Cheryl L. Bowman

Glenn Research Center, Cleveland, Ohio

Michael G. Jones

Langley Research Center, Hampton, Virginia

Simulated Tip Rub Testing of Low-Density

Metal Foam

NASA/TM—2009-215818

December 2009
NASA STI Program . . . in Profi le

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scientifi 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 Offi 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 signifi cant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of signifi cant 

scientifi 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. Scientifi c 

and technical fi 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. Scientifi c and 

technical fi ndings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientifi c and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scientifi 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 scientifi c and 

technical material pertinent to NASA’s mission.

Specialized services also include creating custom 

thesauri, building customized databases, organizing 

and publishing research results.

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 via the Internet to help@

sti.nasa.gov

• Fax your question to the NASA STI Help Desk 

at 443–757–5803

• Telephone the NASA STI Help Desk at

 443–757–5802

• Write to:

 NASA Center for AeroSpace Information (CASI)

 7115 Standard Drive

 Hanover, MD 21076–1320
Cheryl L. Bowman

Glenn Research Center, Cleveland, Ohio

Michael G. Jones

Langley Research Center, Hampton, Virginia

Simulated Tip Rub Testing of Low-Density

Metal Foam

NASA/TM—2009-215818

December 2009

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The authors would like to acknowledge the foundational work on metallic foams performed by Dr. Mohan Hebsur, formerly of 

Ohio Aerospace Institute. The tip rub test rig was developed by Jose C. Gonsalez in support of the Rotor Alone Nacelle Phase 

of the Advanced Subsonic Technology Program. System confi guration, controller confi guration, and data acquisition system 

development as well as testing support were provided by Derek Podboy, John DeArmon, and Don Hammett, Jr., of NASA 

Glenn Research Center, TFOME/Jacobs Sverdrup, and TFOME/Sierra Lobo, respectively. Acoustic impedance tests were 

conducted by Carol Harrison of NASA Langley Research Center.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5285 Port Royal Road

Springfi eld, VA 22161

Available electronically at http://gltrs.grc.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

This work was sponsored by the Fundamental Aeronautics Program 

at the NASA Glenn Research Center.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2009-215818 1 

Simulated Tip Rub Testing of Low-Density Metal Foam 

Cheryl L. Bowman 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Michael G. Jones 

National Aeronautics and Space Administration 

Langley Research Center 

Hampton, Virginia 23681 

Abstract 

Preliminary acoustic studies have indicated that low-density, open-cell, metal foams may be suitable 

acoustic liner material for noise suppression in high by-pass engines. Metal foam response under 

simulated tip rub conditions was studied to assess whether its durability would be sufficient for the foam 

to serve both as a rub strip above the rotor as well as an acoustic treatment. Samples represented four 

metal alloys, nominal cell dimensions ranging from 60 to 120 cells per inch (cpi), and relative densities 

ranging from 3.4 to 10 percent. The resulting rubbed surfaces were relatively smooth and the open cell 

structure of the foam was not adversely affected. Sample relative density appeared to have significant 

influence on the forces induced by the rub event. Acoustic responses of various surface preparations were 

measured using a normal incidence tube. The results of this study indicate that the foam’s open-cell 

structure was retained after rubbing and that the acoustic absorption spectra variation was minimal. 

Introduction 

Open-cell, porous metals are used most commonly in battery electrodes, biomedical applications 

filtration systems, certain heat exchanger functions, and a variety of specialty applications (Ref. 1). Lowdensity metal foams are currently being explored for possible incorporation as a multifunctional 

component in an advanced turbine engine case. Metal foams have several characteristics that could be 

used in a sophisticated engine case design. Low-density foams have useful impact-absorption behavior 

because the local deformation is unconstrained by the surrounding material, unlike solid materials in 

which indention plasticity is constrained by surrounding material. This unconstrained deformation can 

result in a stress-strain plateau that leads to good energy absorption. Sandwich panels traditionally are 

high stiffness-to-density structures and metal foams are a viable candidate for sandwich materials at warm 

to high temperatures. Perhaps the most enticing characteristic is that open cell foams can dissipate sound 

energy and therefore offer potential as acoustic treatment in an engine case. Previous testing in a lowspeed, ducted fan test rig indicated potential noise reduction benefits by locating metal foam liners fore, 

aft, and over a fan rotor (Ref. 2). Some of the most promising acoustic absorption results were obtained 

with the metal foam was placed over the rotor or just in front of the rotor. If placed over the rotor, the 

foam would be vulnerable to damage caused by the tips of the fan blades. A rub event potentially could 

smear and effectively close the foam surface. Or damage to the foam surface could propagate and cause 

generalized loss of material. This paper summarizes testing which addressed the physical response of 

metal foam to simulated tip rub conditions. 
NASA/TM—2009-215818 2 

Nomenclature 

cpi Cells per inch 

OASPL Overall sound pressure level 

PACS Probe Actuator Control System 

PV Percent variability 

α Frequency-dependent absorption coefficient 

θ Normalized, frequency-dependent acoustic resistance 

χ Normalized, frequency-dependent acoustic reactance 

Materials 

The metal foams in this study were produced by Porvair Advanced Materials/Selee Corporation 

Hendersonville, North Carolina. These porous materials are formed by a metal sintering process where 

metallic particles are suspended in slurry and coated over an open-cell polymeric foam template. The 

polymeric foam skeleton vaporizes during heat treatment and the metallic particles sinter in its place. 

