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ECHNOLOGY
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Structural Design of FRP Components
May 2013 CTI Bulletin ESG-152 (13)
This document was developed using the consensus procedure outlined in the CTI Operating Procedure 304 and has been approved for publication by the CTI Board of Directors
FOREWORD
This Cooling Technology Institute (CTI) publication is published as an aid to cooling tower purchasers and designers. It may be used by anyone desiring to do so, and efforts have been made by CTI to assure the accuracy and reliability of the data contained herein. However, CTI makes NO WARRANTY OF FITNESS FOR PARTICULAR PURPOSE OR
MERCHANTABILITY nor any other warranty expressed, implied or statutory. In no event shall CTI be liable or
responsible for INCIDENTAL, CONSEQUENTIAL OR COMMERCIAL losses or damage of any kind resulting from this publication’s use; or violation of any federal, state, or municipal regulation with which this publication may conflict or for the infringement of any patent resulting from the use of this publication.
All CTI codes and standards are copyrighted with all rights reserved to CTI. The reproduction of any part of this or any other CTI code or standard is a violation of Federal Law. One must recognize and appreciate commitment by a number of volunteer members who donate their time to develop and update CTI codes and standards. The monies spent for code development, administrative staff support and publication are essential and constitute a substantial drain on CTI. The purchase price of these documents helps to offset these costs. Reproduction and distribution by others, in addition to being unethical, undermines this system and constitutes a further financial drain on CTI. When copies are needed, you are requested to call or write the Cooling Technology Institute, P.O. Box 73383, Houston, Texas 77273, (281) 583-4087. Please request that your associates buy the required codes and standards rather than copy them. Your cooperation in this matter is greatly appreciated.
Nothing contained herein is to be construed as granting any right for the manufacture, sale or use in connection with any method, apparatus, or product covered by letters patent, nor as insuring anyone against liability for infringement of letters patent.
This guideline document summarizes the best current state of knowledge regarding the specific subject. This document represents a consensus of those individual members who have reviewed this document, its scope and provisions. It is intended to aid all users or potential users of cooling towers. CTI STD 137, (R2007), Fiberglass Pultruded Structural Products for Use in Cooling Towers, Cooling Technology Institute, PO Box 73383,Houston, Texas 77273, USA is the materials standard to be utilized in conjunction with this guideline.
Approved by the CTI Executive Board.
Copyright 2013
Approved by the by Cooling Technology Institute CTI-Guideline
CTI Executive Board Printed in U.S.A. ESG-152 (13)
This document has been reviewed and approved as part of CTI's Five Year Review Cycle. This document is again subject to review in 2018.
Structural Design of FRP Components
Preface
FRP composites structural members are the newest building materials available for constructing cooling towers. As with any new material, designers must understand FRP's unique properties and the performance differences from other structural materials.
This Structural Design Guideline provides minimum design standards and cautionary recommendations to designers of FRP structural cooling towers. STD 137(07) is incorporated in this guideline as the material specification for FRP pultruded structural products and is the basis for the various tables of values.
1. MANUFACTURING STANDARDS
1.1 Design:
Material manufacturers provide allowable loads, design equations, reduction factors and service factors for common shapes and loading. This guideline is provided in lieu of the manufacturers data as a means to determine the design allowable loads. The design allowable provided herein is expressed as follows: Design Allowable = Ultimate Strength x Rt x Rm
Service Factors
Design Allowable = Maximum allowable stress
Strength = Material ultimate strength as specified in Section 1.1.1 Ultimate Strength or in the case of long columns critical buckling stress.
Rt = Strength reduction factor due to temperature as specified in Section 1.1.2 Strength Reduction Factors Rm = Strength reduction factor due to moisture as specified in Section 1.1.2 Strength Reduction Factors Service Factors = Strength reduction factors as specified in sections 1.1.3 Minimum Material Service Factors. 1.1.1 Ultimate Strength:
The minimum ultimate strength may be obtained through the material maufacturer, CTI-STD 137 or testing of pultruded components. Where testing is employed the ultimate strength shall not be less than the 5th percentile
value for a weibul distribution with 80% confidence per ASTM D7290-06.
1.1.2 Strength Reduction Factors for Temperature
Composite materials lose strength and stability at elevated temperatures. The resulting strength losses are related to the particular resin system used in fabrication of the member. Reduction factors which reflect the percentage of strength and modulus of elasticity retained at these higher temperatures must be applied to account for this loss of strength. Tables 1 and 2 indicate these
factors for Polyester and Vinyl Ester resin systems. Linear interpolation is permitted for temperatures not shown. These temperature reduction factors do not account for loss of strength of the composite material due to moisture exposure. (Comparisons were done dry at elevated temperatures). Reduction factors for pultruded shapes immersed in water are listed in Table 4 and in CTI STD-137, Table 2.3.
Table 1
APPLICATION CONDITION VS. “AVERAGE” COMPRESSION STRENGTH REDUCTION,
ASTM D695
(See footnote #3 below)
Temperature, °F REDUCTION FACTOR POLY VINYL 77 1.0 1.0 100 0.85 0.90 125 0.70 0.85 150 0.50 0.80 175 NR 0.75 200 NR 0.50
1) 2004 CTI Paper (TP04-08): “Update on the Impact ofWater Immersion for Pultruded FRP”
2) 2005 CTI Paper (TP05-03): “The Impact of Veil Thickness and Coating on Cooling Tower FRP Composites”
3) These reduction factors are for isophthalic polyester resin shapes.
Table 2
APPLICATION CONDITION VS. “AVERAGE” MODULUS OFELASTICITYREDUCTION, ASTM D638(* also reference modulus of elasticity
procedure written for this application)
(See footnote #3 below)
Temperature, °F REDUCTION FACTOR POLY VINYL 77 1.0 1.0 100 1.0 1.0 125 .90 .95 150 .85 .90 175 NR .88 200 NR .85
The reduction factor is multiplied by the Ultimate Strengths or Modulus of Elasticity prior to applying service factors. Thus, the location of the member within the tower will dictate the exposure (i.e. fill support beams will be exposed to a lower temperature environment than the hot water distribution piping support beams). If the tower is to experience a short-term hot temperature excursion in excess of the design temperature this would be considered a live load condition. The following example illustrates the use of temperature effects.
