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Fabrication of lightweight structural panels through

ultrasonic consolidationJ. George

a& B. Stucker

a

aDepartment of Mechanical Engineering, Utah State University, Logan, Utah, 84322-4130,

USA

Published online: 16 Feb 2007.

To cite this article:J. George & B. Stucker (2006) Fabrication of lightweight structural panels through ultrasonic

consolidation, Virtual and Physical Prototyping, 1:4, 227-241, DOI: 10.1080/17452750601106799

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Fabrication of lightweight structural panels through ultrasonicconsolidation

J. GEORGE and B. STUCKER*

Department of Mechanical Engineering, Utah State University, Logan, Utah, 84322-4130, USA

A fundamental investigation of the feasibility of producing lightweight structural panels

using ultrasonic consolidation (UC) was undertaken. As a novel solid freeform

fabrication technology, UC utilizes both additive ultrasonic joining and subtractive

CNC milling to enable the creation of complex aluminum structures with internal

geometry at or near room temperature. A series of experiments were performed to

understand the issues associated with sandwich structure fabrication using UC, including

peel test experiments which evaluated the bond strength for various geometric

configurations. The honeycomb lattice was found to offer the best core configuration

due to its ability to resist vibration from the sonotrode and provide adequate support for

pressure induced by the sonotrode. UC was found to be capable of producing lightweight

and stiff structures, including honeycomb and other sandwich panels, without the use of

adhesives. An effective manufacturing process plan for fabricating structural panels was

developed. A case study was performed on a deck built for the TOROID small satellite

spacecraft. The fabricated deck was tested for mechanical integrity. Finally, the cost and

benefits of utilizing UC for lightweight structural panels versus traditional fabrication

methods are discussed.

Keywords: Ultrasonic consolidation; Honeycomb; Sandwich panel; Lightweight panels;

Composite panel

1. Introduction

In mechanical design, a common design goal is to achieve

maximum load bearing capability or stiffness from a given

structural panel with the least amount of weight. This is

true throughout applications in the transportation and

aerospace industries. One application where this is particu-

larly true is in the area of small satellite design. Due to the

exorbitant costs associated with launching satellites, de-

signers seek greater functionality from their satellites at

reduced overall mass, volume and cost, and with less time

needed for design, production and testing (Kingston 2005,

Rodgers 2005). Over the past few years, small satellites have

emerged as a potentially disruptive technology for many

space missions (Lewin 2004). The potential disruptiveness

of small satellites, however, has been significantly hindered

by the cost and time involved when using traditional

manufacturing techniques for satellite fabrication, which

still predominate in industry (Panetta 1998). Traditional

methods of machining and assembly make every satellite

produced one-of-a-kind. Advanced additive manufacturing

techniques provide a potentially important shift in satellite

design and manufacturing, if an automated and repeatable

process can be developed for manufacturing satellite

systems (Mosher 2004).

One additive manufacturing technique that has tremen-

dous potential for fabricating the lightweight structural

panels needed for small satellites is ultrasonic consolidation

(UC). This technology, developed by Solidica Inc, USA,

uses a sonotrode to join layers of metal foils using

ultrasonic vibration (White 2002). The UC machine is an

*Corresponding author. Email: [email protected]

Virtual and Physical Prototyping, Vol. 1, No. 4, December 2006, 227 241

Virtual and Physical PrototypingISSN 1745-2759 print/ISSN 1745-2767 online # 2006 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/17452750601106799


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integrated machine tool that incorporates an additive

ultrasonic joining head with a computer numerically

controlled milling machine (CNC). Because features may

be machined into the deposited layers and subsequently

covered with more layers, it is possible to create parts with

internal features. This is desirable, as sensors, electronics,

thermal regulators, and simple voids can be integrated to

create a multifunctional lightweight panel. In theory, it ispossible, using UC, to create a printed satellite which

offers reproducibility and functionality never before seen in

the satellite industry. This vision for printing satellite

panels, as illustrated in figure 1 where a functional satellite

panel is fabricated in a series of steps, was the impetus for

the current investigation. As the first step toward this

vision, the capabilities of UC with respect to fabrication of

lightweight structural panels were investigated.

2. Background

2.1 Lightweight structural panel design and fabrication

Over the past 40 years, many designs for lightweight

structural panels have been developed. Vinson (1999) has

applied a methodology to create minimum-weight panels.

He presented the idea that a panel contains many failure

modes, any of which could cause failure of the entire panel.

Different features of the panel have an associated weight

which varies directly with its load carrying capabilities.

When a failure occurs at any one location, any portions

which have not failed are essentially dead weight. Thus it

is apparent that a minimum-weight panel is one in which all

of the failure modes occur simultaneously.Another important aspect of structural efficiency and

weight was presented by Osgood (1966). He noted that an

optimum structure would weigh nothing and posses infinite

strength. Because neither of these is attainable, it is

necessary to define the method which will make optimiza-

tion possible. In many cases, the loading condition can be

well defined, thus imposing a constant strength require-

ment. Since the strength requirement is defined, the weight

must be the variable parameter which will enable optimiza-

tion.

