Page 40 - MetalForming August 2011

The Science of Forming By Stuart Keeler
How Reliable are Your Test Results?
Tooling Technology
Conducting tests forms the foundation of science, engi- neering, education and press shops. We conduct tests to obtain answers to questions, to troubleshoot, and to satisfy spurts of curiosity. Most often, these tests are carefully planned, well constructed, cautiously run and honestly ana- lyzed. Sometimes, however, incorrect answers result because one or more rules of experimentation were overlooked. Let’s review some actual tests gone wrong.
•Laser welding, gaining popularity in many industries, gen- erates a narrow weld and heat-affected zone compared to arc-welding processes. In one case, a study was conducted to evaluate stretching degradation when butt welding two pieces of steel of different thicknesses. To evaluate weld stretchabili- ty, the metalformer clamped a series of welded blanks using a variety of material-thickness combinations, and stretched the blanks using a 4-in.-dia. hemispherical punch. The weld line was centered over the pole of the punch.
(Data reported here are illustrative and should not be used for any other purpose.)
The relative sheet thicknesses: 1.0, 0.9, 0.8 and 0.7 as proportional to the thickest sheet. The as-received sheet also was tested without any welds.
The study concluded that using even small step welds greatly reduced the stretchability of all laser welds. Unfor- tunately, a key measurement was not reported: The largest reduction in weld stretchability occurs when two cut edges are welded, even without a change in thickness. Lacking this measurement, no valid measure of damage due to step welds can be made. However, since we know the large reduc- tion in edge stretchability caused by shear cutting the blank edges, the stretchability of welds in equal-thickness sheets
Stuart Keeler (Keeler Technologies LLC) is best known worldwide for his discovery of forming limit diagrams, development of circle grid analysis and implementa- tion of other press shop analysis tools. Stuart’s sheet- metal forming experience includes 24 years at National Steel Corporation and 12 years at The Budd Company Technical Center, enabling him to bring a very diverse background to this column and the many seminars he teaches for PMA.
Keeler Technologies LLC
P.O. Box 283 | Grosse Ile, MI 48138 Fax: 734/671-2271 keeltech@comcast.net
likely will be around 85 percent. The added increment of cre- ating a step edge most likely is small.
Lessons learned: Be sure to include all variables. Do not overlook the obvious but unseen. Ask two or three experts from another field to review your test plans, your work and your conclusions.
• A fatigue study of wheel rims showed a significant dif- ference in fatigue life between two types of steel. After study- ing metallurgical samples and conducting tests on compo- sition and tensile properties, no difference between the two steels could be identified. Then someone reviewed the ship- ping papers for the steels and found that, although the orders were identical; the steels were shipped with different thicknesses. Steel 1 was thinner. Since fatigue life is a func- tion of the fourth power of thickness, the thinner steel exhib- ited a much lower fatigue life. When the test results were cor- rected to the same steel thickness, steel 1 had the longer life. Lessons learned: Back-engineer your tests. Find out what vari- ables are significant in the final result, then be sure to include them as test variables.
• The relative rate of aging for two types of steel was measured by Olsen tests. These 1-in.-dia. dome-stretch tests recorded maximum dome height at the onset of failure. Tests were performed in an automatic machine to eliminate operator error/bias. Tested were steel 1 (an aging steel) and steel 2 (a non-aging steel). Test results showed the non- aging steel aged more than the aging steel—an unlikely result. Examining the test procedures, it was found that steel 1 samples were prepared the day before testing and were tested at the start of work the next day, while steel 2 samples were prepared and tested after lunch of test day. Once start- ed, the hydraulics of the test machine ran all day.
Examining the test machine revealed that the operating tem- perature of the machine was higher after lunch than during morning startup. The hot hydraulic oil heated the punch and reduced the viscosity of the test-piece lubricant. The hot hydraulic oil also caused the afternoon tests to be run at faster speeds, so valid test comparisons were not being made. Lesson learned: Mix the sets of samples into random sequence for test- ing, so that equal numbers of sample pairs will be exposed to the same test environment independent of time of test.
• Coefficient of friction (COF) tests are conducted routinely to study the relationships between three key components— lubricant, sheetmetal surface and tooling material (see the fig- ure). When testing one component, the other two must remain constant—often a difficult restriction to meet. For example, sheetmetal can change throughout the coil, so a group of samples can vary. Or, the tool can wear during the test, or become contaminated by dirt.
Weld Combination
Stretchability
Non-welded sheet
100 percent
1 to 0.9 ratio
82 percent
1 to 0.8 ratio
79 percent
1 to 0.7 ratio
76 percent
38 MetalForming/August 2011
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