Practice English Speaking&Listening with: Stress-Strain Diagrams

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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

The Description of Stress-Strain Diagrams