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Flashcards in Mechanical 2 Deck (72)
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1

Strength of paper

about 60MPa

2

suffixes like mega, milli, pico, etc

learn off

3

why easier to rip paper with crack

stress is concentrated at tip of crack, material fails there, allowing crack to grow

4

relationship between fracture stress and crack length

σf ∝ 1/√a

5

fracture toughness in tough vs brittle materials

-tough: value of constant high, good at tolerating cracks
-brittle: low, even small cracks weaken them

6

Fracture Toughness equation

Kc = Qσf√(πa)

Q = a constant
σf = fracture stress
a = crack length

7

Values of Q

taking a crack in a flat sheet,
if
a < W and a < H, then Q = 1.0

other values shown on graph on lecture slides 4

8

Total length of crack in a flat sheet

Length of crack = 2a

9

Brittle fracture

a material containing a crack will break at a stress σf

10

Ductile fracture

Failure by yielding

11

Sizes of cracks

-Those having large cracks will fail by brittle fracture
-Those with small cracks (a < a*) will fail by ductile fracture

12

Cracks and strength of material

cracks only affect strength if they are larger than a* (see diagram on lecture slides 4)

smaller cracks are harmless

13

Unit of fracture toughness

MPa√m

14

what a* means

Materials can have cracks up to this length without any loss of strength

15

Static properties of a material

Stiffness
Strength
Toughness

16

Long-Term Failure

Creep
Fatigue
Wear

17

Creep

Plastic strain which takes time

-Sometimes if you apply stress and hold it constant, strain will gradually increase over time
-can happen for stresses above and below σy

18

stages of creep test

primary, secondary, tertiart

19

Time to failure formula

t(subscript f) = C/σᵐ

20

what time to failure depends on

stress
temperature

21

Fatigue

A stress which was not sufficient to cause failure when applied once, can, if repeated enough times, eventually cause failure

22

Cyclic Loading

-tests carried out with given stress range △σ and mean stress σₛₜᵣₑₛₛ
-number of cycles to failure, N(subscript f) counted for each specimen tested

23

Stress-Life curve

-straight line except at high cycles, where line may become horizontal at "fatigue limit"

24

Wear

-Happens when two surfaces rub together

-you need a compressive force F and a shear movement d
-material removed from one or both surfaces

25

Wear testing

-Volume of material lost △V found by measuring/weighing test piece

26

Wear formula

△V = kFd

k = constant, tells you how easily material will wear away
d = shear movement
F = compressive force

27

four classes of materials

Metals
Polymers
Ceramics
Composites

28

Alloys

made by combining elements

29

Metals

strong and tough, but heavy and expensive

30

Polymers

-Light and cheap, can be strong
-fracture toughness poor but have high impact resistance