5.1 The property K_Ic (G_Ic) determined by these test methods characterizes the resistance of a material to fracture in a neutral environment in the presence of a sharp crack under severe tensile constraint, such that the state of stress near the crack front approaches plane strain, and the crack-tip plastic (or non-linear viscoelastic) region is small compared with the crack size and specimen dimensions in the constraint direction. A K_Ic value is believed to represent a lower limiting value of fracture toughness. This value has been used to estimate the relation between failure stress and defect size for a material in service wherein the conditions of high constraint described above would be expected. Background information concerning the basis for development of these test methods in terms of linear elastic fracture mechanics can be found in Refs (1-5).3

5.1.1 The K_Ic (G_Ic) value of a given material is a function of testing speed and temperature. Furthermore, cyclic loads have been found to cause crack extension at K values less than K_Ic (G_Ic). Crack extension under cyclic or sustained load will be increased by the presence of an aggressive environment. Therefore, application of K_Ic (G_Ic) in the design of service components should be made considering differences that may exist between laboratory tests and field conditions.

5.1.2 Plane-strain fracture toughness testing is unusual in that sometimes there is no advance assurance that a valid K_Ic (G_Ic) will be determined in a particular test. Therefore it is essential that all of the criteria concerning validity of results be carefully considered as described herein.

5.1.3 Clearly, it will not be possible to determine K_Ic (G_Ic) if any dimension of the available stock of a material is insufficient to provide a specimen of the required size.

5.2 Inasmuch as the fracture toughness of plastics is often dependent on specimen process history, that is, injection molded, extruded, compression molded, etc., the specimen crack orientation (parallel or perpendicular) relative to any processing direction shall be noted on the report form discussed in 10.1.

5.3 Before proceeding with this test method, reference should be made to the specification of the material being tested. Any test specimen preparation, conditioning, dimensions, or testing parameters, or combination thereof, covered in the relevant ASTM materials specification shall take precedence over those mentioned in this test method. If there are no relevant ASTM material specifications, then the default conditions apply.