08 - Mechanical Tests B0 Flashcards
what are the loading conditions
- Monotonically Increasing (Static)
- high strain rate (impact)
- repeated (fatigue)
- sustained (creep)
what does a material undergo under repeated stresses
internal, progressive, permanent structural changes
how and when to fatigue failures occur
- suddenly, without significant prior deformation
- important to detect fatigue cracks ahead of time
where does fatigue failures usually occur in high cyclic loading
- axles
- drive shafts
- propeller shafts and blades
- crank shafts
where does fatigue failure occur in structural elements
- aircraft wings and fuselage
- structures carrying high live loads (bridges)
- bracing, struts, ties
what are the two basic stages involved in fatigue failure
- crack initiation
- crack propagation
where do cracks typically initiate?
- 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
does tensile loading increase cracks or decrease cracks?
it increases and causes fatigue damage
does compressive load increase cracks or decrease them
it closes cracks so no damage from cyclic compression loads
explain the cause of striations (beach marks)
- 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
what are the common types of fatigue loading
- alternating
- partly reversed
- pulsating (stress goes back to 0)
- random (seismic, wind, waves)
what does the fatigue test determine? what is the end goal
determine the number of cycles that a sample material can safely endure for a given stress
what are the two basic types of machines for fatigue tests
- constant load
- constant displacement
what is the constant load machine
- load cycle remains constant
- strain gradually increases as specimen sustains damage
what is the constant displacement machine
- displacement cycle remains constant
- stresses change as specimen sustains damage
- tests ran at very high speeds
what are the drawbacks of using the constant moment rotating bending machine
- not suitable for nonzero mean stress
- specimen must be circular in cross- section
how does the constant moment rotating bending machine work
- 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
what type of alloy can be cycled for an indefinite # of times at stresses below their fatigue limit or endurance limit without failing
ferrous
the reciprocating-bending machine is for what type of specimens
flat
what is unique about a direct stress machine
direct tension or compression
what causes the variability in fatigue testing results?
- 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
what are some factors affecting fatigue
- 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)
what does the palmgren-miner hypothesis state
- sum #of stress cycles in streses conditions/ #stress cycles to failure in stress condition >= 1
- if < 1 then it does not fail
what are the two major shortcomings of the palmgren-miner hypothesis
- 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
difference between perfect material and real material
- energy irreversibly lost
- stable hysteresis loop
how does stress concentration severity increase
- increasing flaw size
- decreasing radius of curvature of flaw tip
grain size effect on fatigue
finer grain size improves fatigue resistance
surface finish effect on fatigue
- smooth surfaces less susceptible to crack initiation
- grinding, plating, milling, etc, all affect crack initiation potential
residual stresses in surface effect on fatigue
tensile: reduce fatigue life
compressive: improve fatigue life
temperature effect on fatigue
increased temp = decreased fatigue life
- fatigue limits may disappear at high temperature
what is thermal cycling and what are its consequences
thermal cycling can induce stresses that lead to thermal fatigue failure if the material is restrained from expanding/contracting
corrosion effect on fatigue
- surface pitting provides crack initiation points
- fatigue limits disappear in corrosive environments
what is creep
- plastic deformation of a material which occurs as a function of time when that material is subjected to a constant stress or load
what is the creep test
- high- temperature test because at temperatures below 40% of the absolute melting point (creep is negligible in most metals at low temp)
when does concrete undergo creep
any temperature unless fully frozen
how to account for creep
- leave space and clearing for deformation
what is a creep curve
plot of the elongation of a tensile specimen versus time, at a given temp, under either constant load or constant true stress
what are the four stages of elongation shown on a creep curve
- instantaneous elongation as the load is applied
- primary or transient creep
- secondary or steady-state creep
- tertiary creep
what increases the creep rate
- increasing stress or temperature
