6

Question 1

What does a notch do to stress?

Answer 1

• Increase average stress by reducing x-section
• Concentrates Stress (K_t)

Question 2

What is Nominal Stress?

Answer 2

The average stress in the net-section higher that the applied stress

Question 3

Stress Concentration factor, K_t=

Answer 3

σ_peak/σ_nominal

Question 4

For a circular hole, K_t=

Answer 4

3

Question 5

For an elliptical hole, K_t=

Answer 5

1+2(a/r)^½

Question 6

What is Fracture Mechanics?

Answer 6

The general analysis of the failure of structural materials with existing flaws. The main outcome is fracture toughness

Question 7

Why is there such a variation between theoretical and experimental bond strength?

Answer 7

Stress concentration at microscopic flaws. It is amplified at tips of microscratches, voids, notches, surface scratches, corners etc.

Question 8

What are Stress Raisers?

Answer 8

• Microcracks
• Voids
• Notches
• Surface Scratches
• Corners

Question 9

What does the magnitude of ampification of Stress depend on?

Answer 9

Microcrack orientation, geometry and dimensions

Question 10

Define Fracture toughness?

Answer 10

Critical value of stress intensity factor at a crack-tip necessary to produce catastrophic failure under simple uniaxial load

Question 11

Stress Intensity Factor, K_I=

Answer 11

fσ(πa)^½

a= crack length

K_I is a variable

Question 12

Fracture Toughness, K_IC=

Answer 12

fσ_max(πa_max)^½

This is a property and increases with refinement of grain size

Question 13

Design Criterion for K_IC

Answer 13

K_I <=K_IC

Question 14

Brittle materials have

Answer 14

Low K_IC

Question 15

K_IC decreases when

Answer 15

• Strain rate increases
• Temperature decreases

Question 16

K_IC increases when

Answer 16

Grain size decreases

Question 17

Cracks will suddenly grow if

Answer 17

Load is too high, leading to fracture in a brittle manner

Question 18

Ductile fracture occurs in a

Answer 18

Transgranular manner- Through the grains

Question 19

Brittle fracture occurs in a

Answer 19

Intergranular manner- along grain boundaries. Enhanced by impurities

Question 20

How does ductile fracture occur in a simple tensile test?

Answer 20

1. Nucleation, growth and coalescence of microvoids at necked region
2. The stress then causes seperation of grain boundaries
3. As local stress increases, microvoids grow and coalesce into larger cavities
4. Eventually metal-metal contact is too small to support load thus fracture

Question 21

Which is more notch-sensitive under tensile loading; Brittle or Ductile?

Answer 21

Brittle

Question 22

What is Fatigue?

Answer 22

The lowering of strength or failure of a material due to repetitive stress

Question 23

What are the 3 stages of fatigue?

Answer 23

1. A small crack nucleates at the surface and can include scratches, pits, sharp corners, inclusions, grain boundaries or dislocation concentrations
2. Crack gradually propagates as the load continues to cycle
3. Sudden fracture when remaining x-section is too small to support applied load

Question 24

For Fatigue to occurs at least part of the load must be

Answer 24

Tensile

Question 25

Fatigue is common in

Answer 25

Plastics and metals. Not common in ceramics due to low fracture toughness

Question 26

What is Striation?

Answer 26

The cyclic build up of crack growth from a surface intrusion. These are on a finer scale and show the position of crack tips after each cycle

Question 27

What is Granular portion of Fracture surface?

Answer 27

Rapid crack propagtion at the time of catastrophic failure

Question 28

How are the theoretical infinite stresses relieved at tip ends in practice ?

Answer 28

• Ductile materials- Blunted crack tips
• Some polymers- Elongated voids with bridging structure
• Brittle materials- regions of high density tiny cracks

Question 29

What is thermal fatigue?

Answer 29

Repeated heating and cooling can cause cyclic stresses due to differential thermal expansion and contraction

Question 30

Mean Stress, σ_mean=

Answer 30

(σmax+σ_min)/2

Question 31

Stress amplitude, σ_a=

Answer 31

(σ_max-σ_min)/2

Question 32

Stress range, σ_r=

Answer 32

σ_max-σ_min

Question 33

Stress ratio, R=

Answer 33

σ_min/σ_max

Question 34

Stress concentration decreases the

Answer 34

Fatigue limit

Question 35

Low cycle fatigue

Answer 35

High loads, plastic and elastic deformation

Question 36

High cycle fatigue

Answer 36

Low loads, elastic deformation

Question 37

Define fatigue limit

Answer 37

When the S-N curve becomes flat when stress amplitude decreases to a certain level

Question 38

What is fatigue strength?

Answer 38

For materials that don’t show fatigue limit, the stress level at which the failure will occur for a specified nuber of cycles

Question 39

Endurance Ratio=

Answer 39

Fatigue limit/Tensile Strength ≈ 0.25-0.5

Question 40

Factors affecting Fatigue Life?

Answer 40

• Magnitude of stress
• Qualities of surface
• Thermal fatigue
• Corrosion fatigue

Question 41

Ways to improve fatigue life

Answer 41

• Polishing
• Introducing compressive stress by “shot peening”
• Case hardening- Introduces compressive stress
• Optimize geometry- no internal corners etc.

Question 42

Stress intensity factor, delta K=

Answer 42

f(σ_max-σ_min)(πa)^½

Question 43

Selection criteria for fatigue?

Answer 43

• Initiation and growth of small cracks= 80-90% of total life. Remainder of life has fast groth to failure
• Experimental strength justification based on S-N fatigue (safe life)
• Experimental life 3-4 times required life
• Thickness of fatigue critical parts is greater than required for static strength

Question 44

Means to increase life?

Answer 44

• Damage tolerance approach
• Avoid stress concentrations
• Less fatigue senstive materials