Upload
lambert-sherman
View
225
Download
0
Tags:
Embed Size (px)
Citation preview
Properties of materials
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)
Mechanical properties studied as:
• time –independent
• time-dependent
• temperature-dependent
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
If, once removed the applied force, the material gains the initial state, such behavior is said to be ELASTICELASTIC
linear elastic behavior
non linear elastic
behavior (rubber)
Anelastic behavior
Eelastic
Hysteresis
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)
Curiously, the cause (load) is on the abscissa scale)
Covalent or ionic solids
Metals
Polymers
EE TTmeltmelt
Tensile measurements:
fragile (brittle) materials break beyond the elastic limit (ceramics, glasses)
ductile materials (metals, polymers): plastic deformation
Fragile Material
Ductile material
ToughnessToughness
measures the energy a material can store before breaking
Area under the curve!
Indeed, a corrected curve should be used…
striction
Another measure of the cohesive strength of the material: tenacity
Charpy pendulum
Time dependent mechanical properties:
Creep
Fatigue
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)
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
Often, a limit value for the load (FATIGUE LIMIT) is observed
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
Rockwell Method
Ultimate Tensile strength
Relationship between hardness and UTS
THERMAL PROPERTIES OF MATERIALS
Thermal capacity*
Thermal expansion*
Thermal conductivity
Resistance to thermal shocks*
* Not really important in biomaterials
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
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)
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 )()(
Chemical characterization
Often surface only
Others: • HRTEM
• Adsorption (porous systems)
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
lv
sl
sv
BIOGLASS
BIOGLASS
SILANIZED
Ways of measuring contact angles
ESCA
Highly energetic X-rays cause expulsion of the electrons of the inner cores, which have different binding energies, so allowing chemical determination
Infrared Spectroscopy: functional groups in a molecule are recognized through their vibrational features
A well developed technique, very powerful…
Versions of the technique for surface analysis
Scanning tunneling microscope
The end