ASTM E399-20a method is used to measure fracture toughness. Fracture toughness describes the resistance of metals to the propagation of flaws under applied stress, and it assumes that the longer the crack, the lower is the stress needed to cause a fracture. The ability of a crack to cause fracture depends on the fracture toughness of the material.
The specimens are subjected to slowly increasing crack-displacement force. A widely utilized standardized test method is the Charpy impact test, whereby a sample with a V-notch or a U-notch is subjected to impact from behind the notch. Also widely used are crack displacement tests such as three-point beam bending tests with thin cracks preset into test specimens before applying load.
Fracture toughness test by ASTM E399-20a method is performed to quantify the resistance of a material to failure by cracking. The test results are either expressed as a single value or as a resistance curve.
In ASTM E399-20a method, a cracked metal is subjected to slowly increasing crack-displacement force. One of the common test methods used for the application of force is the Charpy impact test, in which a sample with a V-notch or a U-notch is subjected to impact from behind the notch. Also widely used are crack displacement tests such as three-point beam bending tests with thin cracks preset into test specimens before applying load.
During the test, the load and the crack mouth opening displacement (CMOD) are recorded, and the test is continued till the maximum load is reached. The value of KIc is derived from the recorded data.
The most common test specimen configurations are the single edge notch bend (SENB or three-point bend) and the compact tension (CT) specimens.
Video 01: Measuring fracture toughness of metals
The specimen has a sharp crack made by cyclic loading, in which the load is applied, removed, and reapplied. This precracked specimen has a thickness of 1.6 mm or greater. Thinner specimens cannot be tested by this method. The state of stress near the crack front approaches tri tensile plane strain, and the crack-tip plastic zone is small compared to the crack size, specimen thickness, and ligament ahead of the crack.