Lefebvre (Ref. 1) reserves the term metal foam for materials formed by a foaming process, but the term 

metal foam is used here due to the way the metal closely follows the structure of the foam template. This 

processing technique produces an open-cell structure that has a high degree of variation. Within-ligament 

porosity is prevalent because the metal does not fill the space of the original polymeric foam skeleton. 

The cell ligaments have varying thickness and it is common to find cell walls. The advantages of this 

processing technique include its ability to produce a wide range of alloy types, cell sizes, and relative 

densities. Moreover, this approach appears to be capable of near-net-shape production of parts suitable for 

fan case application. The flexibility of this processing route makes it attractive for the design of 

multifunctional components and especially useful for surveying variable responses as a function of alloy 

type, nominal cell size, and relative density. 

The foams studied here were made from the four different alloys listed below with nominal chemistry 

given in weight percent. 

 Haynes 25: Co-51, Ni-10, Cr-20, W-15, Fe-3max, Mn-1.5, Si-0.4max, C-0.10 

 FeCrAlY: Fe-73, Cr-22, Al-5, Y-trace 

 316 stainless steel: Fe-66, Cr-17, Ni-12, Mo-2, Mn-2, Si-1, C-0.1, P-0.04, S-0.03 

 314 stainless steel: Fe-52, Cr-24, Ni-20, Mn-2, Si-2, C-0.2, P-0.04, S-0.03 

Haynes 25 (Haynes International Inc.) was selected because previous research has shown that it has 

very good specific impact resistance as well as oxidation resistance that would allow application in hotter 

sections of the engine (Ref. 3). FeCrAlY, also known as Fecralloy (AEA Technology, Harwell) or 

Kanthal (Kanthal Group companies), is an iron alloy that has good corrosion/oxidation resistance and is a 

common heat exchanger product. The 316 and 314 stainless steels are austenitic alloys with good 

corrosion resistance. 

The cell size of an irregularly shaped foam network can be measured in several ways. The linear 

density of cells (cells per inch) is one convenient expression of cell size but it would not give the same 

value as a statistical average of cell diameters. The cell size of these foams were reported by the 

manufacturer as “pores per inch”. In-house measurements have found that the cell distribution as 

determined by counting cells (a.k.a. pores) along a linear dimension was finer than that reported by the 

manufacturer (higher number of cells per inch). It is possible that the manufacturer values were based on 

the pores per inch of the template foam and that the higher measured values are due to shrinkage during 

sintering. For simplicity, the manufacture’s reported cell size will be used in this report. Therefore care 

must be exercised when comparing the results of this report with foams produced by other manufacturing 
NASA/TM—2009-215818 3 

techniques. The term “cells per inch” (cpi) will be used to distinguish the macroscopic cell architecture 

from the finer pores found within the cell ligaments. 

The relative density is the density of the porous material normalized by the parent alloy density. 

Foams with relative densities from 3 to 10 percent were used in this study. The average relative densities 

measured in-house agreed well with those reported by the manufacturer. However, in-house 

measurements have found that localized density variations of 0.5 percent relative density are not 

uncommon. That is, the average relative density of 1 cm cubes from the same parent panel might be 

6.1 percent and the measured values fall between 6.9 and 5.6 percent. 

Simulated Tip Rub Testing 

Tip Rub Sample Materials 

Metal foam samples were selected to screen the rub response as a function of alloy type, cell size, and 

relative density. Table I summarizes the foam materials tested. The sample identification nomenclature 

includes reference to the alloy type, cells per inch, and an alphanumeric panel designation. In the sample 

identification nomenclature, FEC is FeCrAlY, HAY is Haynes 25, SS314 is 314 stainless steel, and 

SS316 is 316 stainless steel. Rectangular 127 by 38 mm (5 by 1.5 in.) samples were cut from the panels 

using a band saw. In general, only one rub test was performed per sample. Repeat tests, designated by (r) 

in Table I, were performed on three samples by flipping the sample axial in the specimen holder. The cell 

size and relative density values listed in Table I are those provided by the manufacturer; these are nominal 

values that must be applied with caution, as discussed in the previous section. The panels available for 

making rub samples had different initial thicknesses. This difference in sample thickness was considered 

to be an insignificant factor in the rub testing. 

TABLE I.—FOAM SAMPLES USED IN RUB TESTING 

[Alloys used were FeCrAlY (FEC), Haynes 25 (HAY), and stainless steel (SS314 or SS316). 

Cell size (cells per inch) and relative density values are as provided by manufacturer.] 

