Nitinol Cryojoint Design for Fatigue Resistance
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Solution Overview
Problem
Metallic implantable devices face fatigue failures due to stress concentrations caused by sharp dimensional changes, which are difficult to predict and lead to premature failure under cyclical loading.
Innovation Solution
A design that reduces stress concentrations by creating circumferential grooves or indentations in cylindrical components and using a phase-changing material to form a compressive joint, such as the cryojoint design, which utilizes Nitinol's shape memory property to create a uniform and robust connection with reduced strain and distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If traditional crimping or swaging methods are used to join cylindrical components with a tube, then strong mechanical connection is achieved, but stress concentrations occur at the joint leading to fatigue failure
Solution Approach 1:
The patent applies parameter changes by utilizing the phase transition temperature of Nitinol material. The tube is heated to austenite phase (e.g., 50-100°C) where it achieves elastic properties, allowing mechanical expansion without permanent deformation. After assembly, cooling to martensite phase enables the tube to recover elastically and compress onto the cylindrical components, creating a strong interference fit without stress concentrations that would lead to fatigue failure.
Solution Approach 2:
The patent directly employs phase transitions of Nitinol between austenite and martensite phases. During assembly, the tube is heated to transform to austenite phase, expanded mechanically to accept components, then cooled to transform back to martensite phase. This phase transition enables the tube to undergo reversible elastic deformation, creating a compression fit that joins components strongly while avoiding the stress raisers and fatigue failures associated with traditional plastic deformation methods.
2Ease of manufacture
If the tube is mechanically expanded to accept cylindrical components, then assembly is simplified, but permanent deformation and stress concentrations are created
Solution Approach 1:
The patent changes the temperature parameter of the Nitinol tube during assembly. By heating the tube to austenite phase temperature before expansion, the material achieves superior elastic properties that allow large reversible deformations. The tube is expanded mechanically to accept components, then cooled to martensite phase where it elastically recovers, creating compression fit without permanent deformation or stress concentrations that would compromise tube integrity.
Solution Approach 2:
The patent replaces the traditional mechanical expansion system with a thermally-assisted expansion system. Instead of relying solely on mechanical force to permanently deform the tube, the invention uses thermal energy to transform the material phase, enabling the tube to expand and contract elastically. This substitution of mechanical deformation with thermally-induced elastic deformation simplifies assembly while preserving tube integrity by avoiding permanent stress concentrations.
3Strength
If traditional crimping methods are used to create interference fit, then strong connection is achieved, but difficult-to-predict fatigue failures occur due to stress raisers
Solution Approach 1:
The patent changes the material phase parameter during assembly to achieve a fundamentally different stress distribution. By heating the Nitinol tube to austenite phase and then cooling it after expansion, the tube undergoes elastic recovery that creates uniform compression fit. This eliminates the stress raisers and sharp dimensional changes inherent in traditional crimping methods, creating predictable stress fields that do not lead to difficult-to-predict fatigue failures while maintaining strong connection strength.
Solution Approach 2:
The patent converts the typically harmful permanent deformation and stress concentrations of traditional crimping into beneficial elastic deformation and uniform compression. By utilizing the phase transition properties of Nitinol, the invention transforms what would normally be damaging plastic deformation into reversible elastic recovery, creating a connection that is both strong and free from the stress raisers that cause unpredictable fatigue failures.
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 solution effectively reduces stress concentrations and enhances fatigue resistance, maintaining tensile and torsional strengths comparable to crimped joints while improving cyclic fatigue resistance, particularly in hemodynamically loaded devices like heart valves.
Implementation Method 1
using a tube that is formed of a phase-changing material, bringing the temperature of the tube to a point where the tube achieves an elastic phase, mechanically expanding the tube to accept the ends of the cylindrical components, and then allowing the tube to resume a temperature at which the tube is no longer in the elastic phase
Implementation Method 2
This is being referred to herein as a cryojoint design. The invention may also have application for rotating solid shafts.
Data Source
AI summary
A method of reducing the concentration of stress that occurs when joining two cylindrical component ends with a tube. Grooves are placed near the ends of the cylindrical components that alleviate the stress/strain felt by the cylindrical components when undergoing flex cycling.