Table 3
EXAMPLE: FILL SUPPORT BEAMS DESIGN TEMPERATURE – 125°F PARAMETER (Minimum Design Values) Applicable ASTM 77°F 125°F POLY VINYL Ultimate Bearing, PSI D953 30,000 21,000 24,000 Flexure Strength, PSI D790 30,000 21,000 24,000 Ultimate Shear, PSI D2344 4,500 3,150 3,600 Full Section Modulus Elasticity, 106 PSI 2.6 and 2.8 2.34 2.66
NOTE: Although polyester resin system strength is reduced more than a vinyl system, polyester systems can be used and still achieve the desired long term behavior.Additionally, the appropriate service factors must be used to minimize long-term creep regardless of which resin system is used. It is recommended that an extra 10°F be added to the maximum design temperatures to compensate for the unexpected short term upset conditions that may occur.
1.1.3 Strength Reduction Factors for Water Absorption
The FRP Committee of the Cooling Technology Institute sponsored a two year study in 2001-2003 which revealed strength property losses for FRP square tubes due to immersion in water. The resulting loss of strength for FRP shapes and plate for water immersion conditions must be taken into account. These reduction factors were developed and are listed in Table 4.
Table 4
TOTAL WATER IMMERSION
REDUCTION FACTORS
3-Rm
CONDITION AVERAGE SHORT COLUMN 0.80 1, 2 LONG 0.95 1, 2 COLUMN ASTM D695 COMPRESSIVE STRENGTH 0.651 ASTM D695 COMPRESSIVE MODULUS 0.9511) 2004 CTI PAPER (TP04-08): “Update on the Impact of Water Immersion for Pultruded FRP”. 2) 2005 CTI PAPER (TP05-03): “The Impact of Veil
Thickness and Coating on Cooling Tower FRP Composites”.
3) These reduction factors are for isophthalic polyester resin shapes.
The reduction factors for Tables 1, 2, and/or 4 depending upon application and engineering judgment. are multiplied by the Ultimate Strengths or Modulus of Elasticity prior to applying service factors. Thus, the location of the member within the tower will dictate the exposure (i.e. fill support beams will be exposed to a lower temperature environment than the hot water distribution piping support beams). If the tower is to experience a short-term hot temperature excursion in excess of the design temperature this would be considered a live load condition.
1.1.4 Minimum Material Service Factors and Deflection Limits.
Note, absent an approved national code of standards for FRP materials, service factors indicated with an asterisk (*) have been derived based on a history of FRP tower installations, in service inspections, and testing of actual FRP tower components by reputable cooling tower manufacturers/designers. These service factors are listed in lieu of the service factors published by FRP material producers.
1.1.4.1 Dead and Operating Loads
1.1.4.1.1 Bearing Service Factor – 4.0, when applied to a fastener group. The commonly reported bearing values obtained per ASTM D953 (Standard Test Method for Bearing Strength of Plastics) for thermoset pultruded shapes are ultimate’s, expressed in Megapascals or pounds force per square inch.
1.1.4.1.2 Shear Service Factor -- 3.0 min.
1.1.4.1.3 Bending/Flexural Service Factor – 2.5 min. 1.1.4.1.4 Compression Service Factor
1.1.4.1.4.1 Short Column [Material Failure] - 3.0 min.
1.1.4.1.4.2 Long Column Buckling] – 2.0 min. Engineering calculations or testing shall model long columns based upon end conditions. Long columns are columns greater than or equal to that length at which the Euler buckling stress is equal to one half the lesser of the material compressive failure stress
or the local buckling stress. Short columns are those shorter than this length.
1.1.4.1.4.3 Beam-Column: Actual stress shall not exceed the following formula for members subjected to combined compression and bending loads by design or construction practices, including eccentrically loaded members.
σ
c/σ
ac+ σ
bx/σ
abx+
σ
by/σ
aby < = 1σ
c= Actual compressive stressσ
ac = Allowable compressive stressσ
bx = Actual bending stress for strong axisσ
abx= Allowable bending stress for strong axisσ
by = Actual bending stress for weak axisσ
aby= Allowable bending stress for weak axis1.1.4.1.5 Deflection Limits – L/D ratio of 180 – total 1.1.4.1.6 Deck Dead Loads – L/D ratio of 360
1.1.4.2 Short Term Loads and Temporary Construction Loads
Short-term loads include wind, seismic, ice/snow, maintenance and construction loads. Construction or refurbishment loads, which occur when a tower or cell is not operating, often may exceed design seismic / wind or other short term loads in combinations with operational loads.
1.1.4.2.1 Bearing Service Factor—2.5* 1.1.4.2.2 Shear Service Factor – 2.0
1.1.4.2.3 Bending/Flexural Service Factor – 2.0 1.1.4.2.4 Compression Service Factor
1.1.4.2.4.1 Short Column [Material Failure] –3.0, min.
1.1.4.2.4.2 Long Column [Buckling] –2.0, min. Long columns are columns greater than or equal to that length at which the Euler buckling stress is equal to one half the lesser of the material compressive failure stress or the local buckling stress. Short columns are those shorter than this length.
1.1.4.2.5 Deflection Limits – L/D ratio of 180 1.1.4.2.6 Deck Live Load--L/D ratio of 240 . 1.2 Load Analysis (Dead, Live & Operating)
All load combinations shall be per ASCE 7, Design Loads for Building and Other Structures.
1.2.1 Fill Dead Loads – Dry weight of fill material, including water hold up. The estimated weight of fouled fill should be added, should fill fouling be considered probable and the expected fill weight should be stated in the bid documents.
1.2.2 Fill Live Loads – Concentrated loading for maintenance and a live load from fill icing.
1.2.3 Eliminator Dead Loads – Dry weight of eliminators 1.2.4 Eliminator Live Loads – Snow load for region 1.2.5 Distribution System Dead Loads – Distribution system full of water with component weights
1.2.6 Deck Dead Load – Weight of deck materials 1.2.7 Deck Live Load – 60 PSF equally distributed load over entire usable roof deck
1.2.8 Operating Load – Fill Dead Loads, Eliminator Dead Loads, Distribution System Dead Loads, Deck Dead Loads, Stack Weight, Fan Thrust, all framework including walls and Mechanical Equipment (Dead and Rotational Loads), and Header Thrust Loads.
1.2.9 Short Term Loads and Temporary Construction Loads
1.2.9.1 Seismic Load – Seismic Load factor applied to the operating weight.
1.2.9.2 Wind Load – Wind Load applied to all vertical and horizontal surfaces. The casing will not be considered sacrificial.
