1. MECHANICAL PROPERTIES OF METALS

2. 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)

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. Let’s get to the web..

14. 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

15. 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