1.3 Environmental factors
I indicated earlier that many failures occur after a product has been in service for some time: such as the wear of a car tyre, or corrosion of the car body itself. It is also possible for components to fail because of a combination of a manufacturing defect with the applied loading or with the environmental conditions during use. Figure 6 illustrates the link from mechanisms such as corrosion, fatigue (repeated loading) and creep (continuous deformation under load) to failure in some form.
So in addition to knowing the stresses in a material arising from the applied loading, depending on the environment in which the component is used it may be necessary to consider the effects of corrosion, wear, creep and fatigue. The effects of any of these mechanisms can weaken a structure to the point where it can no longer bear the loads for which it was originally designed, as shown in Figure 7.
The study of structures that contain cracks from the day they are made is sufficiently important for us to dedicate a large proportion of this course to it. Fracture mechanics allows us to assess whether cracks will be safe under the applied loads. But even in cases where we can be reasonably sure that there are no cracks of any significant size (say more than a millimetre), there is still the possibility that fatigue loading or another mechanism can cause them to grow, as illustrated in Figures 8 and 9.
There are also complex resonant loading cases that can cause failure. You may well remember the publicity surrounding the ‘wobbling’ of the Millennium footbridge in London when it was opened. More spectacular was the actual collapse of the Tacoma Narrows suspension bridge in America in 1940 (see Figure 10). The structure was perceived in its time as the pinnacle of structural lightness, grace and flexibility in bridge design. However, it met its end within four months of construction because of a woeful inability to cope with even moderate winds – only 45 mph on the day of collapse.
All the examples I have used here are of things that have failed, which is perhaps not the best illustration of the design process. However, learning from failures can give insight into why the failure occurred and how to avoid it in the future. Successful designs that simply do the job rarely make the headlines.
Have another look at that list of failures you made for Exercise 2. Are you any the wiser as to the origin of the failure, in terms of the loading on the component when it failed?
Keep that list tucked away safely, I'm going to ask you to have a look at it again later.