Because the optical caustic technique offers unique opportunities for experimental measurement of fracture toughness under conditions where ordinary instrumentation cannot be used, an effort to correlate the size of the caustic with fracture parameters has been undertaken. The materials of interest are a 4340 steel in a range of thicknesses designed to allow plane-strain, plane-stress, and mixed-mode constraint conditions; an intermediate-strength 5% nickel steel with a high degree of fracture resistance; and a lower-strength, ductile high-strength low-alloy (HSLA) steel, A710. Using compact tension specimens and standard K and J-integral testing procedures, the caustics were photographed as fracture tests were conducted.
Experiments showed that there is a clear correlation between the J-integral and the caustic diameter (D) for all three steels. The values did not conform to expected relationships predictable from existing theory for several reasons. In the 4340 steel, the correlation for the three thickest samples was independent of thickness, while the thinnest (plane-stress) sample was distinctly different when the generating circle was contained inside the plastic zone. Under these conditions, the J-D correlation was distinctly different for the other two steels as well. When the generating circle was maintained outside the plastic zone, a single relationship was obtained for all three steels. This behavior is attributed to differences between the steels in plastic flow behavior and slip systems. As a result, the near-field behavior is strongly influenced by factors that have little or no effect on the far-field behavior.