1 Engineering for purpose
1.1 Safe design
This unit is about the concepts and theories that underpin the field of engineering known as Structural integrity – that is, the safe design and assessment of load-bearing structures in their entirety, including any individual components from which they may have been constructed. Aspects of structural integrity are implemented in almost every engineering design process, even if the engineer or designer does not necessarily think of it in that way. In this unit, we have separated the skills and knowledge associated with expertise in structural integrity under two headings: Stress analysis, which is the study of how applied forces lead to internal stresses in structures; and Fracture mechanics, which is the study of components and structures containing cracks.
(a) Identify four components or structures that experience loads during their operation.
(b) Can you identify components or structures that do not experience significant loads during operation?
(a) A few that spring to mind are: the wing of an aeroplane; the wheel of a car; the keyboard of a computer; the control wheel of an iPod; the leg of a chair; a door handle; a bookcase … I'm sure you get the idea.
(b) This is a bit trickier, and perhaps the question is a little unfair, but I hope that thinking about this will have shown you just how universal the presence of loading is. Something like the casing of a television set may not be highly loaded during use, but will be loaded when the set is carried, for instance; it also has to bear its own weight without deformation. In fact I can't think of any components that won't experience loads at some point during their lifetime, even if they are only handling loads.
In thinking about part (b) you should have come to the conclusion that virtually nothing is entirely load free. At the very least, any component or structure has to bear its own weight, irrespective of any external loads. A designer might make an intuitive judgment that the loads on a product do not need to be considered, but hopefully that would be underpinned by an educated estimate of what the forces are likely to be and what intensity of load, or stress, the assembly can support. And this is why the concept of stress is important: the limiting material property we are dealing with is ‘strength’, and for safe operation the stresses experienced during use need to be well below the material strength.
Engineering failures can be spectacular and highly publicised, especially when they result in death and destruction; but the failure of a household product can be more immediately annoying, and just as indicative of a poor design.
Make a list of three structural integrity failures of which you have experience. In other words, list three single items, assemblies or structures that have snapped, collapsed, fractured or just plain fallen to pieces in your home, car or workplace, for example. Try to make them as different as possible – so don't choose three smashed pieces of china. Try to think why these items might have failed, paying particular attention to how they were loaded during use and whether this loading was different when failure occurred.
You have obviously made a personal list and I can't really help you with that, but the process is useful because it has allowed you to begin to engage with how to assess structural failure.
You may be feeling pleased with yourself, or you may be thinking how little you know. In any case, I would like you to keep your list safe, along with your interpretation of each failure.