Aaron Siddens and Javid Bayandor

Virginia Tech, Blacksburg, Virginia

Mark Celestina

Glenn Research Center, Cleveland, Ohio

Detailed Post-Soft Impact Progressive Damage

Assessment for Hybrid Structure Jet Engines

NASA/TM—2014-218397

December 2014
NASA STI Program . . . in Pro¿ le

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scienti¿ c and Technical Information (STI) 

program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Of¿ cer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI program provides access 

to the NASA Aeronautics and Space Database and 

its public interface, the NASA Technical Reports 

Server, thus providing one of the largest collections 

of aeronautical and space science STI in the world. 

Results are published in both non-NASA channels 

and by NASA in the NASA STI Report Series, which 

includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major signi¿ cant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of signi¿ cant 

scienti¿ c and technical data and information 

deemed to be of continuing reference value. 

NASA counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scienti¿ c 

and technical ¿ ndings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies that 

contain minimal annotation. Does not contain 

extensive analysis.

• CONTRACTOR REPORT. Scienti¿ c and 

technical ¿ ndings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scienti¿ c and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scienti¿ c, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scienti¿ c and 

technical material pertinent to NASA’s mission.

Specialized services also include organizing 

and publishing research results, distributing 

specialized research announcements and feeds, 

providing information desk and personal search 

support, and enabling data exchange services.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question to help@sti.nasa.gov

• Phone the NASA STI Information Desk at

 757-864-9658

• Write to:

NASA STI Information Desk

Mail Stop 148

NASA Langley Research Center

Hampton, VA 23681-2199
Aaron Siddens and Javid Bayandor

Virginia Tech, Blacksburg, Virginia

Mark Celestina

Glenn Research Center, Cleveland, Ohio

Detailed Post-Soft Impact Progressive Damage

Assessment for Hybrid Structure Jet Engines

NASA/TM—2014-218397

December 2014

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The great support provided by the following colleagues at NASA is highly acknowledged and appreciated. Special thanks also 

go to the research team at CRASH Lab for their technical contribution to the work undertaken, the Department of Mechanical 

Engineering and, in particular Dr. Kenneth Ball, as well as the College of Engineering at Virginia Tech, without their generous

contribution and selÀ ess support this work could have not been successfully completed. This project has been partially supported 

through the NGFF Program. 

NASA Glenn Research Center (GRC)

Aeropropulsion Division, Dr. D.R. Reddy

Turbomachinery and Heat Transfer Branch, Dr. J. Heidmann and Mr. S. Kulkarni

Structures and Materials Division, Drs. G. Stefko, M. Bakhla, C. Chamis, M. Melis, M. Pereira

Research Directorate, Drs. J.F. Lei, M.D. Kankam and Ms. B. Beznoska 

Also, Drs. A. Ameri, D. Ashpis, P. Jorgenson, D. Krause, K. Carney, W. To, P. Sockol, J. Verse, and W. Wright

Available from

NASA STI Information Desk

Mail Stop 148

NASA Langley Research Center

Hampton, VA 23681-2199

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identi¿ cation 

only. Their usage does not constitute an of¿ cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 

This report is a formal draft or working 

paper, intended to solicit comments and 

ideas from a technical peer group.

This report contains preliminary ¿ ndings, 

subject to revision as analysis proceeds.
Abstract 

Currently, certification of engine designs for resistance to bird strike is reliant on physical tests. Predictive 

modeling of engine structural damage has mostly been limited to evaluation of individual forward section 

components, such as fan blades within a fixed frame of reference, to direct impact with a bird. Such 

models must be extended to include interactions among engine components under operating conditions to 

evaluate the full extent of engine damage. This paper presents the results of a study aim to develop a 

methodology for evaluating bird strike damage in advanced propulsion systems incorporating hybrid 

composite/metal structures. The initial degradation and failure of individual fan blades struck by a bird 

were investigated. Subsequent damage to other fan blades and engine components due to resultant violent 

fan assembly vibrations and fragmentation was further evaluated. Various modeling parameters for the 

bird and engine components were investigated to determine guidelines for accurately capturing initial 

damage and progressive failure of engine components. Then, a novel hybrid structure modeling approach 

was investigated and incorporated into the crashworthiness methodology. Such a tool is invaluable to the 

process of design, development, and certification of future advanced propulsion systems. 

1 Introduction 

The threat of foreign object impact (FOI), such as bird strike or hail strike, on turbofan engines is a major 

concern for commercial airlines and engine manufacturers. The FAA has set specific airworthiness 

standards under Federal Aviation Regulation (FAR) 33.76 requiring that engines be capable of 

withstanding impact with birds ranging from 0.8 to 8 lbs. without sustaining damage that poses a fatal 

threat to passengers and crew. To meet these standards, jet engine manufacturers rely heavily on a 

multitude of costly physical tests requiring the destruction of full-scale engines by simulating bird strikes 

using birds or bird surrogates in accordance with FAA guidelines. 

Bird strike on a turbofan blade can be defined as a complex, transient fluid-solid interaction between the 

soft bird material and the fan blade [1]. Upon impact, the internal solid structures of the bird are rapidly 

deteriorated as the resultant shockwave propagates through its body, breaking internal bonds of the bird 

and transforming its material properties to that of a viscous fluid contained within a shelled control 

volume. This causes pressure to be built up within the bird material and, as the shockwave progresses, 

leads to the formation of pressure release waves that force the transient fluidic matter out of the initial 

control volume enclosure [2]. The bird material expands nonlinearly beyond the initial point of impact, 

creating a larger contact region and causing greater potential for damage to additional fan blades as well 

as other engine components outside of the initial impact zone. 

