Kutz M. Handbook of materials selection

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1362 USING COMPOSITES

Fig. 3 Co-curing stiffeners with skins.

sandwich structure has been discussed previously. Another concept that has been

evaluated by the automotive industry involves resin injection. Dry ﬁber preforms

representing several parts are assembled from fabric, mat, braided stock, and

knit ﬁbers. Stitching of the dry fabric to enhance normal tension strength is

feasible. The preform is placed in a closed mold and resin is injected. After

cure, the assembly is removed from the mold. Using resins with isocyanate

curatives, an assembly can be made in several minutes. One automaker has

combined 50 stamped steel parts assembled by welding into two composite

chassis members joined by bolting. I-beam cross-section ribs for military aircraft

are molded with at least one attachment clip integral to the web. Each clip saves

making and tracking an individual part through the factory, as well as the cost

of installing several fasteners.

A common method of providing localized stiffness in commercial ﬁberglass

structures involves placing machined pieces of foam or honeycomb within the

lay-up and molding them in. Applications include the keel areas of boats, cir-

cumferential rings on composite gasoline storage tanks, and helicopter fuselage

panels. Many carbon ﬁber/epoxy panels are molded with integral stiffeners or

attachment angles co-cured on the skin. The tooling setup shown in Fig. 3 was

used to mold the panel shown in Fig. 4. The method is suitable for I-blade, tee,

hat, and joy stiffeners. The designer can reproduce in composites conﬁgurations

that have worked well in sheet metal structures. Before committing to a design,

however, the designer must understand how much stiffness is required. The

cheapest and easiest approach is to add strips of unidirectional material to the

skin to react to moderate bending loads. Where bending or ﬂutter is a severe

problem, a stiffened panel is required. Stiffener effectivity is greatly enhanced

by adding unidirectional plies in the cap to increase the inertia so that the number

of stiffeners may be reduced. The most structurally efﬁcient stiffener is the hat

section, but mandrel removal is a problem. Unless access to inﬂatable mandrels

8 PRODUCIBILITY CHECKLIST 1363

Fig. 4 Co-cured fusilage skin panel with integral stiffeners and frames.

is available, other stiffener conﬁgurations should be strongly considered because

they can be molded on reusable tools. The costs of single-use washout or meltout

mandrels are high, and no foam withstands the normal autoclave cures at 85 psi

and 350

⬚F used to maximize graphite/epoxy laminates properties for 260⬚F ser-

vice. Teﬂon mandrels are surprisingly difﬁcult to extract due to the compressive

stress up to 5 ksi exerted by resin shrinkage.

In summary, integrally stiffened panels are both weight and cost effective

when compared to comparable fastened structures. However, these beneﬁts can

be obtained only by designing panels with a minimum number of stiffeners of

a conﬁguration whose mandrels can be removed easily. Fortunately, structures

for helicopters or passenger aircraft can be cured at 250

⬚F, so ﬂyaway foam

mandrels left in the part are a possibility.

8 PRODUCIBILITY CHECKLIST

With a design concept or two in mind, the designer should review a producibility

list to lighten the burden on manufacturing. To meet quality and cost objectives,

1364 USING COMPOSITES

design, materials selection, and process must be coordinated. In the composites

industry, with such varied products, processes, and materials available, gener-

alization is difﬁcult. Thus, the reader is invited to add his or her requirements

to this list and eliminate those not pertinent to his or her situation.

The design must provide for low-cost manufacturing, including:

