Lecture 2: Classification and Properties of Materials PDF

CHAPTER TWO 2.Classification & Properties of Materials 1 2.1 Classification of Materials Materials that are used for construction purpose can be broadly classified based on their: Metallic Property Physical nature Mode of production 2 CLASSIFICATION BA...

CHAPTER TWO 2.Classification & Properties of Materials 1 2.1 Classification of Materials Materials that are used for construction purpose can be broadly classified based on their: Metallic Property Physical nature Mode of production 2 CLASSIFICATION BASED ON METALLIC PROPERTY 1.METALLIC : IN GENERAL METALS CAN BE CLASSIFIED IN TO :FERROUS AND NON- FERROUS. A. FERROUS: IS THE METAL IN WHICH THE PRINCIPAL ELEMENT IS IRON. EXAMPLES:STEEL,WROUGHT IRON & CAST IRON B. NON-FERROUS : IS THE METAL IN WHICH THE PRINCIPAL ELEMENT IS NOT IRON EXAMPLES: COPPER,ALUMINIUM,LEAD,ZINC,ETC 2.NON METALLIC: EXAMPLES;CONCRETE,TIMBER,STONE,LIME ETC. 3 CLASSIFICATION BASED ON PHYSICAL NATURE OF MATERIALS 4 CLASSIFICATION BASED ON MODE OF PRODUCTION a) Naturally Occurring Materials.stone.timber b) Industrially produced materials.Cement.glass C) Materials produced at construction site.Concrete 5.mortar 2.2 PROPERTIES OF MATERIALS Properties Which relate to materials are: 1. Physical properties  Density & specific gravity  Thermal property  Acoustic /sound permeability  Porosity 6 2, Chemical properties  Corrosion Resistance  Combustibility  Toxicity  Decay Resistance 3.Mechanical Properties The resistance of material to:  The action of external static forces (compressive,tensile,bending,shear ,torsional strength)  The action of dynamic external forces(impact and vibratory loads) 7 Behavior of materials under load Application of external force on solid body in equilibrium results in: ….Internal resisting forces are developed in the body which balances the externally applied force. ….The body is deformed to varying degree ….The intensity of internal force is stress and the deformation per unit length is strain 8 Depending on the arrangement & direction of the external forces, the stress produced in the body may be :  Tensile  Compressive  Shear  Bending  Torsional  Various combinations of the above. 9 2.3 Testing of Materials for Mechanical Properties..Mechanical properties are conducted to examine the performance of construction materials under the action of external forces...Mechanical tests are classified : A. With reference to the arrangement & direction of the external forces;  Tension Test  Specimen under tension test is subjected to an axial tensile force  Tensile stress is developed on cross- sectional area perpendicular to the line of action of the force. 10  The specimen increase in length.  Compression Test  Specimen is subjected to an axial compressive force  Compressive stress is produced.  The specimen decrease in length.  Shear test In this test, shearing stress is determined on the x-sectional area parallel to the line of action of the external forces.  Bending Test.  Specimen is subjected to forces that give rise to bending moments  The resulting stresses are compressive on one side of the neutral axis & tensile on the other side.  Shear stress exist throughout the beam.  Torsion Test  This test is conducted to determine the shearing strength of a material 11  The specimens for torsion test are generally cylindrical in shape. B. WITH REFERENCE TO THE RATE & DURATION OF THE LOAD APPLICATION.  Static Tests  Made with gradually increasing load. eg. ordinary tests in tension & compression etc.  Dynamic Tests  Made with suddenly applied loads.  Wear Tests  Made to determine the resistance to abrasion & impact.  Long time Tests  These are made with the loads applied to the object for long period of time.  Fatigue Tests  These tests are made with fluctuating stresses repeated a large number of times 12 C. With Reference to the effect on the specimen.  Destructive Test  The specimens are either crushed or ruptured and made useless at the end of the tests.  Tests conducted on the following materials are best examples..Ultimate strength of steel..Compressive strength of concrete  Non-destructive Tests  Are used to test the strength of members of existing structures without affecting their performance.  Example: hammer test 13 2.4 Stress-Strain Properties in Simple Tension Test  In standard conventional tension test, specimen is subjected to a gradually increasing axial tensile force ‘P’ by means of testing machine. 