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Advanced ceramics provide numerous advantages compared to other materials: durability, hardness, high mechanical strength at high temperature, stiffness, low density, electrical insulation and conductivity, thermal insulation and conductivity, etc. Most ceramic materials are almost entirely immune to corrosion.
Ceramics show a different fracture behavior than that of metals and polymers because of their brittle nature. Therefore, ceramic engineers should be aware of this behavior when designing reliable ceramic components. The design challenge for the use of ceramic materials in technical applications is the computation of their failure probability under prescribed boundary and loading conditions.2
The Weibull TheoryCeramic materials exhibit strength scatter, and the failure of a ceramic begins with pre-existing defects in the material. The material's strength therefore depends on the size of the most critical flaw in the material, which varies from specimen to specimen. Essentially, the scatter of the ceramic strength is caused by the scatter of the flaw size.
Since the strength of ceramics is not a constant value, design with ceramic materials cannot be done deterministically. Instead, a probabilistic approach should be used in which a ceramic component's failure probability is calculated. More than 70 years ago, Weibull derived a statistical theory of brittle fracture.3-4 Since that time, the Weibull distribution function has become the most widely used function in the mechanical design of ceramic components.
The simplest form of the so-called Weibull function5 is given in Equation 1 for a uniaxial homogeneous tensile stress state, where PF,V(σ, V) is the failure probability of a ceramic component due to the volume flaws; V is the volume of the component; V0 is the unit volume containing average number of cracks; σ is uniaxial applied stress; m is the Weibull modulus, which describes the scatter of the strength; and σ0 is the cumulative mean stress, at which the failure probability is 63.2% for a specimen with a volume V = V0.
In general, the stress distribution within a ceramic component is not constant but takes different values at different positions. In addition, the true orientation of the cracks in ceramic components is randomly distributed. As a result, normal and shear stresses acting in the crack plane cause Mode-I, Mode-II and Mode-III loading of the cracks. A proper account of the spatial variation in the stress triaxiality (e.g., effect of shear stresses) must therefore be taken using a multiaxial failure criterion.6-8 In this article, the normal stress criterion will be used for the computation of failure probability.9
Stirling EnginesThe Stirling engine is a thermal engine in which the displacing piston separates the expansion area from the compression area. The displacing piston shifts the work gas from the hot area to the cold area through thin tubes. To be effective, the piston must fulfill two requirements: its contact surface with the cylinder should be gas-tight, since the flow of gas reduces the efficiency; and it should exhibit low heat conductivity, since a large temperature difference exists between the cold and hot areas.
In this work, a reliable design of a displacing piston will be applied (taking into account the second requirement). The heat loss of the displacing piston, the friction between the body of the engine, and the displacing piston will not be considered.
Under these conditions, due to the low heat conductivity and high temperature stability, 3 mol % yttria-stabilized zirconia is particularly suitable for the production of the displacing piston.10 In addition to low heat conductivity, properties such as better wear resistance and lower density compared to conventionally used material (e.g., chromium-nickel stainless steel), enable the use of 3 mol % yttria-stabilized zirconia to offer many advantages in this application. The geometry and the boundary conditions of the displacing piston have been realized according to a beta series Stirling engine,11 as represented in Figure 1, where Th is the temperature of the hot part, Tc is the temperature of the cold part, Ph is the pressure of the hot part, and Ph is the pressure of the cold part.
FEM ModelIn general, Th = 873 K, Tc = 333 K, Ph = 17.67 bar, and Pc = 3.2 bar. The diameter of the displacing piston is Ddp = 55 mm and Hdp = 80 mm. The technical drawing and the geometric variables of the displacing piston are given in Figure 2(a), where t1 = 12.5 mm, t2 = 20 mm, t3 = 10 mm, d1 = 24 mm, d2 = 36 mm and d3 = 24 mm. Using these variables, a parametric study was performed, and the form was optimized related to minimum failure probability.
The stress analysis was performed by ABAQUS, a commercial finite element program, and the failure probability of each model was calculated by STAU (STatistische AUswertung), a post-processor for ABAQUS. STAU was developed by the Probabilistic Group at the IZSM at Karlsruhe University, in cooperation with several partners.M12
The loading and boundary conditions used in FEM analysis are shown in Figure 2(b). The temperature boundary conditions were Th = 900 K and Tc = 300 K. The mechanical boundary conditions were Ph = 18 bar, Pc = 3 bar and Pps = 1100 bar, which is the pre-stressing due to the used screw. In the calculations, an M10 screw was selected, which produces a pre-stressing force of 26 kN according to the strength class of 8.8.
As the ceramic material, 3 mol % yttria-stabilized zirconia was used due to its low heat conductivity, high-temperature strength and good Weibull parameters. The material parameters of 3 mol % yttria-stabilized zirconia used for the FEM and calculation of failure probability include: Young's Modulus E = 210 GPa, Poisson's ratio υ= 0.3, Weibull modulus m = 20, mean strength σ0 = 600 MPa, heat conductivity λ = 1.5 W/mK and thermal expansion coefficient α30-1000 = 12.5 x 10-6 K-1.
Results and DiscussionThe stress distribution in the displacing piston with the boundary and loading conditions given in Figure 2(b) is shown in Figure 3. In order to find the optimized form, all geometrical variables given in Figure 2(a) were changed one by one, and the effects of the variables on the failure probability were investigated.
According to the results, it was observed that the parameters t1 and t2 have the most significant effects on the failure probability. Thus, the failure probability of the ceramic displacing piston was calculated for t1 intervals of 2.5-15 mm and t2 intervals of 10-65 mm. The failure probability values for different combinations of t1 and t2 are shown in Figure 4.
The region where the failure probability is PF,V > 10-4 is shown with a red color, since it is not acceptable for practical applications. According to these results, a displacing piston with t1 = 2.5 mm and t2 = 55 mm gives the lowest failure probability, which is PF,V = 2.7 x 10-7. As previously stated, the main aim of this work was to find the optimum shape of a zirconia displacing piston according to the calculation of the probability of failure, which occurs due to the stresses that are mainly caused by the temperature difference between the hot and cold part of the displacing piston. From a fracture mechanics point of view, it is possible to use 3 mol % yttria-stabilized zirconia for the displacing piston in a Stirling engine; by changing the shape, the failure probability can be reduced to an order of 10-6.
Next, the temperature of the hot reservoir was increased from 900 to 1100 K, and the failure probability of the optimized form was calculated. Here, it is important to take into account the strength change of the 3 mol % yttria-stabilized zirconia with the temperature for the calculation of failure probability. Figure 5 illustrates the strength of the 3 mol % yttria- stabilized zirconia as a function of service temperature.13
It can be seen that increasing the temperature from 900 to 1100 K did not influence the strength of the 3 mol % yttria-stabilized zirconia. Therefore, the same Weibull parameters can be used for this calculation. When the temperature of the hot reservoir is increased by 200 K, the maximum theoretical efficiency of the Stirling engine increases from 66.7% to 72.7%, which results in an approximately 9% increase in efficiency.
The failure probability of the optimized form with t1 = 2.5 mm and t2 = 55 mm is equal to PF,V = 1.4 x 10-5. This means that increasing the temperature by 200 K increases the efficiency while keeping the displacing piston in the reliable region. In the future, a more comprehensive reliability analysis should be done by taking into account the heat loss of the displacing piston and the friction between the body and the displacing piston.
For more information, contact the author at (90) 0342-2116789; fax (90) 0342-2116677; or email firstname.lastname@example.org.
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