Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-2457 April ‘2001 

Non-Polluting Composites Repair 

and Remanufacturing for Military 

Applications: Induction-Based 

Processing 

Bruce K. Fink, Nicholas B. Shevchenko, and James M. Sands 

Weapons and Materials Research Directorate, ARL 

Shridhar Yarlagadda and John W. Gillespie, Jr. 

University of Delaware 

Approved for public release; distribution is unlimited. 
Abstract 

The development of induction-based processing of carbon-fiber (CF) 

thermoplastic-matrix composites and accelerated cure of thermosetting 

adhesives has the potential to provide nonautoclave processing technology for 

manufacturing and repair of polymer-matrix composites (PMCs). In this report, 

the results of recent tests demonstrating bonding of composites. using 

commercial off-the-shelf thermal-cure adhesives that are heat cured via an 

induction field using an inductive susceptor are discussed. This method of cure 

utilizes heat generation within metal screen-based susceptors to cure the 

adhesive makx via a heat transfer mechanism. The mechanical-performance of 

these bonds is presented in comparison with autoclave and thermally cured 

baselines. No substantial loss of mechanical lap-shear strength is observed in 

adhesive bonds processed by induction. In addition, an example of induction 

welding of a thermoplastic-impregnated carbon fiber (A!%) is presented. In 

order to successfully demonstrate induction welding for manufacture of CF 

composites, the degradation of the polymer in the laminates is also investigated. 

No measurable degradation of the polymer, either by dielectric or thermal 

breakdown when heated by induction, was observed. 

ii 
Acknowledgments 

This research was supported in part by the U.S. Department of Defense through 

the Strategic Environmental Research and Development Program (SERDP) under 

contract to the US. Army Research Laboratory (ARL) under the Non-Polluting 

Composites Repair and Remanufacturing for Military Applications program. 

. . . 

lu 
iv 
Contents 

Acknowledgments 

.I. 

111. 

List of Figures vii 

List of Tables ix 

1. Introduction 

2. Induction-Based Repair and Remanufacturing 1 

2.1 Induction-Based Repair of Multifunctional Composite Armor. .............. 2 

2.1.1 Material Systems ............................................................................... 3 

2.1.2 Induction Bonding Setup ................................................................. 4 

2.1.3 Mechanical Performance .................................................................. 7 

2.1.4 Conclusions to Induction-Based Adhesive Bonding.. .................. 9 

2.2 Induction-Based Remanufacture of Thermoplastic Composite 

Laminates .................................................................................................................. 9 

2.2.1 Heating Mechanisms for Carbon/Thermoplastics ..................... 10 

2.2.1.1 Theo&id Heating Model ............................................. 11 

2.2.1.2 Joule Heating of Carbon Fibers [15,X] ........................ 13 

2.2.1.3 Dielectric Hysteresis Heating at Fiber Junctions.. ..... 

..13 

2.2.1.4 Heat Generated by Fiber Contact Resistance ............... 14 

2.2.1.5 Material Properties .......................................................... 15 

2.2.1.6 Two-Ply Heating Model ................................................. -15 

2.2.1.7 Two-Ply Model Results ................................................... 16 

2.2.1.8 Through-Thickness Heating ........................................... 18 

2.2.2 Degradation Studies ........................................................................ 19 

2.2.2.1 Thermal Degradation Study ........................................... 20 

2.2.2.2 Weight Loss Measurements ............................................ 20 

2.2.2.3 Molecular Weight Characterization .............................. 21 

2.2.2.4 Electrical Degradation Study ......................................... 

.23 

2.2.2.5 Mechanical Performance ................................................. 24 

2.2.3 Induction Coil Design ..................................................................... 25 

V 
2.2.3.1 Laminator Coil Design . . . . . . . . . . . . . . . ..__.._.............................. 25 

2.2.4 Conclusions to Induction-Based Thermoplastic Composite 

Lamination . . . . ..____....._.................--......................*..-.-.-.*...........*--.-..............*.. 25 

3. References 

Distribution List 

Report Documentation Page 

27 

29 

47 

vi 
r. Figure 3. Induction coil motion direction and configuration. . . . . . . . . . . ..+_...........-.-....... 5 

Figure 4. Typical bondline temperature profile along the coil motion 

direction . . . . . . ..*....~..__...*.......___.~~...._*............~....~~***..~....f.~.......~...........~....~..........~~........ 5 

Figure 5. Mechanical performance for SW-2214 adhesive system cured at 

250 “F. (A-H are defined in Tables 1 and 2). __..f.~.._..............__f........_.._......~~...~.~.... 8 

Figure 6. Mechanical performance for FM300K adhesive system cured at 

350 “F. .......................................................................................................................... 8 

Figure 7. Mechanical performance for EC-1386 adhesive system cured at 

350 “F ........................................................................................................................... 9 

Figure 8. Schematic of the induction heating process for carbon/ 

thermoplastic composites- ...................................................................................... 10 

Figure 9. Schematic of induced voltage loops in the composite.. ........................... 12 

Figure 10. Heating mechanisms in each individual conductive loop.. .................. 12 

Figure 11. Circuit model for dielectric heating. ...................................................... 1.14 

Figure 12. Circuit model for heating by fiber contact resistance. ........................... 14 

Figure 13. Dielectric properties of PEI (ultem 1000, GE plastics). .......................... 15 

.Figme 14. Schematic of a 2-ply heating model. . . . . . . ..___............__............._..........____... 16 

Figure 15. Comparison of heating patterns for [O/90] 2-ply stack. . . . . . . . . . . . . . . . . . ..__... 17 

Figure 16. Comparison of heating patterns for [O/45] 2-ply stack. . ..___._........____... 17 

Figure 17. Model through-thickness heat generation of an 8-ply carbonfiber laminate with orientation [O/90/O/-90]5. __.+.....___ __ __......,f.._.____.f.......____f..... 18 