Several types of damage occur in the forward fan section of an engine once subjected to an FOI. One type 

of damage is that resulting from a bird directly impacting several fan blades. Disintegration and 

degradation of the fan blades can cause decreased engine performance and further damage to other 

components as fragments or detached blades are ingested into the engine. 

Another significant source of damage is that resulting from contact between the fan blades and engine 

casing leading to high tangential friction and normal impact forces along the blade tips and the inside wall 

of the engine casing. Minute disturbances in the fan assembly position will bring the blades into contact 

with the casing, producing high blade tip stresses as they shear against sections of the engine casing. Bird 

impact along the length of a fan blade induces a moment about the engine axel and can remove mass from 

portions of the fan assembly, shifting the center of mass away from the axis of rotation. This can induce 

NASA/TM—2014-218397 1

'HWDLOHG3RVW6RIW,PSDFW3URJUHVVLYH'DPDJH

$VVHVVPHQWIRU+\EULG6WUXFWXUH-HW(QJLQHV

$DURQ6LGGHQVDQG-DYLG%D\DQGRU

9LUJLQLD7HFK

%ODFNVEXUJ9LUJLQLD

0DUN&HOHVWLQD

1DWLRQDO$HURQDXWLFVDQG6SDFH$GPLQLVWUDWLRQ

*OHQQ5HVHDUFK&HQWHU

&OHYHODQG2KLR
vibrations that move the fan blades through the small blade tip tolerance. Fragments resulting from blade 

and casing damage are ingested into downstream sections of the engine and can cause further destruction. 

Capturing the deterioration of engine components resulting from violent contact between the fan blade 

tips and the engine casing is essential for characterizing damage in turbofan engines. This is a particularly 

challenging task for modern fan blades utilizing composites with reinforcing metal components, with the 

resulting part called a hybrid structure. Tools for accurately predicting damage in these structures are not 

well established. New methods are needed to facilitate the design and analysis of future propulsion 

designs incorporating these advanced structures. 

The goal of the current study is to develop a methodology for predicting bird strike damage in jet engine 

forward fan sections incorporating the true nature of structural dynamics and destructive interactions. 

Critical features under investigation are the degradation and disintegration of the turbofan blades and the 

engine casing. Three-dimensional explicit finite element analysis models will be employed to simulate the 

highly nonlinear, transient response of the forward engine section resulting from impact and capture the 

associated turbofan blade damage, fan assembly vibrations, and subsequent engine casing damage. A 

methodology for predicting these damage effects is imperative to facilitating the design and certification 

processes of future advanced propulsion systems. 

2 Problem Approach 

A review of the literature shows several approaches employed through previous research programs to 

predict bird strike damage to jet engine fan assemblies. Several researchers have conducted in-depth 

studies of damage to an individual fan blade resulting from bird impact to assess the response and damage 

of the structure [3-5]. In addition, some investigators have studied the effects of impact between a bird 

and multiple (three or four) blade configurations in accordance with the distributed impact that occurs 

when a bird strikes several blades as it passes through the front section of the engine [6, 7]. Lastly, there 

are few models that include the entire fan assembly arranged about a central hub in which the fan blades 

rotate about a rigid point. 

While these models capture the damage that occurs to a blade or blades directly impacted by a bird, they 

do not incorporate the engine casing or a deformable axel into the simulation and the highly interactive 

role that their dynamic responses play in sustaining or containing damage. Therefore, these models are 

unable to capture the post-impact damage responses that may result from abrasion of the fan blade tips 

onto the engine casing. Also, these studies are often limited to individual cases in which one simulation is 

developed to match experimental or analytical data without consideration for the effect that modeling 

changes may impose on predicted results. 

The approach chosen for this study was to model the bird impact damage for a jet engine forward section 

capturing several key features: 

Initial impact between the bird and fan blades and the resulting total damage, 

Mass unbalancing, leading to fan assembly and main axel vibration and post-impact dynamic 

damage, 

Blade tip shearing against the engine casing and the resulting damage both the blade and the 

casing. 

NASA/TM—2014-218397 2
Additionally, it is aimed to explore modeling approaches that affect the predicted initial and final damage 

in order to establish general guidelines for bird strike damage assessment on engine forward sections and 

any possible conclusions contributing to the design or certification procedures. 

3 Problem Complexities 

The difficulties of modeling bird strike damage on a full fan section are numerous even for a simplified 

model. In addition to the common challenge of accurately modeling progressive material failure, the 

following major complexities must also be addressed: 

3.1 Bird/Fluid Modeling 

An important complexity was modeling the bird accurately using an approach that can capture large 

deformations and separation of the fluidic bird material following initial impact without incurring heavy 

element distortions that can lead to mesh entanglement and ultimately divergence of the analysis. To 

avoid this, a meshless formulation was utilized for the bird. This approach is discussed in detail in section 

5.

3.2 Contact Modeling 

Essential interactions required to be incorporated into models included bird-blade, bird-casing, bladecasing, blade-blade, and blade-self contact. Contact between the bird and engine components had to 

accurately capture the interaction forces between the bird and component surfaces over time. Since 

contact between interacting parts due to material failure in element eliminated areas was lost, contact 

regions had to be redefined over time as the simulation progressed. FE models had to be able to 

adequately impose contact between eroding parts as contact interfaces continued to evolve. 

