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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The engineering types among us will recognize the classic material failure mode in the image below.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>I installed shelves in my house about 25 years ago. The top shelf was grossly overloaded (and I knew it (but hoped for the best.))<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Yesterday it failed catastrophically, bringing down the shelf which took out the shelf below it.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>This is the primary failure mode, a classic stress concentration serial fail: metal close to its yield strength will have its covalent bonds give up one after another until one day BOOM.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The metal thickness is about 1.5 mm and length of rupture about 6 mm. Just for fun I calculated how many covalent bonds were giving up per second. I calculated about half a million failed bonds per second, day and night for most of my adult life.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Since the other side (not visible in this image) also had a similar failure, I could say a million covalent bonds broke every second for a quarter of a century.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><img width=547 height=582 style='width:5.6979in;height:6.0625in' id="Picture_x0020_1" src="cid:image001.jpg@01D64D5D.0A396840"><o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>spike<o:p></o:p></p></div></body></html>