Figure 18. Predicted through-thickness temperature profiles at various 

time steps. ______....~...._.__.___..f....__.......___....,..~.~......_____f........_____.......~.~~~..~~........____.~~f.f.. 19 

Figure 19. TGA weight loss profile for PEI resin and prepreg (heating 

rate = 15 ‘C/min> _..........f___.......-.-......-.-*......--.-......--._..........._f______........*.-.-......---... -21 

Figure 20. Breakdown voltage measurements for neat PEI films . ..__..........____....___. 24 

Figure 21. Process schematic of lamination stage in a production line. . . . ..____....___ 26 

Figure 22. Schematic of rectangular coil for lamination stage. ............................... 26 

Figure 23. Temperature profiles of S-ply stack with rectangular coil .................... 26 

vii 

List of Figures 

Figure 1. Multifunctional composite armor. __.... ..___ . . . . . . . . . ..:. . . . . . . . .._............. ____+......__.... 3 

Figure 2. Specimen configuration during induction bonding. . . . . . . . . . . ..__...........-.....+. 4 
. . 

vlll 
List of Tables 

Table 1. Baselines for comparison with induction bonding. ..................................... 6 

Table 2. Test cases for induction bonding. ................................................................... 7 

Table 3. Experimental data of neat ultem 1000 resin under various heat 

treatments. ........................................................................................................... -22 

Table 4. Degradation study of PEI and A%/PEI prepreg using GPC ................... 23 

Table 5. Comparison of mechanical properties. ___......._... e . . . . ..__..........._..-.................. 25 

ix 
INTENTIONALLY um BLANK. 

X 
1. Introduction 

The U.S. Army is currently pursuing the development of low-cost nonautoclave 

manufacturing technologies for polymer-matrix composites (PMCs) that will 

allow the U.S. military and contractors to prepare high-performance composites 

for armor, munition, and vehicle applications with reduced costs, reduced 

environmental impact, and increased efficiency. The authors have previously 

introduced the concept of induction-based processing and the environmental 

impacts for Army applications [l]. The initial repair demonstrations involved 

heat generation using a metallic (electrically conductive) mesh to translate 

electiomagnetic energy into thermal energy at an adhesive bond line. In this 

report, the background and physics of induction processing in carbon-fiber 

thermoplastic laminates is reviewed and the successful development of 

induction healing that allows suitable repair of many composite stictures is 

discussed. Specifically, the adhesive bonding using susceptor-based induction 

heating, susceptorless induction-based carbon-fiber laminate consolidation, and 

development of process models for electromagnetic heating of carbon-fiber 

composites, including heat generation by carbon fibers, metal-mesh susceptors, 

and magnetoresistive particles is discussed. 

2. Induction-Based Repair and Remanufacturing 

The search for cost-effective environmentally friendly manufacturing methods 

has led to the study of induction heating for bonding and processing of 

composites [l]. Electromagnetic cure methods involve using induction or 

electrical resistance heating focused directly at the material to be cured. 

Induction heating occurs when a current-carrying body, or coil, is placed near 

another conductor, the susceptor material. The magnetic field caused by the 

current in the coil induces a current in the susceptor. This induced current 

causes the susceptor to heat due to Joule heating, and, in the case of a 

ferromagnetic material, due to hysteresis losses. Carbon-fiber reinforcement in 

composite materials can function as the susceptor. For other material systems, 

the susceptor is a metallic mesh or magnetic particles. Energy can be introduced 

into the precise region to be cured both in the plane of the structure and at the 

specific depth required [2]. 

The ability of induction heating to rapidly process carbon-fiber-based 

thermoplastic composites is a significant environmental asset. Assuming that the 

1 
thermoplastic composite meets the performance and quality requirements of an 

equivalent thermoset counterpart, one can replace the limited shelf-life thermoset 

with the unlimited shelf-life thermoplastic. This completely eliminates 

hazardous raw material wastes at the production level resulting from shelf-life 

expiration and raw-material overages associated with thermoset-based 

composites production. 

Other advantages of induction include reduction of volatile organic compounds 

(VOCs) and nitrous oxide (NO X ) emissions by processing out of the autoclave, 

localizing processing controls for repair and remanufacturing, and eliminating 

processing steps. In addition, induction offers internal noncontact heating; the 

possibility of a moving heat source (the coil) to heat large areas; high efficiency; 

control of the heat generation by coil design or by susceptor design; and 

powerful, portable, and easy-to-operate units [3,4]. 

2.1 Induction-Based Repair of Multifunctional Composite Armor 

This research is motivated by the expanding use of multifunctional hybrid 

materials in military ground vehicles and the increasing need for field-expedient 

and depot-level repair procedures for these thick-section components. 

The critical issue in adhesive-based repair of composites is the application of 

sufficient heat and pressure at the bond line. It is highly desirable that thermal 

generation be localized at the bond line and be evenly distributed (taking into 

account thermal conductive losses). This is especially important with the 

increasing use of multifunctional hybrid composites, such as composite armor. 

These composites typically have several layers, each serving a different function, 

as shown in Figure 1. Repair of such a thick-section composite will require 

heating locally at the appropriate bond lines; one such method is induction 

heating [5,4]. In addition, due to the noncontact nature of induction heating, it 

may be possible to bond several layers at the same time, which reduces 

hazardous waste, energy consumption, arid repair times for the part. 

Conventional repair techniques (e.g., heat blankets) will require bonding of one 

layer at a t&e, resulting in multiple potential hazardous waste streams such as 

trim, consumables, and VOC emissions. 

While the induction-based repair procedure has the potential to reduce 

hazardous waste, it is essential that the repaired part meet the performance 

requirements dictated by the application. Hence, the initial work done under the 

program focuses on evaluating the performance of induction-based repair 

procedures. The goal is to obtain performance similar to that achieved with 

conventional repair proceduresRecent studies [I, 7, 81 have shown similar properties for induction-heated 

adhesive bonds compared to baselines for room-temperature adhesives. 

