Page 186 - Failure Analysis Case Studies II

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Fig. 3. Comparison of dimensions of failed (LHS) and new radiator boxes. Note longitudinal distortion of failed box.

glass-reinforced nylon 6,6 compound to enter the tool cavity smoothly. The failed tank is compared
directly with a new moulding taken from the same batch in the three figures. A small amount of
carbon black had been added to the compound to give a matte black colouration. Both tanks were
date stamped, indicating that they had been moulded only recently.
Direct comparison of the tanks showed the failed tank to be distorted along its greatest axis, the
sidewalls bulging inwards, as shown in Fig. 3. Such distortion can be caused by relaxation of
internal frozen-in strain developed during moulding at temperatures below normal, or low melt
temperatures in the barrel of the moulding machine. The tank had experienced only a few cycles
from ambient temperatures and pressures up to working conditions in excess of 100°C and 25 psi
over atmospheric pressure. Such conditions allow internal chain orientation to relax to the equi-
librium state owing to the extra thermal energy provided for diffusion.

2.2. The crack and adjacent features

The single crack which had led to loss of water pressure and loss of cooling action for the engine,
was situated near an external buttress, used to support a nearby fan. Tidemarks were visible
immediately next to one end of the crack, their position showing the tank to be placed in a vertical,