Induction Bonding for Structural 

Composite Tubes 

by Eric D. Wetzel, William A. Spurgeon, and 

Christian J. Yungwirth 

ARL-TR-2818 September 2002 

Approved for public release; distribution is unlimited. 
NOTICES 

Disclaimers 

The findings in this report are not to be construed as an official Department of 

the Army position unless so designated by other authorized documents. 

Citation of manufacturer’s or trade names does not constitute an official 

endorsement or approval of the use thereof. 

Destroy this report when it is no longer needed. Do not return it to the 

originator. 
Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-2818 September 2002 

Induction Bonding for Structural 

Composite Tubes 

Eric D. Wetzel and William A. Spurgeon 

Weapons and Materials Research Directorate, ARL 

Christian J. Yungwirth

University of Delaware 

Approved for public release; distribution is unlimited. 
ii

Abstract 

Large structural composite tubes, prototype components for a proposed U.S. 

Navy application, are bonded by induction curing of an engineering adhesive. 

Magnetic powder is used as the susceptor material and is directly incorporated 

into the adhesive prior to processing. Different induction power supplies, coil 

designs, and adhesive formulations are investigated. Final demonstration runs 

bond 9-in-long, 3-in-diameter axisymmetric bondlines in 15 min. These results 

demonstrate for the first time the successful induction bonding of large structural 

composite components using magnetic particulate susceptors. 
iii

Acknowledgments 

Christian Yungwirth was funded through the University of Delaware Composite 

Materials Research Program. Partial funding for this research was also supplied 

by the U.S. Navy Undersea Warfare Center. We would also like to acknowledge 

John Brown and Dave Spagnola of the U.S. Army Research Laboratory (ARL) for 

their help in manufacturing composite components, as well as James Sands 

(ARL) for his technical input. 
iv

INTENTIONALLY LEFT BLANK. 
v

Contents 

Acknowledgments iii

List of Figures vii

List of Tables ix

1. Introduction 1

1.1 Application ..................................................................................................... 1

1.2 Induction Bonding ......................................................................................... 1

1.2.1 Magnetic Particles and Adhesive Bonding ................................... 1

1.2.2 Generation of Induction Fields ....................................................... 2

1.3 Objective.......................................................................................................... 2

2. Materials 3

2.1 Composite Tube Material and Geometry ................................................... 3

2.2 Magnetic Materials and Other Adhesive Additives ................................. 4

2.3 Lap Shear Testing........................................................................................... 5

3. Equipment 6

3.1 Induction Equipment .................................................................................... 6

3.2 Induction Coils ............................................................................................... 6

3.3 Temperature Sensors..................................................................................... 9

4. Phase I Experiments 9

4.1 Tuning and Coil Design ................................................................................ 9

4.2 Preliminary Heating Experiments............................................................. 10

4.2.1 Pancake Coil .................................................................................... 10

4.2.2 Solenoid Coil.................................................................................... 14

4.3 Preliminary Bonding Experiments ............................................................ 15

4.4 Final Bonding Run ....................................................................................... 16
vi

5. Phase II Experiments 17

5.1 Tuning and Coil Design .............................................................................. 17

5.2 Preliminary Heating Experiments............................................................. 18

5.3 Preliminary Bonding Experiments ............................................................ 21

5.3.1 Lap Shear Strength Specimens...................................................... 21

5.3.2 Large Bond Area Specimens.......................................................... 24

5.3.3 Adiprene Self-Heating Experiments ............................................ 27

5.4 Final Bonding Run ....................................................................................... 29

6. Discussion and Conclusions 29

6.1 Equipment Considerations......................................................................... 29

6.2 Material Considerations.............................................................................. 31

6.3 Adiprene Self-Heating Phenomenon ........................................................ 31

6.4 Future Applications of Induction Heating ............................................... 32

7. References 33

Report Documentation Page 35 
vii

List of Figures 

Figure 1. Geometry of full prototype tubes to be bonded......................................... 4

Figure 2. Saturation magnetization as a function of temperature for FP95, 

FP110, and FP160 powders. ..................................................................................... 5

Figure 3. Coils used during many of the induction bonding experiments............. 8

Figure 4. RF Current as a function of coil inductance for the Lepel 

induction power supply. ........................................................................................ 12

Figure 5. Heating behavior of insulated FP110 and FP95 powders in the 

Lepel induction power supply, using the 3-turn pancake coil. ........................ 13

Figure 6. Heating behavior of Adiprene adhesive with 10%vol FP110 and 

FP95 powders, in the Lepel induction power supply with the 3-turn 

pancake coil. ............................................................................................................. 14

Figure 7. Heating behavior of insulated FP160 powder in the Lepel 

induction power supply, using the 5//5 solenoid coil...................................... 15

Figure 8. Induction bonding of full tubes using Lepel induction unit. ................. 17

Figure 9. Final bonded tube for phase I experiments. ............................................. 18

Figure 10. Cross section of bonded minitube for phase I experiments. ................ 19

Figure 11. Heating behavior of insulated FP160 powder in the 

Ameritherm induction power supply, using various coils and power 

levels.......................................................................................................................... 19

Figure 12. Assembly geometry for measuring lap shear bondline 

temperature histories. ............................................................................................ 20

Figure 13. Temperature history for lap shears bonded with 20%vol 

FP160, using the Ameritherm induction power supply with the 5//5 

solenoid coil at various power levels.................................................................... 21

Figure 14. Effect of volume fraction of FP160 powder on lap shear 

strength for oven cured Adiprene adhesive. Error bars show ± 

standard deviation. ................................................................................................. 22

Figure 15. Position of lap shears in 5//5 solenoid coil during processing........... 23

Figure 16. Effect of processing conditions on the lap shear strength of 

samples bonded using 20%vol FP160. All samples were induction 

cured using the Ameritherm induction power supply, except for the 

oven-cured sample. Error bars show ± standard deviation. ............................ 24

Figure 17. Position vs. time protocols for bonding runs from Table 9. ................. 25
viii

Figure 18. Power vs. time profile for minitube bonding run 2 in Table 9. 

Also shown is the temperature on the inner surface of the tube at the 

longitudinal center of coil, as measured using the fiber optic 

temperature sensor.................................................................................................. 27

Figure 19. Location of area of overheating during high power bonding 

using the 5//5 solenoid coil in the Ameritherm induction power 

supply........................................................................................................................ 28

Figure 20. Comparison of heating behaviors of glass fiber/Adiprene 

composite minitubes with no adhesive, in the Ameritherm induction 

power supply with the 5//5 solenoid coil at 500 W. ......................................... 28

Figure 21. Induction bonding of full tubes using the Ameritherm 

induction unit........................................................................................................... 29

Figure 22. Final bonded tube for phase II experiments........................................... 30
ix

List of Tables 

Table 1. Hardware configurations and resulting performance for the 

Lepel induction power supply. ............................................................................. 10

Table 2. Measured inductance, capacitance, resistance, and total 

impedance values for various coil geometries. ................................................... 11

Table 3. RF Current readings for various coils on the Lepel induction 

power supply. .......................................................................................................... 12

Table 4. Comparison of heating rates for neat FP110 powder and 10%vol 

FP110 powder in adhesive, for the Lepel induction power supply with 

the 3-turn pancake coil............................................................................................ 14

Table 5. Results of bonding runs for the Lepel induction power supply.............. 16

Table 6. Comparison of heating rates for insulated FP160 powder in the 

Lepel and Ameritherm induction power supplies, with the 5//5 

solenoid coil.............................................................................................................. 20

Table 7. Lap shear strength data for oven-cured Adiprene with various 

volume fractions of FP160 powder. All failures were cohesive in the 

adherend................................................................................................................... 22

Table 8. Lap shear strength data for induction-cured Adiprene with 

20%vol FP160. .......................................................................................................... 23

Table 9. Results of bonding runs for the Ameritherm induction power 

supply........................................................................................................................ 26
x

INTENTIONALLY LEFT BLANK
1

1. Introduction 

1.1 Application 

The U.S. Navy has identified the need for long, flexible, lightweight tubes for a 

future sea-based application. The expected tube lengths are tens to hundreds of 

feet and will need to be able to support moderate loads. 

Glass fiber-reinforced polyurethane composites have been identified as a 

potential material solution. Polymer matrix composites are, in general, 

lightweight and not susceptible to corrosion in salt water environments. 

Polyurethane composites offer low stiffness and high strains to failure, with wide 

tailorability of mechanical properties through composite architecture. 

The most efficient fabrication method for long axisymmetric composite 

components is filament winding. However, the length of the proposed tubes is 

well beyond the capabilities of conventional filament-winding equipment. An 

alternative approach is to filament-wind short sections and then adhesively bond 

these sections to form the longer tube. 

The structural strengths required for this application necessitate the use of 

engineering adhesives. Most of these adhesive systems require elevated 

temperature cures, which are typically achieved using convection ovens or 

infrared heaters. However, these surface heating approaches can be very slow, 

since they rely on through-thickness thermal conduction to transfer heat from the 

part surface to the bondline. This problem is especially severe in low thermal 

conductivity materials such as glass fiber composites. The large size of the 

bonded tubes also presents tooling difficulties, since they are too large to fit in 

conventional convection ovens. 