Electrically conductive mesh susceptors and epoxy-based adhesives were used. 

2 



significantly from bond line temperatures due to the good thermal conduction of 

carbon fibers; this was confirmed by thermocouple measurements. For 

glass/vinyl ester, differentials of up to *lO “C between the bond line and surface 

were noted. Further refinements to determine the ideal motion velocity and 

pattern for minimal temperature gradients are in progress. 

For a carbon/epoxy substrate or adherend, no susceptor is necessary. However, 

bonding tests were performed for both cases (i.e., with and without the mesh 

susceptor) to assess the presence of the mesh on bond strengths. For the 

carbon/epoxy substrate, all three adhesive systems were induction bonded, with 

and without the mesh. For the glass/vinyl-ester system, the SW-2214 adhesive 

was induction bonded with the mesh, as the cure cycle of the other systems 

(350 “F) could cause degradation in the substrate. All the induction-bonded 

specimens were fabricated under vacuum pressure. Table 1 lists the baselines; 

Table 2 lists the test cases. 

Table 1. Baselines for comparison with induction bonding. 

I I ( Number of T -~-- I I 

Case Substrate Specimens Adhesive Cure Cycle 

A Carbon/Epoxy 6 SW-2214 250 “F, Vacuum 

B SW-2214 + mesha 60 min 

A Carbon/Epoxy 6 EC-1386 350 “F, Vacuum 

B EC-1386 + mesh 6omin 

A FMSOOK 350 “F, Vacuum 

B FM3OOK f mesh 6Omin 

Carbon/Epoxy 6 

E FMSOOK 350 “F, 40 psi 

F FM300K + mesh 6OminG Glass/Vinyl Ester 6 SW-2214 250 “F, Vacuum 

SW-2214 + mesh 6Omin 

aMesh = SS304,30 x 30,0.0075-in wire. 

Baseline lap-shear specimens were fabricated according to the manufacturer’s 

recommended cure cycles. For the FM300K adhesive system, two pressures were 

considered: vacuum (-15 psi) and 40 psi. For aII other autoclave specimens, 

vacuum pressure was used. This allows for direct comparison with inductionbonded lap-shear tests, which were performed tider vacuum pressure. 

Test cases of single lap-shear (SLS) specimens were fabricated as described in 

Table 2. The adhesive bond-line thicknesses for the specimens were measured 

by means of a traveling microscope. In all cases where no susceptor or mesh was 

6 
Table 2. Test cases for induction bonding. 

Case Substrate 

Number of 

Snecimens 

I I 

Adhesive r ~~ Cure Cycle 

C 

D Carbon/ Epoxy 6 SW-2214 250 “F, Vacuum 

SW-2214 + mesha 6Oti 

C Carbon/Epoxy 6 EC-1386 350 “F, Vacuum 

D EC-1386 + mesh 60 min 

Carbon/Epoxy I 6 I 

FM300K 350 “F, Vacuum 

FM300K + mesh I 6Omin I 

H Glass/Vinyl Ester 6 SW-2214 + mesh 250 “F, Vacuum 

6Olllill 

a Mesh = F&304,30 x 30,0.0075-in wire. 

used, the thickness of the bond line was consistently about 0.002 in. The mesh 

caused an increase @ bond-line thickness to about 0.013 in. These values were 

very consistent and did not appear to vary greatly between specimens. The 

effect of bond line thickness variation was not considered in the analysis, 

although it is recognized that this variable may be significant. 

2.1.3 Mechanical Performance 

All SLS specimens were tested to failure in an Instron universal testing machine. 

The mean nominal shear strengths and the associated error bars are shown in 

Figures 5, 6, and 7 ,for SW-2214, FM300K, and EC-1386, respectively. Six 

specimens were tested for each case, and in all cases cohesive failure of the 

adhesive layer was obtained. Degree of cure for each case was determined by 

differential scanning calorimetry (DSC) and was greater than 95% for all 

observed specimens. 

In general, for all the adhesive systems, no loss in mechanical performance is 

noted, between induction-cured and autoclave baseline specimens. In some 

cases (Figures 5 and 7), the induction-cured specimens exhibit higher mean shear 

strengths, which may reflect the thinner bond line. The same bend is observed 

for the glass/vinyl-ester system (cases G and H in Figure 5). It is also interesting 

to note the relatively similar scatter in strength data between the induction-cured 

specimens and the autoclaved baselines, despite the temperature differentials 

during induction cure. For the FM300K film adhesive system, higher pressure 

during the cure cycle (40 psi) causes a significant increase in the bond strength 

but not in the presence of a mesh. However, the presence of the mesh does not 

seem to affect performance in vacuum-processed specimens. For the EC-1386 

and SW-2214 paste adhesives, the mesh causes a significant drop in stiength. 

7 
35007 

3000 i T 

2500 

5, 2000 

B 

z 

j 1500 

WI 

500 

0 

’ A ’ B ’ C ’ D ’ G 

Test Cases 

Figure 5. Mechanical performance for SW-2214 adhesive system cured at 250 “F. (A-H 

are defined in Tables 1 and 2). 

Figure 6. Me char&al performance for FM300K adhc :sive system cured at 350 “F. 

'""1 ~ FM300K Adhesive 

Test Cases 

8 
2500 

T 

23 

g 1500 

G 

% 

j 1000 

i 

500 

0 

I 1356 F. Vacuum 

-I T’ ’ 

I A ’ B ’ c ’ D ’ 

Test Cases 

Figure 7. Mechanical performance for EC-1386 adhesive system cured at 350 “F. 

