08 - Mechanical Tests B0

Question 1

what are the loading conditions

Answer 1

• Monotonically Increasing (Static)
• high strain rate (impact)
• repeated (fatigue)
• sustained (creep)

Question 1

what does a material undergo under repeated stresses

Answer 1

internal, progressive, permanent structural changes

Question 2

how and when to fatigue failures occur

Answer 2

• suddenly, without significant prior deformation
• important to detect fatigue cracks ahead of time

Question 3

where does fatigue failures usually occur in high cyclic loading

Answer 3

• axles
• drive shafts
• propeller shafts and blades
• crank shafts

Question 4

where does fatigue failure occur in structural elements

Answer 4

• aircraft wings and fuselage
• structures carrying high live loads (bridges)
• bracing, struts, ties

Question 5

what are the two basic stages involved in fatigue failure

Answer 5

• crack initiation
• crack propagation

Question 6

where do cracks typically initiate?

Answer 6

• at a free surface at a point of high stress concentration like: preexisting flaw, discontinuity (thread root, hole, geometric change)
• can start at any flaw, discontinuity or inhomogeneity

Question 7

does tensile loading increase cracks or decrease cracks?

Answer 7

it increases and causes fatigue damage

Question 8

does compressive load increase cracks or decrease them

Answer 8

it closes cracks so no damage from cyclic compression loads

Question 9

explain the cause of striations (beach marks)

Answer 9

• made over time and add on every cycle
• make cross section smaller and smaller
• when cross section is too small, it can no longer hold the load
• fails instantly

Question 10

what are the common types of fatigue loading

Answer 10

• alternating
• partly reversed
• pulsating (stress goes back to 0)
• random (seismic, wind, waves)

Question 11

what does the fatigue test determine? what is the end goal

Answer 11

determine the number of cycles that a sample material can safely endure for a given stress

Question 12

what are the two basic types of machines for fatigue tests

Answer 12

• constant load
• constant displacement

Question 13

what is the constant load machine

Answer 13

• load cycle remains constant
• strain gradually increases as specimen sustains damage

Question 14

what is the constant displacement machine

Answer 14

• displacement cycle remains constant
• stresses change as specimen sustains damage
• tests ran at very high speeds

Question 15

what are the drawbacks of using the constant moment rotating bending machine

Answer 15

• not suitable for nonzero mean stress
• specimen must be circular in cross- section

Question 16

how does the constant moment rotating bending machine work

Answer 16

• each revolution of the apparatus constitutes a full cycle of stress reversal which is repeated several thousand timers per minute
• specimens tested to failure using different loads
• Sress vs number cycles to failure plotted

Question 17

what type of alloy can be cycled for an indefinite # of times at stresses below their fatigue limit or endurance limit without failing

Answer 17

ferrous

Question 18

the reciprocating-bending machine is for what type of specimens

Answer 18

flat

Question 19

what is unique about a direct stress machine

Answer 19

direct tension or compression

Question 20

what causes the variability in fatigue testing results?

Answer 20

• for real materials, containing imperfections of various kinds, no two samples are identical
• virtually impossible to reproduce precisely the same test conditions over a large number of tests

Question 21

what are some factors affecting fatigue

Answer 21

• stressing conditions (type of stress, mean stress, stress history, frequency etc)
• material properties (type of material, surface conditions, grain size)
• environmental conditions (temperature, thermal fatigue, corrosion)

Question 22

what does the palmgren-miner hypothesis state

Answer 22

• sum #of stress cycles in streses conditions/ #stress cycles to failure in stress condition >= 1
• if < 1 then it does not fail

Question 23

what are the two major shortcomings of the palmgren-miner hypothesis

Answer 23

• assumes that the damage accumulating in each cycle of loading is independent of stress history - not true (if more cracks takes less energy to make next crack)
• assumes that there is no effect due to the order in which different stress levels are applied - not true
• still used to calculate preliminary estimates of fatigue life

Question 24

difference between perfect material and real material

Answer 24

- energy irreversibly lost - stable hysteresis loop

Question 25

how does stress concentration severity increase

Answer 25

- increasing flaw size - decreasing radius of curvature of flaw tip

Question 26

grain size effect on fatigue

Answer 26

finer grain size improves fatigue resistance

Question 27

surface finish effect on fatigue

Answer 27

- smooth surfaces less susceptible to crack initiation - grinding, plating, milling, etc, all affect crack initiation potential

Question 28

residual stresses in surface effect on fatigue

Answer 28

tensile: reduce fatigue life compressive: improve fatigue life

Question 29

temperature effect on fatigue

Answer 29

increased temp = decreased fatigue life - fatigue limits may disappear at high temperature

Question 30

what is thermal cycling and what are its consequences

Answer 30

thermal cycling can induce stresses that lead to thermal fatigue failure if the material is restrained from expanding/contracting

Question 31

corrosion effect on fatigue

Answer 31

- surface pitting provides crack initiation points - fatigue limits disappear in corrosive environments

Question 32

what is creep

Answer 32

- plastic deformation of a material which occurs as a function of time when that material is subjected to a constant stress or load

Question 33

what is the creep test

Answer 33

- high- temperature test because at temperatures below 40% of the absolute melting point (creep is negligible in most metals at low temp)

Question 34

when does concrete undergo creep

Answer 34

any temperature unless fully frozen

Question 35

how to account for creep

Answer 35

- leave space and clearing for deformation

Question 36

what is a creep curve

Answer 36

plot of the elongation of a tensile specimen versus time, at a given temp, under either constant load or constant true stress

Question 37

what are the four stages of elongation shown on a creep curve

Answer 37

- instantaneous elongation as the load is applied - primary or transient creep - secondary or steady-state creep - tertiary creep

Question 38

what increases the creep rate

Answer 38

- increasing stress or temperature