Much of present fracture toughness testing concerns chevron notched specimens (Newman Jr, 1984) since these, in principle, require neither fatigue precracking nor more registration than failure load to determine fracture toughness. Two such specimens which can be made from diamond drilled rock cores with minimal machining are the Short Rod (Barker, 1977) and the Chevron Edge Notch Round Bar in Bend- ing or CENRBB Specimen (Ouchterlony, 1980). Even if the precracking property is especially valuable for a material like rock, practice has made the relative advantages of these chevron notched specimens much less apparent (Ouchterlony, 1982; Ouchterlony and Sun, 1983). Enough remains, however,to have the Commission on Testing Methods of the International Society for Rock Mechanics (ISRM) focus on them as standard configurations in a "Suggested Method for Determining Fracture Tough- ness of Rock Material", which is in preparation. This paper presents material which supports the ISRM effort:

  • An experimental compliance calibration of the Chevron Bend (CENRBB or CB for short) and resulting formulas.

  • Specimen tolerances, evaluation formulas, and loading rate requirements relating to both Short Rod and Chevron Bend Specimens of suggested standard proportions.

  • Some fracture toughness values obtained with the Chevron Bend Specimen.


Preliminaries The specimen and basic notation is shown in Figure 1 together with a straight notched variety (SECRBB). Subscripts c and s are sometimes used below to distinguish between them. Assuming linearly elastic conditions the load point compliance ¿f LPD/F, or g = ¿F.ED in non- dimensional form, be converted to a thickness average stress intensity factor through (mathematical equation) (available in full paper)

Figure 1: Bend specimens with basic notation(available in full paper)

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