PSI - Issue 13

Evgeniia Georgievskaia / Procedia Structural Integrity 13 (2018) 971–975 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

975

5

material, which are determined by experimental method. Thus, the possibility of crack growth under action of high frequency loads can be determined by the formula: Δ K HF ≥ K th . The destruction instant is determined according to the results on modelling of the stage-by-stage crack growth. Considering high level of indefiniteness for external loads, complexity of crack diagnosing at early stages of development and uncertainty of design procedures it is recommended to use the following safety factors:  by crack length n ℓ = 2.0 – 2.5;  by value of fracture toughness n K = 1.5. Figure 3b shows the scheme for determination of allowable crack length, upon achievement of which it is necessary to stop operation of damaged element due to increased risk of its destruction. Considering above mentioned considerations algorithms of durability assessment can be presented as follows: 1. Determination of material crack resistance characteristics K С and K th . 2. Development of finite element 3D-models for considered hydraulic turbine element combining geometric parameters, conditions of mating and fastening of elements, parameters of loading in all operating modes, material properties. 3. Design modelling of cracks considering experience in operation and diagnosing. 4. Determination of stress state for considered element, including local one in the area of crack tip. 5. Construction of design dependence of SIF range from crack length Δ K ( ℓ) due to action of high-frequency loads. 6. Determination of crack length corresponding to threshold value of SIF: ℓ th = ℓ( K th ,). Level of sensitivity for non destructive testing methods used for control shall be so that cracks having length ℓ th confidently defined. 7. Determination of allowable crack length based on crack resistance condition: [ ℓ ] = min{ ℓ 1 /n ℓ ; ℓ 2 }, ℓ 1 = ℓ( K c ); ℓ 2 = ℓ( [ K c ]), [ K c ] = K c / n K .. 8. Determination of allowable number of cycles [ N ] or allowable operation time [ T ] up to achievement of allowable crack length:[ N ] = N ([ ℓ ]) or [ T ] = T ([ ℓ ]). 5. Algorithm of Durability Assessment

6. Conclusion

The proposed approach to design assessment of durability allows considering all required individual features of hydraulic turbine influencing its lifetime , doesn’t specify the excessive requirements for computer facilities, cost and time of design procedures, as well as provides the possibility with the required engineering accuracy to evaluate service life characteristics and plan timely repair or replacement of worn elements.

References

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