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Motorsport Fabrication Fundamentals: Stress Risers

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Stress Risers


00:00 - Just because a component is sufficiently strong to withstand the forces it'll be subjected to one or two times, this doesn't mean that the component can be considered indestructible and often we'll find components that fatigue and fail after being exposed to thousands or perhaps hundreds of thousands of cycles in operation.
00:19 This is why mission critical components in top levels of motorsport and industries like aviation have a specific service life.
00:26 For example, if you happen to be lucky enough to be running a factory built GT3 racecar, you'll find that all of the suspension components have a lifespan that's specified in racing kilometres, typically about 5000 km.
00:39 The manufacturer has validated the components so that they can be certain that they won't fail within their service life if they're used as expected.
00:47 A fatigue failure will typically start with microscopic cracks beginning to form in the areas of the component that are exposed to the highest concentration of stress.
00:57 Once these cracks begin to form, they'll continue to grow and the part will become weaker until a complete failure occurs.
01:03 When it comes to fatigue life, there are many aspects that will affect it.
01:07 The material, the thickness of the material, the overall design of the component as well as the actual stresses it will be subjected to are pretty obvious aspects.
01:16 But there are also some subtle aspects that we should keep in mind with our component designs that'll help reduce the concentration of stress and improve the fatigue life of a part.
01:27 Essentially what we need to understand is that sharp edges are our enemy here and particularly if we have a component that has a step machined into it, we want to ensure that the step is machined with a smooth radius at its root, rather than a sharp square edge.
01:43 The crankshaft in an engine is a perfect example of this concept in action.
01:48 If you look at the transition detail between a crankshaft journal and the rest of the crankshaft, you'll always find a nice smooth radius in this location to reduce the concentration of stresses.
01:59 Even with a proper radius here, it's still the most stressed part on the crankshaft and this is still usually where the first signs of cracks will start forming.
02:08 The inclusion of a radius instead of a sharp edge however will extend the fatigue life of the crankshaft dramatically.
02:15 As an example of the importance of a radius, let's look at an example from Red Bull F1 technical direction Adrian Newey's book, How to Build a Car.
02:24 This involves the steering column design that was fitted to the Williams F1 car that Ayrton Senna was driving when he passed away.
02:31 There was a tonne of controversy over the cause of Senna's death however the accident did highlight that there was a fundamental design error made with the modification to the steering column.
02:42 The column had been lowered in the car by 2 mm in order to provide a little additional room for the driver's hands around the wheel but this subsequently required the diameter of the steering column to be reduced by 4 mm in a local area to satisfy an FIA regulation regarding the required clearance between the driver's body and the steering column.
03:03 In making this change, the column was simply reduced in diameter with sharp steps and square edges where the material was removed.
03:10 The result was that the steering column was found to have a fatigue crack running 1/3 of the diameter of the column where its diameter had been reduced at the time of Ayrton's crash.
03:21 While the column itself ultimately was not the actual cause of Ayrton's death, it was clear that it would have failed in due course if the car had continued to race and it was accepted by those involved as a badly designed component.
03:34 A very good way of demonstrating this exact situation is by looking at the way an axle is machined.
03:40 The overall strength of an axle is dictated by its smallest diameter which is typically the root of the splines.
03:47 Conventional thinking when it comes to making a stronger part would be to increase the diameter of the axle, however unless the root diameter of the spline is also increased, this won't help and regardless of the diameter of the rest of the axle, the failure will always occur at the root of the splines.
04:03 Instead, what you'll typically find is that the diameter of the axle is maintained at the same diameter as the root of the spline and special attention is also paid to any transition in diameter on the axle, particularly at the end of the splines in order to ensure it's nicely radiused to prevent stress concentration and extend component life.
04:24 So the key takeaway from this module is that sharp edges and transitions can lead to concentrated stresses that can result in premature fatigue related failure of the component.
04:36 Instead of sharp edges, we always want to utilise a smooth radius wherever possible.
04:40 This is a fundamental design element we should always keep in mind when we're designing or building any component.
04:47 You'll also want to be mindful of this with any machined components that you're having made by third parties.

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