Ceramics And Polymers Flashcards
Why don’t we use a a tensile test for ceramics (3)
They’re brittle
Difficult to prepare required geometry
Difficult to grip without fracturing
Ceramics show no…. before fracture
Plastic deformation
Fractography for impact/point loading
Crack lines 180 degrees around origin X
Fractography for bending
180 degree across origin then splits in 2 with 45 degree between
V
I
Fractography for torsion
180 degree across origin then splits in 2 with 15 degree between
Fractography for internal pressure
180 degree then splits then splits 90 degree between
Characteristic fracture behaviour of glass (4regions)
Origin
Smooth mirror region
Mist
Hackle
How do ceramics fail in tension
Worst flaw propagates to failure
Crack normal to force
How do ceramics fail in compression
Crack extend in a stable manner
Failure when crush band collapses
How to measure tensile strength in ceramics
3 point bend test
Top in compression
Bottom in tension
How much stronger are ceramics in compression than tension
X10
Porosity….. fracture strength and elastic modulus of ceramics
Decreases
At what strain does the stress strain curve of a polymer become non-linear (yield)
1%
The behaviour of polymers beyond the yield point depends on the temp relative to
The glass transition temperature
Drawing…. and increases…..
Aligns polymer chains
Increases strength and stiffness
Affects of temperature increase on polymers (4)
Strength decreases
Elastic modulus decreases
Reduces tensile strength
Increases ductility
Why are polymers more sensitive to changes in temperature
Room temperature is closer to their melting point
Strain rate=
How fast you deform the material
…. strain rate has the same effect as increasing temperature on the ……
Decreasing
Strength and ductility
4 ways to strengthen polymers
Bending
Drawing
Cross-linking
Reinforcing with particles, fibres, fabrics
