Emmanuel Maillet

Development of an Interlaminar Toughness Test for Ceramic Matrix Composites

Interlaminar fracture properties in fiber-reinforced ceramic matrix composites have been found to be a limiting feature in certain applications, a limitation that intensifies at elevated temperatures. Few if any fibers bridge the cracks in the interlaminar direction, resulting in a weaker toughness and a higher probability of failure in this direction. The main challenge in evaluating interlaminar fracture toughness is the difficulty to detect the crack initiation and to measure the crack length directly at high temperature, primarily because of the limited visual accessibility of the area of interest. A DCB test-method using a ceramic wedge is developed. The method uses electrical resistance to monitor crack growth for melt-infiltrated woven SiC/SiC composites at room temperature, with the goal of doing the same at high temperature. In situ optical measurements of crack growth in addition to micro-CT post-inspection were used to validate and calibrate the resistance method. The estimated crack length was in excellent agreement with the measured crack length in the three considered specimen geometries. Preliminary estimates of Mode I energy release rate were also provided.Copyright

High Velocity Impact Damage Assessment in SiC/SiC Composites

Foreign object damage in gas turbines presents serious safety and financial concerns. As the aerospace industry draws closer to implementing ceramic matrix composites (CMCs) in gas turbines, the corresponding behavior in these materials after such events needs to be understood. To address this requirement, several silicon infiltrated fiber reinforced SiC/SiC coupons were impacted with high speed projectiles with velocities up to 360 m/s with an impact rig built at the University of Akron and NAVAIR. The resulting damage states were assessed using several non-destructive evaluation (NDE) techniques and compared to actual damage condition observed through the sectioning of impacted coupons. Ultimately, the true consequence of the damage was revealed by measuring the post-impact, residual strengths via uniaxial tensile tests to failure at both room and elevated temperatures. Lastly, the NDE results revealed a complicated damage morphology consisting of in-and-out-of plane damage that significantly affected the retained mechanical properties.Copyright

5.12 Nondestructive Evaluation – Use of Acoustic Emission for CMCs

Acoustic emission (AE) is a technique which has been used to detect and monitor damage in ceramic matrix composites (CMCs). An overview of AE itself and how it has been used for damage monitoring in CMCs is highlighted. Some specific examples of the use of AE to monitor and assess matrix cracking in CMCs is demonstrated and how that has been used to supplement mechanical behavior modeling and a relationship for life-limiting stress states caused by oxidation at elevated temperatures. Specific techniques for determining the location of AE events and the limitations of sound travel due to attenuation are outlined in addition to the use of frequency content and other classification techniques to try to specify damage mechanisms with AE content.