Compressor Wheel Bore Cold Working for Low-Cycle Fatigue Life
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Solution Overview
Problem
Centrifugal compressor wheels in turbochargers often fail due to low-cycle fatigue (LCF) cracks starting at the surface of the through bore, with existing treatments failing to effectively manage stresses and extend life.
Innovation Solution
A cold working process is applied to a fractional portion of the bore length of the compressor wheel, inducing compressive residual hoop stresses in the metal, while avoiding high-stress areas to prevent deleterious stress overlap, using mechanical or non-mechanical tools to apply radially outward pressure and create a zone of compressive stress along the bore.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If cold working is applied to the entire bore length, then compressive residual stresses are created throughout the bore, but deleterious overlapping stresses occur at high-stress areas such as blade root fillets, reducing the benefit of cold working
Solution Approach 1:
The patent applies cold working only to a fractional portion of the bore length (typically 50-75% of the total axial length), specifically avoiding the regions adjacent to blade root fillets and other high-stress areas. This localized approach creates beneficial compressive residual stresses in the critical bore regions while preventing harmful stress overlap at the high-stress features, thereby resolving the contradiction between maximizing compressive stress benefits and avoiding detrimental stress concentrations.
2Strength
If cold working is applied to high-stress areas such as blade root fillets, then compressive residual stresses are created in those areas, but the cold working negatively impacts the wheel's overall life due to stress overlap
Solution Approach 1:
The patent specifically excludes high-stress areas such as blade root fillets from the cold working zone by limiting the treatment to a fractional portion of the bore length. This selective approach ensures that beneficial compressive stresses are created in the bore regions that experience high cyclic loading, while avoiding the introduction of harmful residual stresses at the blade root fillets and other critical high-stress features, thereby maintaining wheel reliability and extending service life.
3Strength
If the bore is cold worked along the full axial length, then maximum compressive stress coverage is achieved, but the variable wall thickness and high stress features around the impeller bore create deleterious overlapping stresses
Solution Approach 1:
The patent addresses the complexity of variable wall thickness and high-stress features by applying cold working only to a fractional portion of the bore length that excludes the regions adjacent to these complex features. This localized treatment strategy achieves sufficient compressive stress coverage in the critical bore regions without introducing harmful stress overlaps at the variable wall thickness zones and high-stress features, thereby simplifying the overall stress state of the impeller.
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 process significantly extends the low-cycle fatigue life of the compressor wheel by creating beneficial residual compressive hoop stresses, reducing the risk of crack initiation and improving overall wheel durability.
Implementation Method 1
cold working the metal at an inner surface of the bore beyond the tensile yield strength in a hoop-wise direction about the bore
Implementation Method 2
induce compressive residual hoop stresses in the metal adjacent the inner surface along said fractional portion
Data Source
AI summary
A process for cold working of the inner surface of a bore in a centrifugal compressor wheel along only a fractional portion of the bore length (i.e., along less than a full axial length of the bore), thereby creating a zone of compressive residual hoop stress in the metal surrounding the bore where the wheel needs the beneficial residual stress. The process purposefully avoids cold working of the bore at locations adjacent to high-stress areas and features of the wheel, where cold working in such locations could negatively impact the wheel's overall life.