Detection of the Onset of Necking and Accurate Measurement of Strain Limits of AHSS

John E. Carsley, General Motors Company

A neck is a groove that may develop during plastic deformation that deepens and may lead to fracture during the stamping process.

Onset of necking is defined to be the condition at a neck becomes detectable by tactile or visible sensing of the groove of the neck.

The condition of the onset of necking is important to know because localized necking is an unacceptable manufacturing condition.

The challenge is how to measure the onset of necking without having to measure 100’s of test results over a range of severity in the process of neck formation to find a few tests that are as close as possible to the condition to qualify as at the onset of a neck.

Why is this flawed?
• Result depends on the order of the polynomial.
• Shape of the strain distribution depends on the geometry of the tooling and the blank, and therefore the result of the fit with be affected by this.
• The value might be in the ballpark… it might even be a satisfactory conservative limit, but is that good enough?

Measured strains near fractured edge identified to be within the neck (beyond onset of necking)

Measured strains outside the neck

Correction for Nonlinear Strain Path

Many researchers believe that the DIC recorded strain history can be used to determine onset of necking.

Clearly the record of these strains show an increase in the strain rate as the test approaches fracture.

But strain acceleration also results from inhomogeneous straining, even if a neck does not form.

How does one de-convolute a strain signal from localized neck groove from other factors?

Selected points along the line CD that would
eventually form the neck and later fracture

Fit these points to a circle to determine the curvature

An improved curvature method of detecting the onset of localized necking
in Marciniak tests and its extension to Nakazima tests

Junying Min, Thomas B. Stoughton, John E. Carsley, Jianping Lin
International Journal of Mechanical Sciences, 2017,  123, 238-252

Challenges

Fitting to conditions near
zero curvature.

Add an artificial curve to data to avoid zero.
Solutions
Marciniak and
Nakajima test signals
are different.

Redefine Nakajima surface data to a
spherical coordinate system using radius and
arc length.

Selection of the data to
follow the groove.

A) Expand the analysis to cover the area of the
groove.

B) Adopt the approach proposed by the University of
Waterloo (Warswick, et al.).

Scale of local curvature fit is set to 1.5 times the sheet thickness 

Run the curvature fit from A to B for each frame

Let’s look at what these curvature fits look like in the LAST frame before fracture

Focus on the geometry of the DIC surface data to detect the true onset of necking.

Do not support the ad-hoc strain analyses proposed in Europe and current ISO Standards.

This focus on meaningful measurements will enable us to make wider use of AHSS’s by taking more advantage of their limited ductility through more reliable knowledge of their actual limits.