STRENGTH OF MATERIAL - MATERIAL PROPERTIES

MECHANICAL PROPERTIES

MECHANICAL PROPERTIES:

IN THE COURSE OF OPERATION OR USE, ALL THE ARTICLES AND STRUCTURES ARE SUBJECTED TO THE ACTION OF EXTERNAL FORCES, WHICH CREATE STRESSES THAT INEVITABLY CAUSE DEFORMATION. TO KEEP THESE STRESSES, AND, CONSEQUENTLY DEFORMATION WITHIN PERMISSIBLE LIMITS IT IS NECESSARY TO SELECT SUITABLE MATERIALS FOR THE COMPONENTS OF VARIOUS DESIGNS AND TO APPLY THE MOST EFFECTIVE HEAT TREATMENT. I.E. A COMPREHENSIVE KNOWLEDGE OF THE CHIEF CHARACTER TICS OF THE SEMI-FINISHED METAL PRODUCTS & FINISHED METAL ARTICLES (SUCH AS STRENGTH, DUCTILITY, TOUGHNESS ETC) ARE ESSENTIAL FOR THE PURPOSE.

FOR THIS REASON THE SPECIFICATION OF METALS, USED IN THE MANUFACTURE OF VARIOUS PRODUCTS AND STRUCTURE, ARE BASED ON THE RESULTS OF MECHANICAL TESTS OR WE SAY THAT THE MECHANICAL TESTS CONDUCTED ON THE SPECIALLY PREPARED SPECIMENS (TEST PIECES) OF STANDARD FORM AND SIZE ON SPECIAL MACHINES TO OBTAINED THE STRENGTH, DUCTILITY AND TOUGHNESS CHARACTERISTICS OF THE METAL.

THE CONDITIONS UNDER WHICH THE MECHANICAL TEST ARE CONDUCTED ARE OF THREE TYPES.

(1) STATIC: WHEN THE LOAD IS INCREASED SLOWLY AND GRADUALLY AND THE METAL IS LOADED BY TENSION, COMPRESSION, TORSION OR BENDING. 

(2) DYNAMIC: WHEN THE LOAD INCREASES RAPIDLY AS IN IMPACT 

(3) REPEATED OR FATIGUE: (BOTH STATIC AND IMPACT TYPE) . I.E. WHEN THE LOAD REPEATEDLY VARIES IN THE COURSE OF TEST EITHER IN VALUE OR BOTH IN VALUE AND DIRECTION NOW LET US CONSIDER THE UNIAXIAL TENSION TEST. 

[FOR APPLICATION WHERE A FORCE COMES ON AND OFF THE STRUCTURE A NUMBER OF TIMES, THE MATERIAL CANNOT WITHSTAND THE ULTIMATE STRESS OF A STATIC TOOL. IN SUCH CASES THE ULTIMATE STRENGTH DEPENDS ON NO. OF TIMES THE FORCE IS APPLIED AS THE MATERIAL WORKS AT A PARTICULAR STRESS LEVEL. EXPERIMENTS ONE CONDUCTED TO COMPUTE THE NUMBER OF CYCLES REQUIRES TO BREAK TO SPECIMEN AT A PARTICULAR STRESS WHEN FATIGUE OR FLUCTUATING LOAD IS ACTING. SUCH TESTS ARE KNOWN AS FATIGUE TESTS] 

UNIAXIAL TENSION TEST: THIS TEST IS OF STATIC TYPE I.E. THE LOAD IS INCREASED COMPARATIVELY SLOWLY FROM ZERO TO A CERTAIN VALUE. 

STANDARD SPECIMEN'S ARE USED FOR THE TENSION TEST. 

