Self-Expanding Shape Memory Alloy Fastener for Fatigue Life
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Solution Overview
Problem
Aerospace structures face challenges in joining structural components due to stress concentration at unfilled holes caused by non-interference fasteners, which reduces fatigue life, while interference fit fasteners require specialized insertion and do not create stress raisers but are not commonly used.
Innovation Solution
Self-expanding fasteners made of shape memory alloys that transform from a smaller diameter martensite phase to a larger diameter austenite phase upon heating, filling the hole and creating compressive residual stresses to enhance fatigue life, allowing easy insertion and plastic deformation of the fastener and structural components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If non-interference fasteners are used, then ease of insertion is improved, but stress concentration at hole edges increases reducing fatigue life
Solution Approach 1:
The fastener employs a dynamic shape change mechanism using shape memory alloy material that transitions from a compressed low-temperature state (easy insertion) to an expanded high-temperature state (filling the hole). This dynamic transformation allows the fastener to adapt its dimensions based on operational requirements, resolving the contradiction between ease of insertion and fatigue life.
Solution Approach 2:
The invention utilizes temperature as a controlling parameter to change the physical state of the shape memory alloy fastener. By heating the fastener above its transformation temperature, it expands from a compressed state to a larger diameter state that fills the hole, thereby eliminating stress concentration while maintaining ease of insertion during the compressed state.
2Reliability
If interference fit fasteners are used, then stress concentration is reduced improving fatigue life, but device complexity increases due to specialized insertion requirements
Solution Approach 1:
The fastener exploits the phase transition properties of shape memory alloy between martensite (low temperature, deformable) and austenite (high temperature, rigid) phases. This phase transition enables the fastener to be easily inserted in the martensite phase and then transform to the austenite phase to create the interference fit, eliminating the need for specialized insertion equipment while maintaining fatigue life benefits.
Solution Approach 2:
The invention replaces complex mechanical insertion systems (such as freeze insertion or specialized tools) with a thermal activation system. By using temperature-controlled phase transformation, the fastener achieves interference fit capabilities without requiring complex mechanical insertion devices, thereby reducing overall system complexity.
3Reliability
If shape memory alloy fasteners are used, then fatigue life is improved by filling holes, but manufacturing precision requirements increase
Solution Approach 1:
The shape memory alloy fastener's dynamic expansion capability allows it to adapt to a range of hole sizes. The fastener is inserted in a compressed state through larger tolerance holes and then expands to fill the hole, eliminating the need for high-precision hole machining. This dynamic adaptation resolves the contradiction between fatigue life improvement and manufacturing 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 self-expanding fasteners fill the holes, reduce stress concentration, increase fatigue life, and provide strong, lightweight joints without the need for high-tolerance holes, while being easily inserted and locked into place, enhancing the structural integrity and ease of assembly.
Implementation Method 1
The fastener comprises a shape memory alloy which was first formed into a first shape having a diameter slightly larger than a diameter of the first and the second holes of the joint when the shape memory alloy was in its higher temperature austenite phase. This fastener is formed to a second shape having a diameter less than the diameter of the first and second holes of the joint when the shape memory alloy is in the lower temperature martensite phase.
Implementation Method 2
Heating the element converts martensite into austenite, thereby causing the state transformation. The state transformation causes a longitudinal contraction and/or an expansion in other directions of the element.
Implementation Method 3
Continuing to heat the fastener through the temperature range from the austenite start to austenite finish and beyond allows the fastener to first initiate plastic deformation in the structural components that it is joining, and finally, being plastically deformed itself.
Data Source
Figure 1~2
Figure 3
Figure 4A~5B
AI summary
In one embodiment a method to join a first structure (410) to a second structure (415), comprises aligning a first hole in the first structure with a second hole in the second structure and inserting a fastener (420) into the first hole and the second hole. The fastener (420) comprises a shape memory alloy which was formed into a first shape having a diameter slightly larger than a diameter of the first hole and the second hole when the shape memory alloy was in an austenite state and reduced to a second shape having a diameter less than the diameter of the first hole and the second hole when the shape memory alloy was in a martensite state. The method further comprises heating the fastener above an austenite start transition temperature, such that the fastener initiates free expansion from the second shape back to the first shape and establishes physical contact with the surface of the first hole and the second hole and continuing to heat the fastener such that first structure and the second structure proximate the holes and the fastener undergo plastic deformation.