Recently I was in London travelling on the S7 stock of the London Underground. These trains use rows of 1450mm T5 lamps to light the carriage interior.
Now these are quite long lamps and the fittings they are in have clear plastic clips securing the middle of the lamp but I noticed that several of them had missing or broken clips and over certain sections of track, the vibration of the train was causing the lamps themselves to vibrate and I reckon I could see at least 30mm of deflection in the middle!
So anyway my question is this, are fluorescent lamps tested under rough conditions like this and just how much can they take before shattering?
Seeing a glass tube flexing like that was quite interesting shall we say!

I am not sure that flexural testing like that is carried out on finished fluorescent lamps, but that is definitely done for the raw glass tubing and indeed it's remarkable how much a glass tube can bend before fracturing!  The lamps also pass through a vibration test as part of their development, but that is mainly to check the robustness of the electrodes.  It is very rare that the glass would break.

I know my name obviously isn't James, but I had to read it anyway lol. 30mm is CRAZY! I know it is only T5 diameter and almost 5 feet long, but dang, maybe the tubing is ion-exchange-hardened, but I highly doubt it.
 

All fluorescent tubing is ion-exchange hardened!  I think pretty much all lamp tubing in general.

When the tubing is drawn out of the furnace and has almost cooled to rigidity, it passes through a large flame enriched with sulphur dioxide gas.  The flame cracks the gas and sulphur ions are infused into the outer skin of the glass, where they react with sodium ions that are weakly bound in the form of the sodium carbonate component of all soda-lime glasses.  This forms sodium sulphide, which is volatile and evaporates away, along with the simultaneously produced CO2 gas. 

The result is that the chemical composition of the glass is changed only on its outer skin, which becomes relatively high in silica due to the loss of the other glassmaking components.  This silica-rich sheath has properties similar to quartz tubing, an exhibits greatly improved scratch-resistance.  Due to the reduction of surface flaws and defects, the glass is less likely to fail when it is bent, which would normally put those defects into tension and cause the glass to crack.

These kind of sulphur fires are used in many stages of lampmaking to create a remarkably strong outer surface.  Another commonly used technique (also on FL tubing) is to blast the outer skin of the glass with cold air just before it cools through the strain point.  When the hotter inside surface then cools some seconds later, its contraction pulls the outer skin into compression (while of course stretching the inner skin into tension).  However, since most flaws, scratches and impacts come from the outer surface of the lamp rather than the inner wall, this has a similar effect in improving strength.  This is because glass can only fail when there is the combination of a flaw + tension.  The sulphur fires make the glass more scratch-resistant to reduce the chance of flaws, and the thermal stressing ensures that the outer surface begins in compression - so you have to deform it more before any external flaws are pulled into tension.


In the case of ribbon bulb shells for GLS type lamps the effects are especially impressive.  Our standard test used to be to drop the pre-stressed bulb shells from a height of 1 metre onto a concrete floor, and they are required to bounce without cracking.  However, if you take a small piece of broken glass and drop it into the bulb it will invariably shatter, because the inner surface is already in tension and only a small extra force is required to make it crack.  Fortunately there is not normally anything inside a GLS bulb that touches the inner wall, so its relative weakness inside is not a problem.

WHAAAAAAT?!?! Oh my gosh, that was a complete and utter guess. How come no one ever talks about this? I thought the only way to ion-exchange-harden was dipping in molten potassium nitrate, I didn’t know that you could do it with a sulfur doped(?) flame! That is really interesting, and definitely explains the flexibility.

That is indeed super interesting!

I've been reading a lot of Philips technical magazines, but they never once spoke about that kind of stuff.