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Increased emphasis has been and continues to be placed on attaining energy independence within the United States, both for the

general populus and for its military. With calls for a reduced

dependence on petroleum products, decreased greenhouse gas

emissions, and increased renewable energy usage, research and

development efforts have not only increased in numbers, but also

in intensity. Although energy from biomass, water, wind, and the

sun has been harvested for thousands of years,

it wasn’t until the 1940s and 50s that many

began to design and implement technologies to

convert the energy from each of these sources

into electricity. With the growing call for energy independence,

researchers must find methods to optimize these technologies and

create new and improved manufacturing processes.

In the news, we most frequently hear about ethanol and solar

and wind power. However, these are not the only research areas,

and they are surely not the only means for solving the energy

independence problem. Current R&D efforts are not specifically

focused on one topic because there are numerous avenues that

can lead to the realization of solutions. Even though many would

like for one technology to solve the entire energy crisis, it is nearly impossible. The US populus is too dependent upon energy and

its various forms. Thus, multiple solutions are needed.

In addition to solving the energy problem – or petroleum

dependence problem – we must also find improved waste disposal methods as well as methods to encourage energy conservation.

With recycling efforts gaining momentum, many post-consumer

products can be re-used. However, with continued population

increases there are ever increasing amounts of waste that must be

disposed. The most used waste disposal method is landfilling. To

promote energy conservation, almost everything we use needs to

be made more efficient, and a new “energy conservation” mindset needs to be developed. Efficiency has become a major driver

in production in recent years since manufacturing processes need

to be made more environmentally friendly and more efficient to

survive. This is being done through the use of Six Sigma and Lean

Six Sigma at many locations. These are just a few of the problems

that need to be solved, problems with thousands of solutions.

As the quest for solutions continues, some research areas are

gaining increased attention. One such area is materials research.

Many of the advancements in wind and solar energy were the

result of improved materials. Materials researchers are also

responsible for discovering methods by which post-consumer

materials can be re-used or recycled. And this is just the tip of

the iceberg. Recently, in the aircraft and automobile industries,

an emphasis has been placed on making planes and cars lighter

weight, thus improving fuel economy and reducing emissions.

This has been accomplished through the increased use of

polymer matrix composites (PMCs) versus metals and metal

matrix composites.

However, as PMCs grow in popularity and use, they will be

expected to continuously meet ever increasing design requirements. Polymer matrix composites will have to be lighter and

cheaper while offering the same if not enhanced material characteristics. As these materials become increasingly important, it

becomes even more important to have a basic understanding of

the materials, their failure mechanisms, and the methods by

which quality improvements can be realized.

This themed issue of the AMMTIAC Quarterly focuses on

this important class of materials and these important topics.

The feature article discusses the development and successful

implementation of glass fiber-reinforced thermoplastic composite lumbers, which have been used to solve timber bridge

degradation issues. These recyclable lumbers are not only made

from waste materials, but they are also eco-friendly and support the notion of sustainable development. TechSolutions 18

provides an overview of the common polymer matrix composite material defects that result from their manufacture. Lastly,

the third article shows how nondestructive evaluation methods

can be utilized to improve the quality of polymer matrix composites during their fabrication. With product quality improvements, energy requirements decrease resulting in a more environmentally friendly production process.

Stephanie Knoeller, Editor

Energy and

Materials

A note to our readers:

We recently received notification that misleading information was published in TechSolutions 16 of AMMTIAC Quarterly Volume 5 Number 2. In this

article, we cite that AET is sensitive to damage growth and that it is capable of detecting defects on a picometer scale. Although AET does detect damage

growth, its capabilities to detect this growth are limited by the amount of energy released by a particular material during damage growth, the type of

damage, the distance from the sensor to the damage, and the level of ambient noise. In controlled, laboratory settings AET has been able to detect micrometer sized defects in polymer matrix composites and metals. Thus, in practical engineering structures, AET will be less sensitive due to background noise

and sensor spacing. If you downloaded this issue after the release of AMMTIAC eNews No. 22 (11/11/2010), the information is correct. This note applies to our

print subscribers and those who downloaded the issue prior to the eNews release.

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The AMMTIAC Quarterly, Volume 5, Number 4 3 http://ammtiac.alionscience.com

Table 1. Advantages of Glass Fiber-Reinforced Thermoplastic Composite Lumber.

INTRODUCTION

With hundreds of bridges in its inventory, the US Army spends a

substantial portion of its budget performing periodic maintenance

operations, repairing, and rebuilding degraded bridges. Especially in

hot and humid environments, degradation rates for wood timber

bridges increase due to rot and insect attack. In fact, it is not

uncommon for bridges of all types to receive reduced load ratings or

close as a result of material degradation. Although chemicallytreated wood provides lifespan advantages over untreated wood, it is

disadvantageous because it may leach harmful chemicals into the

soil and groundwater and require special permits upon disposal.

To counter these surmounting, degradation-related bridge repairs

and rebuilds, a cost-effective, structural-grade, wood lumber alternative was developed. The structural capabilities of the developed

glass fiber-reinforced thermoplastic composite lumber have been

demonstrated in the replacement of an existing wood timber bridge

at Camp Mackall (a sub-installation of Fort Bragg), North Carolina.

This article highlights the material development and its implementation at Camp Mackall.

GLASS FIBER-REINFORCED THERMOPLASTIC

COMPOSITE LUMBER

Development and Production

Glass fiber-reinforced thermoplastic composite lumbers are produced using a unique, immiscible polymer blending, extrusion, and

molding process that creates a material with the flexibility of

high-density polyethylene (HDPE) and the stiffness and strength

of fiberglass. The immiscible polymer blending process combines

post-consumer recycled HDPE and automotive bumper scraps

(made of polypropylene and fiberglass) in a manner that specifically

orients the glass fibers, creating a product with properties that exceed

those predicted by the law of mixtures for the individual materials.

This mixture is then pushed through a screw and barrel extruder.

The specialized extrusion process uses 11% fiberglass but produces a product equivalent in strength to that of a material containing 34% fiberglass. This low fiberglass content is critical since

fiberglass is both difficult to dispose of and irritating to the skin.

The extruded material is then pushed into a mold to create the

product lumber. The molding process allows the product lumber to

be produced in a variety of cross-sectional geometries and lengths.

Thus, products can be produced with different moments of inertia

(I), and the flexural rigidity (modulus of elasticity (E) x I) of the

member can be adjusted to achieve the desired mechanical properties and performance.[1]

The resulting composite has a rough, no-slip, grain-like surface,

specific strength greater than steel, and high creep* resistance. This

high creep resistance results from the material design, which incorporates 600 psi of allowable tensile, compressive, and flexural stress.

This value is well below the tested minimum ultimate strength. In

other words, if a tank were to be parked on a bridge constructed of

this material for 25 years, once driven off the bridge would return

to its original shape with inconsequential residual deformation.

The glass fiber-reinforced thermoplastic composite lumber,

which incorporates small quantities of foaming agents and other

additives – to reduce the occurrence of voids and increase creep

resistance – has a modulus of elasticity between 350,000 and

400,000 psi (2,410-2,760 MPa) and a minimum ultimate strength

of 3,500 psi (24 MPa). Comparatively, virgin high-density polyethylene has an elastic modulus and ultimate strength of 160,000 psi

(1,100 MPa) and 3,500 psi (24 MPa), respectively.[2] Pine and oak,

Richard G. Lampo

James Wilcoski

Vincent F. Hock

Sarah B. Nemeth

ERDC-CERL

Champaign, IL

Henry Diaz-Alvarez

ERDC-GSL

Vicksburg, MS

Dr. Thomas J. Nosker

Rutgers University

Piscataway, NJ

• Does not leach harmful chemicals

• Lasts more than 50 years with minimal to no maintenance

• Resistant to ultraviolet light – degradation rates are less than 0.003

inch (0.07 mm) per year

• Carbon pigment (a natural compound) can be added to the lumber to

reduce the effects of ultraviolet light

• Can withstand temperature extremes greater than those observed at

any one location on the Earth’s surface, since it melts and freezes at

125°C (257°F) and -125°C (-193°F), respectively

• Impermeable to water and chlorides

• Resistant to the abrasion that can occur in marine environments due to

salt and sand

• Does not biodegrade or oxidize

• Insect and rot resistant

• Retains key mechanical properties in humid and wet environments

• Construction operations utilize the same tools as required for timber

bridges

• High energy absorption capacity

• Safe from catastrophic failure due to the ductility of the materials

• Diverts HDPE and fiberglass from landfills where they take thousands

of years to decompose

• When used as a bridge construction material, short payback periods

and high returns on investment are realized

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The AMMTIAC Quarterly, Volume 5, Number 4 4 http://ammtiac.alionscience.com

The first composite, vehicular bridge was built at Fort

Leonard Wood, Missouri during the summer of 1998. This

bridge is constructed of standard and structural-grade, thermoplastic composite lumber, and can support more than 30

tons (27,000 kg).

