MECHANICAL PROPERTIES OF METALS

INTRODUCTION Materials subjected to forces/load – Thus need to learn properties to avoid excessive deformation leading to failure when in service i.e. the mechanical behavior – Laboratory testing replicates an almost actual working conditions of materials – Design factors to consider : Nature of load (tensile, compressive, shear) Duration (constant, fluctuate, seconds, years) Environmental (high & low temp., corrosive etc)

STANDARDISED TEST METHODS ASTM – American Society of Testing & Materials BS – British Standard JIS – Japanese Industrial Standard SIRIM – Malaysian Standard DIN – Deutsches Institut fur Normung (Germany) AFNOR – Assc. Francaise de Normalisation UNI – Ente Nazionale Italiano di Unificazione AISI – American Iron & Steel Institute

CONCEPTS OF STRESS & STRAIN For static (or slow changes) load applied uniformly (at constant rate) Normally conducted at room temperature Test: Tension, compression, shear, torsional

Tension (Tensile)Test Most common destructive test Specimen deformed to fracture thru gradual increase of applied load along axis Refer standard for size & types of specimen UTM measures applied load (via load cell) & elongation (via extensometer) simultaneously Result (Stress-Strain curve)plotted on screen

Stress…Strain (Tensile & Compressive): Stress, σ = F / A 0 F is applied load – A 0 is initial area of cross section of specimen – Units N/m 2 or MPa ( 1MPa=10 6 N/m 2 ) Strain ε = Δl / l 0 = l i - l 0 / l 0 – l i is instataneous length, l 0 is initial length – No units

Compression… Similar to tensile but force is compressive along axis Except l 0 > l i Normally performed when – material is brittle in tension – Material to behave under large & permanent strain is desired

Bearing Stress.. Quite similar to compressive stress except that it arises when a member exerts a force on another member Bearing stresses develop on the contact surfaces of the two members Bearing stresses always occur in pair Example: in ribet & bolt joint

Shear & Torsional Test.. Pure shear stress, ז = F / A 0 F is load applied to parallel face of area A 0 Torsion is a variation of pure shear in which structure is twisted Torsional force produces a rotational motion about one end of the member relative to the other

Cont.. Torsion experienced by machine shaft, drive shaft, twist drill Torsional test are done on solid shaft & tubes Shear stress (in torsion) is a function of applied torque T Shear strain (in torsion) related to angle of twist

ELASTIC DEFORMATION Stress – Strain Behavior – Degree of deformation (or strained) depends on magnitude of applied stress – For metals in tension, stress is proportional to strain, σ = E.ε ( Hooke’s Law) Where E is a constant of proportinality known as Modulus of Elasticity Units normally GPa, for metals about 45GPa – Stress-Strain proportional Elastic deformation

Modulus of elasticity.. Slope of Stress-Strain plot gives Modulus of Elasticity Modulus = Stiffness or material’s resistance to deformation The greater the modulus, the stiffer the material i.e. smaller strain results from a given stress Modulus is a critical design parameter for computing elastic deflection Elastic deformation is non permanent, material able to return to original shape when load is released

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Working Stress.. Materials produced are not 100% homogenous Yield Point (YP) Stress & Ultimate Strength (US) in Mechanical Handbook are the average values of each material To derive a SAFE WORKING STRESS, a safety factor N is used either with YP or US

Working stress..(cont) Therefore..for safety reason – σ allowable = σ yield point N yp – σ allowable = σ ultimate N ult Material σ (MPa) Steel 0.8% carbon, hot rolled 830 Copper 380 Brass 380 Bronze 230 Aluminum 90