Note 2: Crack growth in adhesive bond specimens can proceed in two ways: (1) by a slow-stable extension where the crack velocity is dictated by the crosshead rate or (2) by a run-arrest extension where the stationary crack abruptly jumps ahead outrunning the crosshead-predicted rate. The first type of crack extension is denoted flat; the second type peaked because of the appearance of the autographic record. The flat behavior is characteristic of adhesives or test temperatures, or both, for these adhesives where there is no difference between initiation, G_1c, and arrest, G_1a. For example, the rubber modified film adhesives tested above −17.8°C (0°F) all exhibit flat autographic records. Peaked curves are exhibited for all modified materials tested below −73°C (−100°F) and in general for unmodified epoxies.

It should be noted that both peaked and flat behaviors are determined from a crack-length-independent specimen. For other specimens or structures where G increases with a at constant load the onset of crack growth would result in rapid complete fracturing whatever the adhesive characteristics.

5.1 The property G_1c (and G_1a if relevant) determined by this test method characterizes the resistance of a material to slow-stable or run-arrest fracturing 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 tritensile plane strain, and the crack-tip plastic region is small compared with the crack size and specimen dimensions in the constraint direction. It has not been proven that tough adhesive systems fully meet this criteria. Therefore, data developed using equations based on this assumption may not represent plane-strain fracture values. Comparison of fracture toughness between adhesive systems widely different in brittleness or toughness should take this into consideration. In general, systems of similar type toughness (6, 7, 8, 9, 10) can be compared as can the effect of environment on toughness of a single system. A G_1c value is believed to represent a lower limiting value of fracture toughness for a given temperature, strain rate, and adhesive condition as defined by manufacturing variables. This value may be 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 this test method in terms of linear elastic fracture mechanics may be found in Refs (4) and (8).

5.1.1 Cyclic loads can cause crack extension at G₁ values less than G_1c value. Furthermore, progressive stable crack extension under cyclic or sustained load may be promoted by the presence of certain environments. Therefore, application of G_1c in the design of service components should be made with awareness of the G increase for a prior crack which may occur in service due to slow-stable crack-extension.

5.2 This test method can serve the following purposes:

5.2.1 In research and development to establish, in quantitative terms, significant to service performance, the effects of adhesive composition, primers, adherend surface treatments, supporting adhesive carriers (scrim), processing variables, and environmental effects.

5.2.2 In service evaluation to establish the suitability of an adhesive system for a specific application for which the stress conditions are prescribed and for which maximum flaw sizes can be established with confidence.

5.2.3 For specifications of acceptance and manufacturing quality control, but only when there is a sound basis for specification of minimum G_1c values. The specification of G_1c values in relation to a particular application should signify that a fracture control study has been conducted on the component in relation to the expected history of loading and environment, and in relation to the sensitivity and reliability of the crack detection procedures that are to be applied prior to service and subsequently during the anticipated life.

1.1 This test method (1, 2, 3, 4, 5)2 covers the determination of fracture strength in cleavage of adhesives when tested on standard specimens and under specified conditions of preparation and testing (Note 1).

1.2 This test method is useful in that it can be used to develop design parameters for bonded assemblies.

Note 1: While this test method is intended for use in metal-to-metal applications it may be used for measuring fracture properties of adhesives using plastic adherends, provided consideration is given to the thickness and rigidity of the plastic adherends.

1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.

1.4 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.