Structural-grade composite lumber boards, incorporating

polystyrene for added stiffness rather than fiberglass, were

used as the main support joists. These joists were placed over

the steel girders that provided support to the original timber

bridge. The deck was constructed using unreinforced, standard-grade plastic lumber. The figure shows an

Army High Mobility Multipurpose Wheeled Vehicle (HMMWV) crossing the plastic lumber bridge.

The construction of this bridge diverted approximately 13,000 pounds of post-consumer plastics or 78,000

1-gallon plastic milk jugs, and 335,000 8-ounce polystyrene-containing foam coffee cups from landfilling

operations. In addition, because there were minimal maintenance costs, this bridge was able to pay for itself

in less than eight years.

Although successfully constructed, this bridge had higher initial costs compared to treated wood.

the two most common sources for wood lumber, have moduli of

elasticity that exceed 1 million psi (6,900 MPa) and strengths of

2,400 psi (17 MPa) and 3,500 psi (24 MPa), respectively, when

measured along the growth axis.[3]

Accelerated weather tests have demonstrated that this lumber has

an estimated lifespan that exceeds 50 years and requires little maintenance. Table 1 lists the advantages of glass fiber-reinforced thermoplastic composite lumber over chemically-treated wood lumber.

Structural Application Demonstration

An opportunity to demonstrate and validate the performance and

benefits of the glass fiber-reinforced composite lumber was offered

in 2008 as a part of the Deputy Under Secretary of Defense’s

Corrosion Prevention and Control (CPC) Program and the Army

Chief of Staff for Installation Management’s (ACSIM’s) Installation

Technology Transition Program (ITTP).

Innovative I-beam Bridge at Fort Bragg, NC

The original goal of the effort at Fort Bragg, North Carolina was to

build and evaluate a glass fiber-reinforced thermoplastic composite

bridge that could be implemented as a cost competitive replacement

for conventional wood timber bridges. A replacement bridge was

proposed by the US Army Engineer Research and Development

Center (ERDC), Construction Engineering Research Laboratory

(CERL) with an American Association of State Highway and

Transportation Officials (AASHTO) load rating of HS-20.

However, the Fort Bragg Directorate of Public Works (DPW)

turned around and challenged ERDC-CERL to design and build a

bridge that could support a 71-ton (64,400kg) M1A1 Abrams tank

(AASHTO HS-25), while remaining cost competitive with a traditional wood timber bridge on a first cost basis. Figure 1 shows the

original wooden bridge at Camp Mackall that was replaced using

glass fiber-reinforced thermoplastic composite lumber. This degraded timber bridge had a 4.7 ton (4,300 kg) load rating.

To meet the challenge, engineers designed a thermoplastic composite bridge using traditional timber design methodologies but

with slight modifications. The most considerable modification was

the use of lower allowable stresses for the thermoplastic composite

materials. Table 2 provides the material properties for the glass fiberreinforced thermoplastic composite lumber, as well as the values

that were actually used in the design of this bridge.

The final bridge design utilized the I-beam concept, which supports the removal of any material that does not contribute to

the beam’s strength, thus reducing the material costs. The bridge

deflection‡ (or bending) is controlled through the use of I-beams

that have large moments of inertia. Likewise, these I-beams enable

the bridge to support large loads, while staying within the AASHTO safety limits.

This 36-foot (11-m) long bridge was built using 12-inch (305-

mm) diameter pilings, heavy-duty I-beams, up to 18-inches (460-

mm) high, and decking, all made from the glass fiber-reinforced

thermoplastic composite material. Figure 2 shows an I-beam pile

cap covering a series of piles. Stainless steel and other corrosion

Figure 1. The original wood timber bridge located at Camp Mackall,

NC. This bridge was replaced with a bridge constructed of fiberreinforced thermoplastic composite lumber.

Table 2. Design Values for Thermoplastic Composite Bridges.[4]

Elastic Modulus for live load (short duration) E = 350 ksi (2,400 MPa)

Ultimate compression parallel to grain† F‘c =3,500 psi (24 MPa)

Allowable compression parallel to grain† F‘c =1,000 psi (6.89 MPa)

Ultimate flexural strength F’b =2,300 psi (15.9 MPa)

Allowable flexural strength F’b =600 psi (4.1 MPa)

Ultimate shear strength parallel to grain† F’v =1,100 psi (7.58 MPa)

Allowable shear strength parallel to grain† F’v =350 psi (2.4 MPa)

Self weight ωp =0.032 pci (8,686 N/m3)

Coefficient of thermal expansion ε = 0.000052/°F

(2.88889E-05/°C)

Thermoplastic

Composite Lumber

Demonstration

Bridge at Fort

Leonard Wood

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The AMMTIAC Quarterly, Volume 5, Number 4 5 http://ammtiac.alionscience.com

resistant bolts and screw fasteners were used in the construction of

this bridge. After the pilings were driven in, only two workers were

required to finish the project.[1] This ten-week bridge construction effort was completed in May 2009.

This bridge is constructed of more than 94% recycled material,

which means approximately 85,000 pounds of HDPE or 550,000

1-gallon plastic milk jugs were diverted from landfills. In addition,

this bridge provides minimal risk in terms of catastrophic failure

since it was built and designed to have a working stress that does not

exceed 15% of the ultimate strength of the individual materials.

Also, because the materials are ductile in nature, they will exhibit

visible signs prior to failure. Comparatively, steel and concrete

bridges are built to have working stresses that are 40-60% and 33%

of their ultimate strengths, respectively.[1]

Testing and Economic Analysis

On 11 June 2009, initial load tests were conducted

using a 36-ton dual axle dump truck and an M1A1

Abrams tank. With the 36-ton (32,700 kg) dump truck,

the deflection at mid-span was 0.216 inches (5.49 mm),

including pier deflections. With the M1A1 Abrams tank

(Figure 3), the mid-span deflection, including pier

deflections, was 0.525 inches (13.3 mm).

Table 3 highlights the stresses and strains experienced

by the bridge during these initial load tests. After analysis of these initial results, this bridge received a posted

load capacity of 73 tons (66,225 kg) for tracked vehicles

and 88 tons (79,833 kg) for wheeled vehicles.

An economic analysis was performed to compare lifecycle costs

for thermoplastic composite designed bridges, treated wood timber

bridges, and hybrid concrete, steel and wood design bridges. Where

possible, data was collected from actual bridge constructions. The

compiled costs were normalized to account for cost differences

due to locality and the estimated HS ratings of the bridges. A net

present value (NPV) discount factor of 3% was used in the analysis

covering a time period of 30 years. The analysis also took into consideration that a wood timber structure would need to be replaced

at least once during the life expectancy of the thermoplastic composite bridge. The thermoplastic composite bridge design proved to

be cost competitive with a treated wood bridge on a first-cost basis.

In addition, with its very low maintenance expectation over the

NPV lifecycle, the thermoplastic composite bridge design shows

an estimated $300 per square foot lifecycle cost savings compared

to conventional wood timber bridge designs and their associated

maintenance costs (Table 4).

