Page 94 - Plastics Engineering

Mechanical Behaviour of Plastics 77

4500
4000

3500

1500

1000
500
0

Fig. 2.26 Effect of geometry of flexural stiffness

Solution
(a) In order that the two beams (aluminium and acetal) will have the same
stiffness it is necessary that

(EOd = (EZ)aCetul (2.23)

In order to get the 1 month modulus for acetal, the strain in the beam must be
known.
--
-
Strain, CJ MY - W(L/2)(dU1/2) WLdar
EI EI 4EI
where W = total load (200 N) and dd = depth of beam.
Also, deflection
WL3
6=-
8EZ
So, strain,

Using the information for the aluminium
200 x 803 x 12
6= = 5.5 mm
8 x 70 x lo3 x 50 x 23