SMA-FRP Repair Patch for Localized Steel Crack Prestressing
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Solution Overview
Problem
Traditional methods for repairing fatigue cracks in steel structures require heavy equipment and irreversible modifications, and prestressing fiber-reinforced polymer (FRP) patches often necessitate complex fixtures, limiting their practicality for localized crack repair.
Innovation Solution
A self-stressing shape memory alloy (SMA)/FRP composite patch uses NiTiNb SMA wires embedded in an FRP overlay to apply prestressing forces by harnessing the shape memory effect, eliminating the need for heavy equipment and allowing for localized crack repair without permanent modifications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional methods (bolting, welding, peening) are used to repair fatigue cracks, then crack propagation can be halted, but heavy equipment is required and permanent irreversible modifications are made to the structure
Solution Approach 1:
The SMA wires automatically generate prestressing force through their inherent shape memory effect when heated, eliminating the need for external hydraulic jacks, pumps, and complex fixtures. The system serves itself by converting thermal energy directly into mechanical prestress without requiring heavy equipment or manual intervention.
Solution Approach 2:
The patent replaces the traditional mechanical prestressing system (hydraulic jacks, threaded rods, complex fixtures) with a thermally-activated material system (SMA wires). The shape memory effect converts thermal energy directly into mechanical force, substituting a complex mechanical apparatus with a simpler thermal-mechanical material response.
2Reliability
If prestressed FRP patches are applied to halt crack propagation, then fatigue life is extended, but complex fixtures and heavy equipment are required for prestressing
Solution Approach 1:
The SMA wires embedded in the FRP patch automatically generate and maintain prestressing force through their shape memory effect. When heated, the wires recover their pre-strained configuration, generating compressive forces in the patch without requiring external prestressing equipment. The system self-regulates and maintains the prestress throughout the service life of the structure.
Solution Approach 2:
The patent utilizes the temperature-dependent phase transformation of SMA wires to generate prestressing force. By changing the temperature parameter (heating the SMA wires above their transformation temperature), the material undergoes a phase change from martensite to austenite, causing it to recover its pre-strained shape and generate compressive forces in the FRP patch.
3Adaptability or versatility
If localized patching is performed at discrete crack locations, then targeted repair is achieved, but conventional prestressing techniques are not suitable for localized application
Solution Approach 1:
The patent divides the repair system into discrete, localized modules (FRP patches with embedded SMA wires) that can be independently applied at specific crack locations. Each patch is a self-contained unit that can be installed separately without requiring global prestressing of the entire structure, enabling targeted repair of individual cracks while leaving other areas unaffected.
Solution Approach 2:
The localized patches are self-sufficient, generating their own prestressing force through embedded SMA wires that respond to thermal activation. Each patch operates independently without requiring connection to or support from other patches or heavy external equipment, making the system highly adaptable for localized application at discrete crack locations.
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 SMA/FRP composite patch effectively generates and sustains prestressing forces, significantly increasing the fatigue life of cracked steel members by reducing stress ratios and halting crack propagation, with a scalable solution that can generate up to 12 kN of prestressing force using simple activation methods.
Implementation Method 1
Shape memory alloys (SMAs) are a unique class of material which exhibit unique thermo-mechanical characteristics. Below a critical temperature, application of a mechanical strain induces a reorientation of the crystal structure from 'twinned' martensite to 'detwinned' or oriented martensite. This reorientation results in a built-in deformation upon unloading similar to yielding of traditional metals. Upon heating to a level above a critical temperature, the crystal structure undergoes another change from a martensitic to an austenitic state. This transformation relieves the built-in strain, thereby allowing the SMA to return to its undeformed shape.
Implementation Method 2
Upon heating to a level above a critical temperature, the crystal structure undergoes another change from a martensitic to an austenitic state. This transformation relieves the built-in strain, thereby allowing the SMA to return to its undeformed shape.
Implementation Method 3
A similar transformation cycle, which is induced by application and removal of stress above a critical temperature, is responsible for the 'super-elastic' effect of SMAs.
Data Source
AI summary
A self-stressing shape memory alloy (SMA)/fiber reinforced polymer (FRP) composite patch is disclosed that can be used to repair cracked steel members or other civil infrastructures. Prestressed carbon FRP (CFRP) patches have emerged as a promising alternative to traditional methods of repair. However, prestressing these patches typically requires heavy and complex fixtures, which is impractical in many applications. This disclosure describes a new approach in which the prestressing force is applied by restraining the shape memory effect of nickel titanium niobium alloy (NiTiNb) SMA wires. The wires are subsequently embedded in an FRP overlay patch. This method overcomes the practical challenges associated with conventional prestressing.