Remote Sensing and Monitoring

Sensors were added to the bridge at Camp Mackall to monitor the

strains and deflections, temperature, weather, and ultraviolet radiation. More than twelve displacement gauges measure the deflection

and displacement of the bridge under load, and during temperature

changes, to an accuracy of 0.01 inches (0.25 mm). Ten resistancetype strain gauges measure the strain within a range of ±3,000

micro-strain. These gauges compensate for temperature and humidity effects as necessary to provide accurate strain readings.

Temperature is measured at sixteen different locations that

correspond to the installed locations of the displacement and

strain gauges. These gauges determine the temperature at specific

Figure 2. Construction of thermoplastic composite bridge at Fort

Bragg.

Figure 3. M1A1 Abrams tank crossing the bridge during initial load testing.

Table 3. 11 June 2009 Load Test Results.

Dump Truck M-1 Abrams Tank

Micro StrainMax (106) 637 740

StressMax (psi) 255 296

Modulus of Elasticity (ksi) 450

Table 4. Bridge construction material lifecycle cost comparison.[5]

Material N P V Lifecycle Costs Expected Life

Wood Timber Bridge $981 / square foot 15-20 years w/annual maintenance

Hybrid Bridge (steel and concrete $1,101 / square foot 30+ years w/annual maintenance

with laminated wood deck) & future complete deck replacement

Thermoplastic Composite Bridge $675 / square foot 50+ years, minimal maintenance

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locations, and the temperature gradients that exist over the bridge

to an accuracy of 0.1°F. An image capture system records digital

images of all vehicles that cross the bridge. Time and date stamps

are placed on the recorded images so that the data can be correlated to the respective load condition.

A weather station has also been installed near the bridge to accurately report the temperature, wind speed, precipitation, relative

humidity, and ultraviolet radiation intensity at the bridge site. The

atmospheric corrosion is assessed using test coupons made of silver,

copper, steel, and aluminum alloys. These coupons are removed

every three months and their mass loss analyzed. Test coupons of the

glass fiber-reinforced thermoplastic composite have also been

installed to monitor the effects of the environment on the bridge.

The sensors are connected to a data recording system that can

store output data for a minimum of 14 days. During a load event,

data are collected for the duration. The data collected are remotely

transmitted to a computer for data analysis, or to provide an alert

should an overload condition, failed component, or failed sensor or

gauge be detected.

Recent Testing

Additional load tests were conducted in August 2010 using an

approximate 17-ton wheeled M1089 Wrecker and a 70-ton tracked

M88A2 Heavy Recovery Vehicle. Initially, the vehicles were driven

over the bridge at very slow speeds, stopping at the point of maximum deflection. After five minutes, the vehicle was driven off with

recovery observed. The vehicles were then driven over the bridge at

increasing speed increments up to 25 mph to observe the effects of

vehicle speed on the bridge response. The deflections were within

the expected values, and there were no indications of creep or material structural degradation based on previous load tests conducted

during September 2009 and January 2010. A future load test is

planned for July 2011.

CURRENT STATUS

Other Army installations have expressed interest in using thermoplastic construction materials and bridge designs to replace existing

wood timber bridges. In addition, the Fort Bragg DPW has provided funds for the construction of a third fiber-reinforced thermoplastic composite bridge at Camp Mackall.

CONCLUSION

The innovative thermoplastic composite I-beam bridge at Camp

Mackall has demonstrated that glass fiber-reinforced thermoplastic

composite lumbers are viable, cost-competitive alternatives to traditional treated-lumber products. These materials can be used in the

replacement of many of the degraded wood timber bridges in use

throughout the US, both on and off military installations.

GENERAL REFERENCES

Jackson, L.M., and T.J. Nosker, “Technology, Applicability, and Future of

Thermoplastic Timber,” Proceedings of the DoD Corrosion Conference 2009,

August 2009.

Lampo, R.G., S.C. Sweeny, J. Wilcoski, V.F. Hock, V.P. Chiarito, H. DiazAlvarez, and T.J. Nosker, “Thermoplastic Composites as DegradationResistant Material Systems for Timber Bridge Designs,” Proceedings of the

DoD Corrosion Conference 2009, August 2009.

NOTES & REFERENCES

* Creep is the slow, permanent deformation of a material that occurs over

time as a result of prolonged external loads or stresses below the elastic

limit. Creep is influenced by load magnitude, the duration of the applied

load, and temperature.

† For the flow molded thermoplastic composite members, grain is considered to be the direction of material flow in the mold during fabrication.

‡ Deflection describes the degree to which a structural element is displaced

under a load.

[1] Jackson, L.M., and T.J. Nosker, “Technology, Applicability, and Future

of Thermoplastic Timber,” Proceedings of the DoD Corrosion Conference

2009, August 2009.

[2] “Encyclopedia 1996,” Modern Plastics, Vol. 72, No. 12, 1995, pp. B185, B-186.

[3] The Forest Products Laboratory, Wood Handbook, US Department of

Agriculture Forest Service, Agriculture Handbook No. 72, 1955 (rev. 1987).

[4] Lampo, R.G., S.C. Sweeny, J. Wilcoski, V.F. Hock, V.P. Chiarito,

H. Diaz-Alvarez, and T.J. Nosker, “Thermoplastic Composites as

Degradation-Resistant Material Systems for Timber Bridge Designs,”

Proceedings of the DoD Corrosion Conference 2009, August 2009.

[5] Draft ERDC/CERL Technical Report, “Demonstration of Thermoplastic Composite I-Beam Design Bridge at Camp Mackall, NC” to be

published in early 2011.

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Stephanie L. Knoeller

AMMTIAC

Rome, NY techsolutions 18

Polymer Matrix Composite Manufacturing Induced Defects

INTRODUCTION

Polymer matrix composites (PMCs) comprise more than 90% of

the composite materials available. They are used extensively within

industry and the DoD due to their high stiffness and strength, low

density, long fatigue life, corrosion resistance, and crash worthiness.[1, 2] These materials can also be tailored to exhibit stealth

characteristics, sensor capabilities, and high thermal or electrical

conductivity.[3]

With many desirable properties, the potential applications for

PMCs continue to increase. As a result, it is becoming ever more

important to know, and have a basic understanding of, the underlying causes for polymer matrix composite (PMC) material failure,

specifically those that result from the manufacturing process since it

accounts for approximately 44% of the failures in [fiber-reinforced]

PMCs.[4] These failures result from defects that are inherently

introduced during the manufacturing process, either from the reactions that occur within the resin during material processing, or via

mechanical, human, or environmental factors. This article surveys

the common defects that result from the manufacture of PMCs.

BACKGROUND

Polymer matrix composites consist of matrices made of polymeric

materials*, such as plastics† or resins‡ that are either thermosetting

or thermoplastic, and fibrous reinforcements.[1] The reinforcements (e.g., natural, oxide glass, aramid, or carbon or graphite

fibers) can be placed in any direction and used in a variety of

shapes and sizes (i.e., straight and continuous; discontinuous or

chopped; particles or flakes; or continuous and woven, braided, or

knitted).[2] The continuous matrix provides form to the much

stiffer and stronger reinforcements, distributes loads across the

reinforcements, and protects the reinforcements from the environment and weathering.[1, 2]

Thermosetting resins are liquid at room temperature. As the

name implies, thermosetting resins solidify with heat in a process

known as curing. The heat from the curing process creates cross

links between the single and branched polymer chains of the resin

to create a solid 3-dimensional (3-D) structure. Since voids are

inherently introduced during the cure process, PMCs are typically

debulked§ to reduce the void content.[1] Once cured, the component is cooled to its service temperature.[1, 5]

Thermoplastic resins, on the other hand, are solid at room temperature.[1] These resins are heated to a processing temperature

above the melting point, molded, and solidified upon cooling to the

service temperature.[6] This cooling process causes the matrix to

shrink, creating Van der Waals bonds between the fibers and the

matrix.[9] Table 1 compares the common attributes of thermoset

and thermoplastic matrices. Table 2 provides an overview of the

basic PMC manufacturing methods.

