Qualification - Pearson BTEC Level 5 Higher National Diploma in Engineering (Electrical and Electronic)
Unit Name - Engineering Science
Unit Number - Unit 3
Unit Level - Level 5
Assignment Title - Testing materials using scientific method
Learning Outcome 1: Examine scientific data using both quantitative and computational methods.
Learning Outcome 2: Explore the characteristics and properties of engineering materials.
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Case Scenario
You work as a materials engineer and have been tasked with investigating issues with products manufactured by your company. These products are made from polymer, composite and metal components.
Materials are failing in service, but your manager is not sure of the reason for this and has hypothesised that this could be due to degradation caused by fatigue, creep or other means. It has been suggested that types of hysteresis should be considered and/or that there are issues with the specification of materials being used.
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Section 1
In a tensile test on a polycarbonate strip, the following was the procedure.
The initial length, width and thickness of the strip was measured and then the strip was put into the machine. The strip was observed to increase in its length proportionally to the applied force during the elastic phase of the experiment. Following the elastic phase, elongation was observed in the strip without any additional force. Thereafter the strip continued to increase in length when the force was decreased. This phase continued for a long while till the strip got destroyed in a fracture.
The following data outlines the readings obtained before and after the experiment.
Material for test specimen : Polycarbonate strip
Dimensions before test :
Length : 75 mm Width : 10 mm Thickness : 4 mm
Dimensions after test :
Length : 126.6 mm Width : 8.3 mm Thickness : 3.2 mm
1. Use the data above to plot the graph of force vs. extension. Use the graph to locate the elastic region.
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2. Calculate the stress vs. strain for the strip and write your values in the table. Use the values to plot a different graph showing the stress vs. strain for the elastic region. Fit a straight line to the stress vs. strain curve and use the line to calculate Young's modulus Y for the strip.
Solution:
Stress=F/cross Section area ; Cross section area= 10*4 mm2
Strain= Extension /Original Length ; Original Length= 75 mm
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Extension (x) (mm)
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Extension (x) (m)
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Force (F) (kN)
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Force (F) (N)
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Stress(N/m2)
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Strain
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0
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0
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0.15
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150
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0.00000375
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0
|
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0.012164
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1.2164E-05
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0.31
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310
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0.00000775
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0.0001622
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|
0.024328
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2.4328E-05
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0.48
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480
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0.000012
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0.0003244
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0.036492
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3.6492E-05
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0.62
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620
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0.0000155
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0.0004866
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0.048656
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4.8656E-05
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0.78
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780
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0.0000195
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0.0006487
|
|
0.06082
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0.00006082
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0.87
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870
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0.00002175
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0.0008109
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0.072984
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7.2984E-05
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0.96
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960
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0.000024
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0.0009731
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|
0.085148
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8.5148E-05
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1.07
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1070
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0.00002675
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0.0011353
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|
0.097312
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9.7312E-05
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1.23
|
1230
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0.00003075
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0.0012975
|
|
0.109476
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0.00010948
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1.41
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1410
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0.00003525
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0.0014597
|
|
0.12164
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0.00012164
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1.55
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1550
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0.00003875
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0.0016219
|
|
0.133804
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0.0001338
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1.65
|
1650
|
0.00004125
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0.0017841
|
|
0.145968
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0.00014597
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1.76
|
1760
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0.000044
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0.0019462
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By the Best fit line, Young's Modulus =0.0204 N/m2.
3. Write the dimensions of force, extension, stress, strain and Young's modulus.
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Sr. No.
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Quantity
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Dimension
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1
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Force
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MLT-2
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2
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Extension
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L
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3
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Stress
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ML-1T-2
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4
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Strain
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Dimensionless
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5
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Young's Modulus
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ML-1T-2
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4. Use the force-extension graph to calculate the yield stress (σ_Y ), the ultimate stress (σ_U ) and the ductility.
By the Force-extension graph,
Yield Stress (σY) = 1.76*1000 N/(10*4 mm2 ) = 44 N/mm2 = 4.4E-05 N/m2
=Stress value at the end point of elastic range.
Ultimate Stress ((σU) = Highest Stress value
=2.32*1000 N/(10*4 mm2 )= 58 N/mm2 =5.8E-05 N/m2
5. Fit a quadratic curve to the force-extension graph. Use the curve to calculate the error in the last and the middle readings and comment whether the quadratic fit is appropriate to the data.
Middle reading- 2.15 kN(observed) &
calculated= -11.789*0.2919362+8.6246*0.291936+0.5019 =2.0149945 kN
So Error= (2.15-2.014)*100/2.15 %= 6.28%
Last Reading- 1.82 kN(observed) &
calculated=11.789*0.60822+8.6246*0.6082+0.5019 = 1.3865 kN
so Error= (1.82-1.3865)*100/1.82 % = 23.82%
The graph thus isn't a good fit.