2.1.4 Conclusions to Induction-Based Adhesive Bonding 

Following the methodology established in the previous work [9] on thermoset 

adhesive cure, performance studies of high-temperature curing adhesive 

systems, due to induction cure, have shown no loss in bond strengths between 

baselines and induction processing. This study, along with the previous effort 

[9], validates induction cure as an alternative method for processing thermally 

curable adhesives while retaining baseline bond strengths. Advantages of 

induction cure stem from reduction of VOCs and NO, emissions by processing 

out of the autoclave and an increased cure rate due to through-thickness heating. 

It also provides a unique ability to perform multimaterial repairs (as in 

composite armor) in a single step, thus eliminating process steps and 

corresponding hazardous waste. 

2.2 Induction-Based Remanufacture of Thermoplastic Composite 

Laminates 

The Department of Defense (DOD) has increased use of carbon-fiber-reinforced 

thermoset prepregs, resulting in a potentially significant hazardous waste stream 

due to shelf-life expiration and VOC emissions during processing [l]. It is 

estimated that the raw material requirements (i.e., prepreg) for one particular 

application will exceed one million pounds annually when that weapon system 

goes into full-scale production, and the potential waste is estimated at 20% of the 

total [l]. This problem can be avoided by the replacement of thermoset-based 

carbon-fiber composite laminates with carbon-fiber-reinforced thermoplastic 

9 

The primary objective of this study is to investigate the heating mechanisms of 

carbon/thermoplastics and then determine the relationship between the 

induction coil and material parameters and heating, Once this relationship is 

determined, it will be possible to perform parametric studies using the major 

process variables in order to optimize and meet the thermal requirements for the 

potential production of thermoplastic-based composite structures2.2.1.1 Theoretical Heating Model 

Alternating magnetic field lines intersecting the laminate induce emf’s within 

each conductive loop are governed by Faraday’s Law of Induction [12]. Loops 

are formed between adjacent plies through the junctions, where fibers overlap 

each other. As a result, the induced current flows along the carbon fibers and 

either through the polymeric region or by direct contact of fibers, into its adjacent 

ply, as shown in Figure 9. Generally the emf induced in a circuit is directly 

proportional to the time rate of change of magnetic flux through the circuit and is 

calculated from 

emf =m4Bo =2nfABo, (1) 

where Bo is the maximum value of the magnetic field normal to the area of the 

conductive loop, A is the area of the conductive loop, and f is the time rate of 

change of magnetic flux. 

Once the emf values for all conductive loops in the calculation domain are 

obtained, Kirchoff’s voltage and current conservation laws are applied to the 

network of conductive loops. Kirchoff’s voltage law (KVL) [12] requires that the 

algebraic sum of all voltages around the loop should be zero while Kirchoff’s 

current law (KCL) means that current is conserved at each node. In 

mathematical terms, they can be expressed as follows: 

c Voltage Drop = Induced emf ; Kirchoff s Voltage Law 

c (Incoming Current - Outgoing Current) = 0 ; Kirchoff s Current Law 

(2) 

Three heating mechanisms are possible within the composite: 

l Joule heating due to the inherent electrical resistivity of the carbon 

fibers. 

l Dielectric heating of the polymer at the fiber junctions. 

l Contact resistance at the fiber junctions. 

In general, prepregs have nonuniform surface roughnesses, which makes it 

difficult to determine which mechanism is dominant at a certain region in the 

interface between plies. In addition, it is not easy to estimate the elecnical 

contact resistance between carbon fibers of adjacent plies. The heating 

mechanisms are shown schematically in Figure 10 and described in detail 

hereafter. 

11 

2.2.1.2 Joule Heating of Carbon Fibers [15,16] 

Previous authors 113-161 h ave compared joule heating in the fiber and dielectric 

heating in the matrix and shown that dielectric heating is the dominant 

mechanism. The current effort includes the contact resistance mechanism and 

performs a parametric study of all three mechanisms for the process variables 

defined for thermoplastic laminates. Successful modeling will identify the key 

heating mechanism and optimize induction-based processing parameters to meet 

quality and performance requirements. This will enable transition from 

thermosets to thermoplastics, thus eliminating potentially large hazardous waste 

stream due to shelf-life expiration. 

Each carbon fiber is treated as a resistor and the heat generated is calculated from 

(3) 

where $iber is the induced current flowing in the fiber and R+ is the resistance of 

the fiber, which can be expressed as 

(4 

where Afiber is the cross-sectional area of the fiber and I, and I, are the spacing 

distances between fiber intersections in the x and y directions, respectively. Note 

that I, and 1, vary according to the mesh size as shown in Figure 10. 

2.2.1.3 Dielectric Hysteresis Heating at Fiber Junctions [13,14]. 

If the distance between the fibers at the junction is enough to form a capacitor, 

dielectric heating takes place, since the molecular dipoles in the matrix cannot 

rotate with the same frequency of the induced voltages in the fibers. The 

dissipation factor (tan S), which is one of the electrical properties of the matrix, 

determines how much heat will be dissipated. The impedance of the capacitor is 

I/ (oC tan 6), where Q is ,the angular frequency of the alternating current and C is 

the capacitance of the material. Considering the configuration of the fiber 

junction shown in Figure 11, the capacitance of the dielectric material can be 

expressed as follows: 

where K is the relative dielectric constant of the material and EO is the permittivity 

of vacuum (8.85 x lo-12 f/m). A, and h are the projection area and distance 

between the fibers at the junction, respectively. Therefore, the impedance of the 

capacitor (ZC) can be written as 

2, = 

h 

WKE ,(tX+$, ’ 

(7) 

13 
Figure 11. Circuit model for dielectric heating. 

The heating generated by the capacitor is as follows: 

h (8) 

2.2.1.4 Heat Generated by Fiber Contact Resistance 

If the fibers at the junction are in contact or the distance between fibers is very 

short, heating can occur at the contact region due to contact resistance between 

the fibers. However, as mentioned previously, it is hard to quantify the contact 

resistance, as it is a function of surface roughness of prepreg and the laminate 

processing parameters. A simple resistor can model the fiber contact and the 

heating mechanism (Figure 12), and through parametric studies and 

experiments, the contact resistance is estimated. 

