Lap-Welded Joint Heating to Create Compressive Residual Stress
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Solution Overview
Problem
Existing methods for improving the fatigue strength of lap-welded joints, such as heating the lower-side steel sheet, often fail to sufficiently reduce tensile residual stress and may deteriorate the strength of the weld joint.
Innovation Solution
Heating a sub-portion of the overlapping portion of the second steel material until melted, while restraining the steel materials to form a melted portion that extends in parallel to the weld zone, thereby reducing tensile residual stress and enhancing fatigue strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If the lower-side steel sheet is heated until melted to further reduce tensile residual stress, then the residual stress is sufficiently reduced, but the fatigue strength of the lower-side steel sheet itself deteriorates
Solution Approach 1:
The patent utilizes controlled phase transition (melting and solidification) of the lower-side steel sheet. The melting is followed by rapid solidification while maintaining constraints, which generates compressive residual stress that counteracts tensile stress without causing net strength deterioration
Solution Approach 2:
The patent exploits thermal expansion and contraction during heating and cooling cycles. By heating to melting point and then cooling with constraints in place, thermal contraction generates compressive residual stress that improves fatigue strength without deteriorating the steel sheet strength
2Reliability
If heating is applied to the overlapping portion of the second steel material, then compressive residual stress is formed and fatigue strength is improved, but the process complexity increases
Solution Approach 1:
The patent applies heating locally to the overlapping portion of the lower-side steel sheet rather than uniformly heating the entire structure. This localized heating approach achieves the desired compressive residual stress formation while minimizing energy consumption and process complexity
Solution Approach 2:
The patent segments the heating process by applying heat to specific regions (overlapping portions) at specific times during the welding sequence. This segmentation allows controlled creation of compressive residual stress without requiring complex overall process control
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 method effectively improves the fatigue strength of lap-welded joints by creating a compressive residual stress state, inhibiting tensile stress and crack formation, and maintaining the tensile strength of the weld joint.
Implementation Method 1
a sub-portion of the overlapping portion of the second steel material is heated such that a melted portion is formed in the sub-portion of the overlapping portion of the second steel material
Implementation Method 2
a sub-portion of the overlapping portion of the second steel material is heated such that a melted portion is formed
Data Source
Figure 1~2
Figure 3(a)~3(c)
Figure 4(a)~4(c)
AI summary
The fatigue strength of a lap-welded joint 10, in which an overlapping portion 12a of a first steel material 12 and an overlapping portion 14a of a second steel material 14 overlap with each other, and an edge portion of the first steel material 12 is welded to a front face 14b of the second steel material 14 with a weld zone 16 extending along the edge portion, is improved by the following method. First, when a direction perpendicular to an extending direction X of the weld zone 16 and parallel to a front face 14b of the second steel material 14 defined as a reference direction Y, the lap-welded joint 10 is restrained from moving in the reference direction Y, and the first steel material 12 and the second steel material 14 are restrained from moving in their sheet-thickness directions. In this state, a portion 14a of the second steel material 14 is heated such that a melted portion 18 is formed in the portion 14a of the second steel material 14.