Failure as a Design      Criterion

   Fracture Mechanics

   Failure Analaysis


Wire Rope Failure


Undercarriage Leg Failure


Aircraft Towbar Failure


Hail Damage
- Part 1
- Part 2
- Part 3
- Part 4
- Part 5
- Activity 1 - Diurnal Heating Hypothesis
- Activity 2 - Effect of UV Exposure


Insulator Caps


Fractography Resource

Dr David J Grieve ( developed the interactive Java pages for this website.

The Gauteng region of South Africa lies on the highveld, typically at an elevation of 1600 m. It experiences a dry season during autumn and winter (May until September), and has summer rainfall that can be very heavy with thunderstorms and hail. Even in the summer, conditions can be very dusty. The high ground elevation, together with a geographical position just below the Tropic of Capricorn means that there is a high level of ultraviolet (UV) radiation. A map of the Gauteng region, showing the main towns and cities, in given in Figure 1. Gauteng_map.gif (10383 bytes)
Figure 1 Map of the Gauteng region

Commercial flower farming is carried out in greenhouses to protect the plants from hail damage and from exposure during cold winter nights (ground frosts are common on the highveld, although day temperatures are still around 100-150C).
One such commercial flower farming operation existed in the Heidelberg region of Gauteng (see map above) and utilised greenhouses typically 62 m by 137 m. Certain of these greenhouses had, had for reasons of economy, been roofed with sheets of extruded polycarbonate (PC), each about 13 m by 1 m. The extrusion form was standard for roofing sheets (see Figure 2), which were reportedly UV-stabilised, and had been sourced in Europe. Information on the structure and applications of polycarbonate are given on the hyperlinked webpage. PC_Schematic.JPG (21272 bytes)
Figure 2 PC sheet schematic
The extrusion has a depth of 5 mm with top and bottom skins 0.3 mm thick, separated by 0.3 mm thick webs spaced about 5.7 mm apart. These sheets were attached to a mild steel frame via 'J' bolts spaced approximately 900 mm apart along the length of the sheet and 300 mm on the width.

After several years of service, during the second half of the wet season (January - April), farm personnel noticed that the sheets of PC were darkening with consequent implications for growth of the plants. Examination of the PC sheets showed a random distribution of round-to-oval perforations and partial perforations in the upper skin of the sheets. These perforations were allowing ingress of dust into the channels between the top and bottom skins, hence causing the observed darkening. An insurance claim was therefore lodged for the cost of replacement of the roofing sheets, with the claimant stating that the damage was the result of a hailstorm, although they could not be precise about which specific storm caused the damage or prove that the damage had appeared immediately after such a storm.

As the quantum of loss was substantial the insurance company commissioned a report on the damage and its likely cause from a polymer research centre. Staff at this research centre performed a visual examination of the roofing sheets, and linked these observations with the known attachment system to propose a hypothesis for the damage based on ascribing the damage to the result of diurnal heating and cooling cycles. Consider the summary and additional information given in Activity 1 and decide whether this is supportable. Proceed to second part of case study.


Failure Analysis  -  Fracture Mechanics  -  Failure As A Design Criterion