The AMMTIAC Quarterly, Volume 5, Number 4 7 http://ammtiac.alionscience.com

• Develop well-bonded 3-D structures

• Do not melt once hardened

• Require a curing process

Uncured resins are susceptible to moisture and temperature

effects, and contamination

Under-cured resins are too soft to transmit loads efficiently

between the matrix and the fibers

Over-cured resins become embrittled and crack prematurely

• Can be left in a partially cured state

• Most often used in chopped fiber composites

• Examples:

Epoxies, unsaturated polyesters, phenolics, vinylesters,

bismaleimides

• Commonly used for aircraft, space, military, and automotive parts

• Produced by condensation polymerization or by addition polymerization, followed by a condensation rearrangement reaction to form

heterocyclic entities

Both produce water, making it difficult to produce void-free

parts or structures

• 1-D or 2-D in molecular structure

• Soften at high temperatures, showing a discrete melting point

• Become rigid with cooling

• Examples:

polyethylenes, polyesters, polyketones, polysufones,

polypropylenes, polyamides

• Can be crystalline or amorphous

• Creep is a concern, but can be minimized with the proper

reinforcement selection

• No chemical reactions are needed, thus no excess heat or

product gas are released

• The limiting factors in production are time to heat, shape,

and cool

• Materials can be salvaged and re-worked

• Lose strength at high temperatures

Table 1. Comparison of thermoplastic and thermoset matrices.[2, 5]

Thermoset Resins Thermoplastic Resins

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AMMTIAC techsolutions 18 A D V A N C E D M A T E R I A L S , M A N U F A C T U R I N G A N D T E S T I N G

Table 2. Overview of the basic polymer matrix composite manufacturing methods. These methods apply to both resin types; however,

different considerations must be included for each.[1, 2, 7, 8]

Manufacturing Method Description

Hand Lay-up Reinforcing fibers are placed in the mold (after a release film and gel coat in some instances); the resin material is

then rolled into the reinforcing fibers. Hand lay-up is commonly used in the US aircraft industry to produce PMC

parts.

Spray Lay-up The reinforcements are chopped and sprayed simultaneously with the resin into the mold. A roller is then used to

ensure that the resin fully wets the fiber bundles. This technology produces low specific strength structures and is used

to join back-up structures to composite face sheets on composite tools.

Manual Prepreg** Lay-up The prepreg is cut into several layers, as needed for the mold. The shaped prepregs are then layered within the mold.

Automatic Prepreg Lay-up (Applicable to flat panels or parts with moderate curves.) A tape-laying machine lays the prepreg in the mold, cutting

the prepreg when the mold edge is reached. This process is repeated until the part is complete.

Compression Molding A specific amount of uncured resin and reinforcement are placed into the cavity of a matched mold in the open

position. As the mold is closed, the pressure increases, causing the mold to fill and the part to form. This method may

be used with prepregs also.

Liquid Composite Molding LCM processes include Resin Transfer Molding (RTM), Structural Reaction Injection Molding (SRIM), and Injection

Compression Molding. A fiber preform is placed in the mold cavity and a polymeric resin is injected into the sealed

mold. A curing reaction is initiated; the part solidifies and is then removed from the mold. Since LCM has a very high,

non-recurring tooling cost, it is best suited for high production volumes to lower the cost.

Vacuum-Assisted RTM The resin is injected into a mold that contains the reinforcement using pressure that is applied by the atmosphere

against an evacuated system.

Resin Film Infusion The resin film is placed in the bottom of the mold. Heat and pressure are then applied, causing the resin viscosity to

decrease and spread through the preform.

Expansion RTM A material that expands when heated is placed in the preform. The resin is infused and the mold is heated, causing

the core material to expand and subsequently forcing the resin into the remaining parts of the preform.

Transfer Molding Similar to compression molding, but the mold is closed when the resin material is injected.

Filament Winding Fiber spools are mounted to a creel; the strands from each spool are combined and pulled through a resin bath.

The strands are then fully activated with an initiator or hardener and the excess resin is removed. Lastly, they are sent

through a drying device and wound onto a mandrel, forming the desired part. Filament winding is used more extensively to manufacture composites than all other lay-up methods combined.

Fiber Placement Similar to filament winding, but enables all axes of motion.

Pultrusion Continuous fibers are drawn from reels, formed into a general shape, and drawn through a resin bath. The wetted

fibers are then shaped as they converge toward a heated die, where curing occurs. Upon die exit, the formed part

enters a pulling system, which provides the force that pulls the materials through the entire system. The pultruded part

is then cut and trimmed to the desired size.

Thermoforming Utilizes matrices that can repeatedly be softened or melted on heating and hardened or solidified on cooling, and

that can provide increased fracture toughness and higher hot-wet use temperatures (thermoplastics).

PMC MANUFACTURING DEFECTS

Microcracks

Thermal stresses are created during heating and cooling operations

since the matrix and reinforcements expand and contract at different

rates. This behavior results from differing coefficients of thermal expansion (CTE). Often, the matrix has a greater CTE than do the fibers.[10]

When thermoplastic PMCs are cooled to their service temperature following molding, a volumetric shrinkage of the matrix

occurs, exceeding that of the reinforcements. These unequal contraction rates lead to the formation of residual stresses in the fibers

and surrounding matrix.[9] When the magnitude of these stresses

exceeds the yield strength of the composite, interface debonding

may occur, followed by the formation of microcracks (transverse

cracks).[2, 6, 9, 11] Curing, on the other hand, leads to the formation of stresses within thermoset PMCs. These stresses increase with

increasing temperature, leading to the formation of microcracks.

As the stresses increase, the microcracks gain width and form

intricate, meandering patterns within the matrix. This propagation

causes the microcrack density to rise, increasing the likelihood of

delamination.[5] Delaminations that form as a result of microcracking typically occur where the microcracks meet ply interfaces,

an area of increased stress concentration.[10, 12] Microcracks may

also form at the leading point of a main crack, absorbing excess

energy and preventing the propagation of a main crack.[1]

Experimentation has shown that unless multiple microcracks are

present and a critical microcrack density has been reached, delaminations will not occur. Therefore, in practice, microcracks are often

assumed to be tolerable at low densities. Nevertheless, it is critical to

consider delaminations that could be induced by neighboring

microcracks.[11]

Likewise, it can be a detrimental decision to ignore microcracking,

as they are often the gateway to further and more disadvantageous

forms of damage that lead to changes in a PMC’s mechanical properties, such as reduced longitudinal stiffness and thermal expansion

coefficients.[12] When a PMC containing microcracks is placed

under cyclic or fatigue loading, the risk of failure increases greatly.[6]

(LCM)

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AMMTIAC techsolutions 18 A D V A N C E D M A T E R I A L S , M A N U F A C T U R I N G A N D T E S T I N G

Cracks

Cracks are generally characterized by an actual separation of the

material. They are visible on opposite surfaces, extending through

the thickness. Cracks propagate when the local energy is greater than

that which can be absorbed.[1, 2]

Cracking is a processing defect commonly found in the gel coat

of a PMC. The gel coat is a protective layer that is applied to the

mold prior to reinforcement placement. Although this coat contributes minimally to the structural capability of the part, it does

protect the parts. As a result, it must be able to withstand large

stresses, especially during molding and demolding.[1] Demolding

operations add stresses to PMC parts, especially when parts stick

to their mold and require alternate methods of removal.†† When

these stresses exceed the material strength, cracking occurs. To prevent sticking, gel coats are often treated with release agents which,

if prepared properly, should have minimal sticking.[1]

When placed in use, structures and parts experience added stresses that can contribute to cracking as well. These stresses couple with

already-present manufacturing stresses, insufficient to initiate cracks

alone, and lead to the formation of cracks. Under loading, stress

concentrations may be created at the intersections of matrix cracks

and reinforcements.[2] The added stresses cause the cracks to travel

along the edge or surface of the reinforcement, where it must break

interface bonds between the reinforcement and matrix. Eventually,

the load is transferred completely from the matrix to the reinforcement, which absorbs excess energy through elongation. This energy

is dissipated through the formation of internal atomic cracks.[1] As

stresses intensify, delaminations may occur, leading to a rapid decay

in stiffness and strength and, eventually, catastrophic failure.[2]

Moisture Absorption

Composites absorb moisture through the matrix, fiber, fiber-matrix

interface, porous regions, and areas where microcracking and

delamination have occurred. Although matrix materials typically

absorb more moisture than the reinforcements, the reinforcements

do aid in moisture absorption. Water and other liquids often travel

via capillary action along the reinforcement until it is absorbed by

the matrix.[1]

Moisture absorption is unfavorable since it chemically alters

liquid resins, lowering the glass transition temperature‡‡ and

degrading mechanical properties, such as elasticity and strength.

