THE DAMAGE TOLERANCE ANALYSIS

Running Head: DAMAGE TOLERANCE ANALYSIS
1
Part I: Fracture Mechanics Analysis of Engine Fan Blade
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DAMAGE TOLERANCE ANALYSIS 2
Project I: FRACTURE MECHANICS ANALYSIS
INTRODUCTION
The fan components of an aero-engine have seen many design evolutions aimed at reducing the
weight and improving the aerodynamic performance of the plane. Aero-engine fan blades and
discs undergo subjection to high- and low-cyclic fatigue loads around contact points between
adjacent discs and blades. The low-cyclic fatigue (LCF) damage results from the centrifugal
forces that act on the fan blades while airfoil vibrations cause the high-cyclic fatigue (HCF)
damage. In structural integrity assessment, both the load components, that is, the centrifugal
forces and the airfoil vibrations must be considered in combination (Hou, Wicks & Antoniou,
2012).
This paper focusses on the fracture mechanics analysis of a solid fan blade of a small turbofan
aero-engine. The material of the targeted fan blades is the Ti-6Al-4V (Ti 6/4). The centrifugal
force experienced by the fan blade is assumed to be uniformly distributed tensile stresses across
the ends of the panel. The fan blade is assumed to be a center-cracked panel (Kozakiewicz et al.,
2016).

DAMAGE TOLERANCE ANALYSIS 3
LITERATURE REVIEW
The service life of components of an aircraft is determined by the modes of fatigue and
degradation, such as fracture, fatigue, creep, yielding, wear, erosion, corrosion, etc. Fan blades
are crucial components of aero-engine and must be able to endure substantial thermal and
mechanical loading. When a problem arises with the fan blades, the whole aero-engines
performance is significantly affected and, consequently, the safety of the airplane. Broken fan
blades may be contained within the aero-engine casing and could cause puncture of the engine.
Failure and stress analysis of the fan blades of an aero-engine is thus a focus of numerous
research investigations (Amoo, 2013).
Common failures in aero-engine fan blades
The fan-blades may undergo mechanical damage, damage due to high-temperature exposure, and
creep failures. A more in-depth knowledge and understanding are gained on the damage
tolerance of these components through investigations of the crack initiation and propagation
mechanisms (Hou, Wicks & Antoniou, 2012).
High cyclic fatigue (HCF) failures often occur in the rotating parts of the aero-engines, such as
the fan blades. HCF failures result from fatigue loading, especially on materials with damages
sustained from other sources. The damage can be sustained from manufacturing defects or
developed during the service life of the aircraft. The low-cycle fatigue (LCF) has been identified
as the primary source of in-service damage that leads to HCF failures. Fretting and foreign object
damage (FOD) can also alter the HCF resistance in combination with LCF or individually (Zhao
& Zhang, 2010).

DAMAGE TOLERANCE ANALYSIS 4
Fatigue analysis of Aero-engine fan blades
The fan blades are attached to a disc that rotates as the shaft and the blades rotate. The blades are
thus subjected to both centrifugal forces and high vibrational and thermal stresses associated
with the high temperatures inside the engine (Farrokhabadi, Hosseini-Toudeshky &
Mohammadi, 2015).
The damage tolerance approach is often used to relate the remaining life of the engine
components, such as the fan blades based on the crack propagation rates. Vibratory stresses
from structural responses and unexpected drivers may exceed the capability of the material as
determined during sub-component tests of laboratory specimens. Gerber, Soderberg, and
Goodman's theories are some of the mean stress theories used to analyze fatigue failures. The
accidental introduction of damage to the material at the manufacturing level or during in-service
usage causes HCF failures due to fatigue loading on the materials. A threshold HCF resistance
below which no HCF failures will occur during the service operation is determined to account
for the high-frequency vibrational modes in the airfoil (Lin, 2014).