Alloy/ID Cell per inch, 

cpi 

Panel relative density, 

percent 

Thickness, 

mm 

HAY-80-E 80 4.6 12.7 

HAY-80-G 80 6.4 25.4 

HAY-100-F 100 6.5 12.7 

 

SS316-60-B(r) 60 5.3 12.7 

SS314-80-C(r) 80 4.0 8.9 

SS316-80-D 80 5.3 6.4 

SS316-120-H 120 5.2 6.1 

 

FEC-60-607xviii 60 4.7 12.5 

FEC-60-B 60 5.3 10.2 

FEC-60-607Axvi 60 9.7 12.5 

FEC-80-C 80 2.9 6.6 

FEC-80-D(r) 80 4.0 8.9 

FEC-80-607Bxv 80 4.8 12.5 

FEC-80-E 80 5.5 25.4 

FEC-80-607Di 80 10.0 12.5 

FEC-120-807Div 120 4.7 12.5 

 
NASA/TM—2009-215818 4 

Tip Rub Experimental Procedures 

The tip rub test was performed on an apparatus designed to simulate the rubbing of blade tips against 

the surrounding wall material (Ref. 4). The features of this test rig included 

 Two-component load balance for measuring radial and tangential loads in the rub strip 

 Controlled, variable rotor speeds 

 Real-time control of the linear velocity of the sample rub strip material as it contacted the rotating 

disk teeth 

The rotating portion of the test system consisted of a commercially available router motor, a high 

speed spindle, and a rotor disk with twelve Ti-6Al-4V teeth. The rotor assembly balance was monitored 

using two accelerometers mounted on the spindle housing. The rotor disk was designed such that the teeth 

are replaceable and thus other blade materials or geometries could be simulated. The rub-material sample 

was mounted in a two-axis load frame on motorized traverse structure that provides a controlled insertion 

into the rotor plane. The load frame was a set of three plates that were sandwiched together with lowfriction miniature slides. The slides were configured so that the top plate could freely move in twodimensions while the bottom plate remained fixed. Two load cells were attached to the middle plate. 

These load cells were configured to measure force perpendicular to each other. The top and bottom plates 

where then connected to the appropriate load cells to create a two-component load balance that measured 

x and y loads in the sample corresponding to the tangential and radial directions with respect to the rotor. 

Three combinations of load cells were used in these tests. Initial tests were conducted with load cell 

capacities of 445 N (100 lbf) in the x direction and 2224 N (500 lbf) in the y direction. The second set of 

tests was conducted with load cells of 44.5 N (10 lbf) and 445 N (100 lbf), and the last tests were 

conducted with load cells of 44.5 N (10 lbf) in both directions. 

The vertical position of the rub strip sample was controlled by a stepper motor and verified by a linear 

variable displacement transducer. Position commands were sent to the stepper motor using the in-house 

designed Probe Actuator Control System (PACS) that allowed for independent control of position and 

traverse speed. Labview (National Instruments Corporation) software was used to collect the load cell, 

displacement transducer, and tachometer responses of the system. 

Rectangular samples were supported in an adjustable depth frame to accommodate a range of 

specimen thickness. A mounted sample prior to rub testing is shown in Figure 1. The rub tests were 

performed using a rotational speed of nominally 10,000 revolutions per minute (rpm), which corresponds 

to a tip speed of 93 m/s (305 ft/s, 3665 in./s) for the 178 mm (7 in.) diameter rotor. The foam sample 

traversed down along the y-axis (radial direction relative to rotor) at a traverse rate of 6.4 mm/s 

(0.25 in./s). One second rub tests were performed resulting in an incursion that was approximately 6.4 

mm (0.25 in.) at the deepest point of the arc. An illustration of the sample, rotor, and force directions is 

shown in Figure 2. The rotor speed was recorded during the rub event in addition to x and y forces 

induced in the foam sample. 

Tip Rub Results 

The primary observation for these tests was that all samples wore gracefully with minimal loading 

induced in the x and y directions. The foam cut away smoothly and the foam did not appear to smear or 

lose its open cell nature. There were no signs of tearing or gouging regardless of whether the specimen 

thickness was narrower or wider than the 12.7 mm wide rotor tips. Figure 3 shows an example of a thin 

wall of foam that was left behind as the rotating tips rubbed away adjacent foam. The three repeat tests 

were performed on specimen edges that were partially or entirely cut with a band saw blade. These three 

tests were intended to simulate rubbing across a seam. Figure 4 shows the resulting rub surface after 

rubbing across a saw cut. In each case there was no noticeable gouging or crumbling at the cut edges. 
NASA/TM—2009-215818 5 

Figure 3.—FEC-80-607Bxv rub specimen showing smooth, semi-circular wear pattern and ~1 mm thick wall of 

unrubbed foam, indicated with red arrow. The specimen was about the same thickness as the blades were 

wide, ~12.5 mm. However, the specimen was offset sufficiently to leave a thin section of material unrubbed. 

Figure 4.—Repeat rub test on second edge of specimen SS314-80-C. Prior to rub testing a slot was 

machined in the sample to simulate rubbing over a seam. Remnants of slot visable in center of semicircular wear pattern. The specimen is 9 mm thick and the rubbed area is approximately 6.3 mm deep. 

Figure 1.—Metal foam rub sample mounted in two-axis load 

frame. 

Figure 2.—Illustration of rub test 

set up. Dashed area indicates 

area where material is removed.

