2009

than expected, evaluate the analysis conditions and determine what is causing
the discrepancy.
Equivalent Stress
Three-dimensional stresses and strains build up in many directions. A common
way to express these multidirectional stresses is to summarize them into an
Equivalent stress, also known as the von-Mises stress. A three-dimensional
solid has six stress components. If material properties are found experimentally
by an uniaxial stress test, then the real stress system is related by combining
the six stress components to a single equivalent stress.
Maximum and Minimum Principal Stresses
According to elasticity theory, an infinitesimal volume of material at an
arbitrary point on or inside the solid body can be rotated such that only normal
stresses remain and all shear stresses are zero. When the normal vector of a
surface and the stress vector acting on that surface are collinear, the direction
of the normal vector is called principal stress direction. The magnitude of the
stress vector on the surface is called the principal stress value.
Deformation
Deformation is the amount of stretching that an object undergoes due to the
loading. Use the deformation results to determine where and how much a
part will bend, and how much force is required to make it bend a particular
distance.
Safety Factor
All objects have a stress limit depending on the material used, which is referred
to as material yield. If steel has a yield limit of 40,000 psi, any stresses above
this limit result in some form of permanent deformation. If a design is not
supposed to deform permanently by going beyond yield (most cases), then
the maximum allowable stress in this case is 40,000 psi.
10 | Chapter 1 Get Started With Stress Analysis