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Developing Surface Characterization Maps for Microturbine Ceramic Rotors

Microturbines a few inches in diameter are critical components in compact cogeneration units that produce electricity. These modular distributed power systems are intended to operate on-site at manufacturing plants as a source of economical and reliable electrical power.  Advanced structural ceramics, such as silicon nitride, enable microturbines to operate at higher temperatures than conventional metal alloys. This translates into significant fuel savings and emissions reductions. However, ceramics exhibit large variations in fracture strength, particularly with flaws related to various surface treatments.  Accounting for these complex statistical strength distributions will lead to more accurate predictions of expected component life.

Two algorithms work in conjunction with one another to provide the probabilistic design approaches required in determining these ceramic reliability predictions.

One of the challenges facing engineers is defining and implementing a method that establishes Weibull distribution metrics for silicon nitride suppliers based on the particular component.  Service stress states from the various treated surfaces of a rotor blade must be combined with a stipulated component reliability to develop material performance curves. These curves must be scaled to standard ceramic bend bar test specimens, making component requirements more readily understood by material suppliers.

Through the use of ANSYS Parametric Design Language (APDL), surfaces of a rotor component with specified finishes are identified, the ANSYS results file is queried and stresses are mapped to the relevant element surfaces. Failure data is analyzed using the WeibPar algorithm. Using information generated by ANSYS (geometry and stress state), the CARES algorithm computes component reliability. The openness of ANSYS technology and the ease of integration with other software enabled the ANSYS, and algorithm programs to operate together in a smooth and efficient manner.

The resulting design approach allows changes and improvements in system requirements to take place readily in parallel with enhancements in material properties. Material characterization maps can now be generated quickly for a given component under specified operating conditions. The information can influence the goals of a ceramic materials development program and better guide engineers toward an optimal design.