Press-Formed Product Shape Prediction Under Residual Stress Relaxation
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Solution Overview
Problem
Existing methods for predicting shape changes in press-formed products with bends after spring back suffer from accuracy deterioration, particularly in components where the shape change of the punch shoulder portion significantly influences dimensional accuracy.
Innovation Solution
A method involving the generation of a blank model divided into multiple pieces in the sheet thickness direction, acquisition of the shape and residual stress immediately after spring back, setting reduced residual stress values for the sheet thickness surface layer, and determining the balanced moment shape through analysis.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If press forming is performed on high-strength metal sheets to achieve weight reduction and improved collision safety, then strength and safety performance are improved, but dimensional accuracy deteriorates due to larger spring back
Solution Approach 1:
The patent applies preliminary action by predicting the shape change due to spring back before actual press forming production. The prediction method calculates residual stress distribution and spring back amount in advance, allowing manufacturers to compensate for dimensional deviations in the tooling design or forming parameters, thus ensuring final dimensional accuracy despite using high-strength materials with large spring back
Solution Approach 2:
The patent implements feedback by using the predicted spring back results to adjust and optimize the press forming process. The prediction system provides feedback information about expected dimensional deviations, which is then used to modify forming parameters, tooling compensation, or process conditions to achieve the target dimensional accuracy in subsequent production
2Measurement precision
If integrated press forming simulation is used to predict spring back shape, then spring back prediction is achieved, but shape prediction accuracy deteriorates for products with bends due to ignoring stress relaxation over time
Solution Approach 1:
The patent applies parameter changes by introducing time-dependent stress relaxation parameters into the simulation model. The method calculates how residual stress evolves over time after press forming, considering the relaxation of tensile stress on the bending outside and compressive stress on the bending inside. This time-parameter integration significantly improves shape prediction accuracy for bent components compared to traditional instantaneous spring back analysis
Solution Approach 2:
The patent uses segmentation by dividing the metal sheet into different regions (bending inside, bending outside, neutral axis) with distinct stress states. Each region's stress relaxation behavior is analyzed separately, allowing the model to capture the complex stress evolution patterns in bent components and achieve more accurate shape prediction
3Duration of action of stationary object
If conventional creep analysis methods are applied to predict shape change over time, then time-dependent deformation is considered, but prediction accuracy deteriorates because the shape change occurs without external load
Solution Approach 1:
The patent converts the harmful effect of unloading (which causes unexpected shape changes) into a beneficial prediction tool. By recognizing that the removal of external load creates a specific stress relaxation pattern rather than treating it as an anomaly, the method successfully predicts the time-dependent shape change that occurs after press forming, turning a source of dimensional inaccuracy into a predictable and controllable phenomenon
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 allows for accurate prediction of shape changes over time in press-formed products with bends, improving dimensional accuracy and enabling the manufacture of components with excellent shape retention, thus enhancing vehicle body assembly precision and productivity.
Implementation Method 1
A phenomenon in which residual stress generated in a press-formed product at the time when a metal sheet is deformed by press forming serves as driving force and the press-formed product removed from a die instantaneously returns to a shape of the metal sheet before the press forming like a spring is called spring back
Implementation Method 2
the inventors first focused on a stress relaxation phenomenon in which stress gradually decreases with the elapse of time in a case where strain is applied to a metal sheet and kept constant
Data Source
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AI summary
A method of predicting a shape change of a press-formed product according to the present invention includes: a generation step (S1) of generating a blank model being divided into a plurality of pieces in a sheet thickness direction such that the blank model can be analyzed; an acquisition step (S3) of acquiring a shape and residual stress in the sheet thickness direction of the press-formed product immediately after spring back by performing press forming analysis and spring back analysis using the blank model and a die model; a setting step (S5) of setting a value of relaxed and reduced residual stress to the acquired press-formed product immediately after the spring back at an element or an integration point of a sheet thickness surface layer; and a shape analysis step (S7) of determining a shape in which moment of force is balanced in the press-formed product for which a value of residual stress of the sheet thickness surface layer has been set to be relaxed and reduced.