Further Investigations
As noted in the activity, the insurance company repudiated the claim on the basis that thermal fatigue was 'progressive deterioration' and not 'sudden and unforeseen' hail damage. The flower farm management did not accept the insurance company report and commissioned further work aimed at verifying the hail damage hypothesis. This work followed two main routes; firstly, it simulated hail impact on new and weathered PC roofing sheets and, secondly, a report by fracture experts was commissioned. This second part concentrated on stress analysis and fractography.
Impact Testing
Published data on hail storms in South Africa [1] indicated that 75% of all hail stones have diameters < 10 mm. The table below gives typical data for diameter of hail stone and corresponding impact energy [2].
| Diameter (mm) |
5 |
10 |
15 |
20 |
25 |
| Energy (J) |
0.0023 |
0.036 |
0.183 |
0.58 |
1.41 |
|
A miniature Charpy impact testing machine (papers containing information on miniaturised Charpy specimens and testing can be found at the MPM Technologies website) was used for this testing and a typical machine is shown in Figure 1.
|
Figure 1
|
Impact energies between 0.133 J and 2.652 J were applied to sample roofing sheets using a 10 mm ball indenter on a bench top testing machine. On new PC roofing samples, perforation could not be achieved even at the highest energies. On weathered PC sheets from the roof in question, however, even the lowest energy of 0.133 J produced oval impact damage virtually identical with that originally observed on the roof. The conclusions from this part work are considere d in
Activity 2.
Proceed to
third part of case study.
References
-
P Admirat, GG Goyer, L Wojtiw, EA Carte, D Roos and EP Lozowski (1985), A comparative study of of hailstorms in Switzerland, Canada and S Africa, Journal of Climatology Vol. 5 pp.35-51.
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D van der Roos (1980), Hailfall intensities, South African Journal of Science Vol. 76 pp.360-363.
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Failure Analaysis
