Compression loading applied to round double beam fracture specimens. I: Application to materials with large characteristic lengths

The round double beam loaded in tension, RDB(T), or wedging, RDB(W), is part of standard test methods for measuring the fracture toughness of aluminum (ASTM E 1307), cemented carbides (ASTM B 644), and rock. When the combination RDB(T) or RDB(W) is used with materials with a relatively large characteristic length, which is defined as the square of the ratio of fracture toughness to tensile strength, the crack tends to depart from the intended fracture plane rendering the data unusable. Loading the RDB in eccentric compression, RDB(B), has been investigated as an acceptable loading alternative in such cases. Tests of over 100 RDB(B) specimens made from different concrete mixtures using three specimen sizes all produced usable data. In addition, the fracture toughness values obtained from RDB(B) tests are consistent with values obtained from RDB(W) tests. The test equipment, test procedures, and data reduction method are all described in detail for the RDB(B).