Stress / Strain Curves
Use of compression stress/strain curves in design. Urethane elastomers have excellent high load-bearing capabilities. They are often used as fork truck tires and bridge-bearing pads due to their ability to handle heavy loads. The influence of shape or the "shape factor" can greatly impact the compressive strain percentage of a product. Shape factor is defined as the ratio of the area of one loaded surface to the total area of the unloaded surfaces which are free to bulge.
Shape Factor Calculator
For rectangular shaped prisms:
Shape Factor = lw / (2T (l+w)) |
Where:
l = Length
w = Width
T = Thickness
d = Diameter
h = Height
|
For discs and cylinders:
Shape Factor - d/4h |
While the ability to deform under compressive stress and then recover is a characteristic property of elastomers, other factors, notably 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. 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 cylindrilical one because the sides of the rectangular block provide a greater area free to bulge.
No Load
| 4 ln^2 |
Surface Area |
4 ln^2 |
| 16 ln^2 |
Area Free to Bulge |
14.2 ln^2 |
| 0.25 |
Shape Factor |
0.285 |
Under Load
- Surface area is the same.
- Pressure applied is the same
- Shape factor causes a large difference in the percentage of compression.
