Boeing Commercial Airplane. P.O. Box 3707 MS 5L-05. Seattle, Wa. 98124. Abstract. Structural aircraft composite parts designed with honeycomb core for.
3 1st International SAMPE Technical Conference October 26 - 30, 1999
Material Factors Influencing Composite Part Producibility in Relation to Prepreg Frictional Measurement T. L. Pelton, T. L. Schneider and R. Martin Boeing Commercial Airplane P.O. Box 3707 MS 5L-05 Seattle, Wa. 98124
Abstract Structural aircraft composite parts designed with honeycomb core for stiffening and joggled flanges for component assembly frequently experience part producibility problems associatedwith these design elements. Honeycomb core in carbon fiber-epoxy parts are susceptible to ‘core crush’, a non-repairable defect occurring when the honeycomb core walls collapse during cure. Parts with joggled flanges are also susceptible to internal laminate porosity in the joggled regions. Core crush and porosity are two predominant types of defects which lead to costly part rejections since these conditions can rarely be repaired. This paper presentsrecent findings which elucidate key material factors influencing core crush and porosity. However, an in depth follow on paper will addressthe resolution of porosity as it relates to fiber type. Results from prepreg ply friction studies and image analysis of prepreg weave structure after resin impregnation processing reveal that certain carbon fiber morphologies used in woven fabrics are more susceptible to core crush and porosity than other morphologies. This data holds significance towards the development and manufacture of more robust material forms to eliminate these defects in * composite production parts.
Introduction As part of a continuing effort to resolve composite honeycomb core crush in Boeing Commercial Airplane production parts, current research activities focused on the frictional resistanceof carbon and fiberglass prepregs.A test method to measurethe frictional resistanceof the prepreg material was developedby the University of Washington’s Polymeric Composite Lab (PCL) and further optimized by Boeing’s Manufacturing Researchand Development (MR&D) Organization. Current work has indicated that typical friction load-displacementcurves correlate directly to the degreeof core crush in producibility panels. Ultimately this friction test method was used to further establish the hypothesis that fiber type is the predominatefactor that contributes to core movement and ultimately crush in honeycomb composite structures. Previous studies in the attempt to resolve and characterizecore crush identified prepreg friction, permeability, resin content and distribution as the probable factors and mechanisms.The work over the course of the last four years to the present has concentratedin these areas.Historical work and it’s individual contributors helped establish the scientific foundations on which this paper is based.However the leading conclusion from previous work indicated that resin concentration and distribution were the predominantfactors in causing core movement and crush (seereference 1). Data from this study suggeststhe probable causeof core crush in composite honeycomb structuresthat use carbon fabric prepregsis related to the combination of never-twisted (NT) 3K T300 carbon fibers with either a low or medium flow resin system. In turn, this paper will demonstratethat core movement and crush can be minimized to the extent of eliminating ply tiedowns and stabilization techniquesby introducing standardtwist (ST) T300 carbon fiber into the prepregfabric that utilizes the same low or medium flow resin system. It should be pointed out that the frictional resistanceof prepregfabric with ST carbon fiber may have three to four times the frictional resistanceof fabric woven with NT T300 fiber. In addition, it will be demonstratedthat by increasing the frictional resistancein fiberglass prepregs, manufacturing centerscan successfully reduce and eliminate core crush in researchlab parts, as well as in composite production parts.
The objective of this paper is to demonstratethe reduction of core movementand crush in composite parts by selectively using appropriate high friction fiber types, as well as to show the inverse relationship of porosity with certain fiber types. However, an in depth discussion on the minimization of porosity as it relates to fiber type will be addressedin a follow-on paper.It is also the intent to educatethe global composite industry as to the recent resolutions and findings by the Boeing company through disirnination of this paper.
