10-06-2013, 02:32 PM
FATIGUE ANALYSIS OF A PANEL CONSISTING OF WINDOW CUTOUT AND FRAMES IN THE FUSELAGE OF A TRANSPORT AIRFRAME
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PROJECT OVERVIEW:
Aircraft structure is the most obvious example where functional requirements demand light weight and, therefore, high operating stresses. An efficient structural component must have three primary attributes; namely, the ability to perform its intended function, adequate service life and the capability of being produced at reasonable cost. To ensure the safety of aircraft structures, the Air Force requires damage tolerance analysis.
This paper focuses its attention on designing a fail-safe fuselage structure. Two types of damage most frequently associated with the structural integrity of the fuselage are longitudinal cracks under high hoop stresses induced by cabin pressurization and circumferential cracks under stresses from vertical bending of the fuselage. Various analytical and empirical approaches have been used to study the damage tolerance capability of the fuselage structure.
In this paper, fatigue analysis of a panel consisting of window cutout and frames in the fuselage of a transport airframe is carried out through FEA method of analyzing.
Problem Definition:
To analyze the stress tensor and fatigue calculation of fuselage window cut-out using MSC software like MSC Patran which acts as an analyzer and MSC Nastran acts as a linear and non-linear solver.
Geometric Description:
Geometry includes a flat plate with a cutout at the
centre, 2 Z stiffeners, 2 tear plates and rivets along
the cutout.
Tear plates are placed in between the flat plate and
the stiffeners.
The flat plate, the stiffeners and the tear plates are
riveted.
The thickness of the base plate, stiffeners and tear
plate is 2mm and is of aluminium material.
The distance between the two inner arcs is 400mm. The inner arcs are of diameter 360mm
Outer arcs are drawn for meshing purpose. So it can be of any dimension. In this geometry, diameter of the outside arcs is 440mm.
The thickness of the cutout portion is 4mm.
Fuselage Window Cutout:
Window being a small portion of an aircraft
do not create a severe problem. Problem arises
when the pressure is increased within the
fuselage or when the pressurization is lost.
While designing an aircraft an optimum
pressure will be defined. When an aircraft is
ready for a takeoff, the pressure inside the cabin would be equal to the atmospheric pressure. As the altitude increases the atmospheric pressure decreases. So as to maintain the pressure inside the fuselage, pressurization is done within the cabin (fuselage).
The larger part of passenger and freighter aircraft is usually pressurized. The cabin altitude is usually changed quite slowly, beginning pressurizing long before 2500m, which is the normal cabin pressure altitude during cruise, is reached. When the cabin is being pressurized, the cabin gets expanded which results in circumferential stress (or) hoop stress. Continuous and fluctuating loads on the cutout results in fatigue. So window cutout is a major challenge during the lifetime of an aircraft.
Loads and Boundary Conditions
When the aircraft is pressurized the whole
fuselage structure expands radially. The
fuselage window cutout also expands along
the circumference of the fuselage. When the
expansion takes place, the cutout experiences a
stress which is known as circumferential stress (or) hoop stress. If a structure is defined dimensionally, for a known differential pressure, the hoop stress (or) the circumferential stress can be calculated.
Consider a fuselage structure of diameter 3000mm and of thickness 2mm. A differential pressure of 6psi acts along the inner walls of the fuselage. This differential pressure allows the structure to expand radially. This challenges the window cutout to deform. To calculate the stress acting on the cutout, we consider a portion of 1800×1000×2mm which includes a cutout region.
CONCLUSION
The stress analysis and fatigue calculation using the analysis software, MSC Nastran and Patran was successfully completed. A detailed iterative study was carried out based on courser and finer meshing.
The iteration carried out shows that, when the elements size is decreased to an extend, the accuracy of the analysis will be high. Later on the decrease of the element size will not show any changes in the results. Conclusion can be made that while meshing the element size should be considerably small in order to get more accurate value.
For a particular maximum stress the structure will last for infinite number of cycles, as the damage fraction is less than 1and the panel is safe for that particular number of cycles. The analysis carried out here concludes that the structure does not fail for the maximum stress obtained during the analysis of the panel and lasts for infinite number of cycles.