TENSION TEST ON STEEL ROD
To study the behavior of mild steel, tore steel and high tensile steel under tension and find out the Young’s Modulus, Ultimate strength, percentage elongation
UTM, Extensometer, scale Venire calipers, punching tools etc.
DESCRIPTION OF MACHINE
The machine consists of 2 main units, viz, the loading unit and the control panel.
This consists of a robust base. The main hydraulic cylinder is fitted in the centre of the base. A geared motor is fitted to the base and the chain and sprocket driven by the motor rotate the two screwed columns mounted in the base. The lower table is connected to the main piston through a ball and seat joint. This joint ensures axial loading.
This consists of a hydraulic power pack which contains the hydraulic oil. Oil level sight glass is fitted to the oil tank to check the oil tank. A positive displacement type piston pump is provided to assure a continuous high pressure non- pulsating oil flow for the smooth application of the load on the specimen. A pendulum dynamometer is fitted to measure and indicate the force coming on the specimen. A big size load indicating dial fitted with a glass cover is mounted at the side of the control panel. The range indicating dial (located at the back side of the load indicating dial) is to be adjusted for the particular range selected.
THEORY AND PRINCIPLE
The specimen is subjected to tensile load and extension is noted against the load with in the elastic limit. Loads at, Yield point, Breaking point and Ultimate point are noted. With these, stress-strain graph is plotted and following results are calculated.
Modulus of Elasticity = Stress (with in the elastic limit)/Strain.
Yield stress = (Load at yield point /original c/s area)
Ultimate stress = (Ultimate Load/ original c/s area.)
Nominal Wreaking Stress = (Breaking load / Nominal Breaking Stress)
Actual Breaking Stress = (Breaking load / neck area)
Percentage elongation = (Change in Length/ Original length) x 100
Percentage reduction in area= (Change in Length/Original area) x100
Clean the given rod with sand paper. Measure the diameter of the rod. Calculate the gauge -length using the formula . Mark the gauge length (say 20mm) on the rod. The marking are made by punching. Assuming a tensile stress, calculate the maximum expected load on the select the range of the machine. Adjust the ram for the range. Firmly grip the upper end of the specimen by operating the handle. The left valve is kept fully closed position and the right valve in normal open position. Open the right valve and close it after the lower table is slightly lifted. Adjust the load pointer to zero with the zero adjusting knobs. By operating the handle, lift the lower cross head chuck up and grip firmly the lower part of the specimen. Lock the jaws. Fix the extensometer on the specimen and set the reading to zero. Turn the right control valve slowly to open position to get desired loading rate. When the specimen is under load, unclamping the locking handle. Note the extension at a convenient load increment. Extensometer must be removed before reaching the yield point. Again apply the load. At some point, the load pointer will remain stationary. Load corresponding to this indicate yield point. Apply the load again and note the ultimate load before the pointer goes backward and specimen breaks at a particular load. Note the breaking load. Close the right control valve, take out the broken piece. Open the left control valve to pump the oil back. Maximum capacity of specimen can be seen against the red pointer. Measure the diameter of the specimen at the neck.
Assuming working stress = 140N/mm^2.
Factor of safety = 3.
ie Ultimate stress = 140×3 = 420Nmm^2.
Ultimate load = 420 x area of cs.
From the ultimate load, range to be used can be fixed.
1. Young’s Modulus = ______ N/mm^2
2. Yield stress= ______ N/mm^2
3. Ultimate stress = ______ N/mm^2
4. Nominal Breaking stress= ______ N/mm^2
5. Actual breaking stress = ______ N/mm^2
6. % Elongation = ______
7. %reduction for Area= = ______
Questions – Do you know?
1. Why the testing machine is called Universal Testing Machine?
2. What is the maximum capacity of UTM in the laboratory?
3. What are the possible ranges in the UTM and what is the corresponding least count?
4. How do you change the loading capacity of the UTM?
5. How do you grip the specimen for tension test?
6. How do you find set the load pointer to zero in UTM?
7. What is the use of dummy pointer?
8. What is the name of the device used for measuring elongation of the specimen in tension test?
9. What is gauge length? Explain its importance.
10. What is the gauge length used for tension test?
11. Before you mount a specimen on the testing machine for any test, what are the preliminaries you must complete?
12. Identify the moving cross head in UTM?
13. Explain the terms ductility and brittleness. Give example.
14. Differentiate between elastic limit and yield point.
15. Differentiate between elastic limit and proportional limit?
16. What is the purpose of calculating percent4ge elongation and percentage reduction in area in tension test?
17. What is mean by an isotropic material?
18. When is the neck formed in tension test?
19. Why breaking load is less than the maximum load?
20. What is true stress-strain curve?
21. How do you find the area of cross-section of deformed bar?