c1_1 lab report

8/20/2019 C1_1 Lab Report

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1.ABSTRACT:The main objective of this experiment is to

determine the tensile properties of the twometals,brass and steel.In the experiment,a tensometerwas used to increase load supply to the metals untilfracture occured.A revolving drum is used to plot thegraphs of load against elongation for the twometals.From the graphs,we can obtain :The limit of proportionality stress;

Yield or 0.2% proof stress; Ultimate tensile stress; Percentage elongation and Percentage reduction of area. Hence,we can compare the properties of the two metalsfrom the obtained results.


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2.INTRODUCTION: In all engineering construction,the component partsof a structure must be properly proportioned toresist the actual or probable forces that may beimposed upon them.

In many instances,the composite structure needs tobe rigid and not deflect excessively when underimposed loads during operation.So,the ability todetermine the maximum load that a slender rod cancarry before fracture occurs is of great importance.

The study of the tensile properities of ferrous and

non- ferrous metals covers the study of the strengthof materials.The most commonly way to definethe strength of a material is to plot a curve of theload applied to the material(stress) against theelongation resulting from the loadapplied(strain).Futhermore,analytical methods areinvolved in this experiment to determine the strength, ductility and stability of the various load carryingmembers,from the Stress-Strain Curve.


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3.OBJECTIVE:This experiment is to determine the following

material properties of given round Brass and Steel bars by tensile testing according to British

Standard 18: 1) Modulus of elasticity;

2) Limit of proportionality stress;

3) Yield or 0.2% proof stress;

4) Ultimate tensile stress;

5) Percentage elongation;

6) Percentage reduction of areas.


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4.THEORY:

4.1.Tensile Stress:The Tensile Stress is Force applied on the material

per unit area.

Tensile Stress = F/A where F is the force applied;

A is the cross-sectional area;

Dimension is [ML-1T-2]

4.2.Tensile Strain:The Tensile Strain is the elongation per unit length

or the ratio of the deformed length e to theoriginal length l. This is expressed as,

Tensile Strain = e/l

where e is the extension or elongation; l is the original length.

4.3.Young's Modulus Or Modulus Of Elasticity,E:The Young's Modulus of a material is defined as the

ratio of stress to strain.E = Tensile Stress/Tensile Strain

E = (F*l)/(A*e)

Dimension is [ML-1T-2]

Young's Modulus is applicable to elastic materials

within the proportional limit i.e where*Hooke's law

is obeyed.


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*Hooke's law deduce that the extension is proportional

to the force or load on a material if the proportional limit is not exceed.

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4.4.Limit Of Proportionality Stress,fp:

The Limit Of Proportionality Stress is thestress(load divided by original area of cross-

sectional of a test piece) at which thestrain(elongation per unit of original gaugelength) ceases to be proportional to the correspondingstress.

In practice,it can be determined for certain metalsby inspection of a load-elongationdiagram(Fig.1a,p7 ). If A1 = original cross-sectional area of the metal;

Pp1 = the stress at which the strain ceases.

fp = Pp1/A1

4.5.Yield Stress,fy:

The Yield Stress is the stress at which elongationof the test piece first increases without increase ofload(Fig.1a,p7 ). It is defined as the load at yield per unit area.

fy = Py/A1

where Py is the load at yield.

4.6.Proof Stress:

The Proof Stress is the stress which is justsufficient to produce under load,a non-proportional


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elongation equal to a specified percentage of theoriginal gauge length.

In specifying or describing a proof stress the non-proportional elongation should be quoted, eg.0.1 or 0.2% proof stress.

The proof stress is determined from the load-elongation curve by drawing a line parallel to thestraight portion of the curve and distant from it byan amount representing the required non-proportionalelongation,thus determining the load ,Pp,at which

the line cuts the curve(Fig.1b,p7 ).

0.2% Proof Stress = Pp/A1

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4.7.Ultimate Tensile Stress,fu:

The Ultimate Tensile Stress is the maximum load,P ult

reached under the prescribed testing conditions dividedby the original cross-sectional area of the gauge

length of the test piece.

fu = Pult/A1

4.8.Percentage Elongation At Failure:The Percentage Elongation At Failure is obtained by

measurement of the fractured test piece and is equal to

(L2-L1)100/L1

where L1 = the original length of test piece;

L2 = the distance between original gauge marks

obtained by measurement of the fractured testpiece.


