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Long term properties ofplastics
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Long term strengthCREEP- The time dependant
non-reversibledeformation of a material exposed to aconstant
stress
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Plastic under creep
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Tensile CreepThe strain produced by aspecied tensile load after
aspecied time of application
Flexural CreepThe outer ber strainproduced by a speciedexural
load after aspecied time of application
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t is important for the designer to !or" in thesecond stage and
not enter the third stage#
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Creep curve re-plots
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$amily of creep curves at ToC% const
sochronouscurves
sometric curves
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Creep response in log scale
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Question
& polycarbonate sheet of '(( mm in length )L* and
+,(mm in !idth )b* is used for ma"ing a boo" shelf
1.Calculate the minimum reuired !all thic"ness )d*.
if the plastic shelf supposed to be xed at both
ends and to carry a uniformly distributed load of
/, "g0m at service temperature of /1 o C# The
service life of this shelf should be at least 2 years#
3aximum deformation of plastic material must
not exceed +#, 4#
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Fig.4.1 (b)Creep curves
for polycarbonate
/ 5hat !ill be the service life for the abovedesigned shelf. if
the service temperature isincreased by /( oC
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2 years%6',x2%+2'(
days
Constructisochronous curveat +2'( days
6
150
6 Z
12
22dbd
Z
FL ===
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Fig.4.1 (b)Creep curves
for polycarbonate
7 Stress(#84 ,3P&
(#14 +(
+#+ 4 +,3Pa
+#24 /(3Pa
+#'4 /,3Pa
/4 6(3Pa
