Page 86 - MetalForming July 2017

The Science of Forming By Stuart Keeler
Study of 1977 Auto Bumper Quite Revealing
In 1977, a formability team from the steel company supplying steel for an automotive front bumper
was assigned to evaluate the bumper. The goals:
1) Analyze strain distribution found in different auto bumpers made with higher-strength steels. The bumpers were constructed in three sections. Two
distribution and maximum allowable strain.
3) Strains in the corner radius and center section are to be tested with four steels. The yield strengths are:
Ksi (MPa)
• 80 (550)
• 65 (450)–Production
Steel
• 55 (380)
• 45 (310)
Fig. 2 illustrates one way
80
60
40
20
0
No neck
Necked Maximum
allowable stretch
Safety factor
300
(MPa)
400 500 600
Actual peak stretch of corner radius
40 50 60 70 80 90 Yield Strength (ksi)
X
Fig. 1—Two corner sections of the steel bumper. Note that the two-level bumper is separated by a depressed central area.
corner sections (Fig. 1) attach to the center section (I’ll review an analysis of the center section in next month’s column). A highlight of the entire bumper is the deep, flat indentation formed along the center of the entire bumper to add strength.
2) Without changing any process variables, determine the effect of the material yield strength on the strain
Stuart Keeler (Keeler Technologies LLC) is known worldwide for his discovery of forming limit diagrams, development of circle-grid analysis and implementation of other press-shop analysis tools. Keeler’s metalform- ing experience includes 24 years at National
Steel Corporation and 12 years at The Budd Com- pany Technical Center, enabling him to bring a very diverse background to this column and to the sem- inars he teaches for PMA.
Keeler Technologies LLC P.O. Box 283
Grosse Ile, MI 48138 keeltech@comcast.net
to rate the steels. This
method of displaying the interaction of the peak
strain and the limit strain
plots both values as a func-
tion of the yield strength.
The maximum-allowable-
stretch line inserted in the
graph is taken from the forming-limit diagram, which denotes the actual peak strain
of the corner radius. The data
on the graph show a straight-
line plot of peak strain increas-
ing and limit strain decreasing
as the strength of the steel increases and the n-value decreases. A practical forming limit—onset of necking—
occurs when the peak strain in
the stamping exceeds the limit
strain. This relationship is eas-
ily seen in Fig. 2. Steels with
yield strength greater than 77
Ksi would be located in the red
zone, ready to fail. The 65-Ksi produc- tion steel would be located at the yel- low/green boundary line, thus accept- able for production.
During the test, it was discovered that the corner bumpers were being produced at two different plants. A comparison study revealed several operational differences between the
Y
Fig. 2—The maximum-allowable-stretch line (dashed) in this steel-rating graph is controlled by the forming-limit diagram.
Location
Plant A Plant B
Corner radius location
84 MetalForming/July 2017
www.metalformingmagazine.com
Fig. 3—Plant A is producing better corner bumpers than Plant B, with fewer weak areas.
two (Fig. 3). Plant A had a constant percent strain at two different levels for two steels—reflective of constant R/t bending. Plant B had a steep gra- dient similar to one-half of a hemi- sphere—indicating major deformation by stretching. This stretching thinned the corner and weakened the bumper at various locations. MF
Stretch %
Percent Stretch