Chapter 2: Metal Failure Flashcards
Causes of failure
1) Poor Design - stress raisers (sharp corners, holes, keyways)
2) Material Selection - types of stress, rate of wear
3) Imperfections in materials - surface defects, internal flaws
4) deficiencies in processing - high residual stress caused by treatments
5) Misalignment
6) Improper service conditions - speed, loading temperature, environment
7) inadequate maintenance
how will imperfections in materials & deficiencies in processing cause failure?
imperfections in materials: reduces strength and start cracks
deficiencies in processing: produces crack and loses ductility
What is ductile fracture?
application of excessive tensile force to a metal that has the ability to plastic deform before fracture (aka overload failure)
how to identify ductile fracture?
Visual: large plastic deformation
neck with cup and cone region
dull, rough and fibrous
non-visual: transgranular cracks
dimples
causes of ductile fracture
1) material not strong enough
2) service conditions differed from anticipated ones (mistake)
3) abnormal loading (outside source)
what is brittle fracture?
small amount of work absorbed, little deformation
sudden fracture
identification of brittle fracture
visual: little to no plastic deformation
flat, shiny, crystalline
chevron markings
non-visual: trans/intergranular cracks
cleavages
causes/conditions of brittle fracture
1) ambient temp. < transition temp.
2) presence of notch (stress concentration)
3) presence of tensile stress
what is transition temperature?
it is the range of temperature over which the mode of fracture changes rapidly from ductile to brittle in a notched specimen
factors affecting transition temperature
1) size & thickness of specimen
2) rate of loading
3) type of microstructure
which metals have transition temp. and how it is determined?
BCC metals, determined by Charpy V notch test
how to prevent brittle fracture?
removing the causes;
making:
ambient temp > transition temp
remove notch (stress concentrations)
remove tensile stress
how can transition temperature in steels be lowered?
a) decrease in C content to below 0.15%C
b) decrease in rate of loading
c) decrease in depth notch/ increase radius notch
d) increase nickel content to 2 - 5%
e) reduce grain size (adding grain refine elements, Al, Nb)
difference between ductile and brittle failure
ductile failure: above yield point
takes long time
cup and cone structure
fibrous appearance
brittle failure: below yield point
sudden
chevron pattern
crystalline appearance
what is fatigue failure?
failure due to repeatedly applied stress
how to test for fatigue?
using the Wohler machine, specimen in the form of cantilever forms the extension of a shaft, driven by a motor
what is fatigue limit?
aka endurance limit, is the limiting stress below which a metal will withstand an indefinitely large number of cycles of stress without failure by fatigue fracture
stress-cycle curve difference between ferrous and non-ferrous metals
ferrous: curve tends to a straight line after about 10 million cycles which is the endurance limit
non-ferrous: curve all the way
difference between ferrous and non-ferrous metals
ferrous: different steels
non-ferrous: Mg, Cu, Al
what is endurance strength?
repeated cycles at which failure will not occur before a stated number of stress cycles
identification of fatigue failure
little deformation
start to end:
1) crack initiation
2) beach marks (short marks means closer to initiation site) (fatigue zone, crack propagation, smooth)
3) final fracture zone (can be brittle or ductile)
how to reduce stress concentration?
improving:
1) design (use of radius, chamfers, fillets)
2) processing (eliminate voids in castings, improve surface finish)
3) surface conditions (introduce compressive stress e.g, shot peening, cold rolling, case hardening)
describe the methods to introduce compressive stresses on surface.
1) shot-peening - stream of steel shots in made to impinge on the surface
2) cold rolling - metal is compressed and squeezed by rollers
3) case hardening - nitrogen or carbon are allowed to diffuse into the surface at elevated temperatures to produce a hard layer that has nitride or carbide phases in the material
what is creep?
slow plastic deformation of metals under a constant stress
when is creep important?
is important when:
1) operation temp. > 1/3 melting point in Kelvin
2) steam and chemical plants (450 - 550°C)
3) gas turbine at high temp. (800 - 900°C)
4) furnace parts ( > 1000°C)
3 stages of creep description
EP - rapid extension with decreasing rate due to strain hardening/ work hardening
PS - steady rate as work hardening is balanced by thermal softening
SX - strain rate accelerated, formation of voids and structural changes leading to rupture
factors affecting creep
higher ambient temp., faster creep
strain-time curve becomes steeper
methods to improve creep resistance
1) use of high melting point metals (creep occurs above 1/3 Tm in Kelvin, higher the Tm, higher creep temp.)
2) solid solution strengthening - having at least 2 different atoms (1 parent + 1 alloying atom), presence of alloy atoms disturbs parent atoms thus higher strength and creep resistance
3) precipitation/dispersion hardening - introduction of fine dispersed precipitates interferes parent matrix leading to higher strength and creep resistance
how to prevent creep failure?
1) lower working temp./high Tm alloys
2) use coarse grains/ single crystal alloys
3) strengthen alloy (solid solution/precipitation hardening)
Causes of metal failure
1) poor design (stress raisers)
2) material selection (wrong material)
3) imperfection in materials (surface/internal)
4) deficiencies in processing
5) improper service conditions (speed/load/temp)
6) not enough maintenance (wear and tear)
what is stress concentration and the effect of stress concentration on fatigue life
specimens have holes notches that increases stress levels, stress level surrounding the defects is raised above the average, fatigue life will be significantly reduced