The reinforcements often act as barriers against moisture diffusion,

creating additional stresses within the matrix material. Thus, moisture absorption can lead to swelling due to the restrictions. If

allowed to freeze, a separation between the matrix and reinforcing

materials or between the layers in a laminate can occur, leading to

delaminations.[2]

To prevent moisture absorption, susceptible materials should be

stored in low humidity environments. Moisture should be removed

from thermoset PMCs prior to high temperature cure to prevent

expansion and subsequent delamination.[2]

Temperature Effects

Changes in temperature contribute greatly to the stresses and

strains experienced by PMCs. In laminates, heat can cause a free

expansion of the laminate layers. If this expansion is restricted by

the adjacent laminate layers due to differences in reinforcement

direction, internal stresses and failure may result.[2]

At temperatures above the glass transition temperature, thermoplastic matrices can soften to a point at which they no longer

efficiently transfer loads, leading to premature failure.[2, 5]

Voids/Pores§§

Voids form from the entrapment of volatile gasses, air, or non-solid

foreign materials, and they are virtually incapable of transmitting

structural stresses or non-radiative energy fields.[2, 12, 13]

Voids result from dissolved and absorbed water and gases on the

prepreg; residual volatiles (i.e., solvents) in the resin and condensates

that form during cure; the entrapment of gases during resin mixing

operations; or as a result of ply bridging and wrinkling. They can also

be produced from the use of improper pressure during the cure cycle,

inadequate use of adhesives in bonded structures, and the improper fit

of adjacent surfaces.[2] Pores often arise from inadequate wetting of the

fiber by the resin or from the inadequate infiltration of fiber tows.[14]

Voids greatly affect several matrix-dominated properties including

interlaminar, compressive, and transverse tensile strengths.

Interlaminar strength can be reduced by nearly 7% for every 1% of

voids present, up to a void content of 4%.[14] Additionally, for each

1% increase in voids, the fatigue life of a fiber-reinforced PMC is

reduced by 50% and the strength by approximately 5%. These

reductions in strength properties lead PMCs to be more susceptible

to degradation in aggressive environments.[13] The accepted void

limit in practice is between 2 and 2.5%.[2, 12]

Voids can be eliminated from the final PMC product by drying

prepregs in humidity-controlled rooms prior to lamination. Pores,

on the other hand, should be removed during the autoclave process

since they are nearly impossible to remove from the prepreg.[13]

Inclusions

The encapsulation of solid, foreign materials often leads to physical and mechanical discontinuities within a material. Material

inclusions occur most often in uncured thermoset composites that

are susceptible to environmental contamination from a variety of

sources, including oil, grease, and silicone-based products.

Compressed air and the careless application of release agents are

common sources of contamination.[2]

Although inclusions are often capable of transmitting structural stresses and energy fields, the transmission rate is noticeably

different from that of the parent material. This results in internal

stresses that can lead to failure.[2]

Table 3 provides a summary of the common PMC manufacturing defects discussed in this article as well as others that were not

discussed.

Delamination, one of the most commonly observed failure modes in composite materials, is characterized by the separation of the fiber and matrix at

their interface. Although caused by increasing stress concentrations within a composite, poor process control, poor dimensional tolerance, faulty

hole-drilling procedures, and inclusions of the release film during fabrication all contribute to delaminations. Failures often occur near free edges (the

region where the load is transferred from the fibers to a weak matrix), around holes at the end of bonded components, or from impact that causes

separation of the components.[2]

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AMMTIAC techsolutions 18 A D V A N C E D M A T E R I A L S , M A N U F A C T U R I N G A N D T E S T I N G

SUMMARY

An understanding of the basic PMC material defects, which result

from manufacture, is crucial for the prevention of property degradation and catastrophic failure. This article surveyed several of the

common manufacturing-induced PMC materials defects.

NOTES & REFERENCES

* Polymeric materials are composed of several small repeating units known

as monomers.

† Plastics are polymeric materials (usually organic) of large molecular

weight that can be shaped by flow; the term plastic usually refers to the

final product with fillers, plasticizers, pigments, and stabilizers included.

Often, this term connotes thermoplastics.

‡ Resins are any class of solid or semi-solid organic products of natural or

synthetic origin with no definite melting point. Resins generally have high

molecular weights; most resins are polymers. The use of the term resin

connotes a thermoset composite material.

§ The debulking process reduces the void content by removing the excess

air contained within the composite. This process typically follows lay-up

operations.

** Prepreg is shorthand for pre-impregnated material. A prepreg is a readyto-mold material that is impregnated with a resin. The resin is usually partially cured to the B-stage and supplied to the fabricator, where it is formed

into the desired shape and cured completely.[3]

†† In industry, stuck parts are typically discarded as a result of the damage

incurred during removal.

‡‡ The glass transition temperature is the temperature at which the matrix

changes from a glassy state to a viscous state.

§§ The terms void and pore are oftentimes used interchangeably. For the

purposes of this article, voids refer to the empty spaces beneath the surface

of a PMC. The empty spaces on the surface of the PMC are termed pores.

[1] Strong, Brent A., Fundamentals of Composites Manufacturing: Materials,

Methods, and Applications, 2nd Edition, Society of Manufacturing

Engineers, 2008.

[2] ASM Handbook, Volume 21: Composites, ASM International, 2001.

[3] Committee on High-Performance Structural Fibers for Advanced

Polymer Matrix Composites, High-Performance Structural Fibers for

Advanced Polymer Matrix Composites, The National Academies Press, 2005.

[4] Lemasçon, A., P. Castaing, and H. Mallard, “Failure Investigation

of Polymer and Composite Material Structures in the Mechanical

Engineering Industry,” Materials Characterization, Vol. 36, 1996, pp.

309-319.

[5] Weeton, J.W., D.M. Peters, and K.L. Thomas, “Section I: Introduction

to Composite Materials,” Engineers’ Guide to Composite Materials,

American Society for Metals, 1987.

[6] Timmerman, J.F., B.S. Hayes, and J.C. Seferis, “Cure Temperature

Effects on Cryogenic Microcracking of Polymer Matrix Materials,”

Polymer Composites, Vol. 24, No. 1, 2003.

[7] Wilkins, D.J., et al., JTEC Panel Report on Advanced Manufacturing

Technology for Polymer Composite Structures in Japan, Loyola College,

Baltimore, Maryland, 1994.

[8] Bickerton, Simon, et al., “Modeling Manufacturing Issues in Liquid

Composite Molding Process,” Second Conference on NDE Applied to Process

Control of Composite Fabrication, NTIAC, 1996, pp. 49-60.

[9] Parlevliet, P.P., H.E.N. Bersee, and A. Beukers, “Residual Stresses in

Thermoplastic Composites: A Study of the Literature – Part I: Formation

of Residual Stresses,” Composites Part A: Applied Science and

Manufacturing, Vol. 37, 2006, pp. 1847-1857.

[10] Wisnom, M.R., M. Gigliotti, N. Ersoy, et al., “Mechanisms

Generating Residual Stresses and Distortions During Manufacture of

Polymer-Matrix Composite Structures,” Composites Part A: Applied Science

and Manufacturing, Vol. 37, 2006, pp. 522-529.

