Chapter 7 : Mechanical Properties of Metals Flashcards
What is failure?
the inability for a material to
- perform intended tasks
- meet performance criteria
- perform safely and reliably
Fracture is?
separation of a solid under stress into two or more parts
The two types of fracture are?
Ductile and Brittle
Describe ductile fracture.
occurs after extensive plastic deformation
characterized by a slow crack propagation
cup and one shape, dull and fibrous
breaks into big pieces
List the steps in ductile fracture.
- specimen forms a neck, and cavities form within the neck region
- cavities coalesced into a crack in the center and propagate towards the surface perpendicular to the applied stress (microvoid)
- microvoid growth into crack
- close to the surface, crack changes angle to 45 degrees and forms a cup and cone shape
Describe brittle fracture.
little plastic deformation
occurs along crystalline planes (cleavage planes)
rapid crack propagation (speed of sound)
occurs a lot in HCP because of the lack of slip planes
transgranular (cracks propagate across the matrix of the grains
granular, shiny surface with flat faucets
breaks into several small pieces
List the steps in brittle fracture.
- crack propagates perpendicular to applied stress
- no plastic deformation - no warning!
- breaking of atomic bonds, cleavage, transgranular fracture (along grains)
Why would a material fail by a brittle fracture?
defects in the materials (corners, irregularities)
larger area = more defects
What is a stress riser?
the defect location
What is toughness?
the measure of the amount of energy a material can absorb before fracturing
What is DBT?
ductile to brittle transformation
related to temperature (energy absorbance)
marked by a large drop off in impact energy required to fracture the meterial
What can cause DBT
lower temperatures
high stress
fast loading rates
allowing agents (carbon increase = brittle increases)
grain size refinement (grain size decreases, DBTT decreases - smaller grains slow the crack movement)
What is the relationship with carbon and DBT?
low carbon = lower temperature transition and a narrower range of temperatures
more carbon, more brittle, less energy absorbed
What is more susceptible to DBT, FCC or BCC?
BCC
high strength alloys and FCC are not affected ( ~ 6 orders less to move a perpendicular than in BCC)
Where is the stress concentration the highest?
at the tip of the crack
What does a low Kic mean?
little plastic deformation, more brittle
What is fatigue?
failure due to repetitive or cyclic stresses
Describe fatigue failures.
the more fatigue on a material, the less stress it takes to fracture
What are basic structural changes that occur in a ductile metal in the fatigue process?
- crack initiation - early development of fatigue damage
- slip band crack growth (called stage 2)
- crack growth on planes of high tensile stress (stage 3)
- ultimate ductile failure
Extrusions and Intrusions are formed by?
rotation back and forth (fatigue)
What are 4 major factors that affect the fatigue strength of a metal?
- stress concentration - formed from notches, holes edges etc.
- surface roughness - rougher surface finish = more fatigue
- surface condition - most fatigue failures start at surface (carburization increases fatigue life)
- environment - corrosion etc
What is the relationship between crack length, fatigue, stress, and propagation?
- when crack length is small, fatigue crack growth rate is small
- increase of cyclic stress increases crack growth rate
What is creep?
a constant stress that produces a constant progressive plastic deformation over a period of time
What increases creep rate?
higher stresses
higher temperatures
What are the characteristics of the surface of a ductile fracture of a metal?
dull and fibrous surface with a cup and cone shape
what are the characteristics of the surface of a brittle fracture of a metal?
granular, shiny appearance that contain flat faucets that are created during cleavage
How does the carbon content of a plain-carbon steel affect the ductile-brittle transition temp range?
increase of carbon content = temperature and the range width also increase
What is fatigue test SN curve, and how are the data for the SN curve obtained?
plot of fatigue stress versus corresponding cycles including fracture. Data for stress is from repeated fluctuating bending, axial forces, torsional forces, or combined stresses. the cycles are counted and plotted on a logarithmic scale
What is metal creep?
a time dependent strain that produces progressive plastic deformation
What is the relationship between ideal and real materials strength?
engineering materials (real) << perfect (ideal) materials experimental strength anywhere from 10-100 times less
What are the basic steps in fracture?
- crack formation
2. crack propagation (mode of fracture is highly dependent on method of crack propagation)
What is the relationship between temperature and DBT with BCC
low temp = larger shear force needed to fracture
higher temp = easier to move (more energy involved) and is more ductile
What is the relationship between magnitude of amplification to distance from crack tip?
magnitude of stress amplification is inversely proportionate to the distance from the crack tip.
What is the difference in theoretical and experimental strengths?
theoretical - strength you need to pull apart bonds
experimental strength - much lower than theoretical, practical strength, includes impurities
What is Kic?
when a crack will propagate
What materials are immune to fatigue?
BCC (but ONLY after a certain period of time)
FCC applied stress will continually decrease with the number of cycles
The _____ 2a gets, the sharper the _____ becomes.
the BIGGER 2a gets, the sharper the POINT becomes
Where does fracture begin?
where stress concentration is highest
If there is a high fracture toughness, does the crack move?
No, high fracture toughness –> cracks not moving
What does Ki and Kic stand for
Ki = stress intensity factor Kic = fracture toughness
how do you calculate creep rate?
∆strain / ∆t
Is ductile fracture more desirable than brittle fracture?
Yes
Do BCC materials exhibit an endurance limit in fatigue while FCC materials do not?
Yes, BCC has an endurance limit,
FCC does not
What is one sign of ductile fracture?
A lot of microscopic plastic deformation
Is toughness usually higher in slow loading or impact loading?
Higher in slow loading
What does a tensile stress graph look like?
a straight steady positive slop than an inverted semi circle
Creep rate for a given material will increase or decrease if the temperature is increased?
creep rate will increase if the temperature is increased
What does a creep test graph look like?
similar to an x^3 function
Do BCC materials exhibit DBTT?
DO FCC materials exhibit DBTT?
BCC yes
FCC no
axis of impact graph?
energy absorbed vs temperature
axis of fatigue failure graph?
stress vs number of cycles (log10)
axis of creep curve graph?
strain vs time
