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According to EN standards, elongation for each material type has been defined, and it varies with different range of thicknesses for the same material. Why is it so? How is this value calculated?

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A few points; Actually covering the subject requires a book. For steels , the differences in properties vary with thickness because of : 1- inclusions, 2- grain size, 3- hardenability, 4- composition, 5- Actual product thickness, 6- steel making practice, 7- casting technique, And several things I forgot. Inclusions are generally concentrated near the center,inclusions reduce ductility. Grains size , larger sections have larger grains and lower ductility- very strongly dependent on heat-treatment, etc. Hardenability will control strength and ductility of particular thickness - very strongly dependent on heat-treatment. Composition differences mostly a factor for ingot cast. Actual product thickness , a twelve inch thick plate will be very different from one inch plate. Steel practice , electric furnace,oxygen converter,open hearth,etc,plus deoxidation, vacuum degassing, etc. Casting practice, ingot or continuous cast ; continuous cast has revolutionized steel making it more uniform and generally reducing inclusions improving ductility. None of this can be calculated

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A specimen of each material, precisely machined to specific dimensions, is tested under controlled conditions to measure the change in length according to a measured load.

That gives the elongation value available in the charts.

This is a common lab experiment, at least in many engineering courses, and sometimes it is elongation or sometimes bending. One that we did involved a 1 metre bar of aluminium, 3cm wide and 25cm deep. An interesting lab with a lot of hand calculations to do after...

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