Mechanical and Other Testing Flashcards
Describe Dynamic Mechanical Thermal Analysis (DMTA)
Subjects the polymer to an oscillating force while it is heated/cooled. The energy absorbed when the specimen is deformed is measured as a function of the temperature.
A plot of energy loss per cycle as a function of temperature shows a maximum at Tg. The Tg varies with heat.
DMTA is designed to apply a controlled stress/strain to the sample, to measure the 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 then 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 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
Bone is considered a responsive material, formation and resorption of bone occur continuously (bones can be continuously reshaped). Stress of 25-40 MPa is enough to keep the correct levels of bone. Under stress for prolonged periods of time can cause bone wastage, and sudden high levels of stress can lead to increase in bone mass
Bone can remodel according to individual conditions so shows great variation compared to engineered materials
Bones can be considered to consist primarily of collagen fibers and an inorganic matrix, so can be analyzed as a fiber composite. Composites are materials that are composed of 2 or more different components, commonly used in engineering and industry where the combination of the 2 materials have properties that are superior to those of the individual components.
Young’s modulus - The Young’s modulus of aligned fiber composites can be calculated using the rule of mixtures and the inverse rule of mixtures for loading parallel and perpendicular to the fibers respectively
The model predicts that the composite will be stiffer in the axial direction than transverse, so cortical bone will be stiffer in the direction parallel to the osteons (the long axis of the 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.
