construction materials - metal report

University of Technology, Sydney

Tensile testing of metals

August 18, 2010

James Trinh 10863619Hiu Siu 11087552Richard Sung Jong 10891974

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48352: Construction Materials


Contents

INTRODUCTION............................................................................................................................ 3

OBJECTIVE................................................................................................................................... 4

MATERIALS AND EQUIPMENT................................................................................................... 4

TEST PERFORMED...................................................................................................................... 4

PROCEDURE................................................................................................................................ 4

RESULTS...................................................................................................................................... 8

LOAD VS EXTENSION/STRESS STRAIN RELATIONSHIP....................................................... 8

CALCULATIONS......................................................................................................................... 10

FRACTURE PATTERNS............................................................................................................. 12

SUMMARY OF METAL PROPERTIES....................................................................................... 14

DISCUSSION............................................................................................................................... 14

CONCLUSION............................................................................................................................. 15

REFERENCES............................................................................................................................. 16

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IntroductionMetals are solids which have high melting points and densities. Metals are opaque, lustrous elements that are good conductors of heat and electricity. Most metals are malleable and ductile and are, in general, denser than the other elemental substances.

Properties of each metal are different allowing different melting points and densities. In addition, metals have other certain properties which include being ductile or being brittle in which each metal differs because of its properties.

Ductility means the ability of the material to deform easily upon the application of a tensile force, or the ability to withstand plastic deformation without failure. Other terms such as crushability and bendability associate with ductility. It is apparent that a ductile material will deform quite largely before failure. Of course with the change of temperature, the ductility of a metal can be altered, whereby an increase in temperature would increase ductility, while a decrease in temperature would see a decrease in ductility, and more or less turn the metal into brittle behaviour.

A metal being brittle is similarly applied with a tensile load but does not deform as much as a ductile material. Brittle fracture occurs by rapid crack propagation with less expenditure of energy than for ductile fracture. Clear defined cut with a fair flat surface like fracture is observed.

Both ductility and brittleness are important factors in determining which material is to be used for the right situation not only for engineers but other industries as well.

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ObjectiveTo determine the engineering properties of metals and compare the ductile and brittle behaviour of metals in direct tension. The metals being tested are Mild Steel, and Cast Iron.

Materials and Equipment Mild Steel (black) Cast Iron Micrometer/ vernier calliper Universal testing machine (Shimadzu) 30 tonne (300 kN) 120 kN range Instron extensometer 50.00 mm gauge length LVDT RPD LDC 2000 (a 100 mm linear variable differential/displacement

transducer/transformer) P3500 strain amp (Biolab/Davidson) DT605 data taker (data logger 10 channel and 1 sample per second) Laptop computer (any computer with a serial port)

Test PerformedDirect tensile test.

ProcedureRefer to AS1391 for further details on this experiment.

The test shall be carried out at ambient temperature between 10°C and 35°C, unlessotherwise specified. Tests carried out under controlled conditions shall be made at atemperature of 23°C ±5°C.

Metal Test specimens are tested under unaxial tension at room temperature.

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Figure 1: Demonstrates Mild Steel rod and Mild Steel fracture after testing with UTS machine. Observations demonstrate that the fracture type for Mild Steel is ductile, with a cone shape like fracture. Observe the reduction in cross sectional area at fracture point.

1.Preparation of materials: Mild Steel and Cast Iron bars (figure 1)

2.Mark the original gauge length using a jig (figure 2)

3.Ensure extensometer is in place and correctly calibrated in accordance with AS 1545 (figure 3)

4.Clamp material to be tested on the universal tensile test machine (figure 4)

5.Perform Tensile test on material

6.Observe and collect data from the CHT graph

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Figure 2: A jig is used to make a small indent into the metal providing a reference point for determining the original and final gauge lengths of the tested material.

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Figure 3: Extensometer calibrated and attached to UTS machine.

Figure 4: Universal Tensile Machine with computer and digital reading attached.

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ResultsMeasurement Black Mild Steel Cast Iron

Original mean diameter (mm) 9.98 9.92Final diameter (mm) at failure

location5.80 9.91

Original gauge length (mm) 50.51 50.51Final gauge length (mm) 68.17 50.99

Maximum load (kN) 38.06 22.98Failure Load (kN) 12.855 22.934

Proportional limit load (kN)Lower yield load (kN) -

Stress (MPa) 486.539 297.329Strain 0.0035 0.000095

Load vs Extension/Stress Strain RelationshipBlack mild steel

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Cast iron

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Calculations

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%Elongation Black Mild Steel = ((68.17mm – 50.51mm)/50.51mm) x 100

= 34.96%

≈ 35%

%Elongation Cast Iron = ((50.99mm – 50.51mm)/50.51mm) x 100

≈ 0.95%

%Reduction in area Black Mild Steel = [(( x 9.982mm)/4) – (( x 5.802mm)/4)] / x 9.922mm/4)

x 100

= 66.23%

≈ 66%

%Reduction in area Cast Iron = [(( x 9.922mm)/4) – (( x 9.912mm)/4)] / x 9.922mm/4)

x 100

= 0.2015%

≈ 0.20%

ε Black Mild Steel = (68.17mm-50.51mm)/50.51mm

= 0.3496

≈ 0.35

ε Cast Iron = (50.99mm-50.51mm)/50.51mm

= 9.503 x 10-3

= 0.9503

≈ 0.95

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Fracture PatternsBlack mild steel is a ductile metal, with a high modulus of elasticity. When placed under a load using the universal testing machine until fracture, the black mild steel specimen exhibited a ductile failure. There was a significant reduction of area at the fracture point. It also displayed a large degree of necking with a cup and cone type failure thus suggesting that the black mild steel is a ductile metal.

