Fatigue Test

Question 1

define fatigue

Answer 1

it is a progress, localized permanent structural damage that occurs when a material is subjected to cyclic or fluctuating loading

maximum stress values are less than the ultimate tensile stress limit and may be below the yield stress limit of a material…. 90% materials fail from fatigue

Question 2

fracture

Answer 2

is a seperation or fragmentation of a solid body into two or more parts under actions of stress

Question 3

fracture can occur in how many ways?

Answer 3

• slow application of external loads (tension)
• rapid application of external loads (impact)
• cyclic or repeated loading (fatigue)
• time-dependent deformation under constant load (creep)

Question 4

fatigue cracks are generally _________

Answer 4

initiated flaws on a free surface… once initiated cracks grow at accelerated rate

Question 5

when crack become unstable it can cause

Answer 5

sudden catastrophic failure

Question 6

fatigue failure can be divide into the following stages:

Answer 6

1. cyclic plastic deformation
2. nucleation
3. short crack or small crack phase
4. crack propagaion or growth rates
5. final instability or failure

Question 7

the following factors are necessary to cause fatigue fracture:

Answer 7

1. applied stress of sufficent magnitude
2. large variation or fluctuation in applied stress
3. sufficiently large number of cycles of applied stress

Question 8

fatigue is a _____ process

Answer 8

stochastic (random)

Question 9

Name factors that affect fatigue

Answer 9

• type of loading
• surface roughness, scratches, dents
• type of material
• grain size- fine grained parts have longer fatigue life
• environmental conditions
• residual stress- heat and deformation
• direction of loading
• temperature
• geomentry

Question 10

direction of loading is important in case of

Answer 10

non isotropic material

Question 11

define stress concentrator

Answer 11

is a location in an object where stress is concentrated.

Question 12

infinite lifetime concept

Answer 12

design to keep stress below threshold of fatigue limit

Question 13

finite lifetime concept

Answer 13

design for a fixed life after which the user is instructed to replace the part with a new one

Question 14

damage tolerant design

Answer 14

replace part once crack proceeds…“retirement for cause”

Question 15

safe-life design based on the infinite-life criterion reflects the

Answer 15

classic approach to fatigue

Question 16

it is generally categorized as a

Answer 16

high cycle fatigue

Question 17

strain-life criteria is primarily intended to address the

Answer 17

low cycle fatigue area

Question 18

types of fatigue testing

Answer 18

tension-compression
tension-tension
irregular or random
sheet and plate bending