[11] Parlevliet, P.P., H.E.N. Bersee, and A. Beukers, “Residual Stresses in

Thermoplastic Composites: A Study of the Literature – Part III: Effects of

Thermal Residual Stresses,” Composites Part A: Applied Science and

Manufacturing, Vol. 38, 2007, pp. 1581-1596.

[12] Nairn, J.A., “Microcracking, Microcrack-Induced Delamination, and

Longitudinal Splitting of Advanced Composite Structures,” NASA-CR4472, 1992.

[13] Wood, J.R., and M.G. Bader, “Void Control for Polymer-Matrix

Composites (1): Theoretical and Experimental Methods for Determining

the Growth and Collapse of Gas Bubbles,” Composites Manufacturing, Vol.

5, No. 3, 1994.

[14] Matzkanin, G.A., and H.T. Yolken, “Techniques for the

Nondestructive Evaluation of Polymer Matrix Composites,” AMMTIAC

Quarterly, Vol. 2, No. 4, 2007.

[15] Mallick, P.K., Composites Engineering Handbook, No. 14702, Marcel

Dekker, Inc., 1997.

Table 3. Summary of process-induced defects commonly encountered in PMCs.[15]

Contamination Due to foreign particles, extraneous fibers, or pieces of plastic release film (peel ply) not being removed from

prepreg surface, etc.

Undercure or Variation Occurs if the proper temperature and/or time are not used during the cure process

in Degree of Cure

Delamination or Separation of Due to poor consolidation and undercure in the curing operation or created during hole drilling or

Plies within the Laminate machining of a cured composite

Voids Formed by entrapped air, the presence of moisture, an excessive amount of solvent, or the release of gases

during the curing reaction

Resin-rich or Fiber-starved Areas Caused by nonuniform resin distribution or flow during molding

Resin-starved Areas Caused by an uncontrolled, or lack of, resin flow through the reinforcing layers

Fiber Misalignment Due to misoriented fibers, deviation from the preselected layup or filament winding pattern, or fiber washout

from excessive resin flow

Broken Filaments Caused by scratches or cuts, or a drill during hole drilling processes

Fiber Waviness or Kinking Due to improper tensioning during prepreg preparation, filament winding, and pultrusion

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INTRODUCTION

Polymer matrix composites (PMCs) provide properties superior to

those of conventional materials due to their synergistic combination

of high-performance fibers and matrices. In many instances, however, specialized processing operations are required to produce the

unique microstructures and geometries that give PMCs their

desirable properties and characteristics. Rejection rates tend to be

higher than those for more conventional materials, and the unpredictable material property variations often prevent designers from

utilizing these materials to their full potential. To mitigate these

issues, a new method of manufacturing known as intelligent processing emerged in the 1990s.

Contrary to conventional automated processing, intelligent

processing employs nondestructive evaluation (NDE) sensors to

characterize the material being manufactured in real time, rather

than using sensors to monitor certain process variables such as temperature and pressure. Using the data from these in-process NDE

sensors, the process can be optimized, placing the material properties under closed-loop control and the process variables under

adaptive control. This method “builds in quality” and evades the

high costs associated with the rejection of fully processed materials

or parts. This is in contrast to conventional automated processing,

where controllers maintain the desired preset process variables, and

NDE techniques are used after manufacture to “inspect in quality.”

PMC MANUFACTURING PROCESS CONTROL PARAMETERS

Polymer matrix composites are manufactured using a wide range of

technologies that depend upon resin type, fiber type, and end use

application. The most common methods include spray lay-up; hand

lay-up; ply cutting and stacking, in support of hand lay-up operations; automated tape lay-up; filament winding; tow placement;

pultrusion; liquid composite molding (i.e., resin transfer molding

(RTM), structural reaction injection molding (SRIM), and injection compression molding (ICM)); thermoforming; curing; and

forming, stamping, injection molding, and rolling.

The quality of the PMCs that result from these manufacturing

processes can be controlled by regulating the cure (thermosetting

resins only), resin flow, porosity and voids, fiber and matrix distribution, fiber volume fraction, fiber orientation, fiber waviness, and

fiber placement.

Cure: Fiber-reinforced PMCs are typically manufactured using

thermosetting resins. These resins require a cure cycle to transform

the short linear and branched polymer molecules into a single threedimensional (3-D) network of cross-linked chains, producing a

solid material.

Cure cycle improvements can enhance the fabrication time and

the overall quality of the end product and reduce the operational

costs. Although analytical models can be used to determine the optimum processing conditions, batch to batch variations in the properties of the input materials cannot be accounted for using these

methods. To obtain optimum processing conditions, a system is

needed that selects and controls the processing conditions in real

time. Figure 1 shows a common laminated thermoset polymer

matrix composite (PMC) cure cycle.[1] The cure cycle is dependent

upon the rate of the temperature increases, the temperature of the

curing plateau, the time at which pressure is applied, and the post

curing temperature and pressure.[2]

Resin Flow and Mold Fill: Liquid composite molding (LCM)

depends on the rates of resin flow and mold fill. To achieve parts that

have the matrix resin fully infiltrating the preform, proper mold fill,

with either 2- or 3-D preforms, is required. Mold fill is critically

dependent on resin flow and impregnation. Unsuccessful mold filling can lead to the formation of macroscopic and microscopic voids.

When multi-layer preforms are used to make thick parts, the

mold filling process becomes much more complex since the preform

has a 3-D nature. In thick parts, variations in the preforms and the

resin can affect flow rate. Pressure gradient, fluid viscosity, and the

fiber preform permeability can all be correlated to the flow rate of

the resin using Darcy’s Law. Darcy’s Law is typically utilized in the

codes for mold filling simulations.[3]

Porosity and Voids: Pores can be detrimental to the performance

of a PMC, leaving regions of unsupported fibers and areas of stress

concentration. Porosity leads to a decreased density, modulus of elasticity, and strength.[4] Whether in the form of distributed voids

within the matrix, between plies, or at the fiber matrix interface,

porosity is a perennial problem in composite manufacturing, especially for laminated structures such as carbon fiber reinforced PMCs.

Fiber and Matrix Distribution: The fiber and matrix distribution

is critical to the performance of a composite structure. Knowledge of

the size and location of matrix-rich and matrix-starved regions provides important information, which can be used in finite element

codes to calculate performance influences and to adjust processes.

Fiber Volume Fraction: Strength is determined largely by the

interaction between the fibers and the matrix. Since the matrix

distributes the load onto and between the fibers, it is important to

know and control the respective fiber volume fraction.

H. Thomas Yolken

George A. Matzkanin

AMMTIAC

Austin, TX

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Figure 1. Laminated thermoset PMC cure cycle.[1]

Heatup

Cure

Cool

Down

Pressure

0 60 120 180 240 300 360

Time (minutes)

600

400

200

60

30

Temperature

Temperature °C

Pressure (tons)

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Fiber Orientation: In continuous fiber reinforced PMCs, the

fiber orientation should be carefully controlled. If not properly

aligned, further issues could result during ply lay-up. For instance,

a variation of 10° in the ply* orientation can result in a 30%

decrease in the stiffness of a composite laminate.

Fiber Waviness: Fiber waviness is a manufacturing defect that

causes a through-thickness undulation in the fibers of a thick section PMC. Fiber waviness can occur during filament winding since

the overwrapped layers place stresses upon the wet hoop-wound

filaments. This defect can also be introduced through the buckling

of a pre-impregnated material (prepreg) or during the cure cycle as

a result of residual stress buildup. Fiber waviness can cause a significant reduction in the strength and stiffness of the final product.

Fiber Placement: Laminated composite parts can be fabricated

using automated tow† placement to save time and decrease production costs. Tows must be placed flush to one another with no space

or overlap; overlaps or gaps can reduce the mechanical strength to a

value below the design specification.

NDE SENSORS FOR PROCESS CONTROL

The following NDE methods are suitable for the in-process monitoring and control of PMC manufacturing processes.