There are two common solutions to the general design

problem of creating structural panels with high buckling

strength relative to their weight (Larson 2003). The first

solution is the use of a milled pattern, such as an isogrid or

orthogrid pattern, in aluminum or titanium plate metal (a

so-called rib-on-plate solution). The second solution is asandwich panel. Composites are an appealing solution for

sandwich panels due to their light weight and stiffness. They

do, however, present many difficulties due to the level of

required expertise, specialized equipment and expensive

molds needed for manufacturing.

The most common composite solution for lightweight

applications is a honeycomb sandwich panel. This type of

panel provides a large surface area and has a high ratio of

stiffness to weight (Osgood 1966). A simple form of the

sandwich construction consists of two thin, stiff, strong

sheets of dense material (facings) separated by a less stiff

and strong central core (Allen 1969). Generally, the core is

much thicker than the facings to prevent shear deformation

in the panel. The facings of a sandwich panel act similarly

to the flanges in an I-beam: they take the bending load with

one facing in compression and the other in tension. In a

typical I-beam, the flanges cannot be extremely thin

because of buckling at the flange tips. With sandwich

panels, however, the numerous webs which compose the

core support the flange tips, and thin facings will work,

even to their full material yield stress, without buckling

(Bitzer 1997).

The structural efficiency of a honeycomb sandwich panel

as compared to a solid metal sheet has been illustrated by

Hexcel (1999). This manufacturer has shown that asandwich construction twice the thickness of a solid metal

sheet can increase the stiffness by 7 times and the strength

by 3.5 times, while only increasing the weight by 3 percent.

A sandwich thickness of four times that of a solid metal

sheet increases the stiffness by 37 times and the strength by

9.25 times, with only a 6 percent increase in mass.

Honeycomb is widely used in the aerospace industry as a

core material for sandwich panels. Satellites requiring large

Figure 1. a) Metal layers are deposited by a UC machine. b) Milling operation removes material, leaving thin ribs. c) Sensors,

electronics, thermal regulators, and fiber reinforcement are embedded and covered with a facing. d) Final procedure mounts

solar cells and external mechanisms.

228 J. George and B. Stucker


4/16

surface areas for solar cells almost always use some form of

honeycomb sandwich construction. It is typically produced

using one of two methods. The most common method is by

expansion (Hexcel 1999). As shown in figure 2, the

expansion process connects sheets of material with adhesive

lines. The resulting block is then cured and sliced to the

proper thickness for a core. A final procedure expands the

sliced block into a lattice of connecting cells that are very

thin. Later, thin sheets of another material can be glued to

the core as facings to form a sandwich panel.Though honeycomb core can be produced in very high

volumes, there are also many drawbacks to this type of

sandwich construction. Traditional methods require preci-

sion in assembly, as the process is sensitive to variation. In

addition, any bolted or riveted joints cause stress concen-

trations, and special potted inserts are required to prevent

local failures of bolts (Shirgur 2000), adding to the mass

and manufacturing time of the panel. In addition, this

design discourages modularity and increases both time and

cost when any modification to the panel is required. For

this reason, an automated process such as ultrasonic

consolidation is viewed as a valuable solution for creating

sandwich panels.

2.2 Ultrasonic consolidation

Though ultrasonic welding has been performed since the

late 1950s (Daniels 1965, Weare 1959), it has only recently

become a useful additive manufacturing technique. UC

emerged as a direct metal manufacturing technique and

rapid prototyping technology in the late 1990s. Research by

Johnson (1998) at Tufts University found that ultrasonic

welding could be used to make prototypes similar to other

rapid prototyping machines with the added benefits of low

energy consumption, modest space, and no emission offumes. In addition, he found that ultrasonic metal welding

had many advantages over other rapid prototyping methods

due to the fact that bonds can be formed between dissimilar

metals. He also noted that since there is no melting,

dimensional accuracy could be very high. Finally, he noted

that off the shelf materials could be used, which lowers cost.

Johnsons work involved the integration of a simple

ultrasonic metal welder and a high-speed cutter to make

very simple three dimensional dog bones for testing. His

work was followed by Gao (1999) who analyzed the

mechanics of ultrasonic metal welding during rapid proto-

typing. He used analytical modeling, finite element analysis,

and experimental data acquisition to look at static and

dynamic effects in the elastic and plastic flow regions during

welding.

White (2002) was the first to develop a commercial

machine tool to fabricate 3-dimensional structures using

ultrasonic welding. This ultrasonic consolidation (UC)machine uses a custom computer software tool to slice

the three-dimensional CAD model of the component to

be built into a number of horizontal layers, whose

thickness is equal to the metal foils used. These layers

are systematically created and stacked from bottom to

top, producing a three-dimensional object. Figure 3a

shows the UC machine installed at Utah State University

and figure 3b illustrates the basic UC additive manufac-

turing process. In the process a rotating ultrasonic

sonotrode travels along the length of a metal foil placed

over the substrate. The foil is held in contact with the

substrate by applying a normal force via the rotatingsonotrode. The sonotrode oscillates perpendicular to the

direction of welding at a frequency of 20 kHz and at a

user-set oscillation amplitude. The combination of normal

and oscillating shear forces results in the generation of

dynamic interfacial stresses between the two mating

surfaces (White 2003, Daniels 1965, OBrien 1991). The

stresses produce deformation of surface asperities, break-

ing up the oxide film, producing relatively clean metal

surfaces in intimate contact, establishing a metallurgical

bond. After depositing a strip of foil, another foil is

deposited adjacent to it. This process repeats until a

complete layer is placed. After placing a layer, a CNC

mill shapes the layer to its slice contour. This milling can

occur after each layer or, for certain geometries, after

several layers have been deposited. Once the layer is

shaped, chips are automatically blown away using com-

pressed air and foil deposition starts for the next layer. A

number of studies have been conducted to determine the

optimum process parameters for welding Al 3003 using

UC (Kong et al. 2004, Janaki Ram et al. 2006).

