Lecture 8- failure of ceramics and polymers

Question 1

whats special about failure in ceramics?

Answer 1

failure in ceramics is unpredictable; the ceramic could fail at a given stress one time and not fail another time

Question 2

What do we use to predict failure for ceramics?

Answer 2

the Weibell modulus of survival

Question 3

What about the structure of ceramics allows them to fail?

Answer 3

the many surface flaws and small cracks

Question 4

what happens to surface cracks in tension?

Answer 4

The worst crack propagates (gets worse) till failure.

Question 5

What happens to surface cracks in compression?

Answer 5

cracks extend in a stable manner until crush band collapses

Question 6

Why do we not measure the strength of ceramics by tensile testing? 3 points.

Answer 6

difficult to grip material without fracture

difficult to shape into required geometry

ceramics show little plastic deformation before failing

Question 7

in what type of loading do ceramics work?

Answer 7

only compression, not tension

Question 8

how do we measure the tensile strength of ceramics?

Answer 8

3 point bending tests

Question 9

Is 3- or 4-point bending better for ceramics? why ?

Answer 9

4 point bending because theres a constant bending moment between the central supports

Three-point bending puts high stress on the central load point, which can damage specimen

Question 10

what do stress-strain curves for ceramics look like?

Answer 10

straight line through the origin, no curve

Question 11

What is the ratio of compressive to tensile strength in ceramics?

Answer 11

compressive= 10x tensile

Question 12

What is the relationship between the porosity of a material and its strength or modulus of elasticity?

Answer 12

the more porous a material is, the weaker it is and the lower the young’s modulus

Question 13

Why aren’t low-porosity ceramics more popular?

Answer 13

they’re very expensive

Question 14

which polymers show plastic deformation?

Answer 14

semi-crystalline polymers, theyre ductile

Question 15

whats the relationship between the modulus and strength of a polymer and the temperature?

Answer 15

youngs modulus and strength decrease with temperature

Question 16

What happens to polymers at yield strength (semi-crystalline polymers)?

Answer 16

bonds between polymer chains are broken, allowing chains to slide and align

a stable neck forms, which extends in length as stress and strain increase

Question 17

What are the 3 effects of increasing temperature on a polymer?

Answer 17

decreases elastic modulus
reduces tensile strength
increases ductility

Question 18

why are polymers sensitive to temperature?

Answer 18

they have a low melting point

Question 19

whats strain rate? for polymers What is the effect of strain rate on strength?

Answer 19

Strain rate is how fast you deform the material

reduces tensile strength
increases ductility

Question 20

How do we strengthen polymers?

Answer 20

blending, drawing, and cross-linking

Question 21

define blending

Answer 21

mixing 2 or more polymers to combine desirable properties (like an alloy)

Question 22

define drawing

Answer 22

stretching the polymer to align chains, which results in increased strength

Question 23

define cross links

Answer 23

creating chemical bonds between polymer chains, increasing strength and stiffness