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Tensile testing - Lab Report Example

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Firstly specimens of different materials will be used to determine Young’s modulus; secondly specimens will be tested to…
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School of Maritime and Mechanical Engineering Faculty of Engineering and Technology Lab 7 – Tensile Testing Aim The aim of this laboratory is to gain a greater understanding of tensile testing procedures and to undertake a series of tensile tests on a range of materials.
2. Objectives
To familiarise the student with tensile testing procedures and equipment.
To conduct tensile tests on a range of engineering materials.
To determine the tensile properties of materials by analysing test results.
3. Apparatus
In this laboratory a Houndsfield test frame (figure 1) is used to apply a tensile load to a variety of test specimens made from different materials. Firstly specimens of different materials will be used to determine Young’s modulus; secondly specimens will be tested to destruction in order to establish key material properties.
Strain indicator box
LVDT
Test specimen
Load cell
Loading handle
Figure 1: Houndsfield test frame
The apparatus needed for this experiment are:
Houndsfield test frame
Strain gauged steel test specimen
Steel test specimen for destructive test
Aluminium alloy test specimen for destructive test
Brass test specimen for destructive test
Strain indicator box
Computer for results acquisition
%age elongation gauge
%age reduction in area gauge
3. Theory
One of the most common methods of measuring the mechanical properties of a material is through a tensile test. The test consists of loading a specimen of known dimensions in tension, with the loading force being gradually increased up to a predetermined magnitude or until the sample breaks.
Data from the test is either collected through a data acquisition system and subsequently plotted using appropriate software or plotted using an X-Y plotter. The data typically consists of the load applied to the specimen and the extension or the stress and strain (since the initial cross sectional area of the specimen and gauge length are constants both the load v extension and stress v strain graph have the same shape); a typical plot for a metallic material is shown in figure 2.
Figure 2: A typical stress v strain plot from a tensile test
(Ashby, MF; Jones, DRH: Engineering Materials 1 – An Introduction to Properties, Applications and Design (3rd edition), 2005, Elsevier Butterworth Heinemann)
Tensile specimens may have a variety of cross-sections, although rectangular or cylindrical ones tend to be more common, and may be in a machined or unmachined state. The geometry of a typical specimen is shown in Figure 3, where (note that the nomenclature may vary):
ao original thickness of a flat test piece or wall thickness of a tube
bo original width of the parallel length of a flat test piece
Lc parallel length
Lo original gauge length
Lt total length of test piece
Lu final gauge length after fracture
So original cross-sectional area of the parallel length
1 gripped ends
Figure 3: Typical tensile test specimen geometry with a machined rectangular cross-section
(BS EN ISO 6892-1:2009 Metallic materials. Tensile testing. Method of test at ambient temperature)
The mechanical properties typically derived from a tensile test include the yield stress (or more commonly the offset proof stress), tensile strength (stress), elastic (or Young’s) modulus, percentage total extension at fracture and reduction in cross-sectional area.
A more detailed explanation of the range of properties that may be determined from a test and the analysis involved may be found in BS EN ISO 6892-1:2009 and ASTM E8M. It is good practice to test to a standard such as these as they stipulate various parameters, such as the speed of testing (the rate at which the load is applied) and shape of the specimen, which may affect the results obtained, although some companies will have their own procedures that will ensure compatibility between tests within that company.
Stress is usually indicated by the Greek letter σ (sigma) or S and is the measure of the force per unit area. It has the units N.m-2 or Pa (Pascal) (megapascals, MPa, are more commonly used, these being 106 x N.m-2 (106 Pa) or N.mm-2, due to the magnitude of the values encountered).
(See Figure for details of nomenclature)
Strain is usually indicated by the Greek letter ε (epsilon) or e and is the amount a material extends in response to an applied force. Strain is dimensionless and therefore has no units, although it is frequently expressed as a percentage.
(See Figure for details of nomenclature)
The stress-strain graph of many materials frequently consists of a linear region (this is the section depicted as the yield strength in Error! Reference source not found.) and a non-linear section as shown in Error! Reference source not found.. In the linear section the material exhibits elastic behaviour, i.e. if the load is removed the specimen will return to its original size and length (it acts like a spring or an elastic band). In the non-linear section the material exhibits plastic behavior and undergoes permanent deformation. The transition point between the linear and non-linear regions is known as the yield point and the magnitude of the stress at which this occurs is known as the yield strength, σY.
Some metallic materials have a clearly defined yield point, but in more recent times the processing techniques employed in the production of materials have resulted in this becoming less well defined and the proof stress is frequently stated. The proof stress is determined by constructing a line parallel to the linear region, but offset by a small amount of strain, typically 0.1% or 0.2%, the stress at which the intersect between this constructed line and that from the tensile test plot is the proof stress; in Error! Reference source not found. the 0.1% proof stress, σ0.1%, is indicted.
The elastic or Young’s modulus, usually indicated by the capital letter E, of a material relates to how much a material will displace for a given force when behaving elastically and is calculated as the slope of the initial linear section of the tensile graph, i.e. the stress/strain, and usually has the units GPa (109 x Pa or 109 x N.m-2).
