Summary
Highlights
The video introduces the Materials I lab, focusing on materials characterization through tensile and compressive properties. The lab will test one copper sample and one low carbon steel sample in tension using a dog-bone type specimen, and a copper sample in compression.
Tilly, a PhD student, demonstrates the Hounsfield tensometer used for tensile testing. The machine measures force (load cell) and extension (extensometer). Specimens are mounted using collar grips and pins, and slack is removed before testing.
The process of starting the data recording program is shown. The video highlights the inherent uncertainty in sensor readings and the need to calibrate the load cell to ensure accurate measurements starting from zero. The force-extension graph begins to form as the wheel is turned.
The tensile test continues until the specimen breaks. After the test, the data, including extension and force, is exported to an Excel file for further analysis and graphing.
The video explains how to measure the percentage elongation of the broken specimens. For steel, it's 15%, and for copper, it's 35-36%. It also demonstrates how to measure the reduction in area (necking) at the point of fracture for both steel and copper, noting copper's extreme necking.
The setup for the compression test on a copper sample is shown. The specimen is carefully inserted, and the testing wheel is engaged. The test is stopped at 10,000 Newtons to prevent damage to the machine rather than breaking the specimen. The deformed specimen, exhibiting barreling, is then shown.
A test with a stiff bar is performed to account for machine deformation. Since the stiff bar should not deform, any recorded extension is attributed to the machine's internal movements. This data will be used to correct the force-extension graphs of the actual specimens.
The video moves to a whiteboard to explain how to correct force-extension graphs for machine deformation. By using the data from the stiff bar test (a straight line with a specific slope), the extension due to machine movement can be calculated for each force value and subtracted from the specimen's extension data, shifting the curve to the left for a truer representation.
The importance of standardizing graphs using stress (force over initial cross-sectional area) and strain (deformation divided by original length) is explained. This makes the material properties independent of specimen size, allowing for comparison between different materials like steel and copper.
The stress-strain graph for steel exhibits distinct elastic and plastic deformation regions. The elastic region signifies reversible deformation, while the plastic region indicates permanent deformation. The yield point marks the end of the elastic region.
From the elastic region, the Young's Modulus (slope) indicates stiffness. In the plastic region, the ultimate tensile strength (highest stress) can be found. For materials like copper, where the yield point isn't clear, the 0.2% offset strain method is used to determine it. This involves drawing a line parallel to the elastic region from 0.2% strain and finding its intersection with the curve.