Induction Cure of Adhesives For Composite Repair Applications

2003 NCMS/CTMA SymposiumSalt Lake City, Utah1 April 2003

Army Research LaboratoryComposites and Lightweight Structures Branch

Bldg. 4600, AMSRL-WM-MBAberdeen Proving Ground, MD 21005-5069

James M. Sandsjsands@arl.army.mil 410-306-0878

and Eric D. Wetzel

ewetzel@arl.army.mil 410-306-0851

Induction Heating• Induction heating: high frequency electromagnetic field remotely

couples power source to susceptor material– Typical frequencies: 0.10 - 10 MHz

• Sub-microwave frequencies• Pose few risks to human exposure

– Typical amplitudes: 50 - 500 Oe• Field generated by passing high

frequency current through water-cooled copper tubing (induction coil)– Spatial dispersion of field can be controlled through coil design

• Heat generated in susceptor material– Electrical conductors – Magnetic materials

• Metals, carbon fiber composites • Magnetic metals, ceramics• Non-susceptor materials

– Polymers – Most ceramics– Glass fiber, Kevlar fiber composites

induction power supply

induction coil

susceptormaterial

Induction Heating Equipmentand Geometry

• Induction power supply– Ameritherm NovaStar 1M– Operating frequency 10-15 MHz– Magnetic field amplitude: "0-1500 W"– Self-tuning, solid state, water cooled– Relatively recent technology

tube A tube B

induction coil

adhesive with magnetic filler

Bench top

Hand heldCoils

• Requires incorporation of susceptor material in adhesive– Adherend preferably a non-susceptor material

• Advantages over conventional processing (surface heating approaches, e.g., convection oven, radiant heater, heat blanket, heat gun):– Faster heating– Can heat embedded bondlines (e.g., thick-section composites)– Self-regulating - does not require continuous process control

• Promising susceptor material: magnetic particles– High energy density– Automatic temperature regulation

• Ensure proper cure, minimize risk of thermal degradation

Induction Processing of Adhesives

non-susceptor(adherend) susceptor

(adhesive)

induction field

Minimize Operator Training. Minimize Operator Errors.

Magnetic Particle Susceptors• Heating mechanism −> magnetic hysteresis

– Losses associated with magnetization / demagnetization of material

– Requires high ( > 1MHz) frequencies• Primary advantage −> automatic temperature regulation

– Hysteresis heating mechanism disappears at material Curie temperature (Tc)

Temperature Time(material property measurement) (particle heating in AC magnetic field)

Hys

tere

sisE

nerg

yD

ensi

ty

Tem

pera

ture

( ferromagnetic /ferrimagnetic )

( par

amag

netic

)

Particles heat to and dwell at Tc without active feedback control !!

Curie temperature

Temperature Control UsingCurie Limit

• Curie temperature determined by composition of magnetic material

• Choose magnetic material with Tc = processing temperature (example, T = 354°C)

• Incorporate filler into adhesive• Demonstrate constant heating

temperature with power variation

• Demonstrate induction adhesive processing in large-scale structural applications

400

300

200

100

0

Tem

pera

ture

(°C

)

Time (s)0 100 200 300

Pow

er Level

temperature

power

0.25

0.75

0.5

1.0

0

PSU proc. window

Composition: 25% Ni in PolysulphoneCurie Temperature of Ni = 354 °CMagnetic Field Frequency = 6.5 MHz

From Fink et al. “Ferromagnetic Nano-Particulate and Conductive Mesh Susceptors for Induction-Based Repair of Composites.” Proceedings of Army Science Conference. Norfolk, VA. June 15-17 1998.

Induction Particles for Adhesive Repair

• Heating tests performed atf = 6.28 MHz, Ha = 10 kA/m

• No power regulation used!

• Joint ARL-UD patent application in process

0

50

100

150

200

250

300

350

0 8 16 24 32 40 48 56

Tem

pera

ture

(°C

)

Time (s)

x = 0.25

x = 0.0

x = 1.0

x = 0.50

Co2-2xZn2xBa2Fe12O22

x Tc (theory) Dwell Temp.(measured)Tc0.0 340 334 346 0.25 288 281 283 0.5 236 217 222 0.75 183 164 - 1.0 130 105 117

note: all temperatures in °C

Select curie particles that meet processing requirements (limits)!

Induction Particle Susceptors

• Commercial particle susceptors(FP160) demonstrate variability in heating uniformity

• Historic reason for limited usefulness of technology

• Not ideal for controlled bonding

• Synthetic particle susceptors(Zn2Y) demonstrate ideal heating behavior

• Ideal for operator independent performance

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200 250 300

FP160, 100 WFP160, 500 WFP160, 1500 WZn2Y, 100 WZn2Y, 500 WZn2Y, 1500 W

Tem

pera

ture

(°C

)

Time (s)

±5°C ±65°C

Dilution Effect on Curie Temperature

Induction heating behavior of various volume fractions of magnetic powders

dispersed in alumina powder, in 14 MHz field at amplitude of 500 W

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200 250 300

FP160, 5%FP160, 10%FP160, 20%Zn2Y, 5%Zn2Y, 10%Zn2Y, 20%

Tem

pera

ture

(°C

)

Time (s)

• Dilution effect is significant

• Cure rate and cure temperature are poorly controlled

• Rapidly reach dwell temperature

• Minimal thermal gradient in bondline

Induction Cured Adhesive Performance

0

5

10

15

20

25

30

Lap

Shea

r Str

engt

h (M

Pa)

Avg (MPa) 25.392 21.132 20.863 22.103 13.375 23.451 6.9547

Oven, unfilled Oven, filled 500 W,

15 min500 W, 30 min

1 kW, 5 min

1 kW, 15 min

1.5 kW, 3 min

• No time-dependence on adhesive toughness

• Performance is not sensitive to rapid cure

Under-cured

90 minuteoven cure

15 minuteInduction cure

SUCCESSFUL DEMONSTRATION

Importance of Induction Control

• Minimize impact of “heat sinks”• Applicable to rapid cure of thick sections• Effective for use in repair around honeycomb• Uniformity of thermal cure in induction field• Adhesive development necessary to take advantage of technology• Increase cure rate == Decrease repair time == Decrease

turnaround time• Potential to improve mission readiness of aircraft and ground

vehicle structures

Tube Bonding Example• Tubes fully bonded in 15 minutes!

adhesive bondline

mechanical testing at NUWC

Summary

• Significant developments in induction processing of adhesives– Development of advanced magnetic filler materials

• High energy density -> rapid heating rates and consistent dwell temperature at low filler contents

• Curie temperatures from 130°C - 380°C• High purity -> consistent dwell temperature

– Commercial availability of high frequency induction processing equipment

• Induction is a non-contact heating mechanism suitable for use in composite repairs

• Adhesives designed to be insensitive to RAPID induction cure conditions

• Demonstrates ability to increase rate of repair and decrease down-time over room temperature cured adhesives

• Materials can be selectively designed to meet repair performancerequirements allowing rapid non-contact curing of adhesives

Where to go from Here?

• Commercial development of curie limiting particles• Commercial development of adhesives or commercial

implementation of induction technique in current adhesives• Establish “limits” for induction technology

– Electronic interferences– Thermal heating in composite laminates

• Demonstrate advantages to repair applications