Business Profile
test cold and hot ‘soaking’ of the samples prior to test. These temperatures and acclimatisation have now been stipulated in the new rail standard.
Through a rigorous testing regime IG had shown that constructing the laminated pane using toughened (tempered) glass with a single interlayer (0.76mm) produced excellent small ball impact test results. However, the large object tests required by containment criteria showed that if both toughened components of the laminated pane broke, the subsequent total loss of rigidity caused the double glazed unit to de- frame easily when subjected to falling body impact and/or subsequent static load. Testing with annealed glass with a double interlayer (1.5mm) improved the result insufficiently with the indication that thicker leaves of annealed glass were needed in the lamination. With the restraints on overall weight and thickness of the glazing unit this was not possible. During this period IG had sponsored a small research programme at Cranfield University into the breakage characteristics of various types of laminated glass. Some results of which proved useful background data to their own project. IG tested again the 1.5mm interlayer with specially heat treated 3mm glass leaves. This produced excellent successful and constantly repeatable results.
IG already had considerable experience of the pressure pulse test as the major supplier of units to the UK rail industry
for the HST fleets since the 1990’s. It had already established within its toughening processes the parameters needed to produce toughened glass to UK Standard BS857 but which also met the more stringent pressure pulse requirements (BR573). However when the British Rail test equipment was taken out of use, IG decided to complete its own in house suite of test equipment by investing in its own pulse rig.
This equipment was perhaps the most difficult to design and commission. In the commissioning trials a surrogate window unit was used with non-deformable characteristics. The pulse was recorded on each cycle and found to be precise, consistent and repeatable. However when a broken window unit was installed it was found that the pulse decayed as the broken window pane bowed and the internal void between the two panes of glass got bigger. The pressure effect also was observed to be acting on the inner (laminated) pane. In order to mitigate these effects a self-monitoring and correcting programme was devised to control the air delivery cylinder. The equipment now achieves results consistently within the specified limits. RSSB was informed of the problems associated with the creation of the approved pulse, this resulted in a requirement that each pulse is monitored and recorded as proof of compliance, rather than calculation of pulse based on displacement or proof of pulse at point of set up.
Ensuring the required consistency of output requires a rigorous quality regime and system of physical testing throughout the production and assembly process. It is essential that this regime needs to take account of the initial vagaries in the physical properties of glass and that the quality programme ensures sufficient safety margin to ensure compliance of every unit produced.
The equipment that IG now has at its disposal makes it highly suited to aid its customers in optimising their designs and benefiting their future developments. John Devine Snr, IG’s managing director said: ‘What makes us stand out is that we don’t rely on calculation but more on the proof by physical testing of representative samples. Things that were shown to work by calculation didn’t work under physical tests. There is too much to lose if you get it wrong because these are safety critical parts. Getting it right has always been IG’s imperative. That’s the only way to be, which is why we have invested so heavily in our own test equipment. That is our commitment to UK rail.’
Visit
www.independentglass.co.uk Further information and references: IG has produced a memory stick video of the RSSB containment tests which is available on request – email toughened@independentglass.
co.uk quoting RailPro See also
www.rssb.co.uk/RESEARCH/Pages/ ResearchandDevelopmentTool.aspx For details of T424 and T066 and Standard GMRT 2100 Issue5
• December 2013 Page 117
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