Composite Tank Joint Softening Strip for Cryogenic Stress
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Solution Overview
Problem
Composite cryogenic tanks face issues with peak stresses in bonded joints due to line loads, leading to potential leaks and thermally-induced stresses, as conventional softening strips harden at low temperatures and have incompatible thermal expansion coefficients with tank components.
Innovation Solution
A softening strip made of three-dimensional fabric with graphite fibers coated in a compliant fluoropolymer binder, which remains flexible and stable at cryogenic temperatures, is integrated into the joints to linearize stress and reduce peak shear stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a conventional softening strip made of rubber is used in the joint, then the joint strength is improved at ambient temperatures, but the softening strip hardens at cryogenic temperatures and loses its stress-controlling capability
Solution Approach 1:
The patent changes the material parameters of the softening strip by using a fluoropolymer binder instead of conventional rubber, which maintains its soft and compliant properties at cryogenic temperatures. This parameter change ensures the softening strip remains effective in controlling shear stress at low temperatures while maintaining joint strength.
Solution Approach 2:
The patent employs a composite material structure consisting of three-dimensional fabric reinforcement combined with a fluoropolymer binder. This composite material provides both the structural integrity needed for joint strength and the low-temperature flexibility required for cryogenic applications, resolving the contradiction between strength and temperature performance.
2Stress or pressure
If a softening strip material with high flexibility is used, then the stress linearization capability is improved, but the coefficient of thermal expansion becomes incompatible with composite tank components
Solution Approach 1:
The patent selects a fluoropolymer binder with specific physical and chemical parameters that provide both the desired flexibility for stress linearization and a coefficient of thermal expansion compatible with composite tank components. This careful parameter selection resolves the contradiction between stress distribution and thermal expansion compatibility.
3Reliability
If the softening strip material is made highly compliant to reduce peak stresses, then the joint reliability is improved, but the material must remain stable at high curing temperatures of thermosetting resins
Solution Approach 1:
The patent chooses a fluoropolymer binder with a high service temperature and a glass transition temperature above the curing temperature of the thermosetting resin. This parameter selection ensures the binder remains stable during the curing process while maintaining its compliant properties at operating temperatures, thus improving joint reliability without compromising curing stability.
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 peak stresses and thermally-induced stresses in the joints, ensuring the structural integrity and leak resistance of composite cryogenic tanks at extreme temperatures.
Implementation Method 1
The plastic binder may be a fluoropolymer exhibiting elastic compliance at temperatures below approximately −150° F.
Implementation Method 2
The softening strip linearizes the stress in the joint caused by the transfer of line loads through the joint
Implementation Method 3
The plastic binder exhibits stability at temperatures up to approximately 400° F.
Data Source
AI summary
A composite resin tank includes a wall joined to a dome along a joint. A softening strip is located in the joint.