Mechanical Behaviour of Metals Flashcards
Macroscopic structure of matter
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
malleability explained from structure
a load change the position of some nuclei, but external electrons remain shared, so that the structure deforms without breaking
electrical conductivity explained from structure
external electrons can move when a electric field is applied, carrying current
lustre explained from structure
external electrons absord and emit photons
basic crystal structures
- body centered cubic
- face centered cubic
- hexagonal close packed
different structures of the same element give different properties
types of imperfections
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
tensile test: description
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
definition of engineering stress and strain
engineering stress:
s = F/A0
A0 original area
engineering strain:
e = L-L0 / L0
L0 original length
definition of true stress and strain
true stress:
sigma = F/A
A local area
true strain
deps = dl / l
eps = int(L0,L) dl/l = ln(L/L0)
typical engineering stress-strain plot
- elastic region
- Yield point
- plastic region
- Ultimate Tensile Strenght UTS
- necking
- fracture
elastic region
- linear relation between stress and strain: s = E * e
- E Young modulus of elasticity, a measure of the stiffness
yield point
an elastic recovery starting at Yield point gives a residual deformation of 0.2%
plastic region
- after yield point, plastic region starts
- elongation is easier and requires less force
- an elastic recovery gives a permanent deformation
UTS
- the point at which the force applied is maximum
necking
- after UTS, necking starts
- at the center of the specimes, there is a reduction of area, so the force required decreses
true stress-strain plot
- 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
strain hardening
- 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
possible stress-strain relations
- perfectly elastic
- fracture instead of yield point
- described by E
- brittle materials - elastic and perfectly plastic
- after Y, constant curve (no strain hardening)
- metals above recrystallization temperature - elastic and strain hardening
- increasing line after Y
- ductile metals
ductility: definition and measure
ability of a material to plastically deform without fracture
measures:
total elongation: EL = Lf - L0 / L0
area reduction: AR = A0 - Af / A0
toughness
- amount of energy per unit volume dissipated before fracture
- area under the true stress-strain graph
mechanical properties from the engineering stress-strain graph
- E - > tangent in the elastic region - > stiffness
- tangent in Y - > malleability
- UTS - > strength
- area - > toughness
- elongation - > ductility
temperature effect on mechanical properties
increasing temperature: E decrease Y decrease beta decrease UTS decrease
recrystallization
- 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)
hardness: definitiion
resistance to permanent indentation
brinell hardness test
- 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
Vickers hardness test
- 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
Rockwell hardness test
- 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
hardness as a function of temperature
- in general, hardness reduces with increasing temperature
- ceramic materials have low reduction of hardness
impact test
- 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
toughness and ductility as a function of temperature
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
fatigue test
- 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