In addition, further complexity arose due to the close proximity and high velocity of the blade tips relative 

to the casing elements. In post impact, blade elements penetrated the casing due to oscillations associated 

with unbalancing in the fan assembly. Contact models were utilized that detected this penetration and 

applied subsequent interaction forces to remove the penetration and thus model the contact between 

elements. However, if the time step was too large, the blade elements penetrated the casing beyond the 

limits that the contact formulation was capable of detecting and failed to impose the constraint, causing 

unrealistic penetrations between the parts. To prevent this, the initial time step was lowered until it was 

observed that excessive element penetration no longer occurred for the range of fan assembly velocities 

under investigation. 

3.3 Fan Unbalancing and Vibrations 

Impact between the bird and fan blades at a distance away from the center of the hub moved the fan 

assembly off center, causing it to oscillate about the engine’s axis of rotation. In addition, if the blades 

fractured, the center of mass shifted permanently, triggering a strong rattling effect as the fan blade tips 

hit the casing and caused damage to both the blades and the casing. Suitable models were required to 

capture damage not only from direct impact, but resulting from post-impact damage to the rest of the 

structure due to unbalancing and vibrations. 

NASA/TM—2014-218397 3
3.4 Pre-stressing 

The angular velocity of a jet engine fan section causes centripetal forces along the length of fan blades 

that pull them toward the center of the hub and maintain the circular path of the fan stage around the axis 

of rotation. Stresses are greatest at the blade root and decrease along the length of the blade until reaching 

zero at the tip. In order to accurately predict damage, it was necessary to assess the state of stress and 

strain in the blades as a result of both the bird impact and the initial state of stress due to centripetal 

acceleration. 

4 Soft Impact Theory 

At velocities around and above 50 m/s, a bird impacting an object with a much higher stiffness induces a 

shockwave that propagates in the direction of relative velocity through its body and breaks its internal 

bonds. This leads to a multi-stage problem as the bird’s properties, past the shock, change from solid to 

fluid. The several facets of the impact event can be characterized by the following main stages: (1) solidstructure impact, (2) solid-fluid transition, (3) fluid-structure interaction, (4) nonlinear growth of impact 

region, and (5) structural damage. 

Originally, the bird mass is solid, and the initial impact with a fan blade can be labeled as a “hard” impact 

in which contact forces are highly localized about a relatively small impact region. The velocity of 

material over the contact area instantly decreases, and a shock pressure develops in both the projectile and 

the target as material is compressed. Assuming both objects behave elastically, an expression for the 

shock pressure, or Hugoniot pressure, P_H, induced in the bird can be derived from: 

(1)

where and are the densities of the bird and the blade, respectively, and are the shockwave 

speed in the bird and blade, and is the relative impact velocity [1]. In the case of bird strike, the 

following will be valid: , and subsequently the above equation can be simplified to: 

(2)

The shock pressure created inside of the bird is greater than its material strength. Therefore, the structural 

bonds of the bird are broken, and the solid material undergoes a transition to a fluid. 

Concentration of the shock pressure along an interface, bound by the speed of sound in the object, 

produces a shockwave. The shockwave then propagates at a speed much faster than the relative bird/blade 

velocity, at which the bird material properties are rapidly changed into fluid. Consequently, a large 

pressure gradient is formed across the boundary of the surrounding air and the control volume containing 

the bird. Pressure release waves force the fluidic matter away from the bird center, while the shock 

pressure decays to a lower steady pressure [1]. This steady pressure, , is defined by: 

(3)

NASA/TM—2014-218397 4
The theoretical duration of the impact event is the time it takes for the end of the bird to reach the target. 

Since the post shock gelatin like material can be considered incompressible, its speed does not decrease 

significantly throughout the remainder of the impact. Therefore, the initial velocity may be used to 

estimate the impact window as: 

(4)

where is the length of the bird and is the initial bird velocity. 

In high velocity bird or porous Newtonian fluid-compliant gelatin impacts (Fig. 1), a sharp rise in the 

internal pressure of the soft body can be detected as the projectile first comes into contact with the target. 

This triggers a progressive change in the impactor material from solid to fluid followed by shock pressure 

release waves occurring at the solid-fluid interface. 

Fig. 1. Pressure in a porous gelatin projectile high velocity impact [8]. 

Subsequently, a sudden pressure drop is experienced in the forward and transient fluidic portion of the 

projectile. The residual pressure then continues to gradually decrease through the remainder of the impact 

event until it finally reaches zero [1]. 

The force exerted on a target over time by a soft impactor such as a bird is directly related to this timevarying pressure. The applied force can be derived using the relation pressure = force/area. Therefore, 

the peak force and steady pressure are: 

(5)

(6)

The force therefore behaves similar to the pressure by first rising quickly to a maximum value in 

correspondence with the initial impact, decreasing rapidly as the initial shock in the projectile releases, 

becoming steady, and then declining towards zero at the end of the impact event. A linear approximation 

of this process is shown in Fig. 2.

NASA/TM—2014-218397 5
Fig. 2. Linear approximation of the force time history of a soft impact. 

The excitation force, , can be approximated using a piecewise linear function that defines the active 

force over each unique time domain. Using the Heaviside function, , can be defined as 

(7)

where m is the last time piece. The equation determines the applied force with time as different 

force equations are turned “on” and “off” by the time-dependent Heaviside functions. Therefore, for 

, the applied force is zero after the impact has completed. This simplified model can be 

expanded to include additional time domains describing the applied force in more detail than what is 

shown in Fig. 2 in order to more accurately capture the force-time history on the target. An application of 

this concept to bird impact with equations detailing the force over each time domain is discussed 

in [9]. 