1.2 Induction Bonding 

1.2.1 Magnetic Particles and Adhesive Bonding 

An alternative heating approach is to use induction curing of adhesives (Wetzel 

and Fink 2001). An induction field is a high frequency electromagnetic field, 

typically operating at frequencies from 0.1 to 100 MHz. Most materials, 

including glass composites and conventional adhesives, do not interact with 

induction fields. However, “susceptor materials,” such as metals and magnetic 

materials, heat in the presence of an induction field. 

This behavior can be exploited by mixing magnetic fillers into conventional 

adhesives. In the presence of an induction field, the doped adhesive will heat 

inductively and cure. This approach is superior to conventional surface heating 
2

methods because the bondline is heated directly. By selectively heating only the 

bondline, overall process times and energy requirements are reduced. 

Magnetic particles heat in an induction field due to magnetic hysteresis 

mechanisms. The alternating magnetic field causes the material to magnetize 

and demagnetize cyclically, a process which is not thermodynamically 

reversible. The losses associated with this magnetization process are liberated in 

the form of heat. In general, the rate of heat generation is directly proportional to 

frequency, so that it is advantageous to use higher frequency induction fields. 

Heating also generally increases with magnetic field amplitude up to some 

limiting saturation strength. The details of the mechanisms of induction heating 

of magnetic materials can be found in Wetzel and Fink (2001). 

The saturation magnetization of magnetic materials usually decreases with 

temperature. In very pure materials, the magnetic properties will drop 

precipitously at a characteristic temperature called the Curie temperature. The 

decrease in magnetic properties with temperature, in most cases, also causes a 

drop in the magnitude of heat generation in an induction field. This behavior 

limits the temperature to which a magnetic material can be self-heated. In fact, it 

has been shown that the dwell temperature of very pure materials in an 

induction field is closely related to the Curie temperature (Wetzel et al. 2000). 

These inherent temperature limits enable rapid heating while minimizing the 

risk of exceeding critical upper process temperatures. 

1.2.2 Generation of Induction Fields 

Induction fields are generated using a high frequency power supply connected to 

an induction coil. The induction coil is typically a winding of copper tubing, 

through which cooling water is circulated. The power supply sends a high 

frequency current through the copper tubing, which then generates a high 

frequency magnetic field in the vicinity of the coil. 

Coil geometry design can be used to control the distribution of magnetic fields 

produced. In general, magnetic field strength is highest near the coil. Therefore 

coils are designed such that their windings are close to the area to be heated. 

Larger coils allow the production of magnetic fields over larger areas, enabling 

heating of larger parts. However, these large coils usually also have large 

inductances. Since most induction heaters operate under resonant conditions, 

there is only a limited range of coil inductances which can be tolerated. These 

tuning considerations limit induction coil size. 

1.3 Objective 

The objective of this report is to demonstrate the feasibility of induction 

processing for adhesively bonding large composite tubes for a U.S. Navy 

application. Curing structural adhesives using induction fields has been 

demonstrated before, typically using embedded metallic susceptors (McKnight et 
3

al. 1998; Yarlagadda et al. 2000; Tay et al. 1999; Wedgewood and Hardy 1996) or 

heating a metallic substrate (Eagle 1987; Stefanides 1987). A few researchers 

have investigated the use of magnetic particle susceptors for adhesive curing 

(Fink et al. 1998; Hahn et al. 1991), and patents exist on the technology (Guiles et 

al. 2000; Clark et al. 1995). This report is the first to demonstrate rapid structural 

bonding of large components using magnetic particle susceptors. 

The research in this report was performed in two phases. Phase I was performed 

at the University of Delaware Center for Composite Materials (Newark, DE) 

using a Lepel Corp. (Edgewood, NY) induction power supply. This power 

supply, which is relatively crude in design, has limited frequency and amplitude 

capability. Phase II was performed at the U.S. Army Research Laboratory (ARL) 

(Aberdeen Proving Ground, MD) using an Ameritherm Inc. (Scottsville, NY) 

power supply. The Ameritherm unit is the current state of the art in commercial 

high frequency, auto-tuning induction processing equipment. 

During each of the two phases, preliminary experiments are performed to 

properly configure the induction equipment, determine optimal adhesive/filler 

loading ratios, and select appropriate processing protocols. These preliminary 

experiments are followed by bonding of large demonstration tubes, which are 

representative of possible application designs. These prototype tubes will also be 

mechanically tested at the Naval Undersea Warfare Center (Newport, RI) at a 

later date. 

2. Materials 

2.1 Composite Tube Material and Geometry 

The composite tubes are filament-wound glass-fiber-reinforced polyurethane. 

The resin system is Uniroyal Chemical Co. (Middlebury, CT) Adiprene L-100, 

cured using Uniroyal Caytur 21 curing agent (toluene diisocyanate). Adiprene 

composites have a relatively low elastic modulus and high strain to failure, 

allowing the fabrication of highly flexible composite materials. Figure 1 shows 

the tube geometries. Each of the two tubes is ∼9 ft in length with a diameter of 

roughly 3 in and a wall thickness of 0.28 in. The tubes are bonded along a 9-inlong, tapered, male-female joint. A wind angle of 35° was used for the phase I 

experiments, while a wind angle of 25° was used for the phase II experiments. 

For some of the preliminary bonding experiments, “minitubes” were bonded to 

simulate the final bonding runs. These minitubes are ∼2 ft in length with a 

diameter of roughly 3 in and a wall thickness of 0.25 in. The tubes are bonded 

along a 6-in-long, untapered, male-female joint. All minitubes were filament 

wound at wind angles comparable to the final tubes.
4

Figure 1. Geometry of full prototype tubes to be bonded. 

Conventional adhesives, which are relatively stiff compared to Adiprene, were 

considered unsuitable for bonding the tubes. The stiffness mismatch could cause 

unwanted bondline stresses at high bending strains, or limit the flexibility of the 

bondline areas. Instead, Adiprene L-100 resin, cured using Caytur 21, was used 

as the adhesive. Although this material is not designed as an adhesive, it was 

found to produce adequate bond strengths for this application. The 

recommended cure cycle for Adiprene L-100 with Caytur 21 is 1 hr at 120 °C. 

This resin system will also gel at room temperature over ∼24 hr, although the 

mechanical properties will be inferior to the elevated temperature cured material. 

2.2 Magnetic Materials and Other Adhesive Additives 

Nickel-zinc ferrites manufactured by PowderTech (Valparaiso, IN) were used as 

the magnetic susceptor material. Three particular grades, FP95, FP110, and 

FP160, were used in this study. These materials are soft ferrites with relatively 

low Curie temperatures (<200 °C). Figure 2 shows saturation magnetization vs. 

temperature for these materials, measured using a Lakeshore Cryotronics, Inc. 

(Westerville, OH) 7300 vibrating sample magnetometer. The saturation 

magnetizations do not exhibit a sharp drop at a characteristic temperature, as 

would be expected to occur for a pure material at its Curie temperature. Instead, 

the gradual decrease in magnetization with temperature indicates that these are 

probably multiphase materials. This behavior also implies that, in an induction 

field, these materials will not show obvious self-regulating dwells at their Curie 

temperature. Instead, they are more likely to reach an ultimate temperature 

which is highly dependent on the applied field strength and heat transfer 

conditions. 

In all cases, the as-received magnetic particles were dry ball milled for 1–3 hr 

prior to testing, using a 50 mL tungsten carbide vial and balls. Fifty grams of 

particles (~10 mL) were milled each cycle, resulting in an average particle size of 

~10–100 μm. The magnetic material has a density of ~4.5 g/cm3

. When mixed 

with the adhesive (with a density of ~1 g/cm3

), a particle weight fraction of 

33% roughly corresponds to a volume fraction of 10%, and a particle weight 

fraction of 50% roughly corresponds to a volume fraction of 20%. 

17' 6"

9"

5

8 3 " 5

16 2 "

wall thickness ~ 1

4

"

bondline
5

Figure 2. Saturation magnetization as a function of temperature for FP95, FP110, and 

FP160 powders. 

Hollow glass microspheres (Recyclospheres, manufactured by Sphere Services 

Inc., Oak Ridge, TN) were incorporated into some adhesive formulations to help 

control bondline thickness. Because these particles are inert and nonmagnetic, 

they do not significantly influence the adhesive cure kinetics or particle heating 

behavior. In all cases where glass microspheres were used, loadings of 1%wt 

were added. 

All filler materials were incorporated into the adhesive through hand mixing. 

2.3 Lap Shear Testing 

Lap shear samples were assembled using 0.125-in-thick Adiprene/woven glass 

composite adherends 1 in wide × 6 in long. A bond overlap of 1 in was used. 

The bond surfaces were lightly sanded and degreased prior to bonding. 

Mechanical tests were performed in an MTS Synergie (Cincinnati, OH) 

mechanical tester, using a 5 kN load cell and a loading rate of 0.04 in/min. 