Figure 2. Conventional honeycomb production by expansion.

229Fabrication of lightweight structural panels through UC


5/16

2.3 UC design issues

By utilizing UC, a number of potential benefits of tradi-tional honeycomb panels can be adopted without some of

the inherent manufacturing drawbacks of more traditional

processes. A UC panel can conceivably utilize a sandwich

honeycomb configuration of a thick core composed of thin

webs along with rigid facings. Because an integrated

machine tool is used to fabricate the panel directly from

CAD, the process eliminates some of the expertise and

precision required for traditional honeycomb manufactur-

ing. In addition, a UC-built honeycomb panel can adopt

modular bolt patterns, often used in orthogrid configura-

tions, but without the expensive, time-consuming process of

manually potting inserts for each bolt location. Finally, aUC built panel can integrate multifunctional capabilities by

embedding components, including electronics, wiring har-

nesses and thermal management devices, during the build

(Clements 2006). Figure 4 shows potential design ap-

proaches for lightweight structural panels. UC can be

considered a useful fabrication technique for structural

panels if the stiffness to mass ratio can exceed that of an

isogrid panel. It is not necessary for a UC panel to exceed

the stiffness to mass ratio of honeycomb panels for

usefulness, as the UC built panel will also have multi-

functional capabilities which are not possible when using

traditional honeycomb panels.

One of the critical design aspects of a honeycomb-type

sandwich panel is the achievable height to width ratio of the

webs within the core. Due to the physics of the UC process,

a newly deposited layer will not adhere to a previously

deposited layer if the stiffness of the previously deposited

layer is not sufficient to resist the vibrational forces of the

sonotrode (thus producing the differential motion necessary

between the newly deposited layer and the previously

deposited layer to induce plastic deformation at the inter-

face). A tall, thin rib will not resist sonotrode vibration as

well as a short, thick rib, for instance. As the rib height

increases, a cantilever effect allows the part to vibrate freely

(figure 5). Thus there exists a maximum height-to-width

ratio for a freestanding rib above which UC bonding will

not occur (Robinson et al. 2006). The maximum height-to-

width ratio of 1:1 found by Robinson for Al 3003 alloys is

considered inadequate for thin webs in a structural

sandwich panel core, and thus one of the major objectivesof this current work was to ascertain whether patterns of

ribs, as opposed to single, freestanding ribs, could achieve

height-to-width ratios significantly above 1:1.

The height and width of webs in the core are directly

related to the overall mass and stiffness of a sandwich

panel. The thinner the web, the lower the panels mass. The

taller the web, the higher the panels stiffness. This is due to

the effect of core thickness, c, on the bending stiffness, D, of

a sandwich panel (ASTM C 393-00). If the modulus

of elasticity, E, the width of the panel, b, and the sand-

wich thickness, T, are all held constant, the stiffness

increases rapidly with increasing core thickness, as shown

in equation (1).

DE (T3 c3) b

12(1)

Additionally, the core shear stress, t, and the facing bending

stress, s, can be defined (ASTM C 393-00) by:

Increasing Stiffness

Solid

Panel

Ortho

grid

Isogri

d

Honeycom

b

Absolut

eMini

mum-

Weigh

tStru

ctures

Figure 4. Scale to quantify usefulness of new fabrication

technique (assumes identical weight).

Figure 3. a) Solidica Form-ation ultrasonic consolidation

machine. b) Schematic of the ultrasonic consolidation

process.



6/16

tN

(T c) b(2)

s

N L

2 tf(T c) b(3)

where N is the load placed upon the midpoint of the

sandwich panel, andtfis the facing thickness. If c is allowed

to increase independent of the other variables, the stresses

experienced in the panel decrease. It is therefore evident

that a thicker core or web structure is stiffer and capable of

withstanding more stress.

3. Experimental approach

In order to determine the feasibility of creating lightweight

structural panels using UC, a series of experiments were

designed. These experiments were designed to determine the

types of core geometries which were most readily fabricated

using UC as well as the effects of build parameters on the

bond strength between the facing and the core, and

involved: (1) development of a repeatable method for

evaluating the bond strength between face sheets and the

core, (2) determination of a benchmark peel strength based

on established UC optimum parameters, (3) investigation of

the importance of heating the baseplate on bond strength,

(4) an evaluation of the effects of core rib direction on bond

strength between face sheets and the core, (5) and anevaluation of the effects of core lattice shape on bond

strength.

Following these experiments, a case study was performed

where the developed optimum core geometries and process

parameters were used to design and fabricate a functional

structural panel for the TOROID spacecraft. This panel

was modelled and tested to determine its mechanical

properties and its suitability for the launch environment.

