# MATERIAL AND STRUCTURE , STRENGTH OF MATERIAL

Note that All Materials are at the bottom of this Page, just go down

### Material Properties  , Tensile test etc..

We are going to learn what a tensile test is. A tensile test is a fundamental test in material mechanics.  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. 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 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. 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 cGaines, 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.

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