MATERIALS ILECTURE COURSE 2

PROPERTIES OF MATERIALS

Approach from the point of view of the user:

Properties mechanical

physical

chemical

technological

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

MECHANICAL PROPERTIES

B. Mode of applying the stressStatic Progressive

RegressiveOscilatingConstant

Dynamic ShockVariable module /orientation

C. Temperature Cold (<0°C)Ambiant (Room)Hot (<300°C)

MECHANICAL PROPERTIES

TENSILE TESTING

Tensile testing specimen

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 =

MECHANICAL PROPERTIES

Force – elongationtraction curve

(stress – strain)

Tensile characteristic curve of the material

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

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 =

MECHANICAL PROPERTIES

Force – elongationtraction curve(stress-strain)

Tensile characteristic curve of the material

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 [%]

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

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

Ti – base compacts with extra-low elastic modulus (< 10 GPa)

obtained through Selective Laser Melting

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

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

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

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)

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

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.

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

Corrosion prevention methods:B. Material to be protected

Al / Oţel

TECHNOLOGIC PROPERTIES

General properties that characterize the way a material can be worked through various technologies

Ex.: Casting - CastabilityMachining – MachinabilityForming – FormabilityWelding - Weldability

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;

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