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BIOMECH - MATERIALS BACKGROUND (Cyclical Test (Types of Material ((Top…
BIOMECH - MATERIALS BACKGROUND
Hooke's Law
if a force is applied to a spring, material or any other structure its deflection will be proportional to the force
WHY??
solids are made of atoms and deforming a material involves stretching or compressing the inter-atomic bonds...
...seen with the straight line through origin
crystaline solids can be visualised as balls bound with springs - when compressed will repel out
materials respond to forces by deforming until reaction force is equal to the applied force - or at least until they break
Investigating the mechanical properties of a material
mechanical behaviour of the specimen will depend on its size and shape
as well as material properties
so needs to be normalising force and deflection
#
most important is the tensile test on a dumbbell shaped specimen
Stress and Strain Graphs
Stress
Force / Cross Sectional Area
Units (Nm-2)
Strain
Deflection / Original Legnth
No Units
yield strength (top line)
breaking strength (lower line)
young's modulus (Stiffness) = slope of the line
units = NM-2
Energy storage =area under the stress / strain curve
units = Jm-3
Cyclical Test
Not all energy used to deform materials is returned and the energy loss is best measured with a cyclic test
resilience = % energy returned during unloading
hysteresis = % energy lost during unloading
Types of Material
Top Left - perfectly elastic material
Top Right - perfectly plastic material
Bottom Left - elastic - plastic material
Bottom Right - viscoelastic material
Failure & Breaking
materials differ in their breaking properties they maybe tough or brittle
Brittle - left line
Tough - right line
