Fracture Characterization of Advanced 980 MPa Steels

University of Waterloo:

Mike Worswick, Cliff Butcher
Jose Imbert-Boyd
Armin Abedini
Kenneth Cheong
Sante DiCecco
Sam Kim
Amir Zhumagulov
Taamjeed Rahmaan
Kaab Omer

Background
? Objective
? Study Materials
? Fracture Test Methodology and Results
? Component Testing
? Conclusions

Need for Grade Diversification at 980 Strength Level

? Specialized grades now exist at
the 590 and 780 MPa strength
level. (dual phase, TRIP, etc).
? Expand Grades at 980 MPa for
enhanced:
- bending
- flanging
- energy absorption

Microstructures of early DP980 not optimized for bending and edge stretch
? These property limitations restricted the application of DP980.
? Complex part shapes and features could not be formed.
? Energy absorption targets could not be meet due to fracture problems.

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
3. Perform experimental axial and bend crush experiments and assess
fracture performance

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

1.All grades exhibited total elongation typical of 980 level material.
2.Relatively high YS/TS ratios suggested all grades favor local formability.
3.Material #2 had unique yield point elongation behavior.

1.Performance of these grades is consistent with or above current commercial products.
2.VDA bend data is of growing industrial importance as means to evaluate material.

Priority Focus Areas

1) Material characterization at large strains and strain rates
2) Efficient method to determine forming limit strains (FLD) (Global formability)
3) Characterization in tight radius bending (Local formability/fracture)
4) Establish best practices and tests for experimental fracture characterization
*Extensive numerical characterization study pursued in tandem

? 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 model

? Tensile characterization from 0.001 to 1000 s-1
? Scale quasi-static data obtained to large strains for strain rates
? Efficient experimental method for constitutive characterization

Physically-motivated FLD detection methods are needed

.Formability in tight-radius bending indentified as key factor in crash performance
VDA 238-100 bend test promising but only reports bend angle

Plane Strain notch provides lower bound estimate if thinning correction performed
Correction also required for plane strain dome tests

Outer diameter is in uniaxial tension and does not contact the punch
Triaxiality = 1/3 (Butcher et al., SAE, 2013, Pathak et al., JMEP, 2016; Numisheet 2016)

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

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.

Sheared Edge Failure is a  Uniaxial Tensile Mode:
Use Punched Hole Expansion Tests to Obtain  Uniaxial Failure Strain with Sheared Edge

? Sled Mass: 855kg
? Sled Velocity: 25.5km/h
? Total Energy: 21kJ
? Free Crush Distance: 100mm
? Total Crush Distance: 160mm

Axial Crush Dynamic Crash Test at UW
? Sled Mass: 855kg
? Sled Velocity: 25.5km/h
? Total Energy: 21kJ
? Free Crush Distance: 115 mm
? Total Crush Distance: 135mm

Relatively Good
Performance for 3 Grades
Repeatable Energy
Absorption
Material 3 had highest
spot weld strength and
strain rate sensitivity

Efficient and Accurate Experimental Methodologies Established for Characterization
of AHSS
1. Method to experimentally obtain hardening to large strains and strain rates
2. New FLD detection algorithm: Curvature- - based
3. Optical V- - Bend developed: Ideal plane strain test
4. Four tests required to efficiently obtain fracture locus for a material
As with FLD’s,  Use 4 Industrially Friendly Tests  to Construct Fracture Locus to
Compare Material Performance  ? Accounts for Edge Condition
New model to use four tests for rapid FE model implementation  ? Next GDIS