1.2 Component failure
We have all experienced component failures in one form or another. In many cases this is because something has reached the end of its working life due to a slow-acting failure mechanism: car tyres wear slowly and will eventually burst if not replaced; the filament in a light bulb slowly loses material until it cannot sustain the applied voltage and melts. Failures where something has been so badly designed that it cannot withstand its intended loading during normal use are rarer, but they do occur nonetheless. Take a look at Figure 1, which shows the broken handle of a decorative cake knife, the sort that gets used only on ‘special’ occasions. In fact, this example of failure was caused by poor design. Note that a metal ‘tang’ extends from the blade into the handle as a means of reinforcement. In this case the tang was simply too short to strengthen the ceramic handle sufficiently against the bending loads that arose during cutting. The failure occurred while the knife was being used at a wedding reception and resulted in blood-soaked icing.
The elbow connector shown in Figure 2 is another case of poor design having disastrous consequences. The elbow was part of pressurised pipework used in a hydraulically powered waste compactor that ruptured during use, causing severe injury to the operator. Subsequent analysis indicated that the wall thickness and material properties of the connector were just not adequate for the job of containing pressures over 17 MPa (17 × 106 N m−2). You may have had similar experiences of structural failure, although hopefully more mundane. (In this unit we will use the shorthand ‘Pa’ or ‘pascal’ for the units of stress and pressure, N m−2 (newtons per square metre).)
The way the load intensity, or stress, varies within a material is also important. If we can understand and quantify the internal stress distribution then we are some way towards figuring out why the failure occurred. But not only that: we can also redesign structures so that they are less likely to fail next time round. Or better still, we can avoid it ever happening in the first place. In other words, stress analysis is a very useful tool. Often the calculation of stress is relatively straightforward, but for complicated components and systems of components, with multiple loads and varying material properties, more complex analyses are required.
It is often not possible to design something to withstand any foreseeable load. The laptop on which I am writing this is reasonably robust, but I would not expect it to keep working properly after I had reversed my car over it. Figure 3 shows a couple of examples of ‘overload’ failures that occurred because the structures were subjected to a loading for which they were not designed. A bicycle wheel is perfectly capable of carrying the load of two or three merry students provided that the loading direction is approximately in the plane of the wheel itself, where it is strongest. But a sideways impact can easily cause buckling (Figure 3a). Similarly, a stepladder leg is designed to bear heavy vertical loads, but a significant lateral force will cause it to bend (Figure 3b). More extreme, earthquake-induced, overload failures are shown in Figures 4 and 5. Of course, it is extremely difficult to design a building that will withstand any magnitude of earthquake, but there are design philosophies that can make them more ‘resistant’, including methods of damping vibrations and trying to ensure that damage, when it does occur, does not lead to total collapse of the structure. Nevertheless, under such circumstances engineers and designers are often faced with decisions about balancing effective design against cost and the likelihood of catastrophic failure.
Even a single column collapse in a multi-storey building places greater load on underlying storeys, which then collapse in turn leading to multiple floors stacked upon each other, like pancakes. This is due primarily to the use of low-cost structural columns of inadequate strength.