CHAPTER 4 : MECHANICAL PROPERTIES OF METALS

TOPIC 1) TYPES OF MECHANICAL PROPERTIES 2) MECHANICAL TESTS OF METALS 3) HARDNESS TESTS 4) IMPACT TEST Beginning course details and/or books/materials needed for a class/project.

TYPES OF MECHANICAL PROPERTIES Defined as those properties which completely define its behavior under action of external load or forces. Those properties which associated with : Its ability to resist failure Its behavior under action of external forces Knowledge of mechanical properties : Essential for engineers in selecting suitable materials for various applications. Objectives for instruction and expected results and/or skills developed from learning.

Ability to undergo appreciable plastic deformation before rupture. Mechanical properties Strength Ability to withstand or support external forces or load without rupture Hardness Ability to resist deformation by abrasion, indentation or penetration and scratching by harder bodies. Ductility Ability to undergo appreciable plastic deformation before rupture. Brittleness Ability to fracture when deformed . Opposite to ductility Toughness Ability to absorb maximum energy up to fracture. Must be strong & ductile to be tough. Shows ability to withstand impact. Value increase when temperature increases. Elasticity Ability to retain its original shape & size after removal of load Plasticity Ability to experience permanent deformation without fracture when subjected to external forces Resilience Ability to absorb energy when it is elastically deformed Malleability Ability to be deformed into thin sheets by rolling or hammering without fracture Machinability Ability to be cut or removed by cutting tools in various ,machining operations. Weldability Ability of 2 similar or dissimilar metals to be joined by fusion& with or without filler Castability Ability to be formed into different shapes & sizes from its liquid state

MECHANICAL TESTS OF METALS All mechanical properties of metal- established by conducting tests on various testing machines. Types of mechanical test : Tensile test Hardness test Impact test Compressive test Fatigue test Creep test

Tensile test understand mechanical behavior by stress-strain test. 3 ways of load application –tension, shear, compression Most common mechanical stress-strain performed in tension Performed to determined ; Elastic limit Yield point Ultimate tensile strength % of elongation & reduction of area

Standard tensile specimen Circular cross section Reduced section diameter = 12.8 mm Reduce section length = 60mm Gauge length = 50mm Procedure : Specimen held in holding grips of apparatus Load applied gradually at a constant rate Specimen will be elongated until fracture Elongation – measured by extensometer

Data recorded – load vs. elongation Engineering stress : Normalized to engineering stress & engineering strain Engineering stress : where : A0 = original cross sectional area before any load applied (m2) F = instantaneous load (N) Engineering strain : where : li = original length l0 = instantaneous length

Plot a graph – stress vs. strain OA is a straight line Stress & strain proportional (Hooke’s Law) where E = modulus of elasticity Slope corresponds to E Called elastic deformation When applied load is released, specimen returns to its original shape.

AB is a small curve A is a point where elastic deformation end and plastic deformation begin Phenomenon of yielding occurs Called proportional limit Initial departure from linearity To determine yielding point precisely Construct straight line parallel to elastic portion of curve at a specified strain offset (0.002) B is the intersection of parallel line with curve The stress defined as a yield strength,

behavior for some steels Elastic-plastic transition Well defined Occurs abruptly Yield point phenomenon Upper yield point – plastic deformation initiated. decrease Lower yield point – deformation fluctuated at constant Yield strength , average of lower yield point

BC is an upward curve Stress increase to maximum limit (point C) Called tensile strength , TS Corresponds to maximum that can be sustained by a structure in tension . Necking begin to form- decrease of cross-sectional area

CD is a downward curve True stress True Strain Specimen continues to elongate Requires lesser load to continue deformation. decrease. D is a point of fracture or rupture True stress where : Ai = instantaneous cross sectional area True Strain where : li = instantaneous length

Ductility % elongation (% E) Measure degree of plastic deformation that has been sustained at fracture Can be expressed as : % elongation % area reduction % elongation (% E) where : lf = length after fracture lo = original length

where : Af = cross sectional area at % area reduction (%AR) where : Af = cross sectional area at fracture Brittle material Has little or no plastic deformation upon fracture Has less than 5% elongation

Measure hardness by forcing an indenter into materials surface. Hardness test Measure hardness by forcing an indenter into materials surface. Indenter – made of harder material - usually in form of ball. pyramid or cone Early hardness test Done by comparing with 10 standard mineral Increasing hardness on Moh’s scale talc 6) orthoclase Gypsum 7) quartz Calcite 8) topaz Fluorite 9) corundum Apatite 10) diamond

Important test Brinell Hardness Test Rockwell Hardness Test Viekers Hardness Test Knoop Hardness Test

Brinell Hardness Test By Dr Johan August Brinell in 1900 Performed by pressing steel ball into surface of test pieces using appropriate force. Formula : Brinell Hardness Number , HB (or BHN) Where D = diameter of steel ball (mm) d = diameter of indentation (mm) P = applied load (kg)

Rockwell Hardness Test Devised in the USA The most common method: Simple to perform Require no special skills Quick & direct reading Performed when hardness is beyond range of Brinell’s load is smaller than Brinell’s

Viekers Hardness Test The most accurate test Indenter Load Has continuous scale of hardness (10 to 1000) Indenter Diamond square based pyramid with 136ᵒ angle between opposite faces. Load Smaller than Rockwell & Brinell Between 1 and 1000 g Suitable for: Small, thin selected specimen region

Where P = applied load (kg) d = length of diagonal (mm)

Where l = longest diagonal length Knoop Hardness Test Very much similar to Vickers Indenter : Diamond pyramid with short depth and diagonal in ratio in ratio of 7:1 Measure diagonal length under microscope. Knoop hardness number designated by HK Where l = longest diagonal length

Hardness scale for Knoop & Vickers Approximately equivalent Both are referred as micro hardness testing Basis of reload & indenter size Suitable for testing Brittle materials (ceramics) Extremely thin metal Exceptionally hard, very shallow carburized or nitride surface

IMPACT TEST Many machine parts are subjected to sudden applied loads – impact loads Important engineering wise to have : Material that can withstand impact load without fracturing A hard, strong may not be suitable when subjected to sharp sudden load. Capacity of metals to withstands impact without fracture Impact resistance or impact strength Indication of toughness

Method of measuring toughness Impact- testing apparatus Types of impact-testing apparatus Charpy Izod

Question A cylindrical specimen of metal having a diameter of 12.88 mm and a gauge length of 63.50 mm is tested using a tensile testing machine. The elongation measurement are recorded in Table 3. Plot the stress-strain curve on the graph paper provided based on data in Table 3 Based on the stress-strain curved plotted in (i) : Compute the modulus of elasticity Determine the yield strength at a strain offset of 0.002. Determine the tensile strength Determine the ductility in percent elongation and percent area reduction