●

Contour symmetry so a tool can make multiple part numbers

●

Radius of in. on male tools and in. radius on female tools to assist

––

lay-up

●

Cheapest materials that can meet all the design objectives, e.g., don’t use

graphite when glass will do

●

Design lay-ups symmetrical about the centerline of the thickness to reduce

warpage

●

Laying up a minimum number of plies by using thicker materials

●

Easily drapable satin weave fabrics for highly contoured parts

●

Simpliﬁed ply shapes

●

Simpliﬁed preparation of cores for sandwich structure

●

Compatibility with high-rate processes by using automation where pos-

sible

●

Molding a minimum number of parts

●

Minimal curvature to trim

●

Compatibility with advanced tooling concepts

●

Self-contained curing ﬁxtures

●

Removable stiffener mandrels

●

Low thermal expansion tools for dimensional accuracy of surfaces

●

Ease of part removal

●

Tooled surfaces selected to expedite assembly

●

Avoid sacriﬁcial, complex, or hard-to-use tools

●

Simpliﬁed assembly methods

●

co-curing sandwich panels

●

Co-curing integral joints

●

Molding stiffeners on a precured skin with secondary bonded joints

●

Thermoplastic molding

●

Automated drilling and fastening

●

Inspectibility

●

Compatibility with NDI systems (if required). NDI stands for nondestruc-

tive inspection. X ray or processes similar to CAT scans are used to ﬁnd

ﬂaws inside a part without cutting it up.

●

Inspection-critical areas deﬁned

●

Allowable defects deﬁned

●

Repairs for speciﬁc defects documented

Materials selection has a signiﬁcant effect on a project’s success. Factors to

be considered include:

9 ETERNAL QUALITY CONTROL QUESTION 1365

●

Matrix meets service temperature requirements

●

Sufﬁcient resin ﬂow to minimize porosity and enhance ply bonding

●

Cure temperature and pressure compatible with application, e.g., expen-

sive autoclave processing of a small boat is not required

●

Rapid cure to enhance throughput and minimize the need for multiple

tools

●

3D woven or braided dry fabric preforms increase laminate properties in

z direction

●

Reproducibility of batches of material mandatory for part consistency

Lastly, many processes have been optimized for throughput. It is necessary,

however, to review the selected process to see if improvements can be made.

Relevant issues include:

●

Reducing needed workers’ skill levels by simplifying operations

●

Reducing labor-intensive processes by capital investment, e.g., ceiling-

mounted laser projectors that show the location of partial plies eliminate

lay-up tooling and save 50% of the labor cost

●

Reducing long ﬂow times so that multiple tools are not needed

●

Obtaining a tight-tolerance ﬁt of mating surfaces to be joined by bonding

or fastening

●

Joining many parts by making an integrally molded structure instead of

bonding or fastening individual parts

●

Compatibility with automated NDI so that erroneous defects can be found

The intent of this section was to highlight various options and pitfalls in

composite design and processing. Consultation with experts in the various ma-

terial categories or processes will further enhance the technical base of the pro-

ject.

9 ETERNAL QUALITY CONTROL QUESTION

How good do we have to be and how bad can we be to produce a usable part?

The quality control requirements obviously vary according to the end product.

A cafeteria tray is press molded in 2 min from dry ﬁberglass mat, surfacing

plies (called veils), and a cupful of liquid resin. Every part is inspected visually,

and a few are probably structurally tested initially. Periodically, the weight of

the ingredients and trays, and operating parameters of the press, are checked so

that the product that was tested is produced for the entire production run. Any

parts that are too difﬁcult to repair are scrapped. There is a limit on how much

attention can be paid to a cheap product with low structural requirements.

In contrast, manufacturing an advanced composite (CFRP) aircraft structure

is monitored closely. A former chairman of the FAA Composites Committee

was very outspoken that Aunt Minnie should be able to arrive in Peoria as safely

in a composites aircraft as she did in a metal aircraft. Resolving his concern is

a two-step process, with the ﬁrst stage including:

1366 USING COMPOSITES

Fig. 5 Inspection requirements for ﬂight-critical structure.

●

Materials procured to rigid speciﬁcations, with properties deﬁned by al-

lowables

●

Veriﬁcation of design points by the test of coupons made from the spec-

iﬁed material

●

Manufacturing process controlled by a detailed process speciﬁcation

●

Additional design veriﬁcation by testing elements and subcomponents pro-

duced using both speciﬁcations

●

Static and fatigue testing of full-scale articles produced as described above

At this point, the capability of the speciﬁed material and process to produce

a structure that meets all of an aircraft’s requirements has been veriﬁed. For the

production department, the intent is to produce exactly the same product, or a

better product, hundreds of times over a period of several decades, thereby ver-

ifying the design. To ensure meeting this safety requirement, a detailed inspec-

tion plan (Fig. 5) must be implemented. The system works well until a defect

is found. The engineering staff must evaluate the effect of the ﬂaw and determine

the disposition of the part: use as is, repair per a speciﬁed procedure, or scrap.

