Ceramic materials are characterized by being high strength and high modulus materials. However, the brittle nature of ceramics makes them susceptible to impact damage and low fracture toughness. Fracture toughness tests measure the ability of a material, containing a crack, to resist further crack growth and subsequent material failure. Materials with reasonable ductility, such as metals and plastics, typically experience crack origination at material flaws. Ceramics, on the other hand, experience such significant reduction in strength with the presence of materials flaws that manufacture processes for high quality advanced ceramics are very exacting. Ceramic cracks more commonly occur from high force, small point, impact events, thermal shock and cycling, or highly non-uniform loading. Ceramic fracture toughness tests can be conducted with either uncracked or precracked specimens. Precracked specimens have either a machined starter notch or one or more Vickers or Knoop indentations to facilitate crack initiation in the desired beam section. Uncracked ceramic specimens are tested using a chevron notch method, to guide crack extension and facilitate stable crack growth.
Stable crack growth can be very difficult to achieve for extremely high modulus ceramics; if stable crack growth cannot be attained, the specimens should be tested by one of the precracked methods that allow for unstable crack growth. TestResources offers a range of control, data acquisition, and processing software that performs on-line data collection and off-line post processing to calculate the required validation parameters and test results. The software removes the hassle of calculating test results and compiling test reports, allowing quicker and easier verification of proper test procedures and specimen geometries. ASTM and ISO have developed standard test methods for testing the fracture toughness of advanced ceramic materials. ASTM C1421 contains test methods for testing ceramic beam specimens by the precrack, surface crack, or chevron notch method. The variety of test methods allows for characterization of ceramic fracture toughness for unstable and stable crack growth behavior. ISO 18756 is a standard method for testing the fracture toughness of fine ceramics by the surface crack method. Ceramics fracture toughness tests are typically conducted using medium to high force electromechanical test machines, outfitted with high resolution controllers and load cells. The bend fixture required for fracture toughness testing ceramics to ASTM C1421 is made to the same specifications as ASTM C1161.