In the past, Die makers would review the trim or cut edge for signs of increased work hardening during the cutting processes. They would examine the sheet metal for burr...
Companies around the world use many different types of sheet metal to produce their products. When designers and Engineers are trying to decide which material to use to meet the desired attributes for their product, they are considering many different characteristics. Some parts need strength to with stand forces or pressures, some are light weight to be efficient, but all need to meet the formability requirements of a stable product design. One of the key components to proper selection, and process capability is understanding the Sheet Metals Mechanical properties. This leads us to two main questions; how can we get those mechanical properties? what does each mechanical property mean?
To respond to those questions, let start talking about the tensile test. The tensile test is a standardized method to determinate the materials mechanical properties. This method is performed by holding a sample, called specimen, in a rigid device and increasing the load or the stress applied to pulling on the sample until failure occurs. During this process, we record how much stress, or pulling force is being applied to the material, and what shape changes are occurring to the specimen. The shape changes we are looking for are length (elongation) and thickness of the material. The output information is plotted in graphical form and is referred to as the stress-strain diagram. Most of the key mechanical properties come from the tensile test.
The most used mechanical properties determined during the tensile test are:
The YS is a measure of a material’s elasticity, this shows the highest stress that can be applied to a material prior to permanent (plastic) deformation, in other words it is where the elastic deformation ends, and the plastic deformation begins.
The TS is the maximum amount of stress a material incurs prior to failure; it also defines the onset of necking.
The TE is a percentage by which a material can be stretched before it breaks, usually expressed as a percentage over a fixed gauge. Ductility is often reported as % elongation, which is also a rough indicator of formability (a higher number indicates improved formability).
The N Value is also known as strain hardening exponent; it indicates the relative stretch formability of sheet metals and the increase in strength due to plastic deformation. Measured as the slope of the true stress - strain curve generally between 10-20% strain. A higher number indicates better ability to reduce concentrated strain by redistributing the strain over a larger area.
The R Value, sometimes referred to as the plastic strain ratio; indicates the ability of a sheet metal to resist thinning or thickening. A higher number indicates better drawability.
A good understanding and use of these mechanical properties are very important and beneficial to the manufacturing industry, this will allow engineers to predict failures, behaviors and tendencies of metals during the forming processes. These mechanical properties can aid in the decision-making process for material selection by quantifying the information.
-Craig and Wil