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BIOMECH - Biological ceramics (FROM PREVIOUS LECTURE (plant cell walls,…
BIOMECH - Biological ceramics
FROM PREVIOUS LECTURE
plant cell walls
composite of cellulose fibres within a matrix of branches hemicelluloses
gives a stress strain typical of a short fibered composite
the initial high modulus (2-5GPa) falls as the matrix or interface yield and shear
fracture properties of the cell wall
yielding of the matrix around fibres result in energy absorption and a rough fracture surface
Types of wood
young cell walls & walls in expanding tissue only contains hemi-cellulose in the cell wall
older cell walls maybe strengthened by lignification...
how ligninfication works?
like sclerotisation of insect cuticles probably works by excluding water
Evidence - dry cell wall with and without lignin has similar properties
...this increases stiffness (20-30GPa) and stregnth to (0.5GPa) but reduces breaking strain (to 1-2%)
how wood is toughened?
cell wall has reasonable toughness
3-5KJm-2
but wood is toughened by a further mechanism that depends on its cellular structure & fibre orientation
S2 layer fibres at 20 degrees to the long axis
as wood is stretch the cell walls buckle inwards parallel with the fibres, unwinding of the fibres occurs
the result is a very rough fracture structure
toughness - 30-50KJm-2
the problem with fibrous composites
both protein & sugar has a high metabolic cost
maybe better to use widespread, cheaper materials i.e. salts
advantages of salt - v. stiff and strong
disadvantages - v.brittle
therefore best used in combination with a soft matrix
what size and shape should the salt be?
the mechanism of spicule reinforced sponges
sponges contain 2-36% spicule of a range of sizes and shapes, in a mucopolysaccharide matrix
to determine the effect of numbers, size and shape Koehl isolated spicules and made model materials byt setting them in jelly
results
stiffness proportional to volume fraction of spicules- fill and reinforce the matrix
long thin spicules are more effective because higher stresses are transferred
smaller spicules are better than larger because they have bigger surface area & leave smaller gaps
therefore to get a truly rigid ceramic you need a fine array of long tiny crystals in a stiff matrix
the structure of bone
array of collagen with crystals of hydroxyapatite are embedded
needles are around 4nmx200nm & laid down in holes between tropocollagen molecules
the hierarchical arrangement of bone
bone is hierarchical in arrangement and comes in v. different forms for different functions
legnths -> woven -> lamellar -> havesian system
making changes from
fast ->slow growing
low -> high minerals
weak -> strong
time associated changes
the mechanical properties of bone
stiff, strong and fairly tough parallel to fibres, weaker and brittle across fibres
the effect of inperfections
small scratches or internal imperfections result in stress concentrations being built up
stress concentration C = 1+2(a/b)
a = radius at night angles to force
b=radius parrellel to force
this explains why brittle, materials break easily. tough ones dont as they yield as the crack tip, reducing stress
notch sensitivity tests
the stregnth of materials can be investigated when they have notches of different legnths cut into them
brittle materials
strength will fall rapidly with notch legnth
tough material
strength will fall gradually
how tough materials resist function
this energy is the work of fraction
mesuring the work of fracture
double cantilever test
a tattersall cantilever test
a trouser tear
a cutting test
an impact test
the larger the toughness the higher the critical stress and the longer the crack length
in biology toughening involves making the fracture area rougher
energy changes & fracture
the most useful way of understand the behavior of tough material is to consider energy change
breaking a material requires energy to create new surfaces
where does this come from?
ANSWER- energy released from relaxation as cracking breaks bonds
critical crack length
lets consider 2 sorts of energy as the crack extends
critical stress = ROOT2Eg/PIa
therefore critical crack length & stress increase with g
the effect of fibre arrangement
to ensure that bones don't split fibres are arranged in layers with different fibre directions
therefore there is crack stopping & toughening in all directions
mineral content & mechanical properties of bone
stiffnes increases with mineral content but not according to any simple model
however toughness fall with mineral content
different bones are adapted to their functions
mineral content rises with age
therefore stiffness rises but toughness falls
preventing failure - microcracking and bone remodeling
as bone is stressed past its yield it undergoes lots of microcracking,this absorbs energy but over time can lead to weakness and fatigue
haversian systems reabsorbs & lay down new bown removing microcracking and so prevent fatigue
structure of teeth
teeth are modified scales but mineralised like bone and contain dentine and enamal
dentine provides structure
enamal provides its hardwearing cutting edge
structure and properties of dentine
composition is like well mineralised bone (70%) in collagen
crystals hexagonal 3nmx6.4nm
there are no cells only tubules
E=15 GPa, o-=50MPa, toughness= 500Jm-2
but narwhal (possibly elephant) tusks are more like antlers
structures & properties of enamal
v. highly mineralised 97% mineral in protein crystals are large plates 25x100x500nm arraged in prisms (keyhole shape)
in turn prisms are arranged right angles to each other in a mesh separated by proteins
this arrangement stops cracks
because of the high mineral content enamel is very stiff E= 30-80GPa, o- = 200MPa, toughness = low (no figure)
structure of mollusc shells
made of calcium carbonate in the form of argonate or calcite with 0.1-0.5% protein
comes in many forms
prism form 10-200um wide columns
Nacre - 0.3-0.5um tiles
crossed lamellar - 20um thick - plywood like
foliated bundles of long crystals
homogeneous - granular rubble
mechanical properties of mollusc shell
Nacre
plates divert cracks through the matrix, so toughening the material parelell to the shell
toughness across plates = 1650Jm-3
along plates = 150Jm-2
crossed lamellar
the plywood structure diverts cracks like the cracked prisms of enamel creates a rougher fracture surface
mechanical design of razor shells
alternating twin prismatic layers held in precompression between crossed lamellar layers
this causes delamination when a crack hits the prismatic layer, deflecting the crack and absorbing energy
