Blade Root Soft Shoulder Reduces Stress Concentration
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Solution Overview
Problem
Conventional turbine blade root designs experience high stress concentrations due to centrifugal forces, leading to potential fatigue and stress corrosion cracking, limiting the operational life of the blades.
Innovation Solution
A blade root design featuring a plurality of lobes and fillets with a soft shoulder between flanks and fillets, increasing the distance to the rotor disc, which induces compressive stress and reduces tensile stresses at the interface, thereby mitigating stress concentrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional firtree profiles with straight lines and circular arcs are used, then the blade root structure is simple and easy to manufacture, but high stress concentrations occur at the fillets and corners leading to fatigue and stress corrosion cracking
Solution Approach 1:
The patent applies local quality by introducing a soft shoulder with specific radius R2 at the transition zone between the flank and fillet, while maintaining straight lines and circular arcs in other regions. This localized modification reduces stress concentration at the critical interface without changing the overall simple geometry, thereby improving reliability while preserving ease of manufacture.
Solution Approach 2:
The patent changes the geometric parameters by adding the soft shoulder radius R2 and optimizing the fillet radius R1, transforming the conventional sharp transition into a gradual curved transition. This parameter change redistributes the stress field, reducing peak stresses at the fillet-root interface and improving fatigue life while maintaining manufacturability through standard grinding processes.
2Strength
If the blade root is closely fitted to the rotor disc groove, then accurate location and strong centrifugal force resistance are achieved, but high contact stresses occur at the interface leading to stress corrosion cracking
Solution Approach 1:
The soft shoulder introduces a localized geometric feature with radius R2 at the flank-fillet transition, creating a stress-relief zone that reduces contact stresses at the blade-disc interface. This local modification maintains the overall strong fit while eliminating harmful stress concentrations that lead to stress corrosion cracking.
Solution Approach 2:
The soft shoulder acts as a pre-designed stress cushioning element that anticipates and mitigates high contact stresses before they can cause damage. By incorporating the rounded transition zone in advance, the design prevents stress concentration and subsequent stress corrosion cracking rather than addressing it after failure occurs.
3Reliability
If fillets with small radius are used to reduce stress concentration, then fatigue life is improved, but the blade root geometry becomes more complex and manufacturing precision requirements increase
Solution Approach 1:
Instead of reducing fillet radius everywhere, the patent applies a localized soft shoulder with radius R2 only at the critical flank-fillet transition zone. This targeted approach improves fatigue life by reducing stress concentration precisely where needed, while maintaining larger radii in other areas that are easier to manufacture with standard precision.
Solution Approach 2:
The patent applies partial action by introducing the soft shoulder modification only at the specific location where stress concentration occurs most severely, rather than modifying the entire blade root geometry. This selective approach achieves fatigue life improvement without unnecessarily increasing overall manufacturing complexity and precision requirements.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design enhances the fatigue life of the blade roots by distributing loads and reducing stress levels at the contact area between the blade root and the disc, leading to increased operational cycles without failure.
Implementation Method 1
A soft shoulder is provided between the flanks and the fillets to increase the distance to a corresponding lobe of a rotor disc, into which a blade with such a blade root is inserted. The convex surface section and a region of the concave fillet surface section adjoining the convex surface section form a local recess... which induces compressive stress and reduces tensile stresses at the interface
Implementation Method 2
These stresses occur during operation caused by centrifugal forces affecting the blades—the centrifugal load being dependent on the mass of the whole blade
Data Source
AI summary
A blade root including of a plurality of lobes and fillets and flanks in between is provided. A shoulder is provided between the flanks and the fillets to increase the distance to a corresponding lobe of a corresponding rotor disc into which a blade with such a blade root is inserted. A rotor blade having such a blade root is also provided. Furthermore this feature may alternatively or additionally also be applied to a rotor disc slot of a rotor disc, such that a flank of the rotor disc slot merges into a fillet of the rotor disc slot via a soft shoulder to increase the distance to a corresponding lobe of a blade root. A shoulder could also be applied to both the blade root and the corresponding slot of the rotor disc.