A tension test is a destructive test in the sense that the specimen is finally broken or fractured into two pieces. To perform the tensile test, the universal testing machine should be capable of applying that load which is required to break or fracture the material.
The test piece or specimen of the material is generally a straight piece, uniform in the cross-section over the test length and often with enlarged ends which can be held in the machine grips. However, the machine can hold the specimen without enlarged ends also.
Many decades ago before the birth of digital sensor arm or non-contact extensometers, two fine marks were made near the end of uniform test section of the specimen and the distance between these points was termed the "gauge length".
The gauge length is that length which is under study or observation when the experiment on the specimen is performed. The gauge length of a specimen bears a constant standardized ratio to the cross-sectional dimension for certain reasons.
The specimen is placed in the machine between the grips and an extensometer if required can automatically record the change in gauge length during the test. If an extensometer is not fitted, the machine itself can record the displacement between its cross heads on which the specimen is held. However, this method not only records the change in length of the specimen but also all other extending / elastic components of the testing machine and its drive systems including any slipping of the specimen in the grips.
Once the machine is started it begins to apply an increasing load on specimen. Throughout the tests the control system and its associated software record the load and extension of the specimen. Finally, the specimen breaks in the form of cup and cone shape at the fracture point(for ductile metals). Before breaking, the area of cross section becomes very small, so a large stress is produced. The maximum stress which the specimen can bear is called the "ultimate tensile stress". We can also find the modulus of elasticity for the specimen.
LOADING FRAME:
There are many different types of load frame; electromechanical, servo-hydraulic, linear drive, and resonance drive.
CONTROLLER OR CONTROL ELECTRONICS:
It consists of oil tank having a hydraulic oil level sight glass for checking the oil level. The pump is displacement type piston pump having free plungers those ensure for continuation of high pressure. The pump is fixed to the tank from the bottom. The suction and delivery valve are fitted to the pump near tank. Electric motor driven the pump is mounted on four studs of which is fitted on the right side of the tank. There is an arrangement for loosing or tightening of the valve. The four valves on the control panel control the oil stroke in the hydraulic system. The loading system works as described below.
The return valve is close, oil delivered by the pump through the flow control valves to the cylinder and the piston goes up. Pressure starts developing and either the specimen breaks or the load having maximum value is controlled reciprocates. The switches have upper and lower push at the control panel for the downward and upward movement of the movable head. The “on and off” switch provided on the control panel and the pilot lamp shows the transmission of main supply.
METHOD OF TESTING:
Initial Adjustment:
Before testing, adjust the pendulum with respect to capacity of the test (i.e. 8 tones, 10 tones, 40 tones, etc.). For example, a specimen of 6 tones capacity gives more accurate result of 10 tones capacity range instead of 20 tones capacity range. These ranges of capacity are adjusted on the dial with the help of range selector knob. The control weights of the pendulum are adjusted correctly. The ink should be inserted in pen holder of recording paper around the drum and the testing process is started depending upon the types of test as mentioned below.
Tension Test:
Select the proper job and complete upper and lower check adjustment. Apply some Greece to the tapered surface of specimen or groove. Then operate the upper cross head grip operation handle and grip the upper end of test specimen fully in to the groove. Keep the lower left valve in fully close position. Open the right valve and close it after lower table is slightly lifted. Adjust the lower points to zero with the help of adjusting knob. This is necessary to remove the dead weight of the lower table. Then open the left control valve. The printer on dial gauge at which the specimen breaks slightly return back and corresponding load is known as breaking load and maximum load is known as the ultimate load.
Compression Test:
Fix upper and lower pressure plates to the upper stationary head and lower table respectively. Place the specimen on the lower plate in order to grip. Then adjust zero by lifting the lower table. Then perform the test in the same manner as described in tension test.
Flexural or Bending Test:
Keep the bending table on the lower table in such a way that the central position of the bending table is fixed in the central location value of the lower table. The bending supports are adjusted to required distance. Stuffers at the back of the bending table at different positions. Then place the specimen on bending table and apply the load by bending attachment at the upper stationary head. Then perform the test in the same manner as described in tension test.
Brinnell Hardness Test:
Place the specimen on the lower table and lift it up slightly. Adjust the zero fixed value at the bottom side of the lower cross head. Increase the load slowly ultimate load value is obtained. Then release the load slowly with left control valve. Get the impression of a suitable value of five to ten millimeter on the specimen and measure the diameter of the impression correctly by microscope and calculate Brinell harness.
Shear Test:
Place the shear test attachment on the lower table; this consists of cutter. The specimen is inserted in roles of shear test attachment and lift the lower table so that the zero is adjusted, then apply the load such that the specimen breaks in two or three pieces. If the specimen breaks in two pieces then it will be in angle shear, and if it breaks in three pieces then it will be in double shear.
EXTENSOMETRY:
An extensometer is a device that is used to measure small/big changes in the length of an object. It is useful for - measurements. Its name comes from "extension-meter". It was invented by Dr. Charles Huston who described it in an article in the Journal of the Franklin Institute in 1879. Huston later gave the rights to Fairbanks & Ewing, a major manufacturer of testing machines and scales.
There are many types of extensometer but they are mainly split into two categories (Contact and non Contact). Contact extensometers have been used for many years and are also subdivided into two further categories.
1) These are clipped onto a specimen (commonly called "clip-on" extensometers) before carrying out a . These devices are used for applications where high precision strain measurement is required (most ASTM based tests). They come in many configurations and can measure displacements from very small to relatively large (less than a mm to over 100 mm). They have the advantage of lower cost and ease of use, however they can influence small / delicate specimens.
2) For clip-on devices have been largely replaced by digital "sensor arm" extensometers.
