The Science of Forming


 

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When Does Sheetmetal Fail?

By: Stuart Keeler

Sunday, May 01, 2011
 

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 within the diffuse neck until a very sharp localization of deformation occurs, at about 55 deg. to the axis of the tensile sample. This through-thickness neck, or local neck, quickly leads to fracture.

 
 Fig. 1—Load maximum, ultimate tensile strength and uniform elongation define the strain at which failure begins in the tensile sample.
Looking at the tensile-test curve, one would easily choose the diffuse neck as the obvious failure point. All useful deformation 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 elongation changes as the initial gauge length of the extensometer changes. Thus, a single sample can exhibit numerous total-elongation values, depending on the initial gauge length selected.

To further understand formability, a research program dating back to 1957 focused on two questions:

1) When does sheetmetal fail?

2) Can the onset of failure be predicted?

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 sheetmetal 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 elongation. Once increased, the new rate was maintained. Locations 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 terminate useful deformation when stretch forming sheetmetal.

 
 Fig. 2—Graph showing biaxial stretching of aluminum sheet does not terminate as a load maximum when uniform elongation is reached. (Trans ASM, Vol. 56, p. 35).
During the second research phase, scientists searched for a different definition of failure. Again, most would say failure occurs when the specimen tears or fractures. However, a close look at the tensile test (Fig. 1) shows that at the maximum load, all deformation terminates except in the diffuse neck. For practical purposes, when forming a part shaped like a tensile strip, all useful deformation throughout the stamping has stopped. Only a highly localized width neck continues deforming, as the load (stresses) in the stamping decreases. Thus, end of uniform elongation means onset of failure in terms of useful deformation. Fracture comes later.

For biaxial stretchforming over a hemispherical punch, a local or thickness neck occurs when certain modes of deformation are allowed to form. The thickness neck is a highly localized thinning with no deformation along the neck. Therefore, with biaxial stretching the material on either side of the local neck remains rigid with no further deformation. This local neck usually starts with shear bands along the neck traversing at 45 deg. through the sheet thickness. The onset of the local neck formation (start of shear bands) defines when the sheetmetal fails. Fracture follows quickly (Fig. 2).

Having found the mechanism for the onset of failure, the third research phase studied several metal alloys in dead-soft, half-hard and full-hard condition. For the research to be useful, the final step was to determine if the strain at the onset of failure could be predicted. If so, maximum allowable combinations of useful strain would be available for part designers, troubleshooters, virtual press shops and other applications. We’ll present the results of this final research phase next month. MF

 


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