THE CHARPY IMPACT RESULTS
The table of observed Charpy impact energy for real hail stones is repeated below. Weathered roofing sheets showed similar perforation damage to that observed on the roof at impact energies of 0.133 J, while new PC roofing sheets were not perforated even at impact energy of 2.652 J. Apparently, weathered roofing sheets will show perforation damage at impact energies equivalent to impact by hail stones with a diameter between 10-15 mm.
The impact test temperature was the ambient laboratory temperature of 20oC. There are two interesting issues to consider here, namely, what does weathering do to PC roofing and will the test temperature affect the impact results in any way. These two questions form the basis of this activity.
Thermoplastic polymers such as PC exhibit a 'glass transition' temperature TG at which their behaviour tends to change from brittle (below TG) to ductile (above TG). In reality, polycarbonate becomes brittle at around 70% of the glass transition temperature expressed as Kelvin. The glass transition temperature for several polymers is summarised on the hyperlinked page, prepared by the Department of Mechanical Engineering at Duke University.
Effect of Weathering
A literature search  revealed that such craze cracks are the result of exposure to UV radiation which lowers the molecular weight of the polymer by photochemical degradation and renders it liable to fine cracking in the presence of local expansion and contraction.
Consider all the information presented above and test your conclusions in the first activity below.
The second activity considers a fracture mechanics type calculation of the effect of small sharp cracks on the fracture stress. The differences between impact toughness and fracture toughness are amplified nicely in a laboratory manual from the University of Illinois. The plane strain fracture toughness (K1C) of unweathered polycarbonate is about 1.05 MPa m1/2. We can use approximate the situation of craze cracking by considering an edge crack in bend, which has a simple equation relating K1C to crack depth and applied stress. This ignores the shear component which would occur under hail impact. Calculate the effect on the fracture stress of various depths of craze crack (between about 0.1 Ám and 20 Ám). Consider also (extrapolating the data from Figure 1 to fracture toughness values) the consequences of a reduction in toughness by up to 80% following weathering (see the information on toughness after UV exposure at the ATOFINA Chemicals website). We can compare these stress values later with the results of an FE analysis of the stress induced by hail impact.