BEHAVIOR OF CONCRETE ROUND DOUBLE BEAM FRACTURE TOUGHNESS TEST SPECIMENS
Laboratory fracture toughness tests of concrete using wedge-loaded round double beam specimens with a length to diameter ratio of 2:1 show two distinctly different specimen behaviors. Either the crack remains in the plane of the notch, and a valid test result can be obtained, or there is lateral cracking around mid-length which invalidates the results. Results of a controlled study show that the behavior transitions from desired to lateral cracking as the maximum aggregate size in a particular concrete increases. Two theories are proposed to explain this behavior and transition: new crack branching and crack turning. To prevent the lateral cracking behavior, two alternate testing techniques have been developed for use with the RDB specimen.
Stability of crack propagation associated with fracture energy determined by wedge splitting specimen
Abstract The wedge splitting test is performed on notched shaped specimens that enables the determination of energies for large fracture surfaces and material exhibiting brittle behaviour. A stability condition is deduced and found to depend on the Young's modulus and the R-curve behaviour. The latter is defined by the fracture toughness KR and fracture energy Rc both of which depend on the crack length. A stable crack propagation is enhanced by high ratios of K′R/KR and Rc/KR2. The wedge loading tends to behave like raising the rigidity of the testing machine. The results are applied on an example with a special geometry.
Chevron V-Notched Bend Specimen for KIc Measurement of Brittle Materials
A chevron V-notched bend specimen is proposed for plane-strain fracture toughness KIc testing of brittle materials. This specimen has the following distinctive advantages over more conventional toughness specimens: (1) the simple geometry can easily be machined, (2) no fatigue precracking is required, (3) no crack length measurements are required (KIc can be calculated from knowledge of the peak load alone), and (4) the test can easily be conducted at elevated temperatures. The proposed specimen was calibrated with a 7079-T6 aluminum alloy of known fracture toughness. Subsequently, the fracture toughness of the ceramic material Pyroceram® 9606 was determined with the same specimen design. The reproducibility of KIc values was found to be excellent: 2.64 MPa·m1/2 ± 3% (2.39 ksi·in.1/2 ± 3%). The proposed specimen design is also recommended as an inexpensive toughness screening test for other structural materials. The chevron V-notch crack starter used in the bend bar specimen is also recommended for use in other specimen designs to improve the capability for both fatigue and stress corrosion threshold testing. In view of the tremendous cost- and time-saving potential associated with the chevron V-notch design, it is recommended that a detailed finite element analysis be performed on this configuration to further substantiate the applied stress and stress intensity conditions.