Fracture Toughness at ambient temperature ASTM C1421
The ASTM C1421 test is used for determination of fracture toughness of ceramics with flat R-curve behaviour, including few possibilities for materials with rising R-curve. It implements a total of three test methods namely - single-edge precracked beam, chevron notch, and surface crack in flexure.
ASTM C1421 test quantitatively measures fracture toughness (KIc) of various ceramic specimens in surrounding temperature. This test is a collection of three tests – precracked beam test specimen (KIpb), chevron notched beam test specimen (KIvb), and surface crack in flexure (KIsc). All the three methods are based principally on using the existence of a sharp, real crack for detecting any crack extension. Performing all the three tests is essential as the result obtained by one cannot give precise measurements. However, depending on various R-curve possibilities, certain specific tests are preferred over others. For small-crack fracture toughness, KIscmethod is prevalent while for longer-cracks KIvb test results have proven better. The KIpbtest reflects either for short or long-crack length depending upon precracking conditions. While for ceramic specimens with a flat (no) R-curve, all three tests are expected to be the same. Tests conducted by ASTM C1421 deliver specific fracture values that depend upon rate, temperature, and surrounding environment.
ASTM C1421 involves application of force on a given ceramic specimen in three or four point flexure. The ceramic test specimen can either have a crack before the tests (pb, sc) or develop one during the loading (vb). At least five cycles of measurement of each testing condition must be performed. It is advisable for the sc test specimens to be only tested in four-point fixtures.
For the precracked beam method in ASTM C1421, the test specimen must have a cross-section of 3 mm by 4 mm. While for the surface crack in flexure method, in addition to having a similar cross-section it must also have a length of 25 mm or greater. The chevron notch flexure method needs cubic samples of different dimensions.
Data analysis of ASTM C1421 uses values conveyed in accordance with the International System of Units (SI) and IEEE/ASTM SI10. All the values are to be taken in SI units.