Sheared Blank Edge Geometry for Stretch Flange Crack Resistance
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Solution Overview
Problem
Existing techniques for suppressing stretch flange cracking in sheared edges during press forming, such as those described in Patent Documents 1 and 2, lead to fatigue failure due to increased cracked surfaces, which compromise the fatigue strength of workpieces.
Innovation Solution
A sheet-shaped blank with a sheared edge featuring a curved portion that has a longer fractured surface in the sheet thickness direction, reducing the sheared surface fraction, thereby allowing the curved portion to deform without cracking and maintaining sufficient fatigue strength in other areas, is produced using a die assembly with a cutout portion that forms a larger fractured surface during shearing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If shearing is used to cut metal sheets into predetermined shapes for mass production, then productivity is improved, but stretch flange cracking occurs in the sheared edges during press forming
Solution Approach 1:
The invention applies different surface characteristics to different regions of the sheared edge. Specifically, the curved portions (which are prone to stretch flange cracking) have a larger fractured surface area ratio compared to straight portions. This local differentiation in surface quality allows the curved portions to deform more easily during press forming without cracking, while maintaining the overall effectiveness of shearing for mass production.
2Reliability
If existing techniques are used to suppress stretch flange cracking in sheared edges, then reliability is improved, but fatigue strength decreases due to increased cracked surfaces
Solution Approach 1:
The invention selectively applies fracture surface characteristics only to curved portions where stretch flange cracking is prone to occur, rather than creating fractured surfaces across the entire sheared edge. This localized approach ensures that fatigue-critical areas (straight portions) maintain their integrity while curved portions gain improved deformability, thus suppressing stretch flange cracking without significantly compromising overall fatigue strength.
3Reliability
If the fractured surface length in curved portions is increased to prevent cracking, then resistance to stretch flange cracking is improved, but the sheared surface fraction is reduced
Solution Approach 1:
The invention creates a localized difference in surface characteristics between curved and straight portions of the sheared edge. By controlling the cutting process to produce longer fractured surfaces specifically in curved portions, the invention enables these areas to deform more easily during press forming. The straight portions maintain their sheared surface characteristics, preserving overall shape quality while addressing the specific problem in curved areas.
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
This configuration effectively suppresses stretch flange cracking during press forming without decreasing the fatigue strength of the formed articles, as the curved portion is work-hardened during deformation, enhancing its fatigue properties.
Implementation Method 1
a punch 3 is moved toward the surface of the metal sheet 1 in a direction approximately perpendicular thereto (direction indicated by an arrow D) to cut the metal sheet 1
Implementation Method 2
the sheared edge has an edge including, in plan view, a curved portion that is concavely curved
Implementation Method 3
the curved portion is work-hardened during deformation, enhancing its fatigue properties
Data Source
AI summary
A sheet-shaped blank (10) for press forming is produced by shearing a metal sheet (30). The blank (10) includes a sheared edge (14), which includes a sheared surface (14b) and a fractured surface (14c) in the sheet thickness direction and has a loop shape in plan view. In plan view, an edge of the sheared edge (14) includes concavely curved portions (20). The average of lengths of the fractured surface (14c) in the sheet thickness direction in the curved portions (20) is greater than the average of lengths of the fractured surface (14c) in the sheet thickness direction over the entire perimeter of the sheared edge (14).