Sheared-End Surface Stress-Margin Assessment for Delayed Fracture

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Solution Overview

Problem

Existing methods for assessing delayed fracture characteristics in sheared end surfaces of high-strength steel sheets, particularly those with tensile strengths of 980 MPa or more, fail to account for the changes in properties due to plastic deformation and forming strains, leading to insufficient prediction of delayed fractures in actual automotive components.

Innovation Solution

A method involving restraining a metal sheet with a predetermined load stress and placing it in a hydrogen entry environment to determine a stress margin, which is set as an index for assessing delayed fracture characteristics, considering forming strains and external load stresses.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If conventional assessment methods (PTL 1-3) are used to assess delayed fracture, then the assessment process is simplified, but the assessment accuracy is insufficient because they do not account for forming strains and plastic deformation changes

Engineering Contradiction:

Improveassessment accuracyVSAvoidassessment process complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The invention introduces forming strain as a new parameter to the assessment model. By changing the assessment parameters to include forming strain magnitude and direction, the method accurately captures the impact of plastic deformation on delayed fracture characteristics, resolving the contradiction between assessment accuracy and process complexity

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The invention performs preliminary assessment of forming strain characteristics before the actual delayed fracture test. By pre-evaluating the forming strain state of the sheared end surface, the method can predict delayed fracture risk without requiring complex multi-stage testing procedures

Inventive Principle:

Principle #10Preliminary action

2Reliability

If rolling compression processing is applied to the sheared end surface (PTL 1), then the delayed fracture assessment can be performed, but the assessment deviates from the actual forming strain state in press-formed automotive components

Engineering Contradiction:

Improveassessment reliabilityVSAvoidadaptability to actual component conditions

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The invention focuses the assessment on the specific local conditions of the sheared end surface, particularly the forming strain state and plastic deformation characteristics. By targeting the local quality of the sheared end surface rather than applying uniform compression, the assessment becomes reliable for actual press-formed components

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

Instead of applying compression to create a test state (as in PTL 1), the invention inverts the approach by assessing the existing forming strain state directly. This inversion allows the assessment to reflect the actual service conditions of press-formed components without introducing artificial deformation

Inventive Principle:

Principle #13The other way round (Inversion)

3Ease of manufacture

If the sheared end surface is kept as-sheared without additional processing, then the assessment is simpler, but the large remaining tensile stress increases the risk of delayed fracture

Engineering Contradiction:

Improveease of assessmentVSAvoidfracture resistance

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

The invention introduces forming strain assessment as an intermediary evaluation step. By assessing the forming strain state and plastic deformation characteristics as intermediate parameters, the method can evaluate fracture risk without requiring additional compression processing, thus maintaining ease of manufacture while improving reliability

Inventive Principle:

Principle #24Intermediary (Mediator)

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

Enables accurate prediction of delayed fractures in sheared end surfaces, allowing for the expansion of ultrahigh-strength steel sheet applications and reducing automobile body weight by providing a stress-based assessment index.

Implementation Method 1

placing the metal sheet for a predetermined time in a predetermined hydrogen entry environment

Methodology Applied

Scientific EffectHydrogen entry: Absorption (physical)

Data Source

PatentUS20250305919A1Method for assessing delayed fracture characteristics of sheared end surface, program, and method for producing automotive components

Publication Date: 2025.10.02 JFE STEEL CORP

AI summary

A method for assessing the delayed fracture characteristics of a sheared end surface of a metal sheet includes: a test including restraining a metal sheet where a predetermined load stress is loaded to a sheared surface of the metal sheet and placing the metal sheet for a predetermined time in a predetermined hydrogen entry environment in the restrained state; and determining a limit load stress being a load stress at the limit where a delayed fracture in the sheared surface of the metal sheet does not occur based on the results of the test and setting a stress margin to the occurrence of the delayed fracture in the sheared end surface of the metal sheet based on the determined limit load stress, in which the determined stress margin is set as an index of the assessment of the delayed fracture characteristics of the sheared end surface of the metal sheet.