THERE ARE TWO TYPES OF STANDARD SPECIMEN'S WHICH ARE GENERALLY USED FOR THIS PURPOSE, WHICH HAVE BEEN SHOWN BELOW: 

L_G = GAUGE LENGTH I.E. LENGTH OF THE SPECIMEN ON WHICH WE WANT TO DETERMINE THE MECHANICAL PROPERTIES.THE UNIAXIAL TENSION TEST IS CARRIED OUT ON TENSILE TESTING MACHINE AND THE FOLLOWING STEPS ARE PERFORMED TO CONDUCT THIS TEST. 

(I)   THE ENDS OF THE SPECIMEN'S ARE SECURED IN THE GRIPS OF THE TESTING MACHINE.

(II)  THERE IS A UNIT FOR APPLYING A LOAD TO THE SPECIMEN WITH A HYDRAULIC OR MECHANICAL DRIVE.

(III) THERE MUST BE A SOME RECORDING DEVICE BY WHICH YOU SHOULD BE ABLE TO MEASURE THE FINAL OUTPUT IN THE FORM OF LOAD OR STRESS. SO THE TESTING MACHINES ARE OFTEN EQUIPPED WITH THE PENDULUM TYPE LEVER, PRESSURE GAUGE AND HYDRAULIC CAPSULE AND THE STRESS VS STRAIN DIAGRAM IS PLOTTED WHICH HAS THE FOLLOWING SHAPE.

NOMINAL STRESS – STRAIN OR CONVENTIONAL STRESS – STRAIN DIAGRAMS:

STRESSES ARE USUALLY COMPUTED ON THE BASIS OF THE ORIGINAL AREA OF THE SPECIMEN; SUCH STRESSES ARE OFTEN REFERRED TO AS CONVENTIONAL OR NOMINAL STRESSES. 

TRUE STRESS – STRAIN DIAGRAM:

SINCE WHEN A MATERIAL IS SUBJECTED TO A UNIAXIAL LOAD, SOME CONTRACTION OR EXPANSION ALWAYS TAKES PLACE. THUS, DIVIDING THE APPLIED FORCE BY THE CORRESPONDING ACTUAL AREA OF THE SPECIMEN AT THE SAME INSTANT GIVES THE SO CALLED TRUE STRESS. 

SALIENT POINTS OF THE GRAPH: 

(A) SO IT IS EVIDENT FORM THE GRAPH THAT THE STRAIN IS PROPORTIONAL TO STRAIN OR ELONGATION IS PROPORTIONAL TO THE LOAD GIVING A ST.LINE RELATIONSHIP. THIS LAW OF PROPORTIONALITY IS VALID UPTO A POINT A. 

OR WE CAN SAY THAT POINT A IS SOME ULTIMATE POINT WHEN THE LINEAR NATURE OF THE GRAPH CEASES OR THERE IS A DEVIATION FROM THE LINEAR NATURE. THIS POINT IS KNOWN AS THE LIMIT OF PROPORTIONALITY OR THE PROPORTIONALITY LIMIT. 

(B) FOR A SHORT PERIOD BEYOND THE POINT A, THE MATERIAL MAY STILL BE ELASTIC IN THE SENSE THAT THE DEFORMATIONS ARE COMPLETELY RECOVERED WHEN THE LOAD IS REMOVED. THE LIMITING POINT B IS TERMED AS ELASTIC LIMIT . 

(C) AND (D) - BEYOND THE ELASTIC LIMIT PLASTIC DEFORMATION OCCURS AND STRAINS ARE NOT TOTALLY RECOVERABLE. THERE WILL BE THUS PERMANENT DEFORMATION OR PERMANENT SET WHEN LOAD IS REMOVED. THESE TWO POINTS ARE TERMED AS UPPER AND LOWER YIELD POINTS RESPECTIVELY. THE STRESS AT THE YIELD POINT IS CALLED THE YIELD STRENGTH. 

