Composite Fastener Bushing for High-Load Thermal Joints
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Solution Overview
Problem
Mechanical connections for assembling thermostructural composite materials face challenges in applying high pre-loading forces without damaging the composite parts, especially when exposed to high temperatures, due to differences in thermal expansion coefficients and aerodynamic requirements.
Innovation Solution
A fastener system with a bushing having a conical collar that increases the bearing area, allowing for higher tightening forces while reducing crushing pressure and shear stress, and includes a spacer and washer to manage thermal expansion and maintain connection integrity at high temperatures, using standard screws and materials with high thermal expansion coefficients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If a fastener element with a flat head is used to make mechanical connections between composite parts, then aerodynamic performance is improved, but the pre-loading force must be limited to avoid damaging the composite material
Solution Approach 1:
A bushing with a conical collar is introduced as an intermediary element between the fastener head and the composite part. The collar presents an inside face that receives the fastener head and an outside face of conical shape that bears against the countersink in the composite part. This intermediary bushing distributes the pre-loading force over a larger bearing area, allowing high tightening forces to be applied without crushing or shearing the composite material, while the fastener head remains flush with the surface for aerodynamic performance.
2Strength
If a fastener element is used to assemble composite parts, then mechanical connection is achieved, but the difference in thermal expansion coefficients causes connection integrity issues at high temperatures
Solution Approach 1:
The bushing is made from a material whose coefficient of thermal expansion is intermediate between that of the composite part and the fastener element. This allows the bushing to accommodate differential thermal expansion at high temperatures, maintaining connection integrity by absorbing the expansion mismatch rather than transmitting it to the composite part or fastener element.
3Stress or pressure
If the bearing area between fastener head and composite part is increased, then crushing pressure is reduced, but the device complexity increases
Solution Approach 1:
The fastening function is segmented into multiple components: the fastener element (screw or rivet), the bushing with conical collar, and the washer. The bushing collar specifically addresses the bearing area issue by distributing the load, while the washer manages thermal expansion. This segmentation allows each component to perform its specific function efficiently without requiring a complete redesign of the entire fastening system.
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
Enables increased tightening forces without damaging the composite parts, maintains aerodynamic performance, and compensates for thermal expansion, allowing for reliable connections in high-temperature environments.
Implementation Method 1
the outside face of the collar bears against a countersink formed in the first part... the bearing area of the head of the fastener element against the composite material part is increased. This increase in bearing area serves to reduce the crushing pressure and the inter-lamination shear stress in the part
Implementation Method 2
a fastener system having a fastener element with a head from which there extends a shank... the fastener system also having a bushing... the assembly being characterized in that the fastener system comprises a screw... and a nut... the collar presenting an inside face of conical shape that receives the flat head of the screw
Data Source
AI summary
An assembly includes a first part made of composite material and a second part, which parts are held one against the other by at least one fastener system having a fastener element with a head from which there extends a shank. The fastener system further includes a bushing, the bushing including a collar presenting an inside face and an outside face of conical shape. The head of the fastener element bears against the inside face of the collar, while the outside face of the collar bears against a countersink formed in the first part.