curs 2 eng - utcluj.ro 2_eng.pdf · approach from the point of view of the user: properties...
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MATERIALS ILECTURE COURSE 2
PROPERTIES OF MATERIALS
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Approach from the point of view of the user:
Properties mechanical
physical
chemical
technological
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MECHANICAL PROPERTIES
• Characterize the reaction of material specimens with standard shape / size under simple mechanical stresses.
• A mechanical property has always an attached numerical value.
Classification of mechanical testing methods
A. Type of mechanical loading Axial TENSILECompresion
Bending
Shearing
Torsion
Contact pressure
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MECHANICAL PROPERTIES
B. Mode of applying the stressStatic Progressive
RegressiveOscilatingConstant
Dynamic ShockVariable module /orientation
C. Temperature Cold (<0°C)Ambiant (Room)Hot (<300°C)
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MECHANICAL PROPERTIES
TENSILE TESTING
Tensile testing specimen
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MECHANICAL PROPERTIES
Mechanical strength: defined by the reaction opposedby the material (specimen) to the stress that actsupon it and is computed as the ratio between forceand the cross section of the specimen
[R] = MPa =N/mm2
S0 > S → R < σ
SF=s
0SFR =
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MECHANICAL PROPERTIES
Force – elongationtraction curve
(stress – strain)
Tensile characteristic curve of the material
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MECHANICAL PROPERTIES
Characteristic zones: elastic deformation proportional (O-A)
non-proportional (A-B)
plastic deformation yielding (B-C’)cold hardening (C’-D)reduction of the area (D-braking)
Proportional elastic deformation: Hooke’s law
llSEF D
´=
0
es ´= E
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MECHANICAL PROPERTIES
Strength
Ultimate tensile strength
Yield strengthRatio between the force that causes a stated non-
proportional strain and the initial cross section(most frequently 0.2%)
Best results: metals / composites > ceramics > polymers
0SF
R mm =
0
2.02.0 S
FR pp =
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MECHANICAL PROPERTIES
Force – elongationtraction curve(stress-strain)
Tensile characteristic curve of the material
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MECHANICAL PROPERTIES
DuctilityThe ability of the material to deform plastically before failure(connex : plasticity, maleability); opposite of ductility = brittleness
Elongation (at braking) Reduction of the area
Best results: polymers / metals >> ceramics
1000
0 ´-
=SSS
Z u [%]1000
0 ´-
=lll
A u [%]
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MECHANICAL PROPERTIESToughness
Property of materials to absorb energy without failure
Tough material: Strong + Ductile(includes shock resistance – resilience)
Fracture toughness: Ability to stand the propagation of a pre-existing crack
Best results: metals / composites > polymers >> ceramics
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PROPRIETATI MECANICEStiffness
Characterizes the way the material deforms elastically under external stresses
E = tg α
Best results: composites / multi-layered materials > ceramics / metals >> polymers
Polymeri: 0.001 – 4GPaMetals: Pb - 12GPa; steels – 210GPa; Al – 80GPa; Ti – 113GPa;Graphite fibers - 290GPa; Glass fibers– 17-GPa
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Ti – base compacts with extra-low elastic modulus (< 10 GPa)
obtained through Selective Laser Melting
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MECHANICAL PROPERTIESHardness
Ability of materials to oppose the penetration of external indenters
Most frequently measured staticallyMohs – minerals (ceramics)Knoop – all classesBrinell (HB), Rockwell(HRB, HRC,...) – metalsVickers – metals, ceramicsShore – polymers
Best results: ceramics > metals >> polymers
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CHEMICAL PROPERTIES
Essential: chemical stabilityTime (polymers!!!)Heat + other radiations (polymers!!!)Aggressive media (polymers+metals!!!)
Metals: corrosion chemical – reaction with strongly oxidant chemicals
electrochemical– redox reaction in an electrolitic environment
The metal that oxidizes – anode
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CHEMICAL PROPERTIES
Anode: (oxidation)
Cathode: (reduction)
→ M(OH)2 MO
-+ +® eMM 22
-- ®++ OHOOHe 2212 22
¾¾ ®¾drying
MO – protective layer if chemically stableadherentimpermeable
Ex.: Al, Ti, stainless steel
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CHEMICAL PROPERTIESCause of the polarization: difference of the electrode potentials
„0” reference potential – „normal hydrogen electrode” equilibrium, H pressure = 1 atm.-+ +Û eHH 222
Metal Au+ Pt2+ Ag+ Cu+ H+ Ni2+ Co2+ Fe2+ Cr3+ Zn2+ Ti3+ Al3+
Standardpotential[V]
1,692 1,18 0,799 0,521 0 -0,257 -0,28 -0,447 -0,744 -0.76 -1,37 -1.66
Me1 / Me2 = anode / cathodeif V1 < V2 and there is an electric contact Me1 - Me2
→ Me1 corrodes (if Me1O is not protective)
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Corrosion prevention methods:
Methods referring to:
A. External conditionsA.1. Corrosive environment
combined factors: temperature, abrasive particles, stresses,inhibitors, humidity absorbers (desiccants), etc.
A.2. couples of materials in contact (electrochemical corrosion)corrosion occurs at anode (electrode potential!)cathodic protection – sacrifice anodes – Zn / steel, etc.
B. Material to be protected
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Corrosion prevention methods:B. Material to be protected
B.1 Protective coatings
Polymers: paint (primer), polymer layers, powders
Metals: electro-deposition RT - Ni, Cr. Cd, etc.Hot - Zn
chemical deposition – Ni
cladding – Al / Al alloy, Cu / steel, etc.
powder metallisation – HVOF, plasma, laser, etc.
Ceramics: oxides: Ex. Anodisation (Al)
porcelain (enamel) / cast iron, steel, Al, etc.
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B.2. Use of corrosion resistant alloys corrosion resistant steels ( ̴ stainless steels), Al (ATENTION electrochemical couple!), Ni, Co, Ti, etc.
Corrosion prevention methods:B. Material to be protected
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Corrosion prevention methods:B. Material to be protected
Al / Oţel
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TECHNOLOGIC PROPERTIES
General properties that characterize the way a material can be worked through various technologies
Ex.: Casting - CastabilityMachining – MachinabilityForming – FormabilityWelding - Weldability
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Glossary• Epruvetă = specimen;• Solicitare = loading (load);• Încercare la tracţiune = tensile testing;• Încovoiere = bending;• Forfecare = shearing;• Răsucire = torsion;• Rezistenţă mecanică = (mechanical) strength;• Tensiune = stress;• Deformaţie = strain;• Curgere = yielding;• Ecruisare = cold hardening;• Gâtuire = reduction of the area;• Rezistenţa (de rupere) la tracţiune = ultimate tensile strength;• Rezistenţa (limita) de curgere tehnică = yield strength;• Ductilitate = ductility;
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Glossary• Fragilitate = brittleness;• Alungire = elongation;• Tenacitate = toughness;• Rigiditate = stiffness;• Duritate = hardness;• Grund = primer;• Placare (prin presare la cald) = cladding;• Metalizare = metallisation;• Smalt = enamel;• Fonta = cast iron;• Otel inoxidabil = stainless steel