[Music] the 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 mm and an original gauge length of 100 mm 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 cros head into its correct starting position now he can place the threaded ends of the test piece in the lower and upper grips of the testing [Music] [Music] machine in the next step St 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 extensometer sensor arms are carefully pressed onto the test piece this way the gauge length can be measured throughout the whole tensile test the gauge length is displayed at the bottom right hand corner of the screen at the beginning it amounts to 100 mm 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 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 T Cell test an almost constant force level with slight fluctuations occurs this phenomenon is called the Ludas effect after a certain strain known as the Luda 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 [Music] in the recorded Force elongation diagram the force F 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 re is calculated by dividing the force F by the original cross-sectional area s0 the lower yield strength is defined in a similar way the maximum Force divided by the original cross-sectional area is called T Cell strength RM in the last step the tester swings the tensometer back into its resting position and removes the broken test piece on the workt 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% in this example please note that the percentage elongation after fracture depends on the length to diameter ratio by measuring the smallest diameter at the point of fracture the percentage reduction of area can be calculated it describes the reduction of cross-sectional area at the point of fracture in relation to the original cross-sectional area in the second tensile test a material without yield Point phenomenon is to be tested in this case it is a precipitation strength lned aluminium alloy the test piece has exactly the same shape and dimensions as the specimen in the first test it is cylindrical with an original diameter of 10 mm and an original gauge length of 100 mm after fitting the test piece into the testing machine and panning the extensometer into its working position the test can start [Music] [Music] the initial linear curve in the force elongation diagram again shows the elastic behavior of the material but this time the end of the elastic area is not revealed by a sudden drop of force or any distinct change there is a smooth and gradual transition from linear elastic Behavior to plastic deformation in the further course of the tensile test the force increases and the test piece again is strained uniformly along its length at the point of Maximum force a neck develops and all subsequent plastic deformation is confined to this neck until fracture finally occurs there [Laughter] as a common substitute for the yield strength the 0.2% proof strength is used it is the stress that causes 0.2% of plastic deformation in the material and this is the way to calculate it first of all the elongation that corresponds to 0.2% of strain has to be computed for an initial gauge length of 100 mm the result is 0.2 mm in the force elongation diagram a straight line parallel to the elastic line is drawn through the offset point of 0.2 mm of elongation the force at the intersection point of the offset line with the force elongation diagram is designated as FP o.2 this is the force that causes a plastic strain of 0.2% the 0.2% proof strength RP o.2 is equal to FP o.2 divided by the original cross-sectional area s0 tensile strength percentage elongation after fracture and percentage reduction of area are calculated in the same way as in the first test [Music]