3.1 Peel test apparatus development

Typically, honeycomb specimens undergo compressive test-

ing and plate shear testing (Bitzer 1997). These tests help in

measuring the compression modulus as well as the honey-comb shear strengths and moduli. Their applicability to UC

built specimens, however, may be minimal since the core is

not produced by gluing thin pieces of aluminum together.

Another series of tests, such as the flatwise tension test

and climbing drum peel test, are performed on assembled

sandwich panels to test the effectiveness of the bond

between the honeycomb core and the thin facings. The

flatwise edge test pulls the facings in tension to separate

them from the core. The climbing drum peel test peels off a

facing by rolling it around a drum. The failure modes in

both tests are revealed as core tearing, cohesive failure of

the adhesive, or failure of the adhesion to the honeycomb

or facing. Both of these tests are excellent ways ofevaluating the integrity of a honeycomb sandwich panel

(Bitzer 1997).

Kong (2005) found that a test method used for measuring

the resistance of adhesives to peeling was an effective

method for determining weld quality for specimens built

with UC. From his research, he found that as the number

and size of contact points within the welded interface

increased, so did the average resistance to peeling. Though

the peel test results were not as consistent as those for

adhesives, they revealed general trends for weld effective-

ness. Kongs peel test is very similar to the climbing drum

peel test, and thus a modified version of the climbing drumpeel test was adopted for our studies, resulting in an

apparatus much like Kongs (2003).

The Standard Test Method for Floating Roller Peel

Resistance of Adhesives (ASTM D3167-03a) was used to

create a fixture for specimens created on the UC machine.

Some deviations in the dimensions of the specified test

fixture to accommodate plates used in the Solidica machine

were required. Also, the speed was changed from 152 mm/

Figure 5. Effects of different height to width ratios.



7/16


8/16

lightweight properties. Other designs include triangles, sine

waves, and squares. Based on results from the rib direction

studies, discussed below, it was determined that the core

lattice should avoid patterns where the ribs are oriented

parallel to the roller traverse direction, and instead max-

imize ribs perpendicular to the roller traverse direction. Of

the standard core configurations, hexagons and triangles

best meet these criteria. By argument it was difficult to

determine if the hexagon was a better core lattice than the

triangle so an experiment was designed to compare the two.

The dimensions of the patterns were chosen such that both

the triangle and the hexagon enclosed an area of 0.635 cm.

This area was chosen, as it was near the maximum area of a

triangle where the sonotrode would always be over at least 2

ribs at any point during its travel (so as to give it adequate

support). Ribs 1.016 mm thick were used for both pattern

geometries. The specimens were created by milling the

patterns into a 1.27 cm thick aluminum plate to a depth of

2.8 mm. A skin consisting of one tape was applied to each

specimen as shown in figure 8. The tapes were then removed

in a peel test.

4. Experimental results and discussion

4.1 General peel test apparatus results

A number of experiments were performed using the test

apparatus shown in figure 6 to determine whether the setup

gave repeatable and consistent results. A typical baseplate

for the UC machine is 35-by-35 cm, and allows all edges of

the plate to be firmly bolted to the heated platen using

8 fasteners. When using a smaller 10-by-35 cm baseplate

that is capable of fitting in the test apparatus, only the ends

of the baseplate (and not its sides) are bolted to the heated

platen. This reduces the rigidity of the smaller baseplate

during deposition. As three tapes can be consolidated side-by-side on the smaller baseplate, a study was performed to

see if location on the baseplate affected bond strength. This

study determined that the bond strength of tapes consoli-

dated on the outer edges of the small baseplate were always

inferior to the center tape bond strength. Thus, any valid

comparison of bond strength must be done for specimens

fabricated at the same location on the baseplate. This result

is not expected to be true of a firmly bolted, typically-sized

baseplate.

Another specimen preparation aspect that was found to

be of great importance was the direction of the flat-pass

milling operation used to prepare the surface of thebaseplate prior to consolidation. A flat pass milling

operation in the roller traverse direction was found to give

smooth peel data, whereas milling operations perpendicular

to the traverse direction caused spikes in the data.

For bond strength peel tests, there were three failure

mechanisms, based on the strength of the bond. The

weakest bonds would allow a smooth peel where the

resistance to peeling could be observed over the length of

Figure 7. Test specimens for determining the effect of rib

direction on peel strength. Figure 8. Test specimens for comparison of bonding for

hexagons and triangles.



9/16

the experiment without any tearing. In specimens with

extremely good bonding, the aluminium tape would tear

before significant peeling occured. This can be seen in figure

9. On the load plots for such peel tests, there was typically a

sharp incline, a short peak, and a rapid fall to zero. The

third reaction to the peel test was where a small portion of

the tape would tear due to a variation in weld quality across

the tape. In the load versus extension plots for this type of

reaction, the load would increase to a peak and then slowly

slope down to zero. This is because the tear decreases the

effective cross section being tested and all data after the

peak was therefore not comparable with other results.

4.2 Benchmark test

Figure 10 shows the results from peeling two different tapes

consolidated at the optimum parameters to a solid base-

plate. The maximum bond strengths measured were 193 N

and 195 N. The following experiment on the heat effect also

provided a peak value of 195 N. Using the three values, the

average maximum obtainable bond strength for theseparameters was approximately 195 N, which was taken as

a benchmark value for the discussions which follow. It

should be noted that the initial extension length prior to a

load increase was due to the amount of slack present in the

fixture. In order to compare results, the maximum load

reached and the shape of the load-versus-extension curve

were the primary comparison mechanisms, whereas the

absolute extension value was meaningless.

