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LECTURE 12

Compression Test: Machines used for compression testing are basically similarto those used for tensile testing often the same machine can be used to performboth tests.

Shape of the specimen: The shape of the machine to be used for the different

materials are as follows:

(i) For metals and certain plastics: The specimen may be in the from of acylinder

(ii) For buildin materials: Such as concrete or stone the shape of thespecimen may be in the from of a cube.

Shape of stress stain dia ram

(a) !uctile materials: For ductile material such as mild steel, the load Vscompression diagram would be as follows

(1 The ductile materials such as steel, !luminum, and copper ha"e stress #strain diagrams similar to ones which we ha"e for tensile test, there would be anelastic range which is then followed by a plastic region.

($ The ductile materials (steel, !luminum, copper proportional limits incompression test are "ery much close to those in tension.

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(% &n tension test, a specimen is being stretched, nec'ing may occur, andultimately fracture fa'es place. n the other hand when a small specimen of theductile material is compressed, it begins to bulge on sides and becomes barrelshaped as shown in the figure abo"e. )ith increasing load, the specimen isflattened out, thus offering increased resistance to further shortening ( which

means that the stress # strains cur"e goes upward this effect is indicated in thediagram.

"rittle materials ( in compression test )

*rittle materials in compression typically ha"e an initial linear region followed bya region in which the shortening increases at a higher rate than does the load.Thus, the compression stress # strain diagram has a shape that is similar to theshape of the tensile diagram.

+owe"er, brittle materials usually reach much higher ultimate stresses in

compression than in tension.For cast iron, the shape may be li'e this

*rittle materials in compression beha"e elastically up to certain load, and thenfail suddenly by splitting or by cra'ing in the way as shown in figure. The brittlefracture is performed by separation and is not accompanied by noticeable plasticdeformation.

#ardness Testin :

The tem #hardness is one ha"ing a "ariety of meanings- a hard materialis thought of as one whose surface resists indentation or scratching, and whichhas the ability to indent or cut other materials.

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#ardness test: The hardness test is a comparati"e test and has been e"ol"edmainly from the need to ha"e some con"enient method of measuring theresistance of materials to scratching, wear or in dentation this is also used to gi"ea guide to o"erall strength of a materials, after as an inspection procedure, andhas the ad"antage of being a non # destructi"e test, in that only small

indentations are lift permanently on the surface of the specimen.

Four hardness tests are customarily used in industry namely

(i) *rinell

(ii) Vic'ers

(iii) oc'well

($i) Shore Scleroscopy

The most widely used are the first two.

&n the *rinell test the indenter is a hardened steel ball which is pressed into thesurface using a 'nown standard load. The diameter of resulting indentation isthan measured using a microscope / scale.

Units:

The units of *rinell +ardness number in S.& 0nit would ha"e been 2mm $ or Mpa

To a"oid the confusion which would ha"e been caused of her wise +ardnessnumbers are 3uotes as 'gf 2 mm $

"rinell #ardness test:

&n the *rinell hardness test, a hardened steel ball is pressed into the flatsurface of a test piece using a specified force. The ball is then remo"ed and thediameter of the resulting indentation is measured using a microscope.

The *rinell +ardness no. ( *+ is defined as

"#% & '

)here 4 5 Force applied to the ball.

! 5 cur"ed area of the indentation

&t may be shown that

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6 5 diameter of the ball,

d 5 the diameter of the indentation.

&n the *rinell Test, the ball diameter and applied load are constant and are

selected to suit the composition of the metal, its hardness, and selected to suitthe composition of the metal, its hardness, the thic'ness etc. Further, thehardness of the ball should be at least 1.7 times than the test specimen topre"ent permanent set in the ball.

!isad$anta e of "rinell #ardness Test: The main disad"antage of the *rinell+ardness test is that the *rinell hardness number is not independent of theapplied load. This can be reali8ed from. 9onsidering the geometry of indentationsfor increasing loads. !s the ball is pressed into the surface under increasing loadthe geometry of the indentation charges.

+ere what we mean is that the geometry of the impression should not changew.r.t. load, howe"er the si8e it impression may change.

*ic+ers #ardness test:

The Vic'er s +ardness test follows a procedure e actly a identical withthat of *rinell test, but uses a different indenter. The steel ball is replaced by adiamond, ha"ing the from of a s3uare # based pyramid with an angle of 1%; < between opposite faces. This is pressed into the flat surface of the test pieceusing a specified force, and the diagonals of the resulting indentation measuredis using a microscope. The +ardness, e pressed as a Vic'er s pyramid numberis defined as the ratio F2!, where F is the force applied to the diamond and ! isthe surface area of the indentation.

