Goal: To understand some simple stress-strain relationships

Rheology

Relationship between stress and strain

Why did various types of putty deform differently?

– Was stress constant in these experiments?

– Was temperature constant?

– What other variables exist?

Rheology: The study of relationship between stress and strain in deforming materials

– The term rheology is also used as a noun for a material property. For instance: The rheology of glacial ice.

– If two materials have different rheologies, we say they are rheologically (adverb) different

Elastic rheologies

• Example — rubber band

• Exhibit recoverable strain

• Elastic deformation is strain-rate independent

Elastic behavior described by Hook’s Law

σd = E x e

E = Young’s modulus

e = elongation

Can draw strain/time and stress/strain curves for this

Viscous rheologies

• Viscous materials deform by flowing

• Examples: Honey, motor oil, water, and Lava. Silly Putty is an approximately viscous material.

• Exhibit permanent, nonrecoverable strain.

• Strain-rate dependent: Deform Silly Putty as an example

Viscous flow is described by

ė = σd/η

η = material’s viscosity

Can draw strain/time and stress/strain-rate curves for this

Real rocks....

Combination of elastic/frictional-brittle and nonlinear-viscous behavior

• The transition is dependent on both temperature (T) and strain rate (ė)

• Can use silly putty as an example the effects of both T and ė