4.3 Effects of baseplate heating on bond strength

A comparison of results for a heated platen versus a room

temperature platen are shown in figure 11. The average

peak value for specimens processed at 150 8C was 195 N.

The room temperature specimen had a peak value of 71 N,

which was the same as the peak of 71 N reported by Kong

(2005) during his room temperature peel tests. From these

results, it was clear that heating the baseplate to 150 8Ccreated a bond nearly three times as strong as a room

temperature bond. The peak value was the same as the

other benchmark results since the test setup and ultrasonic

consolidation parameters were similar.

4.4 Rib direction

An overlay of three results from the rib direction experi-

ments are found in figure 12. The ribs parallel to the

traversing direction of the sonotrode (08) did not bond well,

as the load necessary to peel the tape was close to zero. The

458 ribs provided a relatively weak bond, but the tapes did

stick to the rib pattern. As had been expected, the ribs

perpendicular to the traversing direction provided a sub-

stantially better bond with a peak load of approximately

182 N before failure. The data shows a repeating pattern of

peaks, which indicate each time the tape pulls away from a

rib and the peel test advances to the next rib. The tape

eventually failed in a manner similar to that shown in figure

Figure 9. Failed specimen after being consolidated to a baseplate and peeled in a peel test.

0

50

100

150

200

250

0 2 4 6 8 10 12 14 16

Extension (mm)

Load(N)

Trial 1

Trial 2

Figure 10. Peel test data for maximum bond strength (150

8C, 16 mm, 28 mm/s, 1750 N).

0

50

100

150

200

250

0 2 4 6 8 10 12 14 16

Extension (mm)

Load(N)

150 Degrees C

27 Degrees C

Figure 11. Peel test data for heat effect (16 mm, 28 mm/s,

1750 N).



10/16

9, illustrating that a facing could be bonded to a rib just as

well as to a solid baseplate under certain conditions.

As illustrated by Robinson et al. (2006), producing tall,

thin free-standing ribs presents a challenge for the UC

process. What this present study illustrates is that an

effective way to minimize web deflection during vibrationis to orient ribs such that the mechanical oscillation of the

sonotrode is applied along the stiffest direction of a rib. As

the sonotrode oscillates perpendicular to the direction of

travel, ribs which lie perpendicular to the traversing

direction are most resistant to vibration, however, rib

patterns in this direction present problems as well, as the

sonotrode will dip into the channels between the ribs if

there are no other structural support members present,

creating a wavy surface. A 458 rib angle relative to the

traversing direction creates a smoother surface and provides

some resistance to vibration, but at a lower overall bond

strength than ribs at 908.

The results of this experiment, as alluded to earlier,

narrowed the options for practical core configurations.

When considering 08ribs, squares, hexagons, and triangles,

a series of perpendicular lines provide rigidity in only one

direction within a sandwich panel, and also produce a wavy

surface. A lattice of squares would provide rigidity in both

directions but only members oriented at 908 would bond

well to the facing. This left hexagons and triangles as the

likely best potential core configurations.

4.5 Core lattice geometry results

The results when comparing hexagonal and triangularpatterns (figure 13) show that the effective bond strength

was similar for both the hexagonal and triangular lattices,

and reached a maximum value of approximately 100N. In

the case of the triangle, the peak load occurred at the

location of maximum bond width (i.e. the width of a tape,

which was 2.4 cm (see figure 8)).

The hexagon, however, had a maximum bond width of

only 1.04 cm. Because this area was only 44% of the area of

a full tape, the equivalent normalized bond strength of the

hexagon was 231 N. This far exceeded the value obtained in

the benchmark test and showed that very good bonding can

occur between segments of honeycomb and a facing.

Because hexagonal structures are more efficient they

use the least amount of material to create a lattice of cellswithin a given volume for any common cell pattern

hexagons represent an excellent shape for a core within a

UC-fabricated sandwich panel. Based on the results from

the rib direction study, a good rule of thumb when using

UC to fabricate honeycomb sandwich panels is to ensure

that the hexagons are oriented such that no cell walls are

parallel to the traversing direction of the sonotrode.

5. Integrating experimental results into design

Once the honeycomb core was confirmed as the best coregeometry for UC-fabricated sandwich panels, solid me-

chanics theory was used to optimize the core geometry to

ascertain the best compromise between what is ideal and

what is realistic for fabrication in the UC machine. Though

the height to width ratio of a rib plays an important role

when fabricating a free standing rib, it has little impact on a

lattice of connecting cells, as the overall stiffness of the

lattice itself is the key to resisting vibration when adding

facings. From a practical standpoint, the thickness of the

panel, therefore, is limited to the maximum allowable

thickness for the application or the maximum depth the

Solidica machine can mill.Other geometric factors that must be determined are the

size of the honeycomb cells and the thickness of the cell

walls. Though the ideal core would posses very thin walls,

the wall thickness in a UC-built panel must be thick enough

to resist buckling under the applied normal force of the

sonotrode. The critical buckling load of the lattice is

determined by the second moment of inertia of the walls

of the cells, which is defined by Gibson (1988) as:

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40 45

Extension (mm)

Load(N)

Hexagonal Pattern

Triangular Pattern

Figure 13. Peel test results for hexagonal vs. triangular

pattern (150 8C, 18 mm, 13 mm/s, 1750 N).