Using this excitation force, the displacement response of an impacted target can be defined. For 

example, if the target was represented as an equivalent one-dimensional mass-spring-damper system, the 

displacement of a mass subjected to the force appeared in Eq. (7) can be defined using the 

convolution integral approach: 

(8)

where is the mass of the target, and are the damped and un-damped natural frequencies of the 

system, respectively, and is the damping ratio. The variable in the equation is the integration variable. 

5 Meshless Lagrangian Bird Modeling Approach 

Several meshless finite element modeling approaches have been developed that represent objects as a 

NASA/TM—2014-218397 6
finite number of discrete particles that interact via an approximation or kernel function rather than a 

structured mesh. The range of influence and interaction with other particles can be defined through using 

this function. In meshless Lagrangian approaches, each node represents a particle with physical properties 

such as mass, volume, and pressure. Each particle experiences fluid forces based on the mass and number 

of surrounding particles and the kernel defining their interaction [10]. Closer particles exert more 

influence than those that are farther away. In this manner, particle interaction can be visualized as an 

inter-molecular attraction type force holding the particles together. The behavior of the particles is 

governed by the Navier-Stokes equations, numerically discretized for use with the particle approach [9].

This might neglect the possible non-Newtonian nature of the transient fluid produced as a result of the 

fluid-solid interaction. 

5.1 Bird Model Details 

The bird was modeled as discrete particles using a Meshless Lagrangian approach on the explicit finite 

element platform Ls-Dyna. Several representative bird geometries have been proposed for both impact 

experiments utilizing bird surrogates and finite element models of high velocity bird impact. One of the 

most common configurations is a cylinder with rounded hemispherical ends [11]. 

A similar geometry was adapted for the employed particle modeling approach in thus work; the particles 

were arranged, evenly spaced, throughout a rounded-cylindrical volume to approximate the shape of a 

bird body. The ratio of total length to diameter for the bird geometry was 2. 

It was necessary to define both a suitable material model and a pressure-volume state equation for the 

bird. As discussed above, the bird material begins changing from a solid to a fluid at the point of impact 

with a blade. Material at the blade/bird interface is rapidly liquefied, and the resulting impact shockwave

propagates through the bird much faster than the relative bird/blade velocity. For the majority of the 

interaction, the blade interacts with the gelatin-like fluid, and only interacts with the solid bird for a brief 

instant at the moment of impact. Therefore, it is justified to model the bird using a fluid material model 

for the entire impact event. 

An appropriate fluid material model was identified that allowed the mass density and the fluid pressure 

cutoff to be defined. In addition, a polynomial equation of state (EOS) was used that defined the inverse 

relationship between the particle volume and pressure, given as: 

.

(9)

The constant is the initial density defined in the material model, is the instantaneous density, and 

is the bulk modulus of the bird material. Through the definition of density, the pressure-volume 

relationship within the above EOS formulation can be established. The bulk modulus of water (2.2 GPa) 

was used in the EOS. The initial material density used was 945 kg/m3

 with a total bird mass of 1.8 kg, or 

4 lb. These properties, along with the stated length-to-diameter ratio, defined the necessary bird geometry. 

NASA/TM—2014-218397 7
The variable is a non-dimensional material constant called the Hugoniot number or compressibility 

constant. This is adopted from the classical linear shockwave speed equation: 

(10)

where is the speed of sound in the material, is the Hugoniot number, and and are the 

shockwave speed and initial velocity respectively, as defined previously. Prior studies have found to 

equal approximately 2 for biological materials including birds [12]. 

Simulations of the bird impacting a rigid plate were conducted and the pressure at the center of the impact 

region was recorded throughout the duration of the impact. The number of particles used to represent the 

bird was varied and the pressure-time history at the center of impact was compared. This was done in 

order to verify that a sufficient particle density was used such that the bird behavior was independent of 

the number of particles used in the simulation. 

5.2 Impact Results 

The pressure-time history of the bird impacting a rigid plate is shown in Fig. 3. The recorded pressure was 

(a)

(b) 

 (c) 

Fig. 3. Pressure-time history of the bird/rigid plate impact scenario for increasingly more particles used to 

represent the bird. 

17,800 particles

34,700 particles

45,300 particles

NASA/TM—2014-218397 8
non-dimensionalized by dividing the results by the theoretical steady pressure given by Eq. (3). Time was 

also non-dimensionalized by dividing each time increment by the theoretical impact duration defined by 

Eq. (4).

Each graph was characterized by a short duration pressure rise, followed by an approximate steady 

pressure state, and a final region where the pressure decays to zero. For Fig. 6 (a) and (b), the predicted 

pressure response included two peaks before settling to a steady pressure. As the number of particles was 

increased, the response changed such that the initial high pressure region was characterized by a single 

pressure rise rather than the two initially observed. 

At around 30,000 to 35,000 particles, the pressure-time history began reaching a consistent response. 

Further increases in the number of particles lead to minor refinements of the response with reduced 

oscillations with the overall response changing only slightly. The peak pressure value was also quite 

consistent with an approximate non-dimensionalized pressure of 8.5. This response was repeated in 

further analyses, up to 150,000 particles. It was concluded that for numbers of particles around 35,000, 

the bird model exhibited an acceptable level of independence from the particle density. To provide 

accuracy and minimize computational time, a bird model comprising of approximately 35,000 particles 

was used for impact damage analysis onto the engine forward section. 

