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C2
Materials Selection
66
Science
Undergraduate 3
06/03/2011

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Cards

Term
what is the difference between young's modulus and stiffness?
Definition
Young's modulus is a material property while stiffness involves geometry
Term
what is the difference between cost, value, and cost effectiveness?
Definition
cost: $$
value: cost in relation to the life of a material
cost effectiveness: extent to which savings can be made by downsizing
Term
what are the three different types of design approaches?
Definition
new/original
adaptive/developmental (change of scale from model or pilot)
variant
Term
what is a fundamental v. ranking property and provide examples
Definition
fundamental: can be measured directly (density)
ranking: must have other materials to compare it with (formabililty, machinability)
Term
what is a performance index?
Definition
a mathematical method of optimizing a combination of materials properties based on building constraints
Term
what is a free variable?
Definition
a material that is not fixed but is also not relevant to the material property of interest
Term
what is an expert system? What is the limiting factor
Definition
computerized materials selection method with ranking/weighting factors which utilize a stored database and interrogate the user to obtain constraining information. Limited by the skill of the programmer and the chosen weights for answers.
Term
What does 'slow crack growth' indicate about potential modes of failure?
Definition
fatigue, creep, stress corrosion cracking
Term
what does 'fast crack growth' indicate about the potential mode of failure?
Definition
brittle fracture, ductile tearing, plastic collapse
Term
what generally determines the life of a material
Definition
properties, design, dominant failure mode, defects
Term
what are the five types of load?
Definition
bending, torsion, buckling, internal pressure, point load
Term
what materials data tends to have low/high error in accuracy?
Definition
low error: density, modulus, thermal expansion, specific heat
high error: thermal conductivity, yield stress of polymers, hardness of ceramics
Term
what materials selection methods can be used to handle multiple constraints?
Definition
1.sequential merit indices. make a selection based on one MI then use the next based on this narrowed selection of materials
2.coupling equations to ultimately have a materials selection map relating two merit indices
Term
what are the steps in creating a merit index
Definition
1.consider what properties are given
2.decide what equation best models constraint (bending/fracture toughness/etc.)
3.use algebra to eliminate free variables
Term
how do you modify merit indices to include cost?
Definition
replace rho with C*rho where C is the cost of 1 Kg of the material of interest divided by the cost of 1 Kg of steel
Term
what are some of the problems associated with materials selection maps?
Definition
-need for independent knowledge to asses dominant properties
-structure sensitive data still not catered for
-must assume likely failure mechanism
Term
Discuss macroscopic and microscopic shape factors
Definition
Macroscopic shape factor: overall bulk shape of a section
φ[Type of loading,Failure mode]
-φ=stiffness of shaped beam/stiffness of circular solid beam of same cross sectional area=I/Io
-φ=1 for circular solid beam

Microscopic shape factor: structural anisotropy within a bulk section
ψ[Type of loadingFailure mode]
-Types-honeycomb, fiber composite, layered, etc.

For the overall structure, multiply shape factors: ψxφ
Term
[image]
Definition

beach marks

indicates a change in fracture mechanism or rate or propagation

Term
[image]
Definition

brittle fracture

looks like fairly straight line cracks in a flat plane (flatter than that of ductile). there are insufficient slip planes to allow for a ductile failure (consider ductile/brittle transition temperature, strain rate, stress concentration, crystal type, etc.)

Term
[image]
Definition

Chevron marks

crack starts at maximum stress then cleaves.  These marks indicate the origin of failure

Term
[image]
Definition

Corrosion

pitted surface with oxides present

Term
[image]
Definition

Creep

voiding and grain boundary sliding observed in stage 3 of creep

there are no microstructural changes in stage 1 or 2

Term
[image]
Definition

Ductile

cup and cone appearance because deformation occurs and then there is necking until edges are pinched off.  Microscopically, looks like a mesh of ridges as triaxial stress about inclusions cause characteristic shear lips

Term
[image]
Definition

Fatigue
75% of failure, characteristic straiations, with varying loads, can get beach marks (initiation>propagation>fails)

Term
[image]
Definition

Intergranular

results in segregation at grain boundary or the formation of different phases at the grain boundary (ie S makes steel brittle so Mn is added to prevent this)

Term
[image]
Definition

River Lines
occurs in inclusion free steels.  When a crack crosses a gb, parallel cracks form with steps between them then they recombine to form single crack

 

Term
[image]
Definition

Stress corrosion

 

 

unexpected failure of a usual ductile material subjected to a tensile stress in a corrosive environment of ten related to residual stresses (stresses that remain after initial stress is applied)

Term

what do the following observations indicate:

color change,

Definition
Term
how can you tell the direction of propagation?
Definition
chevron marks, river lines, shear dimples
Term
what about polymers influences its fracture
Definition
type of bonding, extent of cross linking, chain packing, extent of crystallization, average crystallite size, chain length
Term
what kind of failure modes are exhibited in polymeric materials
Definition

brittle and ductile failure

 

Phenomena similar to metals but by different mechanisms: ductile-brittle transition, toughness, fatigue, stress corrosion

Term
what makes ceramic/glass failure different from metals
Definition
the fact that dislocations are immobile in ceramics
Term
ductile failure in ceramic/glassy materials
Definition
occurs at high temperatures where slip systems become thermally activated
Term
brittle failure in ceramics
Definition
the most likely mode of failure because dislocations are immobile
often the result of service damage, design deficiency, defects during fabrication (pores/inclusions)
Term
fracture initiation sites in ceramics
Definition
-Mirror: crack accelerates from initiation site on one plane
-Mist: as velocity increases, intersects inclusion and shifts in direction of principal stress
-Hackle: larger ridges than mist and transforms to crack branching
Term
[image]
Definition