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60

Extension (mm)

Load(N)

0 Degrees

45 Degrees

90 Degrees

Figure 12. Peel test results for variation in rib direction

(150 8C, 18 mm, 13 mm/s, 1750 N).



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PK E

(1 n2)

t3

l(4)

where K is a constraint factor, E is the modulus of elasticity,

v is Poissons ratio, t is the wall thickness and l is the length

of a single cell wall. Because the UC machine operates at a

specified load during consolidation, and since K, E, and v

are constants which depend on the geometry chosen, t can

be solved as a function of l. The length of a single cell wall

ultimately determines the size of the cells, which is limited

by the need to have the sonotrode always straddling at least

two cell walls in order to provide the sonotrode a flat

surface to which a tape can be consolidated (to help avoid

waviness in the facings). A cell wall size of 0.787 cm was

chosen as an optimal value for maximizing the area of a

honeycomb under these constraints.

The parameter K can be approximated as 4 (Gibson

1988) based on the fact that the honeycomb cell is neither

completely free nor rigidly clamped. Equation (4) can then

be rewritten to represent the elastic collapse stress s3. For

regular hexagons and v/0.3, the formula becomes:

s3

E5:2

t

l

3(5)

The elastic collapse stress is equal to the force applied by

the sonotrode divided by the area of the ribs underneath the

sonotrode. The sonotrode has a contact area of approxi-

mately 5/24 mm. This area, however, acts only on the

honeycomb rib line enclosed by the area. For a cell wall

length of 0.787 cm the minimum required area can be

calculated as:

Alt2 t (0:197 t)

sin(60)

(6)

Plugging in 68.95 GPa for E, 1750 N for the force, and

0.787 cm for the cell wall length gives a minimum cell wall

thickness of 0.61 mm. To allow for a factor of safety, a cell

wall thickness of 1.0 mm was chosen as a conservative wall

size for a honeycomb lattice.

Finally, the dimensions of the facings were determined.

The standard thickness used in regular honeycomb of 0.635

mm corresponded well with the thickness of four consoli-

dated layers and was therefore used. During builds where

facings were consolidated to honeycomb cores, it was noted

that the first couple of layers contained minor defects due

to the sharp interface between the facing and the core whenapplied at high amplitudes. The third and fourth layers,

however, contained negligible defects and therefore repre-

sents the minimum facing thickness for a well built

sandwich panel.

A defined geometry now enables one to use an effective

or equivalent properties method to predict the properties

of the sandwich panel. This method uses the geometry of

the facings and core lattice to create an equivalent solid core

and skin which approximates the properties of the real

sandwich panel. The equivalent single skin plate method

outlined by Paik (1999) considers the rigidity of panels, with

equal facing skin thickness, separately for in-plane tension,

bending, and shear. This method was used to analyze the

deflections in the panel due to single point loading and the

results are compared with experimental data as part of

the case study below.

6. Case study: TOROID

6.1 Panel design

As a demonstration of the capabilities of UC for fabrication

of a lightweight panel, a deck panel was fabricated for the

TOROID spacecraft. Utah State Universitys entry into the

4th University Nanosatellite Competition is the Tomo-

graphic Remote Observer of Ionospheric Disturbances

(TOROID). TOROID will demonstrate both scientific

and technological capabilities as the satellite is fabricated,

tested, and eventually put into orbit around the earth. Thescientific mission of TOROID is to observe scintillations in

the low latitude ionosphere with increased fidelity. This

data will provide the scientific and military communities

with a greater understanding of the morphology and

equatorial phenomena which currently impede accurate

space-based geolocation.

There were several reasons a deck was needed in the

structural design of the satellite. First, the existing Utah

State University Satellite (USUSat) design emphasized the

importance of modularity by using panels. Components for

the various subsystems were attached to the panels which

were, in turn, assembled into a boxlike structure. This

allowed each panel to be tested individually for vibration

and thermal effects. Due to this design, there was very little

space for mounting a new payload, such as the TOROID

science instrument, to an external panel. This was a

significant problem in that the science instrument required

a large area and cantilever support (figure 14). While the

inside of the panels were covered with components and

electrical harnessing, the majority of the interior volume of

the satellite was empty. This empty space, however, left

room to install a horizontal deck panel, upon which the

science instrument could be mounted. It was proposed to

use UC to fabricate this deck as an integral part of the

TOROID design. As the deck employed new fabricationtechniques and the possibility for future multifunctional

capability (i.e. embedded wiring harnesses, thermocouples

or other devices) its development comprised one of the

technological objectives of the TOROID mission.

One major design feature of the USUSat Bus is a

standard bolt pattern. This pattern is repeated on the side

panels, which are orthogrid panels with reinforced, tapped

holes every 3.24 cm. For the UC panel, reinforced cylinders



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were input into the CAD model to allow holes to be

machined and tapped. The pattern was aligned such that

the fasteners used to secure the deck to the satellite bus were

attached to the rim of the deck for added support. These

fastening points were placed on an edge perpendicular to

the direction of the tapes. This was done so that when the

deck panel is loaded, the tapes will not be stressed at theirabutting interfaces.

