Fracture Analysis of All Composite Made Fuselage Shell Under Random Vibration Loading
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Date
2011-07
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Addis Ababa University
Abstract
Aircrafts and its airframes are subjected to various kinds of time dependent
loadings; ranging from flight loads to ground maneuvering loads. In this thesis, the
dynamic responses such as stress, strain or displacement near the crack tips on the all
composite made fuselage structure when landing on various types of take off and landing
pavement are assessed.
Finite element modeling and analysis for all composite made fuselages is done
using the layered shell element 4 node 63. A crack of significant parameter is generated
on the outer top part of the shell for initiation of fracture. One leg model of the landing
gear is used to drive the required mathematical formulations. The crack tips responses to
random excitation caused by road roughness are determined. The excitation includes
smooth, pastured and ploughed take off and landing strips.
In all curves as the crack length increases the displacement and stresses response
near the crack tips increases. We can see from the literature that the applied stress is
directly proportional to the square root of the half crack length. This result was verified
experimentally by Griffith for a wide range of crack length. This confirms both the
analytical and experimental results obtained by Griffith’s and other similar researchers.
From the curves we can observe that the shell responds relatively lowest stress
and displacement response to class A than Class G and Class H pavement. Class H has
the worst stress and strain response near crack tips and much affects and severs the thin
structure. More over, stress and strain response to circumferential crack orientation is
higher than the corresponding longitudinal one. This is probably due to the stress waves
are perpendicular to the orientation of a crack, and which maximizes the local stress.
Generally, the shell structure has higher strength to weight ratio and has higher
stress and strain carrying capability, trends should look for using shells as their primary
structure. Moreover, in the event of forced and emergency landing, the pilot has
recommended to land as much as possible on Class G pavement than Class H pavement
so that the applied stress is optimized.
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Keywords
Composite, Shell fuselage, Crack, Random vibration analysis, Power spectral density, Ground induced excitations, pavement roughness data.