09/17/2024
Today, I spent my time in the lab doing tensile testing, an experiment that’s necessary for understanding the mechanical behavior of metals. In a tensile test, you essentially pull a sample until it breaks, all while recording the force applied and how the sample stretches. The setup follows strict guidelines (like those from ASTM standards), using specific sample dimensions, like a 2-inch gage length and a 0.505-inch diameter for round bar stock. The sample is threaded on both ends, allowing it to be secured in the machine grips for a stable test.
During the test, an extensometer measures how much the sample lengthens as force is applied, and this data is used to calculate stress and strain. Stress is the force applied divided by the original cross-sectional area, while strain measures how much the sample stretches compared to its original length. Plotting these values gives you a stress-strain curve, which shows the material’s response, from initial elastic stretching to permanent plastic deformation, and finally, fracture.
The stress-strain curve has two main sections: elastic and plastic deformation. In the elastic region, the material stretches in a reversible way—take away the force, and it snaps back to its original length. But once you go beyond the elastic limit, the material starts to deform permanently. This is where you start to see a mix of elastic and plastic strain, and if you unload the sample at this point, it won’t fully return to its original shape.
As the test continues, the sample’s cross-sectional area decreases due to necking, where the material thins out at a concentrated spot. This part of the test shows you not just the strength of the metal but also its ductility, or how much it can stretch before snapping, expressed as percent elongation and percent reduction in area.
From these tests, you can determine various material properties like Modulus of Elasticity (how stiff the material is), Yield Strength (the stress at which permanent deformation begins), Ultimate Tensile Strength (the maximum stress the material can withstand), and Fracture Strength (the stress at the point of breaking).
These properties are all very important when selecting materials, as you can imagine.
