Elimination of the Dependence of Forming Limits for AHSS on Testing Process Conditions

? Introduced by Keeler in 1960’s and
adopted by industry in late ‘60s
? Used in tryout and stamping plants
? Applied to assess formability in
“virtual” tryout since mid ‘80s
? Most reliably determined empirically
? Theoretical models require calibration

1) Understand the cause of this ambiguity
2) Find a solution to it
What does a solution look
like?
1) A unique forming limit,
regardless of test
2) An understanding how to
use this forming limit

We can measure the strain path for each test done to determine the FLC

Each test results in different (unique) degrees of nonlinear strain paths

The strain FLC is NOT a static
forming limit.
The strain FLC is a DYNAMIC limit.
Question: Could this fact impact
our measurement of the FLC?

1) 3% difference in strain between
the top and bottom surface
4) Is this intuition correct?
2) What strain defines the limit?
3) Intuition suggests to use the average

Stretch-Bending Study Reported By: Tharrett & Stoughton SAE Report 2003-01-1157

Conclusion of the Study:
In every test specimen in which a neck was detected， the MEASURED strain on the CONCAVE 

side was found to be above the forming limit for in-plane stretching

hese experimental results make perfect sense from a theoretical perspective
Necking is an instability that affects the plastic flow of ALL layers through the sheet thickness
So the instability cannot proceed until all layers satisfy the instability condition

While it is tempting to think
these two FLC’s are close
enough
The Nakazima Test results are
systematically higher
… which suggests we have
missed something in the
compensation
This becomes more obvious
when we add the 50 mm test

… when the degree of nonlinearity,
curvature of the sheet, and contact pressures
that are involved in stamping automotive
components

are more than 10 TIMES LARGER?
We should instead ALWAYS convert the strain limit to
a stress limit, and account for the curvature and contact
pressure in our assessments of the severity of the
process with respect to necking.