By Stephan Favilla 0723668 ME 354 AC
Date of Lab Report Submission: Date of Lab Exercise:
February 11th 2010 January 28th 2010
Tensile tests are fundamental for understanding properties of different materials, and how they will behave under load. This lab tested four different materials, including A-36 hot rolled steel, 6061-T6 Aluminum, polycarbonate, and polymethylmethacrylate (PMMA). Each material was tested three times using an Instron load frame and the BlueHill data acquisition software. The data from each test was used to determine valuable material properties such as ultimate tensile strength, modulus of elasticity, and yield strength. Other calculated properties included true fracture strength, percent reduction of area, and percent elongation. These material properties were used for comparing the materials to each other, and to define the material as brittle or ductile. The true stress and true strain were calculated for one sample of 6061-T6 aluminum to show the difference between the engineering stress and strain, and the true values. The engineering stress is an assumption that uses the initial area of the cross section, ignoring the effects of transverse strain and the changing cross section. This assumption results in the drop of the engineering stress-strain curve after the ultimate tensile strength, where necking occurs. Using the values of the true strain, the true plastic strain was determined for one sample of Aluminum (Sample #2) by subtracting the contribution of the true elastic strain, as outlined in Appendix E. Plotting the logarithm of the true stress versus the logarithm of the true plastic strain allowed the plastic portion of the true stress-strain curve to be modeled by the Ramberg-Osgood model, as detailed in Appendix F. While the model did poorly at low plastic strains near yielding, it did an excellent job just before necking and the ultimate tensile strain. The results of the tensile tests showed that the A-36 hot rolled steel was the strongest material. It had the highest ultimate tensile strength (527.9 MPa), the greatest modulus of toughness (174.6 MPa), and the largest true fracture strength (1047 MPa). The 6061-T6 aluminum had a higher yield (356.3 MPa) than the steel (355.6), but a lower ultimate tensile strength (374.9 MPa) and true fracture strength (571.8 MPa) due to tempering and precipitation hardening. All of the materials besides the PMMA proved to be ductile, especially the polycarbonate, which had a percent elongation of 82.2% The PMMA samples averaged a percent elongation of only 0.7333%. 2
Tensile testing is one of the most fundamental tests for engineering, and provides valuable information about a material and its associated properties. These properties can be used for design and analysis of engineering structures, and for developing new materials that better suit a specified use.
The tensile testing laboratory was conducted using an Instron load frame and the BlueHill data acquisition software. Four different materials were tested, including 6061-T6 Aluminum Alloy, A-36 hot rolled steel, polymethylmethacrylate (PMMA, cast acrylic), and polycarbonate. The samples were cylindrical in cross section, with a reduced gage section. The reduced gage section ensured that the highest stresses occurred within the gage, and not near the grips of the Instron load frame, preventing strain and fracture of the specimen near or in the grips. The reduced gage section of each specimen was about 12.7 mm (0.5 inches). The samples were already machined to the proper dimensions required for the test, according to ASTM standards.
Three samples of each material were tested in the Instron load frame, and the data gathered into an Excel spreadsheet. The data was used to calculate various properties of each material, including the elastic modulus, yield strength, ultimate tensile strength. The data was then...