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&t may be shown that

&n the Vic'er Test the indenters of pyramidal or conical shape are used / thiso"ercomes the disad"antage which is faced in *rinell test i.e. as the loadincreases, the geometry of the indentation s does not change

The Variation of +ardness number with load is gi"en below.

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d$anta e: !part from the con"enience the "ic'er s test has certain ad"antageso"er the *rinell test.

(i) +arder material can be tested and indentation can be smaller / therefore lessobtrusi"e or damaging.

0pto a %<< 'gf 2mm $ both tests gi"e the same hardness number but abo"e toothe *rinell test is unreliable.

Roc+,ell #ardness Test :

The oc'well +ardness test also uses an indenter when is pressed intothe flat surface of the test piece, but differs from the *rinell and Vic'er s test inthat the measurement of hardness is based on the depth of penetration, not on

the surface area of indentation. The indenter may be a conical diamond of 1$<<

included angle, with a rounded ape . &t is brought into contact with the test piece,and a force F is applied.

d$anta es :

oc'well tests are widely applied in industry due to rapidity and simplicity withwhich they may be performed, high accuracy, and due to the small si8e of theimpressions produced on the surface.

-mpact testin :

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&n an #impact test a notched bar of material, arranged either as a cantile"er or asa simply supported beam, is bro'en by a single blow in such a way that the totalenergy re3uired to fracture it may be determined.

The energy re3uired to fracture a material is of importance in cases of #shoc'

loading when a component or structure may be re3uired to absorb the =.> of amo"ing ob?ect.

ften a structure must be capable of recei"ing an accidental #shoc' load withoutfailing completely, and whether it can do this will be determined not by itsstrength but by its ability to absorb energy. ! combination of strength and ductilitywill be re3uired, since large amounts of energy can only be absorbed by largeamounts of plastic deformation. The ability of a material to absorb a large amountof energy before brea'ing is often referred as toughness, and the energyabsorbed in an impact test is an ob"ious indication of this property.

&mpact tests are carried out on notched specimens, and the notches must not beregarded simply as a local reduction in the cross # sectional area of thespecimen, otches # and , in fact, surface irregularities of many 'ind # gi"e riseto high local stresses, and are in practice, a potential source of crac's.

The specimen may be of circular or s3uare cross # section arranged either as acantile"er or a simply supported beam.

Tou hness: &t is defined as the ability of the material to withstand crac' i.e topre"ent the transfer or propagation of crac's across its section hence causingfailures. 9rac's are propagated due to stress concentraction.

Creep: 9reep is the gradual increase of plastic strain in a material with time atconstant load. 4articularly at ele"ated temperatures some materials are

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susceptible to this phenomena and e"en under the constant load, mentionedstrains can increase continually until fractures. This form of facture is particularlyrele"ant to the turbines blades, nuclear rectors, furnaces roc'et motors etc.

The general from of strain "ersus time graph or creep cur"e is shown below.

The general form of ∈ Vs t graph or creep cur"e is shown below for two typicaloperation conditions, &n each case the cur"e can be considered to e hibit fourprincipal features

(a !n initial strain, due to the initial application of load. &n most cases this wouldbe an elastic strain.

(b ! primary creep region, during which he creep rate ( slope of the graph dimensions.

(c ! secondary creep region, when the creep rate is sensibly constant.

(d ! tertiary creep region, during which the creep rate accelerate to finalfracture.

&t is ob"ious that a material which is susceptible to creep effects should only besub?ected to stresses which 'eep it in secondary (st.line region throughout itsser"ice life. This enables the amount of creep e tension to be estimated and

allowed for in design.

'ractice 'roblems:

'R." 1: ! standard mild steel tensile test specimen has a diameter of 1; mmand a gauge length of @< mm such a specimen was tested to destruction, andthe following results obtained.

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Aoad at yield point 5 @7 '

> tension at yield point 5 17% 1; − ; m

0ltimate load 5 1$B '

Total e tension at fracture 5 $B mm

6iameter of specimen at fracture 5 C.@ mm

9ross D sectional area at fracture 5 7E.B mm $

9ross D sectional !rea #! 5 $<< mm $

Compute the follo,in s:

(i) Modulus of elasticity of steel

(ii) The ultimate tensile stream

(iii) The yield stress

(i$) The percentage elongation

($) The 4ercentage reduction in !rea.

'R." 2:

! light alloy specimen has a diameter of 1;mm and a gauge Aength of @< mm.)hen tested in tension, the load e tension graph pro"ed linear up to a load of;' , at which point the e tension was <.<%B mm. 6etermine the limits ofproportionality stress and the modulus of elasticity of material.

%ote: For a 1;mm diameter specimen, the 9ross # sectional area ! 5 $<< mm $

This is according to tables 6etermine the limit of proportion try stream / themodulus of elasticity for the material.

!ns: %< M 2m$

, 7<.E 2m$

solution:

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