These can be applied to the specimen automatically by a motorised system and produce much more repeatable results than the traditional clip-on devices. They are counter balanced and so have negligible effect on the specimen. Better linearity, reduced and synchronisation with the corresponding force data are big advantages due to the lack of analogue to digital converters and associated filters which add time lags and smooth the raw data. In addition these devices can remain on the specimen until failure and measure very high extensions (up to 1000 mm) without losing any accuracy. Theses devices typically have resolutions of 0.3 µm or better (the highest quality devices can read values as low as 0.02 µm) and have sufficient measurement accuracy to meet class 1 and 0.5 of ISO 9513.
For certain special applications, non contact extensometers are beginning to bring advantages where it is impractical to use a feeler arm or contact extensometer.
Working of the Instrument:
The required gauge length on a contacting extensometer is set either by manual adjustment or automatically by electric motoring the upper knife edges. Modern extensometers require no operator intervention and can set their own gauge length, apply and detach themselves to and from the specimen at the start and end of the test. They can also remain on the specimen until specimen break with out damage.
Video and laser extensometers require no contact with the specimen. require marks to be added to the specimen by hand whereas require no contact and also no manually applied specimen marks.
For older mechanical clip-on extensometers, a scale is provided for this purpose. Hold the specimen in the upper and lower jaws of Tensile/ Universal Testing Machine. Position the upper clamp to press upper knife edges on the specimen. The extensometer will be now fixed to the specimen by spring pressure. Set zero on both the dial gauges by zero adjusts screws. Start loading the specimen and take the reading of load on the machine at required elongation or the elongation at required load. Force setter accuracies mean of both the dial gauge readings should be taken as elongation. It is very important to note and follow the practice of removing the extensometer from the specimen before the specimen breaks otherwise the instrument will be totally damaged. As a safety, while testing the instrument may be kept hanging from a fixed support by a slightly loose thread.
Procedure :
Measure and record the initial dimensions of the tensile test specimen; also record the gage length of the extensometer.
After measuring the specimen, record the datas to the tensile test machines computer based data log to start-up the experiment.
Try to look the changes on specimen for taking off the extensometer before the fracture.
Once the experiment start and the device start to pull the specimen from both sides, try to determine the fracture mode.
When the specimen fracture and seperate to pieces take a look at the parts and observe the fracture.
Re-measure the specimen dimension to see the difference.
Look at the graph that the computer-based program produce to us and collect the exact datas to do calculations.
RESULTS and DISCUSION
Tensile Test Questions
1- During loading, the area under the stress-strain curve is the strain energy per unit volume
absorbed by the material. Conversely, the area under the unloading curve is the energy released
by the material. In the elastic range, these areas are equal and no net energy is absorbed. But
if the material is loaded into the plastic range, the energy absorbed exceeds
the energy released and the difference is dissipated as heat.
2- Two measures of ductility are elongation and reduction of area. The conventional means by which these measures are obtained is by pulling a specimen in tension until fracture.
Elongation is defined as the increase in the gage length of a test piece subjected to tensile forces divided by the original gage length. Elongation is expressed as a percentage of the original gage length and is given by:
Elongation (%) = 100 x ∆L/Lo where Lo is the original gage length.
If any portion of the fracture occurs on or outside a gage mark, the elongation and reduction of area may not be representative of the material.
Reduction of area also is expressed as a percentage and is given by:
Reduction of Area (%) = 100 x (Ao – Af)/Ao where Ao is the original cross sectional area.
Af is the cross sectional area after fracture and corresponds to the narrowest part of the neck which occurs at the point of fracture.
3- When a sufficient load is applied to a metal or other structural material, it will cause the material to change shape. This change in shape is called deformation. A temporary shape change that is self-reversing after the force is removed, so that the object returns to its original shape, is called elastic deformation. In other words, elastic deformation is a change in shape of a material at low stress that is recoverable after the stress is removed. This type of deformation involves stretching of the bonds, but the atoms do not slip past each other.
Strain Hardening - Following yielding, additional load may be applied which results in a stress-strain curve that continuously rises up to SUL indicating that the material is becoming stronger. When loaded beyond the yield point, ductile materials plastically deform and are subjected to cold working; this is referred to as Strain Hardening. In this range, the material’s elastic region increases, but its’ ductility decreases.
Necking - When a specimen is loaded beyond its’ ultimate strength the cross-sectional area begins to decrease in a localized region instead of over its' entire length creating a so-called "neck" which rapidly forms in this region as the specimen elongates. Since the cross-sectional area within this region is continually decreasing, the localized stress rapidly increases causing further localized elongation up to rupture.
4-When we look at the specimen, the fracture is starts after little necking. With using this information we can said that the fracture mode is ductile. In ductile materials, the crack moves slowly and is accompanied by a large amount of plastic deformation. The crack will usually not extend unless an increased stress is applied.
5- By looking the mechanical properties of the metal, we can said its a aluminium based component metal.When we look at the graph of stress-strain curve and see the data that we collect, aluminium is the possible answer for the tested material.
ERROR ANALYSIS
There was a error while we moving the extensometer from the specimen. It affects the graph of the curve that the computer-based program did.
CONCLUSION
In this experiment, we observe that the mechanical stress affect the property of the material.The tensile test measures the resistance of a material with appliying static force. It shows us the mechanical properties such as modulus of elasticity, yield strength, toughness, uniform elongation and the reduction in area at rupture by using computer-based datas and graphs.The graph that the computer program shows us the curve which is stress-strain results that are gave us the exact values of the critical point to help us for calculations.There were some error analysis in the experiment because of removing the extensometer. Moving the extensometer from the specimen was needed to careful moving. How ever when we tried to move it from the specimen there was a problem and it affect our graph in the computer-based program as well.