0

5

10

15

20

25

30

35

50 100 150 200 250 300

FP95

FP110

FP160

Moment (emu/g)

Temperature (°C)
6

3. Equipment 

3.1 Induction Equipment 

For the phase I experiments, a water-cooled 2.5 kW Lepel model 7-2.5-1-MC5-BW 

induction power supply was used. The unit is rated for a tunable frequency 

range from 2.5 to 8 MHz. Manual tuning is performed using three internal 

adjustments. A two-position jumper determines whether the unit tunes within a 

range of low frequencies (2.5–5 MHz) or high frequencies 

(5–8 MHz). A 7-position tap setting varies the internal inductance. The internal 

capacitance is altered through swapping capacitors in an internal bank. Two 

control knobs on the unit exterior, each with a range from 0% to 100%, are 

labeled “Grid Control” and “Power Control.” Grid Control provides additional 

fine tuning capability. Power Control scales the coil current, so that maximum 

coil current (and maximum field strength) is achieved at a Power Control of 

100%. The Lepel unit also has four output meters, labeled “RF Current,” “Grid 

Current,” “Plate Current,” and “Plate Voltage.” According to the manufacturer, 

larger values of RF Current imply larger coil currents, although the exact value 

and proportionality of these currents is not known. 

For the phase II experiments, a water-cooled Ameritherm NovaStar 1M 1.5 kW 

induction power supply was used. The unit is rated for a tunable frequency 

range from 10 to 15 MHz. System capacitance is adjusted through an internal 

bank of capacitors, as well as a mechanically tunable variable capacitor. A “Load 

Power” setting of 0–1500 W can be set manually. Larger values of Load Power 

produce larger output field strengths, although it is not clear whether these 

qualities are linearly proportional. The value of Load Power does not indicate 

the amount of power actually being transferred to the workpiece. Instead, it 

should be considered an indication of the relative induction field strength, 

independent of load. 

For the Lepel unit, frequency measurements were performed using an 

Optoelectronics, Inc. (Ft. Lauderdale, FL) 3000A Plus frequency counter. For the 

Ameritherm unit, frequency values are measured internally and displayed as 

system output. 

3.2 Induction Coils 

Custom coils were constructed using 0.25 in outer diameter (OD) (0.030 in wall 

thickness) copper tubing. In some cases tubes were annealed between bending 

steps to allow tighter bend radii. All coils were coated with Sherwin Williams 

Co. (Solon, OH) S00600 dielectric insulating varnish to reduce the likelihood of 

arcing. 
7

Three general types of induction coils were used: (1) pancake, (2) conventional 

solenoid, and (3) parallel solenoid. The pancake coil has a spiral in-plane 

geometry and is useful for imposing magnetic fields where only one-sided access 

is possible. This coil design is common in induction processing and was 

therefore used to initially benchmark particle and adhesive performance. For 

heating the cylindrical tube bondline, an axisymmetric coil is preferred. The 

solenoid is a simple and efficient coil geometry for generating axisymmetric 

fields. 

To bond the composite tubes as quickly as possible, the solenoid coil must be 

designed to maximize the area of bondline heated. This consideration is not 

trivial due to the relatively large bond area in this application. The diameter of 

the coil is fixed at a size just large enough to fully encompass the tube diameter. 

The remaining design variables are the number of turns n in the coil and the 

length l of the coil. The magnetic field strength Hi at the center of the coil and 

inductance Li of an ideal solenoid are given by 

Hi = 

nI

l 

Li = 

n2A

l (1) 

where A is the cross-sectional area of the coil and I is the coil current. H must be 

kept high enough to provide adequate bondline heating, while L must be kept 

low enough to allow for proper tuning. An alternative design approach is to 

connect two identical solenoids in parallel. If each solenoid has an area A, length 

l/2, and n/2 turns, then the total effective magnetic field strength Hp and 

inductance Lp is 

Hp = 

nI

l

1

2 

Lp = 

1

4

n2A

l

. (2)

For inductance-limited tuning, this approach allows longer coils to be designed, 

although at the expense of lower field strength. We will use the notation 

“m//m” to signify a coil composed of two parallel solenoids each with m turns. 

A wide range of coil geometries are considered during tuning of the Lepel power 

supply. However, most of the bonding experiments, both with the Lepel and 

Ameritherm units, focus on four specific coils: (1) a 3-in-diameter 3-turn pancake 

coil; (2) a 2-in-long 3-turn solenoidal coil; (3) a 4.5-in-long 4//4 solenoidal coil; 

and (4) a 9-in-long 5//5 solenoidal coil. These coils are shown in Figure 3. All 

solenoidal coils have a diameter of ~3.5 in. The 3-turn solenoid was the longest 

conventional solenoidal coil that we were able to fully tune with the Lepel unit. 

The 4.5- and 9-in parallel coils represent well-tuned parallel coils of increasing 

length. 

, , (1) 

,, 
8

(a) Pancake Coil

 

(d) 5//5 Solenoid Coil 

Figure 3. Coils used during many of the induction bonding experiments.

(b) 3-Turn Solenoid Coil (c) 4//4 Solenoid Coil 
9

3.3 Temperature Sensors 

Conventional thermocouples do not produce reliable measurements in induction 

fields due to electromagnetic interference and inductive heating of the sensor 

itself. One alternative is a fiber optic temperature sensor (FISO Technologies, 

Sainte-Foy, Quebec). The fiber optic sensor is very accurate and has a response 

time of under 0.1 s. However, it cannot be used for temperatures above 400 °C, it 

is extremely fragile, and costs around $350/sensor. In cases where the fiber optic 

sensor was not feasible, an AGEMA Infrared Systems (Secaucus, NJ) 900 Series 

infrared camera was used to perform temperature measurements. Because the 

infrared camera was not fully calibrated for each material and view angle under 

consideration, its temperature measurements can only be considered accurate to 

within 20 °C. 

4. Phase I Experiments 

Phase I experiments were performed at the University of Delaware using the 

Lepel induction power supply. 

4.1 Tuning and Coil Design 

A variety of tuning configurations were tested in order to determine the best 

settings for induction bonding. In general, the heating power generated in 

magnetic materials increases with induction field frequency and amplitude. 

Therefore, the optimum tuning configuration is the one that produces the highest 

values of frequency and RF Current. 

For all tuning experiments, a 3-turn pancake coil was used, and the internal 

jumper was set to the high frequency position. Major tuning adjustment was 

performed by changing the tap position and bank capacitance. At a particular 

tap position and bank capacitance, the Grid Control setting was adjusted until 

maximum RF Current was achieved. In all cases, Power Control was set at 100% 

to produce maximum induction field strength. 

Table 1 shows the output performance for a wide range of hardware 

configurations. For frequencies above 8 MHz, the measured frequency appeared 

to jump randomly so that a constant and consistent field strength output is not 

guaranteed. Setup 11, with a frequency of 6.28 MHz and an RF Current of 56 A, 

appears to be the best setup. The frequency is below 8 MHz, so consistent field 

strength is assured. This setup is used for all subsequent Lepel induction heating 

experiments. 
10

Table 1. Hardware configurations and resulting performance for the Lepel induction 

power supply. 

Setup 

No. 

Tap 

Position Capacitance 

(pF) 

Power 

Control 

Grid 

Control Frequency 

(MHz) 

RF 

Current 

(A) 

Grid 

Current 

(A) 

Plate 

Current 

(A) 

Plate 

Voltage 

(kV) 

1 6 100 100 0 — a a a a 

2 6 350 100 0 7.9 46 170 0.42 4 

3 4 350 100 20 6.1 40 160 0.42 4 

4 4 250 100 20 7.1 30 200 0.40 4 

5 4 100 100 20 9.2 30 240 0.64 4 

6 5 100 100 35 11.1 34 240 0.52 4 

7 5 350 100 15 6.9 48 180 0.44 4 

8 5 450 100 13 6.5 50 160 0.48 4 

9 5 700 100 13 5.1 50 160 0.48 4 

10 6 700 100 6 5.9 56 110 0.50 4 

11 6 600 100 5 6.3 56 120 0.52 4 

12 6 450 100 5 7.1 51 160 0.48 4 

13 4 450 100 18 5.5 44 160 0.38 4 

14 4 600 100 20 4.8 49 145 0.39 4 

15 4 700 100 19 4.5 50 140 0.40 4 

aFor setup 1, the system output was erratic, and no reliable values could be recorded. 

Using this tuned Lepel setup, a variety of solenoid coils were tested. Table 2 

shows the inductance, capacitance, resistance, and total impedance values for a 

number of coils, as measured using a Hioki 3522 LCR meter. Note that the LCR 

meter is only capable of making measurements of up to 100 kHz, which is below 

our expected operating frequency of 6 MHz. Table 3 shows the measured RF 

Current for some of these coils, for a Power Control setting of 100%. In all cases, 

the measured frequency was ~6.3 MHz. Figure 4 shows RF Current as a function 

of coil inductance. From the plot, it is clear that the output current increases as 

the coil inductance decreases. For the highest current case (3//3 coil), however, 

electrical crackling could be heard inside the unit. Therefore, the more 

conservative 3-turn, 4//4, and 5//5 coils were judged to be best suited for the 

bonding application. 

