Current Attempt in Progress

Suppose that a wing component on an aircraft is fabricated from an aluminum alloy that has a plane strain fracture toughness of $.$ It has been determined that fracture results at a stress of $116$MPa $(16820$ psi$)$ when the maximum internal crack length is $.).$ For this same component and alloy, compute the stress level at which fracture will occur for a critical internal crack length of $5\mathrm{.}5mm(0\mathrm{.}2165$ in$.).$

MPa