Hole Expansion Evaluation for Stretch Flange Crack Prediction
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Solution Overview
Problem
Current methods for predicting stretch flange cracking at the sheared end face of high strength steel sheets in press forming are time-consuming and impractical due to the need for frequent forming analyses and strain measurements, which are complicated by metal sheet warping during conical hole expansion forming.
Innovation Solution
A method involving hole expansion forming analyses on a randomly selected metal sheet to establish a relationship between strain at the hole edge and strain gradient, independent of material conditions, allowing for easy prediction of stretch flange cracking without requiring forming analysis for each different material.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If forming analysis is performed for each metal sheet to evaluate stretch flange cracking, then prediction accuracy is improved, but evaluation time increases significantly
Solution Approach 1:
The patent performs forming analysis in advance on a reference metal sheet to establish the relationship between hole expansion ratio and strain gradient. This preliminary analysis creates a database that can be reused for evaluating different metal sheets, eliminating the need to perform forming analysis for each individual sheet and significantly reducing evaluation time while maintaining prediction accuracy
Solution Approach 2:
The patent uses a reference metal sheet to create a model or database of the relationship between hole expansion ratio and strain gradient. This reference model is then copied or applied to evaluate other metal sheets by comparing their hole expansion characteristics, avoiding repeated forming analyses and reducing computational overhead
2Measurement precision
If direct strain measurement is performed during hole expansion test, then strain gradient data is obtained, but metal sheet warping makes measurement difficult
Solution Approach 1:
The patent replaces direct mechanical strain measurement during hole expansion with a computational approach. Instead of using sensors or measurement devices on the warping metal sheet, the invention performs forming analysis to calculate strain gradient from the hole expansion ratio, substituting physical measurement with computational modeling that is not affected by warping
Solution Approach 2:
The patent introduces hole expansion ratio as an intermediary parameter that can be measured without affecting the warping issue. Rather than directly measuring strain on the warping sheet, the invention measures hole expansion ratio (which is not affected by warping) and then calculates strain gradient from this intermediary measurement through forming analysis
Data Source
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Figure 4A~4E
AI summary
Provided is a technology for more easily evaluating stretch flange cracking at a sheared end face of an evaluation metal sheet. A method includes reference strain gradient information (10c) acquired by performing a forming analysis of a hole expansion test on a randomly selected second metal sheet under predetermined forming conditions and converting a hole expansion ratio to a strain at a hole edge composed of true strain, the information (10c) being a relationship between the strain at the hole edge and a strain gradient along a radial direction, and two or more pieces of the information (10c) being included by changing the forming conditions. Hole expansion forming is performed on the evaluation metal sheet under the same forming conditions as respective forming conditions corresponding to at least two pieces of the reference strain gradient information (10c) to obtain at least two limit hole expansion ratios at a hole expansion limit of the evaluation metal sheet. A formable region (ARA) of the evaluation metal sheet is obtained from the at least two pieces of the reference strain gradient information (10c) and the obtained at least two limit hole expansion ratios at the hole expansion limit. Stretch flange cracking at the sheared end face of the evaluation metal sheet is evaluated by the obtained formable region (ARA).