Fiber 

c 

Contact Regioxl 

Rj = Rjc 

Figure 12. Circuit model for heating by fiber contact resistance. 

14 

I emf Field 1 

1 Apply Kirchhoff’s Laws 1 

Voltage Drop 

in the Domain II Sparse Matrix Solver I 

urrent Segments 

iu the Domain 

Figure 14. Schematic of a 2-ply heating model. 

The process variables used in the model are: (1) coil type (pancake, conical, 

paper clip, and solenoid), (2) coil size (outer dimension, inner dimension, (3) 

number of turns, (4) spacing between turns), (5) distance between induction coil 

and composite, (6) frequency of the current in the induction coil, and (7) size and 

geometry of the composite. 

Variables in the numerical model for parametric studies are mesh size and 

density, fiber-fiber distance at the interface of two plies, and fiber-fiber contact 

resistance or equivalent impedance for fiber junction. 

2.2.1.7 Two-Ply Model Results 

Initial experiments focused on evaluating numerical predictions qualitatively. 

This was done by heating 2-ply stacks at various angles - [O/90], [O/O] -under a 

known magnetic field and comparing measured heating patterns, obtained using 

a calibrated thermal infrared camera, with the 2-ply model predictions. Results 

16 



are subject to potentially critical polymer degradation as a result of a littlestudied phenomenon known as thermoelectric degradation [19]. When polymers 

degrade through any mechanism, they suffer significant losses in strength, 

stiffness, and durability. 

The focus of this effort is on identifying degradation mechanisms during 

electromagnetic induction processing and quantifying their effects on 

performance. There are two possible degradation scenarios associated with 

induction-based processing of carbon/thermoplastics: thermal degradation, and 

electrical degradation due to dielectric breakdown in the matrix. 

2.2.2.1 Thermal Degradation Study 

Weight loss and molecular weight (MW) measurements were used to 

characterize thermal degradation of both neat resin and prepreg. 

Thermogravimetric analysis (TGA) was used for weight loss measurements in 

both air and inert (nitrogen) atmospheres. Gel permeation chromatography 

(GPC) was used to obtain MW measurements. In addition, dissolution times for 

resins in a good solvent (methylene chloride) were also measured. 

2.2.2.2 Weight Loss Measurements 

TGA for both neat PEI and AS4/PEI prepreg indicates no measurable weight loss 

of the bulk material up to 500 “C, as shown in Figure 19. Isothermal TGA data 

also shows. that no weight loss was observed at 350 “C, for up to 1 hr. 

Approximately 2% weight loss was observed at 450 “C after 30 min, which 

indicates significant degradation. Since the normal processing temperature is 

330 OC, thermal degradation of the bulk material is expected to be minimal as 

long as the electromagnetic induction processed material does not exceed the 

processing window. 

Neat PEI samples were exposed to various thermal histories using a TGA 

chamber, and the glass transition temperature was measured using DMA. 

Changes in resin color and dissolving time in a good solvent (methylene 

chloride) were also noted. As shown in Table 3, no significant changes in glass 

transition temperature were observed. However, the color of the resin changed 

from yellow to black, and the dissolving time increased significantly when 

temperature and time increased. Oxygen in the atmosphere also affects the color 

change and dissolving time in the solvent. In several cases (G=K), there was 

some gel left over in the solution, which obviously indicates that crosslinking 

reactions occurred in the polymer. 

The TGA study shows that weight loss alone is not sufficient to identify the 

degree of degradation of the polymer. The initiation of crosslinking in the 

polymer is a better measure of the onset of degradation. Crosslinking on a 

composite surface hinders diffusion of polymer chains during processing of 

complex parts and may result in poor bonding and performance. 

20 

Table 3. Experimental data of neat ultem 1000 resin under various heat treatments. 

Condition of 1 Tg 1 Color 1 Dissolving Time in Solvent 1 

1 Sample 1 Heat Treatient 1 Atmosphere 1 From DMA 1 Change I (Methy!lene Chloride) I 

1 A 1 No treatment 1 I 210 1 None ( Lessthan2hr I 

B 

C 

330 “C, 3omin 

330 “C, 

3omil-l 

Nitrogen 

Air 

210 None 

210 Mild 

Lessthan 2 hr 

3hr 

I I 1 I 

D 350 “C, 

30 min Nitrof ;en I 210 None 4hr I 

I E 

t 

I F 

I G 

-_ --_ 

350 “C, 60 min Nitrogen 210 Mild 5hr 

350 “C, 30 min Air 210 Moderate 6hr 

I 

350 “C, 

hn min I Air I 210 I Moderate I Some left over as a gel I 

I I -- 1.s.s. I I I I I 

I I H 400 “C, 

3omin Nitrogen 1 210 IModerate Some left over as a gel I 

I I I 400 “C, 

60 min Nitcogt m 210 ) Moderate ( Some left over as a gel I 

J 400 “C, 

30 min Air 210 Severe Some left over as a gel 

K 400 “C, 

60 min Air 210 Severe Some left over as a gel 

Results from GPC analysis are shown in Table 4. Neat resin and prepreg 

specimens were tested under various thermal histories in air, nitrogen, and 

vacuum atiospheres. All the GPC curve areas were intensity normalized with 

the neat resin case (sample A). In the neat resin study, no change is observed up 

to 350 “C for 30 min in nitrogen (AeD). However, in the presence of air 

(atmospheric oxygen), significant increases in the GPC-curve areas are observed, 

indicating the presence of crosslinked polymer chains. Samples D and F show 

the effect of atmosphere, F and J show the effect of temperature, and J kd K 

show the effect of time in a reactive atmosphere (air). The GPC area indicates the 

onset of crosslinking (and degradation) at lower temperatures than weight-loss 

tests and hence is a better tool to quantify degradation and establish process 

limits. For prepreg processed under vacuum conditions, some crosslinking 

occurs at 350 “C (1.13 compared to 1.08 baseline) while significant crosslinking 

degradation occurs at 400 “C. In comparison, specimens processed in air exhibit 

higher levels of degradation at both test temperatures, as expected. 