Many programs now have documentation covering allowable defects, standard

repairs, and mandatory scraps to reduce the engineering effort with regard to

disposition of a defective part. So the engineering staff must ﬁrst design an

optimum part, then documentation must be prepared to deﬁne how ‘‘bad’’ it can

be and still meet design requirements. Standard repairs for repetitive defects

must be provided.

10 ENVIRONMENTAL CONCERNS 1367

This concept also applies to other industries. While evaluating the processing

of large ﬁberglass boat hulls, for example, I have observed mechanics making

repairs based on the engineering department’s sketches and instructions. Other

ﬂaws were left as is.

In the case of press-molded parts, a visual examination of the part surface

and tap testing for porosity may be sufﬁcient.

In summary, every product requires a quality control plan suited to the prod-

uct’s function and its value. As structural requirements and costs increase, the

engineering design effort must address defects. Parts with unacceptable discrep-

ancies or inadequate repairs must not be placed in service. Conversely, usable

parts should not be scrapped unnecessarily.

For lightly loaded consumer products, the engineering department can relax

as the manufacturing department struggles to always make the product look

pretty and keep defects within tolerable limits.

10 ENVIRONMENTAL CONCERNS

A composite manufacturing plant inherently has many environmental problems

simply due to the nature of the materials and processes used. Under prodding

by the Environmental Protection Agency (EPA), industrial hygiene has improved

signiﬁcantly since the mid-1980s. Even so, proﬁts from a well-conceived pro-

duction effort can be lost due to EPA penalties. So environmental concerns must

be a part of the materials selection and design effort, as well as a requirement

for manufacturing facilities. There are four problem areas: airborne vapors, skin

contact with allergens, dust, and disposal of waste.

Many resins contain ingredients with low boiling points. Polyester and vinyl

ester contain styrene whose odor permeates even well-ventilated facilities. Phe-

nolic resins have the sweet odor of unreacted phenol and give off formaldehyde.

A group of mechanics working on a highly classiﬁed project were awarded

compensation for medical problems because, reportedly, they were working in

an inadequately ventilated area and absorbed some chemicals.

An early curative for epoxy resin mixes prepared on the shop ﬂoor required

melting prior to use. Brown fumes were produced. I experienced them. They

stained our clothes (I was wearing ‘‘epoxy’’ workpants) and anything else

nearby. The ﬁrst curative for polyurethane castings also required melting and

was a carcinogen. Both compounds where withdrawn after years of use.

Epoxy resins produce dermatitis once the body becomes sensitized. My best

technician did not develop a rash until 17 years of exposure, but I have seen

new employees develop rashes in several months. The only cure seems to be to

avoid contact with epoxy materials prior to cure and with dust from cured panels.

Material suppliers are required to provide materials safety data sheets listing the

ingredients in generic terms. These sheets should be carefully reviewed by the

safety team before new materials are introduced to production. In well-ventilated

areas, in my opinion, problems, except for dermatitis, are not signiﬁcant as long

as normal precautions are taken. Food and drinks, for example, are no longer

allowed in lay-up areas.

Chips are produced when metals are drilled or cut. Composite machining, on

the other hand, produces dust. Recently, water jet cutting of composites, which

does not produce dust, has been introduced into production. Also, water-cooled

drills produce a gritty slurry instead of dust. Maximizing the use of water-cooled

1368 USING COMPOSITES

drilling in modern equipment is cost effective and provides a free safety beneﬁt.

Dust can be controlled by using vacuum hoses next to cutters, spray booth ﬁlters,

and vacuum cleaners to replace brooms.

The EPA has guidelines for the disposal of solvents and paints. Composite

scrap and dust should be buried. This is especially true for carbon/graphite

composites. Once, in Cleveland, scrap carbon ﬁbers were incinerated and the

residue was blown into a transformer that shorted out. Much of the city was

dark until repairs could be made.

Even though a history of effective methods to prevent environmental problems

exists, using new materials or inattention can cause problems. So, to reduce

environmental hazards in the factory, the designer should confer with the safety

department before transmitting a design to the production department.