6 Forward Section Impact Modeling 

The fan blades were modeled with shell elements using a general commercial aviation fan blade 

geometry. The width at the base was 0.2 m, representing a variety of wide-chord fan blade designs. A 

total of 36 blades were modeled with their bases fixed to a central hub via rigid connectors constraining 

both translational and rotational motions against the hub. A total shell thickness of 3.3 × 10-3 m was 

assigned to each blade element. An isotropic plasticity material model was used to define titanium 

material properties with yield and failure strain incorporated into the material behavior. The pre-yield 

elastic and post-yield tangent moduli of 112.5 and 1.6 GPa were respectively selected. The maximum 

strain at which failure would occur was set to 0.1, determined from typical stress-strain curve data for an 

aerospace grade titanium alloy. 

Post-impact unbalancing and vibration were determined through assessing the deformation imposed on 

the engine axel. Translational motion along its axis was restricted via boundary conditions on the end 

opposite of the fan assembly. Supports were placed along the axel to prevent rigid body movement in the 

plane perpendicular to its longitudinal axis. These supports were modeled as rigid “sleeves” that were free 

to rotate about any axis, pivoting on a centrally located reference node, however restricted from rigid 

body translation. This sufficiently held the axel in place while allowing it to bend in accordance with 

forces induced by mass unbalancing at the fan section. 

The casing was modeled as an open cylinder using shell elements surrounding the fan section. Care was 

taken to fit the diameter of the casing close to the fan blade tips in accordance with the tip tolerances that 

are used in modern engine designs. It was essential to allow a small tolerance for the fan blades to deform 

radially as a result of pre-stressing in order to avoid initial penetrations between the blades and the casing. 

An isotropic plasticity material model was used to include strain rate effects, yielding, and failure. 

Aerospace grade aluminum alloy was used for the property definition. The finite element model as a 

whole with the casing is shown in Fig. 4.

NASA/TM—2014-218397 9
Fig. 4. Explicit finite element impact environment. 

An explicit dynamic relaxation method was used to pre-stress the fan blades. This approach allowed 

stresses and displacements to be initialized using prescribed centrifugal body loads by damping nodal 

velocities by a prescribed scaling factor at each time step. The peak distortional kinetic energy was 

calculated at the first time step, and the current distortional kinetic energy was determined at each 

subsequent time step [13]. When the ratio of the current distortional kinetic energy to that of the peak

energy fell below a set threshold, the system was determined to be sufficiently steady, and the analysis 

was considered complete. As the threshold was lowered, the solution began to improve. 

For systems with loads due to rotation, it was found to be extremely important to set this value as low as 

possible. Finding the lowest convergence ratio required a trial and error procedure. After wide-ranging 

experimentation, a value of 1.5 x 10-5 was selected. The results of the pre-stressing are shown in Fig. 5. 

Fig. 5. Fan assembly stresses following the pre-stressing phase. 

Subsequent to the stress and displacement initialization phase, the bird impact was introduced into the 

simulations: the initial velocity of the bird was 350 m/s, while the initial rotational velocity of the fan 

assembly was selected to be 525 rad/s, corresponding to a rotation of 5000 rpm. The bird was positioned 

close to the fan blade, just outside the penalty length, to reduce needless computation. 

NASA/TM—2014-218397 10
6.1 Bird Impact Results 

Initially, an impact scenario without the casing was modeled to establish the response of the fan assembly 

to a bird impact without interference from the fan containment. The bird directly impacted several blades 

and induced high stresses and strains that lead to material failure at the base of two of the blades. From 

this analysis, deformation of the axel was used to establish the mode shape of the vibration response of 

the fan assembly due to mass unbalancing. The presence of the three non-translating sleeves holding the 

axel meant that there were three fixed points representing the engine axel anchor stations within the 

propulsion system. The distributed mass between the sleeves and the large fan assembly constituted a 

system with three degrees of freedom. The mode shape diagram is given in Fig. 6. This mode shape 

corresponds with the first vibration mode of a three degree of freedom system. 

Fig. 6. Mode shape of the axel vibration. 

Consequently, a bird strike event against the full fan assembly with the casing included was simulated to 

observe the damage behavior of the system. Again, the bird first made contact with a single blade and 

began deforming heavily as a result of the interaction. 

The resulting soft body impactor was sliced with portions of it coming into direct contact with three to 

four blades. The force-time history of the impact between the bird and the fan blades is shown in Fig. 7.

Fig. 7. Impact force between bird and fan blades. 

The spread of the bird contributed to a larger contact area between the bird material and the back of each 

blade as the blades sliced through and then collided with clusters of particles. This larger contact area 

appeared to contribute to high deformation across the entire blade, inducing plastic strains through the 

blade and at its base. Eventually, strains at the base exceeded the designated material limit, causing 

failure. This was effectively captured by the analysis in the form of element deletion along the base of 

one or two of the impacted blades. 

NASA/TM—2014-218397 11
Fig. 8. Impact between the bird and fan assembly. 

Following the initial impact, several events occurred that contributed to extensive damage to other 

portions of the fan assembly. First, the detached blades impacted the engine casing and began to interact 

with other nearby blades. Fragments from impacted blades became stuck between the other blades and the 

fan containment, causing further plastic deformation of the casing. 

While the fan assembly rotated through one revolution following the impact, the detached blades caused 

the fan assembly to move off center, pushing blades on the opposite side of the impact zone into the fan 

casing, resulting in high plastic strains and disintegration. 

The response with no casing was much more uniform in nature. While time and computational resources 

prevented the simulation from running until a quasi-steady state was reached, it was clear from the no 

casing scenario that the response of the system was smooth and approaching a limit cycle as the center of 

mass shifted. 

Fig. 9. Fan blade tips impacting and sliding against the casing across from the impact zone. 

