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Computer Aided Manufacturing > Mechanical Behaviour of Metals > Flashcards

Mechanical Behaviour of Metals Flashcards

1
Q

Macroscopic structure of matter

A

atoms and molecules are arrangend in crystalline or non-crystalline structures.
Metals are Always cristallyne in the solid state, with external electrons free to move

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2
Q

malleability explained from structure

A

a load change the position of some nuclei, but external electrons remain shared, so that the structure deforms without breaking

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3
Q

electrical conductivity explained from structure

A

external electrons can move when a electric field is applied, carrying current

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4
Q

lustre explained from structure

A

external electrons absord and emit photons

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5
Q

basic crystal structures

A
  1. body centered cubic
  2. face centered cubic
  3. hexagonal close packed

different structures of the same element give different properties

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6
Q

types of imperfections

A

it is observed that metals resist less than what is predicted by theory.
This is explained by imperfections, that reduce the resistance of metals
1. point defects
- interstitial atom (smaller)
- substitutional atom (similar dimension)
- vacancy
2. linear defects
- edge dislocation
- screw dislocation
3. surface defects
- grain boundaries: a block of metal contains different structures with different orientations
4. volume defects

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7
Q

tensile test: description

A

a specimen is loaded in the axial direction, controlling its deformation speed to be low, and the force applied is measured

  • initial elongation
  • elongation continues
  • necking begins
  • fracture
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8
Q

definition of engineering stress and strain

A

engineering stress:
s = F/A0
A0 original area

engineering strain:
e = L-L0 / L0
L0 original length

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9
Q

definition of true stress and strain

A

true stress:
sigma = F/A
A local area

true strain
deps = dl / l
eps = int(L0,L) dl/l = ln(L/L0)

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10
Q

typical engineering stress-strain plot

A
  • elastic region
  • Yield point
  • plastic region
  • Ultimate Tensile Strenght UTS
  • necking
  • fracture
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11
Q

elastic region

A
  • linear relation between stress and strain: s = E * e

- E Young modulus of elasticity, a measure of the stiffness

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12
Q

yield point

A

an elastic recovery starting at Yield point gives a residual deformation of 0.2%

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13
Q

plastic region

A
  • after yield point, plastic region starts
  • elongation is easier and requires less force
  • an elastic recovery gives a permanent deformation
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14
Q

UTS

A
  • the point at which the force applied is maximum
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15
Q

necking

A
  • after UTS, necking starts

- at the center of the specimes, there is a reduction of area, so the force required decreses

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16
Q

true stress-strain plot

A
  • the curve is monotonically increasing, because locally the stress is Always increasing, since the section decreses
  • sigma = K * eps^n flow curve describes the graph
  • K strength coefficient, n strain hardening exponent
17
Q

strain hardening

A
  • the true stress increases as the deformation increses, it means that the material becomes stronger
  • a new test from the permanent deformed specimen gives higher yield point
18
Q

possible stress-strain relations

A
  1. perfectly elastic
    - fracture instead of yield point
    - described by E
    - brittle materials
  2. elastic and perfectly plastic
    - after Y, constant curve (no strain hardening)
    - metals above recrystallization temperature
  3. elastic and strain hardening
    - increasing line after Y
    - ductile metals
19
Q

ductility: definition and measure

A

ability of a material to plastically deform without fracture

measures:
total elongation: EL = Lf - L0 / L0

area reduction: AR = A0 - Af / A0

20
Q

toughness

A
  • amount of energy per unit volume dissipated before fracture
  • area under the true stress-strain graph
21
Q

mechanical properties from the engineering stress-strain graph

A
  • E - > tangent in the elastic region - > stiffness
  • tangent in Y - > malleability
  • UTS - > strength
  • area - > toughness
  • elongation - > ductility
22
Q

temperature effect on mechanical properties

A 
increasing temperature:
E decrease
Y decrease
beta decrease
UTS decrease
23
Q

recrystallization

A
  • at a certain temperature, most metals have a perfectly plastic behaviour (n=0) so no strain hardening occurs
  • this is due to the new crystal formation with grains free of strain
  • temperature when new grains are fromed in about 1 hour (usually between 1/2 and 2/3 the melting point)
  • this property can be exploited in manufacturing, to form metals easily (hot working)
24
Q

hardness: definitiion

A

resistance to permanent indentation

25
Q

brinell hardness test

A
  • used for metals and non metals of low to medium hardness
  • hard ball (10mm diameter) pressed into the surface of the specimen (F=29400 N) for 15 s
  • diameter of permanent indentation measured Di
  • HB = 0.102*F/S
    S surface of intentation, S = pi * Db * h
  • ideal test Di/Db = cos (136°/2) = 0.375
  • for a test to be valid, Di/Db in [0.25,0.5] range
26
Q

Vickers hardness test

A
  • used for metals and non-metals with medium to high hardness
  • pyramid shape indenter (136°) pressed (F=294 N) for 15 s
  • diagonal of the deformation d measured
  • HV = 0.102*F/S
27
Q

Rockwell hardness test

A
  • two steps: initial indentation with load F0
  • second indentation with added load F
  • elastic recovery h after removal of F
  • HR = N - h/S
    N and S constants
28
Q

hardness as a function of temperature

A
  • in general, hardness reduces with increasing temperature

- ceramic materials have low reduction of hardness

29
Q

impact test

A
  • notched specimen hit by a swinging pendulum
  • from the difference in the height of the pendulum, an estimation of the energy dissipated is obtained
  • brittle materials: initial elastic deformation, then fracture; low energy dissipated
  • tough materials: plastic deformation without complete fracture; high energy absorbed
30
Q

toughness and ductility as a function of temperature

A

many materials have a sharp change of these properties in a certain transition temperature
it is important not to use materials near their transition temperature

31
Q

fatigue test

A
  • materials can broke also from repeated loads: fatigue failure
  • fatigue test measures the number of cycles at a given amplidude at which a material breaks
  • under a certain value of load, some materials never brake: endurance limit
  • some material brakes also with small loads: fatigue strength defined as a function of number of cycles
  • fracture surface: first part planar, it increases dimension until the stress is no more tolerated by the small residual area, and a fracture happens

Computer Aided Manufacturing (15 decks)

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