Fracture Toughness Testing
Fracture toughness testing measures the conditions under which an existing crack in a material will extend. During a fracture toughness test, a specimen with a pre-existing crack is subjected to an increasing tensile load. By measuring the combination of load and displacement as the crack lengthens, the material’s resistance to further crack extension is determined. Fracture toughness is an important material property in design applications since the occurrence of flaws is not completely avoidable. Flaws may appear as cracks, voids, inclusions, weld defects or design discontinuities. The test is also valuable in determining whether there is a danger of component failure when a flaw is discovered in an existing structure.
Laboratory Testing Inc.’s computerized fracture toughness test equipment can generate 55,000 lbs. of tensile or compressive force. Our testing lab has an environmental chamber for controlling the test temperature between -250° F and +400° F. At LTI, we perform fracture toughness testing according to the ASTM E399 and ASTM E1820 standards, where E399 determines KIC and E1820 provides KIC, KJIC and JIC.
The stress-intensity factor, “K”, is used to characterize the fracture toughness of linear elastic materials, which are typically high strength materials that fail in a brittle manner. The J integral, “J” describes elastic-plastic fracture toughness in more ductile materials which are better able to deform and resist crack growth under load. A Roman numeral subscript added to “K” and “J” indicates which of the three modes of fracture is used in the test. Mode I fracture is most common and is the condition in which a tensile load is applied normal to the direction of the crack plane. When a cracked material under Mode I plane-strain conditions reaches a critical value, denoted as KIC or JIC, the crack will begin to grow. Experimental values of KIC and JIC can be applied to the design of structures to ensure that a fabrication does not fail by brittle or ductile fracture. KJIC is derived from the JIC result.
The fracture toughness test requires a precisely machined specimen prepared to specification and with an EDM notch. The Machine Shop at LTI can prepare specimens for all fracture toughness testing performed at our lab. We also have fixturing to test various specimens in a range of sizes, including SE(B) (single edge bend) and C(T) (compact tension) specimens.
The Test Process
A fracture toughness test characterizes the resistance of a material to fracture when a sharp crack is present. The fracture toughness properties are obtained by tests on specimens containing deliberately introduced cracks or notches and subjected to prescribed loading conditions.
Fracture toughness testing according to ASTM E399 applies a continuously increasing load to the specimen. ASTM E1820 uses a rising load with periodic partial unloading to measure the instantaneous crack length.
- Test Methods – KIC per ASTM E399 and ASTM E1820; JIC and KJIC per ASTM E1820
- Force – load cells capable of running from 50 lbs. to 55,000 lbs.
- Temperature – between -250° F and +400° F
- Fixturing – SE(B) single edge bend and C(T) compact tension specimen geometries in a range of sizes
- Test Specimens – specimens are prepared in LTI’s Machine Shop according to standard procedures; EDM notch capabilities
- ASTM E399
- ASTM E1820