Materials in Practice Tensile Test

Topics: Tensile strength, Elasticity, Yield Pages: 6 (1613 words) Published: June 22, 2013
Experiment 1: Mechanical Properties

Part A: Tensile Strength

Group A Loh Jia Jun (1316717)
26 April 2013


(a) Synopsis
The main objective of tensile testing is to determine the tensile strengths and tensile modulus of different materials. Specimens that would be used are General Purpose Polystyrene (GPPS), High Impact Polystyrene (HIPS), High Density Polyethylene (HDPE) and Polypropylene (PP). The thickness of each specimen must be measured and recorded before being placed on the tensile testing machine. Once a specimen is placed on the tensile testing machine, start the machine. When the specimen is stretched and broken, the computer will record down the necessary measurements. By conducting this experiment,measurements can be found in terms of the breaking load, maximum load, tensile strength, tensile modulus and yield strength. Through this experiment, I found out that GPPS has the highest tensile strength while HDPE has the lowest tensile strength. This shows that out of all four specimens, GPPS is the hardest and toughest material while HDPE is the most ductile.

(b) Introduction
The reason that tensile tests are performed is to use the results obtained and apply it in selection of suitable materials for engineering applications. With the results, we can predict how the material is going to react under certain forms of loading. This information is essential for engineers as they have to decide on what type of materials to use under different circumstances by comparing different materials. Tensile tests are also able to measure how much stress a material can withstand before deformation takes place. Ductility can also be determined through tensile tests by measuring how much the material deformed before fracture takes place.

(c) Theory
Tensile testing of thermoplastic materials is the simplest test you can perform. It is to measure how much force a material can withstand before breaking. By using a tensile testing machine, a PC and a printer, results of the test can be recorded and be plotted into a stress –strain graph. Through a stress-strain graph, many mechanical properties data like elastic limit, proportional limit, modulus of elasticity (E), yield point, ultimate strength, rupture strength, ductility and toughness can be obtained. Stress can be obtained by dividing the force by the area of the specimen whereas strain can be obtained by dividing the length of extension over the initial length of the specimen. σ= ForceArea ε= ∆LL

Example of a stress-strain graph:
Fig. 1
Point A represents the point of proportional limit. For point O up to point A, linear relationship between stress and strain obey Hooke’s Law σ α ε ⇒σ=E ε
Hooke’s Law states that the stress imposed on a solid is directly proportional to the strain produced. From the equation, Young’s modulus of elasticity, which represents the stiffness of the material, can be found. Point B represents the point of yield point. When the graph reaches the yield point, the specimen starts to yield and plastic deformation begins to occur without any increase in stress

Yield Strength = Yield ForceCross-Sectional Area

Point D represents the point of ultimate strength, where the highest stress a specimen can withstand while being stretched before it breaks. Point E represents the point of rupture stress, at which the material breaks. Out of all 4 thermoplastics used in this experiment, only GPPS is unable to exhibit yielding. This is because GPPS is hard and brittle. The other 3 thermoplastics are able...

References: (i) J. R. Davis, E-book on Tensile Testing (2nd Edition), 2004 edition
(ii) Vasileios Koutsos, Tensile Testing of Polymers,
Centre for Materials Science and Engineering, The University Of Edinburgh
(iii) Jeannie Chew, Materials in Practice – Tensile Strength Test (2009)
(v) Ho Thiam Aik, Materials in Practice (2010)
School of Chemical and Life Sciences, Singapore Polytechnic
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