A STUDY A STRESS – STRAIN DIAGRAMS SHOWS THAT THE YIELD POINT IS SO NEAR THE PROPORTIONAL LIMIT THAT FOR MOST PURPOSE THE TWO MAY BE TAKEN AS ONE. HOWEVER, IT IS MUCH EASIER TO LOCATE THE FORMER. FOR MATERIAL WHICH DO NOT POSSES A WELL DEFINE YIELD POINTS, IN ORDER TO FIND THE YIELD POINT OR YIELD STRENGTH, AN OFFSET METHOD IS APPLIED. 

IN THIS METHOD A LINE IS DRAWN PARALLEL TO THE STRAIGHT LINE PORTION OF INITIAL STRESS DIAGRAM BY OFF SETTING THIS BY AN AMOUNT EQUAL TO 0.2% OF THE STRAIN AS SHOWN AS BELOW AND THIS HAPPENS ESPECIALLY FOR THE LOW CARBON STEEL.

(E) A FURTHER INCREASE IN THE LOAD WILL CAUSE MARKED DEFORMATION IN THE WHOLE VOLUME OF THE METAL. THE MAXIMUM LOAD WHICH THE SPECIMEN CAN WITH STAND WITHOUT FAILURE IS CALLED THE LOAD AT THE ULTIMATE STRENGTH. 

THE HIGHEST POINT ‘E' OF THE DIAGRAM CORRESPONDS TO THE ULTIMATE STRENGTH OF A MATERIAL. 

S_U = STRESS WHICH THE SPECIMEN CAN WITH STAND WITHOUT FAILURE & IS KNOWN AS ULTIMATE STRENGTH OR TENSILE STRENGTH. 

S_U IS EQUAL TO LOAD AT E DIVIDED BY THE ORIGINAL CROSS-SECTIONAL AREA OF THE BAR. 

(F) BEYOND POINT E, THE BAR BEGINS TO FORMS NECK. THE LOAD FALLING FROM THE MAXIMUM UNTIL FRACTURE OCCURS AT F. 

[BEYOND POINT E, THE CROSS-SECTIONAL AREA OF THE SPECIMEN BEGINS TO REDUCE RAPIDLY OVER A RELATIVELY SMALL LENGTH OF BAR AND THE BAR IS SAID TO FORM A NECK. THIS NECKING TAKES PLACE WHILST THE LOAD REDUCES, AND FRACTURE OF THE BAR FINALLY OCCURS AT POINT F ]

NOTE: OWING TO LARGE REDUCTION IN AREA PRODUCED BY THE NECKING PROCESS THE ACTUAL STRESS AT FRACTURE IS OFTEN GREATER THAN THE ABOVE VALUE. SINCE THE DESIGNERS ARE INTERESTED IN MAXIMUM LOADS WHICH CAN BE CARRIED BY THE COMPLETE CROSS SECTION, HENCE THE STRESS AT FRACTURE IS SELDOM OF ANY PRACTICAL VALUE.

PERCENTAGE ELONGATION: ' D ':

THE DUCTILITY OF A MATERIAL IN TENSION CAN BE CHARACTERIZED BY ITS ELONGATION AND BY THE REDUCTION IN AREA AT THE CROSS SECTION WHERE FRACTURE OCCURS.

IT IS THE RATIO OF THE EXTENSION IN LENGTH OF THE SPECIMEN AFTER FRACTURE TO ITS INITIAL GAUGE LENGTH, EXPRESSED IN PERCENT. 

L_I = GAUGE LENGTH OF SPECIMEN AFTER FRACTURE(OR THE DISTANCE BETWEEN THE GAGE MARKS AT FRACTURE) 

L_G= GAUGE LENGTH BEFORE FRACTURE(I.E. INITIAL GAUGE LENGTH) 

FOR 50 MM GAGE LENGTH, STEEL MAY HERE A % ELONGATION D OF THE ORDER OF 10% TO 40%. 

ELASTIC ACTION: 

THE ELASTIC IS AN ADJECTIVE MEANING CAPABLE OF RECOVERING SIZE AND SHAPE AFTER DEFORMATION. ELASTIC RANGE IS THE RANGE OF STRESS BELOW THE ELASTIC LIMIT.