4.2 Preliminary Heating Experiments 

4.2.1 Pancake Coil 

To downselect optimum adhesive/filler ratios for induction bonding, a series of 

preliminary experiments were performed using the 3-turn pancake coil. 

The heating behavior of FP110 and FP95 powders was evaluated by placing ~5 g 

of each powder in a DuPont Kapton polymide film envelope. A fiber optic 

temperature sensor was embedded into the powder, and the envelope was 

insulated with ~1 cm of glass wool insulation. This insulated envelope was 

placed on a 3-mm-thick plaque of glass composite, which was then placed 
11

Table 2. Measured inductance, capacitance, resistance, and total impedance values for 

various coil geometries. 

Coil 

Geometry Length 

(in) 

f 

(kHz) 

L 

(μH) 

C 

(μF) 

R 

(mΩ) 

Z 

(mΩ) 

3 2 10 1.3 200 2.4 79 

 50 1.2 8.2 4.5 390 

 100 1.2 2.1 3.1 770 

4 2 10 1.8 140 3.4 120 

 50 1.8 5.6 6.6 570 

 100 1.8 1.9 11 1100 

4 3 10 1.8 150 3.2 110 

 50 1.7 5.9 6.4 540 

 100 1.7 1.5 10 1100 

4 4 10 1.6 160 3.1 100 

 50 1.6 6.3 6.5 500 

 100 1.6 1.6 10 1000 

5 2.5 10 2.2 140 3.6 140 

 50 2.1 4.6 6.5 680 

 100 2.2 1.2 11 1400 

8 9 10 3.9 64 6.0 250 

 50 3.9 2.6 14 1200 

 100 3.9 0.66 20 2400 

15 9 10 8.2 30 12 0.52 

 50 8.2 1.2 26 2.6 

 100 8.2 0.31 35 5.1 

3//3 6 10 0.75 330 2.0 47 

 50 0.71 14 3.0 220 

 100 0.69 3.7 4.0 440 

4//4 4.5 10 1.2 200 3.0 80 

 50 1.2 8.2 7.0 380 

 100 1.2 2.1 9.0 770 

5//5 9 10 1.2 220 2.2 72 

 50 1.1 8.9 4.0 350 

 100 1.1 2.3 2.0 710 

8//8 9.5 10 2.3 110 4.0 1.4 

 50 2.3 4.5 8.0 710 

 100 2.3 1.1 12 1400 

directly on top of the coil. The distance from the top of the coil to the center of 

the sample is ~8 mm. Figure 5 shows the heating behavior of the FP110 and FP95 

powders. Both powders show relatively rapid heat and dwell behavior, 

achieving within 10 °C of their ultimate temperature in less than 1 min. The 

FP110 sample reaches an ultimate temperature of ~220 °C, while the FP95 sample 

reaches an ultimate temperature of ~180 °C. 
12

Table 3. RF Current readings for various coils on the Lepel induction power supply. 

Coil 

Geometry Width 

(in) 

L at 100 kHz 

(μH) 

RF Current 

(A) 

3 2 1.2 52 

4 2 1.8 0 

4 3 1.7 10 

4 4 1.6 35 

15 9 8.2 0 

3//3 6 0.69 61 

4//4 4.5 1.2 52 

5//5 9 1.1 52 

Figure 4. RF Current as a function of coil inductance for the Lepel induction power 

supply. 

These heating behaviors represent the highest thermal efficiency scenario, 

insulated neat powders. In the actual bonding application, the powders will be 

dispersed in an adhesive, and spread as a thin layer on an adherend. To measure 

the heating behavior under these more realistic conditions, Adiprene adhesives 

were prepared with 10%vol FP110, and with 10%vol FP95. Layers of the 

0

10

20

30

40

50

60

70

0.6 0.8 1 1.2 1.4 1.6 1.8

RF Current ( A )

Inductance ( μH )
13

Figure 5. Heating behavior of insulated FP110 and FP95 powders in the Lepel induction 

power supply, using the 3-turn pancake coil. 

adhesive were spread on the surface of a 3-mm-thick Adiprene/glass composite 

adherend, which was then placed directly on the pancake coil. Thick layers 

(~3 mm) of both adhesives were tested, as well as a thin layer (~0.5 mm) of the 

FP110 adhesive. Thermal histories were measured using the infrared camera. 

Figure 6 shows the results of these adhesive heating experiments. The thick 

layers of adhesive exhibit rapid heat and dwell behavior, achieving within 10 °C 

of their ultimate temperature in less than 1 min. The FP110 adhesive sample 

reaches an ultimate temperature of ~150 °C, while the FP95 adhesive sample 

reaches an ultimate temperature of ~115 °C. Both temperatures are ~70 °C lower 

than the temperatures reached for the insulated powders alone. The thin layer of 

FP110 adhesive heats very gradually, taking over 5 min to reach 100 °C and still 

showing a moderate heating rate at 10 min. Comparing this behavior with the 

heating history for the thick layer of FP110 adhesive and the insulated FP110 

powder gives a broad view of the significance of efficiency losses associated with 

the typical adhesive bonding geometry. These heating rates are compared 

quantitatively in Table 4. The behavior of the thin adhesive layer also shows that 

the rapid heat and dwell behavior possible for Curie temperature—limited 

heating will only be observed if the generated induction power greatly 

overmatches thermal losses. 

0

50

100

150

200

250

0 20 40 60 80 100 120 140

FP110

FP95

Temperature ( °C )

Time ( s )
14

Figure 6. Heating behavior of Adiprene adhesive with 10%vol FP110 and FP95 powders, 

in the Lepel induction power supply with the 3-turn pancake coil. 

Table 4. Comparison of heating rates for neat FP110 powder and 10%vol FP110 powder 

in adhesive, for the Lepel induction power supply with the 3-turn pancake coil. 

Material 

Temperature 

at 15 s 

(°C) 

Average 

Heating Rate 

(°C/s) 

Neat FP110 196 11.07 

FP110 in Adhesive (Thick Layer) 80 3.33 

FP110 in Adhesive (Thin Layer) 38 0.53 

4.2.2 Solenoid Coil 

Figure 7 shows the heating behavior of FP160 in the 5//5 solenoid coil, 

measured using the same insulated Kapton envelope approach described in 

section 4.2.1. The sample shows clear heat and dwell behavior, reaching an 

ultimate temperature of ~175 °C. In identical induction fields, we would expect 

the FP160 material to heat to higher temperatures than the FP110 or FP95, due to 

its higher Curie temperature (as specified by the manufacturer). However, the 

temperatures reached by the FP160 in the 5//5 coil are less than or comparable 

to the temperatures reached by the FP95 and FP110 powder in the 3-turn 

pancake coil (see Figure 5). Therefore, we can infer that the field strength 

produced in the 5//5 coil is less than that produced by the 3-turn pancake coil. 

This result is consistent with the theoretical coil relations of section 3.2. 

20

40

60

80

100

120

140

160

0 100 200 300 400 500 600

Adiprene w/ FP110 (thick)

Adiprene w/ FP95 (thick)

Adiprene w/ FP110 (thin)

Temperature ( °C )

Time ( s )
15

Figure 7. Heating behavior of insulated FP160 powder in the Lepel induction power 

supply, using the 5//5 solenoid coil. 

4.3 Preliminary Bonding Experiments 

Bonding experiments were run in 3-turn, 4//4, and 5//5 solenoid coils to better 

model process performance during tube bonding. In all cases, the adhesive was 

loaded with 20%vol FP160. This volume fraction represents an approximate 

upper bound on filler content that allows complete mix wetting and 

incorporation into the adhesive. Glass microspheres were incorporated into 

most of the adhesives, although the particle size was reduced in later runs. A 

variety of adherends were bonded to approximate the final bonding geometry. 

Two types of conventional planar adherends were used. “Thin” adherends are 

roughly 0.125 in thick and 1 in wide, while “thick” adherends are roughly 

0.1875 in thick and 1 in wide. These different adherend thicknesses are intended 

to represent the range of effective adherend thicknesses which occur over the 

length of the final tapered tube joint. The planar samples were bonded in lap 

shear geometries, although the large amount of overlap is some cases is 

unconventional. The samples were placed inside the solenoidal coils on a piece 

of 0.005-in-thick Kapton film set directly on the coil, to closely match the 

bondline position expected during final tube bonding. Minitubes, as described 

in section 2.1, were also bonded. For cases where the bondline overlap is longer 

than the coil length, the bondline was indexed through the coil in a stepwise 

manner. 