22 
Table 4. Degradation study of PEI and AS4/PEI prepreg using GPC. 

Area Ratio Under 

Sample Process Temperature Time Atmosphere the GPC Curve 

(“c> (fi) 

A Resin No treatment - - 1.00 

I P I Oven I 330 I 20 I Air I 1.21 I 

Q Oven 350 30 Air 1.31 

R Oven 350 60 Air 1.39 

S Oven 400 30 Air 1.37 

T Oven 400 60 Air 1.48 

U 1 Induction ] 309 1 1 Vacuum 1 1.07 

V 1 Induction ] 319 1 I Vacuum I 1.07 

w Induction 330 1 Vacuum 1.08 

X Induction 387 1 Vacuum 1.05 

Y Induction 405 1 Vacuum 1.06 

The induction-processed samples were subjected to high frequency magnetic 

fields for approximately 1 min, which was chosen based on design cycle times in 

electromagnetic-induction-based process for manufacture of laminates. 

Magnetic field parameters were selected to mimic process conditions in the 

manufacturing process. Preliminary tests performed under vacuum atmosphere 

indicate no measurable polymer degradations under these conditions, even 

though the composite laminate does reach the degradation temperatures of 

380 “C and 400 “C (samples X and Y, respectively). 

2.2.2.4 Electrical Degradation Study 

Dielectric breakdown of polymers results in localized damage, which leads to 

deterioration of the mechanical properties of the composite [20]. Several 

mechanisms can occur and lead to breakdown, such as discharge breakdown and 

intrinsic breakdown. In this effort, the purpose is to identify electromagnetic 

parameters that produce breakdown; not to elucidate the mechanisms for this 

breakdown. Thus, it is show that dielectric breakdown is not likely to occur 

during electromagnetic processing of AS4/ PEI. 

23 
Breakdown measurements were performed on neat PEI films of various 

thicknesses. The junction of fiber overlap is the region of expected breakdown 

and the thickness of PEI in these regions is small (submicron). Thin films, as 

small as 100 run, were fabricated using a solvent-based spin-coating technique. 

The dielectric breakdown voltages for neat PEI films of various thickness are 

presented in Figure 20. The breakdown voltage increases as the sample thickness 

increases. The voltage required for breakdown in a 100~run film of PEI is 

-350 V. The voltage drop in the induced current loops during electromagnetic 

processing is an order of magnitude smaller. For example, the induced voltage 

due to a three-turn coil with a 10-A current at a frequency of 10 MHz and 

affecting a 0.1-m square loop in the composite is only 40 V. It may be concluded 

that dielectric breakdown of the matrix is unlikely during electromagnetic 

processing of AS4/PEI composites. 

500 

450 

z 400 

.i 350 

z 300 

’ 

g 

250 

200 

z 150 

5 100 

50 

0 

Breakdown of PEI film 

9 

e---+------“--- 

0.00 0.10 0.20 0.30 0.40 0.50 

Thickness irl pm 

0.60 0.70 0.80 

Figure 20. Breakdown voltage measurements for neat PEI films. 

2.2.2.5 Mechanical Performance 

Short beam shear (ASTM D 2344) [21] and compression (ASTM 695) [22] tests 

were performed with autoclaved and electromagnetic-induction-processed 

AS4/PEI specimens. These two properties are directly related to the matrix 

properties in the composite and are sensitive to matrix degradation. High 

pressure (75 psi) was used in order to eliminate the void content effect on 

properties. Measured void contents were less than 1% for both samples. The 

mechanical test results are shown in Table 5 and indicate no loss in performance 

due to electromagnetic induction-based processing of AS4/PEI. 

24 
Table 5. Comparison of mechanical properties. 

Process Apparent Shear Strength Compressive Strength 

(psi) CW 

Electromagnetic-Induction 

Processing (A)” 1,130o f 500 120.1 + 1.2 

Autoclave (B)b 1,150o + 500 118.3 f 1.8 

+‘rocess A: Autoclaved at vacuum; induction heated; autoclaved at 75 psi. 

W’rocess B: Autoclaved at vacuum; autoclaved at 75 psi. 

2.2.3 Induction Coil Design 

A major advantage of induction heating technology is coil-design flexibility. The 

size and shape of an induction coil can be “fit,” or matched, to the composite part 

that is to be heated, even for geometrically complex shapes. It is also possible to 

use a simple coil design and heat complex geometric shapes using programmed 

motion with a robot. Based on induction coil models, coil designs were 

developed for a 12-in wide laminate process. This involves lamination or 

consolidation of an 8-ply prepreg in the desired orientation into a consolidated 

laminate with specified quality. This is achieved by induction heating the 

prepreg stack up to process temperature, followed by consolidation under 

pressure. 

2.2.3.1 Laminator Coil Design 

The function of the laminator or the lamination stage is to fabricate 8-ply 

thermoplastic laminates at high throughputs (-20 ft/min) and desired quality. 

Thus, the induction heating stage of. this process step has to uniformly and 

rapidly heat the incoming material (8-ply prepreg stack) up to the process 

temperature while allowing continuous material flow, as shown in Figure 21. 

The challenge is to handle incoming prepreg stacks of various orientations and 

still meet the rapid and uniform heating requirements. 