In contrast, the simulation with the casing present shows the effect that blade-casing contact imposes on 

the vibration response. The fan assembly experienced quick changes in its motion as the complex 

interaction between blades and the casing occurred. Progressive damage of the fan blades and other 

components is shown at various times throughout the analysis in Fig. 10.

NASA/TM—2014-218397 12
t = 2 ms t = 5.5 ms

t = 10.5 ms t = 13.5 ms

t = 16.5 ms t = 20 ms

Fig. 10. Progressive damage at selected times. 

The dynamic oscillation response of the fan assembly with the casing surrounding the fan assembly 

was compared with the simulation without the casing in order to observe the effect that the casing had on 

the vibration. The x- and y-displacements of the center of the hub were plotted through time for each 

model, generating two orbit plots shown in X-Displacement (m) 

Fig. 11. No Casing Y-Displacement (m)

With Casing

X-Displacement (m) 

Fig. 11. Center hub displacement orbit plots. 

NASA/TM—2014-218397 13
the fan casing and experienced yielding and failure at the blade tips as well as along their length. The 

casing was heavily deformed due to this interaction, causing material failure in several locations as a 

result of high strain levels (Fig. 12).

Fig. 12. Casing plastic deformation after impact. 

Additional simulations with the casing present were conducted in which the number of elements used to 

represent each fan blade was varied. This established the mesh dependence in fan blade damage 

assessment as well as the overall predicted damage. Fig. 13 shows the results of three simulations 

utilizing different fan blade mesh densities immediately after the initial hit. 

(a)

(b)

(c)

Fig. 13. Initial fan damage for (a) 12420, (b) 16524, and (c) 21600 blade elements. 

NASA/TM—2014-218397 14

As the unbalanced rotation continued, addition blades around the entire fan assembly made contact with 
medium and high density models predicted detachment of two blades. This comparison indicated that a 

degree of fineness equal to or greater than the medium density model was necessary throughout the 

middle section of the blade to accurately characterize the bird/blade interaction between the meshless 

particles and the structured Lagrangian mesh used to construct the fan blades. Fig. 14 shows the results of 

the three simulations further in the analysis after several rotations of the fan assembly. 

Fig. 14. Damage after several rotations for (a) 12420, (b) 16524, and (c) 21600 blade elements. 

The coarse density blade mesh model predicted a lower rate of damage than the other models, indicating 

detachment of about half of the blades as predicted by the medium and high mesh density models at the 

same time step. The high density model determined a few more detached blades than the medium density 

model. All models predicted substantial damage to the blade tips as they sheared against the casing. 

The larger elements of the coarse mesh model failed at the blade tips as the simulation progressed, 

resulting in a large gap between the remaining blade material and the casing. This gap prevented further 

interaction between the blades and casing, which particularly lead to prediction of less overall damage 

and fewer detached blades. The medium and high mesh density models experienced smaller gaps as 

elements at the blade tips failed, allowing further interaction between the blades and casing. Therefore, a 

level of mesh refinement equal to or greater than that of the medium density model was deemed necessary 

at the blade tips to accurately characterize damage in the entire forward fan section. 

A preliminary assessment of the damage predicted by the model with a real turbofan engine subjected to 

bird strike is shown in Fig. 15. This comparison supported the argument that the widespread damage to 

the fan stage in its entirety, as determined by the numerical model, is possible in cases of real bird strike 

damage. 

NASA/TM—2014-218397 15

The low mesh density model predicted that the initial hit only caused detachment of one blade, while the 
Fig. 15. Comparison of predicted damage with a real bird strike case. 

7 Hybrid Fan Blade Modeling Approach 

In modern engine designs, composite fan blades are manufactured using unidirectional carbon fiberreinforced epoxy pre-preg material [14]. Layers of the material are arranged and built up on preforms and 

then bonded together to create the fan blade structure. Metal reinforcements, such as a titanium sheath 

that wraps around the leading edge of a composite blade, are added to improve the impact resistance. The 

resulting assembly is referred to as a hybrid structure. 

7.1 Composite Blade Damage 

Failure in hybrid structures is characterized by numerous complex damage mechanisms occurring over 

multiple interacting length scales and among materials with vastly dissimilar properties. Material models 

that provide a ply-level assessment of composite damage were explored in this initial investigation. 

Models that attempt to predict micro-level fiber and matrix failure, called micromechanics models, are the 

subject of current research and will be explored in future studies. 

Ply-level failure mechanisms are broadly defined in terms of strength in the longitudinal (fiber), 

transverse (in-plane perpendicular to fiber), and through-thickness (out-of-plane perpendicular to fiber) 

directions, including shear properties relative to these directions. Failure of an individual ply layer that 

degrades the structural performance but does not cause a complete loss of load-carrying capacity is 

referred to as damage. Once all of the ply layers in a given location are degraded, the complete failure 

occurs. The latter is referred to as failure. 

An orthotropic material model with a strength-based failure criterion was used to model the behavior of 

the composite portion of each blade. Ply failure was determined by calculating generalized stresses in 

both the longitudinal and transverse directions for each layer. Once the stress state satisfied the failure 

criterion, the load-carrying capacity of the layer was removed from the calculation, thus capturing damage 

through the reduced stiffness and strength properties. 

This material was implemented within shell elements with a laminated plate formulation. Integration 

points in the through-thickness direction represented each ply layer and allowed each layer’s contribution 

to the laminate stiffness to be included via the Labotto integration rule [13]. Once each layer satisfied the 

failure criterion, the element was removed, thus underlining the local composite failure. 

