# MATERIAL AND STRUCTURE , STRENGTH OF MATERIAL

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### 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.

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