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• 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 videoWhy 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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