Fracture Toughness Testing Methods
Authors: Ebrahimi Fereshteh 
Published: October 2003

With increasing use of steel structures such as railroads, bridges and tanks in the nineteenth century, the number of casualties due to the catastrophic failure of such structures also increased, and hence, attention was drawn to the brittle fracture of steels.

Fracture toughness is a measure of resistance to cracking in notched bodies. In general, during fracture toughness testing, the load‐versus‐displacement behavior of a pre‐cracked specimen is recorded. The load‐displacement curve depends on the size and geometry of the specimen as well as the loading mode. In practice, the goal of fracture toughness testing is to measure a single parameter (e.g., similar to yield strength) that is a material property and can be directly used in design. Fracture mechanics is a tool to translate the results of laboratory fracture toughness testing into engineering design. Depending on the material and the size and geometry of the specimen, different parameters can be evaluated. Stress intensity factor,K, is a parameter that describes the stress level as well as the stress distribution ahead of a crack. Crack driving force,G, is a measure of the elastic energy released per unit thickness of the specimen and per unit length of the crack. When extensive plastic deformation precedes crack initiation, the flow of energy to the crack tip can be evaluated using the J‐integral approach. The amount of strain at the crack tip, known as crack tip opening displacement, is another parameter that can be calculated from the fracture toughness testing results.

To correlate the results of fracture toughness testing with a parameter that represents a material property, a fracture criterion needs to be established.

Fracture toughness testing is a relatively expensive method in comparison to notched impact testing.

There are many standard procedures established by ASTM where the reader can find the details of fracture toughness testing procedures. The purpose of this article is to summarize briefly various fracture toughness testing techniques and to provide the basic principles of fracture behavior and fracture mechanics necessary for understanding and selecting the proper testing technique. Emphasis will be placed on macroscopic standard testing techniques for metallic materials. Special techniques for mixed‐mode fracture, indentation fracture, crack arrest, creep fracture, and stress corrosion cracking as well as the atomistic concepts are not discussed here. Furthermore, the discussion is limited to elastically isotropic materials.

There are mechanical testing methods that yield data related to the toughness of a material, but they do not provide an actual toughness value. For example Charpy V‐Notch impact testing and notched tensile testing measure the energy absorbed during fracture and the stress required for fracture, respectively. Such tests may be used for screening materials based on toughness.

To conduct fracture toughness testing, a mechanical testing system, apparatus for measuring and recording load and displacement, as well as fixtures for holding the specimens, are required.