1.2.9.3 Ice and Snow Loads -- Consult local codes for time duration loads,
1.2.9.4 Maintenance and Construction loads ---Each horizontal tie line or beam framing level shall be designed for a minimum load of 20 PSF. Every beam and beam to column connection shall be capable of a minimum concentrated load of 600 lbs.
1.3 Life of Structure
A reasonable anticipated life of 30-35 years can be expected from an FRP structured tower with normal loads, proper maintenance, proper sizing of the structural members, and use of proper service factors, with improper use, problems may occur. For example, the fill could reach weights 8-10 times the original design weight if the tower fill is allowed to clog with debris. 1.4 Creep
Creep is the continued deformation that occurs over time after the initial and immediate deformation upon application of load. With higher service factors creep become less of a problem, as overall strain levels are reduced. The stated service factors are sufficient to achieve this behavior for most laminates. The life of the structure then becomes more a factor of normal maintenance, rather than creep.
1.5 Fatigue
Fatigue is the reduced capacity of a member or structure due to repeated cyclical loading. This "continued elastic" reduction will happen primarily when the member is under higher strain levels. With higher service factors fatigue become less of a problem, as overall strain levels are reduced. The stated service factors are sufficient to achieve this behavior for most laminates. The life of the structure then becomes more a factor of normal maintenance, rather than fatigue.
1.6 Operating Environment
The operating environment of the cooling tower must be considered in selecting the correct resin system. The cooling tower Manufacturer should refer the expected water chemistry to the FRP pultrusion supplier for their recommendation about the correct resin system. The water chemistry information should include the various chemicals with percentages, ph, water temperature, duration of unexpected upset conditions, and any other information about the operating environment known by the manufacturer or owner.
1.7 Design Documentation
Upon request, the cooling tower manufacturer shall provide the design loads, design allowable loads, design equations, reduction factors, service factors and material properties to the purchaser. As a minimum, the cooling tower manufacturer shall provide the design allowable loads and estimated loads for column dead loads, wind resistance components (i.e., ties, anchors, and diagonals) and provide to the customer.
1.8 Glossary of Terms (See Appendix “A”)
2. FABRICATION STANDARDS
2.1 Shop Drawings
Shop drawings shall be prepared by the cooling tower manufacturer in advance of the actual shop fabrication. The shop drawings shall give complete information necessary for the fabrication of the component parts of the structure including the location, size, and tolerance of all cuts, holes, copes and blocks, any cut, drilled, or punched edge sealing requirements, and adhesive preparation requirements as applicable.
2.1.1 Shop drawings shall be developed in conformity to the structural design with best practice and due regard to speed and economy in fabrication and field erection. 2.1.2 FRP ladders, platforms, gratings, etc. – Refer to applicable OSHA, BOCA, UBC, or local building standards for direction or design parameters.
2.2 Equipment
The actual tools and equipment selected by the fabricator is directly dependent upon the type of work, quantity, and the facilities engaged.
For best results, carbide tipped or plated diamond grit saw blades, bits, files, and planers are recommended. It is recommended that adequate dust collection and or ventilation be employed when fabricating FRP structural shapes or plates.
Water jet cutting may be utilized; however, precautions must be made for adequately cleaning and preparation of the fabricated part for edge sealing.
Other methods of shearing or punching may be
employed provided that the FRP material adjacent to the work is not cracked, crushed, or otherwise harmed or the
lubricants that may be required do not contaminate the material.
2.3 Connection Design Criteria
2.3.1 General Criteria - Each of the methods for joining fiberglass has advantages and disadvantages. It should be left to the discretion of the cooling tower manufacturer to determine the best joint for his specific application. In either case the emphasis should be placed on the method of determining the strength of a specific joint. This method should be through full scale testing rather than coupon.
Table 5
Joining Method Considerations
Loads must be transferred, or required joint efficiency fraction of strength of weaker part to be joined.
The region within [the structural elements] where this must be accomplished.
Geometry of members to be joined.
Suitability for fabrication, considering dimensions of components, and numbering of components in production run.
Service environment and life of structure. Reliability of joint.
Need for disassembly.
Need for fluid or weather tightness. Aesthetics
Cost target and weight considerations.
Table 6
Comparison of Joining Techniques
Mechanical Adhesive
Bonds Stress concentration at
joint High Medium
Strength/weight ratio Low Medium
Use with non-rigid
plastics Inserts Yes
Seal (water tightness) No Yes
Thermal Insulation No Yes
Electrical Insulation No Yes
Aesthetics (smooth joints) Bad Good
Fatigue Endurance Bad Good
Sensitive to peel loading No Yes Non-Destructive
Disassembly Yes Impossible
Inspection Easy Difficult
Skill required of Fabricator
Low High Heat and pressure
required No Yes
Tooling Costs Low High
2.3.2 Mechanical – (Such as bolted, pinned, riveted, sleeved, etc., but not limited to these listed). Bolted connections should be tested for the specific bolt pattern being used and a loading capacity determined based on mimicking the actual connection utilized in the structure. A series of connection tests shall be performed that accurately portrays the materials and connection details of the joint being utilized in the structure. The test shall be conducted in a lab environment with hole tolerances typical of the manufacturing process. The ultimate capacity of the connection is defined as the highest recorded value prior to failure. The number of tests and data reduction shall be per ASTM D7290-06. The 95% confidence value, derived per ASTM D7290-06 shall be utilized as the ultimate design value. The appropriate service and reduction factors shall be applied to the 95% confidence value during the design process. When bolting hollow members such as square tubes, a positive means shall be provided to prevent tube cracking or a means provided to ensure cracking does not exist without joint disassembly. If a crack is present, the member has failed and should be replaced. The equipment owner’s engineer may, at his sole discretion, elect to terminate a minor crack and repair the member provided the owner’s engineer has determined the repair reduces the risk of failure to an acceptable level.
Table 7
Types of Mechanical Fasteners Bolts
Rivets Screws
Pin and sleeve or pin and collar fasteners Spring Clips
Cam Locks Shrink Fit Threaded
2.3.3 Bearing Capacity—The bearing capacity of pultruded FRP material around the fastener is affected by edge and end distances and the distance between fasteners in a line (pitch). Unless test data for the FRP material’s bearing capacity is provided, Table 2.3.3-1 lists the recommended ratio in minimum distance from the center of fastener to edge/end of member and distance between fastener centers in a line (pitch) to the fastener diameter.
Table 8
Recommended Minimum Fastener Edge Distances to Fastener Diameter (Ratio).
