Finite Element Analysis of a Fuselage Crack

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  • Topic: Finite element method, Fracture mechanics, Elasticity
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13/09/2009

Analysis Of A Fuselage Crack

Anoop Retheesh

Fracture Mechanics and Fatigue

CONTENTS

Title Page Contents Abstract List of Figures List of Tables

i ii iii iv iv

1. Analysis of a Fuselage Crack
1.1 Introduction 1.2 State of Stress in the absence of the Crack 1.3 Geometrical Stress Intensity Factor at the Crack Tip 1.4 Fracture Analysis using Finite Element Methods 1.4.1 Finite Element Model of the Fuselage Crack 1.4.2 The Solution 1.4.3 Grid Independence Study 1.5 Variation in Stress Intensity Factor with Crack Length 1.5.1 Conclusion

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1 2 3 5 5 8 9 10 12

2. References

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Analysis of a Fuselage Crack

ii

Fracture Mechanics and Fatigue

ABSTRACT
Fracture & Fatigue are the most common engineering concerns that limit the useful life of mechanical components. In fact, it was estimated that the occurrence or prevention of fatigue failures costs the US economy about 3% of its gross national product (R P Reed, J H Smith, B W Christ; 1983). The term ‘fatigue’ represents the permanent structural changes occurring in a material subjected to fluctuating stresses that builds up cracks in it and leads to its complete fracture after a sufficient number of fluctuations (ASTM E-1150; 1987). Fracture mechanics, meanwhile deals with the microscopic aspects of fracture and the failure of metals due to fracture. During the past 50 years, the subject has evolved a lot and it helped in understanding and predicting not only fracture failure but also crack growth processes such as fatigue. Fracture mechanics, combined with the conventional fatigue design modals now became an integral part of mechanical engineering design. In this report, an analysis is carried out on the cracks developed on an aircraft fuselage skin using Linear Elastic Fracture Mechanics (LEFM) assumptions. The cracks, emanating from each side of the circular rivet holes in the fuselage is assumed to be in a state of stress equivalent to a wide thin plate subjected to uniform stress (σ). A finite element analysis is a carried out for the same configuration to obtain the stress intensity factor. The analysis is then repeated for different lengths of cracks and it is compared with the values obtained from theoretical calculations.

Analysis of a Fuselage Crack

iii

Fracture Mechanics and Fatigue

LIST OF FIGURES

Figure 1.1 Typical Fuselage Structure of an Aircraft Figure 1.2 Cracks in the Fuselage Figure 1.3 State of Stress without crack Figure 1.4 Correction Factor f(a/r) for a double crack at a circular hole Figure 1.5 Simplified Model of Crack Figure 1.6 Mesh near the Crack Tip Region Figure 1.7 Loads and Boundary Conditions for the Fuselage Crack Model Figure 1.8 Stress Intensity near the Crack Tip Figure 1.9 Stress Intensity Factor Vs the Crack Length

1 2 2 3 5 7 8 9 11

LIST OF TABLES

Table 1.1 Variation of KI with respect to the size of the Singular Element Table 1.2 Stress Intensity Vs the Crack Length

10 11

Analysis of a Fuselage Crack

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ANALYSIS OF A FUSELAGE CRACK
1.1 Introduction
An aircraft fuselage assembly is the main structure or body of the aircraft to which all other components (such as wings and landing gears) are attached. It provides the space for power plant, cargo, controls, accessories, passengers and other requirements depending on the purpose (i.e., civil, military or transport). It consists of a thin shell stiffened by longitudinal axial elements called stringers and longerons; and supported by traverse frames or rings (bulkheads) along its length. The stringers and longerons are usually riveted to the fuselage skin which is made up of Aluminum alloys. The figure 1.1 shows the typical riveted fuselage structure of an Airbus A 380 aircraft.

Figure 1.1 Typical Fuselage Structure of an Aircraft (Courtesy: Airbus Corporation) Due to their criticality, the aircraft fuselages are periodically inspected using NonDestructive Testing (NDT) methods for any potential flaws....
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