CHAPTER 3 OUTLINE PROPERTIES OF MATERIALS PART 1

Transcription

1 CHAPTER 3 PROPERTIES OF MATERIALS PART 1 30 July OUTLINE 3.1 Mechanical Properties Definition Factors Affecting Mechanical Properties Kinds of Mechanical Properties Stress and Strain Elastic Deformation Plastic Deformation & Plasticity Strength Brittleness, Toughness, Resilience & Ductility Fatigue Creep and Shrinkage Design and Safety Factors 3.2 Electrical Properties 3.3 Optical Properties 3.4 Magnetic Properties 3.5 Thermal Properties 3.6 Corrosion Properties 30 July

2 3.1 MECHANICAL PROPERTIES DEFINITION Properties or deformation observed when a material is subjected to an applied external force (F = ma) to a mechanical force of stretching, compressing, bending, striking are called the mechanical properties. e.g. Mechanical properties of airplane wing made of aluminum alloy Mechanical properties of a bridge made of steel. 30 July FACTORS AFFECTING THE MECHANICAL PROPERTIES Nature of the applied load, e.g. Tensile, compressive, shear Magnitude of the applied force The duration (application time): may be less than a second, may extend over a period of many years. 30 July

3 3.1.3 KINDS OF MECHANICAL PROPERTIES Elasticity Stiffness Plasticity the ability of a material to deform under load and return to its original size and shape when the load is removed. the slope of the linear segment of stress strain curve is Elastic Modulus or Young s Modulus. The value of the Modulus is the measure of STIFFNESS, material s resistance to elastic deformation (MPa) the property of a material to deform permanently under the application of a load. Yield Strength the stress level at which the plastic deformation begins. (MPa) Tensile Strength the stress at the maximum on the engineering stress-strain curve.the ability of a material to withstand tensile loads without rupture when the material is in tension (MPa) the ability of a material to withstand compressive (squeezing) loads without being crushed Compressive Strength when the material is in compression. (MPa) Fracture Strength corresponds to the stress at fracture (MPa) 30 July KINDS OF MECHANICAL PROPERTIES Toughness the ability of a material to withstand shatter. A material which easily shatters is brittle. The ability of a material to absorb energy (J/m 3 ) Resilience The capacity of material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered (J/m 3 ) Ductility the ability of a material to stretch under the application of tensile load and retain the deformed shape on the removal of the load. Measure of ability to deform plastically without fracture (no units or m/m) Brittleness brittle materials approximately have a fracture strain of less than about 5%. the property of a material to deform permanently under the application of a compressive load. A material Malleability which is forged to its final shape is required to be malleable Fatigue Strength Hardness the property of a material to withstand continuously varying and alternating loads the property of a material to withstand indentation and surface abrasion by another hard object. It is an indication of the wear resistance of a material. 30 July

4 3.1.4 STRESS & STRAIN Types of force(load) applied on the object Tension Compression Shear Torsion Reference: Callister, Material Science and Eng., 5th Ed., p July ENGINEERING STRESS (σ): (Gerilme) Stress is defined as force F applied over the original crosssectional area A o. For a tensile test the stress is given by, Stress, (MPa or psi) Where, F = applied tensile force (N or lbs) A 0 = original cross-sectional area of the test specimen (m 2 or in 2 ) Units for Engineering Stress: US customary: pounds per square inch (psi) SI: N m -2 = Pascal (Pa) 1psi = 6.89 x 10 3 Pa 30 July

5 ENGINEERING STRESS (σ): (Gerilme) Example: A 1.25 cm diameter bar is subjected to a load of 2500 kg. Calculate the engineering stress on the bar in megapascal (MPa) Sol n: F= ma = 2500 x 9.81 = N A o = π r 2 = π ( / 4 ) σ = Ft / A o = 2 x 10 8 Pa = 200 MPa 30 July ENGINEERING STRAIN: (Şekil Değiştirme) When an unaxial tensile force is applied to a rod, it causes the rod to be elongated in the direction of the force. Engineering strain is the ratio of the change in the length of the sample in the direction of the force divided by the original length. ε = ( l l o ) / l o = l / l o Where, l = l - l o is the change in length l0 = original length of the specimen In engineering practice it is common to convert engineering strain into percent strain or percent elongation % engineering strain = engineering strain x 100 % = % elongation Unit of engineering strain: Inch / inch or m/m which is dimensionless 30 July