Ratio Range Common
Ratio Edge Distance - End 2.0 to 4.5 3.0 Edge Distance--Side 1.5 to 3.5 2.0 Pitch (Center to center
distances of fasteners in a line)
4.0 to 5.0 5.0
2.3.4 Chemical - Bonding of a joint is adhesive specific and the adhesive manufacturer should be consulted as to the proper bonding method, and curing instructions. This should include surface preparation, proper temperature and moisture exposure ranges for application. As with any connection the joint should be tested full scale and tested for the proper application environment. Minimum service factors listed in Section 1.3 are to be applied for chemical bonds/connections as well as mechanical connections.
2.3.5 Column Splices – Column splices shall be designed for transfer of location specific loads, including compression, tension, bending, and shear; singularly or in combination as is appropriate for the specific location of the splice and the loading condition considered or 50% of the effective strength of the member, whichever is greater.
2.4 Fabrication Tolerances
Fabrication tolerances unless otherwise indicated on the shop drawings shall be as indicated in Table 2.4-1
Table 9
FABRICATION TOLERANCES Cut lengths 1/8”
Square cuts 1 (Measured from face of part) Hole location 1/16” for top side hole on all shapes up
to 30’ inclusive
Hole location 1/16” for bottom side hole on all shapes up to 30’ inclusive
Hole location 1/8” for topside holes on all closed shapes greater than 30’
Hole location 3/16” for bottom side hole on closed shapes greater than 30’
Hole diameters 1/32” (Holes ½” to 1” diameter) Hole diameters 1/16” (Holes greater than 1” diameter) Slot dimensions 1/8” (Any dimension)
2.5 Quality Control
The fabricator shall provide quality control procedures to the extent that it deems necessary to assure that all work is performed in accordance with the shop drawings and this specification. The quality control procedures shall be provided to the purchaser upon request. In addition to the fabricator’s quality control procedures, material and workmanship at all times may be subject to inspection by qualified inspectors representing the purchaser. If such inspection by representatives of the purchaser will be required, it shall be so stated in the contract documents and on the shop drawings. The inspecting agency should provide 24 hours’ notice of its intention to perform an inspection
PULTRUSION GLOSSARY OF TERMS
A Glass - A soda-lime glass similar to window or bottle glass
with generally poorer chemical and water resistance than E glass. Used primarily as a surface mat on pultrusions.
Accelerator - A chemical additive that hastens cure or
chemical reaction.
Additive - An ingredient mixed into resin to improve
properties. Examples include plasticizers, initiators, light stabilizers and flame retardants.
Anisotropic - Fiber directionality in which different properties
are exhibited when tested along axes of different directions.
Aramid - A high-strength, high-stiffness aromatic polyamide
fiber.
Aspect Ratio - Ratio of length to diameter of a fiber.
B-Stage - An intermediate stage in the reaction of certain
thermosetting resins in which the material softens when heated and is plastic and fusible but may not entirely dissolve or fuse; sometimes referred to as resistol. The resin in an uncured prepreg or premix is usually in this stage.
Balanced Laminate - A composite laminate in which all
laminae at angles other than 0 degrees and 90 degrees occur only in +- pairs (not necessarily adjacent). Balance is symmetrical about the composite centerline.
Barcol Hardness - A surface hardness value obtained by
measuring the penetration resistance of a given material to a sharp steel point under a spring load. The Barcol Impressor is an instrument that measures hardness on a 0-100 scale.
Batch (or Lot) - Material made with the same process at the
same time having identical characteristics throughout.
Biaxial Material - Material having fibers oriented in both the
warp (0 degrees) and weft (90 degrees) directions.
Binder - The agent applied to glass mat or preforms to bond
the fibers prior to laminating or molding.
Blister - A rounded elevation of the pultruded surface with
boundaries that may be more or less sharply defined.
Bond Strength - The degree of adhesion between bonded
surfaces. The stress required to separate a layer of material from the base to which it is bonded, as measured by load/bond area.
Bow - A conditional longitudinal curvature in pultruded parts. Breaker Bars - A series of bars in the resin bath or
impregator which separate the planes of roving and mechanically break the roving bundles into individual filaments for thorough wetting by the resin.
Bromine - A fire retardant additive (halogen) used to reduce
or eliminate a resin’s tendency to burn.
Bundle - A general term for a collection of essentially parallel
filaments or fibers.
Buckling - A failure mode usually characterized by fiber
deflection rather than breakage due to compressive action.
Carbon Fiber - Reinforcing fiber known for its light weight,
high strength and high stiffness. Fibers are produced by high temperature treatment of an organic precursor fiber based on PAN (polyacrylonitrile), rayon or pitch in an inert atmosphere at temperatures above 1,800°F. Fibers can be pyrolized by removing still more non-carbon atoms via heat-treating above 3,000°F.
Catalyst - A substance that promotes or controls curing of a
compound without being consumed in the reaction.
Catalyzed Resin - A resin mixture possibly still in the
workable state, after it has been mixed with the catalyst or hardener.
Catenary - Uniformity of strand length in a specified length
of roving stretched under tension. Poor catenary means some strands in the roving length are longer than others.
Cavity - The female portion of a split cavity or gun barrel
drilled mold; that portion of the mold that encloses and shapes the molded article (often referred to as the die). (Depending on the number of such depressions, molds are designated as single-cavity or multiple-cavity).
Circumferential Winding - In pultruded reinforced polymers,
a winding with the filaments essentially perpendicular to the longitudinal process axis (90 degrees or level winding).
Cocure - The act of curing a composite laminate and
simultaneously bonding it to some other prepared surface.
Coefficient of Expansion (COE) - A measure of the change
in length or volume of an object.
Coefficient of Thermal Expansion (CTE) - A material’s
fractional change in length for a given unit change of temperature.
Composite - A material that combines fiber and a binding
matrix to maximize specific performance properties. Neither element merges completely with the other.
Compressive Strength - The capacity to resist a crushing or
buckling force; the maximum compressive load a specimen sustains divided by its original cross-sectional area.
Contaminant - An impurity or foreign substance that affects
one or more properties of composite material, particularly adhesion.
Continuous Filament - An individual, small-diameter
reinforcement that is flexible and indefinite in length.
Continuous Roving - Single or multiple strands of parallel
filaments coated with sizing and wound into a cylindrical package. It may be used to provide continuous reinforcement in woven roving, filament winding, pultrusion, prepregs, or high-strength molding compounds. It may also be chopped. (See Chopped Strand.)
