week 2 mechanical testing

7/30/2019 Week 2 Mechanical Testing

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EAT104

MANUFACTURING AND MATERIALS

WEEK 2

MECHANICAL TESTING


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Mechanical testing includes:

1. Tensile test2. Hardness test3. Impact test

4. Creep test5. Fatigue test6. Other mechanical tests


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The following MATERIALPROPERTIES can be evaluated /determined by TENSILE TESTING: STRENGTH

DUCTILITY

ELASTICITY

STIFFNESS

Tensile Testing


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STRENGTH - the greatest stress that the material canwithstand prior to failure.

DUCTILITY - a material property that allows it to undergoconsiderable plastic deformation under a load beforefailure.

ELASTICITY - a material property that allows it to retain itsoriginal dimensions after removal of a deforming load.

STIFFNESS - a material property that allows a material towithstand high stress without great strain.

Material Properties


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A machine which applies a tensile force (aforce applied in opposite directions) to thespecimen, and then measures that forceand also the elongation:

This machine usually uses a hydraulic

cylinder to create the force. The appliedforce is determined by system pressure,which can be accurately measured.

The Tensile Tester


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Stress - Strain Curve


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THE SAMPLE

Area of measured test


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Before After


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A Stainless Steel sample is loaded in thetester.


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Necking : The localized decrease in diameter in aspecimen near the rupture point.

Elastic Deformation Region: The area of a stress -strain curve where the specimen will deform under load, yetreturn to its original shape when the load is removed.

Plastic Deformation: Deformation that occurs oncethe object has been stressed past its elastic limit. Thedeformation is no longer reversible.

Terms


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Necking occurs asthe sample leavesthe elasticdeformation regionand begins to

deform plastically.

Necking


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The classic cup &

cone shape of afairly ductile tensilefracture is visiblehere.


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Upon completion of the test, the sample isreassembled and final measurements for totalelongation and minimum diameter are made

using a vernier caliper.


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Compare the material properties of

these three metal samples.

All three failed under tension


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PLASTIC DEFORMATION REGION

ELASTIC DEFORMATION REGION


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In the elastic region, the slope of thisline is the materials Modulus of Elasticity.

The point of departure from the elasticdeformation region is known as the yieldpoint.


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Stress - Strain Plot


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Tensile Test

The tensile test of a material involves a test-piece of known crosssectional area being gripped in the jaws of a testing-machine and thensubject to a tensile force which is increased in increments.

Force-extension diagrams


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Typical force-extension diagrams for both carbon steels and non-ferrousmaterials.


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The determination of 0.1 % proof stress.


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The percentage elongation

The determination of the percentage elongation


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Hardness tests

Definition of surface hardness: the capacity of the surface to resist abrasion

Moh 's scale of Hardness


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The Brinell hardness test

A hardened steel ball is forced into the surface of a test-piece bymeans of a suitable standard load.


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The Vickers Hardness Test

This test uses a square-based diamond pyramid as the indentor.

The Vickers pyramid hardness test, (i) The diamond indentor. (ii) Theangle between opposite faces of the diamond is 136 . (iii) Theappearance of the impression, when viewed, in the microscopeeyepiece.


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The Rockwell Hardness Test

Rockwell hardness = E - hWhere E is a constantdetermined by the formof the indentor; for adiamond cone indentor E is 100, for a steel ball130 (BS 891: Rockwell Hardness Test; BS 4175:Rockwell Superficial Hardness Test).


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Scale in Rockwell Hardness Test

1. Scale B, which is used in conjunction with a 1/16 inchdiameter steel ball and a 100 kgf load. This is usedmainly for softer metals, such as copper alloys,aluminium alloys, normalised steel and mild steel

2. Scale C, which is used in conjunction with a diamondcone of 120 angle and a 150 kgf load. This is usedmainly for hardened steels and other very hard materialssuch as hard cast irons.

3. Scale A, which is used in conjunction with the diamondcone and a 60 kgf load. This is used for extremely hardmaterials, such as tool steels.


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Rockwell Hardness test


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Impact tests

These tests are used to indicate the toughness of a material,and particularly its capacity for resisting mechanical shock.

The Avery-Denison universal impact-testing machine. This machine can beused for either Charpy or Izod impacttests. For Izod tests, the pendulum isreleased from the lower position, to give astriking energy of 170 J and for the Charpytest it is released from the upper positionto give a striking energy of 300 J.


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Details of standard test-pieces used in both the Izodand Charpy tests.


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The nature of the fractured surface in the Izod test.


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Creep test

1. When stressed over a long period of time, some metalsextend very gradually and may fail at a stress well below the tensile strength of the material.

2. This phenomenon of slow but continuous extensionunder a steady force.

3. Such slow extension is more prevalent at hightemperatures, and for this reason the effects of creepmust be taken into account in the design of steam andchemical plant, gas and steam turbines and furnace

equipment


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The variation of creep rate with stress and temperature.


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Fatigue test

(i) The principle of a simple fatigue-testing machine, (ii) A typical S/N curveobtained from a series of tests, (iii) The appearance of the fractured surface of a shaft which has failed due to fatigue.


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Other mechanical tests

The Erichsen cupping test


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Bend tests

Simple bend tests, (i) The material is bent back upon itself, (ii) Here it isdoubled over its own thickness, the second bend being the test bend. (iii) Herea specific radius R is used.


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Compression tests

These tests are used mainly in connection with cast ironand concrete.

The behaviour of brittle and ductile materials during a compression test.


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Torsion tests

The test consists of twisting a piece of wire in the samedirection round its own axis until it breaks, or until aspecified number of twists has been endured.


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References

R.A. Higgins, 2006, Materials for Engineers andTechnicians, 4 th Editions, Butterworth-Heinemann.

week 2 mechanical testing

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