The rim around the perimeter of the deck panel acts as a

stiffener in the satellite. This helps maintain rigidity in the

in-plane axis. The overall dimensions of the final deck panel

were governed by the footprint of the TOROID spacecraft.

The maximum thickness of the panel was limited to

1.27 cm. Small holes were used to perforate the honeycomb

sections, as completely enclosed cavities have a tendency to

rupture in space due to low pressure in the space environ-

ment. This could have also been easily accomplished by

milling a tiny channel through the centroid of the honey-

comb cell walls during fabrication. The final CAD model ofthe TOROID deck panel without its top facing is shown in

figure 15.

6.2 Manufacturing process plan

Solidicas proprietary software, RPCAM, was used to

generate the G-code for the toolpaths and tape lays for

the solid model. A configuration file in the software enabled

the user to modify the weld speed, amplitude of oscillation,

and force for each model. To prepare the machine, an

ultrasonic couplant was applied to one face of the

aluminum baseplate. This couplant enhanced thermal

conduction between the heated platen and the aluminum

baseplate while mitigating differential motion between thesurfaces. The plate was bolted to the heated platen and a

flatpass milling operation was used to clean the surface of

the plate and to zero the plate with respect to the machine,

as shown in figure 16a.

The files for the first model were uploaded into the

controller for the Solidica machine and the process was

initiated. Thirteen foils were welded side-by-side to form a

layer (figure 16b). For each new layer deposited, the tapes

were offset to avoid the creation of a seam. Every four

layers the machine was programmed to use the milling head

to trim excess tape. After building up sufficient thickness

for the panel, the honeycomb pattern and bolt pattern were

milled into the UC-deposited structure. After a final

flatpass (figure 17a), the facing was consolidated to the

honeycomb core (figure 17b) and trimmed.

After adding the facing, the UC machine proceeded to

drill the holes for the bolt pattern and mounting brackets.

The perimeter of the deck was purposefully built larger than

necessary, in order to enable a final trim toolpath to remove

any poorly bonded edges, as our practical experience and

initial modelling efforts indicate that the edges of structures

are prone to non-optimal bonding (Li 2006). The trimmed

deck is shown in figure 18a. It should be noted that the

pattern visible in this figure is due to the fact that in

locations where there is no material below the facing, thedeposited tape remains shiny, whereas in the regions directly

above the honeycomb and perimeter pattern, the resistance

to vibration causes the sonotrode to roughen the surface of

the deposited layer, thus turning the surface white instead

of shiny. In addition, by running your finger over the

surface, you can feel the underlying honeycomb structure

through the facing, as the web locations are higher than the

open regions.Figure 15. CAD of final deck panel.

Figure 14. TOROID spacecraft with simplified science instrument.



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Finally, excess baseplate material was removed by turning

the plate over and milling the back down until the desired

facing thickness remained on the deck panel, as shown in

figure 18b. This operation also resulted in a flat surface

suitable for mounting payloads. The deck was then removed

from the Solidica machine and cleaned up using a band saw

and manual mill. A secondary operation involved threading

the mounting points and installing helicoils for addedstrength within the bolt pattern. The final deck panel for

the TOROID spacecraft is shown in figure 19.

6.3 Mechanical Testing

The three beam-flexure test is often used to evaluate overall

sandwich panel performance (Hexcel 1999). This test, often

called the 3-point bend test, is particularly important since

it verifies how the core and facings work together to achieve

the overall mechanical performance of the panel. The test

can be performed with a single or double point load. The

stiffness of the panel can be calculated using the imposed

force and deflection at the mid span of the panel (Bitzer1997). The ASTM Standard Test Method for Flexural

Properties of Sandwich Constructions can be used to

determine the properties of flat sandwich constructions

subjected to flatwise flexure. Such an experiment can be

carried out in a quasi-static manner with a very low loading

speed (Paik 1999).

With the deck panel fabricated, it was possible to test the

panel for comparison with the finite element results. The

load cell used was a Tinius Olsen load cell with an 50 kN

capacity. It had the capability of measuring a force to within

9/0.5% of the indicated load. The extension was accurate to

within 9/0.01 mm. A 3-point bend test was performed by

placing the panel on two supported cylinders which created

a simply supported line in the same location where it was

applied in the finite element model. A third supported

cylinder was attached to the load cell and brought down tothe middle of the panel. The force and extension were

referenced at zero and the machine was programmed to

lower at a rate of 0.254 mm/min. The resulting data is

shown in figure 20.

The plot shows a nonlinear stiffness for the first 0.75 mm

and then a linear trend for the remainder of the test. The

nonlinear portion was due to the fact that the apparatus

was not touching the deck when the experiment

was initiated in addition to some minor settling in the

deck and fixture. The linear region showed a stiffness of

1511 N/mm.

For comparison with the finite element model, the

deflection at 1334 N was noted. The correct deflectionwas obtained by noting when the force measurements began

in the recorded data and using that as the reference for zero

deflection. The experimental data showed a deflection of

1.285 mm. The finite element analysis predicted 1.458 mm.

