Figure 1: Picture of specimen from Welding Journal Article.

In the previous parts of this series, we examined the fundamentals of ductility and shear stresses were identified as critical to ductile behavior. In this final column on ductility, we consider the factors that inhibit ductility to learn what conditions should be avoided or minimized in design or fabrication. As would be expected, factors that inhibit ductility are those that restrict the formation of shear stresses.

Constraint
Previously, we used Mohr's circle of stress to demonstrate whether multi-axial stress can reduce or even eliminate the shear stresses that are critical to ductility. While applied tensile loads were used in this illustration, restraint can have the same effect.

My personal interest in ductility goes back over 60 years. During WWII, I was Welding Superintendent at Globe Shipbuilding in Duluth, Minn. During a cold winter, we were fabricating a ship hull. One morning we returned to work to discover that a six-foot crack apparently had formed during the night. The welders were inclined to just gouge out the crack and reweld it, but I was concerned that we might have a piece of "brittle" steel, so I had the plate replaced. Instead of scrapping the steel, I sent it out to be tested.

I suspected that there might be some directionality associated with the steel. To test my suspicion, I oriented the test specimen so that the tensile loads would be applied perpendicular to the crack that had occurred in the shipyard. When it was tested in the laboratory, the steel that had been "brittle" in the yard achieved a respectable 48 percent elongation in the lab. There was nothing wrong with the steel. While is it true that the steel was very cold when it cracked during fabrication, and that the steel was warm when tested in the lab, temperature alone does not provide a complete explanation of what transpired.

The difference in behavior can be attributed to constraint. With the steel restrained in the vertical and longitudinal direction, when the weld and surround hot plate tried to volumetrically shrink, the restraint created triaxial stresses that reduced the shear stresses that are required for ductility. When the same material was tested in the laboratory, the simple uniaxial stress resulted in sufficient shear stresses for ample ductility to occur.

Thus, constraint is one factor that inhibits ductility. Constraint is increased with thicker members and with welds that intersect from multiple directions. Experienced fabricators and erectors have learned to minimize constraint during welding. On a complicated assembly, it generally is better to weld relatively fixed joints first, saving the somewhat flexible connections for welding at a later point. For highly restrained connections, increased preheat helps to mitigate cracking tendencies.

Notches
It is well known that the presence of sharp notches, and even 90 degree corners, provide conditions that raise stress and inhibit ductility.

The reason for such behavior is less well understood, however. Granted, as the term implies, the localized stress is elevated near a stress raiser, but this alone does not fully explain the lack of ductility. Consider the tensile specimens as shown in Figure 3. In part a, the length of the reduced section is equal to the width of the resulting cross-section. This permits shear stresses to act on a 45-degree plane, and yielding occurs at 35 ksi.

In part b, the length of the reduced section is decreased to 80 percent of the width. No longer can shear act on a continuous, 45-degree plane. Some restraint to yielding now occurs in the vertical direction, reducing the magnitude of the shear stresses. The measured yield strength is now 35.9 ksi, even though the material is unchanged from the material used to make the tensile specimen in part a.

As the length of the reduced section is progressively reduced to 40 percent of the width, the measured yield strength has increased to 50.7 ksi, as shown in part d. When the length is reduced to 20 percent, no yielding is discernable, and the specimen fractures at 91 ksi. Part f illustrates a severe notch in the material, and no ductility would be expected from such configurations.

Consistent with the paraphrased words of Gensamer "No shear, no ductility", both constraint and notches restrict ductility.

In this series, we discussed the basics of ductility as they relate to weldment design. To reiterate, for a material to exhibit ductility:

• The loading must result in a shear stress.
• The shear stress must exceed a critical value.
• The shear stress must result in movement in a favorable direction.
• There must be a sufficient volume of material to result in substantial movement.

Finally, in the absence of shear stresses, ductility is impossible.

Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Co., struck his first arc on his grandfather's welder at the age of ten. He is the author of Design of Welded Structures and Design of Weldments, and an internationally recognized expert in the field of weld design. In 1999, Blodgett was named one of the "Top 125 People of the Past 125 Years" by Engineering News Record. Blodgett may be reached at (216) 383-2225.