6 ENGINEERING STRAIN: (Şekil Değiştirme) F σ Engineering = = A stress δ 2 F σ F ε = = Engineering = = 2 A L (normal) strain A δ ε F = σ L = A 2 δ δ ε = = 2 L L 30 July STRESS STRAIN TESTING Tension tests: they are common, since they are easier to perform for most structural materials, steel etc. Compression tests: are used, when a material s under large and permanent strains is desired, or when the material is brittle in tension, concrete Shear and torsion tests: Torsion test are performed on cylindrical solid shafts or tubes, machine axles and drive shafts Typical tensile Specimen 30 July

7 STRESS STRAIN TESTING Typical tensile test machine Schematic representation of the apparatus used to conduct tensile stress - strain tests Hydraulic Wedge Grips Extensometer Specimen 30 July YOUNG'S MODULUS (E) During Elastic Deformation: Stress / Strain = a constant σ / ε= E =Modulus of elasticity (Young s Modulus) (Elastisite Modülü) (MPa) Modulus of Elasticity gives an idea about material s resistance to elastic deformation. 30 July

8 STIFFNESS:Material s resistance to Elastic Deformation. Atomic Origin of Stiffness E Net Interatomic Force df dr ro Strongly Bonded Weakly Bonded Interatomic Distance The value of the Modulus of Elasticity is the measure of STIFFNESS 30 July YOUNG'S MODULUS (E) Metal Alloy Aluminum Brass Copper Magnesium Nickel Steel Titanium Tungsten Modulus of Elasticity, E ( GPa) July

9 YOUNG'S MODULUS (E) Total Elongation Engineering Stress, σ = F/A o E Engineering Strain, ε = L/Lo) 30 July ELASTIC DEFORMATION Elasticity, or elastic deformation is defined as ability of returning to an initial state or form after deformation. In most engineering materials, however, there will also exist a timedependent elastic strain component. That is, elastic deformation will continue after the stress application, and upon load release some finite time is required for complete recovery. This time-dependent elastic behavior is known as ANELASTICITY, and it is due to timedependent microscopic and atomistic processes that are attendant to the deformation. For metals the inelastic component is normally small and is often neglected. However, for some polymeric materials its magnitude is significant; in this case it is termed VISCOELASTIC BEHAVĐOR. P A simplified view of a metal bar's structure The same metal bar, this time with an applied load. After the load is released, the bar returns to its original shape. This is called elastic deformation. 30 July

10 3.1.5 ELASTIC DEFORMATION EXAMPLE: A piece of copper originally 305 mm (12 in.) long is pulled in tension with a stress of 276 MPa (40,000 psi). If the deformation is entirely elastic, what will be the resultant elongation? Sol n: σ = Eε Since the deformation is elastic, strain is linearly dependent on stress the magnitude of E for copper is 110 GPa ε= (l lo ) / lo = l / lo l = (276 MPa) (305 mm)/ 110 x 103 MPa = 0.77 mm 30 July PLASTIC DEFORMATION & PLASTICITY For most metallic materials, elastic deformation exists only to strains of about As the material is deformed beyond this point, the stress is not proportional to strain. And permanent, nonrecoverable deformations, PLASTIC DEFORMATION, occurs. 30 July

11 3.1.6 PLASTIC DEFORMATION & PLASTICITY 30 July STRENGTH YIELD STRENGTH ( Y ) ( MPa or psi ) Stress at which noticeable plastic deformation has occurred. The magnitude of the yield strength for a metal is a measure of its resistance to plastic deformation. A straight line is drawn parallel to the elastic deformation part of the curve from the engineering strain value of The stress corresponding to the intersection point of these two lines is YIELD STRENGTH. Yield strengths may range from 35 MPa for a low strength Al to over 1400 MPa for high strength steels. Comparison of Yield Strength : σy (ceramics) >> σ y (metals) >> σ y (polymers) >> σ y (composites) 30 July

