Mat-for-Build-02b-short recall of main Mechanical properties of Materials.pdf

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4 MATERIALS FOR MECHANICAL PROPERTIES BUILDINGS Fundamental to project structures which can afford only elastic strain.  Stiffness  Resistance (strength) Mechanical properties  Ductility Prof. Sara Goidanich  Toughness Dip. di Chimica, Materiali e Ingegneria chimica “G.Natta” [email protected] Mechanical properties Tel. 02.23993148 - http://midar.chem.polimi.it/ Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 6 _mechanical properties Mechanical Properties _mechanical properties: types of deformation ELASTIC: REVERSIBILE TEMPORARY Describe the response of a material to external applied LOAD DEFORMATION stress of different nature, including: F ▪ Compressive ELASTIC PLASTIC ▪ Flexural APPLIED STRESS ELASTIC ▪ Shear ▪ Torsion RUPTURE MATERIAL RUPTURE DEFORMATION It is important to know:  Entity of the load F PLASTIC  Type of material PLASTIC: IRREVERSIBLE  PERMANENT  Geometry of the element 00_Introduction to the course 00_Introduction to the course Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 _testing methods _tensile strength test _tensile strength test Mechanical properties depend on the way they are measured. F=10 N F=10 N F=10 N Comparisons can be mode only if the values result from the same type of Two key parameters L L / 2 L * 2 testing on specimens with similar shape  information on strength, 1. engineering stress:  = F/A0 (Pa = N/m2) stiffness, ductility… 2. engineering strain:  = L/L0 (%) A1 A2 A3  25  10 N/cm2 N/cm2 N/cm2  5 F  applied force L0  initial length A0 L0 L (L0 + Dl) A0  initial cross‐section 2 cm2 4 cm2 F 1 cm2 L  final length F F F Uniaxial Three-points A0 [N/m2] o [Pa] compressive flexural test A test 00_Introduction to the course 00_Introduction to the course 00_Introduction to the course Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148  _stress-strain curve _stress-strain curve _elastic deformation  = F/A0 (Pa = N/m2)  = F/A0 (Pa = N/m2)  = F/A0 TENSILE TESTING (Pa = N/m2) Ao lo Ao lo lo lo+l  = l/l0 (%)  = l/l0 (%)  = l/l0 (%) 00_Introduction to the course 00_Introduction to the course 00_Introduction to the course Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 15 _elastic deformation STIFFNESS _stiffness  = F/A0 Material Young Modulus L (Pa = N/m2) Hooke’s law modulus of elasticity (GPa) (mm) Elastic  =  or Rubber 0,007 143 deformation  Young's Modulus Plastics 1,4 0.7 Stiffness (GPa) Wood 14 0.07 E  stiffer material Concrete 17 0.06 Young's modulus measures the Bones 21 0.05 resistance of a material to elastic E = modulus of elasticity or (recoverable) deformation under load. A Common glasses 70 0.014 =E stiff material has a high Young's modulus (HOOKE’s law) Young's Modulus: Alluminium 73 0.014 and changes its shape only slightly under characteristic of the material elastic loads (e.g. diamond). A flexible Steel 210 0.004  material has a low Young's modulus and Diamond 1200 0.0008  = l/l0 (%) stiffness (E) changes its shape considerably (e.g. rubbers). 00_Introduction to the course 00_Introduction to the course Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 _plastic deformation _plastic deformation _plastic deformation: necking and rupture  = F/A0  = F/A0  = F/A0 (Pa = N/m2) PLASTIC DEFORMATION (Pa = N/m2) PLASTIC DEFORMATION (Pa = N/m2) PERMANENT!!! PERMANENT!!! Rs 0.2% Necking lo lo+l Yield strength lo lo+l (yield point)  = l/l0 (%) 0.2%  = l/l0 (%)  = l/l0 (%) 00_Introduction to the course 00_Introduction to the course 00_Introduction to the course Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148  21 22 _ultimate tensile strength Tensile Strength Mechanical resistance (strenght) DEFINITION: capability to bear applied forces without breaking and without When  > ts  the material breaks forming new surfaces TS permanent deformation  Metals: very high! ts = MAX of stress-strain curve fragile materials very low tensile strength, quite good compression strenght CHARACTERISTIC QUANTITIES: Two possible behaviors: Yield Strength (σy) (elastic limit, tension)  DUCTILE FRAGILE is the stress at which it first suffers permanent (inelastic) deformation in tension. Its units are MPa in the Metric system Tensile Strength (σts)  = l/l0 (%) Lo Lf The tensile strength is the nominal stress at which a round bar of the material, Lo Lf loaded in tension, separates. For brittle solids - ceramics, glasses, and brittle polymers - it is equal to the elastic limit. For metals, ductile polymers, and most When  > TS  the material breaks forming new surfaces composites it is larger than the elastic limit by a factor ranging from 1.1 to 3. Its Depends on: structure, T, presenca of defects, environment,... units are MPa in the Metric system Ultimate tensile strength TS = MAX00_Introduction Sara Goidanich – [email protected] – tel. 02 2399 3148 of