Tensile Testing of Metals
Objective:[FONT='TimesNewRomanPSMT','serif']Stress-strain analysis of a material is one way to determine many of its physical properties. With the information gained through much analysis, one can predict how a part will react when placed under various working loads.[/FONT]
[FONT='TimesNewRomanPSMT','serif']The major objectives are:[/FONT]
[FONT='TimesNewRomanPSMT','serif'](1) Understand the basic process of deformation due to tensile loading[/FONT][FONT='TimesNewRomanPSMT','serif'](2) Characterize the physical properties of various metals from their stress-strain curves[/FONT][FONT='TimesNewRomanPSMT','serif'][/FONT]Equipment:1- Universal Testing MachineA sturdy frame supports a hydraulic jack from which a moving platform is suspended on ball joints. The space below the platform is used for tensile tests and that above it for compression tests. For tensile tests the specimens are fitted into chucks which screw onto ball joints at the top and bottom of the test section. This arrangement ensures purely axial loading. A pair of flat compression platens is supplied enabling compression tests to be performed on a wide range of material.
A high-impact strength clear plastic guard is provided to protect the user during testStrain gauged loaded cells are incorporated into the structure and a direct reading digital read-out load indicator is supplied. This strain measurement system is calibrated at the factory against an NPL (National Physical Laboratory) traceable standard.
A supporting table with integral cupboard, ref. SMl00h, is available.
2- Dial Gauge and Magnetic Stand:
A dial gauge provides a simple and accurate means of making general measurements of deflection.
For most purposes, a gauge reading to 0.01 mm is sufficient, but, for accurate tensile test, extensometers should be used. For normal use, clamp the gauge to the magnetic stand and position such that the anvil bears on the underside of the moving platform (preferably on one of the smooth metal cover plates). Always position the gauge such that movement is away from the gauge in order to protect it in the event of sudden movement, such as due to specimen failure. Don't use sensitive dial gauges when testing the specimen to failure.
3- Hand-operated pump:
The SM100 machine has been designed for use with a hand operated hydraulic pump. This pump is supplied separately and is intended for mounting on the base of the SM100.
Theory:[FONT='TimesNewRomanPSMT','serif']When a load is applied to a specimen, deformation will be obtained. This deformation is elastic if it is completely recovered immediately after the load is removed. Purely elastic deformation is associated with the stretching of primary bonds in materials. Stress is the force per unit area [/FONT]σ[FONT='TimesNewRomanPSMT','serif'] = F/A, and strain is the elongation per unit length ε[FONT='TimesNewRomanPSMT','serif'] = [/FONT]Δ[/FONT][FONT='TimesNewRomanPSMT','serif']L / L. The stress and elastic strain are directly proportional and related by the Modulus of Elasticity (or Young's Modulus) which is related to the potential energy of the inter-atomic bonds. Hooke’s law relates these parameters,[/FONT]σ = E[FONT='TimesNewRomanPSMT','serif'] ε[FONT='TimesNewRomanPSMT','serif'][/FONT][/FONT][FONT='TimesNewRomanPSMT','serif'][/FONT]
Where E [FONT='TimesNewRomanPSMT','serif']is Young's modulus. It is implicit here that only axial stresses and strains are of interest. [/FONT][FONT='TimesNewRomanPSMT','serif']If permanent deformation occurs, it is called plastic. The onset of plastic deformation corresponds to a stress level necessary to initiate the motion of dislocations (a type of defect) in crystalline materials. The stress necessary to produce permanent deformation is the yield strength of the material. Some materials exhibit a sharp yield point, whereas others show a slow change in slope at the end of the elastic range. In the latter, the yield strength is conventionally defined as the stress necessary to produce a plastic strain of 0.2% (elongation). In ductile materials, the strain to fracture is relatively large compared with brittle materials. Plastic deformation of ductile materials can require progressively higher stresses because dislocations multiply in the process and their motion becomes more difficult due to the increased degree of interaction among them. This process is called work-hardening. Uniform elongation of the gauge length occurs when the hardening rate is faster than the decrease in cross sectional area:[/FONT][FONT='TimesNewRomanPSMT','serif']If the hardening rate is too low, a runaway situation called necking develops. This corresponds to the load reaching the maximum value, and at this point the tensile deformation is inhomogeneous and strain is no longer uniform. The corresponding stress is called the ultimate tensile strength or UTS. The elongation to failure, which is the permanent engineering strain after fracture, is an expression of material ductility. It does not include elastic strain but does include a uniform localized strain, necking strain. The elongation to failure is usually stated as percent strain over a given gauge length.[/FONT][FONT='TimesNewRomanPSMT','serif']The deformation process is terminated by fracture. In a brittle material this occurs by the propagation of cracks initiated at the microscopic flaws in the material. Cracks propagate by cleavage, which involves breaking of atomic bonds along specific crystallographic planes, with the work of fracture spent primarily on creating a new surface (i.e., surface energy). The area under the engineering stress-strain curve is a measure of the energy needed to fracture the specimen and is sometimes a measure of a material's toughness.[/FONT][FONT='TimesNewRomanPSMT','serif'][/FONT]
[FONT='TimesNewRomanPSMT','serif']Engineering stress[/FONT] is the force per unit original [FONT='TimesNewRomanPSMT','serif']cross-sectional area of the specimen[/FONT] σ[FONT='TimesNewRomanPS-ItalicMT','serif'] = F/Ao. [/FONT][FONT='TimesNewRomanPSMT','serif'][/FONT][FONT='TimesNewRomanPSMT','serif']Engineering strain[/FONT] is the elongation per unit original [FONT='TimesNewRomanPSMT','serif']length of the specimen[/FONT] Δ[FONT='TimesNewRomanPSMT','serif']L/Lo. The true stress and strain are determined from the instantaneous dimensions during the test. Consequently, the engineering stress-strain curve does not give a true indication of the deformation characteristics of a metal because it is based entirely on the original dimensions of the specimen, and these dimensions change continuously during the test. Also, a ductile metal which is pulled in tension becomes unstable and necks down during the course of the test. Because the cross-sectional area of the specimen is decreasing rapidly at this stage in the test, the load required continuing deformation falls off.[/FONT][FONT='TimesNewRomanPSMT','serif'][/FONT][FONT='TimesNewRomanPSMT','serif'] [/FONT][FONT='TimesNewRomanPS-BoldMT','serif'][/FONT]
Procedures:1- Load the specimen slowly and as uniformly as possible. Keep tightening the locking screws initially to prevent the specimen slipping.2- Record the extension at load increments of at least every 5 kN in the elastic region.3- At the yield point, it is suggested that extra readings are made for a metal exhibiting an upper and lower yield point. Take readings at increments of 0.2 kN, otherwise recording readings every 0.5 kN should suffice.4- Continue loading and recording extensions at 0.5 kN increments up to a safe value below the forecast fracture load.5- Set the digital load meter to peak hold. Increase the load, slowly and uniformly, and fracture the specimen.6- Remove the specimen for study of fractured area. Fit the two pieces together and measure the final length between the marks and the diameter in the "neck" using a vernier.7- Calculate values for stress and strain and plot against each other.8- Determine value for young's modulus (E) from the graph.9- Calculate values for percentage reduction in area and elongation.