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General Climbing Discussion

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dropped draws and boltplates 18-Dec-2004 At 10:00:58 PM AlanD

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On 18/12/2004 tmarsh wrote:

>Hmmn... What you've stated, Shaggy, seems to be a commonly held belief,
>
>but an utterly unfounded one. HT bolts are *proportionally* more brittle
>than
>mild steel bolts but in absolute terms they are still streets ahead. ie
>a high
>tensile bolt, even with its 'brittleness' is still considerably stronger
>and more
>durable than the equivalent lower grade bolt.
>
>I'm not an engineer, but I think it works something like the ratio of
>the yield
>strength of the HT bolt to the ultimate tensile strength is lower than
>mild
>steel bolts, but the yield strength of the HT bolt is still much higher.
>I had
>some data around here *rummaging on desk* on this from a bolt
>manufacturer... I'll post it if I find it.
>
>tim
>
Brittleness really isn't the correct term, the term we should be looking for is toughness or fracture toughness. I'm a bit rusty on my mechanical metallurgy but...

The strength of a material is usually quoted / determined under a static load, i.e. the load is slowly increased. However, the strength of a material can significantly lower if it's loaded dynamically. An important thing happens when you increase the strength of a material, it will tend to become more sensitive to strain rate. This sensitivity is measured when they measure toughness, lovely machine too :)

I could wander of into the crystallography of what's happening to explain, but in short, part of the reason above will relate to the mechanism of the crack growth that leads to the ultimate failure of the material. This mechanism means that any crack in a high strength material will act as a stress concentrator and the material is less able to absorb the energy of a sudden loading.

This lack of toughness also affects properties like fatigue resistance as the fatigue cracks once initiated, the cracks will propagate, a more ductile, lower strength material will "deaden" the crack growth.