An Efficient Methodology for Fracture Characterization and Prediction of DP980 Steels for Crash Application

Steel Marketing Development:  Hesham Ezzat, Dave Anderson

ArcelorMittal: Steve Lynes, Tim Lim

AK Steel: Kavesary Raghavan

Nucor: Dean Kanelos, Andy Thompson

Honda Research of Americas: Jim Dykeman, Skye Malcolm

University of Waterloo:

PI’s: Cliff Butcher and Mike Worswick

Research Team :

Research Team :  Jose Imbert-Boyd

Armin Abedini

Kenneth Cheong

Sante DiCecco

Sam Kim

Amir Zhumagulov

Taamjeed Rahmaan

Kaab Omer

1. Each Supplier Submits One DP980 to SMDI Sample Bank

Materials can generally be described as DP with fine, uniform microstructure.

Grades represent recent optimization in processing / chemistry (but are not Gen 3 level).

Performance of these grades is consistent with or above current commercial products.

Better local formability relative to other DP980’s.

1. Characterize properties of various Dual Phase 980 grades selected by Steel Marketing Development Institute (Blind Study).

2. Investigate optimized fracture testing methodology for Advanced High Strength Steel Industrial Friendly and Efficient Methods Required  (GDIS 2017)

3. Perform experimental axial and bend crush experiments and assess fracture performance (GDIS 2017)

4. Numerical characterization for CAE application to dynamic tests  (GDIS 2018) 

5. Efficient methods needed to transition from coupons to crash simulations.

Limited hardening data available in tensile tests.

Inverse FE modeling used to identify hardening at large strains for fracture.

Hardening data becomes a function of numerical model assumptions...

UW developed simple method to use tensile & shear test data to obtain hardening to large strain levels.

DP980 data to 60% strain!

Not related to FE mode!

• Conflicting limits provided by different specimen types if thinning correction not applied

Min. of 4 Tests can describe the fracture locus

Four Relatively Simple Tests:

1. Mini-shear

2. Hole expansion (reamed)

3. V-Bend

4. Biaxial/Bulge

Four tests can be used to generate physically- - meaningful fracture loci

Not the product of a simulation exercise – Real material performance can be assessed

• Relatively comparable fracture loci

• Mat 2 had the lowest hardening rate,highest hole expansion and v-bend

• How do we use this for CAE?

Tensile-Based Characterization Tests are Employed

X – Strong localization

X – Through-Thickness Strain Gradients

X – Fractures at mid-thickness

No DIC strain measurement

X – Requires 3-D solid elements

X – Requires fine mesh: ~ 0.10 mm

X – Non-linear 3-D stress state develops

Solid element models are great for academic research but less so for industry.

CAE models for forming & crash use plane stress shell elements from 0.5 – 7.0 mm

Extracting the plane stress fracture locus from a calibrated 3-D solid model

works in theory…in practice the element mechanics are different

Relatively simple tests that most labs can perform and are comfortable with
Since sheet is thin, the logic is that these samples are plane stress….
Deformation rapidly localizes, violating plane stress assumption but creating a desired change in the stress state

Shell models cannot resolve strong local thinning and localization   O O verestimate the stress response, underestimates strain
Methods exist to add  damage- - induced softening  to improve the shell solutions. Not a damage issue but element type.
Can create problems for cases when shells are appropriate

Shell element models for sheet metal forming and structural component models can be very accurate
Use of  Nakazima dome tests for CAE characterization is more consistent with the end applications

Mechanics of shear deformation creates a Plane Stress-Plane Strain loading condition
Shell elements provide an accurate description

Regularization factor depends upon:

1. Coupon geometry
2. Element type: some geometries are poorly described by shells
3. Deformation mode: Bending mode is not well described by large elements relative to stretching mode
4. Stress State: Uniaxial tension is different than biaxial tension

Regularization atones for any experimental and modelling sins
Issues of modelling taste Different fracture methodologies can lead to similar
results in component tests after each is regularized…

Have developed an industrially-focused methodology for efficient fracture
characterization

The results are promising but much work remains:
• Application to sheet metal forming with severe non-proportional loading
• Application to sheet metal forming through to crash of an AHSS component
• Spot weld failure and potential un-zipping of weld groups
• Improve physics of damage model
• Need some physics to help guide regularization