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4.9.Percentage Reduction Of Area At Failure:The Percentage Reduction Of Area is obtained by

measurement of a fractured test piece and is equal to

(A1

-A2

)100/A1

where A1 = the original cross-sectional area;

A2 = the minimum cross-sectional area obtained by

measurement of the fractured test piece.

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5.EQUIPMENT:

1) 2 plotting graph papers;

2) Vernier capiler;

3) Test piece a.Brass;

b.Steel;

4) Tensometer.

5.1.Tensometer:


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The general arrangement of the tensometer is shownin Figure 2(p9) and a simplified layout showingthe basic mechanice is shown in Figure 3(p9).Thetensometer is an instrument used to obtain theforce-extension relationship of a metal specimenin the laboratory.

A)Manner in which the specimen is positioned.The chucks holding the ends of the specimen are

pinned to attachments which are spherically mounted toensure non- eccentric,axial loading.

B)Manner in which the load is applied.The load is applied to the specimen by turning the

handwheel(Fig.2,p9).The motion is transmitted through ascrew guided by a cross-head.There is a spring-beam whichdeflects under load.The deflection is proportional to the

magnitude of applied loading and is indicated by themovement of the mercury column in a glass tube.Differentcalibrated scales are available for each spring-beam,toread the amount of load applied at any stage.

C)Manner in which the specimen is extended.The extension of the specimen is indicated by the

amount of rotation of the loading screw.This rotationis transmitted through gears and screws toa rotary drum carrying a graph paper.The pathis traced out at regular intervals.Themagnification ratio for the specimen extension can be

altered by changing into different gear-wheel at oneend of the drum.The magnification ratio is at 16 times.

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6.PROCEDURE:

1) A piece of supplied graph paper was wrapped andclipped onto the recording drum.


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2) The load scale and extension magnification wererecorded.

3) The dimensions of the brass were measured andidentified.

4) The brass supplied was mounted using the correctchucks.

5) The mercury column was rid of air bubbles and thecolumn was adjusted to fall on the zero scalemark.

6) Load was applied by turning the small movementhandwheel.

7) Records were taken on the graph at appropriate

interval.

8) This process was continued until the brassfractured. 9) The graph paper was changed and the process was

repeated for the steel.

10) The tensile properties of each specimen werecalculated and shown on the sample load-elongation diagrams.

7.OBSERVATIONS:It is observed from the graph for the steel,the

Ultimate Tensile Stress is too low.This is because inthe process of the experiment there was air trappedin the mercury column.In order to save time,theexperiment was redone by adjusting the mercurycolumn(to get rid of the air bubbles),and withoutsetting the mercury column and cursor back to zeromark.So,the assumptions do not stand andresulting in a lower value of the ultimate tensile stressfor the steel.Modification is being made by extending thegraph down until it cuts the y-axis(which is the stress

axis).

Assumption:Through the procedure followed,it is assumed that

there is no initial tension.If there was,it wouldnot be recorded as both the mercury column and thecursor were set to the zero mark.


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8.CALCULATIONS:

For Brass:

Limit of proportionality stress, fp = Ppl/A1 = 4.00kN/[3.14*(5.00/2)

2]

= 197.40 N/mm 2

0.2% proof stress = Pp/A1 = 222.82 N/mm

2

Ultimate strength, fu = Pult/A1 = 445.75 N/mm

2

Percentage elongation= [(L2-L1)/L1]*100

= [(37.05-27.05)/27.05]*100 = 36.97%

Percentage reduction of area = [(A1-A2)/A1]*100

= [3.14(2.52

-1.52

)/3.14(2.5)2

]*100 = 63.99%

For Steel:

Limit of proportionality stress, fp = Ppl/A1

= 6.875 kN/3.14(5.05/2)2

= 343.24 N/mm 2

Yield stress = Py/A1

= 7.375 kN/3.14(5.05/2)

2

= 368.20 N/mm 2

Ultimate strength = Pult/A1 = 9.625 kN/3.14(5.05/2)

2

= 480.54 N/mm 2


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Percentage elongation = [(L2-L1)/L1]*100

= [(37.00-27.10)/27.10]*100 = 36.53%

Percentage reduction of area= [(A1-A2)/A1]*100

= 68.15%

The slope of the load elongation curve in theelastic range = 6.875 kN/(20/16) = 5.5 kN/mm

'Apparent' Elastic Modulus

= (5.5 kN/mm * 27.10)/3.14(5.05/2)2

= 7.44 kN/mm 2

9.RESULTS AND LOG SHEET:

