1. Plane Stress Fracture Prediction for Aluminum and High Strength Steel Sheets Using Digital Image Correlation Yannis Korkolis University of New Hampshire
UNH - 28 October 2016
Sheet Metal Forming

2. 2.5 Plane Stress Fracture Prediction for Aluminum and High Strength Steel Sheets Using Digital Image Correlation
Objective: Determine the most useful plane stress fracture prediction models for sheet metal forming applications Address this by developing comprehensive understanding of mechanisms and materials-process interactions, e.g., for non-linear strain paths incl. unloading
Technical Benefits:
Identify key plasticity and fracture models for use by industry
Identify standard types of testing needed to develop & to completely characterize useful models
Understand key primary failure mechanisms associated with HS aluminum & HS steels
Develop insights how HS materials differ from traditional aluminum and steel
UNH - 28 October 2016
Sheet Metal Forming

3. 2.5 Plane Stress Fracture Prediction for Aluminum and High Strength Steel Sheets Using Digital Image Correlation
Technical and Non-Technical Obstacles – AHSS:
AHSS products have significantly different forming characteristics that challenge conventional mechanical and hydraulic press forming, such as:
Lower formability/ductility
Variations in through-coil and coil-to-coil properties
Difficulty in fracture predictions (e.g., shear-type fractures in AHSS)
Deformation-induced heating
High contact pressures cause higher temperatures at die-steel interface, requiring high performance lubricants and tool-steel inserts with advanced coatings
Increased springback, which is relevant in multi-step processes.
Advanced High-Strength Steels Application Guidelines, WorldAutoSteel Version 5.0, May 2014, World Steel Association.
UNH - 28 October 2016
Sheet Metal Forming

4. 2.5 Plane Stress Fracture Prediction for Aluminum and High Strength Steel Sheets Using Digital Image Correlation
Technical and Non-Technical Obstacles – HS Aluminum:
Lower formability/ductility than mild steel
More complex plasticity than mild steel. Need advanced plasticity models.
Warm forming
Increased springback, which is relevant in multi-step processes.
UNH - 28 October 2016
Sheet Metal Forming

5. 2.5 Plane Stress Fracture Prediction for Aluminum and High Strength Steel Sheets Using Digital Image Correlation
Technical and Non-Technical Obstacles – Digital Image Correlation:
No universally accepted standard for thin sheets (necking and fracture)
FLD: ISO is deemed too complicated and too conservative
Many necking criteria have been proposed. No consensus yet.
Fracture itself is often rapid – difficult to observe/record.
Multiple DIC system vendors, incompatible systems and limited or no access to algorithms (incl. post-processing, smoothing, etc.)
UNH - 28 October 2016
Sheet Metal Forming

6. 2.5 Plane Stress Fracture Prediction for Aluminum and High Strength Steel Sheets Using Digital Image Correlation
Path to Implementation:
Plasticity
Select and validate a robust model for plastic flow, incl. non-linear strain paths
Fracture
Determine benchmark experiments for probing fracture locus
Select candidate fracture models
Round-robin to evaluate models against experiments
Identify standard experiments for fracture calibration in industry, compare them to the benchmark ones
DIC
Understand uncertainty in DIC measurements
Agree on standard, universally-accepted algorithms for necking and fracture
UNH - 28 October 2016
Sheet Metal Forming

7. Anything missing for active industry support?
2.5 Plane Stress Fracture Prediction for Aluminum and High Strength Steel Sheets Using Digital Image Correlation
Discussion:
Anything missing for active industry support?
Estimated benefits realistic & complete?
Technical and non-technical obstacles complete?
Alternative implementation paths or better approaches?
Conflicts with intellectual property or trade secrets?
Round-robins?
UNH - 28 October 2016
Sheet Metal Forming