Master of Philosophy
School of Civil, Mining and Environmental Engineering
Hail damage is responsible for significant economic losses in Australia, and the damage will likely be greater in the future due to increased incidence of severe hailstorms. One of the major costs comes from damaged roofs. Compared to the conventional asphalt shingle roofing and concrete tiled roofing, steel roofing is becoming popular due to its long service life, low maintenance cost, and better resistance to natural disasters. However, there is a lack of knowledge regarding the dent resistance of steel sheet to natural hailstone impact as a function of its yield stress and thickness.
In the literature, either steel projectiles or ice balls are used as the artificial hailstones in the hail impact tests. However, there has been no study to correlate the indentation caused by a steel ball to that caused by a natural hailstone. On the other hand, the ice balls used by previous researchers shattered on impacting steel sheeting at velocities close to the terminal velocities of the natural hailstones, while some natural hailstones remain intact after impacting steel roof sheeting at their terminal velocities. When a hailstone remains intact on impact, more energy is available to damage the steel sheet.
In this thesis, a new method to make water based artificial hailstones that remain intact after impacting flat steel roof sheeting at terminal velocities has been successfully developed with a combination of 88% water and 12% PVA (Polyvinyl alcohol). The indentation results of the present artificial hailstones have been validated against those of pure clear ice balls that happened to remain intact after impact at similar velocities. Five sheet thicknesses (0.35 mm, 0.42 mm, 0.55 mm, 0.75 mm and 1.00 mm) and two sheet steel grades (G300 and G550) are tested under the impact of five sizes of artificial hailstones (25 mm, 33 mm, 38 mm, 45 mm and 50.8 mm) at three designated impact velocities (20 m/s, 30 m/s, 40 m/s). Each sheeting is screwed to timber battens spaced at 600 mm from each other, and the projectile is aimed perpendicularly at the middle between the two battens.
The dent depths caused by the PVA ice balls that remained intact after impact are significantly greater than those caused by the PVA ice balls that disintegrated upon impact. For the case involving intact artificial hailstones, the dent depth varies linearly with the square root of the impact energy, and is inversely proportional to the square roots of the sheet thickness and the yield stress. The findings (based on experimental observations and theoretical derivations) regarding the effects of the sheet thickness and the yield stress are believed to be original.
Additional experimental findings are that the rebound energy of hailstones impacting steel roof sheeting is negligible (less than 1% of the impact energy), and that most energy loss of the impact energy is in the form of flexural vibration of the flat steel sheeting. Provided that denting takes place, the energy lost to flexural vibration is a function of the elastic flexural stiffness of the steel sheeting, and varies linearly with the impact energy.
An empirical equation is proposed in this thesis to determine the proportion of impact energy that is lost to flexural vibration of the steel sheeting, based on the sheet thickness and the spacing between the battens. Once the flexural vibration energy and therefore the net impact energy is determined, the dent depth can be estimated from the sheet thickness and the yield stress under the assumption of a (partly) spherical dent.
Wu, Yufei, Determining the Effects of Hailstone Impact on Flat Cold-Reduced Steel Roof Sheeting, Master of Philosophy thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, 2018. https://ro.uow.edu.au/theses1/192
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.