Equipment: Universal testing
Familiarize yourself with the machine. Measure and record the exact diameter of each specimen using the vernier calipers. Mount a test specimen in the upper part of the testing machine. Turn all of the threaded ends into the grips at both ends of the machine head. Adjust the handwheel below the lower platter to take up all the free travel in the specimen mounting but taking care not to apply any load. Measure the distance between the grips and record this as the original length of the specimen. Install the dial gauge appropriately to record change in specimen length during loading. Record the initial dial gauge reading.
Zero the load indicating gauge. For better accuracy, swing the red maximum-load indicating pointer out of the way. Close the hydraulic valve on the front of the machine. Apply load slowly and at a uniform rate by very gradually pumping the operating handle. Simultaneously read and record the load gauge and dial gauge readings at approximately equal small load intervals. Constantly observe the specimen. Record the load at which the specimen begins to "neck-down". If the load remains fairly constant while the dial gauge reading is increasing, record the load value at small intervals of change in length. Record the maximum load attained and the corresponding dial gauge reading. Continue loading and recording the load and deformation at intervals until the specimen fails. Immediately after failure, touch the specimen at the region of failure and note how warm it feels.
- Repeat the procedure above for two other specimens of the same material.
For the test specimens,
the load-deformation diagrams for the three specimens tested on one chart.
2.) Draw the stress-strain diagram for the three specimens tested on one chart.
3.) Determine the yield strength, ultimate strength and rupture strength for each specimen.
4.) Determine the average value for the yield strength, ultimate strength and rupture strength for each material.
5.) Based on your results and material properties given in your textbook, identify the materials of the specimens, giving reasons for your answers.
6.) The strain for this test was determined by considering the entire length of the specimen between the grips as the original length. Would the strain be different if the smaller standard gage length of two inches had been used? Explain.
7.) Why is it necessary to test more than one specimen to obtain a reliable estimate of the physical properties of a material?
8.) List the factors that influence the results obtained from this test.
9.) What is the difference between true stress-strain diagram and nominal stress-strain diagram. Illustrate your answer using the stress-strain diagram for mild steel.
10.) Answer true of false and state why: In tensile tests, (a) True stress is always smaller than nominal stress; (b) True strain is always smaller that nominal strain