What are the stages of a tensile stress-strain curve?
uniform elastic elongation
uniform plastic elongation
necking
fracture
What are the reference conditions for a strain test?
room temperature
quasi static conditions: 10^-3-10^-4 s^-1
How is the true plastic strain calculated?
How is uniform plastic deformation described? How is this equation called?
Ludwig Hollomon equation:
K=strength coefficient
n=strain hardening coefficient
What are the two types of dislocations?
Edge and screw dislocations
What are the three most common crystal structures in metals?
body centered cubic
face centered cubic
hexagonal close package
Name one example for a metal of the Bcc, Fcc and Hdp structure.
bcc: alpha-Fe, beta-Ti
fcc:gamma-Fe, Cu, Al, Ni
Hcp:Mg, alpha-Ti
How is the length of a stacking fault calculated? Name the components.
G=shear modulus
b=burgers vector
SFE=stacking fault energy
What are the typical strain hardening coefficients for the most common crystal structures?
BCC: n typically lower than 0.3
FCC: n typically lower than 0.4
HCP: 0.05-0.15
in which kind of steels occurs Lüders dehnung?
low density of dislocations and specific (comparatively small) atoms
What carries the plastic deformation in materials with small stacking fault energy?
Twinning
What is the meaning of TRIP?
Transformation induced Plasticity
<=> transformation through strain and changing plastic properties
Describe the function of the yield stress over temperature.
Describe the influence of temperature on the stress/strain for BCC and FCC
BCC:
decreasing T: decreasing dislocation movement => faster reaching of saturation conditions and decreasing of n
FCC with high SFE:
way less influence of temperature
More occurence of twinning
Dynamic recovery at high T
FCC with low SFE:
TRIP effect at low T
How is the increase in strength for increase in strain rate described?
m=strain rate sensitivity factor
What is the Ludwig-Hollomon equation with included strain rate sensitivity?
What is the Portevin-Le Chatelier effect? (=dynamic strain aging (DSA))
A serrated stress strain diagramm for 0.15-0.4 Tm
What causes the Portevin-Le Chatelier effect?(=dynamic strain aging (DSA))
The higher diffusion speed of alloying atoms than the dislocation movement speed. This leads to the elements pinning down the dislocations and them needing to be broken up again.
How does the strain rate sensitivity factor behave during the Portevin-Le Chatelier effect?(=dynamic strain aging (DSA))
It is negative
How can a deforming operation be adiabatic?
Plastic deformation generates heat. if that heat is not given the chance to diffuse away from the place it is generated, due to a high strain rate for example, the process can be assumed to be adiabatic. This can be quantified by the diffusion distance d:
a= the thermal diffusivity
t= the deformation time
How can the temperature rise be described under adiabatic conditions?
beta= the fraction of plastic work dissipated into heat
rho=density
Cp=heat capacity
u=specific energy
epsilon=true plastic strain
sigma=mean stress
How and when are shear bands generated?
when plastic deformation localizes in a narrow region through
microstructural imperfactions+ low m(<0) and n values
in regions with forced shear
How are forced shear band generated?
Large shear deformations in narrow reagions by external loading. For example during machining operations.
What is dynamic recovery in plastic deformation?
Anihilation of opposing dislocations and rearanging of dislocations to small angle grain boundaries
healing of 0-dim defects
What eases the plastic flow at high temperatures?
easier dislocation motion
dynamic recovery
dynamic recristallizytion
For which metals applies dynamic recovery?
BCC, FCC with high stacking fault energy
For which metals is dynamic recovery less likely and what is the result?
FCC metals with low stacking fault energy, HCP
=>Dynamic recrystallization
How is dynamic recristallization described mathematically?
A, p, q=material constants
d_0=initial grain size
Z=Zener-Hollomonn Parameter
Q=activation energy
What is dynamic recrystallization favoured by?
large plastic deformation
high T
low deformation rate
low initial grain size
low content of alloying elements
What is understood under grain boundary sliding?
Deformation mechanism of Polycrystalline materials
Sliding along grain boundaries
thin liquid forms to help sliding
Elevated temperatures
Accommodates strain
Creep deformation
What is understood under Diffusion creep?
The diffusion of atoms in the direction of the stress. Applicable if stress too low for dislocation creep.
How is diffusion creep described mathematically?
What is the yielding criterion for three dimsensional bodies and uniaxial stress?
What is the effective strain after fracture?
under assumption that
What is the generalized Ludwik Hollomon equation?
What is understood under uniaxial strain?
=>
What is understood under plane strain?
What is understood under plane stress?
What is understood under pure shear?
Describe the ductility over the temperature for steels.
What is the main reason for ductile cracks?
When neighbouring voids around inclusions touch eachother
Describe ductile deformation mechanics based on temperature and strain.
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