MANY ENGINEERING MATERIALS BEHAVE AS INDICATED IN FIG(A) HOWEVER, SOME BEHAVES AS SHOWN IN FIGURES IN (B) AND (C) WHILE IN ELASTIC RANGE. WHEN A MATERIAL BEHAVES AS IN (C), THE S VS Î IS NOT SINGLE VALUED SINCE THE STRAIN CORRESPONDING TO ANY PARTICULAR ‘S ' WILL DEPEND UPON LOADING HISTORY. 

FIG (D): IT ILLUSTRATES THE IDEA OF ELASTIC AND PLASTIC STRAIN. IF A MATERIAL IS STRESSED TO LEVEL (1) AND THEN RELASED THE STRAIN WILL RETURN TO ZERO BEYOND THIS PLASTIC DEFORMATION REMAINS. 

IF A MATERIAL IS STRESSED TO LEVEL (2) AND THEN RELEASED, THE MATERIAL WILL RECOVER THE AMOUNT ( Î₂ - Î_2P ), WHERE Î_2P IS THE PLASTIC STRAIN REMAINING AFTER THE LOAD IS REMOVED. SIMILARLY FOR LEVEL (3) THE PLASTIC STRAIN WILL BE Î_3P. 

DUCTILE AND BRITTLE MATERIALS:

BASED ON THIS BEHAVIOR, THE MATERIALS MAY BE CLASSIFIED AS DUCTILE OR BRITTLE MATERIALS

DUCTILE MATERIALS:

IT WE JUST EXAMINE THE EARLIER TENSION CURVE ONE CAN NOTICE THAT THE EXTENSION OF THE MATERIALS OVER THE PLASTIC RANGE IS CONSIDERABLY IN EXCESS OF THAT ASSOCIATED WITH ELASTIC LOADING. THE CAPACITY OF MATERIALS TO ALLOW THESE LARGE DEFORMATIONS OR LARGE EXTENSIONS WITHOUT FAILURE IS TERMED AS DUCTILITY. THE MATERIALS WITH HIGH DUCTILITY ARE TERMED AS DUCTILE MATERIALS. 

BRITTLE MATERIALS:

A BRITTLE MATERIAL IS ONE WHICH EXHIBITS A RELATIVELY SMALL EXTENSIONS OR DEFORMATIONS TO FRACTURE, SO THAT THE PARTIALLY PLASTIC REGION OF THE TENSILE TEST GRAPH IS MUCH REDUCED.

THIS TYPE OF GRAPH IS SHOWN BY THE CAST IRON OR STEELS WITH HIGH CARBON CONTENTS OR CONCRETE.

CONDITIONS AFFECTING MECHANICAL PROPERTIES:

THE MECHANICAL PROPERTIES DEPEND ON THE TEST CONDITIONS 

(1)  IT HAS BEEN ESTABLISHED THAT LOWERING THE TEMPERATURE OR INCREASING THE RATE OF DEFORMATION CONSIDERABLY INCREASES THE RESISTANCE TO PLASTIC DEFORMATION. THUS, AT LOW TEMPERATURE (OR HIGHER RATES OF DEFORMATION), METALS AND ALLOYS, WHICH ARE DUCTILE AT NORMAL ROOM TEMPERATURE MAY FAIL WITH BRITTLE FRACTURE. 

(2)  NOTCHES I.E. SHARP CHARGES IN CROSS SECTIONS HAVE A GREAT EFFECT ON THE MECHANICAL PROPERTIES OF THE METALS. A NOTCH WILL CAUSE A NON – UNIFORM DISTRIBUTION OF STRESSES. THEY WILL ALWAYS CONTRIBUTE LOWERING THE DUCTILITY OF THE MATERIALS. A NOTCH REDUCES THE ULTIMATE STRENGTH OF THE HIGH STRENGTH MATERIALS. BECAUSE OF THE NON – UNIFORM DISTRIBUTION OF THE STRESS OR DUE TO STRESS CONCENTRATION. 

