Page 80 - Plastics Engineering

Mechanical Behaviour of Plastics 63

Steel end-plate rigidly
fixed to steel bobbin Acetal
/

LG-4

Fig. 2.16 Metal bobbin with plastic sleeve

Solution Under free conditions, the acetal would expand more than the steel
but in the configuration shown they will both expand to the same extent.
Hence, the acetal will effectively be put in compression by an amount given
by eqn (2.17)

S = (ao - CY,)L. AT
= (80 - 1 1)10-6(40)(80)
= 0.22 mm
0.22
strain = -- x 100 = -0.55%
40
0.55
stress = EE = -1.5 x 10 x -
100
= -8.3 MN/m2

Thus there will be a compressive stress of 8.3 MN/m2 in the acetal. It should
be noted that the above analysis ignores the effect of the constraining effect
which the acetal has on the thermal expansion of the steel. However, as the
modulus of the steel is over 100 times greater than the acetal, this constraining