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.
| Diameter (mm) | 5 | 10 | 15 | 20 | 25 |
| Energy (J) | 0.0023 | 0.036 | 0.183 | 0.58 | 1.41 |
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.
| Materials like polycarbonate are very notch sensitive in the brittle state. Very useful information regarding the mechanical properties and modes of failure of polymers can be found on web pages at the Polymer Interdisciplinary Research Centre at the University of Leeds. Information on notch sensitivity of polymers can be found on a laboratory manual prepared by the Theoretical and Applied Mechanics Department at the University of Illinois at Urbana-Champaign and the website of San Diego Plastics considers the reduction in impact energy in PC due to the presence of sharp defects. Additional information on mechanical properties, strength and effect of notches can be found on the Polymer Engineering website at the University of Nottingham.Figure 1 shows a diagram indicating the reduction in impact energy for PC containing a sharp crack-like notch. If the PC contained such defects, then the observations of the impact testing are readily understandable. |
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Effect of Weathering
| Sample pieces of polycarbonate roofing were cut and prepared for examination in a scanning electron microscope by gold sputter coating. High magnification examination revealed a network of fine cracks dividing the surface of the PC into polygonal regions (Figure 2). Other longer cracks were also present and the implication of these will be considered in the next part of the case study. |
Figure 2 Crazing observed on PC sheet |
A literature search [1] 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 µm and 20 µm). 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.
Reference
- A Blaga and RS Yamaski (1976), Surface microcracking induced by weathering of polycarbonate sheet, Journal of Materials Science Vol. 11 pp.1513-1520.