Experimental As part of a manufacturing evaluation processto screenpotential prepregsuppliersfor qualification to Boeing material specifications, MR&D constructsa discriminator panel that quantifies degreeof core crush. This discriminator panel was developedby a thirty-two run Design of Experiment (DOE) in collaboration with the PCL (seereference 1 for design and schematic).Note: in order to assessprepreg influence on core crush, discriminator panelsare manufacturedwithout core stabilization and ply tiedown methods.During a recent carbon fabric weaver qualification to BMS 9-8, the discriminator panel exhibited 0% core crush. In order to assess weaving acceptancecriteria, the woven fabric was impregnatedwith BMS 8-256 resin system by supplier (A). All historical data from past years,as well as data producedjust prior to this weaver qualification had indicated that all discriminator panels of this type with this sameresin system exhibited rejectablelevels of core crush up to 20%. As this 0% core crush panel was consideredan anomaly, two other verification panels were manufacturedfor reproducibility. These verification panels also exhibited 0% core crush. The preliminary conclusion was that this weaver possessed an extraordinaryability to weave 3K T300 carbon tow into PW fabric, in a way that exhibited no core crush. As a result, MR&D started an in-depth material characterizationinvestigation that included: 1) raw material study of the weaver, as well as that of supplier (A), 2) physical propertiestests of the resin system, 3) quantitative analysis of the prepreg, that incorporatedtack, friction and degreeof impregnation, 4) fabrication of core crush discriminator panels.
During the sametime as the weaver qualification, MR&D was conducting: 1) a porosity study of 90” angled solid laminate structural p.artsthat utilized the samepregpregsystem as the discriminator panel in the above weaver qualification, and 2) core crush studies of 250 degreeF curing fiberglassprepregs.As a result of the material evaluations during the investigationsof theseabove producibility issues,hypothetical assumptions were initially formed as follows: 1) high friction prepregs( carbon and fiberglass) minimize core movement,but may increaserisk of porosity. 2) If the resin system is low flow high viscosity then the following applies if used in combination with carbon fiber: a) StandardTwist (ST) T300 fiber minimizes core movement b) Never Twist (NT) T300 fiber maximizes core movement c) ST fiber maximizes potential for porosity d) NT fiber minimizes potential for porosity 3) Medium flow resin systemscombined with: a) ST fiber overcomeboth core movement and porosity b) NT fiber maximizes core movement In generalthe higher the frictional resistanceof fiberglass and carbon prepregsthe less the likelihood of core movement and crush. High friction in fiberglassprepregsis a function of the fiberglass bed and the ability of the resin systemto flow sufficiently exposing the bed. Thus high flow-low viscosity resin increasesthe likelihood of adjacent fiberglass beds or structureto nest and lock to eachother. It is this mechanical locking ability that minimizes movementduring an autoclave cure cycle.
Fiberglass Prepreg Optimization Severalyears ago Boeing commercial experienceda 45% scrap rate due to core crush on four secondaryand primary structure parts that incorporateda 250°F curing fiberglassprepreg from supplier (B). The immediate resolution was to use the alternatequalified supplier (A). As a result, the core crush rejection rate went to 0% with supplier (A) material. It was at this time the material investigation processstarted. Due to historical researchon core crush the investigative processfocused on the resin
chemistry of both suppliers. Thus physical property tests and complex viscosity profiling were initiated (seeref 4). In addition, the University of Washington’s Polymeric CompositeLab agreedto participate in the evaluation of the resin to fiberglass interface. (Note: the PCL and Boeing have proprietary agreements).The resultant viscosity data indicated that supplier (B) possessedthree times the minimum viscosity of supplier (A) (seefigure 1). This delta in viscosity indicates that more resin remains on the surface bed of supplier (B)‘s material when comparedto that of supplier (A) during a cure cycle. Due to the high viscosity of supplier B the interface and contact of adjacently stackedplies was solely that of resin (ie: no mechanical lock of the fiberglass beds could take place). This ultimately resulted in a slip plane. In other words, the nesting/locking ability of the beds was decreaseddue to the higher viscosity simply becausemore resin remainedon the surface.The PCL further substantiatedthis by indicating that the micrometer thickness of supplier (B) fiberglass was 10% greater than that of supplier (A). Due to the fact that both suppliers incorporated style 7781 fiberglass and that in both casesit was woven at the same manufacturersubstantiatesthat the increasein thickness of supplier (B) was due to the high viscosity resin. This in turn indicates that the impregnation processof resin into the fiber bed differed betweenthe suppliers. The PCL also stated that the fiber bed of supplier (B) cannot interlock with adjacent prepreg plies during the friction processdue to the excesssurface resin which behaveslike a lubricant. They also indicated that supplier (A) possessedthree times the coefficient of friction of supplier (B) ( see figure 2 ). At this time Boeing commercial first becameaware of the PCL friction testing device. This method illustrates the test procedureused to measure the frictional resistanceof prepregmaterials as a function of temperature and pressure(seereference2 for completedescription of equipment and technique).In collaboration with the PCL, MR&D has implemented and optimized the friction test apparatusand procedureover the course of the last several years. In addition, and as part of an effort to optimize qualification of prepregs,MR&D has implementedthis device at both supplier (A&B), in which round-robin enhancementtesting has been initiated. As a result of the Boeing feedbackon core crush sensitivity of supplier (B)‘s 250°F curing fiberglass, the supplier worked to correct this problem and thus manufacturea more robust material that minimizes movement. .