X

Y

Foam sample

Bladed rotor

Rubbed surface

Unrubbed surface
NASA/TM—2009-215818 6 

The forces aligned with the tangential (x) direction, radial (y) rub direction, and the tachometer 

responses are summarized in Table II and plotted in Appendix A. An example of the rub-induced loading 

forces is shown in Figure 5. In most cases, the measured load response was low relative to the overall 

signal. In order to characterize the change due to the rub event, the average signal before nominal touch 

(negative position on abscissa) was compared to the average signal in the last 1.3 mm (0.05 in.) of the 

rub. This increase in average load and decrease in average rotor speed is listed in Table II and with the 

appendix graphs. Initially, relatively high capacity load cells were used to ensure that load cells would not 

be damaged. The respective load cell capacities are listed for test sample since the measurement fidelity is 

best for the tests with the lower capacity load cells. The tachometer readings were not available for five of 

the rub tests. The three samples that were tested twice, once on each of the 127 mm sample edges, are 

indicated in Table II with (r) and had similar force and tachometer responses. 

TABLE II.—SAMPLE RUB RESPONSE FOR FeCrAlY (FEC), HAYNES 25 (HAY), AND STAINLESS STEEL 

(SS314 OR SS316) FOAMS IN ASCENDING ORDER OF RELATIVE DENSITY; RELATIVE DENSITY BASED 

ON RUB SAMPLE MASS AND DIMENSIONS 

Alloy/ID Sample relative density, 

percent 

Load cell x-force increase, 

N 

y-force increase, 

N 

Speed decrease, 

rpm 

FEC-80-D 3.4 a1 0.5 0.9 21 

FEC-80-D(r) 3.4 c3 0.4 0.6 19 

FEC-80-C 3.5 1 0.1 1.0 8 

SS314-80-C 4.3 b2 0.9 1.0 39 

SS314-80-C(r) 4.3 3 0.8 1.1 22 

FEC-60-B 4.4 2 0.4 0.5 38 

FEC-60-607xviii 4.5 3 1.3 2.2 --- 

FEC-80-607Bxv 4.6 3 1.3 2.4 --- 

FEC-80-E 4.8 2 1.5 2.3 65 

SS316-60-B 5.0 1 1.0 1.0 54 

SS316-60-B(r) 5.0 3 1.8 2.6 74 

FEC-120-807Div 5.1 3 0.6 0.5 --- 

HAY-80-G 5.3 2 1.6 2.9 65 

SS316-120-H 5.4 2 0.8 0.3 38 

SS316-80-D 5.9 1 1.8 3.0 75 

HAY-100-F 6.2 1 4.1 7.9 109 

HAY-80-E 6.6 1 4.9 10.4 254 

FEC-60-607Axvi 9.5 3 6.8 20.0 --- 

FEC-80-607Di 9.9 3 9.5 32.3 --- 

a

445 N load cell x-axis, 2224 N load cell y-axis b

44.5 N load cell x-axis, 445 N load cell y-axis c

44.5 N load cell x-axis, 44.5 N load cell y-axis 

 
NASA/TM—2009-215818 7 

Figure 5.—The tangential (x) and radial (y) rub forces measured on sample HAY-80-E. 

Red boundary lines illustrate regions for averages of the signal prior to touch and of 

the final 1.3 mm that were compared to determine the mean force increase. Red 

arrows indicate magnitude of shift. 

In addition to observing the surface damage, the general loading responses were compared based on 

the alloy type, cell size, and relative density. The relative densities calculated from the measured density 

of the 127 by 38 mm rub samples were used in analyzing the data rather than that quoted by the 

manufacturer for the entire panel and it is this sample relative density that is listed in Table II. Figure 6 

shows the response of the induced tangential and radial forces with respect to cell size and alloy. There 

are no discernable trends in these plots. Figure 7 shows the load forces plotted instead as a function of 

sample relative density. There is a distinct increase in tangential and radial loads as the sample density 

increases. Table II lists the samples and test results in order of ascending sample relative density. From 

this data set it appears that relative density is a stronger controlling factor than either cell size or alloy 

type, but interaction effects certainly cannot be ruled out based on this small sample set. 

Figure 8 plots all the FeCrAly alloy tangential (x) and radial (y) rub forces as a function of sample 

relative density. The filled symbols and the solid regression lines represent the 80 cpi data only. The open 

symbols include the 60 and 120 cpi samples and the dashed regression lines include all the data. The 

coefficients of simple determination (R2

 values) are all above 0.9, where 1.0 would be a perfect fit. It is 

interesting that the regression fits are better if only the 80 cpi data are considered. Also, all the 60 and 

120 cpi data points fall below the 80 cpi regression lines. This implies that for a given relative density, the 

80 cpi samples have stronger ligaments than the other samples. One possible explanation is that the 

processing parameters are better optimized for the 80 cpi foam thus producing foams with better ligament 

integrity and strength. 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 4.9 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Position (mm)

Increase in mean = 10.4 N
NASA/TM—2009-215818 8 

Figure 6.—Rub induced (a) tangential and (b) radial forces as a function of foam cell size. 

Figure 7.—Rub induced (a) tangential and (b) radial forces as a function of sample relative density.

