Due to the complexity of the two parameter fracture criteria for tearing fracture behavior, engineers sometimes obtain preliminary estimates for the residual strength using a single parameter fracture toughness criterion.  Figure 4.2.10 describes the stress-crack length levels associated with the onset of cracking (K = K_ONSET) and fast fracture (K = K_cr) conditions for a tearing material.  Intermediate between the two curves established from material observations is a third curve referred to as the apparent fracture curve.  The apparent fracture toughness (K_APP) is established from the same data employed to derive K_ONSET and K_cr.  The calculation procedure uses the onset (or initial) crack length (a_i) and the final recorded stress level (s_cr) for the tests conducted.  Thus, K_APP represents a fracture toughness level bounded by the onset and fast fracture levels.

Figure 4.2.10.  Description of the Three Fracture Toughness Criteria that are Utilized to Estimate Residual Strength Under Tearing Fracture Conditions

For lower bound estimates of the residual strength for fast fracture of a tearing material, one could equate the level of applied stress-intensity factor (K) to the apparent fracture toughness (K_APP), i.e., assume that fracture occurs when

in order to determine the critical level of stress.  Equation 4.2.3 is an abrupt failure criterion for a tearing fracture.