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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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This video is going to bit different to my usually videos, but I want to talk about material

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B1 中級

材料特性101 (Material Properties 101)

  • 70 8
    Lynn Lai に公開 2021 年 01 月 14 日
動画の中の単語