In this video we will provide an overview of engineering stress-strain
diagram instrumentals Engineering stress-strain diagrams are developed from physical testing. A carefully
prepared test specimen is subjected to a tensile test in a universal testing
machine A testing procedures is typically guided by
a standard such as ASTM American Society for
Testing and Materials This ensures that everyone who’s performing the tests follows the same procedure Stress-strain diagrams allow us to plot the
results from tensile tests and graphically identify important
mechanical properties As a reminder, stress is force per unit area that results from an
applied load. Now these these applied loads could be in tension, compression, shear, torsion or any combination Strain is the physical deformation
response of a material to stress So an example of strain would be elongation of material When a test specimen is first placed
in a tension testing machine the force is increased and the strain is proportionally increased If we look on our stress-strain diagram
this is the linear portion of our graph If you were to release the specimen in this
region the specimen would return to its original shape, so no deformation has
taken place If we think about the specimen at the
micro-level the bonds are stretching This is considered the elastic region
and the stress-strain diagram The slope of the line in the elastic
region is the modulus of elasticity, also know as the material stiffness As we increases the load being applied to the test specimen we willl reach a point where there is no longer linear behavior this is our proportional limit. With a
little more loading past our proportional limit we will have noticeable permanent
deformation that takes place in our test specimen This point is our yield stress. Yield stress is the stress associated with this point. Yield starts at 0.2 percent strain for most metals What this means is if the stop our
tensile test and unload the specimen we find the
engineering strain is 0.002 Note since there is a degree of recovery we do not draw the unload line straight down. We draw parallel to the elastic curve. One thing to also realize is at a
microscopic level we have dislocation motion and permanent deformation that has
occurred that is why we are now in what we would
call our plastic region With additional force the test specimen
will eventually begin to neck or rather the diameter or thickness of the test specimen decreases in size We call this point the ultimate tensile
strength or UTS. This is the maximum possible engineering stress that the specimen can take in tension If we continue to load our specimen with additional force we will find that this system will
eventually fail At this point our bonds have completely
broken. As we did with the yield stress if we want to understand engineering
strain at fracture we need to unload parallel to our modulus of elasticity line For designers modulus of elasticity yield strength and ultimate tensile strength are all important mechanical properties We need to make sure we are designing
components as well as products that can withstand the strength of the materials that are being used within the product We need to make sure our component as well as our products do not go beyond the yield strength
or ultimate tensile strength when they’re in service or failure will end up occurring. These are some of the elements that design engineers need to
take into account from mechanical properties