Several different coil configurations were modeled and tested resulting in the 

selection of a rectangular (or paperclip shaped) coil for the laminator. The coil 

geometry and resultant temperature profiles are shown in Figures 22 and 23, 

respectively. 

Work is in progress to optimize the rectangular coil geometry to further reduce 

temperature gradients and improve final laminate quality. 

2.2.4 Conclusions to Induction-Based Thermoplastic Composite Lamination 

Work to date has established that induction heating is a key technology 

component for the use of carbon/thermoplastics in Army composite structures. 

25 

3. References 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

Fink, B. K. “Non-PoButing Composites Repair and Remanufacturing for 

Military Applications.” Annual Technical Report - Sttategic Environmental 

Research and Development Program (SERDP), PPl109, Arlington, VA, 

December 1998. 

Bourban, P. E., E. Karamuk, R. C. Don, and J. W. Gillespie, Jr. “Induction 

Heating for Rehabilitation of Steel Structures Using Composites.” Proceedings 

ASCE Mufetils Engineering Curzfirence, New Materials and Methods for 

Repair, San Diego, CA, 1994. 

Benatar, A., and T. G. Gutowski. “Methods for Fusion-Bonding 

Thermoplastic Composites.” ,SAMPE Quarterly, vol. 18, no. 1, pp. 35-42, 

October 1986. 

Buckley, J. D., and R. L. Fox. “Rapid Electromagnetic Induction Bonding of 

Composites, Plastics and Metals.” Materials Research Society Symposium, 

vol. 124, Boston, MA, 1988. 

Lawless, G. W., and T. J. Reinhart. “Study of the Induction Heating of 

Organic Composites.” International SAMPE Conference, Toronto, Canada, 

October 1992. 

Buckley, J. D., R. L. Fox, and J. R. Tyeryar. “Seam Bonding of Graphite 

Reinforced Composite Panels.” NASA Advanced Composites Con.rence, 1986. 

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

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

Journal of Composite Materiak, vol. 33, no. 17, pp. X43-1664,1999. 

M&night, 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 ANTEC 98, Society of Plastics 

Engineers, Brookfield, CT, 1998. 

9. Yarlagadda, S., J. W. Gillespie, Jr., and B. K. Fink. “Resistive Susceptor 

Design for Uniform Heating During Induction Bonding of Composites.” 

JoumaZ of Thermoplastic Composite Materials, vol. 11, no. 4, pp. 321-337, July 

1998. 

10. Firko, J., S. Yarlagadda, B. K. Fink, and J. W. Gillespie, Jr. “Optimization of 

Heat Generation in Induction Bonding Using Metal Mesh Susceptors.” 

Proceedings of the American Society for Composites Thirteenth Technical Conference 

(CD-ROM), pp. 468-480, edited by A. J. Vizzini, published by American 

Society for Composites, Los Angeles, CA, and distributed by Composites 

Research Laboratory, University of Maryland, College Park, MD, 1998. 

27 
11. American Society for Testing and Materials. Standard Test Method for 

Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens 

by Tension Loading (Metal-to-Metal), ASTM D 1002-94, Conshohocken, PA, 

1994. 

12. Plonsey, R., and R. E. Collin. Principles and Applications of Electromagnetic 

FieZds. New York, NY: McGraw-Hill, 1961. 

13. Fink, B. K., R. L. McCullough, and J. W. Gillespie Jr. “Experimental 

Verification of Models for Induction Heating of Continuous-Carbon-Fiber 

Composites.” PoZymer Composites, vol. 17, no. 2, pp. 19%209,1996. 

14. Fink, B. K., J. W. Gillespie, Jr., and R. L. McCullough. “Induction Heating of 

Cross-Ply Carbon Fiber Thermoplastic Composites.” Proceedings of ANTEC 

92, Society of Plastics Engineers, Brookfield, CT, 1992. 

15. Lin, W., A. K. Miller, and 0. Buneman. “Predictive Capabilities of an 

Induction Heating Model for Complex-Shape Graphite Fiber/Polymer 

Matrix Composites.” 24th International SAMPE Technical Confirence, 

20-22 October 1992. 

16. Miller, A. K., C. Chang, A. Payne, M. Gur, E. Menzel, and A. Peled. “The 

Nature of Induction Heating in Graphite-Fiber Polymer-Matrix Composite 

Materials.” SAMPE Journal, vol. 26, no. 4, p, 37, August 1990. 

17. Fink, B. K., S. Yarlagadda, and J. W. Gillespie, Jr. “Design of a Resistive 

Susceptor for Uniform Heating During Induction Bonding of Composites.” 

ARL-TR-2148, US. Army Research Laboratory, Aberdeen Proving Ground, 

MD, January 2000. 

18. Sarjeant, W. J* “A Cursory Examination of the Nature, Effects and Control of 

Electromagnetic Fields.” ARL-TR-2232, U.S. Army Research Laboratory, 

Aberdeen Proving Ground, MD, May 2000. 

19. Fink, B. K., R. L. McCullough, and J. W. Gillespie, Jr. “Induction Heating of 

Carbon-Fiber Composites: Electrical Potential Distribution Model.” 

ARL-TR-2130, U.S. Army Research Laboratory, Aberdeen Proving Ground, 

MD, November 1999. 

20. Fink, B. K., R. L. McCullough, and J- W. Gillespie, Jr. “On the Influence of 

Moisture on Dielectric Properties of Polyetheretherketone (PEEK) CarbonFiber Composites.” ARL-TR-2236, U.S. Army Research Laboratory, 

Aberdeen Proving Ground, MD, June 2000. 

21. American Society for Testing and Materials. “Standard Test Method for 

Apparent Interlaminar Shear Strength of Parallel Fiber Composites by ShortBeam Method.” ASTM 2344-84, Conshohocken, PA, 1984. 

