🚀 go-pugleaf

ps02@ns1.cc.lehigh.edu (PENELOPE E. SMITH) writes:

>>probably the reason for the demise in use of  of glass bookshelves,
>>at least in damp (UK!) houses. Big elastic load - long time - moisture.

>Question:  Suppose I want to build glass bookcases anyway.  How fast will
>those cracks grow?  Can I make my shelves heavy enough, say, to last at least
>100 years?  Does it help to smooth the stress concentrations at the supports?

Yes you can.  Whether you call the phenomenon "environmental stress cracking"
or "static fatigue" [just a little dig there!], it does depend on stress.
As one of the previous articles pointed out, crack growth rate is dependent
on stress.  So, all you have to do is ensure that the stresses are low,
which can be achieved, as you suggest, by making the shelves thick enough.

Better than that, though, I would suggest spacing your supports a bit closer.
The trick is to avoid _tensile_ stresses (and bending, of course, sets up
tensile stresses on one side);  _compressive_ stresses are not a problem,
since they tend to cause cracks to close up, not grow.  So! if you find it
aesthetically pleasing, you can happily make the uprights out of glass as
well.  And, since they are glass, no-one will feel you have over-designed
if you add a few more uprights than normal in order to reduce the bending
loads on the shelves.

As for the stress concentrations at the supports, it probably does matter,
but _not_ because such stress concentration points will _initiate_ the
cracks;  glass has quite enough surface flaws already without needing to
initiate any more.  But stress concentrations matter for the reason I gave
above  --  that it simply increases the _stress_ on any given crack in that
vicinity.

We have a really natty lab exercise for the undergrad students here,
to teach them the basic principles of fracture mechanics.
We have a simple apparatus for putting glass rods into 3-point bending
under a dead-weight load.  We then take some of the glass rods (ordinary lab
glass, 6 mm diameter) and etch them in 50% hydrofluoric acid
	* WARNING!  For anyone not familiar with HF:  It attacks glass,
	so as you can imagine, it also attacks bone, and does
	indescribably horrible things to skin and eyes.  And it's not
	easy to wash off, either.
	DON'T USE IT UNLESS YOU KNOW WHAT THE SAFETY MEASURES ARE! *
This removes the surface, including all surface flaws.  What this does to
the bend strength is really quite dramatic.  An as-received rod will typically
support about 6 kg;  etched rods will support as much as 60 kg.  The modulus
of rupture comes out well in excess of 1000 MPa!  We get the students to
calculate the flaw size from the Griffith equation (given a value for the
surface energy).  We have estimated the flaw size in the as-received
glass and it agrees well with the calculation (about 5 micron, I think).
It is interesting to note that the edge of the central beam in the loading
jig must damage the surface, yet this doesn't matter because that side
of the rod is in compression.

Students often ask about the potential engineering applications for this
"wonder glass".  We have to disillusion them, of course  --  10 minutes
of usage would introduce new surface flaws.  Unless resin-coating immediately
after etching would prevent damage .............?

Happy book-case building!

|  jeff@minmet.uq.oz.au	       |  Jeff Gates
|			       |  Dept Mining & Metallurgical	       _--_|\
|  Ph. direct: +61 7 365 3726  |		     Engineering      /      B
|  Ph. Dept: +61 7 365 3738    |  University of Queensland	      \_.--\_/
|  Fax: +61 7 365 3888	       |  Brisbane   Qld   4072   Australia	    v