Linear-Elastic
Plane-Strain Fracture Toughness KIC
of Metallic Materials
is most often tested according to ASTM E 399
specifications. The KIC test or
KIC, or K1C, as it is also known, is used to determine the fracture
toughness of metallic materials.
![(Click to play video clip of a KIC Test at WMT&R)]() The test consists of fracturing a
specimen of a specified geometry that has
had a sharp defect or fatigue precrack already introduced into it.
The compact tension (CT) and 3
point bend are the most common specimen geometries used. Due to its
economical use of material, the CT specimen is the more commonly employed.
Specimens are machined to
particular dimensions with recommended sizes; any deviation from the
recommended sizes should be agreed upon with the appropriate Standards body.
Plain-strain fracture toughness test
of sufficiently thinner materials can be performed using other types
of specimens. There exists no standard test method for such thin materials.
Provided the machine is powerful
enough, the fatigue precrack and the fracture tests itself are usually
performed on the same machine saving valuable setup time.
The precracking is performed
in cyclic load control and must be conducted in accordance with the relevant
international Standards.
The fracture test is carried out in
displacement control at a constant rate of increase of stress
intensity while recording load and crack opening displacement data,
until the specimen breaks.
 A Crack Opening
Displacement gauge is used to measure the Crack opening displacement.
Knife edge fixtures are affixed to the specimen to accommodate the gage.
After the specimen fails, the fatigue precrack and any crack extension are
measured accurately at intervals along the crack front and then recorded.
Using a secant modulus technique,
the load and crack opening displacement data are examined to obtain a
critical load value. The load value is converted to a stress intensity (Kq)
value utilizing the previously recorded crack lengths. Provided a series of
validity checks is passed, the Kq value may be quoted as a KIC
value. This test may be conducted across the full range of temperatures.
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