This is a difference of 12%. The deflection at 890 N for the

3-point bend test was 0.99 mm, compared to a predicted

value of 0.97 mm, a 2% difference. Differences were

expected due to assumptions made in the equivalent skin

Figure 16. a) Clean baseplate, b) First layer of consolidated aluminum tapes.

Figure 17. a) Milled honeycomb core. b) First layer of top facing consolidated on honeycomb core.



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method for the finite element model as well as discrepancies

between the setup of the model and the setup of the 3 point

bend test and due to settling in the fixture. For the purposes

of showing a general trend between experimental and

numerical results, however, the results give sufficient

correlation to enable some level of usefulness for predicting

UC sandwich panel performance using an equivalent skin

method.

6.4 Economic Considerations

The total time for the deck panel build was approximately

56 hours. As much of the build was performed unattended,

the deck was completed within a 40 hour work week. The

majority of the time was in machining the honeycomb grid

and bolt pattern. These tasks consumed a significant

amount of time due to the inefficiency of the milling

apparatus on the UC machine at Utah State University.

This machine is not optimized for removing large amounts

of material, as it possesses a small spindle. The bolt pattern

also took considerable time due to excessive lengths fortoolpaths. The time required for both of these operations

could be greatly reduced by performing them in a CNC

with a more powerful spindle.

The total cost of the build was $1597 USD. This cost was

comparable to the cost incurred when a professional

machine shop machined one of the panels of the USUSat

bus. An informal estimate was done by a commercial small

satellite producer as a comparison. They estimated that a

panel similar to the deck plate but fabricated out of

honeycomb composite would cost $2200 to $3200 USD.

Material costs would have been similar but labor costs

would have greatly surpassed those for fabricating the

TOROID deck. This illustrates one of the main advantages

of the UC-built panel over a traditionally fabricated panel,namely the lack of touch-labor required.

The mass was estimated for an equivalent composite

panel with potted inserts as 0.22 kg compared to 0.626 kg

for the UC panel. This illustrates one of the disadvantages

of UC-built aluminium honeycomb panels. In small satel-

lites, such as TOSOID, however, this mass difference has

very little impact on its handling and launch costs, as the

satellite is inherently small and light. The UC built deck has

the added benefit that it can very easily be made into a

multifunctional structure.

7. Conclusion

Ultrasonic consolidation has been demonstrated as a useful

technique for fabrication of lightweight structural panels.

The peel test has been found to be an effective method for

evaluating bonding between an aluminum tape and a solid

or lattice substrate. A benchmark load of 195 N has been

measured as a benchmark bond strength when using

Figure 18. a) Deck panel with bolt pattern milled out. b) CNC mill removing excess material from baseplate.

Figure 19. Finished deck panel.

0

500

1000

1500

2000

2500

0 0.5 1 1.5 2 2.5

Displacement (mm)

Load

(N)

Figure 20. Stiffness of prototype deck panel from 3-point

bend test.



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optimum parameters (Jankai Ram 2006). Heating the

baseplate to 150 8C during consolidation has been found

to enhance bond strength by 3X over room temperature

bonding. Core rib direction has a significant impact on the

bond strength and ribs perpendicular to the traversing

direction of the sonotrode have been found to give the best

bond. This has led to the implementation of an optimized

honeycomb lattice for the core of sandwich panels. Thisconfiguration has been found to allow benchmark quality

bonding between the facing and core while maximizing the

volume within the cells.

The design of the TOROID UC sandwich panel has

integrated the experimental results and solid mechanics

theory. Though this design does create a panel with greater

mass than a similar panel made with composite honey-

comb, its method of fabrication is possible without the use

of significant touch-labor, epoxy or inserts. UC also has the

added benefit of enabling the embedding of useful compo-

nents. The TOROID deck panel was found to interface well

with other structural elements in the satellite and has

provided significantly more mountable area for scienceinstruments. Its testing has shown a stiffness of 1,511 N/mm

and its suitability for the launch environment.

8. Future work

There is a significant amount of future work possible to

improve the quality of structural panels produced using

UC. The most significant may be the implementation of a

support material apparatus in the UC machine. Support

material would allow each layer of the facing to bond fully

to the layer below it. Currently bonding between facing

layers is only achieved directly over the honeycomb cell

walls. In addition, support material would enable the use of

thinner web structures (as they would support against

buckling) and remove some of the honeycomb size restric-

tions.

The facings could also benefit from fiber reinforcement.

Utilization of a fiber-reinforced tape as the facing would

make the sandwich panel much stiffer. Also, the overall

mass of the panel could be reduced by using a stronger

material, such as a 6061 aluminum alloy. In addition, 6061

is more accepted in the aerospace community due to its

extensive use in successful missions.

The time it takes to produce a deck panel could be cutfrom one week to approximately one day with the use of a

more powerful mill, more machinable alloys, and with

optimization of the toolpaths for machining as well as when

laying tapes.

One additional area for future work is the embedment of

subsystems into a structural deck panel. Items such as heat

pipes, antennas, wiring, thermocouples, low profile heaters,

embedded computers, connectors, and printable batteries

are being tested for integration into functional deck panels.

Their integration may one day be automated using direct

write technologies and pick-and-place heads, greatly in-

creasing the automation possible when fabricating future

multi-functional panels.

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