Page 40 - MetalForming May 2011

The Science of Forming By Stuart Keeler
When Does Sheetmetal Fail?
Diffuse Neck
Load Maximum Ultimate Tensile Strength
Uniform Elongation
Engineering Strain
Asking when sheetmetal fails evokes a variety of answers:
• The material workhardens excessively, becomes hard and brittle, and breaks.
• The sheetmetal becomes too thin and breaks up due to the high stresses created.
• Stretching the material causes internal defects to become cracks.
• The material begins to fail when the stretch exceeds the uniform elongation. This answer is the most common, because many people understand the sequence of events that occurs during a tensile test.
The typical load-elongation curve from a tensile test (Fig. 1) shows a maximum load (ultimate tensile strength) that signals the end of deformation along the entire length of the tensile sample, and the onset of a zone of concentrated deformation at one location—usually at the center of the sample. This deforming zone is called a width neck or, more formally, a diffuse neck. Deformation then continues with- in the diffuse neck until a very sharp localization of defor- mation occurs, at about 55 deg. to the axis of the tensile sam- ple. This through-thickness neck, or local neck, quickly leads to fracture.
Looking at the tensile-test curve, one would easily choose the diffuse neck as the obvious failure point. All useful defor- mation along the sample outside the diffuse neck stops. However, the uniform elongation for many lower-strength steels ranges from 20 to 27 percent. Most parts have areas of stretch exceeding those numbers. Others argue that the material continues to deform in the neck until the specimen tears. Therefore, the total elongation must be the failure limit of the material. Unfortunately, the value of the total elon- gation changes as the initial gauge length of the exten- someter changes. Thus, a single sample can exhibit numer- ous total-elongation values, depending on the initial gauge length selected.
To further understand formability, a research program dat- ing back to 1957 focused on two questions:
1) When does sheetmetal fail?
2) Can the onset of failure be predicted?
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
Fig. 1—Load maximum, ultimate tensile strength and uniform elongation define the strain at which failure begins in the tensile sample.
We’ll focus on question one here, and address question two next month.
The research program noted above gathered pertinent data by testing 8-in.-dia. circular blanks locked by a 5-in.-dia. circle of lock bead. Blanks were formed by various sizes of hemispherical punches, as large as 4-in. dia. Each blank was marked with a polar grid of 20 circles/in. for strain (stretch) measurements.
The first research phase attempted to identify how sheet- metal reacts when reaching a strain equal to the uniform elongation in the tensile test. Was there some type of visible diffuse neck, a termination of strain or load maximum? The answer to all three questions: No. The domes continued forming in a well-controlled manner to reach strain values well above the uniform elongation (Fig. 2). Measuring strain rate at the eventual failure site showed that a modest rate increase occurred at a strain approximating uniform elon- gation. Once increased, the new rate was maintained. Loca- tions above and below the eventual failure site showed the same increase in strain rate when they reached strains equal to the uniform elongation.
How does stretching sheetmetal over a hemispherical punch differ from a tensile test? A tensile test is similar to stretching a length of chain or wire—one link can become weak and begin deforming independently. The rest of the chain will stop deformation as a maximum load forms, and eventually the weak link will fracture.
Stretching sheetmetal acts similarly to deforming a chain- link fence. The weak spot can only stretch as much as the areas surrounding it—no localization of strain can take place. The research showed that the diffuse neck did not ter- minate useful deformation when stretch forming sheetmetal.
During the second research phase, scientists searched for a different definition of failure. Again, most would say failure
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Engineering Stress