MultiMechanics Blog

If You Can't Stand the Heat: Advanced Analysis of Ceramic Matrix Composites

Posted by MultiMechanics on Apr 21, 2015 1:20:21 PM

"Everything has its limit--iron ore cannot be educated into gold" - Mark Twain

While Mr. Twain - in his witful ways - has a point, the response of a materials engineer might be: "you could use reinforcements to give it similar behaviors".  

There is no better better example of property manipulation than looking at the development of Ceramic Matrix Composites. CMC's are improving the performance of some of the most complex and important apparatus on the planet.

10-salvaged_cracked_plate_piecesFor instance - a heat shield on a space vehicle will experience temperatures around 1500°C for a few minutes during reentry. Iron and Steel melt around 1400°C (not too mention they are very heavy). For extremely high temperature applications, the only structural materials available are ceramics.

However, as anyone who has dropped a dinner plate knows, ceramics are bad at withstanding abrupt disturbances like brunt impact, reentry vibrations, and thermal shock. In come Reinforce Ceramics or Ceramic Matrix Composites. By combining the tough, thermally resistant properties of ceramics with a more ductile and tenacious reinforcement (like carbon fiber), you can adequately handle thermal shocks and vibrations and mitigate the growth of cracks.

Types and Uses of CMCS

Aerospace is currently the largest application of ceramic matrix composites. They are used in the industry due to their damage tolerance, fracture toughness, and ability to withstand (and not deform) at high temperatures, wear and corrosion resistance.  The market is projected to grow at a CAGR of 14.06% between 2014 and 2019. 

In the propulsion area, gas turbines have long been dominated by the use of nickel-based superalloys and titanium alloys. Engine manufacturers are now taking a closer look at CMCs for use in engine sections that experience extremely high heat (and can theoretically perform better, the hotter they can get). Silicon carbide-based composites can handle temperatures to 1200°C while reducing weight and cooling requirements, resulting in reduced fuel burn and improved performance. For instance, GE's LEAP engine uses SiC-reinforced CMC parts that don’t require cooling, enabling air that would normally be diverted (to keep superalloy components from melting) to be used to generate thrust.

leap

Supersonic and hypersonic flight vehicles present unique challenges for primary hot structural materials, and ultrahigh-temperature ceramics (UHTCs) have been emerging as a promising class of materials for leading edges for hypersonic vehicles. 

Reinforced carbon–carbon (RCC), has long been used in the nose cap and other areas of the space orbiter. During reentry, temperature in these areas exceeds 1,260 °C (2,300 °F). These materials, however are expensive and have high maintenance requirements. 

Oxide/oxide composites are the fastest-growing segment of ceramic matrix composites - the consumption of which is projected to reach $0.78 billion by 2019. North America is currently the largest consumer for oxide/oxide composites and is estimated to grow at a CAGR of 13.50% between 2014 and 2019.  These composites are low-cost alternative for silicon carbide/silicon carbide composites and allow OEMs to customize light-weight and high-temperature resisting composites for their applications. [1]

Predicting Behavior

Thermal_protection_system_spaceshuttle

Predicting the behavior of CMC's has proven to be difficult, and has been the subject of much research and discussion [2-4].

In the realm of multiscale analysis, most analysis softwares have difficulty capturing local irregularity and randomness when modeling parts, saying "in brittle ceramics composites (CMC), the microcracks are often randomly distributed and characterization of their interface properties is difficult. In this case, the use of fine scale models may not be desirable" [5]. This challenge comes from the fact that they employ "model reduction approaches for periodic heterogeneous media", in order to reduce computational complexity. The problem is that very few advanced materials are actually periodic in nature.

To account for this defficiency, these tools use stochastic methods to introduce randomness into their periodic models. But identifying the boundaries of that randomness simply adds another variable to the modeling fold. The use of these stochastic tactics is a little like trying to educate an iron-ore software into being a gold-standard one. That said, there are techniques being developed that can use the use of topology or a material to characterize stochastic microstructures. This could improve these approaches [6].

Another complication with CMCs is their damage mechanisms. Damage for these materials include multiple matrix cracking, followed by fiber fracture and pullout. In these cases it is extremely important to model the frictional bonding between the matrix and fiber.

ceramic matrix crackMultiMechanics believes that accurate modeling of material microstructures is possible without significant computational overhead. For CMCs, MutiMech offers unique approaches for the distribution of material heterogeneities and non-uniformities within a part. Further MultiMech's proprietary "crack insertion" and "cohesive zone modeling" technologies mean that the fiber-matrix interactions can be easily captured.

Numerical techniques will always involve some underlying assumptions and statistical techniques, but the more realistic you can model your part and the more physics-based your approach, the better.

That reminds us of another Twain quip: "Facts are stubborn, but statistics are more pliable."

References

1. http://www.marketsandmarkets.com/PressReleases/ceramic-matrix-composites.asp
2. http://www.sbir.gov/sbirsearch/detail/415493
3. https://www.sbir.gov/sbirsearch/detail/689177
4. https://www.sbir.gov/sbirsearch/detail/381175
5. https://books.google.com/books?id=sXuwAAAAQBAJ
6. http://www2.lbl.gov/ritchie/Library/PDF/2014_Cox_AnnRevMatRes_StochasticVirtualTests.pdf

  MultiMechOpener

Topics: Composite Analysis, Ceramic Matrix Composites