To the untrained eye, operating a press brake or plate roll may just seem as simple as pushing a button. However, it doesn’t take long for even a novice operator to understand that it’s not that simple. While CNC machinery has taken a lot of the guesswork out of the fabrication process, understanding the yield strengths of different metals is an important step as you grow. This understanding will help guide your manufacturing to new heights and understand why certain metals react differently under different conditions.

What is Springback in Metal Fabrication?

One of the earliest things to learn as a press brake or plate roll operator or engineer is how to deal with springback. Springback, as the name suggests, is how the material once bent will attempt to return, or springback, to its original prefabricated state. Despite the perception of metals being rigid and completely solid, most structural metals such as steel, aluminum, copper, and even titanium is somewhat elastic during the fabrication process. This means that these metals will bend under a certain load pressure but try to revert once that pressure has been removed.

Understanding Metal Yield Strength

However, when understanding yield strengths, you can adjust the load pressure to effect a change in the material, so the deformation becomes permanent. The part of the bend or roll that remains elastic during this process is the cause of springback. Usually, operators and engineers will compensate for this springback by over bending to a smaller angle with a press brake or rolling a tighter diameter on a plate roll. This allows the material to loosen to the desired angle or radius when the part is removed from the machine’s pressure.

Understanding Metal Strength

One of the areas to understand is the use of strength as a way of describing different metals. A metal may be hard but not strong since a hard metal may also pose the risk of being brittle. Hardness is how resistant a metal is to penetration. So, materials like AR400 steel which is specifically designed for abrasion and wear-resistant applications, are very hard and able to withstand the impact of large rocks for instance but the same material must be formed very carefully to not crack during fabrication.

What is the Tensile Strength of Metals?

One of the most common gauges of strength is tensile strength. Tensile strength is the measure of how resistant a metal is to break or fail when put under tension. Tension is the fabrication process of either bending or stretching the metal materials. A different measurement is known as yield strength, which indicates the highest stress any metal can handle before permanent change happens and that metal loses some of its elasticity. Understanding the yield strength and tensile strength of the metal used in your fabrication and manufacturing processes is crucial.

What is the Stress Point of Metals?

The importance of understanding tensile and yield strength cannot be understated. However, the most immediate information needed for a metal fabrication or manufacturing shop is to understand the stress or pressure point that a piece of metal can endure before each limit is reached. For bending, rolling, and similar fabrication processes, the piece of metal must pass the yield strength limit without coming too close to the limit of tensile strength.

Are Metals Elastic and Plastic?

Another set of words to understand when looking at metallurgy is elastic and plastic. In metal engineering elastic references the point of stress under which the material will revert to its original form when the amount of stress has been removed. Plastic deformation conversely describes the point of stress needed to cause a permanent change.

There is a correlation between the amount of stress exerted on a piece of metal and the amount of deformation left in the piece of metal when the stress alleviates. As this stress builds, the metal enters the plastic phase of deformation. The correlation is proportional and exists between stress and strain until the material ultimately fails if the stress continues to climb to the point of failure of the metal material you are forming. There is a slight curvature of a straight-line proportion known as the “Proportional Limit”, even though elasticity remains at this point.

Once the stress of the forming process reaches the elastic limit, the metal enters the plastic phase of deformation. Another term for elastic limit is yield point, as it begins to yield or give in, to a new permanent shape. The stress required to reach this point is the yield strength of the material.

This exact transition point from elastic to plastic is hard to identify in many different materials due to its gradual onset. Due to this, an offset method is commonly used to define yield strength. This offset yield, commonly called proof strength, is represented by the percentage of plastic strain at that point, often 0.2%. This method is used to provide a more consistent and reliable method for establishing yield strength.

Upper and Lower Yield Strength of Metals

To complicate things even further, the crystalline structure of some metals such as mild steel, possess both an upper yield point (where plastic deformation starts) and a lower yield point (where strain hardening begins). Due to the lower yield point being generally more stable than the upper yield point, it is typically used in structural engineering for design calculations.

Does Every Metal Have Yield Strength?

Every solid material is measurable in terms of strength. However, because certain inflexible materials exist such as marble, not every solid material has yield strengths. These materials do have specific tensile strengths though since they can break. For reference, human skin has a yield strength of approximately 15 MPa (megapascals) compared to Kevlar which has a yield strength of 3620 MPa.

What is the Yield Strength of Metal?

Different metals have a wide range of yield strengths. Obviously, we cannot go into every metal yield strength but we have included a list below of some of the common metals we see in fabrication and manufacturing. Additionally, annealing is the process of heating metal and allowing it to cool slowly, to remove internal stresses and make it easier to work. For more information about yield strength of different metals, visit The Engineering Toolbox here.

  • Annealed Aluminum 15-20 MPa
  • Annealed Copper 35 MPa
  • Aluminum 35 MPa
  • Copper 70 MPa
  • UNS C93200 Bearing Bronze 125 MPa
  • AISI 1020 Hot-rolled Steel 210 MPa
  • 6061-T6 Aluminum Alloy 241 MPa
  • AISI 1018 Mild Steel 370 MPa
  • Titanium 450 MPa
  • AISI 302 Cold-rolled Stainless Steel 520 MPa
  • AISI 4340 Oil-quenched and Tempered Steel 1620 MPa
  • Tungsten 1725 MPa

A quick note: alloys such as steel have no single yield strength. The strength of each variation is dependent on the different percentages of each alloying element that makes up the material. The procedure used to create the material also affects the yield strength of the metal. Different processes such as annealing, tempering, and hardening will all affect the final yield strength in different ways.

Conclusion

The job of the metal fabricator or operator is to make metal yield to your will! By learning how to properly overcome the yield point of your workpiece without fatigue or failure (of the metal) you can increase the precision and capabilities of your bending fabrication. Does all this sound good? It’s a lot to take in but when you partner with Moore Machine Tools, you partner with metal bending experts.