1) Log sheet;

2) Graph 1 ( Brass );

3) Graph 2 ( Steel );


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10.DISCUSSIONS:

10.1.Describe the fracture of the different materials:

Ductile Fracture which is a fracture preceded byconsiderable plastic deformation.The fracture ofspecimens,steel and brass,belongs to the DuctileFracture. For both the materials,when the critical value ofstress is reached,the specimen underwent a suddenenormous elongation with an increment of relativelysmall applied load.When the maximum value of load isattained,the diameter of the specimen startedto reduce as the elongation continued to a particular

weak section,which is called the necking.Rupture willoccur and resulting in a cup and cone configuration.Thethree diagram below illustrated the above process.


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10.2.Compare the stress-strain curves of the differentmaterials:

From the simple calculations,we obtain that:The percentage elongation of Brass = 36.97% The percentage elongation of Steel = 36.53%

which can be deduce from the stress-strain curves,theplastic deformation for brass is larger than steel.But in

reality steel is a more ductile material than brass.Thisopposite deduction can be due to some impurity addictiveadded on during manufactured.Hence,the structure of themolecular bonding in this experiment is altered andstrengthened the brass specimen.

It is not easy to find the yield point for the brassas there is an absence of significant increment ofelongation on the curve.So,0.2% Proof Stress wasused to represent the yield stress.From thegraph,steel has a higher yield stress than brass.

From the curves,the steel has a higher UltimateStress than the brass.This means that the steel hasa higher safety factor than the brass.This is not trueas brass should have a higher Ultimate Stress thansteel.This may be due to human error duringexperiment.

10.3.Relate the properties to the utilization of thedifferent materials:

Due to steel's flexibility of its tensile stress,viathe variation of the carbon content,steel has got itsgeneral applications on daily life.Examples are the

making of gas pipe,cooking utensils and reinforcedconcrete.


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For the brass,its high ultimate stress and its non-corrosive nature make it most suitable for themanufacture of plumbing fitting,screws etc.

Materials with low percentage elongation or lowpercentage reduction of area has poor formability.Inother words,the materials are rigid and cannot bebent and stretch very much before failure.So,it isimportant for one to take note of the ductility forvarious materials.

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10.4.Comment on this measured value for E and discussthe suitability of the equipment for carryingout the tests:

The 'Apparent' Elastic Modulus,E ,for steel was found

to be 7.44 kN/mm2.The specific E for steel is

206700*106 N/mm

2.

There is a great difference between the twovalues.There are several causes which lead to thisdifference:

1) The material for the tensometer is slightlyelastic.When load is applied,the frame of thetensometer will expand.

2) The mercury column used to indicate the stresslevel, whether it is calibrated accurately is veryimportant too.

3) The test specimen may contain some flaws like airbubbles trapped in between the specimen during massproduction.

This may give rise to uneven cross-sectional areathroughout and unbalanced applied load.


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4) The human parallax error contributes to theinaccurate measurement of length and diameter ofthe specimens.

5) Lastly,the plotting of the graph may not beaccurate, because it is manually operated.

Therefore,the result obtained should not be takentoo seriously for industrial reference.Whereas,itwill be good enough for academics purposes inunderstanding of the properties of thematerials.

In order to achieve a closer specific result,weshould use an automatic recording tensometer,sothat the reading taken will be more

accurate.Futhermore,we can use sensitivedevice(eg.CRO)to monitor the elongation of the specimen.

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11.CONCLUSIONS:

From the above experiment,we have achieved theobjectives as follows:

For Specimen A Brass:

The Limit Of Proportionality Stress was 197.40 N/mm 2

The 0.2% Proof Stress was 222.82 N/mm 2


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The Ultimate Strength was 445.75 N/mm 2

The Percentage Elongation was 36.97 %

The Percentage Reduction Of Area was 63.99 %

For Specimen B Steel:

The Limit of Proportionality Stress was 343.24 N/mm 2

The Yield Stress was 368.20 N/mm 2

The Ultimate Strength was 480.54 N/mm 2

The Percentage Elongation was 36.53 %

The Percentage Reduction Of Area was 68.15 %

The Slope Of The Load Elongation was 5.5 kN/mm Curve

The 'Apparent' Elastic Modulus was 7.44 kN/mm 2

12.REFERENCES:

1) Vernon John,"Dictionary of Material andManufacturing", Macmillan.

2) Dr Boey,"Engineering Material Notes".

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c1_1 lab report

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