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Aerostructures Final

TermDefinition
T/F: Twist only comes from internal torque. True
T/F: If Vz passes through shear center, then the resulting shear has no twist. True
Sigma_0 Farfield normal stress/remote stress
Kt Stress Concentration
T/F: Energy released by the system =/ strain energy stored False. They are the same.
G Strain Energy Release Rate
Gc Critical Strain Energy Release Rate Griffith Criterion Fracture toughness of a material. How brittle the material is Material Property.
T/F: G the critical strain energy release rate is a material property. True
T/F: If G < Gc the crack would grow. False. Greater than or equal to.
W Elastic strain energy
Pcrit Load required to grow the crack.
Mode I Opening Mode
Mode 2 Shear Mode
Mode 3 Scissor Mode
Fracture mechanics for plane stress At the crack-tip the Poisson effect allows for stress relief in the z-direction
Fracture mechanics for plane strain At the crack-tip the surrounding material constrains the Poisson effect and stresses exist in the z-direction.
T/F: When w>>a then Y ~ 1. True.
Fatigue Failure The failure of a mechanical component due to repeated loads.
High cycle fatigue Stresses are generally due to repeated loads.
Low cycle fatigue Stresses can be in the plastic (non-linear) material range.
T/F: Low cycle fatigue is more common for an aircraft. False. High cycle fatigue.
Aircraft cycle examples Wings flap and go up while in the air and fall on the ground. The fuselage has cabin pressure that can cause high or low cycle fatigue.
T/F: Fully reversed loading, i.e. alternating from tension to compression is particularly damaging. True.
Completely reversed loading. At the same sigma_max, failure occurs in fewer cycles. For the same number of cycles to failure the sigma_max must be lower
N or N_f Fatigue life.
R Stress ratio. min/max.
Endurance limit 1/2 ultimate stress which is also the fatigue limit. This limit is lower for fully reversed loading. R = -1. This applies to most steel alloys.
T/F: For stress ratio of R = 0, once max stress is less than or equal to 1/2 ultimate stress, failure will not occur no matter how many cycles are applied. True.
T/F: For aluminum alloys there is no endurance limit. For practical purposes the stress amplitude @ 10^8 cycles is considered the fatigue life. True.
T/F: Aluminum will fail at high stress amplitudes given enough cycles. Therefore parts that are exposed to 100s of millions of cycles are generally not made from aluminum. False. Aluminum will fail at low stress amplitudes.
S_a Fully reversed fatigue strength at a desired number of cycles.
S_e Endurance limit. For infinite life set we set S_a = S_e.
What happens when S_a = 0? The sample fails due ultimate stress.
Safety factors? You divide by F.S.
n_i No. of cycle for block n.
S_i Fully reversed alternating stress for block n.
N_i
Fatigue Crack Growth phases 1) Crack initiation 2) Stable crack growth 3) Un-stable crack growth
a Crack length. Usually given as 2a.
T/F: Crack growth is a function of stress intensity factor K_I. More specifically it is a function of the range of stress intensity factor during a cycle. True
T/F: Long columns usually fail from bending buckling and short ones fail from torsion buckling. True
Where does buckling occur on an aircraft? Buckling can happen on compression members loaded off-axis, (eccentric loading), on webs and flanges of structural elements, and on plates and shells.
T/F: We use steel because it is less prone to buckling. False. We use aluminum because steel is more prone to buckling.
Load control Means you increase the load and monitor displacement. Its equivalent to adding weight to a structure.
Displacement control Means you increase the displacement and monitor loads. This is typically how we test materials for E, yield stress, ult stress, yield strain, and max strain.
T/F: A column under displacement control can buckle in a non-catastrophic manner. A column under load control will buckle in a catastrophic manner. True
Created by: ncam0720
 

 



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