(3)  GRAIN SIZE: THE GRAIN SIZE ALSO AFFECTS THE MECHANICAL PROPERTIES.

HARDNESS:

HARDNESS IS THE RESISTANCE OF A METAL TO THE PENETRATION OF ANOTHER HARDER BODY WHICH DOES NOT RECEIVE A PERMANENT SET.

HARDNESS TESTS CONSISTS IN MEASURING THE RESISTANCE TO PLASTIC DEFORMATION OF LAYERS OF METALS NEAR THE SURFACE OF THE SPECIMEN I.E. THERE ARE BALL INDENTATION TESTS.

BALL INDENTATION TESTS:

ITHIS METHOD CONSISTS IN PRESSING A HARDENED STEEL BALL UNDER A CONSTANT LOAD P INTO A SPECIALLY PREPARED FLAT SURFACE ON THE TEST SPECIMEN AS INDICATED IN THE FIGURES BELOW:

AFTER REMOVING THE LOAD AN INDENTATION REMAINS ON THE SURFACE OF THE TEST SPECIMEN. IF AREA OF THE SPHERICAL SURFACE IN THE INDENTATION IS DENOTED AS F SQ. MM. BRINELL HARDNESS NUMBER IS DEFINED AS : 

BHN = P / F

F IS EXPRESSED IN TERMS OF D AND D

D = BALL DIAMETER

D = DIAMETRIC OF INDENTATION AND BRINELL HARDNESS NUMBER IS GIVEN BY-

 

THEN IS THERE IS ALSO VICKER'S HARDNESS NUMBER IN WHICH THE BALL IS OF CONICAL SHAPE. 

IMPACT STRENGTH

STATIC TENSION TESTS OF THE UNNOTCHED SPECIMEN'S DO NOT ALWAYS REVEAL THE SUSCEPTIBILITY OF METAL TO BRITTLE FRACTURE. THIS IMPORTANT FACTOR IS DETERMINED IN IMPACT TESTS. IN IMPACT TESTS WE USE THE NOTCHED SPECIMEN'S 

 

THIS SPECIMEN IS PLACED ON ITS SUPPORTS ON ANVIL SO THAT BLOW OF THE STRIKER IS OPPOSITE TO THE NOTCH THE IMPACT STRENGTH IS DEFINED AS THE ENERGY A, REQUIRED TO RUPTURE THE SPECIMEN, 

IMPACT STRENGTH = A / F     

WHERE F = IT IS THE CROSS – SECTION AREA OF THE SPECIMEN IN CM² AT FRACTURE & OBVIOUSLY AT NOTCH. 

THE IMPACT STRENGTH IS A COMPLEX CHARACTERISTIC WHICH TAKES INTO ACCOUNT BOTH TOUGHNESS AND STRENGTH OF A MATERIAL. THE MAIN PURPOSE OF NOTCHED – BAR TESTS IS TO STUDY THE SIMULTANEOUS EFFECT OF STRESS CONCENTRATION AND HIGH VELOCITY LOAD APPLICATION 

IMPACT TEST ARE OF THE SEVEREST TYPE AND FACILITATE BRITTLE FRICTION. IMPACT STRENGTH VALUES CAN NOT BE AS YET BE USED FOR DESIGN CALCULATIONS BUT THESE TESTS AS RULE PROVIDED FOR IN SPECIFICATIONS FOR CARBON & ALLOY STEELS.FUTHER, IT MAY BE NOTED THAT IN IMPACT TESTS FRACTURE MAY BE EITHER BRITTLE OR DUCTILE. IN THE CASE OF BRITTLE FRACTURE, FRACTURE OCCURS BY SEPARATION AND IS NOT ACCOMPANIED BY NOTICEABLE PLASTIC DEFORMATION AS OCCURS IN THE CASE OF DUCTILE FRACTURE.