Page 194 - Failure Analysis Case Studies II

179

to the exterior, so allowing seepage of cooling fluid to the outer surface of the tank. This occurred
during use of the car, so pressurising the cooling system, subjecting the outer skin to tension and
initiating cracks. It suggests that the several cracks propagated until penetration to the reservoir
occurred, leading to small spurts of fluid being ejected onto the adjacent buttress.
It is tempting to suggest, judging by the size of the brown stain, that the initial leak occurred
just by the left-hand corner of the buttress (Fig. 11). Not necessarily so. The tank in use is vertical,
so if leaks were occurring anywhere above this corner, liquid would tend to collect here as a bead
since it would be adhering to the corner created by the buttress and the adjacent tank surface.
So what defects were visible on the fracture surface? One feature was the several smooth,
irregular zones, most clearly seen in the optical macrographs of Fig. 12. There were several areas
where such features occurred: a smaller cluster under the first buttress corner; a group near the
second buttress corner and a linear, shallow zone on the underside of the fracture surface. The
irregular form of the first two groups suggested that they may represent fragments of the original
pellets used in the moulding process which have not fused together, and thus represent lines of
weakness within the solid material. They could thus be most closely related to the cold slug defect
found on the inner surface of the moulding, near to the sprue (Fig. 6) and one of the radiographs.
The linear zone was the clearest indication of a ‘true’ weld line, which would be formed when the
pellets have lost their original shape due to melting, but then two streams of molten plastic have
impinged without fusing. The smooth areas in the interior could also represent internal weld zones.

3.3. SEM examination of.fracture surface

High magnification SEM examination of an area near the first buttress showed a widely varying
microstructure [Fig. ll(a)]. Some areas appeared free of fibres, while others possessed a dense
distribution of broken fibre ends. Fibre orientation in the area below the left-hand corner of the
fan buttress appeared to be uniform, and oriented to the buttress and neighbouring external
surface. At a slightly higher magnification, Fig. 1 1 (b) shows the virtually fibre-free part at the inner
edge of the fracture immediately below the first buttress corner. A crack branching directly into
the bulk delineates the internal edge of this feature, which represents the linear weld line mentioned
above. The smooth surface of this zone contrasts sharply with the very rough surface immediately
above, where numerous fibre ends protrude from the surface. Voids may be present just above this
zone. The final plate [Fig. 1 l(c)] shows the lower weld line next to the inner surface below the
buttress.
So where did the cracks start? There are numerous points or zones which could represent origins:
the most likely positions are the two zones near to the corners of the fan buttress, which is a fairly
severe stress concentration, where extra stress magnification will have been created by latent defects
such as voids, cold slugs or weld lines.

4. Mechanical tests on tank material

It was important to conduct some simple tensile tests on bars cut directly from both new tanks
and the failed tank to determine the intrinsic strength of the material, in both the failed and new