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Conversely, using the universal testing machine. It displayed no necking, and was a clean brittle failure. There was very little reduction of area at the fracture indicating that the cast iron is a brittle metal.

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Summary of Metal PropertiesProperty Black Mild Steel Cast Iron

Density (kg/m3) 7850 7850Proportional Limit (MPa)Tensile strength (MPa) 486.539 297.329Failure strength (MPa) 486.549 297.332

Upper Yield Strength (MPa) -Lower Yield Strength (MPa) -Proof strength Rp 0.2 (MPa) - 226Proof strength Rt 0.5 (MPa) - 565

Percentage Elongation 35% 0.95%Percentage Area Reduction 66% 0.202%

Experimental Young’s Modulus (GPa)

139 113

Expected Young’s Modulus (GPa)

200 120

% Error in Young’s Modulus 30.5 5.83Elastic Resilience (MJ/m3) 0.226

Modulus of Toughness (MJ/m3)

DiscussionThe two metals demonstrate different mechanical behaviors during the tensile test.

Beginning with mild steel, as the load was increased the stresses increased and therefore necking of the material was observed. Necking in this case demonstrates that the Mild Steel is a ductile material. As the load was still increased, necking was more apparent and a section of the mild steel began to reduce size in cross sectional area. At point of fracture, the mild steel had a cone shaped fracture as demonstrated in the sketches. This observation coincides with the materials properties as in the calculations; a clear reduction in area is present along with the necking observation of the material.

Cast Iron was different to mild steel in terms of physical observations. As the load increased there was no apparent physical deformation of the material. Until the cast iron snapped almost instantaneously, it is clear that the cast iron material is a brittle material. After observing the cross sectional area which did not really change, there was a clear tooth like cut across the cross sectional area, which is shown in the sketches. The properties of cast iron show that there is no yield strength therefore has no plastic region. This is demonstrated during the experiment by the material not deforming like the Mild Steel but instead fail instantaneously at its maximum stress.

The black mild steel was particularly ductile, displaying a cup and cone failure with a large degree of necking. The mild steel also had a large reduction in area (around 66%), which is a property of ductile metals. Ductile metals tend to have a low carbon content (0.15%-0.25% for mild steels) and a high Young’s modulus. Black mild steel is strong and also very easy to weld. Cast iron has a very good

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compressive strength, but a low tensile strength. Its high carbon content (around 4%) makes it very brittle which is evident when it was tested. It has a brittle failure with no necking and very little elongation, indicating limited ductility and low Young’s modulus (100-150GPa). It had a low reduction in area (around 0.20%) further emphasising its brittleness. Its high carbon content also makes it impossible to weld.

Mild Steel can be used for various engineering purposes. Examples such as: nuts & bolts , chains, hinges, and pipes. Advantages of using mild steel is that it has high strength, high toughness, and having a low percentage of carbon allow alloying with other metals easier and additionally increasing the strength and toughness. Workability with metal is an important factor in engineering, and mild steel has a trait to not only be easy to work with, but is cost effective as well which is vital in the engineering sector. An example of mild steel application is pipe work. Pipes made by mild steel contain carbon contents of less than 0.18%, and is therefore not hardened because of low carbon content. Mild steel is available in a variety of structural shapes that are easily welded into pipe, tube, tubing etc. Mild steel pipes and tubes are easy to fabricate, readily available, and cost less than most other metals. In well protected environments, the life expectancy of mild steel pipe is 50 to 100 years.

Cast iron can be split in various types of iron in which each vary in mechanical and physical properties, i.e tensile strength and hardness. It has a reasonable strength to weight ratio, and has a relatively low cost factor and low cost per unit of strength than other materials. This allows workability, i.e. moulding the cast iron to be simpler and therefore has flexibility from design to final product is easily achieved. An example is cast iron being produced in a range of different sizes with complex geometries, from ounces, to over a hundred tonnes. Chemical modifications can improve specific properties such as corrosion resistance, oxidation resistance or wear resistance, therefore provide longer life span of the cast iron. Finally cast iron has a reduced tendency towards residual stresses than other material, so is very effective.

Universal Tensile Machines can perform a variety of stress tests on material samples. A UTM may be equipped to perform a bending test, a compression test, and a tensile test on a single sample of material. These machines incorporate computers and highly sensitive electrical component such as the use of an extensometer. These more advanced machines enable scientists and engineers to obtain extremely accurate measurements. Results determined in this experiment are considered reliable and fairly accurate with very low margin of error due to the quality of the machine.

ConclusionIn this test, we have determined the mechanical properties of black mild steel and cast iron as a result of comparing the ductile and brittle behaviour of both metals in direct tension. By the means of this testing result, we have learnt that the given species of black mild steel is a ductile metal, with a high modulus of elasticity; and the cast iron exhibited a brittle failure when placed under a load.

The results of these tests on each sample of material that we obtained may be stored in a database for future use. This database allows analysis to be performed using statistical methods to correlate the mechanical properties data with other information about the material.

http://www.wisegeek.com/what-is-a-compression-test.htm

http://www.wisegeek.com/what-is-a-bending-test.htm

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References1. Engineers Edge, 2010, Ductility – Strength (Mechanics) of Materials, viewed 30 August 2010,

<http://www.engineersedge.com/material_science/ductility.htm>

2. Anne Marie Helmenstine, Ph.D., 2010, Metals – Properties of Element Groups, viewed 30 August 2010, <http://chemistry.about.com/od/elementgroups/a/metals.htm>

3. AS 1391 (2007), Australian Standard Metallic materials – Tensile Testing at ambient temperatures, viewed 30 August 2010

4. Dr K. Vessalas, (2010) 48352: Construction Materials Lecture notes, University of Technology, Sydney

construction materials - metal report

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