Toughened glass used in our 'A'-rated windows
Double glazed unit composition: 28mm overall width : 4mm Toughened low iron glass / 20mm thermobar space / 4mm Toughened soft coat glass
Method - attack the middle of the glass pane. The glass will not smash easily in the centre and the hammer will bounce back. We then attack a corner of the glass and hopefully the glass will then smash easily. Instead of large shards of glass the unit shatters into thousands of harmless pieces.
Trying to demonstrate that there is better inherent glass strength in a toughened unit but it's still easily breakable if you know what you are doing. But also that the pieces that are left are not dangerous to the homeowner / intruder - hence it's use in 'safety' critical applications.