Material properties for IM7/8551-7 graphite fiber composite were used for the material definition. A 

composite layup of [0°/+45°/-45°/0°] was used for this initial study, with the 0° fiber orientation directed 

along the length of the blade. The thickness varied across the blade from 6.35 mm (0.25 in) at the tip and 

NASA/TM—2014-218397 16

50.8 mm (1 in) at the base, which is consistent with hybrid fan blade dimensions [14]. 
7.2 Leading Edge Delamination 

Another critical damage mode for hybrid structures is ductile failure and separation of the metal leading 

edge from the composite edge when subjected to impact loads. In order to capture the damage to the 

leading edge and separation of the sub-structure from the composite blade, the edge was explicitly 

modeled using shell elements that wrapped around the blade edge. 

Fig. 16. Hybrid fan blade model with leading edge explicitly modeled. 

The decohesion technique was chosen for both bonding the leading edge to the blade and capture debonding of the component. A mixed-mode bilinear traction-separation law was explored that reproduced 

softening of the interlaminar bond between surfaces as they displace, followed by failure as they separate 

a critical distance. 

In this model, the displacement between bonded shell elements at which the softening began was 

determined by the equation 

(11)

where is the displacement for onset of softening, and are the mode I and mode II damage 

initiation separations, is the mode mixity, and are the peak normal and tangential tractions, and 

and are the cohesive normal and tangential stiffness, respectively. The displacement at which failure 

occurred was defined by the equation 

(12)

where is the ultimate displacement, and are the mode I and II energy release rates, and is 

the mixed-mode exponent. 

NASA/TM—2014-218397 17
This cohesive model was implemented in the explicit FE code using tied a constraint between the titanium 

leading edge and the portion of the composite surrounded by the metal component. With the tied 

constraint approach, parts being coupled do not need to share similar geometries or meshes. This provides 

a convenient method of defining cohesive behavior between parts with very different shapes. 

Cohesive parameters for the composite/metal bond were not available at the time of this study. As an 

approximation of the peak tractions and the mode I and II energy release rates, properties from carbonfiber composite delamination experimental studies were obtained from the literature [15] and decreased 

by one-fourth before being applied, with the assumption that the composite/metal bond would form a 

weaker interface. 

The titanium leading edge incorporated an elastic-plastic material model with a strain-based failure 

criterion. Properties for Ti-Al6-4V ASTM Grade 5 titanium alloy were used to define the leading edge 

behavior. 

7.3 Pre-stress Analysis 

Centripetal loads were applied to the hybrid fan blade using the same explicit DR analysis approach 

utilized for metal blades. Again, it was important to decrease the convergence tolerance as much as 

possible to arrive at a pre-stressed state sufficiently close to the true stresses for a given rotational speed 

to reduced unwanted stress oscillations at the beginning of the transient analysis. 

8 Hybrid Fan Impact Modeling 

The fan assembly was formed using an array of hybrid fan blades arranged around a central hub and 

connected via rigid constraints at their bases. Half of the fan assembly was modeled with a total of 9 

blades. The 4 lb. bird model developed earlier was introduced to impact at about the mid-span of the 

blades. Contact was defined between the particles and the hybrid blades using the same penalty-based 

approach. 

The fan assembly was set rotating with an initial speed of 2400 rpm, a typical maximum speed for similar 

high bypass ratio turbofans with wide-chord fan blades. The bird was assigned an initial relative velocity 

of 100 m/s. 

Fig. 17. Explicit FE model of a bird strike scenario for a hybrid fan assembly. 

NASA/TM—2014-218397 18
8.1 Bird Impact Results 

The bird was sliced into two parts upon impact with the first blade. Part of the bird material remained in 

contact with the blade, while the remaining material continued to impact additional structures. The soft 

body was separated into multiple impactors due to slicing by subsequent blades, with portions coming 

into direct contact with four fan blades. Contact between the structures and the bird involved both a 

slicing action with the blade edge and a blunt impact with the rear face of the blade. The impact event is 

shown in Fig. 18. 

None of the hybrid blades experienced failure in any part of the structure as a result of the impact. 

However, critical damage mechanisms were captured throughout several of the blades impacted by the 

bird. The damage analysis showed fiber tensile failure occurring at the base of two blades as a result of 

high stress at their respective roots, as shown in Fig. 19.

Fig. 18. von Mises stresses induced by the bird impact at progressive time steps. 

These blades were subject to impact with the largest portions of the bird. The plot shows the percentage 

of plies intact/failed, with blue indicating 100% of plies intact and red indicating 100% of plies failed. 

NASA/TM—2014-218397 19
Fig. 19. Fiber tensile failure initiated at the root of two impacted blades. 

In addition, matrix failure was predicted throughout each of these blades, especially where the bird 

impacted the face of the blade rather than the leading edge. This damage is shown in Fig. 20. 

Fig. 20. Extensive matrix damage and failure evident throughout the two impacted blades. 

It was observed that damage was more prevalent in the outer plies. The outer plies are subjected to higher 

normal strains and stresses when subjected to bending caused by the bird impact. This tendency towards 

outer ply damage is shown in Fig. 21. 

NASA/TM—2014-218397 20
Matrix Tension Fiber Tension

Ply 4 Ply 3 Ply 2 Ply 1

Fig. 21. Matrix and fiber tensile damage within each layer for an impacted hybrid fan blade. 

Leading edge delamination was predicted in several blades by the cohesive model. The delaminated 

region is shown as red in Fig. 22. Blue indicates fully bonded areas. Other regions indicate softening prior 

to full de-bonding. 

Fig. 22. Leading edge delamination following bird impact. 

