7) Mechanical properties of solids and tissues Flashcards
Interactions between molecules of solids
Molecules of solids interact through Van Der Waals forces
The spacing of molecules is 10^-10
Density of solids is larger
Spatial arrangements of molecules of solids
Determined by interaction forces between molecules
Most stable has minimum potential energy of interaction forces
Crystal lattice
An arrangement of points repeating periodically in three dimensions
They consist of identical unit class that fill the space of the lattice
Lattice order
Long range order
-arrangement of unit cells is continuous and unbroken
Short range order
-the crystal like structure is unbroken only for short distances
Single crystal
The crystal lattice is continuous and unbroken in the whole body
Single crystals are anisotropic -properties are directionally dependent . Mechanical strength .thermal conductivity .velocity of light
Polycrystals
Polycrystals- bodies made out of smaller monocrystals with random orientation
Polycrystals are isotropic- they’re homogenous in all directions
Amorphous bodies
No repeating structures
Isotropic in all their properties
No definite boundary between solid and liquid states =supercooled liquids
Classification of crystal lattices
230 space groups
32 classes
7 system
Polymorphism when a solid material can exist in more than one form of crystal lattice
Eg:carbon in graphite and diamond
Classification of crystal lattice 2
Ionic
Metallic
Molecular
Network (atomic)
Ionic crystal lattice
Salts have ionic crystal lattice
DRAWING
NACL
Metallic lattice
A sea of delocalised electrons around positive charge
.high electrical conductivity
.high thermal conductivity
Eg: zinc
DRAWING
Network crystal lattices
Elements in group 14 of the periodic table have network crystal lattices
eg:carbon
DRAWING
Molecular crystal lattice
Crystal lattice is made by interaction of weak Van Der Waals forces
Eg: ice, co2 bromine and iodine
Polymers
Long chains composed of repeating units called monomers
Protein is a naturally occurring polymer
Eg:cotton and skin
Properties of polymers
Strong and capable of large elastic deformations
Very viscous
Don’t exist in gaseous state
Conformations of polymers
Thermal motion and external forces cause reconfiguration of the polymer
Liquid crystals
Liquid crystals
- can flow
- are anisotropic
Have narrow temp range
Properties are determined by structure or by shape
Types of liquid crystals
Nematic
Smectic
Cholesteric
Nematic
Long range orientational order
No positional order
Smectic
Long range orientational order
Long range positional order In one direction
Cholesteric
Arranged in layers
Orentiation is rotated by small angle from one layer to the next
Sensitive to external factors
Deformation
Change in the size and shape of the body due to applied force
Mechanical properties of solids depend on their ability to resist deformation
Types of deformation
Elastic
Then body recovers to its original shape and size
Plastic
Permanent deformation occurs
Types of deformations 2
Classification: Tensile Compression Bending Shear Twisting
Examples
Muscles: stretching and compressing
Bones: stretching, bending and twisting
Mechanism of elastic deformation
The distance between molecules change
Internal elastic forces arise trying to restore initial state
Mechanical stress
Ó= F/S
F is deforming force
S is area of cross section
Hooke’s Law
Applied stress ó and strain ę are directly proportional
Ó= Eę
E is Young’s modulus
Strain
ę=🔺L/L
Modulus of elasticity
E is Young’s modulus and is a temperature dependant constant
Equal strain means higher stress in materials
Deformation is crystalline solids
DRAWING
C is strength
B is elastic limit
A is proportional limit
Other mechanical properties
Plasticity and brittleness
Hardness is determined by material resistance to scratching
Strength is determined by ultimate strength
Deformation of polymers
Wrapping of long molecular chains is changed by deformation forces
Tensile force strengthens chain
Elasticity of polymers
Rubber can experience strain of 300% without breaking but steel breaks at 0.3%strain
Creep
Is the time dependant time stress and strain
Drawing x 3
F
ę
Ó
Bone tissue
Made from hydroxyapatite and collage in ration 2:1
Fast deformation due to hydroxypatite and collages due to creep
Skin
Main components
Collagen 75% and elastin 4%
Elastin stretches up to 300
Collage up to 10
Muscles
Built of elastin and collagen
Deformation due to straightening of collagen molecules
Walls of blood vessels
High collagen means less elasticity and higher strength
Elastin: collagen
2: 1 in carotid artery
1: 2 in the femoral artery
