ASTM E399 Standard Test for Linear-Elastic Plane-Strain Fracture Toughness of Metals

Scope:

ASTM E399 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 load application.

Test Procedure:

Fracture toughness test by ASTM E399 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 method, a cracked metal is subjected to a 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 load application. 

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 K_Ic 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.

Specimen size:

In ASTM E399, 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 the 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.

Data:

Plane-strain fracture toughness of metals is calculated in ASTM E399 by the following formula: 

Conclusion:

ASTM E399 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.