Fold-Back Joint Structure for High-Strength Steel Spot Welds
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Solution Overview
Problem
Joining parts between high-strength sheet steel components in vehicle bodies are prone to fracture due to increased nugget brittleness and reduced cross tension strength, limiting the overall strength of the joint structure.
Innovation Solution
A joint structure is designed with a third sheet member having a fold-back portion that overlaps and folds back around the edges of the first and second sheet members, forming joining parts at intervals to suppress separating direction forces and prevent openings, thereby enhancing the strength of the joint.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high-strength sheet steel with tensile strength exceeding 780 MPa is used to reduce vehicle body weight, then vehicle body weight is reduced, but cross tension strength of spot welds decreases due to increased nugget brittleness
Solution Approach 1:
The joining structure is divided into multiple segments: a first sheet member, a second sheet member, and a third sheet member with fold-back portions. This segmentation distributes the stress across multiple joining parts rather than concentrating it in single spot welds, thereby maintaining joint strength even when using high-strength sheet steel with reduced weldability
Solution Approach 2:
The third sheet member extends in the sheet thickness direction, creating a multi-layered joining structure. The fold-back portions are folded back in the sheet thickness direction to form joining parts, transforming a planar joining problem into a three-dimensional structure that resists delamination cracking and enhances overall joint strength
2Productivity
If spot welding is used to join high-strength sheet steel, then joining efficiency is maintained, but fracture resistance at joining parts deteriorates due to nugget brittleness
Solution Approach 1:
Instead of relying on a single large spot weld, the structure uses multiple smaller joining parts distributed along the edge. This segmentation reduces the stress concentration at each individual joining part while maintaining overall joining efficiency through the distributed configuration
Solution Approach 2:
The fold-back portions are configured beforehand to prevent delamination cracking before it can propagate through the joining parts. This pre-configured structural feature acts as a cushion against the harmful effects of nugget brittleness, preventing fracture even when spot welds are used on high-strength steel
3Strength
If sheet steel composition is controlled to improve spot weld strength, then joining part strength is improved, but degrees of freedom to set composition at locations away from spot welds is limited
Solution Approach 1:
The invention applies local quality control only where necessary - at the joining parts where spot welds are formed. The third sheet member's fold-back portions are specifically configured at joining locations to prevent delamination cracking, while allowing flexible composition design for the main body of the sheet members away from these localized joining zones
Data Source
AI summary
A joint structure includes a first sheet member, a second sheet member, a third sheet member, and plural joining parts formed at locations where the first sheet member, the second sheet member, and the third sheet member overlap, so as to join the first sheet member, the second sheet member, and the third sheet member together. The plural joining parts are formed at an interval along an edge of the first sheet member and an edge of the second sheet member. The third sheet member includes a fold-back portion provided at least between the plural joining parts by folding the third sheet member back around the edge of the first sheet member and the edge of the second sheet member such that the fold-back portion is disposed on an opposite side of the first sheet member from a side overlapped with the second sheet member.