Core - In sandwich construction, the central component to
which inner and outer skins are attached. Foam, honeycomb, paper and wood are all commonly used as core material.
Crack - visual separation that occurs internally or penetrates
down from the pultruded surface to the equivalent of one full ply or more of reinforcement.
Craze, Hairline - Multiple fine pultrusion surface separation
cracks that exceed ¼” in length and do not penetrate in depth to the equivalent of a full ply of reinforcement.
Craze, Resin - Multiple fine separation cracks at the
pultruded surface not penetrating into the reinforcement.
Crazing - A region of ultrafine cracks that may develop on or
under a resin surface.
Creel - A device for holding the required number of roving
spools or other supply packages of reinforcement in the desired position for unwinding.
Cross-Linking - Applied to polymer molecules; the setting-up
of chemical links between the molecular chains. When extensive as in most thermosetting resins, cross-linking makes one infusible supermolecule of all the chains. The higher the cross-link density, the more rigid the material.
Cure - To irreversibly change the molecular structure and
physical properties of a thermosetting resin by chemical reaction via heat and catalysts alone or in combination, with or without pressure.
Cure Temperature - The temperature at which a material
attains final cure.
Cure Time - The time needed for liquid resin to reach a solid
state after the catalyst has been added, and initiation has progressed.
Curing Agent or Hardener - A catalytic or reactive agent
that brings about polymerization when added to a resin.
Delamination - The separation of ply layers due to adhesive
failure or the separation of layers of fabric from the core structure. A delamination may be associated with bridging, drilling and trimming.
Demold - To remove a part from a tool, or a tool from an
intermediate model.
Die (Tool) - The mold, either one- or two-sided and either
open or closed, in or upon which composite material is placed to make a part.
Dielectric - Nonconductor of electricity; the ability of a
material to resist the flow of an electrical current.
Dielectric Curing - The curing of a synthetic thermosetting
resin by the passage of an oscillating electric field produced from a high frequency generator through the resin. Commonly used is radio frequency (RF) energy.
Dielectric Strength - The voltage required to penetrate
insulating material. Material with high dielectric strength offers excellent electrical insulating properties.
Die-Parting Line - A lengthwise flash or depression on the
surface of a pultruded plastic part. It is only at the surface and does not weaken the part.
Dimensional Stability - Ability of a plastic part to retain the
precise shape to which it was molded, cast, or otherwise fabricated.
Draft - The taper or slope of the male sections of a mold
designed to facilitate shrinkage of pultruded profiles.
Dwell Time - In pultrusion, a pause in the normally
continuous pulling motion which allows the material to cure under static condition. The pause allows a cleaning or “purging” action which removes tenacious material from the die wall.
E-Glass (Electrical Glass) - Borosilicate glass fibers most
often used in conventional polymer matrix composites.
Elastomer - A material that substantially recovers its original
shape and size at room temperature after removal of a deforming force.
End - A strand of roving consisting of a given number of
filaments gathered together. The strand is considered an end or strand before twisting.
End Count - An exact number of strands contained in a
roving.
Epoxy Plastics - Thermoset resins made by the reaction of
epoxides or oxiranes with other materials such as amines and alcohols; used as a matrix resin in reinforced composites and structural adhesives.
Epoxy Resin - A common thermoset material used as a
bonding matrix to hold fibers together. When mixed with a catalyst, epoxy resins are resistant to chemicals and water and are unaffected by heat or cold.
Exotherm - Heat released during a chemical reaction (e.g.,
curing).
Exothermic - Characterized by a chemical reaction to heat. Extenders - Low cost materials used to dilute or extend high
cost resins with little lessening of properties.
Fabric, Nonwoven - A material formed from fibers or yarns
without interlacing (e.g., stitched nonwoven broadgoods).
Fabric, Woven - A material constructed of interlaced yarns,
fibers or filaments
Fiber - Filamentary material.
Fiber Architecture - The design of a fibrous part in which the
fibers are arranged in a particular way to achieve the desired result. This may include braided, stitched or woven fabrics, mats, rovings or carbon tows.
Fiber Blooming - A pultrusion surface condition exhibiting a
fiber prominence or fiber show that usually has a white or bleached color and a sparkling appearance. This condition usually is the result of surface degradation by ‘UV’ on a pultruded part where a synthetic surfacing veil was not used.
Fiber Bridging - Reinforcing fiber material bridging an inside
radius of a pultruded product. The condition is caused by shrinkage stresses around such a radius during cure.
Fiber Content - The amount of fiber in a composite expressed
as a ratio to the matrix.
Fiber Prominence - A visible and measurable pattern of the
reinforcing material on the surface of a pultruded part.
Fiber Reinforced Polymer (FRP) - A composite material or
fibers such as glass or carbon having greater strength or stiffness than the resin. The term FRP is most often used to denote glass fiber-reinforced polymers; the term “advanced composite” usually denotes high-performance aramid or carbon fiber-reinforced plastics.
Fiberglass – 1) An individual filament made by attenuating
molten glass. A continuous filament is a glass fiber of great or indefinite length; a staple fiber is a glass fiber of relatively short length (generally less than 17 inches). 2) General description of polymetric (plastic) matrix reinforced with fibrous glass.
Filaments - Individual fibers of indefinite length used in tows,
yarns or roving.
Filler - Material added to the mixed resin to increase
viscosity, improve appearance and/or lower density and cost.
Finite Element Analysis (FEA) - A process of selecting the
optimum combination of materials in a composite based on software analysis.
Flexural Modulus - The ratio, within the elastic limit, of the
applied stress on a test sample in flexure to the corresponding strain in the outermost fibers of the sample.
Flexural Strength - The strength of a material in bending
expressed usually in terms of force per unit area, as the stress of a bent test sample at the instant of failure.
Fracture - A rupture of the surface of a laminate, due to
external or internal forces; may or may not result in complete separation.
Gel - A partial cure of polymer resins; a semi-solid, jelly-like
state similar to gelatin in consistency.
Gel Time - The period of time from initial mixing of liquid
reactants to the point when gelation occurs, as defined by a specific test method.
Glass Fiber - See Fiberglass.
Glass Transition - The reversible change in an amorphous
polymer between a viscous, rubbery condition and a hard, relatively brittle one.
Glass-Transition Temperature (Tg) - The approximate
temperature above which increased molecular mobility causes a material to become rubbery, rather than brittle. The measured value of Tg can vary, depending on the test method.
