Samples of the damaged roofing were prepared for examination in a scanning electron microscope by gold sputter coating
these were then examined at high magnifications (up to 1 000 x). In particular, there was interest in identifying the various crack patterns and in examining the fracture surfaces of the perforations. The samples were also inspected using stereo binoculars at low magnifications (up to 40 x). Figures 1 and 2
show whitish semicircular rings which were observed around perforations, partial perforations and in areas where no macrodamage had occurred. These rings were usually located in slight surface depressions and often showed segments of discrete cracking at higher magnification, even if no apparent perforation had occurred (Figure 3
The interpretation of these is that they are the result of Hertzian stress field imposed during impact by a round object. It is necessary to explain why they are seen centred between two web positions (e.g. Figure 2
). Although hail can impact the surface at any point, it is reasonable to presume that the patterns of rings will form preferentially when the impact is centred between two webs. As they show up in light microscopy, this implies that local yielding (crazing) has occurred. This is supported by the rounded appearance of the perforation fracture surface at the upper edge of the outer skin (seen in Figure 4
The scalloped appearance of the fracture surface in Figure 4
is also interesting. A similar scalloped fracture surface is given in reference 1 (Figure 5
) and was obtained from a laboratory PC specimen subject to a controlled mixture of bend and shear; precisely the type of loading anticipated during impact from a hail stone.
The last piece of fractographic evidence to be acquired was the existence of fairly extensive fine cracks running parallel and perpendicular to the webs in the roofing (Figure 6). These could well be the result of the diurnal heating and cooling cycles proposed by the first investigators. This network of fine straight cracks is superimposed on the network of crazing cracks and were often located next to a web as the high local stiffness there gives rise to a strain concentration during hail impact.
Sufficient evidence had now been collected to support a coherent hail damage hypothesis. However, an FE analysis of stress due to impact was performed to compare the fracture mechanics calculations of fracture stress with likely impact stresses.
Proceed to fourth part
of case study.
NJ Mills (1976), Plastics - Microstructure, Properties and Applications, Arnold, London.