12 TENSĐLE STRENGTH (TS) ( MPa or psi ) The stress at the maximum on the engineering stressstrain curve. This corresponds to the maximum stress that can be resisted by a structure in tension. It is the maximum stress without fracture. Examples: metals: occurs when noticeable necking starts ceramics: occurs when crack propagation starts polymers: occurs when polymer backbones are all aligned and about to break. Tensile Strengths may vary from 50 MPa to 3000 MPa 30 July COMPRESSIVE (CRUSHING) STRENGTH It is important in ceramics used in structures such as buildings or refractory bricks. The compressive strength of a ceramic is usually much greater than their tensile strength. Tensile, compressive and bending testing for materials 30 July

13 COMPRESSIVE (CRUSHING) STRENGTH Comparison of Stress - Strain Curves for Metals, Ceramics, Polymers and Elastomers 30 July COMPRESSIVE (CRUSHING) STRENGTH The Relationship between Elastic Modulus and Melting Temperature 30 July

14 3.1.8 BRITTLENESS, TOUGHNESS, RESILIENCE & DUCTILITY BRITTLENESS A material that experiences very little or no plastic deformation upon fracture is termed brittle. Ductile vs Brittle Materials Only Ductile materials will exhibit necking. Ductile if EL%>8% (approximately) Brittle if EL% < 5% (approximately) Engineering Stress A X X B D X C Brittle A & B Ductile C & D X 30 July 2007 Engineering Strain BRITTLENESS 30 July Brittle Fracture Surfaces 14

15 TOUGHNESS A measure of the ability of a material to absorb energy without fracture. (J/m3 or N. m/m3= MPa) It is a measure of the ability of a material to absorb energy up to fracture. Energy needed to break a unit volume of material. Area under stress-strain curve For a material to be tough, it must display both strength and ductility. Often ductile materials are tougher than brittle ones. Examples: smaller toughness (ceramics), larger toughness(metals, PMCs) smaller toughness unreinforced ( polymers) 30 July TOUGHNESS Toughness, U t Engineering Stress, S=P/Ao Sy U t = e f Sde o (S y +S u ) 2 S u EL% 100 X Engineering Strain, e = L/Lo) 30 July

16 TOUGHNESS Toughness is really a measure of the energy a sample can absorb before it breaks. 30 July RESILIENCE A measure of the ability of a material to absorb energy without plastic or permanent deformation. (J/m 3 or N. m/m 3 = MPa) Resilience, U r Engineering Stress, S=P/Ao E S y e y e y U r = Sde o S u S ye y 2 = S y 2 2E X 30 July 2007 Engineering Strain, e = L/Lo) 32 16

17 DUCTILITY (% EL) Ductility is another important mechanical property. It is a measure of the degree of plastic deformation that has been sustained at fracture. 30 July DUCTILITY (% EL) Stress-Strain diagrams for typical (a) brittle and (b) ductile materials Stress- Strain Curves for Brittle and Ductile Materials 30 July

18 DUCTILITY (% EL) Ductile Materials Brittle Materials 30 July DUCTILITY (% EL) 30 July

19 STRESS STRAIN CURVES CURVE EXAMPLE A. Stiff but Weak: CERAMIC B. Stiff and Strong: CERAMIC C. Stiff and Strong: METAL C'. Moderately Stiff and Strong: METAL D. Flexible and Moderately Strong: POLYMER E. Flexible and Weak: POLYMER Stress- Strain Curves for Different Materials 30 July FATIGUE If placed under too large of a stress, metals will mechanically fail, or fracture. This can also result over time from many small stresses. The most common reason (about 80%) for metal failure is fatigue. The most common reason (about 80%) for metal failure is fatigue. 30 July

20 FATIGUE MECHANISM 30 July FATIGUE MECHANISM This front brake assembly broke off under braking and severely injured the cyclist. Poor maintenance had allowed the brake bolt to loosen and allow the assembly to "chatter" when braking imposing cyclic loads instead of steady stress on the fastening 30 July 2007 bolt

21 MECHANICAL PROPERTIES Typical Mechanical Properties Metals in annealed (soft) condition Material Yield Stress (MPa) Ultimate Stress (MPa) Ductility EL% Elastic Modulus (MPa) Poisson s Ratio 1040 Steel Steel Al Alloy Stainless Steel /30 Brass Ti Alloy AZ80 Mg Alloy July