If you want to know more about this subject I recommend you read Engineer To Win by Carrroll Smith. In fact I recommend you read all his books.
When we build our own cars we want them to be safe and reliable (in fact reliability is the main criteria for my cars). If we can make them light then we will go faster. If we make them too light then they will break. Things break at points of highest stress, so we should try to minimise these areas of high stress by making sure the stress is as evenly distributed as possible. We can do this by removing material from places of low stress and / or add material to areas of high stress.
FEA and computers can do this or we can learn to identify stress raisers. Stress raisers basically occur anywhere there is a change in section that interrupts the free flow of stress along the component. Common raisers are holes, scratches (scribe lines), notches, abrupt changes in section (square edges or corners, internal and external), machining marks, threads, splines, joints (including welds which is why top bicyles use butted tubes, ones that are thicker wall near the joints), corrosion etc.
So wherever we have a stress raiser we should take steps to reduce the stress in that area. Sometimes this means adding material and other times removing material. For example if we have splines on the end of a half shaft or drive shaft then these splines should be on a larger radius than the rest of the shaft, or we can reduce the rest of the shaft diameter, taking care to have a nice blend between the section changes. The same applies to bolts. You can generally identify a highly optimised bolt because the shank will be a smaller diameter than the threads.
If we are putting a hole to bolt a part in a shaft or piece of plate the area around the bolt is thicker than the rest of the area.
However sometimes we can remove material to reduce stress. If we are cutting sheet material into a corner you should cut to a drilled hole, the bigger the better. The end of the cut leaves a sharp notch, a stress concentration. The hole distributes the stress over a larger area.
Components generally fail due to an overload, how long this takes to happen depends on the size of the component. If you have made the part too small then it will fail first time the load is put onto it. If you make it stronger we run into fatigue. This is basically a reversal of load and will eventually cause a small crack to appear, this crack will propagate until the part is weakened and it fails. If you inspect the part often enough you will spot the crack and avoid failure. This is why components are lifed.
I try to design and make my parts to avoid failure by fatigue because I have better things to do than crack test my critical components every week / month. Consequently my parts are heavier than they could be but they meet my criteria of reliability.
The following drawings have come from Carroll Smiths Engineer To Win. The subject is covered in greater detail. It is a good book as are all his books.
Finally a couple of drilled brake pedals I have seen in magazines.
JB
