Chapter 7.1 Aerospace Structure Design Flashcards
3 safety guidelines philosophies
- safe life
- fail safe
- damage tolerance
safe life design
structure is designed to have “infinite” life or to be removed from service after specific design life
used for safety critical and/or difficult to inspect and/or replace components
Wöhler curve (S-N curve) relates the magnitude of cyclic stress to the logarithm of the number of cycles to failure
used most often
fail safe design
component is assumed to fail during service life safely, without leading to a catastrophic event, hence redundant load paths are designed
rarely used nowadays
– complex structure designs might occur
– component might be lighter than in safe life design, but there’re weight penalties for redundant load paths
– maintainance program must be determined increasing the cost
damage tolerance design
structure contains damages and works with them
cracks are allowed, but will not grow in an uncontrollable way until the next inspection (good understanding of crack propagation in metallic structures)
damaged composite possesses a specific residual strength if the loads don’t exceed certain strain level
not a good strategy for composites
failure mechanisms
- material strength
- local and global stability
- fatigue and damage tolerance
- joints
reserve factor
design allowable / applied design load
von Mises (maximum distortion energy)
ductile materials begin to yield when the maximum shear strain energy per unit volume equals to the shear strain energy at the yield point in the uniaxial tension test
if the 3 principal stresses are equal then they are hydrostatic stresses and they don’t cause yielding in ductile materials
limit loading
loads that an aircraft might see in life
might cause plasticity
equals yield strength when no plasticity is allowed
ultimate loading
limit loading x 1.5
bifurcation point
load level in structure where it (theoretically) abruptly looses stability and collapses
practically, due to imperfections, the structure failure occurs gradually and is accompanied by significant deformation
column buckling
not material failure
the load can be theoretically increased over the limit, but the structure will be unstable
column slenderness
informs whether the area of the column is big/small compared to its span
small slenderness (thick column) -> big critical stress -> for such columns we use Euler-Johnson
B-Value
90% of the samples show a higher value of the property
A-Value: 99% of the samples show a higher value (used for single load paths)
Euler-Johnson buckling
empirically based equation for calculating the critical buckling stress of a column covering the material strength failure
Ramberg-Osgood
formula for describing non-linear stress-strain curve
epsilon = epsilon_elastic + epsilon_elastic
critical buckling stresses are computed with Euler, but the elastic modulus is obtained from Ramberg-Osgood
depending on the Stress Office - either Euler-Johnson or Euler-Ramberg-Osgood is used
