A pressure vessel is constructed with a diameter of d = 18 and a length of L = 6. The vessel is to be capable of withstanding an internal pressure of p = 1000 psi, and the wall thickness is such as to keep the nominal hoop stress under 2500 psi. However, the vessel bursts at an internal pressure of only 500 psi, and a micrographic investigation reveals the fracture to have been initiated by an internal crack 0.1 in length. Calculate the fracture toughness (KIc) of the material.