The role of alignment for valid tensile testing of ceramic matrix composites

The determination of tensile properties of ceramic matrix composites (CMCs) is crucial for material development and assessment. Despite the damage tolerance of CMCs, the efforts required for proper tensile testing are similar high than for ceramics. This study describes a method to determine the sample alignment prior every single tensile testing and discusses the required amount of alignment for valid testing. For this purpose, carbon fiber-reinforced silicon carbon (C/C-SiC) samples were evaluated by only two alignment criteria. Approximately 80 % of the tensile samples failed valid with a percentage bending less than 6 % and no valid testing was achieved at a percentage bending higher than 12.3 %. Invalid testing showed an increase of strength scattering of more than 50 %, but only a minor change of the strength. The preliminary alignment check and subsequent realignment was able to increase the rate of validly tested samples by about four times. • Two verification criteria allow to identify detrimental misalignment prior testing. • C/C-SiC was somewhat less sensitive to misalignment than monolithic ceramics. • Valid failure was not observed with percentage of bending higher than 12.3 %. • Invalid failure leads to 50 % increase of the scattering of strength. • Alignment criteria predict valid and invalid failure before final tensile testing.