20

40

60

80

100

120

140

160

180

0 50 100 150 200

Temperature ( °C )

Time ( s )
16

Table 5 shows results from these experiments. In general, bonding with the 

3-turn coil was successful, while bonding with the 4//4 and 5//5 coils was 

unsuccessful. One possible reason why the parallel coils are unable to produce 

complete bonding is inadequate field strength, since the parallel coils have 

winding currents equal to one-half of the input current. To test this theory, a 

bonding run was performed using the 3-turn coil at one-half of the Lepel full 

current value (run 4 in Table 5). Since the 3-turn and 5//5 coils have 

approximately the same number of turns per unit length, this experiment should 

simulate the magnetic field strength produced by the 5//5 coil operating at full 

current (although the field is produced over a much reduced area). As expected, 

this run produced an unbonded sample. Further confirmation of this field 

theory is found in the results of the 4//4 coil, which produced partially bonded 

samples (runs 5 and 6 in Table 5). This coil has a moderately higher turn density 

than the 5//5 coil, so it should produce a moderately higher field strength for 

the same input current. This difference in field strengths explains why the 4//4 

coil bonded the samples slightly more effectively than the 5//5 coil. 

Table 5. Results of bonding runs for the Lepel induction power supply. 

Run 

No. 

Coil 

Geometry 

Coil 

Length 

(in)

RF 

Current 

(A)

Glass 

Microspheres 

wt%, size 

Adherend 

Geometry, 

Overlap 

Time 

(No. Steps at 

Time per Step) Result 

1 5//5 9 52 0% Thin/Thin, 1 in 10 min Not Bonded 

2 5//5 9 52 1%, 500 μm Thin/Thin, 1 in 15 min Not Bonded 

3 3 2 52 1%, 500 μm Thin/Thin, 1 in 15 min Bonded 

4 3 2 26 1%, 500 μm Thin/Thin, 1 in 15 min Not Bonded 

5 4//4 4.5 51 1%, 300 μm Thin/Thin, 9 in 2 at 10 min Semi-Bonded 

6 4//4 4.5 51 1%, 300 μm Thin/Thin, 1 in 10 min Semi-Bonded 

7 3 2 52 1%, 300 μm Thin/Thin, 3 in 3 at 10 min Bonded 

8 3 2 52 1%, 300 μm Thin/Thin, 3 in 3 at 10 min Bonded 

9 3 2 52 1%, 300 μm Thin/Thin, 6 in 6 at 10 min Bonded 

10 3 2 52 1%, 300 μm Thin/Thin, 9 in 10 at 10 min Bonded 

These results indicate that with the Lepel induction power supply, only the 

3-turn coil is capable of producing fully bonded joints for the adhesive and 

adherend materials under consideration. For this 3-turn coil, bonding is best 

performed in 1 in increments, with 10 min of field exposure per increment. 

4.4 Final Bonding Run 

Based on the results of the preliminary experiments, Adiprene resin with 20%vol 

FP160 and 1%wt 300 μm glass microspheres was selected as the adhesive for the 

final tube bonding run. Both tube bond faces were lightly sanded and degreased 

with acetone, and then evenly coated with the adhesive. The tubes were forced 

together, and the excess adhesive flash was wiped off, both at the internal and 

external joint butts. The viscoelastic properties of the adhesive tended to force 

the compressed joint apart longitudinally, so mechanical restraint was used to 

hold the tubes together during the first few minutes of bonding. 
17

The tube was bonded in the Lepel unit using the 3-turn solenoid coil. The coil is 

~2 in long, while the tube bond area is ~9 in long. The bond area was fed 

through the coil in a series of 10 steps of 1 in of displacement each with a process 

time of 10 min/step. Some tube rotation may have also occurred between each 

step. The total bonding time was ~100 min. Figure 8 shows the bonding process 

in progress. 

Figure 8. Induction bonding of full tubes using Lepel induction unit. 

After bonding, the tube was allowed to cool and was then inspected. The flash 

was fully cured. The joint could not be separated by twisting or pulling on the 

tubes by hand. 

This tube is shown in Figure 9. Figure 10 shows the cross section of the minitube 

bonded under the same protocol, which we expect looks very similar to the cross 

section of the final bonded tube. 

5. Phase II Experiments 

Phase II experiments were performed at ARL, using the Ameritherm induction 

power supply. 

5.1 Tuning and Coil Design 

The same 3-turn, 4//4, and 5//5 solenoid coils used in the phase I experiments 

were used with the Ameritherm unit in the phase II experiments. With a 100 pF 

capacitor, these three coils tuned at 12.78, 12.75, and 12.92 MHz, respectively. 

For each coil, the unit was capable of being operated over its full range of Load 

Power, from 0 to 1500 W, with only minor changes in frequency. 
18

Figure 9. Final bonded tube for phase I experiments. 

5.2 Preliminary Heating Experiments 

Figure 11 shows temperature histories for FP160 in the 3-turn, 4//4, and 5//5 

solenoid coils at Load Power settings of 100, 500, and 1500 W. These 

measurements were performed using a fiber optic temperature sensor embedded 

in an insulated envelope of 5 g of powder, identical to the setup described in 

section 4.2.1. The 3-turn solenoid heats the powder faster than the parallel coils, 

and the 4//4 coil heats the powder faster than the 5//5 coil. Heating rates are 

higher at higher power levels. Note that, even for the 100 W setting and the 5//5 

coil, the powder heats to 100 °C in under 1 min. As a rough comparison of 

heating rates, Table 6 shows the temperature of the powder at a time of 30 s, for 

both the Ameritherm unit and the Lepel unit (see Figure 7). In all cases, in 

Table 6 the 5//5 coil was used to generate the induction field. The Lepel unit, 

operating at full power, heats the powder at a rate comparable to the 
19

Figure 10. Cross section of bonded minitube for phase I experiments. 

Figure 11. Heating behavior of insulated FP160 powder in the Ameritherm induction 

power supply, using various coils and power levels. 

Ameritherm unit operating at between 100 and 500 W. The Ameritherm unit 

operating at full power heats the powder approximately twice as fast as the 

Lepel unit operating at full power. 

100 W

500 W

1500 W

3-turn

4//4

5//5

0

50

100

150

200

250

300

0 20 40 60 80 100 120

Temperature ( °C )

Time ( s )
20

Table 6. Comparison of heating rates for insulated FP160 powder in the Lepel and 

Ameritherm induction power supplies, with the 5//5 solenoid coil. 

Induction Unit Power Setting 

Temperature at 

30 s 

(°C) 

Average 

Heating Rate 

(°C/s) 

Lepel 100% 140 3.67 

Ameritherm 100 W 70 1.33 

Ameritherm 500 W 180 5.00 

Ameritherm 1500 W 220 6.33 

 

Special lap shear specimens were prepared to allow for direct measurement of 

representative bondline temperature histories. The adherends were 

Adiprene/woven glass composite 0.125 in thick H 1 in wide H 6 in long. A 

0.0625-in-wide H 0.0625-in-deep H 1-in-long groove was cut into one of the 

adherends in the bondline area, as shown in Figure 12. A thin layer of Kapton 

film was placed over this groove, and then the adherends were bonded together. 

The samples were bonded using a 1 hr oven cure at 120 °C, using Adiprene with 

20%vol FP160 and 1%wt 300 μm glass microspheres as the adhesive. Once 

bonded, a fiber-optic temperature sensor was inserted into the groove, and 

induction heating experiments were performed. Note that the groove width and 

depth dimensions provide a tight fit for the fiber optic sensor, so that the sensor 

is well insulated and in good thermal contact with the adhesive bondline. Also 

note that, because the adhesive is precured, these experiments only measure the 

heating effect due to the magnetic particles and do not include any exothermic 

heating due to the adhesive cure reaction. 

Figure 12. Assembly geometry for measuring lap shear bondline temperature histories. 

Figure 13 shows the lap shear temperature histories for power settings of 500, 

1000, and 1500 W, using the 5//5 solenoid coil. The ultimate temperatures 

reached for each case are ~160°, 180°, and 210 °C, respectively. These 

temperatures are lower than the temperatures reached for the neat powder (see 

Figure 11), but are still well above the required cure temperature of 120 °C. 

adherend

adherend

adhesive

Kapton film

fiber optic temperature sensor

groove
21

Figure 13. Temperature history for lap shears bonded with 20%vol FP160, using the 

Ameritherm induction power supply with the 5//5 solenoid coil at various 

power levels. 

5.3 Preliminary Bonding Experiments 

5.3.1 Lap Shear Strength Specimens 

The effect of filler volume fraction on bond strength was investigated to 

determine acceptable filler loadings. Lap shear specimens were bonded using 

Adiprene with loadings of 0%, 5%vol, 10%vol, and 20%vol FP160. All samples 

also contained 1%wt 300 μm glass microspheres and were oven-cured at 120 °C 

for 1 hr. Five samples of each adhesive formulation were bonded and 

mechanically tested. Table 7 and Figure 14 show the results. Lap shear strength 

increases with increasing filler content, with the strength at 20%vol FP160 ~50% 

higher than the sample with no filler. In all cases, failure was mostly cohesive in 

the adherend, located directly below the bondline. The increasing strength with 

filler content is probably due to a stiffening effect in the adhesive, which may 

improve load transfer and decrease peel stresses. Since high bond strengths are 

achievable at the highest filler loading, and heating power increases with 

increasing filler loadings, 20%vol FP160 was used for all subsequent bonding 

experiments. 

