Intermediate temperature strength degradationin SiC/SiC composites

In‐Plane Cracking Behavior and Ultimate Strength for 2D Woven and Braided Melt‐Infiltrated SiC/SiC Composites Tensile Loaded in Off‐Axis Fiber Directions

“…For this study, several AE criteria were used to evaluate key stress levels near initiation of matrix cracking since these values typically represent upper design limits beyond which CMC moduli and axial thermal conductivity irreversibly decrease and the possibility exists for adverse environmental effects for SiC/SiC composites [11]. These stress levels, which are indicated in Figure 6 and Table II, are associated with (1) the first AE event, (2) the first loud AE event which is defined as an AE event with an energy of at least one tenth the highest energy event not corresponding to failure, and (3) the effective AE onset event which is determined by extrapolation of the high energy noise distribution down to the zero noise axis.…”

Design Guidelines for In‐Plane Mechanical Properties of SiC Fiber‐Reinforced Melt‐Infiltrated SiC Composites

“…The HNS composites [5] show similar normalized retained strengths at 1315 o C (Figure 10d), but maintained the relatively high retained strengths out to larger total strains, and did not show strength reduction in the range of strains explored in that study. Also note that two of the ZMI composites, which were tested at room temperature after creep, failed in the radius of the dogbone (marked with up-arrows in Figure 10a), or in the area of the specimen that was outside the hot zone region of the furnace, and ~800 + 100 o C. Evidently these specimens suffered from intermediate temperature embrittlement [21,22].…”

Stress-dependent matrix cracking in 2D woven SiC-fiber reinforced melt-infiltrated SiC matrix composites

“…For non-oxide composites, such as the SiC/SiC system, the presence of matrix cracks enables oxidizing environments to diffuse into the interior of the composite and cause strength-degradation, especially at intermediate temperatures [4-51. I n addition, for some BN interphase composites, the degree of strength-degradation at intermediate temperatures is related to the number of matrix cracks [6]. Therefore, it is essential that a good understanding of the stress-dependent matrix crack properties of a viable composite system be wellcharacterized.…”

“…For example, the stressdependent matrix crack distribution for the higher epi composites (> 16 epi) was fit with a simple linear relationship for ominimatrix above 95 MPa ( Figure 5): where pc is the stress-dependent matrix crack density. This data was then used to model the G/E curve over the appropriate stress range for two specimens (012 and 068) with similar interfacial shear stress, T, of N 70 MPa (Figure 6) from the approach taken in reference 12 based on references 19 and 20: where the sliding length 6 = a r (a + om) / 22 (4) r is the fiber radius, and…”

“…The stress-dependent matrix cracking behavior at room temperature can be used to model the time-dependent elevated temperature life expectancy at stress [6]. The stress-strain curve of these composites upon initial loading varies only slightly up to 12OOOC [2].…”