6. Failure of materials Flashcards
ductile tensile fracture surface
is rough
define toughness
the quantity of mechanical energy needed to break a given cross-section of a material
what is stable crack growth?
in ductile fracture where the crack propagates by being continually supplied by energy
stages of tensile fracture of a ductile material
- in the necking region, microvoids form around oxide inclusions and at grain boundaries
- these microvoids join up (coalesce) to form a large central void which propagates towards the surface
- Final fracture occurs along ~45 degree shear planes to give a typical ‘cup-and-cone’ fracture
Why is brittle fracture associated with pre-existing cracks or flaws(pores)?
local stresses at stress-concentrating cracks or flaws can exceed the engineering fracture stress of the material, causing it to fracture well below the yield point
What does the impact test measure?
toughness
Why is a notch involved in the impact test’s sample?
to concentrate the stress, making the component more susceptible to brittle failure even if the material is ductile TO SIMULATE AN EXTREME TEST
Effect of carbon content, manganese content and impurities respectively on the transition temperature of steel
higher carbon content and more impurities increase transition temperature. Increasing manganese content decreases transition temperature.
effect of grain size on transition temperature
decreasing grain size decreases transition temperature
How does faster loading affect transition temperature?
Faster loading increases transition temperature (as dislocations need time to move to produce slip)
length of edge crack and internal crack in terms of a
edge crack length = a; internal crack length = 2a
What is K1?
Stress Intensity Factor. Measure of the degree to which an external stress is amplified at a crack tip
What is K1C?
The critical value of K1 where the crack will propagate rapidly (at the speed of sound in the material) and result in fast, uncontrollable fracture.
What is K1C dependent on?
The material
What else is K1C known as?
Fracture toughness
Why are fracture mechanics important to an engineer? 3 reasons
- Engineer can design with imperfect materials (accounting for K1C) so real world engineering
- Material’s strength (UTC) and fracture toughness must be considered when designing load-bearing components
- Fracture may occur before the material’s yield strength is reached
Situations where you can get fatigue failure
when the component is exposed to a corrosive environment
when there is a oscillating or cyclic pattern of stress (fatigue loading)
four requirements for fatigue failure to occur
cyclic stress
tensile component (to open crack)
maximum cyclic stress is less than yield stress
a crack must either pre-exist or form and it must grow to critical size (for brittle fracture)
What is an fatigue/endurance limit and what metals does this apply to?
below this endurance limit fatigue will not occur, applies to ferrous metals (iron/steel etc)
define fatigue strength
the max stress that will allow a minimum number of cycles N to be achieved before failure
What are ‘beach’ marks?
periods of major crack advancement, can be seen by eye usually
what are fine fatigue striations?
microscopic advance of the crack - shows how much the crack grows with each cycle
What type of stress cycle produces a shiny or polished fracture surface
reverse stress component, the compression components remove the beach marks and fatigue striations, producing a polished part.
3 ways of improving fatigue properties
improving surface finish (polishing, grinding)
putting surface in residual compression (reducing effective tensile strength)
good design and manufacturing
what is shot peening
firing tiny pieces of metal at the surface, deforms it slightly and puts it into compressive.
When does a pure metal begin to creep, and when does a ceramic begin to creep?
pure metal: service temp > 0.3-0.4 of melting temp
ceramic: service temp > 0.4-0.5 of melting temp
TEMPERATURES IN KELVIN
four parts of a creep curve
- Instantaneous elastic strain
- Primary non-linear creep (small bend)
- Secondary/ steady state creep (most important)
- Tertiary creep (small bend) ends in creep failure
What is the failure time of a creep curve called?
time to rupture
What is dislocation climb?
At sufficiently high temperature (creep temp), atoms move via diffusion into vacancies which can then cause the dislocation to move up from a slip plane with an obstacle, effectively bypassing it
What is stress-induced migration of atoms and vacancies and how does this cause creep?
At high temperatures vacancies increase and atoms have enough energy to diffuse - atoms diffuse in direction towards the tensile stress ELONGATING THE SAMPLE. vacancies diffuse perpendicular to the tensile stress, REDUCING THE CROSS-SECTIONAL AREA
Where is diffusion the most rapid?
at grain boundaries, as these are regions of disorder and they have greater energy due to strained inter-atomic bonds
Describe grain boundary sliding
Rapid diffusion at grain boundaries leave trail of vacancies (as grain boundaries are not smooth like slip planes), These vacancies grow and combine to form voids or microscopic cracks leading to creep fracture.
How to make creep resistant material?
High melting point alloys (creep occurs at > 0.3-0.4 of melting point)
maximum obstacles to dislocation movement and dislocation climb
prefer alloys with covalent bonding (stronger bonds, harder for dislocation movement)
LARGE grain size so atoms have to diffuse further before reaching a grain boundary (where diffusion is faster) also less sliding
USE single crystal (above reasons)
precipitates at grain boundaries (makes sliding difficult)
