German armour-piercing shells were far more effective than the British ones, which often failed to penetrate heavy armour. The issue particularly concerned shells striking at oblique angles, which became increasingly the case at long range. Germany had adopted trinitrotoluene (TNT) as the explosive filler for artillery shells in 1902, while the United Kingdom was still using a picric acid mixture (Lyddite). The shock of impact of a shell against armour often prematurely detonated Lyddite in advance of fuze function while TNT detonation could be delayed until after the shell had penetrated and the fuze had functioned in the vulnerable area behind the armour plate. Some 17 British shells hit the side armour of the German dreadnoughts or battlecruisers. Of these, four would not have penetrated under any circumstances. Of the remaining 13, one penetrated the armour and exploded inside. This showed a 7.5 per cent chance of proper shell function on the British side, a result of overly brittle shells and Lyddite exploding too soon.