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When a load is applied to an elastomer, it “flows”
in accordance with the force exerted on it and within the
limits provided by the mass of the material itself or by the
dimensions of its container. In rubber technology this is
called compression. Although this term is correct in the framework
of normal rubber usage, it may be misleading to the engineer.
It does not mean that the elastomer will undergo a change
in volume under pressure. Rather, it means the elastomer will
deflect, or undergo a change in shape. This distinction is
important. An elastomer is an incompressible fluid, capable
of changing its shape to the limit of its strength under load.
It will react to a load placed upon it by tending to exert
force uniformly in all directions. This is illustrated in
Figure 1. Even though the elastomer is changed in shape under
load, it is compelled by the characteristic of elasticity
to return to its original shape once the load is removed.
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FIGURE 1 URETHANE
ELASTOMERS IN EFFECT AN INCOMPRESSIBLE “FLUID”,
REACTS TO LOAD BY EXERTING FORCE IN ALL DIRECTIONS.
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Compression strain can be considered
to be extensions of tensile strain which are continuous through
the origin. However, the compressive samples must be free
to move, i.e., the faces must be lubricated. Generally, rubber
in compression is bonded to the surface or surface friction
restricts movement. Compression curves are usually smooth
and do not exhibit the “S” shape usually found
in tensile tests. Compressive strain (Figure 2) is limited
to less than 100% and, therefore, the curve becomes asymptotic
to the 100% line.
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FIGURE 2 CONTINUTITY
OF TENSION AND COMPRESSION TESTS |
While the ability to deform under compressive
stress and then recover is a characteristic property of elastomers,
other factors, notably the shape of the part, affect the way
an elastomer deforms in compression. To illustrate, consider
two blocks cut from the same piece of rubber. One is a cylinder
with the proportions of an ice-hockey puck, the other is a
block of the same height and cross-sectional area, but rectangular
in shape. If equal weights are placed on the blocks, subjecting
them to the same compressive stress, the rectangular block
will deflect more than the cylinder (Figure 3). Since the
blocks will not change in volume, the reduction in height
is caused by the freedom of the sides to bulge. The rectangular
block deflects more than the cylindrical one because the sides
of the rectangular block provide a greater area free to bulge.
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FIGURE 3 SHAPE
FACTOR: RATIO OF THE AREA OF ONE LOADED SURFACE TO THE
TOTAL AREA OF UNLOADED SURFACES FREE TO EXPAND
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The designer of elastomeric parts allows for this behavior
by using a concept called shape factor. Shape factor describes
the role of the shape in determining how a part with parallel
load faces will behave under compressive forces.
The concept of shape factor is useful for the design engineer.
If the elastomeric part does not deflect enough to do its
job, the designer can reduce the shape factor by increasing
the thickness of the pad. In reality, he does no more than
increase the area free to expand under load. If the pad deflects
too much, he may decrease the area free to expand or he may
increase the hardness of the elastomer.
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| Other
Engineering Properties |
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Die-Thane Tooling 