0

50

100

150

200

250

0 100 200 300 400 500

500 W

1000 W

1500 W

Temperature ( °C )

Time ( s )
22

Table 7. Lap shear strength data for oven-cured Adiprene with various volume fractions 

of FP160 powder. All failures were cohesive in the adherend. 

Vol% Filler 

Lap Shear Strength 

Average ± Standard Deviation 

(MPa) 

0% 3.61 ± 0.70 

5% 4.54 ± 0.64 

10% 5.46 ± 0.55 

20% 5.84 ± 0.89 

Figure 14. Effect of volume fraction of FP160 powder on lap shear strength for oven 

cured Adiprene adhesive. Error bars show ± standard deviation. 

The effect of processing conditions was investigated using lap shear specimens 

bonded with 20% vol FP160 in Adiprene, with 1%wt 300 μm glass microspheres. 

In addition to oven-cured samples, lap shears were processed in induction fields 

at settings of 500, 1000, and 1500 W for 5–30 min. All induction samples were 

processed using the 5//5 coil, with the lap shear bond area centered laterally in 

one of the two parallel coils (Figure 15), and placed inside the coil on a piece of 

0.005-in-thick Kapton film set directly on the coil. This location closely matches 

0

1

2

3

4

5

6

7

0 0.05 0.1 0.15 0.2

Lap Shear Strength ( MPa )

Volume Fraction of Filler
23

Figure 15. Position of lap shears in 5//5 solenoid coil during processing. 

the bondline position expected during the final tube bonding. Because Adiprene 

is susceptible to gelling at room temperature, all samples were mechanically 

tested within 6 hr of bond processing. Five samples of each processing condition 

were bonded and mechanically tested, except for the 500 W samples for 20 min 

and 30 min, for which only four samples were tested. 

Table 8 and Figure 16 show the lap shear strengths for these specimens. Note 

that none of the induction-cured samples reach the full oven-cured strength, 

although at 500 W for 20 min and 30 min the average strengths are only 20% 

below the oven-cured samples. Also note that, for the 500 W samples at 20 min 

and 30 min, failure is mostly in the adherend. At 1500 W, increasing cure time 

causes a decrease in average lap shear strength. This trend implies that thermal 

degradation is occurring at this power level. This result is consistent with the 

temperature measurements (see Figure 13) which showed that temperatures as 

high as 210 °C were reached at 1500 W, which is well above the recommended 

cure temperature. 

Table 8. Lap shear strength data for induction-cured Adiprene with 20%vol FP160. 

Process Conditions Failure Mode 

Lap Shear Strength 

Average ± Standard Deviation 

(MPa) 

500 W for 10 min Cohesivea 2.32 ± 0.70 

500 W for 20 min Adherendb 4.81 ± 0.47 

500 W for 30 min Adherend 4.75 ± 0.68 

1000 W for 10 min Cohesive 2.47 ± 0.38 

1500 W for 5 min Cohesive 2.78 ± 0.23 

1500 W for 10 min Cohesive 2.31 ± 0.70 

aCohesive indicates a cohesive failure within the adhesive. 

bAdherend indicates a cohesive failure in the adherend. 

Although the maximum induction lap shear strengths are not as high as the 

oven-cured filled sample, the values still exceed the lap shear strength of the 

unfilled Adiprene adhesive (see Figure 13). This strength is considered adequate 

for the final tube prototype application. Note that, unlike the Lepel induction 

power supply, the Ameritherm induction power supply with the 5//5 coil 
24

Figure 16. Effect of processing conditions on the lap shear strength of samples bonded 

using 20%vol FP160. All samples were induction cured using the Ameritherm 

induction power supply, except for the oven-cured sample. Error bars show ± 

standard deviation. 

produces adequate heating for adhesive bonding. In fact, the strength data may 

indicate that at full power the generated heating at the bondline exceeds 

allowable levels. Due to the large area over which this coil generates induction 

fields, the success of the 5//5 coil provides an opportunity to bond the final tube 

bondline much faster than was possible with the Lepel induction power supply. 

5.3.2 Large Bond Area Specimens 

Bonding experiments were run in the 5//5 coil to determine optimum conditions 

for tube bonding. “Thick” 0.1875-in-thick × 1-in-wide and “thin” 0.125-in-thick 

× 1-in-wide planar adherends were used. The adherends were bonded with 9-inlong bondlines, to simulate the length of the final tube bondline. The samples 

were placed inside the solenoidal coil on a piece of 0.005-in-thick Kapton film set 

directly on the coil, to closely match the bondline position expected during final 

tube bonding. Minitubes, as described in section 2.1, were also bonded. All 

Adiprene adhesives contained 20%vol FP160 and 1%wt 300 μm glass 

microspheres. 

0

1

2

3

4

5

6

7

Oven 500 W

10 min

500 W 

20 min

500W

30 min

1000W

10 min

1500W

5 min

1500W

10 min

Lap Shear Strength ( MPa )
25

Three different positional protocols were investigated, labeled A, B, and C in 

Figure 17. Protocol A is stationary, with the 9 in bondline centered in the 90 in 

coil. Protocol B is a 20 min protocol, beginning with 5 min centered on the coil, 

then 5 min off-center to the right, and then 10 min off-center to the left. The last 

stage is given extra time because of the significant cooling which occurs to the far 

right of the sample during the second 5 min stage. Protocol B is an inefficient 

approach and was replaced by protocol C. Protocol C involves three 5 min 

stages, with the bondline displaced by one-half of its length in the same direction 

in each step. Note that in protocol C, each area of the bond is exposed to the 

induction field for 10 consecutive min. 

step 1: 15 minutes or 20 minutes

induction coil

bond 

area

(a) Protocol A. 

step 2: 5 min

step 3: 10 minutes

step 1: 5 minutes

induction coil

bond 

area

(b) Protocol B.

step 1: 5 min

step 2: 5 minutes

step 3: 5 minutes

induction coil

bond 

area

(c) Protocol C.

Figure 17. Position vs. time protocols for bonding runs from Table 9.
26

Table 9 shows the results of these tests. The first minitube, processed at 500 W 

with protocol A (run 1), appeared well bonded after processing. However, also 

evident was a series of large blisters inside the tube, ranging from 0.5 to 1.5 in in 

diameter. The second minitube, initially heated at 500 W and then reduced 

power levels (run 2, also Figure 18), was also well bonded. The reduction in 

power levels decreased the severity of blistering, but some small blisters were 

still evident. In runs 3–5, planar adherends were bonded at 500 W using protocol 

B. All samples were well bonded. However, all of the samples had thin strips on 

the adherends, within a few inches of the bondline, that appeared more visually 

translucent than the virgin matrix (see Figure 17). There was also evidence of 

matrix flow at the adherend edges near this thin strip, where it appeared that 

matrix resin had “oozed” out of the adherend. The translucency and flow are 

evidence of matrix material melting. We expect matrix material melting to be 

detrimental to the mechanical performance of the composite, so the power level 

was reduced. Runs 6–8 were performed on planar adherends at 400 W using 

protocol C. All adherends were well bonded, with no signs of melting. Run 9, 

performed on planar adherends at 300 W and using protocol C, produced a 

partial bond with no evidence of melting. A minitube was then bonded at 400 W 

using protocol C, producing a well-bonded tube with no blistering. This 

approach, 400 W and protocol C, was adopted for the final tube bonding runs. 

Table 9. Results of bonding runs for the Ameritherm induction power supply. 

Run 

No. Power 

(W) 

Adherend 

Geometry Time 

(min) 

Protocola Result 

1 500 Minitube 15 A Bonded, Blistered 

2 500–150b Minitube 20 A Bonded, Blistered 

3 500 Thin/Thin 20 B Bonded, Some Melting 

4 500 Thin/Thick 20 B Bonded, No Melting 

5 500 Thick/Thick 20 B Bonded, No Melting 

6 400 Thin/Thin 15 C Bonded, No Melting 

7 400 Thin/Thick 15 C Bonded, No Melting 

8 400 Thick/Thick 15 C Bonded, No Melting 

9 300 Thin/Thin 15 C Semi-Bonded, No Melting 

10 400 Minitube 15 C Bonded, No Blistering 

11 400 Final Tube 15 C Bonded, No Blistering 

12 400 Final Tube 15 C Bonded, No Blistering a Bonding protocol details are given in Figure 17. 

b Full power vs. time profile is given in Figure 18. 
27

Figure 18. Power vs. time profile for minitube bonding run 2 in Table 9. Also shown is 

the temperature on the inner surface of the tube at the longitudinal center of 

coil, as measured using the fiber optic temperature sensor. 

Note that, using protocol C and 400 W, each area of the bond is subjected to 

400 W for ~10 min. According to our earlier lap shear strength data (Figures 13 

and 15), under these conditions the actual bond strength may be less than the 

standard oven-cured bond strength. However, we expect that there are less 

thermal losses during tube bonding than during conventional lap shear bonding, 

since the large bond area eliminates many of the lap shear end effects. Therefore, 

tube processing at 400 W probably corresponds to lap shear processing at higher 

power levels. Direct mechanical testing of these tube bonds would be necessary 

to determine if full strength is truly achieved. 