Ultrasonics/Acoustics: Ultrasonic testing is the most widely

used NDE method for inspecting PMCs during manufacturing,

after manufacturing, and while in-service. Ultrasonics can detect

and evaluate delaminations, voids and porosity, and at times broken

fibers and ply slippage.[5, 6]

Acousto-ultrasonics and acoustic emissions testing methods

have been used to inspect composites at bonded joints and to locate

failure sites, respectively.[7, 8]

Radiography: X-ray radiography is used to nondestructively

evaluate PMCs after manufacture and while in-service. The development of digital radiography detectors has led to continued

improvements in the digital radiography and computed tomography of PMCs. Digital radiography can be used in real time, or

near-real time, to provide feedback on operations such as lay-up,

consolidation, and curing.[9]

Thermography: Thermography maps regions of equal temperature (isotherms) on a surface. There are several different types of

thermography, categorized as contacting or non-contacting, with

either active or passive heating methods. Thermography inspects for

disbands in adhesive joints, delaminations, and inclusions.

Optical and Spectroscopic: There are a number of optical and

spectroscopic NDE techniques that are used to inspect PMCs after

manufacture and while in-service. These techniques include: visual,

penetrants, photoelasticity, coherent light, holography, laser

shearography, and embedded optical fibers. Several of these techniques are also applicable for PMC process monitoring, especially

fiber optic strain sensors. One demonstration used an in-situ fiber

optic strain sensor, coupled to a miniature strain sensor, to monitor

the changing rheology of a curing resin and thus track the cure cycle

progression.[10]

Optical imaging and scattering techniques have also been used

for the in-process monitoring of PMC manufacture. Laser optical

triangulation has been utilized in measuring the gap and tow width

during automated layup.[11]

Electromagnetic: Microwave NDE in the frequency range from

225 megahertz (MHz) to 100 gigahertz (GHz) is sensitive to the

dielectric properties of PMCs and is particularly effective in monitoring their internal structure (fiber orientation), resin-rich or resinpoor areas, the state of cure, moisture content, porosity, thickness,

and surface waviness. Microwave sensors have been used to monitor

resin curing and porosity.[12]

ON-LINE PROCESS CONTROL APPLICATIONS

NDE methods are most commonly used to monitor the state of

resin cure, void formation, resin flow and mold fill operations, and

reinforcement placement. The technologies that have been developed to monitor each of these processes are discussed.

Monitoring the Resin Cure Cycle

A commercial ultrasonic system for the in-process monitoring and

control of the cure cycle has been developed to monitor in-mold

PMCs at temperatures below 260°C. The temperature and ultrasonic sound speed responses of this system are shown in Figure 2

for an epoxy-graphite fiber prepreg during a compression molding

process. The changes in the ultrasonic sound speed directly correlate

to the cure cycle progression. For the example in Figure 2, the minimum in sound speed occurs as the temperature cycle enters a

121°C hold period. This is followed by an increase in sound speed

as the temperature is increased to 177°C. The rate of increase in

sound speed slows as the cure reaction nears completion.[13]

Embedded acoustic wave-guides have also been employed to

monitor the state of cure in PMCs in this work; the transmission

velocity of an acoustic signal within the wave-guide at final cure was

measured, revealing the final acoustic impedance (density multiplied by wave velocity) of the host composite. Since the velocity of

sound is directly proportional to the square root of the bulk modulus and inversely proportional to the square root of the density and

Poisson’s Ratio, the acoustic wave-guide sound velocity is a relative

measure of the material modulus. In turn, these quantities are all

related to the material’s state of cure and ultimately the internal

residual stresses.[14]

Differences in the residual strain within the composite were also

measured by embedding several identical acoustic wave-guides in a

composite part and determining the differences in acoustic waveguide sound speed. Figure 3 shows the system used for the acoustic

wave-guide cure and stress monitoring of composites. Although

these embedded wave guides are effective as cure cycle monitors,

their use has been avoided in the past, as some view the wave-guides

as defects or inclusions.[14]

Embedded electrodes have been developed to obtain dielectric

Sound Speed

Temperature

0 50 100 150

Time (minutes)

900

800

700

600

500

180

160

140

120

100

80

60

40

20

Sound Speed (m/s)

Temperature °C

Figure 2. The changes in ultrasonic sound speed and temperature

for an epoxy-graphite fiber prepreg during a compression molding

process.[13]

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measurements as a means of monitoring the cure rate of resins as

well. The dielectric properties of a resin typically vary over a large

range as the resin cures.[15, 16] These embedded electrodes are

also seen by some as defects, limiting the use of this method in

production process control. Nevertheless, this method has found

widespread use in process design and optimization.

Microwave-based nondestructive methods that measure the

permittivity and permeability of a composite during cure have been

developed to indicate whether the fully cured material will have the

desired properties. It is important to tightly control the complex

permittivity and permeability of PMCs that are to be used for

particular electromagnetic applications, such as shielding or

absorption of electromagnetic radiation. Often, however, the electromagnetic parameters depend on the environment in which the

PMC is cured.[17]

Monitoring Void Formation

Laser ultrasonic methods have been used to nondestructively quantify graphite-epoxy porosity when only one surface of a sample, or

product, is available. This method enables the inspection of PMC

porosity both during fabrication and while in-service.[18]

Infrared thermography has also demonstrated capabilities for

the in-process monitoring and control of porosity during thermoplastic composite fabrication. This method focuses on thermoplastic automated tow placement as a means for minimizing

surface porosity.[19]

Monitoring Resin Flow and Mold Fill

Electromagnetic NDE sensors have been developed to monitor the

real-time flow of resin into molds during the RTM process. These

sensors consist of two planes of electrically conducting threads

placed in an orthogonal, non-conducting grid that are separated

by plies of insulating preform. They are typically arranged such that

the location of the resin can be determined. As the resin fills the

preform, the two planes of sensors form a series of connected electrical circuits that can be easily monitored using a 12-volt DC

current.[20] These sensors were used during the production of the

Army XM194 Ballistic Shield (a thick section composite which

protects part of the 155-mm cannon) to monitor the flow-front in

the mold and control the inlet and vacuum ports of the process.[21]

Ultrasonic techniques have also been used to monitor the flowfront during RTM mold filling operations. With this method, a

laser is used to generate ultrasonic waves, which are then detected

using a Fabry-Perot interferometer or laser probe. Figure 4 shows a

schematic of the mold geometry and the key reflected ultrasonic

pulses for the in-situ monitoring of this RTM process.[22]

Ultrasonics have been used for the in-process analysis of polymer

melts as well. Ultrasonic monitoring of polymer melts can improve

quality, enhance process control, and enable new materials with specific properties to be produced. In-process ultrasonic analysis techniques that accurately measure melt viscosity, filler content, and

molecular weight have been demonstrated. With this technology,

ultrasonic pulses, from 2 MHz to 10 MHz, are guided into

and out of the polymer melt by metal buffer rods that protect the

transducer from heat and pressure damage. A second transducer

receives the transmitted sound that echoes after passage through the

melt. These signals are amplified and digitized. The time of flight

and amplitude are extracted and used to calculate the velocity and

attenuation of the ultrasonic pulses via correlation techniques. A

calibration model combines the ultrasound results with temperature

and pressure data to calculate the material properties. This ultrasonic polymer analysis technology has been demonstrated in many

polymer manufacturing and production processes. A wide variety of

property measurements have been made in real time with excellent

precision relative to laboratory analysis methods.[23]

Monitoring Reinforcement Placement

Optical imaging, based on video camera technology, has been successfully utilized to monitor a commercial 3-D automated braiding

process. Three video cameras, each with 120° of coverage, were used

to obtain a total 360° of coverage. A commercial frame grabbervideo array processor and a desktop computer were used for the data

processing activities. A block diagram of this video imaging system

is shown in Figure 5.[24]

Eddy current NDE has been coupled with a horseshoe type

probe to determine fiber orientation, and it has been proven useful

in monitoring the cure of graphite epoxy composite laminates.[18]

In order to use eddy current NDE, however, the PMCs must have

The AMMTIAC Quarterly, Volume 5, Number 4 13 http://ammtiac.alionscience.com

Figure 3. Schematic of the system used for the acoustic wave-guide

cure and stress monitoring of composites.[14]

Wavetek Model 191

Pulse Function Generator

Signal Generator (Pulse Burst Mode 150/sec, 32 Cycle Bursts)

60 - 80 kHz

To Scope Pulsed

Wavetrains

Shell 815 Resin

Specimen 10 x 2 x 1 cm.

