Page 175 - Plastics Engineering

158 Mechanical Behaviour of Plastics
Williams, M.L., Landel, R.F. and Ferry, J.D., J. Amer. Chem Soc., 77 (1955) p 3701.
Thomas, D.A. Uniaxial compressive creep studies, Plasrics and Polymers, Oct(1969) p. 485.

Questions
Where appropriate the creep curves in Fig. 2.5 should be used.
2.1 A plastic beam is to be subjected to load for a period of 1500 hours. Use the 1500 hour
modulus values given below and the data in Table 1.5 to decide which of the materials listed
would provide the most cost effective design (on a stiffness basis).

Poly-
Material PP uPVC ABS Nylon 66 Polycarb. Acetal sulphone

1500 hr
modulus 0.3 2.1 1.2 1.2 2.0 1 .o 2.1
(GN/m*)

2.2 An extruded T-section beam in polypropylene has a cross-sectional area of 225 mm and a
second moment of area, I, of 12.3 x lo3 mm4. If it is to be built-in at both ends and its maximum
deflection is not to exceed 4 mm after 1 week, estimate a suitable length for the beam. The central
deflection, 6, is given by
6 = WL3/384EI
where W is the weight of the beam. Use a limiting strain of 2%.
2.3 In the previous question the use of the 2% limiting strain will produce a conservative
estimate for the beam length because the actual strain in the beam will be less than 2%. If the
T-section is 25 mm wide and 25 mm deep with a general wall thickness of 5 mm, what is the %
error incurred by using the 2% modulus?. Calculate the likely beam deflection after 1 week. The
central bending moment on the beam is given by W4.
2.4 A polypropylene pipe with an outside diameter of 80 mm is required to withstand a
constant pressure of 0.5 MN/mZ for at least 3 years. If the density of the material is 909 kg/m3
and the maximum allowable strain is 1.5% estimate a suitable value for the wall thickness of the
pipe. If a lower density grade of polypropylene (p = 905 kg/m3) was used under the same design
conditions, would there be any weight saving per unit length of pipe?
2.5 Show that for a simply supported beam of length, L, subjected to a point load, W, at
mid-span the maximum strain, E, in the material is given by
66d
E=-
L2
where d is the beam depth and S is the central deflection.
2.6 A piece of thin wall polypropylene pipe with a diameter of 300 mm is rotated about its
longitudinal axis at a speed of 3000 rev/min. Calculate how long it would take for the diameter
of the pipe to increase by 1.2 mm. The density of the polypropylene is 909 kg/m3.
2.7 The maximum strain in a vertical liquid storage tank is given by E = pgHR/2Eh where
H is the level above the base of the liquid of density p, and h is the wall thickness of the tank.
If a polypropylene tank of radius, R = 0.625 m, and height 3 m is to be filled with water for a
period of one year, calculate the thickness of the tank material so that the change in its diameter
will not exceed 12.5 mm. The density of the polypropylene is 904 kg/m3.
2.8 The value of the external pressure, P,. which would cause a thin wall sphere of radius, R,
to collapse is given by
Pc = 0.365E (Iz/R)~
where h is the wall thickness.