The tensile strength, σTS, of a material is the maximum stress it can withstand and is typically measured in MPa (the tensile strength is sometimes indicted as the ultimate tensile strength and given the abbreviation UTS). It is calculated by:
The tensile strength occurs at the onset of necking. Necking is the result of the plastic deformation becoming localised to one section of the specimen, the result being a reduction in the cross-sectional area in this region. Stress v strain plots often then appear to exhibit a region where the strain increases, but the stress decreases. This occurs because most results do not take into consideration the reduction in area that
occurs due to the tensile loading; in reality, if this reduction is taken into consideration the stress would continue to increase:
When the effect of necking is ignored the calculated stress is known as engineering stress, whereas if necking is taken into consideration it is known as the true stress.
Figure 4: Tensile test results from mild steel indicating both true and engineering stress v strain plots
Continued application of the load will result in fracture of the specimen. If a line is drawn from the point of fracture, parallel to the linear region initially exhibited at the beginning of the test, to the x axis the plastic strain after fracture may be calculated as shown in Error! Reference source not found.
5. Experimental Method
1) Accurately measure and record the diameter of the test specimens ( ).
2) Load the strain gauged steel specimen into the test frame using the split collets, connect the strain gauge into the strain indicator box.
3) Set the virtual strain indicator box on the PC to acquire data.
4) Load the specimen slowly using the loading handle to a maximum load of 4kN.
5) Load the un-instrumented steel specimen into the test frame using the split collects, connect the strain gauge into the strain indicator box.
6) Set the virtual strain indicator box on the PC to acquire data.
7) Load the specimen slowly using the loading handle to destruction.
8) Repeat 10) above for the aluminium alloy and brass specimens.
9) Plot a graph of load ( ) against deflection (∆) in excel for each strain gauged specimen (on the same axes).
10) Plot a graph of stress ( ) against strain ( ) in excel for each strain gauged specimen (on the same axes). Calculate the gradient to determine the Young’s modulus for each material.
11) Plot a graph of load ( ) against deflection (∆) in excel for each un-instrumented specimen (on the same axes) in order to compare their material properties.
12) For each failed specimen determine, using the gauges provided, the %age elongation and the %age reduction in area.
6. Health and Safety
Although there is a little risk from this experiment you need to be careful when using the equipment. It is essential that immediately after the specimen is loaded into the machine the Plexiglas guard is placed over the working section of the test machine. Be careful not to place fingers in the test area when conducting the test and avoid any loose clothing becoming trapped in the mechanism, if this should happen stop winding the loading handle immediately.
7. Discussion
The material properties for each material used are defined by the tensile strength and the modulus of elasticity. The values for these properties are as recorded below.
Material
Ultimate strength
Modulus of elasticity
Aluminium
560.17 Mpa
0.043
Carbon steel
461.5 Mpa
0.029
Brass
371.71 Mpa
0.2
The test results for the three materials show a great level of consistency. The strain curves for the three materials increase upwards. The aluminium did not exhibit ability to harden as shown on the strain curve. It is important to note that the strain does not account for the changes that occur on the cross section area. Aluminium exhibits the highest measure of ultimate strength and it can be said to be the strongest material at an ultimate strength of 560.17. It is followed by carbon steel at 461.5 Mpa and brass at 371.71 Mpa. The modulus of elasticity determines the amount of energy in a material. Aluminium displays the highest value of modulus of elasticity while brass has the lowest value of the modulus of elasticity. Carbon steel shows a great degree of ductility. These results differ to a certain degree to the expected values which would be 103 Mpa for brass, 69 Mpa for aluminium and 200 Mpa for carbon steel.
The elastic part of the graph for the test to destruction is not the same at that of the instrumented Young’s Modulus test because the strain experienced is not uniform across the material hence the disparity in the values recorded.
Errors incurred could have resulted from different sources including:
Faulty instruments: The instruments used in the experiments could be erroneous. Instruments such as grips and gauges may be faulty hence causing erroneous readings.
Faulty materials: The materials used to conduct this experiment may be faulty such that they have a shape that does not allow uniform distribution of stress. This can lead to erroneous results.
Misalignment of the materials and the equipment: This can lead to erroneous recording of the results obtained during the experiment.
Speed of testing may be too fast hence causing some values to be missed out during the experiment.
8. Conclusions
From the experiment carried out above, the following conclusions can be made:
The calculated modulus of elasticity for aluminium is 560.17 Mpa and its modulus of elasticity was 0.043
The calculated modulus of elasticity for carbon steel is 461.5 Mpa and its modulus of elasticity was 0.029.
The calculated modulus of elasticity for brass is 371.71 Mpa and its modulus of elasticity was 0.2.
Aluminium is the strongest material and brass has the least strength.
The experiment was completed successfully since the objectives of the experiment were since all the required values were computed.
References
Hannah and Hillier (1995) Applied Mechanics, 3rd Edition, Longman, 0-582-25632-1
Clifford, M., Brooks, R., Howe, A., Kennedy, A., McWilliam, S., Pickering, S., Shayler, P.
& Shipway (2009) An Introduction to Mechanical Engineering: Part 1, 1st Edition, Hodder Education, 9780340939956 Read More
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