70 kHz Acoustic

Transducer

Transmitted Signal

Teflon Mold

TV Monitor

VCR

TV Camera

30 kHz

High Pass Filter From

Signal

Generator

Oscilloscope (Tektronix 7844)

Received Signal

Nichrome AWG, 20 cm. long

7A

22 7A

22

Figure 4. A schematic of the mold geometry and key reflected ultrasonic pulses for the in-situ monitoring of RTM process.[22]

Inner Walls of Aluminum Mold

Fiber Preform

Generation and

Detection Laser Beams

Resin

A

A

B

B

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The AMMTIAC Quarterly, Volume 5, Number 4 14 http://ammtiac.alionscience.com

a high concentration of electrically conducting fibers (e.g., carbon

or boron), since the fibers establish a continuous conducting path

that enable the movement of eddy currents.

CONCLUSIONS

NDE is playing an important role throughout the life cycle of

PMCs from the design of parts to process optimization. NDE

techniques are used for in-process control, after manufacturing

inspection, and in-service inspection. This article discussed NDE

techniques for the in-process monitoring of PMC fabrication,

specifically of the resin cure cycle, void formation, resin flow and

mold fill, and reinforcement placement.

The resin cure cycle can be monitored using ultrasonic wave

propagation techniques based on contact transducers, non-contacting lasers, and embedded wave-guides; dielectric measuring equipment using embedded electrodes; and microwave sensing. Void

formation has been monitored using contact ultrasonics with traditional transducers, non-contact ultrasonics utilizing laser-based

techniques, and lock-in infrared thermography using rugged

equipment. Electrical circuit type sensors and embedded optical

fibers are capable of monitoring resin flow and mold fill.

Reinforcement placement monitoring uses optical imaging with

computer image processing and contact ultrasonics.

It is expected that, as composite parts continue to become larger

and more complex, the demands for in-process NDE sensing techniques will continue to expand.

REFERENCES

* A ply is a piece of laminate used to capture the features of a 3-D object.

Plies are often placed atop one another in differing directions to give the

component the desired properties.

† A tow is an untwisted bundle of continuous fibers.

[1] Sih, G.C., A. Carpinteri, and G. Surace, Advanced Technology for Design

and Fabrication of Composite Materials and Structures, Kluwer Academic

Publishers, 1995, pp. 1-14.

[2] Geier, M.H., Quality Handbook for Composite Materials, Chapman &

Hall, 1994.

[3] Bickerton, S., et al., “Modeling Manufacturing Issues in Liquid

Composite Molding Processing,” Second Conference on NDE Applied to

Process Control of Composite Fabrication, NTIAC, 1996, pp. 49-60.

[4] Kardos, J.K., R. Dave, and M.P. Dudukovic, “Voids In Composites,”

Proceedings of Manufacturing Int., 1988, pp. 41-48.

[5] Parker, I., “Looking Inside a Structure,” Aerospace Composite Materials,

May/June 1989, pp. 14-22.

[6] Leonard, L., “Inside Story on Composites: NDI Looks Sharp,”

Advanced Composites, Vol. 56, January/February, 1990.

[7] Talreja, R., “Application of Acousto-Ultrasonics to Quality Control and

Damage Assessment in Composites,” 4th Int. Conference Composites

Structures, 1987.

[8] Drouillard, T.F., “A Comprehensive Guide to Literature on AE From

Composites,” Proceedings 3rd International Symposium Acoustic Emission

Reinforced Composites, Supplement II, July 1989, RFP-4297.

[9] Yancey, R.N., and C. Mitchell, “Nondestructive Evaluation of

Composites Using Digital Radiography and Computed Tomography,”

Conference Proceedings: Nondestructive Evaluation Applied To Process Control

of Composite Fabrication, NTIAC, 1994, pp. 111-114.

[10] May, R.G., and R.O. Claus, “In-Situ Fiber Optic Sensor for

Composite Cure Monitoring Through Characterization of Resin

Viscoelasticity,” SPIE, Vol. 2948, 1996, pp. 24-34.

[11] Folsom, T.C., “A Sensor to Measure Gap Width During Fiber

Placement,” Second Conference on NDE Applied to Process Control of

Composite Fabrication, 1996, pp. 93-102.

[12] Zoughi, R., and S.I. Ganchev, “Application of Microwave

Nondestructive Testing and Evaluation to Thick Composites,” Conference

Proceedings: Nondestructive Evaluation Applied To Process Control of

Composite Fabrication, NTIAC, 1994, pp. 77-90.

[13] Shepard, D.D., and K.R. Smith, “A Complete Ultrasonic

Measurement System for In-Process Cure Monitoring and Control of

Composites,” Second Conference on NDE Applied to Process Control of

Composite Fabrication, NTIAC, 1996, pp. 61-68.

[14] Harrold, R.T., et al., SPIE Proceedings: Nondestructive Evaluation for

Process Control in Manufacturing, Vol. 2948, 1996, pp. 12-23.

[15] Senturia, S.D., and N.F. Sheppard, “Dielectric Analysis of Thermoset

Cure,” Advanced Polymer Science, Vol. 80, 1986, pp. 1-47.

[16] Rice, B.P., and C.W. Lee, “Novel, Low-Cost Sensors for Intelligent

Process Control,” 41st International SAMPE Symposium, March 1996, pp.

1518-1529.

[17] Bowler, N., and E.R. Abram, “Monitoring the Effect of Relative

Humidity During Curing on Dielectric Properties of Composites at

Microwave Frequencies,” Review of Quantitative Nondestructive Evaluation,

Vol. 25, 2006, pp. 469-476.

[18] Karabutov, A.A., I.M. Pelivanov, and N.B. Podymova, “Laser

Optoacoustic Method for Nondestructive Porosity Evaluation of GraphiteEpoxy Composites,” SPIE, Vol. 6162, 2005.

[19] Steiner, K.V., et al., “Infrared Thermography and Laser-Based

Ultrasonic Methods for On-Line Porosity Sensing During Thermoplastic

Composites Fabrication,” Second Conference on NDE Applied to Process

Control of Composite Fabrication, NTIAC, 1996, pp. 115-122.

[20] Walsh, S.M., “In-Situ Sensor Method and Device,” US Patent:

5,210,499 (May 1993).

[21] DuBois, C.J., S. Malghan, W.O. Ballata, and S. Walsh, “Resin Flow

Sensing: A Precursor to Successful Composite Control,” Conference

Proceedings: Second Conference on NDE Applied to Process Control of

Composite Fabrication, October 1996, pp. 81-92.

[22] McKie, A.D.W., R.C. Addison, Jr., and L.T. Liao, “In-Situ Process

Monitoring Using Laser-Based Ultrasonics,” Conference Proceedings

Nondestructive Evaluation Applied to Process Control of Composite

Fabrication, NTIAC, 1994, pp. 115-126.

[23] Seto, D., and W. Weber, “Developments in Using Ultrasound for

QNDE of Polymer Melts,” Review of Quantitative Nondestructive

Evaluation, Vol. 21, 2002, pp. 1727-1734.

[24] Cauffman, S., R. Brown, and A. Bartos, “Video Monitoring of a

Large-Scale Commercial Three Dimensional Braider for Advanced

Composite Preform Process Control,” Conference Proceedings Nondestructive

Evaluation Applied to Process Control of Composite Fabrication, NTIAC,

1994, pp. 127-142.

Figure 6. Video System Block Diagram.[23]

Braiding Room Computer Control Room

Control

Camera #1

Camera #2 Camera #3

Mouse

Braider

Video Image Analysis PC

Process

Control

Computer

Serial

Card

Network

Card

Host (PC) BUSS

Frame

Grabber

Board

Video

Array

Processor

Video

Image

Monitor

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