http://citycyclingedinburgh.info/bbpress/ CityCyclingEdinburgh Forum » Topic: Metal fatigue en Wed, 13 May 2026 05:09:28 +0000 http://citycyclingedinburgh.info/bbpress/topic.php?id=16150#post-214012 Fri, 11 Mar 2016 11:49:41 +0000 neddie 214012@http://citycyclingedinburgh.info/bbpress/ <p>When a metal is bent backward and forward e.g. under vibration, it gets harder and therefore more brittle - this is known as work-hardening.</p> <p>This happens very quickly in certain metals. A copper pipe can really only be bent once - if you try to bend it back it will break. Aluminium is poor in this respect too. Steel is much better - see how many times you have to bend a paper clip back and forth until it breaks.</p> <p>The shape of the bracket can also have an effect. Internal corners that are "too square" will increase the stress, whereas more rounded corners will cope better. The case of the DeHavilland Comet fuselage break-ups was attributed to the windows being "too square" and being fixed with rivets instead of glue - both working to increase the stress concentrations. The aircraft was redesigned with rounder windows after several crashes, but by then it was too late...</p> <p><a href="https://en.wikipedia.org/wiki/BOAC_Flight_781" rel="nofollow">https://en.wikipedia.org/wiki/BOAC_Flight_781</a></p> <p><a href="https://en.wikipedia.org/wiki/Stress_concentration" rel="nofollow">https://en.wikipedia.org/wiki/Stress_concentration</a> </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=16150#post-214010 Fri, 11 Mar 2016 11:46:00 +0000 Darkerside 214010@http://citycyclingedinburgh.info/bbpress/ <p>As adding extra supports is far beyond my humble skills, I instead returned to the B&amp;Q rummage bin and now have a suitable bit of steel. I shall report back... </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=16150#post-213998 Fri, 11 Mar 2016 11:01:17 +0000 wingpig 213998@http://citycyclingedinburgh.info/bbpress/ <p>My old sparebike's front porteur rack was held to the fork crown with an aluminium bracket which snapped at one of its bends. The bends came with it and were not an aftermarket application by me. I noted that a few of the equally-light and non-ferrous brackets in my tub of bits of brackets I've fished out of the Bike Station have wee extra perpendicular buttressy sort of bits on the inside of some of their bends. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=16150#post-213989 Fri, 11 Mar 2016 10:27:21 +0000 le_soigneur 213989@http://citycyclingedinburgh.info/bbpress/ <p>Steel/ferrous metals &amp; titanium- you need to bend them more than that (repeatedly) and they will fail. You can keep bending them less than that as many times as you like and they will not fail.<br /> Aluminium, copper etc do not have this fatigue threshold, you just have to bend them a small amount &amp; do it more often to achieve failure.<br /> <img src="https://upload.wikimedia.org/wikipedia/commons/3/30/S-N_curves.PNG"> </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=16150#post-213986 Fri, 11 Mar 2016 09:53:30 +0000 steveo 213986@http://citycyclingedinburgh.info/bbpress/ <p>I had a (diy) lamp go like that. I consulted a mechanical engineer (colleague with misapplied education) he suggested a rubber washer at the mounting points for the bolt to mate with cuts out most of the vibration. My bracket was fine after that. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=16150#post-213983 Fri, 11 Mar 2016 09:40:42 +0000 I were right about that saddle 213983@http://citycyclingedinburgh.info/bbpress/ <p>One of us will maybe extemporise on the crystal structures involved but I'll just say that thin alloy isn't really strong enough for brackets, especially once it's been bent. Retain bits of stainless steel in your 'oh god I've turned into my dad' coffee tin of odd screws and useful things. </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=16150#post-213976 Fri, 11 Mar 2016 01:03:00 +0000 chdot 213976@http://citycyclingedinburgh.info/bbpress/ <p>go back to the inner-tube rubber bungee I used for 000s of happy miles before </p> http://citycyclingedinburgh.info/bbpress/topic.php?id=16150#post-213975 Fri, 11 Mar 2016 00:51:53 +0000 Darkerside 213975@http://citycyclingedinburgh.info/bbpress/ <p>Dear material scientists of this parish.</p> <p>400km ago I made a bracket to support the front of the rear mudguard on the recumbent. This consisted of a 20cm-ish piece of aluminium 2x16mm strut (from a B&amp;Q rummage box...), with a 70-ish degree bend near one end.</p> <p>Today, I noticed that it had snapped at the bend. There's no massive force going through the strut as the guard is supported by two more pairs of stays, but it did have to flex a touch with the bounce of the guard.</p> <p>Thoughts? Is aluminium particularly vulnerable to back and forth flexing (would I be better with steel?), or should I get something slightly thicker? Or go back to the inner-tube rubber bungee I used for 000s of happy miles before.</p> <p>Ta much. </p>