Chapter 2: Metal Failure

Question 1

Causes of failure

Answer 1

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

Question 2

how will imperfections in materials & deficiencies in processing cause failure?

Answer 2

imperfections in materials: reduces strength and start cracks

deficiencies in processing: produces crack and loses ductility

Question 3

What is ductile fracture?

Answer 3

application of excessive tensile force to a metal that has the ability to plastic deform before fracture (aka overload failure)

Question 4

how to identify ductile fracture?

Answer 4

Visual: large plastic deformation
neck with cup and cone region
dull, rough and fibrous

non-visual: transgranular cracks
dimples

Question 5

causes of ductile fracture

Answer 5

1) material not strong enough
2) service conditions differed from anticipated ones (mistake)
3) abnormal loading (outside source)

Question 6

what is brittle fracture?

Answer 6

small amount of work absorbed, little deformation
sudden fracture

Question 7

identification of brittle fracture

Answer 7

visual: little to no plastic deformation
flat, shiny, crystalline
chevron markings

non-visual: trans/intergranular cracks
cleavages

Question 8

causes/conditions of brittle fracture

Answer 8

1) ambient temp. < transition temp.
2) presence of notch (stress concentration)
3) presence of tensile stress

Question 9

what is transition temperature?

Answer 9

it is the range of temperature over which the mode of fracture changes rapidly from ductile to brittle in a notched specimen

Question 10

factors affecting transition temperature

Answer 10

1) size & thickness of specimen
2) rate of loading
3) type of microstructure

Question 11

which metals have transition temp. and how it is determined?

Answer 11

BCC metals, determined by Charpy V notch test

Question 12

how to prevent brittle fracture?

Answer 12

removing the causes;
making:
ambient temp > transition temp
remove notch (stress concentrations)
remove tensile stress

Question 13

how can transition temperature in steels be lowered?

Answer 13

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)

Question 14

difference between ductile and brittle failure

Answer 14

ductile failure: above yield point
takes long time
cup and cone structure
fibrous appearance

brittle failure: below yield point
sudden
chevron pattern
crystalline appearance

Question 15

what is fatigue failure?

Answer 15

failure due to repeatedly applied stress

Question 16

how to test for fatigue?

Answer 16

using the Wohler machine, specimen in the form of cantilever forms the extension of a shaft, driven by a motor

Question 17

what is fatigue limit?

Answer 17

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

Question 18

stress-cycle curve difference between ferrous and non-ferrous metals

Answer 18

ferrous: curve tends to a straight line after about 10 million cycles which is the endurance limit

non-ferrous: curve all the way

Question 19

difference between ferrous and non-ferrous metals

Answer 19

ferrous: different steels
non-ferrous: Mg, Cu, Al

Question 20

what is endurance strength?

Answer 20

repeated cycles at which failure will not occur before a stated number of stress cycles

Question 21

identification of fatigue failure

Answer 21

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)

Question 22

how to reduce stress concentration?

Answer 22

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)

Question 23

describe the methods to introduce compressive stresses on surface.

Answer 23

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

Question 24

what is creep?

Answer 24

slow plastic deformation of metals under a constant stress

Question 25

when is creep important?

Answer 25

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)

Question 26

3 stages of creep description

Answer 26

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

Question 27

factors affecting creep

Answer 27

higher ambient temp., faster creep

strain-time curve becomes steeper

Question 28

methods to improve creep resistance

Answer 28

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

Question 29

how to prevent creep failure?

Answer 29

1) lower working temp./high Tm alloys
2) use coarse grains/ single crystal alloys
3) strengthen alloy (solid solution/precipitation hardening)

Question 30

Causes of metal failure

Answer 30

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)

Question 31

what is stress concentration and the effect of stress concentration on fatigue life

Answer 31

specimens have holes notches that increases stress levels, stress level surrounding the defects is raised above the average, fatigue life will be significantly reduced