22. American Society for Testing and Materials. “Standard Test Method for 

Compressive Properties of Rigid Plastics.” ASTM 695-96, Conshohocken, 

PA, 1996. 

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S DETERESA 

F MAGNESS 

M FINGER MS 313 

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36 

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J MARTIN MS 8621 

D DUTHINH MS 8611 

100 BUREAU DR 

GAITHERSBURG MD 20899 

1 HYDROGEOLOGIC INC 

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SWALSH 

1155 HERNDON PKWY S-I-E 900 

HERNDON VA 20170 

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DIV 8241 

J HANDROCK 

YRKAN 

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8650 COBB DR 

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MARIETTA GA 30063-0648 

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1011 LOCKHEED WAY 

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1745A W NURSERY RD 

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991 ANNAPOLIS WAY 

WOODBRIDGE VA 22191 

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H FAIR 

I MCNAB 

I? SULLIVAN 

S BLESS 

W REINECKE 

C PERSAD 

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SOUTH BURLINGTON VT 

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EEL AIR MD 21014-3613 

1 UCLA MANE DEFT ENGR IV 

HTHAHN 

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2 UNIV OF DAYTON 

RESEARCH INST 

RYKIM 

AKROY 

300 COLLEGE PARK AVE 

DAYTON OH 45469-0168 

1 MIT 

P LAGACE 

77 MASS AVE 

CAMBRIDGE MA 01887 

1 IIT RESEARCH CENTER 

D ROSE 

201 MILL ST 

ROME NY 13440-6916 

1 GA TECH RSCH INST 

GA INST OF TCHNLGY 

P FRIEDERICH 

ATLANTA GA 30392 

1 MICHIGAN ST UNIV 

MSM DEFT 

R AVERILL 

3515 EB 

EAST LANSING MI 48824-1226 

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763 ANDERSON HALL 

LEXINGTON KY 40506-0046 

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UNIV OF WYOMING 

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R MCNITT 

C BAKIS 

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UNIVERSITY PARK PA 16802 

PENN STATE UNIV 

R S ENGEL 

245 HAMMOND BLDG 

UNIVERSITY PARK PA 16801 

PURDUE UNIV 

SCHOOL OF AERO & ASTRO 

CTSUN 

W LAFAYETTE IN 47907-1282 

STANFORD UNIV 

DEFT OF AERONAUTICS 

& AEROBALLISTICS 

S TSAI 

DURANT BLDG 

STANFORD CA 94305 

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J M WHITNEY 

COLLEGE PARK AVE 

DAYTON OH 45469-0240 

UNIV OF DELAWARE 

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J GILLESPIE 

M SANTARE 

G PALMESE 

5 YARLAGADDA 

S ADVANI 

D HEIDER 

D KUKTCH 

201 SPENCER LABORATORY 

NEWARK DE 19716 

DEFT OF MATERIALS 

SCIENCE & ENGINEERING 

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AT URBANA CHAMPAIGN 

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41 
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3 THE UNIV OF TEXAS AT AUSTIN 

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3 VA POLYTECHNICAL 

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K REIFSNIDER 

R JONES 

BLACKSBURG VA 24061-0219 

1 UNIV OF MARYLAND 

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42 
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A TEETS 

RYIN 

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T BOGET-TI 

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5 CORNELISON 

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D HENRY 

RKASTE 

M KLUSEWlTZ 

M LEADORE 

R LIEB 

E IUGAS 

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POSTBUS 6006 

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S 172 90 STOCKHOLM 

SWEDEN 

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GROUND 

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GENERAL HERZOG HAUS 

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45 
46 
REPORT DOCUMENTATION PAGE Form Appmved 

0M6No.0704-0188 

rduction-Based Processing 

, Nicholas B. Shevchenko, James M. Sands, 

hridhar Yarlagadda,* John W. Gillespie, Jr.* 

Lberdeen Proving Ground, MD 2 1005-5069 

AGENCY REPORT NUMBER 

UniversiQ of Delaware, Newark, DE 19716 

2a. DlSfRlBUTlONlAVAllABlLl~ STATEMENT 12b. DISTRIBUTION CODE 

Lpproved for public release; distribution is unlimited. 

3. ABSTRACT(Maximum 200 words) 

The development of induction-based processing of carbon-fiber (CF) thermoplastic-matrix composites and accelerater 

:ure of thermosetting adhesives has the potential to provide nonautoclave processing technology for manufacturing an 

repair of polymer-matrix composites (PMCs). In this report, the results of recent tests demonstrating bonding c 

zomposites using commercial off-the-shelf thermal-cure adhesives that are heat cured via an induction field using a 

inductive susceptor are discussed. This method of cure utilizes heat generation within metal screen-based susceptors t 

Sure the adhesive matrix via a heat transfer mechanism The mechanical performance of these bonds is presented i 

comparison with autoclave and thermally cured baselines. No substantial loss of mechanical lap-shear suength 

observed in adhesive bonds processed by induction. In addition, an example of induction welding of 

thermoplastic-impregnated carbon fiber (AS4) is presented. In order to successfully demonstrate induction welding fc 

manufacture of CF composites, the degradation of the polymer in the laminates is also investigated. No measurab 

degradation of the polymer, either by dielectric or thermal breakdown when heated by induction, was observed. 

14. SUBJECTTERMS 15. NUMBER Of PAGES 

composite material, pollution prevention, induction-based processing, lamination, polyether 53 

imide 16. PRICE CODE 

17. SECURITY CLASSIFICATION 18. SECURITY ClASSIFICATION 19. SECURITY CLASSIFICATION 

OF REPORT 

20. LIMITATION OF ABSTRAC 

OF THIS PAGE OF ABSTRACT 

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL 

NSN754041-280-5500 

47 

Standard Form 298 (Rev. Z-83) 

Prescribed by ANSI Std. 239-l B 298-102 
48 
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Date of Reporl April 200 1 

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