stress-strain to the course Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 23 24 Mechanichal characteristics DUCTILITY _brittle VS ductile Capapability of a material to be plastically deformed  BRITTLE  DUCTILE   A B C  It can be measured by:  0  % = elongation = (l0-lf)/l0 Resistance – Mechanical stregth   Z% = reduction of area = (A0-Af )/A0 00_Introduction to the course Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 _mechanical properties _ductility Ductility - Elongation “STRENGTH” - TYPE OF DEFORMATION - ENTITY OF THE Capability of a material to undergo plastic deformation before reptu DEFORMATION  Elongation A The elongation  is the extension in the length of a BRITTLE / DUCTILE BEHAVIOUR B tensile specimen at fracture, expressed as a percentage of the original gauge length where L0 is Rupture of the Plastic (permanent) the original gauge length and Lf is the final gauge material occurs C deformation occurs length. It is normally expressed as a percentage. without permanent before rupture deformation  It can be measured by: % = elongation = (l0-lf)/l0 00_Introduction to the course 00_Introduction to the course 00_Introduction to the course Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148  _elastic deformation _plastic deformation _brittle behaviour CLAY ball GLASS ball RUBBER ball NEGLIGIBLE ELASTIC DEFORMATION VERY SMALL ELASTIC DEFORMATION (stiff WIDE ELASTIC DEFORMATION Plastic deformation prevails material) Plastic deformation is not possible No plastic deformation  fragile rupture 00_Introduction to the course 00_Introduction to the course 00_Introduction to the course Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 33 34 36 Toughness Toughness toughness Elastic energy Plastic energy Difficult concept toughness Its opposite is fragile  It is increased by:  is the ability of a material to absorb energy by  Stiffness (??) deforming without fracturing  Ductility  Strength the amount of energy per unit volume that a material can absorb before rupturing material's tolerance to defects inside.   Toughness requires a balance of strength and ductility Toughness can be determined by integrating the stress-strain curve. It is the energy of mechanical Toughness  Reliability deformation per unit volume prior to fracture. Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 37 Toughness Fracture toughness Characteristic quantities_mechanical parameters Plastic deformation  There is no direct correlation between strength Fracture toughness stress (σ,R), ductility (A%) e toughness (KIc) The fracture toughness Kc’, is a measure of the resistance of a Ultimate tensile material to the propagation of a crack. It can be measured by strength loading a sample containing a deliberately-introduced crack of  Some high strength materials may become length 2c and then recording the tensile stress σ at which the Yeld Strength Rupture crack propagates. Elastic extremely fragile if defects are present deformation 𝐾 𝑌𝜎 𝜋𝑐 Young Modulus E  metals >> polymers where Y is a geometric factor, near unity, that depends on details of the sample geometry. Its units are MPa.m1/2 in the Metric system  Ceramics e glasses very low (fragiles) strain Ductility 00_Introduction to the course 00_Introduction to the course Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148  45 MECHANICAL PROPERTIES Other mechanical properties Material properties are important for function  Stiffness Young’s Modulus hardness :  Ductility Elongation is a measure of how resistant solid matter is to various kinds of permanent shape change when a  Resistance (strength) compressive force is applied. Tensile Yield Strength there are different measurements of hardness: Ultimate Tensile Strength scratch hardness, indentation hardness, and rebound hardness.  Toughness Hardness is dependent on ductility, elastic stiffness, strength, toughness,... Mike Ashby, 2015 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 50 Material properties are important for function Economic issues Mechanical properties Stifness Strength Ductility Toughness Material Steel  Material’s cost StainlessSteel Alluminum Copper Polyethilene PE  Working cost Policarbonate Concrete Glass FiberGlass composite CarbonGlass composite  Maintainence cost wood Rubber The most The  Disposal cost stiff stronger The most ductile The most Mike Ashby, 2015 tough Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 Sara Goidanich – [email protected] – tel. 02 2399 3148 CAUTION ALL PICTURES IN THIS FILE BELONG TO THE PHOTO ARCHIVE OF TEACHERS AND COLLEAGUES OR THEY WERE ACQUIRED FROM BOOKS AND MAGAZINES OR DOWNLOADED FROM WEBSITES: THEREFORE, THEY ARE PROTECTED BY COPYRIGHT. THEY CAN ONLY BE USED IN UNIVERSITY TEACHING ENVIRONMENT AND FOR NO REASON THEY CAN BE REPRODUCED AND DISCLOSED IN ANY OTHER WAY. Sara Goidanich http://midar.chem.polimi.it Sara Goidanich – [email protected] – tel. 02 2399 3148