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CIVE 202 construction materials > 08 - Mechanical Tests B0 > Flashcards

08 - Mechanical Tests B0 Flashcards

1
Q

what are the loading conditions

A
  • Monotonically Increasing (Static)
  • high strain rate (impact)
  • repeated (fatigue)
  • sustained (creep)
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1
Q

what does a material undergo under repeated stresses

A

internal, progressive, permanent structural changes

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2
Q

how and when to fatigue failures occur

A
  • suddenly, without significant prior deformation
  • important to detect fatigue cracks ahead of time
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3
Q

where does fatigue failures usually occur in high cyclic loading

A
  • axles
  • drive shafts
  • propeller shafts and blades
  • crank shafts
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4
Q

where does fatigue failure occur in structural elements

A
  • aircraft wings and fuselage
  • structures carrying high live loads (bridges)
  • bracing, struts, ties
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5
Q

what are the two basic stages involved in fatigue failure

A
  • crack initiation
  • crack propagation
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6
Q

where do cracks typically initiate?

A
  • 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
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7
Q

does tensile loading increase cracks or decrease cracks?

A

it increases and causes fatigue damage

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8
Q

does compressive load increase cracks or decrease them

A

it closes cracks so no damage from cyclic compression loads

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9
Q

explain the cause of striations (beach marks)

A
  • 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
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10
Q

what are the common types of fatigue loading

A
  • alternating
  • partly reversed
  • pulsating (stress goes back to 0)
  • random (seismic, wind, waves)
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11
Q

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

A

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

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12
Q

what are the two basic types of machines for fatigue tests

A
  • constant load
  • constant displacement
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13
Q

what is the constant load machine

A
  • load cycle remains constant
  • strain gradually increases as specimen sustains damage
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14
Q

what is the constant displacement machine

A
  • displacement cycle remains constant
  • stresses change as specimen sustains damage
  • tests ran at very high speeds
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15
Q

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

A
  • not suitable for nonzero mean stress
  • specimen must be circular in cross- section
16
Q

how does the constant moment rotating bending machine work

A
  • 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
17
Q

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

A

ferrous

18
Q

the reciprocating-bending machine is for what type of specimens

A

flat

19
Q

what is unique about a direct stress machine

A

direct tension or compression

20
Q

what causes the variability in fatigue testing results?

A
  • 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
21
Q

what are some factors affecting fatigue

A
  • 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)
22
Q

what does the palmgren-miner hypothesis state

A
  • sum #of stress cycles in streses conditions/ #stress cycles to failure in stress condition >= 1
  • if < 1 then it does not fail
23
Q

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

A
  • 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
24
Q

difference between perfect material and real material

A
  • energy irreversibly lost
  • stable hysteresis loop
25
Q

how does stress concentration severity increase

A
  • increasing flaw size
  • decreasing radius of curvature of flaw tip
26
Q

grain size effect on fatigue

A

finer grain size improves fatigue resistance

27
Q

surface finish effect on fatigue

A
  • smooth surfaces less susceptible to crack initiation
  • grinding, plating, milling, etc, all affect crack initiation potential
28
Q

residual stresses in surface effect on fatigue

A

tensile: reduce fatigue life
compressive: improve fatigue life

29
Q

temperature effect on fatigue

A

increased temp = decreased fatigue life

  • fatigue limits may disappear at high temperature
30
Q

what is thermal cycling and what are its consequences

A

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

31
Q

corrosion effect on fatigue

A
  • surface pitting provides crack initiation points
  • fatigue limits disappear in corrosive environments
32
Q

what is creep

A
  • plastic deformation of a material which occurs as a function of time when that material is subjected to a constant stress or load
33
Q

what is the creep test

A
  • high- temperature test because at temperatures below 40% of the absolute melting point (creep is negligible in most metals at low temp)
34
Q

when does concrete undergo creep

A

any temperature unless fully frozen

35
Q

how to account for creep

A
  • leave space and clearing for deformation
36
Q

what is a creep curve

A

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

37
Q

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

A
  • instantaneous elongation as the load is applied
  • primary or transient creep
  • secondary or steady-state creep
  • tertiary creep
38
Q

what increases the creep rate

A
  • increasing stress or temperature

CIVE 202 construction materials (10 decks)

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