CHAPTER 6

MECHANICAL PROPERTIES OF METALS

"An Introduction to Strength of Materials for Middle School and Beyond," Nancy L. Denton and Vernon S. Hillsman, NEWU 93, pp. 147–151 and EMSET. This paper describes demonstrations that illustrate several basic mechanics principles using common equipment and supplies—viz., fishing line, plastic straws, a foam bar, hex head bolts, etc. These principles include axial force and deflection, buckling, torsion, and bending. Students may also be provided an opportunity to build and strength test structures using craft sticks and glue.

"Craft Stick Beams," Alan K. Karplus, NEWU 95, pp. 95–101 and EMSET. This experiment is designed, using craft sticks or popsicle sticks, to: (1) provide a phenomenological "hands-on" experience that shows how geometry influences the load carrying capacity of a material used in construction; (2) demonstrate that different materials have different failure characteristics; and (3) demonstrate how the performance of a composite material is affected by its construction.

"Designing, Engineering, and Testing Wood Structures," Thomas M. Gorman, NEWU 91, pp. 121–128, EMSET, and J. Mater. Educ., Vol. 13, p. 473 (1991). A standard bridge-type structure of specified configuration and having a span of 0.9 m (3 ft) is to be constructed using wire cross bracings and interchangeable wood pieces that have four different cross-sectional dimensions/geometries; cost per piece is a function of wood used. The goal of the exercise is to design a structure using these materials that will support a load of at least 23 kg (50 lb) at a minimum cost. The class is divided into groups, each of which is given a supply of lumber pieces and other requisite materials. Fifteen minutes are allowed for the groups to decide on a design, at which time the structures are assembled. Weights are loaded onto each structure in 4.5 kg (10 lb) increments until failure occurs or the optimum load is reached.

"Design and Construction of a Tensile Tester for the Testing of Simple Composites," Mark A. Borst and F. Xavier Spiegel, NEWU 93, pp. 53–99 and EMSET. The relatively inexpensive apparatus described in this paper is constructed of the following: (1) a threaded rod mounted on a hand wheel, which, when turned, applies the tensile force; (2) laminated maple support beams that make up part of the frame; (3) steel support rods that are attached to the maple support beams; (4) an oak drive beam; (5) a load cell; (6) a rail assembly; and (7) a set of grips that are machined from aluminum onto which faces sheets of rubber are attached. Calibration and verification of accuracy are necessary. Furthermore, the apparatus also allows measurement of lateral displacement of the specimen as it is elongated. Tests are performed on aluminum fiber-transparent plastic matrix and glass fiber-transparent plastic matrix composites, and fibers are embedded at several different orientations. Data are collected and presented as force versus elongation, and lateral displacement versus elongation. Moduli of elasticity are determined for the several orientations, and value comparisons are made.

"An Affordable Material Testing Device," Nikhil K. Kundu and Jerry L. Wickman, NEWU 94, pp. 201–209 and EMSET. This paper describes the design and construction of an apparatus that may be used to perform the following tests: tensile, punching shear, column buckling, beam bending, and spring rate. A metal frame is constructed from channel iron pieces and threaded rods; load is applied using a hand-operated hydraulic power pack. Other accessory equipment is also necessary and must be built. Also described is the manner in which the several tests are conducted; and actual test data are plotted.

"Table-top Experiments for Material Property Determinations," R. E. Smelser, E. M. Odom, and S. W. Beyerlein, NEWU 99, pp. 169–186. This paper describes how tensile, bending, and torsion tests are conducted using table-top apparatuses. The design and construction of this equipment were a components of senior design projects conducted by undergraduate students.

"Elasticity, Plasticity, and Anelasticity: Demonstrations," F. Xavier Spiegel, NEWU 96, pp. 293–295. Various combinations of the three phenomena—elasticity, plasticity, and anelasticity—are demonstrated using a coat hanger, a rubber band, a spring, modeling clay, and a plastic beverage container.