Cell Size (cpi)

50 60 70 80 90 100 110 120 130

Tangential force (N)

0

1

2

3

4

5

6

7

8

9

10

(a)

FeCrAlY SS Haynes 25

Cell Size (cpi)

50 60 70 80 90 100 110 120 130

Radial force (N)

0

4

8

12

16

20

24

28

32

(b)

Sample Relative Density (%)

4 6 8 10

Tangential force (N)

0

1

2

3

4

5

6

7

8

9

10

FeCrAlY SS Haynes 25

(a)

Sample Relative Density (%)

4 6 8 10

Radial force (N)

0

4

8

12

16

20

24

28

32 (b)
NASA/TM—2009-215818 9 

Figure 8.—Rub induced tangential (a) and radial (b) forces as a function of sample relative density for FeCrAlY 

foams. Trendlines have coefficients of simple determination (R2

 values) of (a) 0.988 for 80 cpi, 0.953 for all 

samples and (b) 0.967 for 80 cpi, 0.910 for all samples. 

Acoustic Characterization Testing 

Acoustic Characterization Materials 

Acoustic test coupons were obtained from a single 25.4 mm (1 in.) thick panel of Haynes 25 foam 

with nominally 80 cpi and 4.5 percent relative density. The samples were 50.8 by 50.8 mm (2 by 2 in.) 

with approximate thicknesses of 20 mm (0.8 in.) and 10 mm (0.4 in.). Two thicknesses were used in these 

tests to allow determination of characteristic impedance and propagation constant via the two thickness 

techniques described by Smith and Parrott (Ref. 5). Determination of these key material acoustic 

properties can be used to establish the metal foam’s acoustic uniformity. The square faces of the 

specimens were prepared by five techniques. The as-received surface is the as-cast surface as provided by 

the manufacturer. Wire electro-discharge machining (EDM), plunge EDM, and band-saw cutting 

represent three possible machining techniques. Table III describes the face preparation of the thirteen 

specimens used in the acoustic study. 

TABLE III.—IDENTIFICATION, SURFACE CONDITION, AVERAGE THICKNESS, 

AND RELATIVE DENSITY FOR IMPEDANCE TUBE SAMPLES 

ID Face A Face B Thickness, 

mm 

Relative density, 

percent 

H01 Band saw As–received 20.8 5.6 

H02 Band saw As–received 11.2 4.3 

H03 Band saw As–received 11.3 3.8 

H04 Plunge EDM Wire EDM 20.4 6.2 

H05 Plunge EDM Wire EDM 20.4 6.7 

H06 Plunge EDM Wire EDM 10.2 6.1 

H07 Tip rub Wire EDM 20.2 6.7 

H08 Tip rub Wire EDM 20.7 7.2 

H09 Tip rub Wire EDM 10.1 4.8 

H10 Wire EDM As–received 20.3 4.5 

H11 Wire EDM As–received 20.9 4.5 

H12 Wire EDM Wire EDM 10.1 4.4 

H13 Wire EDM Wire EDM 10.6 6.5 

Sample Relative Density (%)

4 6 810

Tangential force (N)

0

1

2

3

4

5

6

7

8

9

10 (a)

Sample Relative Density (%)

4 6 810

Radial force (N)

0

4

8

12

16

20

24

28

32

36 (b)
NASA/TM—2009-215818 10 

In order to achieve a rubbed surface over the square face, the two-axis load frame was replaced with a 

square specimen holder that was translated in two axes by PACS controlled stepper motors. Approximately 

2 mm of the specimen surface was rubbed away in sequential 10 mm wide passes. Figure 9 compares 

scanning electron micrographs of as-received, wire EDM, plunge EDM, band-saw cut, and rubbed surfaces. 

Both the as-received and wire EDM samples had cell walls and ligaments that ended abruptly at the surface. 

The plunge EDM process resulted in localized melting that gave the appearance of coarsening the surface. 

The band-saw cut and rubbed surfaces appeared to have some cell walls or ligaments that were folded over. 

 (a) (b) 

 (c) (d) 

 (e) 1 mm 

Figure 9.—Haynes 25 metallic foam, 80 cpi, sample surfaces (a) as-received, (b), wire EDM, (c) plunge EDM, 

(d) band saw, and (e) tip rubbed. 
NASA/TM—2009-215818 11 

Acoustic Characterization Experimental Procedures 

The frequency-dependent acoustic impedance was determined using the NASA Langley Normal 

Incidence Tube (Ref. 6). Tests were conducted using a broadband noise source at two overall sound 

pressure levels (OASPL), 120 and 140 dB. The impedance spectra were calculated using the multipoint 

method which employs fitting techniques to model acoustic wave propagation based on measured 

pressure data. Once the standing wave generated by the reflection process is described, the complex 

reflection factor and related acoustic parameters can be calculated. The impedance spectra then were used 

to compute the corresponding absorption coefficient spectra using Equation (1): 

2 2 ( 1)

4

   



  (1) 

where  represents the frequency-dependent absorption coefficient and  and  represent the 

corresponding normalized, frequency-dependent, acoustic resistance and reactance, respectively, as 

defined in Reference 6. Acoustic resistance and reactance are the real and imaginary components of 

acoustic impedance and are normalized by the density and sound speed of air, 415 MKS Rayls. 

