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

Summary and Conclusions

These efforts to prove that the damage was due to hail provide a cogent and coherent case in support of the hail damage hypothesis, and can be summarised as follows:
  • The network of weathering cracks would provide nuclei for the initiation of other subsequent cracks, e.g. from thermal fatigue of the roofing sheets.
  • Weathering also reduces the impact toughness of PC dramatically, and this effect is compounded by the induced network of fine, sharp craze cracks.
  • Impact resistance in the presence of hail would be reduced by the freezing temperature induced on the PC skin.
  • The outer skin would probably be at a significantly lower temperature than the inner skin, as the thermal conductivity of PC is low. This would induce tensile stresses in the outer skin, again aiding fracture.
  • The scalloped appearance of the lower part of the fracture surface in the perforation holes is a result of mixed bending and shear acting during the fracture. Such loading would arise during hail impact, as shown in the FE analysis.
  • The shape of the holes, elongated along the webs, correlates with the shape of the stress field associated with hail impact, and this tendency would be enhanced if the crack can follow pre-existing thermal fatigue cracks for a short distance.
  • The white semi-elliptic and semi-circular bands of cracks conform to the shape expected from an applied Hertzian contact stress field.

The insurance company, faced this this report and its well substantiated and coherent conclusions, accepted liability for the 'sudden and unforeseen' damage and negotiated a settlement. Nonetheless, the client had to pay for the failure investigations, which would not be covered. This type of result raises ethical questions around the nature of insurance, and whether litigation to recover costs could be avoided through more equitable insurance cover.


Failure Analysis  -  Fracture Mechanics  -  Failure As A Design Criterion