Difference Between Stress and Strain
Date: Fall 2011
What is the difference between stress and strain?
Stress is the force applied over an area of an object. Strain is
the object's distortion (shrinking, expanding, stretching, twisting,
compressing) in response to a stress.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
Yep, what Dr. Barrans said. I might only add that strains are measured
in distances (e.g. millimeters), while stresses are forces (e.g.
Stress relates to force. It is pounds or Newtons of force divided
by the area the force is applied over. Strain, on the other hand,
relates to deflection. It is millimeters or inches of deflection
divided by some characteristic dimension, like the length.
Some people with a lot of work and a busy life often complain about stress or strain, which means a general feeling of being under pressure, having little time to relax, etc. They are advised not to stress themselves. or not to strain over things. So, in that sense, the words strain and stress are used interchangeably.
But I think you mean stress and strain in the mechanical sense. If so, I explain both by an example.
Consider a rubber band that you cut and stretch by attaching a weight of at one end holding the other end. Let us assume, for example, that it stretches to one and half times its original length.
Strain is defined as the change in length divided by the original length. In this example, it is 0.5 (or 50% stretch) in the rubber band.
Stress is defined as the pulling pressure that the rubber band feels. This pressure or stress is obtained by dividing the applied force (the magnitude of the weight attached to the rubber band) by the cross-section area of the rubber band.
Hope this helps.
Ali Khounsary, Ph.D.
Argonne National Laboratory
Stress is the force applied to the object.
Strain is the resulting deformation of the object.
Stress is either force,
or force-per-unit-area of the material's cross-section.
Pascals would be a common unit.
Strain is usually proportional to the original dimensionality,
such as percent stretching
or a dimensionless number such that 1.0 means the length has doubled.
Usually said number is much less than1.0, like 0.01. (1%)
Many metals yield up to 10% strain before breaking.
Brittle glass will often break at 0.1% strain.
Stress is pressure, force divided by the area over which it is
applied, force per unit area. Some refer to it as the "density of
force". Common units for stress are Newtons per square meter and
pounds per square inch.
Strain is the "percent distortion". For compression or extension,
strain is change of length divided by original length. For a
compressed gas, it can be change of volume divided by original
volume. Strain has no unit. The unit divides out.
Another way to view and remember the difference between the two is as follows:
Stress is the pressure, and strain is the pain.
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012