Hello again everyone. Isn’t this fun? Isn’t this just spectacularly magnificent? Nope. Memorizing all of this is boring, but we have to do it to get to the fun stuff. Trust me when I say that it’s a lot easier for you to read all this than for me to type all this.
Anyways.
We are going to go over some important material properties. They are:
- viscosity
- viscoelasticity
- creep
- stress relaxation
- brittleness
- ductility
- resilience
- hardness
- toughness
Viscosity:
the resistance of a fluid to deformation under shear stress
A simpler way of understanding viscosity is that the higher the viscosity, the “thicker” it is, and the less easily it will flow.
The above equation relates the shear stress to the shear strain wile taking into account a viscosity n. When n < 1, the fluid is a pseudoplastic and we observe shear thinning. When n > 1, the fluid is dilatent and we observe shear thickening. If n=1 the fluid is referred to as a Newtonian fluid. An example of a Newtonian fluid is water.
Looking at the equation, we see that there is no direct proportionality between the stress or shear and the viscosity. Rather, viscosity is directly proportional to the shear rate.
For a pseudoplastic, I mentioned that we observe shear thinning. The phrase itself describes a situation where the fluid thins out with an increasing shear force. As the shear rate increases, the viscosity decreases, because n<1 and thus the fluid appears “thinner” and easier to flow through a volume.
The opposite is true for dilatent fluids. With shear thickening, we can see that as we increase the shear rate, the viscosity becomes exponentially higher. This ultimately increases the entire right side of the equation and causes the need for greater increases in shear stress to deform the fluid and get it to flow.
The higher the viscosity, the more resistant a fluid is to flow, and vice versa.
Viscoelasticity
the property of a material to demonstrate both viscous and elastic behavior while undergoing deformation
The figure above contrasts an elastic response to a viscous response to deformation. The viscous response differs in that the shear stress will be applied, and then once removed the permanent deformation continues to occur. What happens is that the shear stress will induce flow. So even after the stress is removed, flow continues.
The elastic response is reversible. And thus, once the stress is removed, the strain reverts back to its original state as well.
Creep
time dependent permanent deformation to relieve stress under a constant load
Creep is the technical term to describe the physical phenomenon of getting “stretched out”. For example, imagine a 5lb weight is being held by some sort of bar. Initially, the extra force of the weight will pull down the bar and create an elastic response. In this situation, the load changed from 0-5lbs. But after the bar has time to elastically adjust, there is still a deformation due to the constant 5lb load. This deformation which is essentially “stretching” the bar out, is termed creep.
Stress Relaxation
the time decay of stress after a sudden deformation due to strain.
Whereas creep described the aftermath of strain on a material, stress relaxation describes the aftermath of stress (well that’s an easy one!). Note: stress relaxation refers specifically to viscoelastic materials.
If a material were elastic and followed Hooke’s Law then under a constant strain, the stress would stay constant as well. But in viscoelastic materials, if a material is under a constant strain, the force required to maintain the deformation decreases with time and thus the stress decreases with time.
Brittleness
Brittle materials experience little or no plastic deformation. They fracture quickly.
Ductility
a measure of the degree of plastic deformation upon fracture
Resilience
capacity to absorb and release energy during elastic deformation
The above shows us how to calculate the modulus of resilience. Sigma is the stress and epsilon is the strain. Essentially it is the area under the stress strain curve during elastic deformation. Recall that
Toughness
capacity to absorb and release energy prior to fracture
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