I. ObjectiveThe primary objective of this lab was to analyze how several different specimens reacted under increasing uniaxial load. Samples of Aluminum 6061-T6, Steel 4140, PS-4 Rubber and Polycarbonate were subjected to increasing axial loads using a mechanical testing machine. In performing these experiments, we as engineers were able to experience first-hand the derivations of materials formulas that are largely empirical. By reviewing the respective stress vs. strain graphs, we can easily calculate the Young’s modulus, 0.2% offset yield,and ultimate tensile strength of the material. II. Experimental Methods:2.1 Aluminum 6061-T6We performed our first test on a cylindrical rod of Aluminum 6061-T6. The tensile testing machine that we used was called ADMAT (Advanced Materials Tester), which has a maximum tensile load of about 10,000 Newtons. The aluminum specimen was a cylindrical rod, characterized by its larger diameter for the top and bottom portions (~12.7 mm) and a thinner centralized diameter (~6 mm). It was constructed in this way so that deformation would take place in the center and not the outsides, where the instrument clamps would make it difficult to measure. After we secured the specimen in the machine, we calibrated the extensometer, which ultimately measures local deformation in the central region of the specimen. We initialized the test, and ADMATbegan applying a steadily-increasing uniaxial load. Numerical data was uploaded to a computer in the form of a Force (N) vs. Deformation (mm) plot, and also in raw data file that can be exported into Microsoft Excel for further review. The machine continued to apply uniaxial load until the specimen experienced failure. This failureoccurred in the narrow central section, as expected, within the region of the extensometer’s knives. 2.2 4140 SteelWe tested two different specimens of steel using the mechanical testing apparatus. The first specimen was a cylindrical rod of steel that had been annealed by the lab technician prior to the test. This process of annealing consisted of heating the specimen in an oven to alter its mechanical properties. The annealed specimen had an initial central diameter of 6.15 mm. The second specimen was a cylindrical rod of as-received steel that had not been altered prior to testing. The as-received specimen had an initial diameter of 6 mm. We tested the annealed specimen first. We clamped the specimen into the machine, calibrated the extensometer, and initialized. We followed a similar procedure afterwards for the piece of as-received steel. Both specimens were subjected to increasing uniaxial loads until failure. Results of both tests were uploaded numerically and in the form of a Force vs. Elongation graph on the computer.2.3 PS-4 RubberThe specimen of rubber was cut from a thin, flat polymer sheet, so instead of a circular cross-section, it had a rectangular cross-section (3.14 mm x 1mm). The width of the specimen was greater on the top and the bottom with a narrower central-region, so that deformation would be localized to the center. This particular test was performed by a smaller, more accurate electronic tensile-tester (Zwick/ Roell Z 2.5) with a maximum tensile load of about 2,500 Newtons. We measured deformation in this specimen by using a camera that was calibrated to measure the differences in positions of markings drawn on the specimen itself. Once the test was initialized, the machine underwent a cyclic pattern of loading/unloading, and data was uploaded to the computer in a Force (N) vs. Elongation (mm) graph. The specimen was not tested until failure.2.4 PolycarbonateIn order to analyze the mechanical properties of Polycarbonate, we used a cylindrical specimen with asimilar shape to that of the aluminum and steel specimens. The cross-sectional area at both ends was greater than the area of the center so that deformation would be constrained to the narrower central region. The initial diameter of this central region was approximately ~9mm. We clamped the specimen into place on the tensile testing apparatus and calibrated the extensometer. We placed the extensometer towards the very center where the1specimen was likely to deform, and the test was initialized. The machine applied a steadily increasing uniaxial load, but the rate at which load increased was much slower so that the specimen would experience maximum deformation. Numerical data of the test was uploaded to a computer, as well as a graph of Force vs. Elongation of the specimen. Uniaxial load continued to increase steadily until the specimen experienced failure.III. ResultsLab Data ConversionsAs stated earlier, Lab data from each test was uploaded in the form of a Force (N) vs. Elongation (mm) graph. In order to obtain Engineering Stress vs. Engineering Strain relations, the following equations were used:sengineering=LoadAfffffffffffffffff [1] eengineering=L @L0L0ffffffffffffffffffff [2]In addition, True Stress vs. True Strain relations can be derived from known properties (as well asEngineering Stress and Engineering Strain) using the following relationships:strue=PAfffff=PA0ffffffffAll0fffff=seng1 +eengb c [3] etrue=Rde =ZL0LdLLffffffff=lnLL0ffffffff g=ln 1 +eengb c [4]True Stress and True Strain differ from their Engineering counterparts in that True Stress and Strain take into account the instantaneous cross-section and length, as opposed to the undeformed cross-section and length. Formulas [3] and [4] assume that volume stays constant. Graphs of Engineering Stress vs. Engineering Strain, aswell as True Stress and True Strain, for each material are attached to the end of this lab report and will be referenced throughout.3.1 Aluminum 6160 T-6As is evident in Figure A-1, the aluminum specimen initially deformed linearly with respect to the applied force. This occurred because the material itself was in the elastic regime, and molecular bonds remained intact but continued to flex under increasing uniaxial tension. If the load was suddenly removed while the material was still in this regime, the specimen would have returned back to its original length. However, around 9000 N, Figure A-1 shows that the trend is no longer linear, and bonds are beginning to separate at the molecular level. The specimen begins to visibly deform (as made evident by the formation of a localized neck) and weakens under increasing