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3D Modeling & CAD for Motorsport: Stress Risers

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Stress Risers


00:00 - Just because a component can withstand the forces it'll be subject to a handful of times, doesn't mean that it's indestructible.
00:07 It's normal for even the highest quality components to fatigue and fail after thousands or even hundreds of thousands of cycles in operation.
00:15 This is why critical components in high level motorsport as well as industries like aviation have a specific service life.
00:23 A factory built GT racecar for example uses suspension components that have a lifespan specified in racing kilometres which is usually about 5000.
00:33 The manufacturer has validated the components so that they can be certain they won't fail within their service life if they're used as expected.
00:41 A fatigue failure will typically start with microscopic cracks forming at what are known as stress risers which are areas of the component where the stress is significantly higher than the surrounding regions.
00:52 Once these cracks begin to form, they'll continue to grow and the part will become weaker until we see a complete failure.
00:59 The fatigue life is affected by a range of different aspects like the material, the thickness of the material, the overall design of the component, as well as the actual stresses the part will be subject to.
01:13 There are also some more subtle aspects that need to be kept in mind with our designs that'll help reduce the concentration of stress and improve the fatigue life of the part.
01:24 Essentially, sharp edges are our enemy.
01:27 Particularly if we have a component that has a step machined into it.
01:30 We need to make sure the step is machined with a smooth radius at its root, rather than a sharp square edge.
01:36 An engine's crankshaft is a good example of this concept in action.
01:40 If you look at the transition between the journal and the rest of the crankshaft, you'll always notice a nice smooth radius which reduces the concentrations of stresses and greatly extends the fatigue life of the part.
01:53 With that said, even with a proper radius, these transitions are still the most stressed part of the crankshaft and it's usually where the first signs of cracks will appear.
02:02 Adrian Newey, Red Bull's F1 technical director, discusses a good example of how using a step transition can go wrong in his book How to Build a Car.
02:12 Newey details the steering column design that was used in the Williams F1 car that Ayrton Senna was driving when he suffered his fatal accident at Imola in 1994.
02:22 While there is a lot of controversy around what actually caused Senna's death, the accident did highlight that there was a fundamental design error made with the modification to the steering column.
02:33 The column had been lowered in the car by just 2 mm to give a little additional room for the driver's hands around the wheel.
02:41 But this meant reducing the diameter of the steering column by 4 mm in a small area to satisfy FIA regulations.
02:48 The team simply reduced the diameter of the column with sharp steps and square edges where the material was removed and the result was that the steering column was found to have a fatigue crack running a third of the diameter of the column where the diameter had been reduced at the time of Senna's crash.
03:05 So while the column itself wasn't the actual cause of death, it was very clear that a big failure was imminent if the car had continued to race.
03:13 The same concept can also be demonstrated by looking at the way an axle is machined.
03:18 The overall strength of the axle is dictated by the smallest diameter which is typically at the root of the splines.
03:26 Conventional thinking when it comes to making a stronger part would be to increase the diameter of the axle but unless the root diameter of the spline is also increased, it's not going to help.
03:35 Regardless of the diameter of the rest of the axle, the failure will always occur at the root of the splines.
03:42 Instead, what you'll usually find is that the diameter of the axle is maintained the same as the root of the spline and attention is also paid to any transition in diameter of the axle.
03:53 Particularly at the end of the splines in order to ensure it's nicely radiused to prevent stress concentration.
04:00 To summarise this module, the key takeaway here is that sharp edges and transitions can lead to concentrated stresses that can result in premature fatigue related failure of the component.
04:11 Instead of sharp edges, we always want to utilise a smooth radius wherever possible.
04:16 This is a fundamental design element we should always keep in mind when we're designing or building any component.

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