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Tensile testing
This unit addresses aspects of the following syllabus outcomes:
Outcomes:
H6.1 demonstrates skills in research and problem-solving related to engineering
H6.2 demonstrates skills in analysis, synthesis and experimentation related to engineering
Source: Board of Studies (1999) Stage 6 engineering studies syllabus preliminary and HSC courses. Sydney : Board of Studies
This unit addresses aspects of the HSC Engineering Application Module 1 - CIVIL STRUCTURES and the section contained under Engineering Mechanics and Hydraulics which covers Stress and strain.
Background to mechanical testing
Mechanical testing is carried out to provide information that may be used for the design of engineering components, structures or mechanisms.
The tensile test is an excellent example of mechanical testing that may be used either to determine the yield strength of a steel for use in design calculations or to ensure that the steel complies with a particular set of specifications.
Mechanical tests 
may be divided into quantitative or qualitative tests.
Activity 1
Visit the mechanical tests website given above to identify the differences between quantitative and qualitative tests then complete the statements below:
A quantitative test is one that provides data that will be used for:
A qualititive test is one where the results willbe used for making:
Tensile Testing
Visit the following web site,
http://www.instron.com.au/wa/applications/test_types/tension/default.aspx
A tensile test, also known as the tension test, is probably the most fundamental type of mechanical test that can be performed on a material.
Why Perform a Tensile Test or Tension Test?
Tensile tests are simple, relatively inexpensive, and fully standardised. By applying a force on a material using a uniaxial load, the reaction of the material can be readily recorded and analysed. This data can then be used to predict how the material will react to forces being applied in practical situations such as in bridges, or in airframes.
A lot can be learned about a material from tensile testing. As the material is stretched until it breaks, a comprehensive tensile profile will result producing a curve showing how it reacted to the forces being applied. This curve is commonly referred to as a “Load-Extension” diagram. The load at which the material fails is of much interest on these diagrams as is the maximum load the material can withstand - the Ultimate Load.
A “load-extension” diagram, however has limited use, because its data can only be used to analyse specimens of exactly the same size and shape. To overcome this problem ‘load’ is converted into ‘stress’ (load per unit cross sectional area), and extension is converted into a percentage of the original specimen length, or ‘strain’. In this way, direct comparisons can be made from the results carried out on different size and shape specimens tested on any machine in the world. It is important to note that the shape of the “load-extension” diagram does not change when converted to a stress-strain diagram.
The measuring units for tensile testing are:
- Load Newtons
(N), or more commonly kN.
- Extension metres
(m), or more commonly µm
- Stress Pascal
(Pa), or more commonly MPa
- Strain percentage (%)
When a "Load-Extension" diagram is obtained from a tensile test, it soon becomes clear that a relationship exists between the applied load and the elongation of the specimen upon which the test was conducted. Similarly with the relevant "stress-strain" diagram.
Activity 2
Visit this web site,
http://www.instron.com.au/wa/applications/test_types/tension/default.aspx
Identify the name of this relationship and explain it using the spaces below to insert your answers.
RELATIONSHIP NAME:
RELATIONSHIP DESCRIPTION:
This feature is clearly illustrated on the unlabelled stress-strain curve below where the straight-line section can be seen on the diagram.
Activity 3
Visit the following website,
http://www.twi.co.uk/j32k/protected/band_3/jk69.html
Locate the image of the stress-strain curve and identify its parts. Insert the names of areas and critical points on the incomplete diagram given below.
Conducting tensile tests
Tensile tests are carried out by gripping the ends of a suitably prepared standardised test piece in a tensile testing machine and then applying a continually increasing uni-axial load until such time as failure occurs.
The test pieces must be standardised so that results can be replicated and compared.
Before the test, the gauge length (Lo), and the cross-sectional area (Ao) are measured to enable calculations of strain and % reduction in area to be made.
Reference: http://www.twi.co.uk/j32k/protected/band_3/jk69.html
A typical tensile testing machine
In a typical tensile testing machine the specimen is held in place by special grips,. One end of the specimen is held firm, whilst a hydraulic piston forces the other grip away from it. Thus producing the tensile load within the specimen. The specimen and grips can be seen on the machine below. Note that an electronic device for measuring the specimen extension, an extensometer, is mounted on the specimen. This has to be removed once the specimen approaches its proportional limit or it will be damaged when the specimen breaks.
Visit the following web site
http://www.twi.co.uk/j32k/protected/band_3/jk69.html
Following the failure of the material, the specimen is re-assembled. It has the appearance as shown below when compared to the original pre-test specimen. The gauge length at failure (Lf), and the final cross sectional area (Af) are measured. The original gauge length and the final gauge length are used to calculate the percentage elongation of the specimen at failure. The original cross sectional area and the final cross sectional area are used to calculate % reduction in area.
Information that can be obtained from a tensile test:
As well as calculating % elongation at failure and % reduction in area at failure, the load-extension graph is converted to an engineering stress/strain curve by converting the graph axes from load to stress (load divided by cross sectional area), and by converting extension to strain (% elongation).
Activity 4
What can be learned from a tensile test?
Visit the web site,
http://www.twi.co.uk/j32k/protected/band_3/jk69.html
Identify and explain the data, which can be obtained from the results of a tensile test. Complete the omitted sections of the table.
| Aspect of the Tensile Test | Explanation | Calculation Formula |
|---|---|---|
| Tensile strength |
||
| Yield Point (YP) |
||
| Percentage elongation, El% |
||
| Percentage reduction of area |
||
| Young's Modulus of Elasticity |
Stress-strain graphs typical for certain materials.
The stress-strain curve above is typical of a material that has a well-pronounced yield point. However, it should be noted that only annealed carbon steel exhibit this sort of behaviour.
Metals that are strengthened by alloying, by heat treatment or by cold working do not have a distinct yield and another method must be found to determine the 'yield point'.
This method is carried out by measuring the proof stress, which is the stress required to produce a small specified amount of plastic deformation in the test piece. The small amount of plastic deformation is considered to be a safe level for design purposes.
The proof stress is measured by drawing a line parallel to the elastic portion of the stress/strain curve at a specified strain, this strain being a percentage of the original gauge length, hence 0.2% proof, 0.1% proof.
Types of fracture
Fracture causes a physical separation, or tearing, of the material, through either an internal or external crack. There are two basic types of fracture:
- ductile fracture, and
- brittle fracture.
Ductile fracture Brittle fracture
Ductile fracture:
Ductile fracture is characterised by plastic deformation that precedes failure
of the part. Ductility is usually understood to mean the ability of a material
to accept large amounts of deformation (mainly tensile) without fracture. It
is the antithesis of brittleness
Brittle fracture:
Brittle fracture occurs with little or no gross plastic deformation occurring
in the component. It is characterised by the very small amount of energy,
which is absorbed, and by the crystalline appearance of the surfaces of the
fracture (break). Brittle fracture mostly results in catastrophic failure.
Brittle fracture is influenced by defects, fatigue, stress-corrosion,
and hydrogen embrittlement.
Other Web Addresses for Further Research
http://www.lloyd-instruments.co.uk/
http://www.matweb.com/reference/tensilestrength.asp
http://www.ptli.com/testlopedia/tests/tensile-plastics-D638-ISO527.asp
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