Grooving - Long, narrow grooves or depressions in a surface
of a pultrusion parallel to its length.
Halogenated Resin - A resin combined with chlorine or
bromine to increase fire retardancy.
Hardener (or Curing Agent) - A substance that reacts with
resin to promote or control curing action.
Heat - A term used colloquially to indicate any temperature
above ambient (room) temperature to which a part or material is or will be subjected.
Heat-Distortion Temperature (HDT) - The temperature at
which a test bar deflects a certain amount under specified temperature and stated load.
Hybrid Composite - A composite made with two or more
types of reinforcing fibers.
Hygroscopy - A material’s readiness to absorb or retain
moisture.
Ignition Loss - The difference in weight before and after
burning. For glass reinforcements, the burning of the binder or size. For composites, the burning off all organic constituents.
Impact Strength - A material’s ability to withstand shock
loading as measured by fracturing a specimen.
Impregnate - To saturate the voids and interstices of a
reinforcement with a resin.
Impregnated Fabric - See Prepreg.
Inclusion - Any foreign matter of particles that are either
encapsulated or imbedded in the pultrusion.
Inhibitor - A chemical additive that slows or delays cure
cycle.
Initiator - A material used as a source of free radicals.
Initiators are used in free radical polymerizations, curing thermosetting resins, as cross-linking agents for elastomers and polyethylene, and for polymer modification. Frequently referred to as “catalyst”.
Insufficient Cure - A pultrusion abnormality created by lack
of, or incomplete, cross-linking of the resin.
Interface - The surface between two materials (in glass fibers,
for instance, the area at which the glass and sizing meet; in a laminate, the area at which the reinforcement and laminating resin meet.)
Interlaminar - Existing or occurring between two or more
adjacent laminae.
Interlaminar Shear - A shearing force that produces
displacement between two laminae along the plane of their interface.
Isophthalic Polyester - An unsaturated polyester resin
prepared with isophthalic acid as the starting acid constituent.
Isotropic - Fiber directionality with uniform properties in all
directions, independent of the direction of applied load.
Isotropic Laminate - A laminate in which the strength
properties are equal in all directions, such as contact-molded laminates or metals.
Knitted Fabrics - Fabrics produced by interlooping strands of
yarn, roving, and so forth.
Laminate - To unite layers with a bonding material, usually
via pressure and heat.
Laminate Ply - A fabric/resin or fiber/resin layer that is
bonded to adjacent layers in the curing process.
Lap Joint - A joint made by overlapping two parts and
bonding them together.
Loss of Ignition - Weight loss, usually expressed as percent of
total, after burning off an organic sizing from glass fibers, or an organic resin from a glass fiber laminate.
Low Profile - Resin compounds formulated for low or zero
Mandrel - An elongated mold around which
resin-impregnated fiber, tape or filaments are wound to form structural shapes or tubes.
Mat - A fibrous reinforcing material composed of chopped
filaments (for chopped-strand mat) or swirled filaments (for continuous-strand mat) with a binder applied to maintain form; available in blankets of various widths, weights, thicknesses and lengths.
Matrix - The material in which the fiber reinforcements of a
composite system are imbedded. Thermoplastic and thermoset resin systems, as well as metal and ceramic, can be used.
Microcracking - Cracking in composites at points where
thermal stresses exceed the strength of the matrix.
Mil - The unit used in measuring the diameter of glass fiber
strands, wire and so forth (1 mil = 0.001 inch).
Modulus - The physical measurement of stiffness in a
material, equaling the ratio of applied load (stress) to the resultant deformation of the material, such as elasticity or shear. (A high modulus indicates a stiff material.)
Moisture Absorption - A material assimilation of water
vapor from air, as distinguished from water absorption by immersion, which results in weight gain.
Mold - The cavity or matrix into or on which the resin/fiber
material is placed and from which it takes form.
Mold Release (External) - A substance used to coat the mold
to prevent sticking of the resin that will be used to make the part.
Mold Release (Internal) - A lubricant, liquid or powder, used
to prevent sticking of molded articles in the cavity by incorporation into the resin formulation. Referred to as an internal rather than external mold release.
Mold Release Agent - A lubricant used to prevent a part
from sticking to a mold.
Molding - The forming of a resin/fiber material into a solid
mass of prescribed shape and size.
Monomer - A single molecule which can react with like or
unlike molecules to form a polymer; the smallest repeating structure of a polymer(s); for addition polymers, this represents the original unpolymerized compound. Styrene is the predominant monomer used for reinforced polyester material.
Multifilament - A yarn consisting of many continuous
filaments.
Nondestructive Inspection (NDI) - Determination of material
or part characteristics without permanent alteration of the test subject. (Nondestructive testing (NDT) and nondestructive evaluation (NDE) are generally considered synonymous with NDI.)
Nonwoven Roving - A reinforcement composed of
continuous fiber strands loosely gathered together.
Orthophthalic Resin - An unsaturated polyester resin of
which phthalic anhydride was the starting point.
Outgassing - The release of solvents and moisture from
composite parts under a vacuum.
Part Consolidation - A process of composites fabrication in
which multiple discrete parts are designed and fabricated together into a single part, thus reducing the number of fabricated parts and the need to join those parts together.
Parting Line - A continuous mark on a pultruded piece where
the sections of a mold have met in closing. Tight mold tolerances can minimize this objectionable mark.
Peel Ply - Layer of material applied to a lay up surface that is
removed from the cured laminate prior to bonding operations, leaving a clean, resin-rich surface ready for bonding.
Peel Strength - Strength of an adhesive bond obtained by
stress that is applied “in a peeling mode.”
Phenolic Resin - Thermosetting resin produced by
condensation of an aromatic alcohol with an aldehyde, particularly phenol with formaldehyde.
Pin Holes - Small holes caused by the mold used.
Pitch - Residual petroleum product used in the manufacture of
certain carbon fibers.
Plastic - A high molecular weight thermoplastic or
thermosetting polymer that can be molded, cast, extruded or laminated into objects. A major advantage of plastics is that they can deform significantly without rupturing.
Platens - The mounting plates of a pultrusion machine in
which the entire mold is contained for the dual purposes of restraint and heating.
Ply - One of the layers that makes up a stack or laminate.
Also, the number of single yarns twisted together to form a plied yarn.
Ply Schedule - Lay up of individual plies or layers to build an
FRP part laminate. Plies may be arranged (scheduled) in alternating fiber orientation to produce a multi-directional strength part. (See Fiber Architecture).
