What are the types of sheet metal testing?
- Tensile test
- Fatigue test
- Hardness testing
- Hole expansion test
In sheet metal forming, many techniques are implemented in order to provide the metal with a specific shape, size, or quality. As one of the oldest metal manufacturing technologies, sheet metal forming continues to be reliable, and innovations in the industry continue to define its landscape. But before any sheet metal-based product can be fit for assembly or wider distribution, they need to undergo several sheet metal testing types as a way of defining their characteristic values. As more industries continue to leverage this form of production, these testing methods provide manufacturers with a way to gauge metal formidability, strength, quality, and the like.
The types of sheet metal testing differ mainly in the machinery and the property of the material being tested. Some of the most popular techniques include tensile, fatigue, hardness, and hole expansion. In order to understand the limits of material deformity before any stress or breakage exhibits, these tests are performed under controlled conditions for more accurate results. Read on to learn more.
By definition, tensile is defined as the capability of a material to become stretched or drawn out until cracks or stresses begin to show. Another more common term is “tensile strength” which is the resistance of breaking under impacts or stresses. This term is used to describe the limit at which steel or any ductile material transforms from temporary elasticity to permanent deformation. To put it simply, when a material has been stretched past its tensile strength rating, it will break apart.
In sheet metal fabrication, undertaking tensile tests or tensile strength tests is important because it predicts the reproducibility of a given product. This is especially useful for the mass production of metal goods, wherein each product must have relatively the same measurements for tensile strength. For example, even if a single sheet of a metal coil is formed in the same facility, material characteristics will still vary, affecting the quality of the part and scrap rate.
As one of the most common methods for testing metal, tensile strength tests are widely available and can be done on universal testing machines (UTMs) that are also capable of other types of mechanical tests. In this case, a small sample of sheet metal is loaded into the machine and drawn out. The operator records the specimen’s maximum load values, as shown on a computer screen.
Another type of metal testing method is known as the fatigue test. Unlike tensile strength tests where a specimen is subject to only a single complete execution, fatigue testing is done under a cyclical load that constantly adds stress to the material. This is done at a certain frequency or alternating load tests in order to measure tension or compression.
Material failure in fatigue testing takes place when damage begins showing on the specimen after being subjected to frequent repetitions of stress. This type of testing is crucial in understanding why metal components that have been used for long periods may suddenly fail. Oftentimes this failure occurs not because of a single overload, but a continuous pattern of cyclical stress drawn out for a certain timeframe.
Fatigue testing methods can be further subcategorized into a high cycle or low cycle testing. For the high cycle test, the finite life fatigue strength and the high cycle fatigue strength are determined. Some examples of these two types in action can be found in turbine blades or stationary power-generating turbines that undergo disc strain when in constant use.
Most mild steel or low carbon steel sheet metals that are 1.5 millimeters in thickness will most likely have a Rockwell B hardness rating. Rockwell hardness is simply the measuring range that determines the resistance of a material to permanent deformity and penetration by another material. This is usually done for certain types of steel, such as tool and cutting steel which is engineered to be more durable than the typical.
As mentioned before, mild steel will record a Rockwell B hardness rating that falls in the mid to high 70s. Three main components are involved in this type of testing — the indenter, anvil, and the specimen. Here’s a brief illustration of the process:
- The minor load is pressed onto the specimen and generates a reference depth for the measurement. For Rockwell B, around 10 kg/cm2 of force is used.
- To achieve a deeper penetration, an additional load is pressed onto the surface of the sheet metal. It is removed then a minor load is re-applied.
- The Rockwell B hardness rating is calculated by measuring the difference between the depth and the reference depth done on the material.
Hole Expansion Test
Hole expansion testing is specific for punched sheet metals and is done to assess the ductility (the ability of the material to be formed into a wire without breaking) on the sheet metal’s edges. This method is applicable especially for high-strength steel products, which face challenges on edge cracks, especially when sheared.
The hole expansion test starts off with shearing a 10mm-diameter hole and widened using a conical punch at 60°. The resulting ratio of the expanded diameter to the initial measurement is subsequently known as the hole expansion ratio. Since shearing creates significant alterations to the material’s forming properties for sheet metal edges, this technique proves to be a fast and economic way of measuring the change.
As discussed, sheet metal testing types vary on what the manufacturer wants to measure regarding certain properties of a steel or metal sheet. Tensile strength tests, for example, determine the point at which the material starts displaying cracks after stretching. Fatigue testing determines cyclical material failure, while hardness testing and hole expansion tests measure resistance to permanent deformity and ductility respectively.
All of these tests have been crucial for the sheet metal fabrication industry to continue coming up with quality materials for different manufacturing sectors. These methods will continue to be reliable and will no doubt pave the way for further innovations in sheet metal manufacturing.