5.3.3 Adiprene Self-Heating Experiments 

During the preliminary bonding runs of section 5.3.2, tube blistering and planar 

adherend melting were observed. Both effects are likely evidence of some form 

of matrix softening, in which the matrix begins to flow. In the case of the tubes, 

this softening causes blistering due to the residual radial stresses induced during 

filament winding. In the case of planar adherends, the effect is to allow local 

flow and changes in the appearance of the matrix. In all cases, both the blisters 

and melting occurred over a very thin (~0.5 in) strip of material. This strip of 

material was always located at the very center of the 5//5 coil, as shown in 

20

40

60

80

100

120

140

160

100

200

300

400

500

600

0 200 400 600 800 1000 1200

Temperature

Power

Temperature ( °C )

Power ( W )

Time ( s )
28

Figure 19. This local melting was also observed in the lap shear strength 

specimens of section 5.3.1, in which the melting zone was located several inches 

away from the inductively heated bondline. 

area of overheating

Figure 19. Location of area of overheating during high power bonding using the 5//5 

solenoid coil in the Ameritherm induction power supply. 

One possible cause of softening is excessive heating of the matrix. To determine 

if heating was the cause of softening, a fiber optic temperature sensor was placed 

on a minitube, centered in the 5//5 coil, with no adhesive. The resulting 

temperature history at 500 W is shown in Figure 20. 

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600

Virgin tube

Pre-heated tube

Dried tube

Temperature ( °C )

Time ( s )

Figure 20. Comparison of heating behaviors of glass fiber/Adiprene composite 

minitubes with no adhesive, in the Ameritherm induction power supply 

with the 5//5 solenoid coil at 500 W.
29

The tube heats by ~25 °C in 10 min. To determine if the rate of heating is higher 

at higher tube temperatures, a tube was preheated to 100 °C and placed in the 

5//5 coil, again at 500 W. In this case, tube heating was not evident, and the 

tube merely cooled towards ambient temperature. A third tube was dried at 

120 °C for 4 hr, then placed in the 5//5 coil at 500 W. In this case, very slow 

heating was observed. 

These results are discussed further in section 6.3. 

5.4 Final Bonding Run 

Based on the results of the preliminary bonding experiments, 20%vol FP160 in 

Adiprene with 1%wt 300 μm glass microspheres was chosen as the final bonding 

adhesive. The mating tube surfaces were lightly sanded and degreased with 

acetone prior to bonding. The adhesive was spread evenly on both surfaces, 

which were then forced together. Excess adhesive was wiped clean before 

bonding. 

The tubes were bonded using the 5//5 coil at a setting of 400 W, using protocol 

C. The tubes were rotated by 90° halfway through each of the three steps in the 

protocol. The total bonding time was 15 min. Two sets of tubes were bonded in 

identical fashion. Figure 21 shows the bonding process in progress. 

Figure 21. Induction bonding of full tubes using the Ameritherm induction unit. 

After bonding, the tubes were allowed to cool and were then inspected. The 

flash was fully cured. The joint could not be separated by twisting or pulling on 

the tubes by hand. These tubes are shown in Figure 22. 

6. Discussion and Conclusions 

6.1 Equipment Considerations 

Comparison of the phase I and phase II experiments demonstrates the 

importance of induction power supply characteristics for enabling bondline 

heating using magnetic particles. The Lepel unit, with its restricted tunability 
30

Figure 22. Final bonded tube for phase II experiments. 

and limited magnetic field strength output, was not capable of generating 

adequate heating power over large areas. The Ameritherm unit, on the other 

hand, was able to heat very large areas at only a fraction of its full power 

capabilities. The success of the Ameritherm unit can be attributed both to its 

advanced auto-tuning characteristics, as well as its relatively high frequency of 

operation. It is also important to note that this unit has only been commercially 

available since 1999. Earlier researchers, who concluded that induction bonding 

using magnetic particles was infeasible, were restricted to much less 
31

sophisticated equipment, such as the non-water-cooled, 50 kHz unit used by 

Hahn et al. (1991). 

The Lepel tuning experiments and heating rate comparisons underscore the 

importance of proper coil design, especially for large bond areas. Achieving 

large areas of field coverage with reasonable spatial uniformity must be balanced 

with tuning considerations. In fact, the 3-stage, 15 min process used for the final 

tube bonding in phase II could probably be simplified even further through 

improved coil design. A single stationary bonding step was not possible with 

the 5//5 coil because its length (9 in) was identical to the length of the bond area. 

Centering the bond within the coil produced non-uniform heating, due to 

fringing effects at the ends of the coil. It is likely that a longer (e.g., 12 in) coil 

could have been designed which would have allowed a single stationary 

bonding step, perhaps reducing the 15 min process to only 10 min. 

6.2 Material Considerations 

While the final bonding demonstration was successful, the adhesive and 

magnetic materials used were far from optimal. The lap shear strength data 

indicates that full strength is not achieved until the adhesive is heated for 20 min, 

even at higher processing temperatures. This limitation prevents full utilization 

of the rapid heating provided by induction processing. It is possible that 

through customization of adhesive chemistries, full strength bonds can be 

achieved in much less time. 

The magnetic materials used in these experiments were also far from optimum. 

These soft ferrites were used because they are commercially available, although 

not intended for induction heating applications. The primary limitations of these 

materials are their low energy density and poor Curie temperature behavior. 

Higher energy density materials would allow adhesive formulations with lower 

magnetic particle loadings. Full utilization of Curie temperature control would 

prevent bondline overheating and allow more rapid heating. These features 

would reduce process times and improve thermal uniformity. In fact, the 

potential for improved thermal uniformity would also simplify coil design, since 

the bondline thermal history would be much less sensitive to local variations in 

induction field strength. We have already designed improved magnetic 

materials and demonstrated their superior heating performance (Wetzel et al. 

2000). We hope to scale-up production of these materials and utilize them in 

subsequent induction bonding applications. 

6.3 Adiprene Self-Heating Phenomenon 

The Adiprene self-heating experiments (section 5.3.3) provide a number of 

confounding results. Most importantly, the self-heating of a non-conductive, 

non-magnetic glass fiber/polymer composite at submicrowave frequencies is 

completely puzzling. At these low MHz frequencies, typically only conductive 
32

and magnetic heating mechanisms can produce heating, as is the case with 

carbon-fiber composites and magnetic particle-loaded adhesives. Most glass 

fiber/polymer composites will show absolutely no heating at these frequencies. 

Some polymers will heat significantly at microwave (GHz) frequencies due to 

dielectric mechanisms. These same dielectric mechanisms are active at the lower 

inductive frequencies, but their magnitude is rarely, if ever, large enough to 

produce a measurable heating effect. 

A second puzzling effect is that the two minitubes that blistered were subject to 

the same 500 W field as the unbonded tubes in Figure 18. However, after 10 min 

in the field, the unbonded tubes only self-heated by 25 °C. This small 

temperature rise would not be expected to be significant enough to cause 

blistering during a bonding run. It is possible that the combined effect of 

adhesive heating and Adiprene self-heating caused the local overheating that led 

to blistering. However our preheated, unbonded tube (Figure 18) did not show 

significantly accelerated heating. In fact, it did not appear to heat at all. Perhaps 

there are more subtle thermal and moisture interaction effects at work. 

The extremely localized nature of the overheating is also puzzling. Overheating 

always occurred in a very thin strip of material located at the very center of the 

coil. In fact, the strip of material seemed to lie directly below the center coil 

winding and had the same width as the copper tubing. The center of the coil 

could be the location of the highest induction field strength. However, with the 

parallel solenoid coils, the center of the coil is also the boundary between the end 

of one semi-coil and the beginning of the second semi-coil. This transition may 

play a role in the localized heating. At the center of the coil, there will be a large 

potential gradient between the two semi-coils, since one semi-coil is at its highest 

local potential while the other is at its lowest local potential. This potential 

gradient should produce an electric field, which will oscillate at the induction 

operating frequency. It is possible that this electric field causes dielectric heating 

in the polymer. We are not sure how to modify the parallel induction coil design 

to alleviate these effects, or what other material systems will be susceptible to it. 

6.4 Future Applications of Induction Heating 

Although there is significant room for process improvement, both in terms of 

material and equipment, the tube bonding demonstrations confirm the feasibility 

of using induction processing for rapid curing of large, structural bonds. The 

experiments also show that magnetic particle susceptors are viable alternatives to 

traditional metallic susceptors. In fact, recent advances in magnetic particle 

susceptor design should lead to significant process improvements which will 

make this approach clearly superior to other induction bonding approaches. We 

also expect that continuing process demonstrations will lead to wider application 

of this technology to other Army and Department of Defense systems, including 

composite structural manufacturing and repair. 
33

7. References 

Clark, W. G. Jr., R. E. Shannon, and W. R. Junker. “System and Method for 

Remotely Heating Polymeric Materials to a Selected Temperature.” U.S. 

Patent 5,391,595, 21 February 1995. 

