Laboratory testing was conducted to evaluate safety concerns for integrally - stiffened tanks that were found to have developed cracks during pressurization testing . Cracks occurred at fastener holes where additional stiffeners were attached to the integrally - stiffened tank structure . Tests were conducted to obtain material properties and to reproduce the crack morphologies that were observed in service to help determine if the tanks are safe for operation . Reproducing the cracking modes observed during pressurization testing required a complex loading state involving both a tensile load in the integrally - stiffened structure and a pin - load at a fastener hole .