Material Properties 101
Articles,  Blog

Material Properties 101

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. I wouldn’t have the opportunity to make
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