Mechanical Testing Flashcards
If a property of a material is to be claimed, it must be backed by what two things?
- a standard method
- statistical analysis (standard deviation)
What are the two main types of material behavior?
- deformation behavior
- fracture behavior
What are some measured values for deformation behavior?
- Young’s modulus
- Yield stress
- Poisson’s ratio
- Elongation
- Strength
What are some measured values for fracture behavior?
- Toughness
- K1c
- Izod
- Charpy
What kinds of test are all mechanical behavior values derived from?
- tension test
- compression tests
- bending test
Tension tests are the simplest but…
- has gripping/slipping issues with dog bone ends
- difficult to calculate accurate strains
Compression has no gripping problems, but…
- there is friction between sample and compression plates
- it is made for materials that tend to yield
What are some properties of bending?
- good for modulus
- variability in strain
- inhomogeniety - when strain is not uniformly applied (flexural modulus)
How many regions of viscoelastic behavior are there, and what are they?
5
- Glassy
- Glass transition
- Rubbery plateau
- Rubbery flow
- Flow
What are some properties seen in a material in the glassy region?
- Brittleness
- High moduli
- Shear yielding
What contributes to the high modulus (little motion) you see in the glassy region?
- bond stretching
- valence angle deformation (bending of bonds)
- local segmental rotation
What does shear yielding in the glassy regions tell you?
- plastic deformation starts to occur
- high yield stress = tough material
- Shear bands play a role in crack initiation
Explain the ductile behavior of a glassy thermoplastic.
- shear yielding shown by micro-shear bands
- shown in region of compression with low shear rate + higher temperature
Explain the brittle behavior of glassy thermoplastics.
- crazing occurs
- show in tension region with high shear rate and lower temperatures
- environmental stresses and solvents can cause brittle failure
What are the main properties seen in a material in the rubbery plateau region?
- Nearly constant modulus
- Exhibit rubber-elasticity
What is seen with linear polymers with increasing temperature in the rubbery plateau region?
- modulus slightly decreases
- chain entanglements, associated with elasticity, decrease
What are the properties seen in the rubbery flow?
- Modulus decreases rapidly in T
- Both rubber elasticity and flow interplay
- Example: silly putty
What are the properties seen in the liquid flow?
- lots of liquid flow
- rapid reptation of chains
- polymer liquids are Newtonian
What is drawing and the effects of it?
- aligns polymer chains by stretching
- increases E and TS but decreases ductility
What are the results of annealing after drawing?
- decreases chain alignment
- reverses effects of drawing
What is an elastomer?
- It can undergo very large deformations and return to its original shape once the applied force is removed
What are some properties of liquid crystal polymers and on example of one?
- excellent thermal stability
- high impact strengths
Example: Kevlar
What is seen with cross-linked polymers with increasing temperature in the rubbery plateau region?
- modulus slightly increases
- elasticity associated with cross-linking sites
