PSI - Issue 14

Ashok Saxena / Procedia Structural Integrity 14 (2019) 774–781

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Ashok Saxena/ Structural Integrity Procedia 00 (2018) 000 – 000

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4. Considerations of Microstructural Gradients

Microstructural gradients are common in welded structures that are exposed to elevated temperatures in the creep regime. Weldments are designed to have matched, under-matched or over-matched strengths that can result in significantly differential creep deformation rates due to differences in chemistry of the weld and base metals. In addition, microstructural gradients can result from cooling rates during solidification of the molten metal and transition layers in the form of heat-affected-zones in the parent material. Post-weld-heat-treatments (PWHT) are designed to homogenize/restore the microstructure and release residual stresses, but gradients in the microstructure remain and can have different crack growth properties compared to the parent metal. Figure 5(left) shows the correlation between time rate of crack growth under creep and creep-fatigue conditions in specimens extracted from over-matched and under-matched weldments, as well as from base metal regions. In the weldments, the crack plane of the specimens was chosen to be along the fusion plane as seen in the photomicrograph in Fig. 5 (right). The correlation between crack growth rates and the (C t ) avg parameter used commonly for correlating creep-fatigue crack growth rates is surprisingly good. The rationale for these good correlations must be fully explored.

Fig. 4- Fatigue crack growth rates, da/dN , as a function of ∆K for Inconel 718 at various frequencies at elevated temperatures showing a good correlation with ∆K if the loading frequency and load -ratios are held constant [Floreen and Kane, 1980]

5. Considerations for Directionally Solidified (DS) and Single Crystal (SX) Materials

Directionally solidified and single crystal materials are commonly employed in gas turbine blades to reduce or eliminate grain boundaries that can reduce the creep strength of these materials. The creep deformation behavior in these materials can be orthotropic and thus cause different creep rates in crystals that are oriented differently. Figure 6 (left) [Gardner, Saxena, and Qu, 2001] shows the evolution of the creep zone in a C(T) specimen where there are crystallographic differences between the upper and lower halves of the specimen. This figure shows that the differences between the creep zone evolution in the two halves of the C(T) specimen. Figure 6(right) [Ibanez, Saxena, and Kang, 2006] shows the creep crack growth behavior in DS-GTD-111 at two temperatures. Excellent correlation exists between creep crack growth rates and the C t parameter in spite of the observation that the crack traversed across different grains. A good theoretical basis for such correlation and its limitations must be developed.

6. Conclusions and Recommendations for Future Work

 A rigorous analytical frame work is needed to extend the TDFM concepts to creep-brittle materials and DS and SX materials. For DS and SX materials, we need to account for grain boundaries and difference in