"Egg Bungee Cord Drop," Robert A. McCoy, NEWU 99, pp. 207–212. In this design exercise, groups of students design, construct, and test bungee cord apparatuses. The apparatus consists of an egg, contained in a plastic bag that are attached to a bungee cord. Groups of students compete to see whose egg, when dropped from a height of approximately 20 feet, comes closest to the floor without hitting it. Full-scale practice drops are not allowed; rather, students decide on cord length by conducting and analyzing static and dynamic drop data on short pieces (of lengths between 0.25 and 0.5 m).

"A Simple Experiment for Determining Elastic Properties of Polymers," P. K. Mallick and Subrata Sengupta, NEWU 99, pp. 491–498. This paper describes an experiment that may be used to measure the modulus of elasticity and Poisson's ratio of the plastic of which a two-liter soft drink bottle is made. The bottle is strain gauged and internally pressurized using a bicycle pump. The pressure-strain data are used to determine values of E and o.

"From Rugs to Demonstrations in Engineering Materials Class," Neda S. Fabris, NEWU 96, pp. 229–237. This paper describes several simple classroom demonstrations that illustrate the following concepts: modulus of elasticity, Poisson's ratio, shear stresses introduced by tensile loading, normal stresses in shear deformation, grain size strengthening, and dislocation movements.

"Measurement of the Modulus of Elasticity Using a Three-Point Bend Test," Richard B. Griffin and L. Roy Cornwell, NEWU 97, pp. 75–80. This paper describes a simple technique that may be used to measure the modulus of elasticity of various materials (viz. stainless steel, brass, graphite fiber-epoxy matrix composite) in three-point loading. Equipment required includes a bending fixture, a dial indicator, a micrometer, and weights (20 g to 200 g).

"Three-Point Bend Testing of Poly(Methyl Methacrylate) and Balsa Wood," Wayne L. Elban, NEWU 93, pp. 375–390 and EMSET. The modulus of elasticity for PMMA is measured in three-point loading. Loads are applied by placing weights on a platform that is in contact with the plate specimen, and specimen deflection is measured using a linear variable differential transfo rmer (LVDT). The second portion of the experiment involves measuring the modulus of elasticity of balsa wood (also in three-point bending), so as to determine whether or not it meets the design specifications for a bridge truss.

"Testing Rigidity, Yield Point, and Hardenability by Torque Wrench," Edward L. Widener, NEWU 93, pp. 501–502 and EMSET. This paper describes demonstrations of rigidity, yield point, and strain hardening. For the first demonstration, a large-loop paper clip is gripped in a vise; using a fish scale the clip is twisted 90o in one direction and then 90o in the opposite direction. Strain hardening is represented by the difference in pull forces for the two deformations. Quantitative measurements are conducted in a similar manner on a rod of a ductile metal alloy that is twisted using a torque wrench. Elastic return after twisting through a small angle is noted. Next, torque values are measured at various angles of twist in one direction (as the rod is plastically deformed), and then in the opposite direction; torque values are now plotted versus twist angle.

"A Device for Measuring the Elastic Modulus of Spherical Solids," Mtrook Al Homidany and Brian L. Weick, NEWU 97, pp. 515–524. Measurement of elastic moduli of spherical specimens is possible by measuring the contact diameter that is produced by pressing the spherical surface against a glass plate. The load is applied using a simple lever arm apparatus. Data are presented for two rubber materials.

"Learning More From Tensile Test Experiment," Neda S. Fabris, NEWU 97, pp. 169–182. Using results from tensile tests for metals, students are asked to generate true stress-true strain plots, compute strength coefficient and strain hardening exponent values, and estimate true toughnesses.

"Properties of Metals in Tension," Jack Kayser, NEWU 99, pp. 629–630. In this experiment, students perform tension tests on steel, aluminum, and cast iron specimens. Stress-strain data are plotted and the following mechanical properties are determined for each metal: ductility (both reduction of area and elongation), tensile and yield strengths, modulus of elasticity, and moduli of resilience and toughness.

