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Stress relaxation of a polymer lab report
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A lab report about the stress relaxation of polymers including data from the lab and associated calculations and theory. Received an A grade
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EG:2184 MATERIALS AND PROCESSES 2Stress relaxation in
polymers
Table of Contents
Introduction and objectives ................................................................................................................... 2
Research .............................................................................................................................................. 3
Readings .......................................................................................................................................... 8
Procedure and conclusion .................................................................................................................... 11
Sample data, graphs and calculations.......................................................................................... 12-20
References .................................................................................................................................... 21
,Abstract
Polyethylene was first invented as a solid Great Britain in 1935 by Eric Fawcett and
Reginald Gibson (now referred to as Low Density Polyethylene, LDPE), with its first
main use coming during World War II. In 1953 a higher density Polyethylene was
invented by German chemists Karl Ziegler and Erhard Holzkamp commonly known
as HDPE (High Density Polyethylene), (Lester, no date). Polyethylene is used widely
within modern industry, both LDPE and HDPE provide different characteristics that
can are used in everyday life. LDPE is much softer than HDPE and so has a variety
of uses including ‘screw caps and lids… carrier bags, packaging materials…’ (SABIC
Europe, 2005), whilst HDPE having less malleable properties makes it more suitable
for more robust products ‘It is used for making containers for fruit, vegetables and
garden waste…’ (SABIC Europe, 2005).
Products that require to be applied under force during their life cycle should have a
stress relaxation time analysis completed. The relaxation time of a material is the
period from which the material returns elastically after a load has been applied.
Stress relaxation times of a material are important to industry as it provides a clear
indication as to the length of the components life ‘.Relaxation time provides a more
accurate understanding of how long the product will last.’ (Lewis, 2012). This can
allow manufacturers to reduce risk to safety, even saving money on materials that
can be manufactured cheaper and still fulfil the requirements of the product.
Objective
This report will investigate quantitatively the relaxation time of Polyethylene to the
dimensions of EN ISO 527-2:1996 under a load of 5kg.
Three pieces of identical samples were tested using a load of 5kg placed within a
Zwick/Roell Tensile Testing Machine. Micrometres measured the area of the
samples (precise to ±0.01mm) in order to determine a more accurate stress figure
for the material. The sample was placed within the Zwick/Roell Tensile Testing
Machine and zeroed in order to start the readings from the exact point where the
stress of the sample is zero. Benefits of using a Zwick/Roell Tensile Testing Machine
included the option of printing the exact values of the results in a stress/time graph.
This graph was then used with the formula provided to gain the stress relaxation
time.
2
, Research
Health and safety
The experiment took place in a lab, the lab has certain rules and regulations which must be
followed at all time. These include health and safety measures placed for the well-being
everyone present. Part of health and safety is the proper conduct, meaning no disruptive
behaviour which may put everyone at risk. The proper clothing must be worn this includes a
lab coat, steel toecap boots, and other necessities such as goggles if needed. All of the
above mentioned fall under the "1974 health and safety at work act".
Industry application
It is important to know the way materials behave before they are able to be used in
applications of any sort, this is so that accidents may be avoided.
Stress relaxation
Stress relaxation is the explanation of how polymers dismiss the strain applied on to them.
The results of the stress relaxation test are placed on a stress vs time graph. A process called
creep is expected to take place due to the behaviour of stress relaxation
Creep
Creep is a process in which a solid body deforms. This is usually caused by an applied stress
which is more than the yield strength of the material. Creep is not only caused by stress,
when high temperatures combined with time are taken into account, creep can tae place
faster.
Yield strength/ yield point
The Yield point is the point in which deformation of the material starts to take place.
Polyethylene
Polyethylene is a type of plastic and is formed when catalysts come in contact with the
molecule Ethylene, ethylene polymerizes, and this is how polyethylene is formed.
Polyethylene is usually used in packaging such as plastic bottles and plastic bags.
Equations
The equation used to calculate changes in stress
3
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