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
|
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
|
Definition
beach marks
indicates a change in fracture mechanism or rate or propagation |
|
|
Term
|
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
|
Definition
Chevron marks
crack starts at maximum stress then cleaves. These marks indicate the origin of failure |
|
|
Term
|
Definition
Corrosion
pitted surface with oxides present |
|
|
Term
|
Definition
Creep
voiding and grain boundary sliding observed in stage 3 of creep
there are no microstructural changes in stage 1 or 2 |
|
|
Term
|
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
|
Definition
Fatigue
75% of failure, characteristic straiations, with varying loads, can get beach marks (initiation>propagation>fails) |
|
|
Term
|
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
|
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
|
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
|
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
|
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
|
Definition
|
|
Term
|
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
|
Definition
|
|
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
|
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
|
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
|
|
Term
|
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
|
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
|
|
Term
| name some dilational failure modes |
|
Definition
crazing
voids
micro cracks |
|
|
Term
| name a non dilational deformation mode |
|
Definition
|
|
Term
| what effects ductile failure in polymers |
|
Definition
- mechanical overload
- effects of liquids
- particulate fillers
|
|
|
Term
| what effects brittle failure in polymers |
|
Definition
- choice of polymer
- design (stress concentration, poor joining)
- processing (defects, non-uniform, welds)
- service factors (degradation, corrosion, etc.)
here you often see mirror, mist, heckle |
|
|
