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Gravity
Terms in this set (28)
What are the eqns. for stress, strain, and elastic modulus?
σ = Force/Area
ε = Change in length/Original Length
E = σ/ε
What is the ultimate tensile strength? What is yield strength?
UTS is the maximum stress a material can withstand before breaking. It is at the top of the σ/ε curve. Yield strength is the value at which a material reaches its elastic limit and begins to deform plastically.
What is the difference between engineering and true stress/strain?
Engineering stress/strain assumes there is no change in cross sectional area. True stress/strain takes into account any change in the cross sectional area
What is poisson ratio?
tells you strain in a plate in y caused by a load in x, or vice versa
ν = - ε(transverse)/ε(axial)
Where axial is the direction in which load is taking place
What are the equations for biaxial stress?
Biaxial stress in loads in 2 directions (x and y)
Using poisson ratio (ν = - ε(transverse)/ε(axial)) to find relationship between εx and εy, and hence relationship between σx/E and σy/E.
εy = 1/E (σy - νσx)
εx = 1/E (σx - νσy)
What are the equations for shear stress, shear strain, and shear modulus?
τ = F/A, where A is CSA parallel to applied force F.
shear strain γ = deltax/L
τ = G*γ
G is shear modulus, G = E/2(1+ν)
What is the difference between axial and hoop stress in a cylindrical tank?
Axial - along length of tank - σz
Hoop - around radius of tank - σθ
What is the assumption that eqns for cylinders/spheres rely upon?
thin-walled, so r>>b
How do you derive eqns for hoop and axial stress?
take sections and Balance forces
e.g. external force = internal force
What is a stress tensor? Give a 2D one
like a matrix of all the stresses.
E.g. σ={σxx, σxy // σyx, σyy}
How do you transform stress tensors?
Use loads of cos and sin
What are principle stresses and stress invariants?
Stress invariants, in every body 3 planes with no shear stress, these are stress invariants. stresses normal to these planes are principle stresses.
What is the von mises criterion?
This finds equivalent uniaxial stress, allowing yield to be calculated.
What is the difference between ferritic and austenetic metals?
Ferritic --> Based on iron, magnetic, any alloying does not alter basic BCC structure
Austenitic --> Based on iron, but alloyed so FCC, not magnetic, more expensive, better at high temp/corrosion
What is stress concentration factor?
How much stress is concentrated compared to farfield
Kc = stress at tip of notch/stress far away
What is stress intensity factor? What is the equation for centre crack in infinite plate? What are the units?
Stress intensity factor predicts the stress near a crack from a remote load
Ki = Yσ
root(pi
*a)
Where y is non-dimensional const, a = crack length, σ = stress (Y =1 for centre crack infinite)
Units are MPa * root(m)
What is a materials fracture toughness? How do you determine failure with it?
The critical stress intensity factor for which a material breaks/crack propogates
calc stresses, compare with stress intensity factor, compare with fracture toughness (critical stress intensity factor) hence see how likely it is to fail
What is the R value for fatigue? What is the mean stress?
R = max stress/min stress
Mean stress = (max + min)/2
What is fatigue strength? What materials do or do not have this?
The same as the fatigue endurance limit.
Range of stress that can be applied that will not cause fatigue failure (can be endured inifinitely)
Steel has this, aliminium does not.
What are the components of the paris equation? da/DN = A * deltaK^m
m = constant, usually 2 to 4, a is crack length, N is number of cycles, Delta K is the range of the stress intensity factor (Kmax - Kmin), A is a constant.
What is creep rupture?
Failure due to creep
What is the difference between primary and secondary loads/stresses?
Primary - from applied forces e.g. pressure
Secondary - due to applied displacements or strains, e.g. temperature gradients, residual stresses
Secondary stresses can relax, primary stresses are fixed (due to external load) and cannot relax
What is creep ductility? What is ductility exhaustion? What does it depend on?
Creep strain at failure.
Ductility exhaustion is creep strain/creep ductility, i.e. how far away it is from failure.
depends on temp, strain rate, stress state (triaxiality etc), pre strain
At what temperature does creep become significant?
30% of melting temp (in Kelvin)
What is creep relaxation? In equation form?
What is stress redistribution?
Secondary stresses will relax due to increased creep strains. Total strain remains the same, so elastic strain + creep strain = overall strain. As creep strain increases, elastic strain relaxes.
Stress redistribution just moves the stresses around to make everything even.
What is creep cracking? Why is it more important?
the initiation and growth of cracks due to creep
Unlike creep rupture (only primary), both primary and secondary stresses can cause creep cracking.
3 types,
cracks under primary
cracks under secondary
cracks due to cyclic creep loading (creep-fatigue)
What is reheat cracking?
creep cracking where the driving stress is residual stress from welding
What is a rule of thumb for whether a weld is likely to initiate a reheat crack?
likely when Z/S > 10
S = spindler fraction
Z = elastic follow up factor
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