Blind Fastener Hole Edge Modification for Fatigue Resistance
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Solution Overview
Problem
Blind fasteners create high stress areas and are difficult to install in composite structures due to the challenge of accessing the blind side of the workpieces.
Innovation Solution
A machining tool with a cutting tip is used to form a surface modification on the workpiece edge, allowing a fastener to be installed by forming a bulb through deformation of a sleeve and insert, reducing stress and enhancing the fatigue resistance by creating a compressive stress area, which serves as a countermeasure to fatigue crack initiation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If blind fasteners are installed in composite structures, then workpieces can be secured together when blind side access is difficult, but high stress areas are created that reduce fatigue resistance
Solution Approach 1:
The machining tool performs preliminary action by forming a surface modification (such as a countersink or chamfer) at the hole entrance before the fastener is installed. This pre-prepared geometry allows the fastener to be installed from the accessible side only, while the surface modification distributes the stress more evenly, reducing the harmful stress concentrations that would otherwise compromise fatigue resistance in composite structures.
2Ease of operation
If a deformable sleeve is used to engage the blind side surface, then workpieces can be held together, but high stress concentrations are created during installation
Solution Approach 1:
The surface modification creates a localized change in geometry at the hole entrance, providing a gradual transition zone that distributes the deformation stress over a larger area. Instead of the deformable sleeve creating a sharp stress concentration point, the pre-formed surface modification (such as a countersink) provides a gradual taper that reduces peak stresses while still allowing the sleeve to engage and secure the workpieces effectively.
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 method reduces stress concentrations and improves fatigue resistance by forming a bulb during installation, ensuring secure and reliable fastening of composite structures.
Implementation Method 1
the cutting tip includes a cutting portion... forming a surface modification at the edge with the cutting tip
Implementation Method 2
forming a bulb through deformation of a sleeve and insert, reducing stress and enhancing the fatigue resistance by creating a compressive stress area
Data Source
AI summary
A method includes inserting a machining tool through a hole of at least one workpiece that includes a first surface and a second surface opposite the first surface and extends from the first surface to the second surface. The hole forms an inner wall. The machining tool includes a shaft and a cutting tip proximate to one end of the shaft. The cutting tip includes a cutting portion. The method includes positioning the cutting portion of the cutting tip to abut an edge of the hole. The edge is located at a junction of the second surface of the one of the at least one workpiece and the inner wall. The method includes forming a surface modification at the edge with the cutting tip. The method includes removing the machining tool from the hole. The method includes installing a fastener within the hole. The fastener engages the surface modification.