Wallner lines simultaneous propagation of crack front and elastic shock wave (you will not find this in a metal)

-principal stress momentarily deviated siturbaed

-the curvature of theses lines are approximate shape of crack front (assuming wave intersects with entire fracture front

-direction of crack propagation and indication of stress distribution (distance of each part of line from crack origin

-if a material is bent, the side in tension will be on the concave side and compression-convex

Term
[image]
Definition

Craze

-this is a dilational form of failure and principally occurs in amorphous polymers

-these absorb energy and improves toughness

-there is a whitening effect due to scattering of light

-precedes crack, therefore plastic zone is ahead of crack in metals

-~10microns

Term
[image]
Definition
mist hackle
Term
[image]
Definition

shear bands

-this is a non-dilational form of polymer failure

-most polymers in compression exhibit this behavior

-microscopic localized deformation along shear planes at 45* to applied compressive load

Strain magnitude ~2-3 locally

Term
[image]
Definition
Wallner lines
Term
what is the problem with accelerated tests?
Definition
they can be misleading when extrapolating due to a change or changes in the failure mechanism
Term
what are the tree different examples of discussed methods of using expert systems
Definition
1-go/no go (accept or reject options based on a set of criteria)
2-degree of merit (have a numerical ranking system for criterion)
3-weighting factors but the limitation is is very subjective and depends strongly on weighting factors/rankings
Term
what are limitations to expert systems examples discussed in class
Definition
1-real data are not shown
2-VERY subjective
3-weighting factors are somewhat arbitrary
Term
what is the value of materials selection charts
Definition
they allow for visual comparisons of properties and combinations of properties
materials on teh same line have the same normalized property
allows fast evaluation of suitable materials including relative merits
Term
what are the problems with materials selection charts
Definition
need for independent knowledge to assess dominant properties for a given design and application
structure sensitive data still not catered for
have to assume likely failure mechanism from simple tests or assessments
Term
what are some microscopic shape factor structures?
Definition
honeycombe
fiber composite
concentric cylindrical
layered
Term
what is the significance of using shape in materials design
Definition
may optimize a design by choosing sections wiht higher values of second moment of area, which can then help to reduce weight
must be aware that this may introduce a different failure mode (ie buckling v. plastic yielding)
Term
describe the ductile brittle transition temperature
Definition
S shape graph where brittle has low amounts of energy absorbed and ductile has high amounts of absorbed energy. the graph has an S shape.
Term
describe the steps in creep
Definition
1-crack intitation
2-steady state creep
3-voiding and grain boundary sliding
Term
how can you predict the direction of propagation in ductile materials
Definition
the shear lip dimples are facing the direction of propagation

If dimples looked like
C C C C C
C C C C C propagation was to the left
C C C C C
Term
why do riverlines exhibit the features they do
Definition
-when the crack crosses a boundary, especially tilt, many small paralleel cracks can form weith steps between them
-these continue to grow and run into each other to form a single crack with larger step
-this can show the direction of flow
-this is a clear sign of brittle failure
Term
what methods are needed to detect cracks in a material?
what is the typical size of a crack
Definition
ultrasonics
cracks are less than 100microns therefore hard to see optically
Term
what material features can change the direction of crack propagation
Definition
-machining marks
-change in section thickness
-hole or internal porosity
-internal defect, impurity
Term
describe the propagation direction of fracture when there is a single impact site
Definition

[image]

usually initiates at a single point then proceeds to branch (bifurcation) as vcrack>1/2vspeed of sound in material

 

 

Term
what is bifurcation
Definition

the branching of cracks

[image]

Term

Fracture initiation site

How does teh site appearance change as vcrack increases?

 

Just list the 3 and describe what they look like

Definition

1-mirror 2-mist 3-heckle

[image][image][image]

Term
Discuss "mirror"
Definition
crack accelerates from initaiton site.  Initiatially proceeds on one planeand the size of mirror is usally material independent, related to fracture toughness, and is affected by defects
Term
what are some potential sites for crack initiatoin
Definition

surface defect

inclusion

stress raiser

Term
what is an approximate speed of a crack
Definition

speed of sound

300m/s

Term
discuss "mist"
Definition

as crack velocity increases, may intersect inclusion or shift in direction of principal stress

slight deviation form original plane

small radial ridges (hard to see in crystalline ceramics)

Term
discuss "heckle"
Definition

larger ridges than mist and trasforms to crack branching

if abrupt change in stress field, points in new direction of crack movement

Term
name some other modes of failure in polymer materials (other than dilational/non-dilational)
Definition

necking

voiding

fatigue

Term
name some dilational failure modes
Definition

crazing

voids

micro cracks

Term
name a non dilational deformation mode
Definition
shear bands
Term
what effects ductile failure in polymers
Definition
  • mechanical overload
  • effects of liquids
  • particulate fillers
Term
what effects brittle failure in polymers
Definition
  1. choice of polymer
  2. design (stress concentration, poor joining)
  3. processing (defects, non-uniform, welds)
  4. service factors (degradation, corrosion, etc.)

 

here you often see mirror, mist, heckle

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