Recently I read the excellent piece about shell damage on Nathan Okun's website, which I recommend. Since then I have come across two accounts of damage to US AP shells that hit Jean Bart that are somewhat at variance with Mr Okun's conclusions.
Mr Okun says that because of their sheath hardening US AP shells formed a happy medium between being flexible enough not to break at oblique impact but not so flexible as to bend so much that the shell stuck in the plate. He attributes the former behaviour to German shells which had a sudden drop in hardness in the shell side and the latter behaviour to British shells which bent too much. He also says there was a problem with the fuzes of US AP shells up to about the middle of 1943. Their explosive D fillers reacted with the shell metal which caused a reaction in the fuzes. So, while these worked well in new shells after about six months deterioration had occurred rendering 50% of shells duds.
In their account of the performance of these shells at Casablanca in US Battleships (1976) Dulin and Garzke say on p. 84:
'Detailed analysis of the damage to Jean Bart indicated that the two duds both struck the ship at oblique angles, which caused them to lose their base plugs, fuzes and filler charges. Final ammunition was that the ammunition was not faulty.'
[The latter rather surprising statement probably means that the shells themselves were not significantly damaged.] At any rate, the reason for the fuze failure is different to that given by Mr Okun. It could' of course, be the case that both failure mechanisms existed in these shells, only one of these was need to render the shell a dud.
In French Battleships 1922-1956 (2009) Jordan and Dumas say on p. 158:
'A second shell struck the barbette armor of turret II to starboard: it broke up, and the base of the shell ricocheted through several compartments, pushing in the armoured deck in section J and ending up close to the munitions trunk for turret V (starboard after 152 mm turret).'
So this AP shell that hit thick calibre sized (405 mm) armor at oblique impact did break up. It was probably mid-body damage with the bending forces breaking the shell in half. This is at variance with what Mr Okun says. Of course, it could be that he is talking about what usually happened in such impacts and that individual cases could vary from this average behaviour significantly. The impact velocity at around 24000 yds would have been between 1500 and 1550 fps. The US tests that Mr Okun describes may have been at higher velocities than this. And at lower velocities the impact duration is longer so that the bending forces act on the shell for longer and are so more likely to seriously damage or break it,
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