Page 54 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 2 Materials: How to Choose and Where to Find Them 35

Material Failure: Stress, FIGURE 2-1 Tension, compression, shear, and
Buckling, and Fatigue torsion
The yield strength is the name given
to the specific stress a material can
experience before it gets, um, stressed
out. Stress is just a force applied over
a certain area, commonly expressed in
pounds per square inch (psi). The stress
at which a material actually breaks is
called the ultimate strength.

There are actually four different
kinds of stresses, and therefore four
different ways a material can fail (see
Figure 2-1):

1. Tension Tension is a fancy word for stretch. Think of the chains that hold up
a child’s swing on a playground. These chains are in tension when someone
sits on the swing because they are being stretched. If someone very heavy sits
on a swing designed for a two-year-old, the chain will break or fail in tension.
The force or weight of the person divided by the cross-sectional area of the
chain is the stress the chain feels in tension.

2. Compression Compression is a fancy word for squish. When you look at
material properties on a website like matweb.com, you will sometimes see
two different numbers for tensile strength and compressive strength. Consider
this when you are designing support structures for your mechanisms—
especially ones that could hurt someone if they break. There’s a reason why
buildings don’t have foundations made of cheese: The compressive strength
just isn’t high enough. The force of the building divided by the area of the
foundation is the total stress the cheese would feel.