This video is going to bit different to my usually videos, but I want to talk about material
properties and the words that we use to describe them, this is information you are going to
need to fully understand future videos.
By the end of this video I want you to understand 6 key words we use to describe material.
These are stiff, strong, ductile, brittle, tough and hard.
With this words you will be able to describe pretty much any material and better understand
why certain materials are used in different applications.
More technical videos like this will be uploaded to my second channel from now on, which you
can subscribe to by following this link.
First we are going to learn what a tensile test is.
A tensile test is a fundamental test in material mechanics.
It’s performed by pulling a sample of material apart until failure, while measuring the force
and displacement.
It provides us with something called a stress/strain curve.
In this scenario the stress is defined by the force applied to the test sample divided
by the cross-sectional area.
This gives us units of Newtons per metre squared, which you may recognise as the metric unit
for pressure Pascals.
Stress goes on the Y-Axis.
Strain describes how much deformation has occurred with that applied force and it is
found by dividing the change in length by the original length.
This is placed this on the x axis.
Let’s watch this test again and see what information we can get from the stress/strain
graph.
As the stress rises the material begins to deform, this initial linear region is elastic
deformation.
That means that if we remove the force the material will regain its original shape, think
of how a rubber band can be deform hugely and still come back to it’s original shape.
The end of this linear elastic deformation is marked by the yield point, from here out
any additional stress will cause permanent deformation.
This is called plastic deformation.
The stress continues to rise until it hits the ultimate tensile strength point.
This is the ultimate strength of the material, the most stress it can handle.
From here less stress is needed as the material begins to decrease in cross section, which
you can see happening here, this is called necking.
This continues until the material fractures.
We can get a lot of really useful information from this graph, the first is Young’s Modulus,
otherwise known as the elastic modulus.
This describes how stiff the material is and it is obtained by finding the slope of this
linear region.
A steeper slope means a stiffer material, for example a high carbon steel may look like
this.
Whereas a flexible material with a low Young’s modulus, like rubber would look like this.
This graph is not to scale, but it should give you an idea of how this information is
represented.
Young’s modulus is one of the most used properties in engineering as we can use it
to predict deflection in a huge range of scenarios.
Yield strength and ultimate tensile strength are two other important properties.
An engineer will divide the yield or ultimate strength by the safety factor to achieve the
max allowable stress, which is used to influence the design of the product.
Usually engineers will aim to keep the max possible stress well below failure, but safety
factors differ between industries.
So we have seen a stiff material and a flexible material.
Now let’s look at a material in between, this material can be described as tough and
ductile.
Tough simple means the material can absorb a lot of energy without breaking.
The area under the graph here defines how much energy is absorbed.
Ductile means it deforms under pressure.
The two previous materials could also be considered ductile.
Spring steel is a tough and ductile material, with a high yield strength, which is why it
is used in springs.
Springs need to absorb and release energy without permanently deforming.
The opposite of ductile is brittle.
A brittle material is a material that breaks with very little deformation.
Glass, ceramics and cast iron all fall into this category.
You can actually tell if a material is brittle or ductile by examining the fracture surface
after they have broken.
A ductile material will have this characteristic cup and cone fracture surface, whereas a brittle
fractures have granular flat looking fracture surfaces.
Some materials can go from ductile to brittle when their temperature is lowered.
This was actually a massive problem during world war two with the liberty ship.
Several of these ships literally broke in half with no warning, including the SS John
P. Gaines, which broke in half in the frigid waters of the bering sea.
It was later discovered that the grade of steel being used became brittle at lower temperatures.
This problem was made worse by stress concentration at the hatches, which you learned about in
my first video “Why are plane windows round”.
This embrittlement is also thought to have also contributed to the fracture of the Titanic's
hull.
The final material property I want to talk about is hardness.
It is directly related to the stiffness and yield strength of the material.
But it is used to describe how difficult it is to dent, scratch and abrade materials.
One way this material property is measured is with the rockwell hardness test.
This test involves three steps, first a minor load is applied to the material by an indenter.
This establishes a zero point.
For the second step a major load is then added which indents the material and for the final
step the major load is removed while maintaining the initial load.
The difference in depth between the first and third step is then used to calculate the
hardness of the material.
Diamond is a very hard material, which is why it is used in cutting tools.
One of fantastic properties of steel is it’s ability to be heat treated to have it’s
hardness tailored in different areas.
For example with swords you want your cutting edge to be hard, but the core of the blade
to be ductile.
This means the sword can bend under pressure without breaking, while the cutting edge can
resist damage.
As always thanks for watching.
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