Uses of fatigue and creep?

Hi folks,

I'm working on a book and am taking a close look at the mechanisms which damage and destroy machines: wear, corrosion, fatigue, creep, brittle fracture, plastic deformation, and so on.

Some of these mechanisms have useful applications too: the wearing in of bearing surfaces so until they match, slow corrosion causing a beautiful patina, the sudden cracking of a fire-sprinkler tube releasing the extinguishing water, and so on. But I'm stuck for useful applications of fatigue and creep.

Can any of the knowledgeable guys here give me interesting examples? The closest I can get for creep is the steam bending of wood, and I have no ideas for fatigue.

Any help would be much apprecated!

Best wishes,

Chris ============================================ Creep, or permanent deformation;

Fatigue is useful to limit service life so the customer has to buy a new product.

That's talking about the energy required to cause collapse by sudden plastic deformation, because it's easier than calculating the maximum stresses, isn't it? At least that's the way I remember it.

To me, no right-thinking engineer would see planned obsolescence as genuinely "useful" :-).

I was thinking more that there might be some kind of timing device or cycle counter based either on creep or fatigue of a material, or perhaps a slow cutting or shaping process?

Thanks!

Chris

That's talking about the energy required to cause collapse by sudden plastic deformation, because it's easier than calculating the maximum stresses, isn't it? At least that's the way I remember it.

To me, no right-thinking engineer would see planned obsolescence as genuinely "useful" :-).

I was thinking more that there might be some kind of timing device or cycle counter based either on creep or fatigue of a material, or perhaps a slow cutting or shaping process?

Thanks!

Chris

=======================================

You can break steel wire or sheet metal with your hands by repeatedly bending it in the same place. First it work-hardens and wants to be bent elsewhere so clamping it in a vise or with pliers is necessary, then it cracks. Polypropylene is notable for its very long fatigue life and is used to mold plastic boxes with integral lid hinge strips.

"Three years after the film, and six years after the publication of Nevil Shute's original novel (No Highway), there were two fatal crashes of the world's first jet passenger airliner, the de Havilland Comet. Investigation found that metal fatigue was the cause of both accidents, albeit in the main fuselage and not the tail section."

I was thinking more that there might be some kind of timing device or cycle counter based either on creep or fatigue of a material, or perhaps a slow cutting or shaping process? =======================================

Superplastic forming is a controlled desirable form of Creep. Titanium can have superplastic warm / hot forming properties. I think it was crucial to the "Eurofighter", which is unstable-in-flight and its little canard "flippers" continuously jiggle under computer control to sythesise apparently-stable flight - at the expense of high cyclic (fatiguing) airframe loadings for which the titanium airframe structure is necessary.

Fatigue - it is used to form an atomically-sharp notch, grown from the tip of a machined notch, for some toughness tests. Realistic repeatable initiation point for fracture on increasingly loading the sample in test. I think CTOD is one.

In general creep and fatigue are nothing but undesirable.

That said, it is a good thing that metals, instead of suddenly fracturing in a low-energy (essentially - "brittle") mode, make the failure process slug-it-out over a long time in a high-energy mode...

Regards, Rich Smith

The only things I can think of that might fall into creep as a plus are fusible links for fire doors, or safety plugs for pressure cookers. There are clear warnings for the fusible links that a minimum load is required for the device to separate. It doesn't entirely melt into a puddle of weird alloy allowing the chains to just separate. I can't find a clear definition of how long something takes to deform to be considered creep, but there are mentions of it being more common at temperatures nearing the melting point.