The percent variability, PV, between the sample faces was computed using Equation (2): 

*100

( )

( ) ( ) PV

3000 Hz

1500 Hz



 

   

f A

A B

f

f f

 (2) 

where and B represent the absorption coefficients at the A and B faces, respectively. The 

computations are based on the results at and above 1500 Hz, where the results were most consistent since 

an insufficient number of averages were acquired to achieve smooth results at the lower frequencies. 

Acoustic Characterization Results 

Acoustic absorption coefficient comparisons found little variation due to surface preparation. 

Figure 10 shows the spectra for one of the samples with one tip-rubbed and one wire EDM surface. 

Graphs comparing the calculated absorption coefficient spectra for all the samples are provided in 

Appendix B. Comparisons of the educed impedance at the two overall sound pressure levels demonstrated 

that the impedance of this foam is nearly linear and therefore the acoustic properties are independent of 

the sound pressure level. This result agrees with previous measurements on similar foam (Ref. 2). Note 

that the absorption coefficient for these samples was affected by the specimen thickness with the thicker 

specimens having greater absorption. 

A review of the figures provided in Appendix B reveals that the absorption coefficient spectra 

measured on each face were nearly identical for each specimen, with the exception of sample H08. This 

sample, with tip-rubbed and wire EDM faces, was the only specimen with faces that yielded notably 

different absorption coefficient spectra. Moreover, the companion tip-rub specimens (specimen H07 and 

H09) appeared to have relatively low face-to-face variation. 

As a means of quantifying these face-to-face comparisons, Figure 11 provides percent variability 

between absorption coefficient spectra measured on each face for each of the thirteen (13) test specimens. 

For all but one sample (H08), the variability is shown to be less than 5 percent between the two faces of 

each sample. Of particular interest is the comparison of results for the H07 and H08 samples, which were 

configured to be virtually identical. The H07 sample is shown to have only 2.7 percent variability 

between the absorption coefficient spectra measured on each face, while the H08 has 7.8 percent 

variability. The H04 and H05 samples were also configured to be virtually identical, each with plunge 

EDM and wire EDM faces. The H04 and H05 samples have 1.2 and 2.7 percent variability, respectively. 

The two specimens with two wire EDM faces (H12 and H13) had 3.7 and 3.2 percent variability between 
NASA/TM—2009-215818 12 

Figure 10.—Absorption coefficient spectra for 20 mm thick specimen H07 comparing tip rub 

(Face A) and Wire EDM (Face B) faces. 

Figure 11.—Face-to-face absorption coefficient spectra variability acquired with 140 dB 

OASPL broadband noise source. 
NASA/TM—2009-215818 13 

 

Figure 12.—Absorption coefficient spectra of “A” faces for 20 mm thick specimens 

with 140 dB OASPL broadband noise source. 

their two faces. This inconsistency, in which some of the identical samples give the expected similar 

levels of variability while others do not, is believed to be at least partially due to the thickness of the 

samples used in this study. By using relatively thin samples, the absorption coefficient is relatively low. 

Hence, very small changes can result in noticeable discrepancies in the percent variability estimates. 

Regardless, these results indicate that acoustic attenuation properties are not greatly dependent on surface 

preparation. This result is consistent with the surface being only a small fraction of the sample volume 

interacting with the sound energy. 

Comparison of the absorption coefficient spectra also shows that the response was similar for 

specimens with the same nominal thickness. Figure 12 shows that the absorption response for the “A” 

Faces of the 20 mm thick specimens. Because the specimens were from the same plate, similar behavior 

is expected. The 20 mm thick specimens with a wire EDM surface for Face A also happened to have a 

slightly lower density and that is probably the reason that their absorption response fell slightly lower 

than the corresponding results for the other three surfaces shown in Figure 12. 

Conclusions 

A bench-top test rig was used to simulate the interaction between rotating fan blades and candidate 

metal foam liner materials. The small magnitude of loading and the minimal surface damage suggest that 

metal foams could be used over an aircraft engine rotor without a cover sheet. The forces induced by the 

rubbing were less than 3 N (~0.65 lbf) in both the x and y (tangential and radial) directions for foams with 

nominally 5 percent relative density and up to 30 N (~6.5 lbf) in the radial direction for foams with about 

10 percent relative density. Force variations based on alloy type and cell size were not readily apparent 

and appeared to be secondary effects compared to the relative density. 

Visual inspection of the rubbed surfaces revealed no gross change in the cell structure, but the 

surfaces certainly appeared more rough and dull than the as-received panel faces or surfaces cut by a 

precise method such as wire electro-discharge machining (EDM). There was no apparent gouging or 

smearing, even when rubbing over gaps that simulated seams. Microscopy observation of the rubbed 

surfaces showed some folding of ligaments and cell-wall segments. 
NASA/TM—2009-215818 14 

One potential way to quantify significant changes in surface morphology is to measure the change in 

acoustic impedance, which is an intrinsic acoustic property of the metal foam and can be used to compute 

the absorption coefficient. The frequency-dependent acoustic absorption coefficients of rubbed surfaces 

were compared to that of as-received, wire EDM, plunge EDM, and band-saw cut surfaces. All samples 

had nearly the same response except for one of the rubbed surfaces, which was deemed to be an outlier. 

These results suggest that there is no appreciable change in acoustic behavior for a simulated tip-rub 

event. Likewise, candidate machining techniques were equally successful based solely on acoustic 

absorption response. 