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Section 2
Your site manager wants you to write a formal report on the major differences in the structural properties of metals and non-metals as observed by the following tests carried out on them : (a) Destructive test on metals (bend test and/or tensile test) (b) Destructive test on non-metals (tensile test) and (c) Non-destructive test on metals. Your report should describe the salient aspects of each of the tests mentioned and proceed to differentiate between the structural properties between metals and non-metals.
You should ensure that your report contains the following :
1. At least two differences (toughness and ductility) between metals and non-metals backed by numerical data from the tests.
2. Any three additional differences between metals and non-metals independent or derived from the tests carried out on them.
Solution:
Destructive test on metals: Tensile test-
A metallic specimen is subjected to external tensile force in this type of test. This procedure is generally carried out in automatic machines that plot the force vs elongation plots automatically for the specimen. The tensile force is increased gradually. The metal shows elastic region initially up to a certain point then it becomes plastic like upon increasing the force. After certain force, a neck like structure develops in the metal and then the cross section of the neck starts reducing until the rod breaks.
Tensile test on non-metals (polymers): Polymers show similar properties to metals initially but after the neck develops in them, instead of continuous cross section reduction at the same place, the neck width increase and the entire length of the polymer takes the same thickness while increasing in the size. Later at much larger tensile force values, it breaks apart.
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Non-destructive test on metals: Dye penetrant test-
This test is used to detect internal cracks in the non-porous materials. The specimen under testing is cleaned by a oil solvent initially. The cleaned material is then sprayed with a colored dye(usually red color). A developer (Talc powder) is then applied on the material as the base. It's then left to dry. The red colored dye comes out of the cracks and gets deposited onto the talc base showing the cracks in the metal.
Toughness is opposite of being brittle. Toughness is a measure of energy required to crack a material. Metals are mostly more tough than non-metals like Glasses, ceramics, foams etc. Toughness of a Nickel (a metal) is around 100 kJ/m2 whereas toughness of Diamond (a ceramic, non-metal) is around 0.01 kJ/m2
Ductility is the property of a material to keep on elongating without it breaking when external tensile force is applied. Ductility of Copper (Metal) is 0.62 whereas ductility of Silicon (a non-metal) is 0.20
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Property
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Metals
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Non-Metals
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|
Electric conductivity
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Mostly good conductors
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Mostly bad conductors
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|
Thermal Conductivity
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Almost all are good conductors
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Mostly low thermal conductivity
|
|
Lustre
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Have metallic lustre
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Mostly lack it due to absence of free electrons.
|
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Section 3
Your site manager wants you to write a formal report on the types of degradation and failures observed in metals and non-metals. Your report should include the following elements :
1. An explanation and comparison of the types of degradation and failures observed in metals and non-metals.
2. Ensure that your report contains a reflection of the scientific method (as shown in your laboratory report) for the testing that was carried out.
3. A description of the phenomenon of hysteresis observed in the domain of electrical, magnetic and elastic applications.
Solution:
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Metals
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Polymers
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|
Degradations
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Wet Corrosion
Dry Corrosion
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Solvent degradation
Solar degradation
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|
Failures
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Ductile failure
Brittle failure
Creep failure
Fatigue failure
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Creep failure
Fatigue failure
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Corrosion when taking place on metal surfaces at low temperatures in presence of water is called wet corrosion. At high temperatures, metals react with atmospheric oxygen and corrode.
Whereas polymers undergo degradations in presence on solvent. They dissolve in it. In the presence solar UV radiations, they go under various degradations as well.
When the component is no longer fit for use, it considered to have failures.
Ductile failure occurs when a material have undergone more elongation or load than it's yield strength levels. Brittle failure occurs in brittle material like cast iron. These materials suddenly break without undergoing deformation like metals. Creep failure occurs when a material is subjected to loads for very long duration of time. Fatigue failures, on the other hand occur when a material is subjected to repeated loading-unloading.
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Hysteresis phenomenon:
When a material is exposed to external force, the induced property inside the material due to that external force first increases with it and then gets saturated. Now if the external force is reduced, the induced property values decreases but doesn't follow the same graph. It shows certain resistance. At zero external forces, the induced property doesn't become zero when reducing external force. We need to provide some negative force to make it zero. Also when the external force is removed, the induced property doesn't become zero. This characteristic of materials is known as hysteresis. Such hysteresis is observed in various domains like magnetism, electricity and even elasticity of materials.