relative stiffnesses - foams are almost universally softer, or more compressible, than solids - but also in their "recovery" behavior during and after an impact.

Quick-recovery materials will return to their original height immediately upon removal of a compressional load. Being highly resilient, they also return a fairly high percentage of the stored compressional energy in the process.

These high-rebound materials include natural rubber, sponge rubber foams, neoprenes and some types of urethane products. Materials of this type are used in applications requiring a combination of cushioning and resilience, such as shoe inserts, gaskets and constant-load contact switches.

Slow-recovery materials do not instantaneously recover their full thickness and therefore do not return stored energy. This low resilience makes them desirable for applications requiring low rebound and high energy absorption, or damping.

Slow-recovering foams conform gradually to static loads while retaining a natural resistance to over compression, or bottoming out, during higher, short duration dynamic loading. This performance characteristic proves advantageous in applications such as transportation seating, and medical, athletic and orthotic products.

In addition, due to the nature of their polymer chemistry, slow-recovery foams often exhibit a degree of temperature sensitivity, softening as temperatures rise. Particularly useful in medical applications. This temperature-softening behavior provides a desirable softer zone adjacent to the skin, supported by a stiffer region away from the skin.

Data for materials selection

In general, shock-cushioning materials are selected by matching material firmness, or load/compression behavior, to impact severity. Material firmness is often represented by graphical display of compression force deflection (CFD), which indicates the resistance to compression (lbs. force of static load) at a given deflection (% compression).

CFD data prove most useful when evaluating quick-recovery materials. CFD values increase as both firmness and compression percentage increase. Such information can be used in engineering calculations to determine a product's behavior under various loadings.

A similar test method, used almost exclusively for slow-recovery foam materials, provides indentation load deflection (ILD) values for material, which indicate its compression resistance after it has been allowed to adjust to the applied load. ILD and CFD values usually are interchangeable only when evaluating quick-recovery materials, and when tested with the same apparatus.

Perhaps the most direct method for selecting an impact-cushioning material is the use of cushioning efficiency curves, or J-curves (see figure 1).

Using J-curves

A typical J-curve plots cushioning efficiency, J, as a function of impact energy density, U. A small shock has a small value of U, and U-values increase as a shock becomes more severe. Use of J-curves allows the easy selection of materials to accommodate an actual shock event.

J-curves are generated using standard impact tests, and can be obtained for practically any type of material or component. An idealized impact cushioning material should exhibit a J-value of 1. Real, physically realizable materials will always produce values higher than 1, but can approach this value when properly selected.

Flexible foams and high-density foams typically have minimum J-values between 2 and 4, whereas solid materials may only rarely achieve results below a value of 5.

J-curves prove invaluable in the selection of materials, as they clearly indicate the energy density, or impact severity, over which a material is effective.

PHOTO : Figure 1 - J-curves, These J-curves demonstrate the cushioning efficiency, J, as a function of impact energy density, U, for three densities of foam and a solid material. Note that as the severity of the impact increases, use of a stiffer material is required.

Daniel T. Lilley is Director of Product Line Engineering for E-A-R Specialty Composites, a division of Cabot Safety Corp., Indianapolis, IN.