Simulated engine testing has suggested that metal foam acoustic liners offer absorption benefits when 

positioned directly over the rotating fan blades (Ref. 2). The results presented here indicate that the 

metallic foams would not be adversely affected by a rub-event. These results suggest that metallic foam 

acoustic treatments can be considered in future system studies as an over-the-rotor treatment without a 

protective face sheet. 

References 

1. Louis-Philippe Lefebvre, John Banhart, and David C. Dunand, “Porous Metals and Metallic Foams: 

Current Status and Recent Developments,” Advanced Engineering Materials, 10(9), 2008, pp. 775–

787. 

2. Daniel L. Sutliff and Michael G. Jones, “Foam-Metal Liner Attenuation of Low-Speed Fan Noise,” 

AIAA–2008–2897, presented at 14th Aeroacoustics Conference sponsored by American Institute of 

Aeronautics and Astronautics (AIAA) and Council of European Aerospace Societies (CEAS), 

Vancouver, British Columbia, Canada, May 5–7, 2008 (also available as NASA/TM—2008-215227 

from http://gltrs.grc.nasa.gov). 

3. Mohan G. Hebsur, unpublished research, NASA Glenn Research Center. 

4. Jose C. Gonsalez, “Turbomachinery Rub Strip Impact Tests,” presented at AIAA Turbine Test 

Facility Working Group (TTFWG) Meeting, NASA Glenn Research Center, Cleveland, Ohio, 

November 3, 1999. 

5. Charles D. Smith and Tony L. Parrott, “Comparison of Three Methods for Measuring Acoustic 

Properties of Bulk Materials,” Journal of the Acoustic Society of America, 74(5), 1983, pp. 1577–

1582. 

6. Michael G. Jones and Tony L. Parrott, “Evaluation of a Multi-Point Method for Determining 

Acoustic Impedance,” Mechanical Systems and Signal Processing, 3(1), 1989, pp. 15–35. 
NASA/TM—2009-215818 15 

Appendix A.—Tip Rub 

HAY-80-E: 80 cpi, panel * = 4.6 percent, specimen * = 6.6 percent, 12.7 mm thick 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 10.4 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 4.9 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft Speed (rpm)

Position (mm)

Drop in mean = 254 rpm
NASA/TM—2009-215818 16 

HAY-80-G: 80 cpi, panel * = 6.4 percent, specimen * = 5.3 percent, 25.4 mm thick 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 1.6 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 2.9 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft Speed (rpm)

Position (mm)

Drop in mean = 65 rpm
NASA/TM—2009-215818 17 

HAY-100-F: 100 cpi, panel * = 6.5 percent, specimen * = 6.2 percent, 12.7 mm thick 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 4.1 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 7.9 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 109 rpm
NASA/TM—2009-215818 18 

SS316-60-B: 60 cpi, panel * = 5.3 percent, sample * = 5.0 percent, 12.7 mm thick 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 1.0 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 1.0 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 54 rpm
NASA/TM—2009-215818 19 

SS316-60-B: 60 cpi, repeat 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 1.8 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 2.6 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 74 rpm
NASA/TM—2009-215818 20 

SS314-80-C: 80 cpi, panel * = 4.0 percent, sample * = 4.3 percent, 8.9 mm thick 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 0.9 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 1.0 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 39 rpm
NASA/TM—2009-215818 21 

SS314-80-C: 80 cpi, repeat 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 0.8 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 1.1 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 22 rpm
NASA/TM—2009-215818 22 

SS316-80-D: 80 cpi, panel * = 5.3 percent, sample * = 5.9 percent, 6.4 mm thick 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 1.8 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 3.0 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 75 rpm
NASA/TM—2009-215818 23 

SS316-120-H: 120 cpi, panel * = 5.2 percent, sample * = 5.4 percent, 6.1 mm thick 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 0.8 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 0.3 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 38 rpm
NASA/TM—2009-215818 24 

FEC-60-607xviii: 60 cpi, panel * = 4.7 percent, sample * = 4.5 percent, 12.5 mm thick 

(shaft speed not recorded) 

 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 1.3 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Position (mm)

Increase in mean = 2.2 N
NASA/TM—2009-215818 25 

FEC-60-B: 60 cpi, panel * = 5.3 percent, sample * = 4.4 percent, 10.2 mm thick 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 0.4 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 0.5 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 38 rpm
NASA/TM—2009-215818 26 

FEC-60-607Axvi: 60 cpi, panel * = 9.7 percent, sample * = 9.5 percent, 12.5 mm thick 

(shaft speed not recorded) 

 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 6.8 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Position (mm)

Increase in mean = 20 N
NASA/TM—2009-215818 27 

FEC-80-C: 80 cpi, panel * = 2.9 percent, sample * = 3.5 percent, 6.6 mm thick 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 0.1 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 1.0 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 8 rpm
NASA/TM—2009-215818 28 

FEC-80-D: 80 cpi, panel * = 4.0 percent, sample * = 3.4 percent, 8.9 mm thick 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 0.5 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 0.9 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 21 rpm
NASA/TM—2009-215818 29 

