Lloyd: as Miller said, the obstruction causes the pressure in the barrel to rise above the elastic limit, the steel begins to stretch, and if the pressure exceeds the ultimate tensile strength, it bursts = a ductal fracture. Steel can also break as a brittle fracture.
Short version of tensile testing
The tension stress which causes a sample to break is called the maximum strength or ultimate tensile strength or simply tensile strength. The ultimate tensile strength is measured at point of fracture
or the reversal point of the stress-strain curve; the specimen may stretch more before fracturing.
Yield strength or elastic limit is the stress which gives a permanent deformation of 0.2%.
Percent elongation is the lengthening of the sample to fracture expressed as a percentage: (final length – initial length) / initial length) x 100. It is a measure of ductility.
A specimen is precisely machined per specifications
Then loaded in the tensile machine and fitted with an extensometer
This is (unfortunately the low resolution) stress-strain curve for the 1898 Smith Armor Steel specimen: Yield - 57,500 psi; Ultimate - 101,000 psi; % elongation - 12.3
