Mechanical and Other Testing - WIP Flashcards
Describe Dynamic Mechanical Thermal Analysis (DMTA)
Measures the energy absorbed when a specimen is deformed cyclically as a function of the temperature, and a plot of energy loss per cycle as a function of temperature shows a maximum at Tg ???
Describe Volume methods of testing
The changes in conformation that occur above Tg require more volume, so plotting a graph of specific volume or thermal expansion coefficient against temperature will give a value for Tg
The actual volume of the molecules stays the same through Tg, but the free volume (the volume through which they can move) increases
What does Mechanical testing show?
Shows how the material responds to stresses, can be applied in tensile testing, rheology or DMTA. Stress-stress curves allow important information such as a material’s elastic modulus and yields stress to be determined
Polymer stress-stress curves are produced by stretching a sample at a constant rate through the application of a tensile force.
What is Hooke’s Law?
The relationship between stress and strain is linear, and independent of time
Defines ideal elastic solids, but most materials are Hookean only at small strains (<1%)). Metals show elasticity for very small strains (<0.2%). In this region, extension is linear and recoverable.
Many engineering materials show Hookean behaviour but only a few biological materials approximate it
What is the Yield stress point?
When stresses up to the yield point are applied and the removed the material recovers along the same curve. When stress exceeds the yield stress the material exhibits plasticity (becomes ductile and flows under near constant stress). The material will eventually break
What are some deformation examples?
Tension is the most commonly used testing mode.
Other deformations include compression, torsion, 3-point bending and shearing
Describe a Hookean material
Tensile stress applied is proportional to the resultant strain.
Hookean materials show linear elasticity, where stress-strain curves on loading and unloading are identical (within range of Hookean elasticity)
Describe the elasticity of a Rubbery material
Show an S-shaped stress-strain curve, and are prone to elastic instability (inconsistent behaviour)
Describe the elasticity of a Tough Biomaterial
Show a J-shaped stress-strain curve:
Initial small increases in stress give large extensions, but in response to larger stress the material stiffens and is more difficult to extend.
Loading and unloading occurs along the same curve (recoverable/reversible) and there is no conformational changes
What is the stress-strain model?
What is a Viscoelastic Material?
Demonstrates both ideal solid and ideal liquid properties.
Non-Hookean elastic materials (strain is recoverable but not linear) have different stress-strain curves for loading and unloading. The area absorbed during a cycle is given by the area within the loop
Biological example of elasticity - Blood Vessels
Weakened arteries can show elastic instabilities such as aneurysms.
As tension increases or radius of the balloon decreases, the pressure increases, forming an S-shaped stress-strain curve. At some point there are different radii, leading to the formation of aneurysms, introducing instability as it tries to lower the pressure
Biological example of elasticity - Bone
Biological example of elasticity - Hair
Hair consists of keratin which have 2 main forms - alpha helices (in hair) and beta sheets.
As hair stretches the hydrogen bonds between the alpha-helices rupture, causing them to unravel into beta-sheets (this is a high energy absorbing process).
When stress is removed, the helices reform over time.
The energy is therefore unavailable for fracture, giving it a high toughness, which is important for nails, hooves and horns.
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