FEC-80-D: 80 cpi, repeat 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 0.4 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 0.6 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 19 rpm
NASA/TM—2009-215818 30 

FEC-80-607Bxv: 80 cpi, panel * = 4.8 percent, sample * = 4.6 percent, 12.5 mm thick 

(shaft speed not recorded) 

 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 1.3 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Position (mm)

Increase in mean = 2.4 N
NASA/TM—2009-215818 31 

FEC-80-E: 80 cpi, panel * = 5.5 percent, sample * = 4.8 percent, 25.4 mm thick 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 1.5 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Increase in mean = 2.3 N

9600

9700

9800

9900

10000

10100

10200

-2 -1 0 1 2 3 4 5 6 7

Shaft speed (rpm)

Position (mm)

Drop in mean = 65 rpm
NASA/TM—2009-215818 32 

Fe-80-607Di: 80 cpi, panel * = 10.0 percent, sample * = 9.9 percent, 12.5 mm thick 

(shaft speed not recorded) 

 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 9.5 N

-5

0

5

10

15

20

25

30

35

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Position (mm)

Increase in mean = 32.3 N
NASA/TM—2009-215818 33 

Fe-120-807Div: 120 cpi, panel * = 4.7 percent, sample * = 5.1 percent, 12.5 mm thick 

(shaft speed not recorded) 

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Tangential force (N)

Increase in mean = 0.6 N

-15

-10

-5

0

5

10

15

20

25

-2 -1 0 1 2 3 4 5 6 7

Radial force (N)

Position (mm)

Increase in mean = 0.5 N Increase in mean = 0.6 N
 
NASA/TM—2009-215818 35 

Appendix B.—Tip Rub 

Sample H01 

ID Face A Face B Thickness

H01 Band Saw As-Received 20.8 

Sample H02 

ID Face A Face B Thickness

H02 Band Saw As-Received 11.2 
NASA/TM—2009-215818 36 

Sample H03 

ID Face A Face B Thickness

H03 Band Saw As-Received 11.3 

Sample H04 

ID Face A Face B Thickness

H04 Plunge EDM Wire EDM 20.4 

 
NASA/TM—2009-215818 37 

Sample H05 

ID Face A Face B Thickness

H05 Plunge EDM Wire EDM 20.4 

Sample H06 

ID Face A Face B Thickness

H06 Plunge EDM Wire EDM 10.2 
NASA/TM—2009-215818 38 

Sample H07 

ID Face A Face B Thickness

H07 Tip Rub Wire EDM 20.2 

Sample H08 

ID Face A Face B Thickness

H08 Tip Rub Wire EDM 20.7 

 
NASA/TM—2009-215818 39 

Sample H09 

ID Face A Face B Thickness

H09 Tip Rub Wire EDM 10.1 

Sample H10 

ID Face A Face B Thickness

H10 Wire EDM As-Received 20.3 

 
NASA/TM—2009-215818 40 

Sample H11 

ID Face A Face B Thickness

H11 Wire EDM As-Received 20.9 

Sample H12 

ID Face A Face B Thickness

H12 Wire EDM Wire EDM 10.1 
NASA/TM—2009-215818 41 

Sample H13 

ID Face A Face B Thickness

H01 Wire EDM Wire EDM 10.6 
REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188 

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB 

control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 

1. REPORT DATE (DD-MM-YYYY)

01-12-2009 

2. REPORT TYPE

Technical Memorandum 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Simulated Tip Rub Testing of Low-Density Metal Foam 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Bowman, Cheryl, L.; Jones, Michael, G. 

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

WBS 561581.02.08.03.15.03 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

8. PERFORMING ORGANIZATION

 REPORT NUMBER

E-17080 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

10. SPONSORING/MONITOR'S

 ACRONYM(S)

NASA 

11. SPONSORING/MONITORING

 REPORT NUMBER

NASA/TM-2009-215818 

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited 

Subject Categories: 26 and 71 

Available electronically at http://gltrs.grc.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

Preliminary acoustic studies have indicated that low-density, open-cell, metal foams may be suitable acoustic liner material for noise 

suppression in high by-pass engines. Metal foam response under simulated tip rub conditions was studied to assess whether its durability 

would be sufficient for the foam to serve both as a rub strip above the rotor as well as an acoustic treatment. Samples represented four metal 

alloys, nominal cell dimensions ranging from 60 to 120 cells per inch (cpi), and relative densities ranging from 3.4 to 10 percent. The 

resulting rubbed surfaces were relatively smooth and the open cell structure of the foam was not adversely affected. Sample relative density 

appeared to have significant influence on the forces induced by the rub event. Acoustic responses of various surface preparations were 

measured using a normal incidence tube. The results of this study indicate that the foam’s open-cell structure was retained after rubbing and 

that the acoustic absorption spectra variation was minimal.

15. SUBJECT TERMS

Metal foams; Porous materials; Abrasion resistance; Acoustic attenuation 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF

 ABSTRACT

UU 

18. NUMBER

 OF 

 PAGES

46 

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email:help@sti.nasa.gov) 

a. REPORT

U 

b. ABSTRACT

U 

c. THIS 

PAGE

U 

19b. TELEPHONE NUMBER (include area code)

443-757-5802 

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18