1369

CHAPTER 45

COMPOSITES IN CONSTRUCTION

Ayman S. Mosallam

Division of Engineering

California State University

Fullerton, California

1 INTRODUCTION 1369

1.1 General 1369

1.2 Advantages 1370

2 CONSTRUCTION APPLICATIONS

OF COMPOSITES 1370

2.1 Aggressive Environments 1370

2.2 Repair and Retroﬁt Infrastructure

Systems 1371

2.3 Internal Reinforcement of

Concrete Members Using

FRP Composites 1399

2.4 All-Composites Structural

Applications 1401

3 DEVELOPMENT OF CODES

AND STANDARDS 1416

4 NEW STRATEGY AND

RECOMMENDATIONS 1418

BIBLIOGRAPHY 1420

1 INTRODUCTION

1.1 General

For the past few decades, the aerospace industry was the major user for ﬁber-

reinforced polymer (FRP) composite materials. Recently, civil engineers and the

construction industry began to realize the potential of these materials in provid-

ing remedies for many problems associated with the deterioration and corrosion

of infrastructures.

Civil engineers have dealt with different types of composite materials for

decades, including wood (natural composites), plywood (laminated natural com-

posites), and concrete (particulate composites). Polymer composites are ‘‘engi-

neered’’ materials that encompass a wide range of materials where two or more,

physically distinct and mechanically separable, components are combined to-

gether to form a new material that possesses properties that are notably different

from those of its individual constituents. The primary load-carrying component

is the ﬁbers, while the matrix acts as a binder, an environmental protector, and

stress distribution phase of the laminate. Fibers are available in different types,

grades, and shapes. Typical types of structural ﬁbers include glass (E, S, AR

grades), aramid (Kevlar), and carbon ﬁbers with different grades. Two types of

polymers are available, namely thermoplastic and thermoset polymers. In most

of the structural application, thermoset resins are preferred. Examples of ther-

HandbookofMaterialsSelection.EditedbyMyerKutz

1370 COMPOSITES IN CONSTRUCTION

Fig. 1 Seismic upgrade of columns using composites.

[Courtesy of Structural Composites Construction Inc. (SCCI)]

moset resins include epoxies, unsaturated polyesters, vinylesters, aminos, phe-

nolics, and urethane resins.

1.2 Advantages

Some of the attractive and unique features of these materials are their durability

and resistance to the marine environment, their toughness, particularly at low

temperatures, their vibration damping capabilities, their energy absorption under

earthquake loading, their electromagnetic transparency, their low value of co-

efﬁcient of thermal expansion, pigmentability and decorative characteristics, and

their high strength-to-weight ratio. These unique properties can be used to pro-

duce an optimum structural system with minimum life-cycle cost, fabrication

and construction cost, and time.

2 CONSTRUCTION APPLICATIONS OF COMPOSITES

Applications where composite materials can show their superiority over other

conventional materials are discussed below.

2.1 Aggressive Environments

●

Waterfront structures

●

Water and wastewater treatment plants structural elements

●

Water declination plants

2 CONSTRUCTION APPLICATIONS OF COMPOSITES 1371

Fig. 2 Reinforced concrete corroded column repair applications using composites.

(Courtesy of Sigma Composites, LLC)

●

Off-shore structures (off-shore oil rig platforms, marine risers, etc.)

●

Cooling towers

●

Petrochemical and nuclear power plants

●

Paper and pulp mills

●

Chimneys

●

Pipes

2.2 Repair and Retroﬁt Infrastructure Systems

Due to their unique properties, composites can provide structural engineers with

the answers to many structural problems. The two major applications are seismic

repair and rehabilitation (Fig. 1), and corrosion repair (Fig. 2). These apply to

buildings, bridges, and other infrastructure systems. Composites can also be used

to upgrade the structural performance and capacity of reinforced concrete, steel,

and wood and masonry structural members inland and off-shore. In this process,

several laminates of composites are bonded to the ﬁnished surfaces of the struc-

tural member in the speciﬁed directions. In early 1990s, the majority of the

applications focused on the ductility enhancement of concrete columns, espe-

cially in seismic areas such as California. In this particular application, the ﬁbers