Tensile testing of metals

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TITLETensile testing of metals.OBJECTIVETo determine the tensile strength of metals.INTRODUCTIONThe tensile test is one of the most basic tests of engineering. This test is provides valuable information about a material and its associated properties. These properties can be used for construction filed (analysis of structures). The resistance of a material to a static or slowly applied force is measured using this test. A sample is subjected to a controlled (tensile) stress until the object fails (i.e. breaks).).This test can directly measure maximum elongation, which is the maximum stress the material can withstand, Ultimate tensile stress (UTS) and reduction in area. Young’s Modulus, Poisson’s ratio, yield strength, and strain-hardening can be calculate this test. The applied tensile load and extension are recorded during the test for the calculation of stress and strain. THEORYTensile testing is a way of determining how something will react when it is pulled apart - when a force is applied to it in tension. Initially, the dimensions such as area of cross-section, and the gauge length of the test specimen are measured. The force is applied at a steady rate on the specimen. The applied force and the extension of the specimen are recorded for the calculation of stress and strain.Engineering stress: Engineering strain:F – Applied load. Δl – Extended lencgthA0 – Initial cross-sectional area. l0 – Initial lengthThe specimen undergoes deformation throughout the course. Initially, the specimen undergoes elastic deformation, where if the tensile load is removed, the specimen would restore to its original dimensions. In more scientific terms this could be interpreted as the strain (deformation) of the specimen is proportional to the stress (load) applied. This is how Hooke’s law is expressed in modern theory of elasticity. If the load is continuously applied, it will lose its elasticity and enter the plastic region. The point at which the specimen shifts to the plastic region is known as the yield point. The plastic behaviour will continue until the specimen fractures. Young’s Modulus can be calculated this formula: .Figure 1:The wire diagram of the specimenFigure 2:Stress-strain behavior of the specimen with the specific points and regionsMATERIALS AND APPARATUS Sample of medium carbon steel.Manual Tensile-Testing Machine.Vernier Caliper. Area reduction gauge. Percentage elongation gauge.PROCEDUREFirst we got diameter of the gauge section was measured using the large callipers and the length of the gauge was measured using the smaller callipers. The specimen was then fitted to the tensometer using the grip sections of the specimen, the readings taken i.e. the area and the length were inputted to the data logger in the tensometer.The computer system connected to the machine and The specimens in the manual rotated tensile testing machine. (constant rate). This applied a tensile load on the specimen causing it to stretch.The rotation was continued until the specimen fractured. Throughout, this process, the load applied and the extended length was recorded by the data-logger present in the tensometer and the computer system was then prepared to record data and output necessary load-deflection graphs.The fractured specimen was removed and the percentage reduction in area of the gauge and at the necking point and the percentage elongation was using the area gauge and the elongation gauge respectively. CALCULATIONS Diameter of the gauged (d) = = RESULTSDiameter of the gauged (d) = Gauge length = Initial area of the gauge (A0) = Percentage elongation of the specimen =Percentage reduction in area of the gauge = Percentage reduction in area at necking point = Ultimate Tensile stress = Discussionthe tensile test first test material with yield point phenomenon in the first tensile test a plain carbon steel with yield point phenomenon is to be tested this is the test piece it has a cylindrical test region with an original diameter of 10 millimeters and an original gauge length of 100 millimeters within this test region distance marks have been drawn at regular intervals they help to visualize and measure the plastic behavior of the specimen using a hand control the tester moves the Upper cross head into its correct starting position now we can place the threaded ends of the test piece in the lower and upper grips of the testing machine in the next step he swings the extensometer into its working position and checks that everything is correctly prepared then he selects all necessary testing parameters on the control computer ready the test starts and the extents amitis sensor arms are carefully pressed onto the test piece this way the gauge length can be measured through out the whole tensile test the gauge length is displayed at the bottom right hand corner of the screen at the beginning it amounts to a hundred millimeters during the tensile test the test piece is slowly and constantly elongated with a standardized speed the force that the test piece opposes to the imposed elongation is recorded and can be seen at the bottom left hand corner of the computer display the material behavior can best be observed in a force elongation diagram the force F is being plotted upwards on the vertical axis the elongation Delta L towards the right on the horizontal axis at first the force rises rapidly force and elongation are proportional and form a steep straight line in the diagram in this area the material behaves elastically if the test piece were to be unloaded from this area it would spring back completely to its original length in materials with yield point phenomenon the end of the elastic area can be seen clearly the plastic deformation starts abruptly and is accompanied by a sudden drop of force if the test piece were to be unloaded now it would not spring back to the original length but instead show a permanent elongation in the next stage of the tensile test an almost constant force level with slight fluctuations occurs this phenomenon is called the ludus effect after a certain strain known as the ludar strain the force increases again the material opposes an increasing force against the imposed elongation it strain hardens up to the point of maximum force the test piece is strained uniformly along its length this means that the test piece gets longer and thinner but keeps a cylindrical shape as soon as the maximum force is reached a neck begins to form at one point of the test piece all further plastic deformation now only takes place at the neck and eventually the test piece fractures there in the recorded force elongation diagram the force F eh at the upper yield point can clearly be seen this is the highest force the test piece can sustain elastically fel is defined as the force at the lower yield point f max as the maximum force using these forces the strength properties of materials can be calculated the upper yield strength are eh is calculated by dividing the force F eh by the original cross-sectional areas 0 the lower yield strength arielle is defined in a similar way the maximum force divided by the original cross sectional area is called tensile strength RM in the last step the tester swings the extensometer back into its resting position and removes the broken test piece on the work table he puts the fragments back together again with the help of the distance marks the percentage elongation after fracture can be determined this is the permanent strain after fracture and amounts to about 30 percent in this example please note that the percentag

 

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