"Measurement of Springback Angle in Sheet Bending," Jon Hilden, Kathleen Lewis, Ali Meamaripour, and Guna Selvaduray, NEWU 96, pp. 77–91 and J. Mater. Educ., Vol. 19, p. 185 (1997). The student determines springback ratios of sheet metal specimens as a function of bend radius. These values are compared to ratios that are predicted using a mathematical model.

"Simple and Inexpensive Method for Testing Shear Strength of Adhesive Bonds," L. Roy Bunnell, NEWU 99, pp. 299–302. This paper describes a technique that may used to measure the shear strength of an adhesive (e.g. epoxy). The adhesive is applied to the surface of a coarse-threaded 7/16 x 1-1/2 inches machine screw onto which a nut is threaded; the epoxy is then allowed to cure. Shear strength is expressed in terms of the torque required to move the nut; the twisting force is applied and torque measured using a common torque wrench.

"Composite Column of Common Materials," Richard J. Greet, NEWU 91, pp. 105–107 and EMSET. Several aluminum soda-pop cans are collected and their tops removed. Plaster of paris that has been mixed with water is poured into some of the cans and allowed to set up for one to two days. Top rims and dished can bottoms are removed; aluminum skins are then cut and peeled from about half of these cans. Compression tests are then performed on at least one empty can, at least one plaster of paris core material, and at least one plaster of paris core-aluminum skin composite. Students are to observe the mode of failure for each, and also the load at which failure occurs. These results are then discussed.

"Ductility and Fracturing," DemoBase

"Measurement of Strain Rate Sensitivity in Metals," Y. Y. Yang and R. G. Stang, NEWU 90, pp. 91–99 and EMSET. Wires of 60Sn-40Pb solder are deformed in tension and at room temperature using five different crosshead speeds. A true stress-strain curve is generated for each strain rate. Logarithm of true stress versus logarithm of strain rate data are plotted, from which the strain rate sensitivity coefficient of the solder may be computed.

"It's Hard to Test Hardness," Edward L Widener, NEWU 90, pp. 161–167 and EMSET. This paper explores some unconventional hardness testing techniques, and how their hardnesses compare with values measured using traditional techniques.

"A Novel Approach to Hardness Testing," F. Xavier Spiegel and Harvey A. West, NEWU 95, pp. 325–328, EMSET, and J. Mater. Educ., Vol. 17, p. 91 (1995). In this experiment a simple rebound time-measuring device is used to determine the relative hardnesses of a number of common engineering materials, and to develop relationships between rebound times and hardnesses.

"Scleroscope Hardness Testing," Patricia J. Olesak and Edward L. Widener, NEWU 93, pp. 453–455 and EMSET. The "elastic" and "plastic" hardnesses of several common materials are measured using a scleroscope hardness testing apparatus. Elastic hardness is determined by measurement of rebound height of a steel ball bearing that is dropped from an elevated position--plastic hardness by measuring the diameter of the dent that results. For aluminum, rebound heights are compared to hardness measurements made using an Equotip hardness tester.

"Application of Hardness Testing in Foundry Processing Operations: A University and Industry Partnership," Donald H. Martin and Bruce Lash, NEWU 92, pp. 223–232 and EMSET. The first phase of this experiment is a plant tour, wherein students are shown facilities that are used to precipitation harden commercial alloys. In the second phase, the instructor demonstrates indentation and hardness (Brinell) computation procedures. Students perform their own hardness tests, and then covert Brinell hardnesses to values in other hardness scales, and also to tensile strength. Limitations on hardness testing procedures are also discussed.

"The Underlying Structure of Engineering Materials," Mike L. Meier and Karl Ewald, NEWU 98, pp. 209–215. This paper presents a technique that may be used to tensile test a metal specimen while simultaneously video taping changes in its microstructure. One face of a tensile specimen (of annealed C26000 brass) is polished. After mounting in a tensile-testing apparatus, a stereo-zoom microscope is mounted and focused on the specimen surface. During the tensile test, load-elongation data are recorded and saved to disk. The microstructure is simultaneously monitored and recorded using a video camera that is directed through the microscope. Video capture and processing techniques using a PC are described.

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