Poisson’s Ratio - When a material is stretched, its cross
sectional area changes as well as its length. Poisson’s ratio is the constant relating these changes in dimensions, and is defined as the ratio of the change in width per unit width to the change in length per unit length.
Polyester Resin – a) Thermosetting resins produced by
dissolving unsaturated, generally liner, alkyd resins in a vinyl-type active monomer such as styrene. The resins are usually furnished in solution form, but powdered solids are also available. b) The resin used for unsaturated polyesters. Formed by the reaction of dibasic organic acids and polyhydric alcohols. Also meant to include the cross-linking diluent included with the polyester molecules.
Polymer Alloy (or Polymer Blend) - A blend of polymers,
copolymers or elastomers.
Polymer - A high-molecular weight organic compound,
natural or synthetic, whose structure can be represented by a repeated small unit, the “mer”; for example, polyethylene, rubber, polyester, cellulose. Synthetic polymers are formed by addition or condensation polymerization of monomers. Some polymers are elastomers and some are plastics. When two or more different monomers are involved, the product is called a copolymer.
Polymerization - A chemical reaction that links monomers to
form polymers.
Porosity - The presence of visible voids within a solid
material into which either air or liquids may pass.
Post Cure - Additional elevated temperature cure, usually
without pressure, to improve final properties and/or complete the cure. In certain resins, complete cure and ultimate mechanical properties are attained only by exposure of the cured resin to higher temperatures than those of curing.
Postforming - The forming, bending, or shaping of fully
cured, C-stage thermoset laminates that have been heated to make them flexible. On cooling, the formed laminate retains the contours and shape of the mold over which it has been formed.
Pot Life - The length of time in which a catalyzed
thermosetting resin retains sufficiently low viscosity for processing.
Precursor - For carbon fibers, the rayon, PAN or pitch fibers
from which carbon fibers are made.
Preform - A fibrous reinforcement preshaped on a mandrel or
mock-up to the approximate contour and thickness desired in the finished part.
Preheating - The heating of an impregnated material prior to
molding, to facilitate the operation or to reduce the molding cycle.
Prepreg - Resin impregnated fibers, fabric or mat in flat form
that is impregnated with resin before being stored for later use in molds and hand lay up.
Promoter (or Accelerator) - A chemical which hastens the
reaction between a catalyst and a resin.
Prototype - The process of creating a test article not intended
for commercial release that establishes design, material and fabrication parameters for a new product. May entail multiple iterations to arrive at final/ commercial part design.
Pultrusion - An automated, continuous process for
manufacturing composite rods, tubes and structural shapes having a constant cross-section. Roving and other reinforcements are saturated with resin and continuously pulled through a heated die, where the part is formed and cured. The cured part is then cut to length.
Puncture - A break in composite skin in sandwich structure
that may or may not go through to the core material or completely through the part.
Pyrolysis - The decomposition or transformation of a
compound caused by heat.
Quasi-isotropic - Approximating isotropy by orienting plies
in several directions.
Ramping - A gradual programmed increase/decrease in
temperature or pressure to control cure or cooling of composite parts.
Reagent - A substance used in a chemical reaction to produce
other substances.
Regrind - Scrap composites (both thermoset and
thermoplastic) collected in-plant or from post-consumer
sources that are reground into pellets or fine powder for use in new parts, either as new base material or in combination with virgin materials.
Reinforcement - The key element added to matrix to provide
required properties (primarily, strength and stiffness); ranges from short fibers and continuous fibers through complex textile forms.
Release Agent - A substance usually sprayed or painted on
mold to prevent cured matrix material from bonding to tooling.
Release Film - An impermeable film layer that does not bond
to the composite during cure.
Resin - A solid or pseudosolid material with indefinite and
often high molecular weight and a softening or melting range that exhibits a tendency to flow when subjected to stress. (As composite matrices, resins bind together reinforcement fibers.)
Resin Rich - Localized area filled with excess resin, as
compared to consistency of resin/fiber ratio.
Resin Starved - Characterizing a localized area lacking
sufficient resin for fiber wetout.
Resin System - A mixture of resin and ingredients required
for an intended processing method and final product.
Resin Viscosity - The viscous property of a resin system, or
solid-to-liquid transition resistance to flow, which can be altered by temperature and pressure to achieve desired flow characteristics.
Roving - A collection of bundles of continuous glass fiber
filaments, either as untwisted strands or as twisted yarn.
Roving Yield - A unit of measure of the size of a fiberglass
roving. Given in yards per pound, the number of yards of the product which equal one pound. The lower the yield, the larger the size of the roving bundle.
S-Glass - Magnesia/alumina/silicate glass reinforcement
designed to provide very high tensile strength. (Commonly used in high-performance parts.)
Sandwich Structure - A composite composed of lightweight
core material to which two relatively thin, dense, high-strength, functional or decorative laminate skins are adhered.
Saw Burn - Blackening or carbonization of a cut surface of a
pultruded section. Fiberglass parts do not dissipate heat as quickly as metal, so the speed of cutting fiberglass must be controlled to prevent saw burns.
Scale - A condition wherein unreinforced, cured resin particles
exit the die on the surface of the part.
Sealant - A paste or liquid applied to a joint that hardens in
place to form a seal.
Secondary Bonding - The joining by adhesive of two or more
already cured composite parts.
Separator - A permeable layer that separates and also acts as
a release film (e.g., porous Teflon-coated fiberglass). Often placed between lay up and bleeder to facilitate bleeder systems’ removal from laminate after cure.
Shear - An action or stress resulting from applied forces that
causes or tends to cause two contiguous parts of a body to slide relative to each other.
Shelf Life - The length of time a material, substance, product,
or reagent can be stored under specified environmental conditions and continue to meet all applicable specification requirements and/or remain suitable for its intended processing function.
Shrinkage - The relative change in dimension between a
dimension measured on the mold when it is cold and the dimension of the molded object 24 hours after it has been molded.
Sizing - A solution of chemical additives used to coat
filaments. The additives protect the filaments from water absorption and abrasion; they also lubricate the filaments and reduce static electricity.
Skin - A relatively dense laminate comprising the outer
surfaces (layers) of the core in a sandwich structure.
Solvent - A liquid used to dissolve and clean materials. Spec - Specification of the properties, characteristics or
requirements a particular material or part must have to be acceptable to a potential user of the material or part.
Specific Gravity - The density (mass per unit volume) of a
material divided by that of water at a standard temperature.
