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 / A0 F is applied load

– A0 is initial area of cross section of specimen

– Units N/m2 or MPa ( 1MPa=106 N/m2 )

Strain ε = Δl / l0 = li - l0 / l0– li is instataneous length, l0 is initial length

– No units

Compression…

Similar to tensile but force is compressive along axis

Except l0 > li 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 / A0

F is load applied to parallel face of area A0

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

Let’s get to the web..

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– σallowable = σyield point

Nyp

– σallowable = σultimate

Nult

Material σ (MPa)

Steel 0.8% carbon, hot rolled 830

Copper 380

Brass 380

Bronze 230

Aluminum 90