properties of materials. the behaviour of a given material is characterised by the response to a...
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Properties of materials
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The behaviour of a given material is characterised by the response to a stimulus. • Mechanical properties (behaviour under a set of forces) • Physical properties (behaviour under action of temperature, electrical or magnetic fields or radiation)• Chemical properties (behaviour under the action of chemicals)
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Mechanical properties studied as:
• time –independent
• time-dependent
• temperature-dependent
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Applying a force to a structure causes a stress bringing about a strain.
STRESS or TENSION : the ration between force F and the surface A to which is applied (Nm-2 o Pa). = F/A Three main types of stress: TENSILE, COMPRESSION and SHEAR
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If, once removed the applied force, the material gains the initial state, such behavior is said to be ELASTICELASTIC
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linear elastic behavior
non linear elastic
behavior (rubber)
Anelastic behavior
Eelastic
Hysteresis
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All materials, for small stresses, show a LINEARLINEAR elastic behavior (Hooke’s law)
σ = E ε
E = elastic modulus(Young modulus,
dimensions of a pressure)
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Curiously, the cause (load) is on the abscissa scale)
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Covalent or ionic solids
Metals
Polymers
EE TTmeltmelt
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Tensile measurements:
fragile (brittle) materials break beyond the elastic limit (ceramics, glasses)
ductile materials (metals, polymers): plastic deformation
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Fragile Material
Ductile material
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ToughnessToughness
measures the energy a material can store before breaking
Area under the curve!
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Indeed, a corrected curve should be used…
striction
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Another measure of the cohesive strength of the material: tenacity
Charpy pendulum
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Time dependent mechanical properties:
Creep
Fatigue
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CREEPCREEP
A constant static load may cause deformation
Not so important at ambient temperature, i.e. with biomaterials
Relevant process when T > 0,3-0,4Tmelt (Metals and ceramics)
T > Tg (Polymers and glasses)
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FATIGUE
Degration in mechanical properties when a material is subjected to cyclic stresses
Samples are subjected to different loads, and the number of cycles cause breakdown is measured at each load
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Often, a limit value for the load (FATIGUE LIMIT) is observed
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HARDNESSHARDNESS
Property of the external layers of a material: resistance to scratching (Mohs’ scale), to abrasion and to plastic deformation upon compression.
Measure: i) formation of an indentation by applying a static constant load for a definite time; ii) evaluation of the dimension
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Rockwell Method
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Ultimate Tensile strength
Relationship between hardness and UTS
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THERMAL PROPERTIES OF MATERIALS
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Thermal capacity*
Thermal expansion*
Thermal conductivity
Resistance to thermal shocks*
* Not really important in biomaterials
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THERMAL CAPACITY
Attitude of a body to store heat
Ratio between exchanged heat and change in temperature
kgK
J
mdT
dQC
When normalised to unit mass SPECIFIC SPECIFIC HEATHEAT
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THERMAL CONDUCTIVITY
Attitude of a body to transfer heat
The thermal conductivity coefficient is defined through Fourier’s law: the heat flux across a unit surface is proportional to the temperature gradient (with inverted sign)
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THERMAL EXPANSION
Usually all solids expand when heated
Coefficient of linear thermal expansion (Coefficient of linear thermal expansion ()=)=
Tl
l
TTl
llt
t
0
00
0 )()(
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Chemical characterization
Often surface only
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Others: • HRTEM
• Adsorption (porous systems)
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Contact angle:
Measures the wettability of a surface by a liquid
Usually water or aqueous solutions (hydrophobicity/hydrophilicity)
Also the surface tension of the solid
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lv
sl
sv
BIOGLASS
BIOGLASS
SILANIZED
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Ways of measuring contact angles
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ESCA
Highly energetic X-rays cause expulsion of the electrons of the inner cores, which have different binding energies, so allowing chemical determination
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Infrared Spectroscopy: functional groups in a molecule are recognized through their vibrational features
A well developed technique, very powerful…
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Versions of the technique for surface analysis
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Scanning tunneling microscope
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The end