14  At various increments of load ,the change in length ∆L of the specimen is measured. ∆L=L-Lo where L=new length Lo =original length  It is assumed that the stress is uniformly distributed for all points on each x-section. This stress is computed as follow; σt =p/Ao where σt =tensile stress Ao=X-sectional area P= applied load 15  Theuniform stress will produce a uniform elongation ∆L.The elongation per unit length is strain & expressed as: ε= ∆L / Lo Where ε =strain ∆L=elongation Lo=original length of the specimen 16 17  Properties in the elastic range  The parameters which are used to describe the mechanical properties of a material in the elastic range are:  proportional limit,  elastic limit,  modulus of elasticity, stiffness etc 1.Proportional Limit: is the greatest stress which a material is capable of withstanding without deviation from the law of proportionality of stress to strain. 2.Elastic Limit: is the greatest stress which a material is capable of withstanding without a permanent deformation remaining up on the release of stress. 3.Yield Point :is the stress at which there occurs a considerable increase in strain without an increase in stress. Only ductile materials have both lower & upper 18 yield points.  4.MODULUS OF ELASTICITY(YOUNG’S MODULUS)  is the slope of the initial linear part of stress-strain diagram.  The greater the modulus of elasticity, the smaller the elastic strain resulting from the application of a given values. Methods of determining modulus of elasticity, E. i. For ductile materials, with linear stress – strain portion, E= ∆ σt ∕ ∆ ε 19  II.FOR MATERIALS WITH NON-LINEAR STRESS-STRAIN CURVES  The slope of the stress- strain curve varies and the modulus of elasticity cannot be readily determined.  The following three methods are employed to define E: a. Initial-Tangent modulus: The slope of the stress –strain curve at the origin which has a value of E 1 =tanф1 Stress (σt ) ф1 20 Strain(ε ) b. Secant modulus : the slope of the line joining the origin and the selected point on the stress-strain curve with the value of E 2 =tanф2 Stress (σt ) Strain(ε ) ф2 21 c. Tangent modulus: The slope of the tangent to the stress –strain curve at the selected point with the value of E 3 =tanф3 ф Stre ss 3 (σt ) Strain(ε ) 22 5.Stiffness  is the measure of the ability of material to resist deformation.  The higher the modulus of elasticity, the stiffer the material.  A material has a higher stiffness value when its deformation in the elastic range is relatively small.  Comparing steel alloys with E=210Gpa and aluminium alloys with E=70Gpa,the steel alloys are about three times as stiff as the aluminium alloys i.e steel alloys will deform about one-third as much as aluminium alloys for the same stress. 6.Poisson’s Ratio  Is the ratio of the unit deformations or strains in transverse direction to the longitudinal direction within proportional limit. µ= ε’∕ ε where ε’ = transverse strain, ε =longitudinal strain  Poisson’s ratio is a measure of the stiffness of the material in the direction at right angle to applied load. 23  PROPERTIES FOR THE PLASTIC RANGE  The characteristic at the plastic range is that there is a permanent deformation in the stressed body after complete removal of the load.  The parameters which are used to describe the mechanical properties for the plastic range are; ultimate strength, ductility and toughness.  1.Ultimate Strength:-is the maximum strength a material can possibly resist before failure.  Depending on the stress strain relationship of a particular material, the plastic strength will correspond to the ultimate strength or to the fracture(rupture)strength. 24 Stres s Stres Ultimate or Fracture Ultimate s (σt ) fracture strength strength (σt ) strength Strain(ε ) Strain(ε ) Ductile material Brittle material 25 2.Ductility:-  represents its ability to deform in the plastic range.  Ductile materials show relatively higher plastic deformation, i.e they are capable of being drawn-out ,before rupture occurs.  Ductility is measured by the percentage elongation or percentage reduction in area. De=Lf –Lo *100 Da =Ao-Af *100 Lo Ao 3.Toughness  Is the ability of material to absorb energy in the plastic range.  A material with high toughness can absorb high values of strain energy in the plastic range. 26 27