5.6 Mechanical tests
What about the mechanical strength of the material? It was important to test the material directly, and compare new and failed polymer samples. Tensile testing would provide basic mechanical properties, which could, for example, be used for comparison with the specification provided by the material supplier.
In a material showing orientation, either of fibres or the polymer molecules themselves, it is an obvious ploy to test the samples both parallel and at right angles to any flow marks. Such tests could reveal any anisotropy and fundamental flaws such as weld lines, and so corroborate independent results from microscopy. The raw load-elongation curves are shown in Figure 69, and the results of analysis are on page 192 of Paper 3. They show the best results fall below the ideal suggested by a specification from the material supplier – about 80 MN m−2 compared with 140 MN m−2. In one case, the tensile test showed the sample to fall well below the ideal value, when tested laterally to the flow marks in the sample. The material appeared rather brittle, although of high modulus. The results therefore seemed to confirm the material could be weakened seriously by flow or weld lines present. Could mechanical analysis throw any light on the reason or reasons for failure?
List the features that could be defects within, and on, the structure of the failed radiator reservoir. Draw an FTA diagram to describe the likely causes of the failure, and indicate the most probable cause of failure, stating the evidence for your opinion. In hindsight, what were the most useful experimental methods used in the analysis described in Paper 3?
The following represent features that could be defects within the structure, and on all the visible surfaces of the failed reservoir:
weld and/or flow lines visible on the inner surface;
cold slugs both on the inner surface and inside the material, as exposed in the fracture surface;
sharp external corner on the outer buttress;
voids within the structure of the reservoir;
change in fibre density or orientation.
Figure 70 shows the possible causes of failure. Low initial water levels, lack of topping up or other unsupported theories have been omitted.
The most probable cause or causes of failure must explain the premature failure of the radiator under apparently normal driving conditions. The evidence of the tidemarks on the corner of the buttress suggests progressive failure from small cracks that grew in the wall of the structure. In the absence of any known external stresses, it is most likely that such cracks grew under the influence of the internal pressure acting on the most serious defects in the composite structure of the wall. Although the material showed poorer mechanical properties than the specification from the manufacturer, it seemed less likely than defects within the structure concentrating stress to unacceptable levels.
The basic reason is that new reservoirs did not show the same level of defects such as weld and flow lines, cold slugs and so on. Defects in the material could clearly lower the strength of the material (sample No. 4 on page 192 of Paper 3) .In addition, there appeared to be a high level of frozen-in strain when the failed and new reservoirs were compared (Figure 3, Paper 3). The evidence thus appears to point to either faulty design or manufacture, or perhaps a combination of both possible causes.
The most useful methods of analysis were simple comparison of new and failed reservoirs (to show distortion and flow and weld lines); microscopy of the fracture surface and immediate surroundings (to show the defects within the material); and tensile testing to show the variation in strength due to defects.