Page 178 - Plastics Engineering

Mechanical Behaviour of Plastics 161

40 mm

Fig. 2.85 Nylon bush in metal housing

Confirm whether or not this data obeys a law of the form
8=Ad
and if so, determine the constants A and n. When a stnss of 5 MNlm2 is applied to this material
the strain after 106 seconds is 0.95%. Predict the value of the strain after 9 x 106 seconds at this
stress.
2.24 For the grade of polypropylene whose creep curves are given in Fig. 2.5, confirm that
the strain may be predicted by a relation of the form
&(t) = At"
where A and n are constants for any particular stress level. A small component made from this
mnterial is subjected to a constant stress of 5.6 MN/m2 for 3 days at which time the stress is
completely removed. Estimate the strain in the material after a further 3 days.
2.25 A small beam with a cross-section 15 mm square is foam moulded in polypropylene. The
skin has a thickness of 2.25 mm and the length of the beam is 250 mm. It is to be built in at
both ends and subjected to a uniformly distributed load, w, over its entire length. Estimate the
dimensions of a square section solid polypropylene beam which would have the same stiffness
when loaded in this way and calculate the percentage weight saving by using the foam moulding.
(Density of skin = 909 kg/m3, density of core = 450 kg/m3).