Splice - The joining of two ends of glass fiber roving or mat. Spun Roving - A bulky, low-cost glass fiber strand consisting
of filaments that are continuous but doubled back on each other providing some level of off-axis reinforcement.
Stiffness - A material’s ability to resist bending; relationship
of load to deformation for a particular material.
Starved Area - An area in a plastic part that has an
insufficient amount of resin to completely wet out the reinforcement.
Strain - Elastic deformation resulting from stress.
Strand - A collection or bundle of continuous glass filaments. Stress - Internal resistance to change in size or shape,
expressed in force per unit area.
Stress Concentration - The magnification of applied stress in
the region of a notch, void, hole or inclusion.
Stress Corrosion - Preferential attack of areas under stress in
a corrosive environment, that alone would not have caused corrosion.
Stress Crack - External or internal crack in a composite
caused by tensile stresses; cracking may be present internally, externally or in combination.
Styrene - A water-thin liquid used to thin polyester resins
prior to cure and which serves as the chemical which links polyester chains together during the curing process.
Substrate - A material on which an adhesive-containing
substance is spread for any purpose, such as bonding or coating.
Surfacing Agent - An oily or waxy material which rises to the
surface of a polyester resin during cure.
Surfacing Veil - Accompanying other reinforcing mats and
fabrics to enhance the quality of the surface finish. Designed to block out the fiber patterns of the underlying reinforcements, it often adds ultraviolet protection to the structure.
Synthetic Fiber - Fiber made of materials other than glass or
carbon, such as polyester.
Tensile Strength - The maximum stress sustained by a
composite specimen before it fails in a tension test.
Terephthalic Resin - An unsaturated polyester resin prepared
with terephthalic acid as the starting acid constituent.
Thermal Conductivity - The ability to transfer heat.
Thermal Stress Cracking - Crazing and cracking of some
thermoplastic resins from overexposure to elevated temperatures.
Thermocouple - Wire assembly used with a control device to
sense temperature.
Thermoplastic - A composite matrix capable of being
repeatedly softened by an increase in temperature and hardened by a decrease in temperature.
Thermoset - Composite matrix cured by heat and pressure or
with a catalyst into an infusible and insoluble material. Once cured, a thermoset cannot be returned to the uncured state.
Thickeners - Materials added to the resin to thicken it or raise
the viscosity index of the resin so that it will not flow as readily. Used most frequently to produce sheet molding compound (SMC).
Thixotropic - A consistency that is gel-like at rest, but fluid
when agitated. Having high static shear strength and low dynamic shear strength simultaneously. Losing viscosity under stress.
Tool - The mold, either one- or two-sided and either open or
closed, in or upon which composite material is placed to make a part.
Toughness - The ability of a material to absorb energy. Tow - An untwisted bundle of continuous filaments (usually
carbon), typically designated by a number followed by K, indicating multiplication by 1,000 (for example, 12K tow has 12,000 filaments).
Twist - A condition of longitudinal progressive rotation found
in pultruded parts.
Ultraviolet Stabilizer - Any chemical compound which,
when admixed with a thermoset or thermoplastic plastic resin, selectively absorbs UV rays.
Undercure - A condition or reverse draft on the mold
resulting from the allowance of too little time and/or temperature for adequate hardening of the mold.
Unidirectional (UD) Laminate - A reinforced polymer
laminate in which substantially all of the fibers are oriented in the same direction. In pultrusion, unidirectional products are produced with all continuous rovings.
Veil - An ultrathin mat often composed of organic fibers as
Vinyl Ester Resin - A family of thermosetting resins having
no ester linkages along the polymer chain but present only at the ends of the molecule providing improved corrosion resistance.
Viscosity - The tendency of a material to resist flow. As
temperature increases, the viscosity of most materials decreases.
Voids - Pockets of entrapped gas that have been cured into a
laminate. (In a composite that has been cured properly, void content is usually less than 1 percent.)
Volatiles - Materials in a sizing or resin that can be vaporized
at room or slightly elevated temperatures.
Volatile Organic Compounds (VOC) - Carbon-containing
chemical compounds (e.g., solvents and styrene) that evaporate readily at ambient temperatures. Environmental, safety and health regulations often limit exposure to these compounds, so low VOC content is preferable.
Volatile Content - The percent of volatiles that are driven off
as a vapor from a plastic or an impregnated reinforcement during cure.
Warp - The yarn running lengthwise in a woven fabric; a
group of yarns in long lengths and approximately parallel.
Warpage - Dimensional distortion in a composite part. Water Absorption - The ratio of weight of water absorbed by
a material to weight of dry material.
Water Jet - High-pressure water stream used for cutting
polymer composite parts.
Weave - The pattern by which a fabric is formed from
interlacing yarns. In plain weave, warp and fill fibers alternate to make both fabric faces identical. In satin weave, the pattern produces a satin appearance with the warp roving crossing over several fill rovings and under the following one. (For example, eight-harness satin would have warp roving over seven fill rovings and under the eighth.)
Weeping - A slow passage of fluid through an FRP laminate
that can occur when a leak path is established by extensive cracking.
Weft - The transverse threads or fibers in a woven fabric;
those fibers running perpendicular to the warp. Also called fill or filling yarn.
Wet-out - Saturation with resin of all voids between strands
and filaments.
Wetting Agent - A surface-active agent that promotes wetting
by decreasing the cohesion within a liquid.
Whisker - A short single crystal fiber or filament used as a
reinforcement in a matrix.
Woven Fabrics - Fabrics produced by interfacing strands at
more or less right angles.
Woven Roving - Heavy, coarse fabric produced by weaving
continuous roving bundles.
Wrinkle - Imperfection in the surface of a laminate that looks
like a crease in one of the outer layers. This occurs in vacuum-bag molding when the vacuum-bag is improperly placed.
X-, Y-, Z-Axis - The axis in the plane of a laminate used as 0°
reference. The y-axis is the axis in the plane of a laminate perpendicular to the x-axis. The z-axis is the through-the-plane thickness.
Yarn - Continuously twisted fibers or strands suitable for
weaving into fabrics.
Yield - The first stress in a material, less than the maximum
rate attainable stress, at which the strain increases at a higher rate than the stress. The point at which permanent deformation of a stressed specimen begins to take place.
Young’s Modulus - The ratio of normal stress to the
corresponding strain for tensile or compressive stresses less than the proportional limit of the material.
Zero Bleed - A laminate fabrication procedure that prohibits
