MECHANICAL TESTING

Fracture Toughness of Metals (K-value) - ASTM E 399

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ASTM E399 covers the determination of the plane-strain fracture toughness (KIc) of metallic materials by tests using a variety of fatigue-cracked specimens having a thickness of 0.063 in. (1.6 mm) or greater.

E399 also covers the determination of the specimen strength ratio Rxx where x refers to the specific specimen configuration being tested. This strength ratio is a function of the maximum load the specimen can sustain, its initial dimensions and the yield strength of the material.

ASTM E399 involves the testing of notched specimens that have been precracked in fatigue by loading either in tension or three-point bending. Load versus displacement across the notch at the specimen edge is recorded autographically. The load corresponding to a 2% apparent increment of crack extension is established by a specified deviation from the linear portion of the record. The KIc value is calculated from this load by equations that have been established on the basis of elastic stress analysis of specimens of the types described in this method. The validity of the determination of the KIc value by this test method depends upon the establishment of a sharp-crack condition at the tip of the fatigue crack, in a specimen of adequate size. To establish a suitable crack-tip condition, the stress intensity level at which the fatigue precracking of the specimen is conducted is limited to a relatively low value.

The property KIc determined by this test method characterizes the resistance of a material to fracture in a neutral environment in the presence of a sharp crack under severe tensile constraint, such that the state of stress near the crack front approaches tritensile plane strain, and the crack-tip plastic region is small compared with the crack size and specimen dimensions in the constraint direction. A KIc value is believed to represent a lower limiting value of fracture toughness. This value may be used to estimate the relation between failure stress and defect size for a material in service wherein the conditions of high constraint described above would be expected.

Reference: ASTM E 399 - Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials

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