A combined NDT/Finite element technique to study the effects of matrix porosity on the behavior of ceramic matrix composites

Ceramic matrix composites are being considered as candidate materials for high temperature aircraft engine components to replace the current high density metal alloys. Ceramic matrix composites are engineered material composed of coated, two dimensional, woven, high strength fiber tows and melt infiltrated ceramic matrix. Matrix voids are commonly generated during the melt infiltration process. The effects of these matrix voids are usually associated with a reduction in the initial overall composite stiffness and a decrease in the thermal conductivity of the component. Furthermore, the role of the matrix, as well as the coating, is to protect the fibers from the harsh engine environment. Hence, the current design approach is to limit the design stress level of ceramic matrix composite components to always be below the first matrix cracking stress. In this study, the stress concentrations around observed macroscopic matrix voids are calculated using a combined nondestructive testing (NDT)/finite element scheme. Computed tomography is utilized as the NDT technique to characterize the initial macroscopic matrix void's locations and sizes in a ceramic matrix composite tensile test specimen. The finite element is utilized to calculate the localized stress field around these voids, based on the two dimensional computed tomography images. The same specimen was also scanned after tensile testing to a maximum nominal stress of 150 MPa (21756 lb/in.(2)) to depict any growth of the previous observed voids. The computed tomography scans taken after testing depicted an enlargement and some coalescence of the existing voids.