Skip the craft foam armor kits and buy floor mat foam instead
I picked up a $45 pre-cut foam kit at a con in Cleveland and the plates cracked apart at the elbow after one afternoon, while $12 of interlocking gym floor tiles cut with my kitchen knife held up through two full weekends of wear. If you are cutting foam for a build, what thickness are you using for curved shoulder pieces?
Do the elbow and shoulder keep cracking because the foam is the same thickness as the flat plates? Curved pieces take way more stress when you bend your arm, so the foam needs to be a bit thinner so it can flex instead of splitting at the crease. I go with 6mm for the curved parts and save the thicker stuff for the flat chest and back plates.