Eagle, G. “Induction Heat Curing of Adhesives.” Proceedings of the SPE 

ANTEC 1987, Detroit, MI, 1987. 

Fink, B. K., S. H. McKnight, J. W. Gillespie Jr., and S. Yarlagadda. 

“Ferromagnetic Nano-Particulate and Conductive Mesh Susceptors for 

Induction-Based Repair of Composite Materials.” Proceedings of the 21st 

Army Science Conference, Norfolk, VA, 15–17 June 1998. 

Guiles, M. A., R. Haghighat, A. Singh, and R. Lerner. “Temperature-Controlled 

Induction Heating of Polymeric Materials.” U.S. Patent 6,056,844, 2 May 

2000. 

Hahn, G. L., L. K. Bergstrom, J. N. Border, and B. E. McIlroy. “Induction Heating 

Repair of Structures.” Final Report F33657-88-C-0087, McDonnell Douglas 

Corporation, St. Louis, MO, October 1991. 

McKnight, S. H., B. K. Fink, S. Wells, S. Yarlagadda, and J. W. Gillespie Jr. 

“Accelerated Curing of Epoxy Paste Adhesives for Repair of Composites 

Using Induction Heating.” Proceedings of SPE ANTEC 1998, Atlanta, GA, 

26–30 April 1998. 

Stefanides, E. J. “Epoxy Cured by Induction Heating Gives Strong Sheet Metal 

Joint.” Design News (Boston), vol. 43, no. 12, pp. 102–103, 22 June 1987. 

Tay, T. E., B. K. Fink, S. H. McKnight, S. Yarlagadda, and J. W. Gillespie Jr. 

“Accelerated Curing of Adhesives in Bonded Joints by Induction Heating.” 

Journal of Composite Materials, vol. 33, no. 17, pp. 1643–1664, 1999. 

Wedgewood, A. R., and P. E. Hardy. “Induction Welding of Thermoset 

Composite Adherends Using Thermoplastic Interlayers and Susceptors.” 

Proceedings of the 28th International SAMPE Technical Conference, Seattle, WA, 

pp. 850–861, 1996. 

Wetzel, E. D., and B. K. Fink. “Feasibility of Magnetic Particle Films for Curie 

Temperature-Controlled Processing of Composite Materials.” ARL-TR-2431, 

U.S. Army Research Laboratory, Aberdeen Proving Ground, MD, March 

2001. 
34

Wetzel, E. D., B. K. Fink, Y. F. Li, and J. Q. Xiao. “Advanced Magnetic Materials 

for Curie Temperature-Controlled Bonding.” Proceedings of the 22nd Army 

Science Conference, Baltimore, MD, 11–13 December 2000. 

Yarlagadda, S., B. Gourichon, B. K. Fink, S. H. McKnight, and J. W. Gillespie Jr. 

“Induction-Assisted Bonding of Elevated Temperature Curing Adhesive 

Systems.” Proceedings of SPE ANTEC 2000, Orlando, FL, 7–11 May 2000. 
bonding, adhesive, induction, magnetic 43

Large structural composite tubes, prototype components for a proposed U.S. Navy application, are bonded by induction

curing of an engineering adhesive. Magnetic powder is used as the susceptor material and is directly incorporated into

the adhesive prior to processing. Different induction power supplies, coil designs, and adhesive formulations are

investigated. Final demonstration runs bond 9-in-long, 3-in-diameter axisymmetric bondlines in 15 min. These results

demonstrate for the first time the successful induction bonding of large structural composite components using magnetic

particulate susceptors. 

*University of Delaware, Center for Composite Materials, Newark, DE 19716

ARL-TR-2818

U.S. Army Research Laboratory

ATTN: AMSRL-WM-MB

Aberdeen Proving Ground, MD 21005-5069

Eric D. Wetzel, William A. Spurgeon, and Christian J. Yungwirth*

Induction Bonding for Structural Composite Tubes 622618.H80

September 2002 Final, September 2000–August 2001 

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 WALTHAM MA 02354-1196 

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 R O MEARA 

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 NEWPORT RI 02840 

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 1449 MIDDLESEX ST 

 LOWELL MA 01851 

 1 O GARA HESS & EISENHARDT 

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 9113 LESAINT DR 

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 1 CONNEAUGHT INDUSTRIES INC 

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 3 PACIFIC NORTHWEST LAB 

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 RICHLAND WA 99352 

 8 ALLIANT TECHSYSTEMS INC 

 C CANDLAND MN11 2830 

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 600 SECOND ST NE 

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 1 SAIC 

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 MCLEAN VA 22102 

 1 APPLIED COMPOSITES 

 W GRISCH 

 333 NORTH SIXTH ST 

 ST CHARLES IL 60174 
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 1 CUSTOM ANALYTICAL 

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 13000 TENSOR LANE NE 

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 3 ALLIANT TECHSYSTEMS INC 

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 515 GILES ST 

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 5 AEROJET GEN CORP 

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 1 BRIGS COMPANY 

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 2668 PETERBOROUGH ST 

 HERNDON VA 22071-2443 

 1 ZERNOW TECHNICAL SERVICES 

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 425 W BONITA AVE STE 208 

 SAN DIMAS CA 91773 

 1 GENERAL DYNAMICS OTS 

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 3 GENERAL DYNAMICS OTS 

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 2350 E EL SEGUNDO BLVD 

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 2 CYTEC FIBERITE 

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 2 UDLP 

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 SANTA CLARA CA 95052 

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 2 UDLP 

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 2 BOEING ROTORCRAFT 

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 1 MATERIALS SCIENCES CORP 

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 500 OFC CENTER DR STE 250 

 FT WASHINGTON PA 19034 

 1 NORTHRUP GRUMMAN CORP 

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 & SYSTEMS DIV 

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 3925 W BRAKER LN STE 400 

 AUSTIN TX 78759-5316 

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 1233 SHELBURNE RD STE D8 

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 05403-7700 
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 1 R EICHELBERGER 

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 409 W CATHERINE ST 

 BEL AIR MD 21014-3613 

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 8500 NORMANDALE LAKE BLVD 

 SUITE 1610 

 BLOOMINGTON MN 55437-3828 

 1 UCLA MANE DEPT ENGR IV 

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 LOS ANGELES CA 90024-1597 

 2 UNIV OF DAYTON 

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 R Y KIM 

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 300 COLLEGE PARK AVE 

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 1 UMASS LOWELL 

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 N SCHOTT 

 1 UNIVERSITY AVE 

 LOWELL MA 01854 

 1 IIT RESEARCH CENTER 

 D ROSE 

 201 MILL ST 

 ROME NY 13440-6916 

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 R S ENGEL 

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 1 PURDUE UNIV 

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 & AEROBALLISTICS 

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 1 UNIV OF MAINE 

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 R LOPEZ ANIDO 

 5793 AEWC BLDG 

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 1 JOHNS HOPKINS UNIV 

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 11100 JOHNS HOPKINS RD 

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 D HEIDER 

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 NEWARK DE 19716 

 1 DEPT OF MATERIALS 

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 UNIVERSITY OF ILLINOIS 

 AT URBANA CHAMPAIGN 

 J ECONOMY 

 1304 WEST GREEN ST 115B 

 URBANA IL 61801 

 1 UNIV OF MARYLAND 

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 A J VIZZINI 

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 1 DREXEL UNIV 

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 32ND & CHESTNUT ST 

 PHILADELPHIA PA 19104 

 3 UNIV OF TEXAS AT AUSTIN 

 CTR FOR ELECTROMECHANICS 

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 J KITZMILLER 

 10100 BURNET RD 

 AUSTIN TX 78758-4497 

 3 VA POLYTECHNICAL 

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 BLACKSBURG VA 24061-0219 

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 SAN ANTONIO TX 78228-0510 

ABERDEEN PROVING GROUND

 1 US ARMY MATERIEL 

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 AMXSY TD 

 APG MD 21005-5071 

 1 US ARMY ATC 

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 400 COLLERAN RD 

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 AMSRL WM BD 

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NO. OF NO. OF 

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ABERDEEN PROVING GROUND (CONT)

 AMSRL WM BR 

 C SHOEMAKER 

 J BORNSTEIN 

 AMSRL WM M 

 D VIECHNICKI 

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 W DRYSDALE 

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 S GHIORSE 

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 D HENRY 

 R KASTE 

 M KLUSEWITZ 

 M LEADORE 

 R LIEB 

 E RIGAS 

 J SANDS 

 D SPAGNUOLO 

 W SPURGEON 

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 E WETZEL 

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 A WERECZCAK 

 J LASALVIA 

 J WELLS 

 AMSRL WM MD 

 W ROY 

 S WALSH 

 AMSRL WM T 

 B BURNS 

 M ZOLTOSKI 

ABERDEEN PROVING GROUND (CONT)

 AMSRL WM TA 

 W GILLICH 

 T HAVEL 

 J RUNYEON 

 M BURKINS 

 E HORWATH 

 B GOOCH 

 W BRUCHEY 

 M NORMANDIA 

 AMRSL WM TB 

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 1 LTD 

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