Tensile Test Report

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Practical Report
Name: Loh Khin Keong
Admin No.: 1237078
Group number: A
Date: 11/5/12
Class: DMLS/1A01
Experiment 1A: Determination of Mechanical Properties of Selected Materials (CDIO)

1. Synopsis
The objective of this CDIO experiment was to determine the mechanical property of polymeric materials which was the tensile properties .The testing standard to adopt for the tensile testing was ASTM D638. Four different types of polymers were placed into the tensile testing machine. The different specimens would then be stretched till it breaks. The tensile testing machine would give the information on the force that was required to stretch the different specimens until it breaks. The average reading of Tensile modulus of all four specimens tested range from 500.00 -1600.0N/mm2. The average reading of Tensile Strength and Yield Strength ranges from 21.000N/mm2 – 37.000N/mm2 and 22.000N/mm2 – 34.000N/mm2 respectively.

2. Introduction
Tensile test is the most common mechanical test to determine several important mechanical properties such as modulus of elasticity, yield strength, ultimate tensile strength, ductility and toughness. Typical tensile applications of tensile testing are usually in packaging, textiles, medical, aerospace industries etc. The Lloyd tensile testing machine is used in this experiment. The tensile machine consist of two clamps, movable and stationary, load cell (a transducer that is used to convert a force into electrical signal) and grip distance (the length between the two clamps that holds the specimen).The specimen will be place into the tensile test machine, and then pulled at a constant rate and has devices to continuously measure and record the applied load and elongation of the specimen. The device will determine the yield strength, tensile strength, tensile modulus, breaking load and maximum load.

3. Theory
1) Molecular arrangement
The polymeric materials that are put to the test are General Purpose Polystyrene (GPPS), High Impact Polystyrene (HIPS), High Density Polyethylene (HDPE) and Polypropylene (PP). The different polymeric materials have different molecular arrangement. GPPS and HIPS have amorphous molecular arrangement. There is no regular spacing between molecules (molecules are randomly arranged and entangled), due to the irregular, non-symmetrical structure of the monomer units. HDPE and PP have semi-crystalline molecular arrangement. The molecules are arrange in a regular pattern and are closely packed. This is due to the monomers which are symmetrical in structure as the monomers are arranged in regular position during polymerisation. 2) Mechanical properties

Mechanical properties describe the behaviour of a material subjected to mechanical forces, which reflects the relationship between its response and deformation to an applied load or force. The tensile test curve which is the stress-strain graph says it all. Tensile stress, σ=fa

Strain,ε=∆LL,
Where:
∆L=the deformation (m) or the change in length (m).
L= the original length (m).

Tensile properties to know before reading the stress-strain graph: * Elastic limit- When stress is removed before reaching this point, materials will return to its original length * Proportional limit - Up to this point, linear relationship between stress and strain obeys Hooke’s Law Hooke’s Law:

σ=Eε
Where:
E=Young’s modulus of elasticity
This implies that stress is directly proportional to strain
Stress vs Strain graph: Slope= Modulus of elasticity
* Yield Point-An increase in strain without a corresponding increase in stress.

* Tensile Strength-A measure of the strength of resistance to plastic deformation Tensile strength=yield force ÷cross sectional area

* Ultimate Strength or Stress- The maximum stress that the material can withstand Ultimate Strength= Highest tensile force ÷ Original cross sectional area

* Rupture Strength or Stress-The stress at which the...
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