The J_Ic can be used as a toughness value at the initiation of crack tearing from a sharp fatigue crack in metallic materials.  This toughness value can serve as a basis for screening tough materials or for evaluating materials utilized in sub-K_Ic thicknesses.  Requirements for a valid J_Ic value according to ASTM E1820 are based on the ratio of the J_Ic to yield strength, i.e.,

where B is thickness and b₀ is the initial ligament.  The relationship between the J-integral and the stress-intensity factor was given in Section 11 as,

where E¢ = E, the elastic modulus, for plane stress, and E¢ = E/(1-n2) for plane strain, and n = Poisson’s ratio.  Thus, using Equations 7.2.5 evaluated at the critical condition (J = J_Ic, K = K_JIc) and Equation 7.2.4, the thickness requirement becomes after some algebra

For the typical condition where the ratio of yield strength to elastic modulus (s_ys/E) is below 0.1, J_Ic values can be obtained using specimens thinner than that required by the K_Ic standard (ASTM E399).

The K_JIc value in Equation 7.2.6, however, does not normally correspond to the K_Ic value that would be obtained using the plane-strain fracture toughness standard.  The K_Jic value based on J_Ic measurements is typically lower and thus leads to conservative estimates of the ASTM E399 K_Ic value.  The differences in K_JIc and K_Ic arise as a result of differences in the amount of allowable physical crack growth associated with the two standards; there is less growth allowed for the J_Ic value than for the K_Ic value.

While the use of a toughness standard for sub-K_Ic thickness specimens provides additional opportunities for characterizing material resistance to fracture, the J_Ic concept appears somewhat limited relative to the design of aerospace structures.  A single test of a J_Ic type specimen might be similar in cost to a K_Ic type test; but a number of J_Ic type specimens must be tested to develop the required crack resistance data prior to estimating the J_Ic value.  Through unloading compliance testing, it is possible to reduce the number of tests.