9 Comprehensive Crashworthiness Methodology 

This work and complementary studies have demonstrated that interactions among forward section 

components contribute to additional damage, not sufficiently captured by a stand-alone assessment of bird 

impact on a structure [16]. As an additional step towards developing a comprehensive crashworthiness 

NASA/TM—2014-218397 21
methodology for hybrid engines, a fully coupled fluid-structural module is proposed. The requirements 

for this module and the platforms utilized will be detailed in the next report. 

10 Conclusion 

Research efforts focusing on the development of a detailed predictive modeling methodology for 

comprehensive analysis of bird strike and post-impact damage assessment in structural-hybrid jet engine 

forward sections are reported. The development of a transformative, aerospace propulsion design platform 

for evaluating the crashworthiness of future advanced structural engines is one of the main objectives of 

these studies. The novel methodology proposed offers the ability to capture the extent of intricate soft 

foreign object damage to fan blades resulting from direct initial impact, subsequent collisions with a 

number of other engine forward section parts and sub-components as well as associated fragmentation 

containment analysis. The full dynamic response prognosis in this methodology further encompasses 

blade-out, fan assembly unbalancing and vibrations, and subsequent engine casing shearing and shaving 

damage analyses, which remain unaddressed in many previous studies. 

A novel numerical method for predicting damage and failure in hybrid jet engine fan blades subject to 

soft impact has also been developed. Critical damage mechanisms have been captured, such as composite 

fiber and matrix failure mechanisms, leading edge delamination, and material failure. This work shows 

promise for extending the forward section engine damage methodology to include hybrid fan structures. 

The outlook of the investigations will include validations of the modules developed thus far by means of 

comparison with data obtained from variety of crashworthiness experiments. Overall, it is aimed to 

develop a comprehensive analysis platform for the assessment of hybrid engine designs that will reduce 

reliance on physical tests and can support future developments of advanced aerospace propulsion systems. 

Fluid-structural coupling will further enable the methodology to assess the performance loss caused by 

structural deformation and failure. 

NASA/TM—2014-218397 22
References 

[1] Wilbeck, J. S., and Barber, J. P. Bird impact loading. Shock and Vibration Bulletin, No. 47, 1978, 

p. 7. DOI: 0362-3874 

[2] Siddens, A., and Bayandor, J. A soft impact predictive methodology for hybrid/composite 

propulsion systems. Proceedings of the 16th International Conference on Composite Structures,

Nashville, Tennessee, January 9-12, 2012. 

[3] Mao, R. H., Meguid, S. A., and Ng, T. Y. Finite element modeling of a bird striking an engine fan 

blade. Journal of Aircraft, Vol. 44, No. 2, 2007, pp. 583-596. DOI: 10.2514/1.24568 

[4] Yupu, G. Foreign object damage to fan rotor blades of aeroengine part i: Experimental study of 

bird impact. Chinese Journal of Aeronautics, Vol. 20, No. 5, 2007, pp. 408-414. 

[5] Meguid, S. A., Mao, R. H., and Ng, T. Y. Fe analysis of geometry effects of an artificial bird 

striking an aeroengine fan blade. International Journal of Impact Engineering, Vol. 35, No. 6, 

2008, pp. 487-498. DOI: 10.1016/j.ijimpeng.2007.04.008 

[6] Chevrolet, D., Audic, S., and Bonini, J. Bird impact analysis on a bladed disk. RTO AVT 

Symposium, Paris, France, 22-25 April, 2002. 

[7] Sinha, S. K., Turner, K. E., and Jain, N. Dynamic loading on turbofan blades due to bird-strike. 

Journal of Engineering for Gas Turbines and Power, Vol. 133, 2011. 

[8] Johnson, A. F., and Holzapfel, M. Modelling soft body impact on composite structures. Journal 

of Composite Structures, Vol. 61, 2003, p. 10. DOI: 10.1016/S0263-8223(03)00033-3 

[9] Siddens, A., and Bayandor, J. A discrete meshless lagrangian based approach for soft impact 

damage modeling in advanced propulsion systems. ASME International Mechanical Engineering 

Congress & Exposition, Vancouver, British Columbia, 12-18 November, 2010. 

[10] Liu, G. R., and Liu, M. B. Smoothed particle hydrodynamics: A meshfree particle method. World 

Scientific Publishing Co. Pte. Ltd., 5 Toh Tuck Link, Singapore, 2003. 

[11] Bayandor, J., Johnson, A. F., Thomson, R. S., and Joosten, M. Impact damage modelling of 

composite aerospace structures subject to bird-strike. 25th International Congress of the 

Aeronautical Sciences, Hamburg, Germany, 2006. 

[12] Nagayama, K., Mori, Y., Motegi, Y., and Nakahara, M. Shock hugoniot for biological materials. 

Shock Waves, Vol. 15, No. 3, 2006, pp. 267-275. DOI: 10.1007/s00193-006-0030-5 

[13] Ls-dyna theory manual, Livermore Software Technology Corporation, 2006. 

[14] Marsh, G. Composites get in deep with new-generation engines, Reinforced Plastics, Vol. 50, No. 

11, 22 December, 2006, pp. 26-29. 

[15] Ilyas, M., Lachaud, F., Espinosa, C., and Salaün, M. Dynamic delamination of aeronautic 

structural composites by using cohesive finite elements. 17th International Conference on 

Composite Materials, Edinburgh, Scotland, 27-31 July, 2009. 

[16] Siddens, A., and Bayandor, J. An extensive crashworthiness methodology for advanced 

propulsion systems - Part II: Detailed damage & vibration instability analysis of jet engine 

forward sections. 49th AIAA Aerospace Sciences Meeting, Orlando, Florida, 2010. 

NASA/TM—2014-218397 23