Many fabrication projects involve bending sheet metal into what can be quite complex shapes. This reduces part counts, simplifies assembly and increases strength. It may also be done to add visual interest to a design. (For an example of this last point, just compare a curvy Tesla Model S with a Cybertruck.)
Hard metals tend to be brittle and are more likely to crack than bend. To bend readily they need the property of ductility. Some metals have this, others don’t. In addition, many that start ductile can lose that property through the process of work hardening.
Ductility and work hardening are important in metal fabrication, and there are occasions when we’ll ask if a less ductile material can be replaced by one that’s more ductile. This blog explains why we might want to do that.
Understanding Ductility
Glass and ceramics are brittle. Drop them and they’ll shatter. Ductility is the flipside of brittleness. Ductile materials will bend without breaking, but importantly, they stay in the position they were bent into.
This deformation has two components; elastic and plastic. These terms relate to how the grains in metals, or cells in polymers move under load.
When a ductile material is placed under tension, first it deforms elastically. That is, when the load is removed it returns to its original shape. As the load is increased the deformation progresses beyond the elastic limit to become plastic. At this point, when the load is removed the material stays in its new shape.
The load at which deformation becomes plastic is the yield strength of the material. As the tensile load is increased further, the material reduces in cross-section (a process called “necking”). Eventually, it will shear, and the load this happens at is the ultimate tensile strength.
An important point to note is that, when bent repeatedly, many metals work to harden. This is where the structure becomes less mobile and more locked into place. (It’s why a piece of metal can be snapped by repeatedly bending it back and forth.) Work hardening means we only get one shot at putting in the deformation or shape needed before the metal starts to crack.
Temperature and speed of deformation are other factors affecting ductility. Take rubber and plastic for example. At room temperature, they bend easily, but in extreme cold, they’ll snap without bending. That’s because the polymer structure is unable to move to handle the load imposed. Likewise, bend them quickly and they’ll snap rather than deform. Metals are less susceptible to the effects of cold, but deformation speed can be an important factor.
Ductility in Fabrication
Bending metal, as we do on a press brake, needs the metal to be ductile. It also takes a feel for the speed at which it can be deformed. Bend it too fast and it will crack. (Grain structure is also a factor here. The sheet behaves differently in X and Y directions, due to how the rolling process deforms the crystals.)
Conversely, if we’re punching metal, ductility is a bad thing. Rather than have it bend around the punch, we need the metal to fracture as it’s hit. (That’s one reason punch presses work quickly.)
Ductility Data
There are two parameters commonly considered in deciding how a metal will behave during fabrication: hardness and tensile strength. In general, harder materials are less ductile and more likely to fracture. One indicator of ductility is the difference between yield strength and ultimate tensile strength. A large gap shows the material can deform before failure, while a small gap indicates little ability to take a plastic deformation.
An alternative, and often more useful tool in metal fabrication, is the forming limit diagram. This shows how a material responds to deformation in two directions. This is important because, as noted previously, sheet metal grain structure is not axisymmetric. A forming limit diagram is produced by bending a sheet of metal over a dome. Circles etched into the metal deform into ellipses, showing how the metal surface will move in every direction.
Applying Ductility Know-How
Once we understand a material’s ductility we can figure out how to bend it to the form we want. We know how much deformation it will take before necking, and how far we can move it before it starts to crack.
Every metal has a different level of ductility, depending on how it’s been processed or treated, what condition it’s in, and even what part of the coil it came from. As fabricators, it’s our job to understand the ductility of every piece of metal we work with because that tells us what we can do with it.
Some of this metal behavior is documented but much depends on having a good “feel” for metal deformation. As people who live and breathe fabrication, we believe this is something we possess. If ever you have a question about what can be done, contact us, and let’s talk.