Standard Test Methods for Plane-Strain Fracture Toughness and Strain Energy Release Rate of Plastic Materials
5.1 The property KIc (GIc) 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 KIc 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 KIc (GIc) 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 KIc (GIc). Crack extension under cyclic or sustained load will be increased by the presence of an aggressive environment. Therefore, application of KIc (GIc) 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 KIc (GIc) 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 KIc (GIc) 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.
1.1 These test methods are designed to characterize the toughness of plastics in terms of the critical-stress-intensity factor, KIc, and the energy per unit area of crack surface or critical strain energy release rate, GIc, at fracture initiation.
1.2 Two testing geometries are covered by these test methods, single-edge-notch bending (SENB) and compact tension (CT).
1.3 The scheme used assumes linear elastic behavior of the cracked specimen, so certain restrictions on linearity of the load-displacement diagram are imposed.
1.4 A state-of-plane strain at the crack tip is required. Specimen thickness must be sufficient to ensure this stress state.
1.5 The crack must be sufficiently sharp to ensure that a minimum value of toughness is obtained.
1.6 The significance of these test methods and many conditions of testing are identical to those of Test Method E399, and, therefore, in most cases, appear here with many similarities to the metals standard. However, certain conditions and specifications not covered in Test Method E399, but important for plastics, are included.
1.7 This protocol covers the determination of GIc as well, which is of particular importance for plastics.
1.8 These test methods give general information concerning the requirements for KIc and GIc testing. As with Test Method E399, two annexes are provided which give the specific requirements for testing of the SENB and CT geometries.
1.9 Test data obtained by these test methods are relevant and appropriate for use in engineering design.
1.10 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
Note 1: This standard and ISO 13586 address the same subject matter, but differ in technical content.
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