Ceramic Matrix Composites (CMCs) have been used in various high performance fields owing to their non-brittle mechanical attributes and high resistance in severe environments such as high temperature, corrosive atmospheres, etc. However, this potential has been faced with lack of standards and insufficient databases, which limit the implementation of these materials.
For continuous fiber reinforced CMCs, there is an extensive knowledge of the influence of coupon geometry on the determined properties and failure mode in bending tests. However, for the short fiber reinforced CMC, just a few studies have focused on this topic: it has been identified that under shear loading, various coupon sizes of short fiber reinforced carbon/silicon carbide (C/SiC) composites result in varying failure modes; there is no clear tendency of flexural strength as a function of specimen size by three-point (3P) and four-point (4P) bending modes; for tensile property measurements of siliconized short carbon fiber reinforced carbon (C/C-SiC) a dependency on cross section has been reported and the notch sensitivity of same material is extremely low. Due to these varying failure behaviors and attributes, neither the testing standards for CMC with continuous fibers nor for monolithic ceramics are suitable for determining the bending properties of these materials.
Yuan Shi and colleagues at the Institute of Structures and Design at German Aerospace Center (DLR) and at Fraunhofer Center for High Temperature Materials and Design in Germany presented a research paper focusing on the effect of coupon geometry, test conditions and fiber orientation on the average value and distribution of flexural strength of short fiber reinforced CMC. They carried out extensive analyses with varying thickness as well as coupon length under bending load. Their research work is published in Journal of the European Ceramic Society.
Based on the results of previous studies, 3P bending test on short fiber reinforced CMCs is not recommended because the coupon fracture does not always occur under the loading roller, which results in inaccurate computation of the bending strength values. For this reason, the authors adopted 4P bending test with the particular advantage of freedom from shear forces in the mid-span area. The results of the mechanical tests indicated that, no significant influence of different numbers of specimens on the investigated properties can be observed, however, the experimental data show a strong dependence of reliability and a large scatter of strength values of CMC with homogeneously distributed short fiber reinforced in relation to the fiber length and coupon dimensions. The comparison between standard deviation, scatter of the flexural strength and coupon cross section showed a considerable increase of the scatter of the bending attributes at smaller cross-sections. Furthermore, volumetric analysis of the coupons using micro computed tomography indicated that all the coupons had a random distribution of the fiber bundles as well as silicon carbide matrix. A significantly reduced volume of an extracted coupon sample contained locally non-homogeneous distribution of short fibers. Failure occurred in areas containing high silicon carbide content and along fiber bundles that were oriented perpendicular to the length of the coupon, therefore, not exploiting the high strength of the fiber.
The scatter of strength values was important in setting out the minimum threshold size of coupon for testing. An increase in coupon cross section results in a larger volume, which results in an increase in randomization of fiber and matrix distribution. In this way, the effects of local inhomogeneities are not as dominant and lead to a reduced scatter of mechanical properties. For composites with shorter fiber length, the limit of coupon size was double the thickness of fiber length, the width should be at least triple the fiber length and the span-to-thickness ratio should be at least 10. For material with long fibers, it is proposed that the threshold limit of coupon geometry should be as follows: coupon thickness at least equal to fiber length; coupon width at least twice the fiber length; the span-to-thickness ratio is min. 6. The results of the study by Yuan Shi et al. indicate that, with the particularly suitable testing conditions the 4P-bending strength of short fiber reinforced CMCs is a meaningful mechanical characteristic when the mandatory geometry is tested. Then these strength values can be used for prediction and evaluation of load bearing capacity at component level. The presented outputs have been proposed to a future standard for bending test of short fiber reinforced CMC materials with different fiber length.
Shi, J.-M. Hausherr, H. Hoffmann, D. Koch. Inspection of geometry influence and fiber orientation to characteristic value for short fiber reinforced ceramic matrix composite under bending load. Journal of the European Ceramic Society, volume 37 (2017), pages 1291–1303.Go To Journal of the European Ceramic Society