Ultimately, they developeda material (patentpending) that exhibits five to six times the frictional resistanceof their original material without affecting drapability (seefigure 2 ). This substantially higher friction prebreg, in turn has: 1) minimized core crush, and 2) led to a reduction in the use of ply stabilization techniques.As an end result, a major cost savings for the Boeing commercial airplane company is realized. The implementation phase of this material is currently in progress.
Carbon Prepreg Optimization When combining a low or medium flow resin system with 3k standard twist (ST) or untwisted (UT) carbon fibers, the end result is a minimization of core movement and crush. Combining this samelow or medium flow resin system with NT fibers maximizes movementand crush. Over one hundred discriminator panels have beenconstructedfrom suppliers (A) and (B) substantiatingthis claim. From figure (3), ten batchesof supplier (B) indicate that when this medium flow resin systemis combined with NT fiber the end result is a rejectable non-repairablelevel of core crush as determinedby the discriminator panel. It should be pointed out that four fiber manufacturersparticipated in this evaluation and the resulting data was consistent.In addition, when combining this sameresin system with UT or ST fiber the percentageof core crush has either beenreducedto acceptable levels or is non-existent.Note: percentcore crush is quantified from a parabolic summation of the discriminator panel (seeref 1). For supplier (B) the critical ply friction load indicates: 1) that above a friction load of approximately fifty pounds core crush is substantiallyreducedand 2) helow forty pounds core crush is maximized. The friction test apparatuscan be utilized to screenprepregsfor the probability of core crush. Boeing, supplier (A) and (B) currently use the friction apparatusto do so. In depth work has indicated that the critical friction value where no core crush occurs is dependenton the fiber type. For example, supplier (A)‘s low flow resin system (note: description of flow is public domain information) combined with NT fibers results in frictional load values below thirty pounds and when combinedwith ST fiber the friction results are approximately above forty pounds. . Once again all discriminator‘panelsmanufacturedwith NT fiber and supplier (A)‘s resin system result in rejectablenon-repairablecore crush and the sameresin system combined with ST fiber indicates acceptableor no core
crush. The point is the friction test will indicate the acceptableload values where no core crush occurs for both suppliers. During the fiber morphology investigation it becameevident that the roundnessassociatedwith the ST fiber tow and flatness of the NT fiber tow were two of the characteristicsthat affected the suppliers ability to impregnateresin into the fabric bed (see figure 4 ). Thus began the evaluation of the fabric weave openessor degreeof openess.Initial data indicated a linear relationship between fabric weave opennessand frictional load value(seefigure 5 ). When supplier (A) impregnatedtheir resin system into a fabric woven with ST fiber the end result was a two-fold increasein degreeof opennesswhen comparedto fabric woven with NT fiber. Note: degreeof opennessis measuredby image analysis pixel count method (see figure 6 ). Clearly the NT fabric is more closed when comparedto the ST fabric. This closure results in more resin residing on the prepreg surfacefor NT fabric. The surface resin thus createsa slip plane (ie: less friction) when plies are adjacently stacked.The end result due to the slippage is movement and the movement during an autoclave cure cycle results in core crush. The opposite occurs with the use of ST fabric. The degreeof opennesscan be three fold over NT fabric. Large opennessresults in a greaterability to nest or mechanically lock adjacentplies. This increasein mechanicallocking ability results in a greaterfrictional resistance.It is this increasein frictional resistancewhich results in a reduction of slippage and movement. Thus the end result is reducedcore crush. During round robin friction method verification with the PCL, a delta of approximately two to three pounds occurred between our results. Further degreeof openness( ie: image analysis) investigation acrossthe width of the prepregindicated an unexpectedvariation (see figure 7 ). The degree of weave opennessfor ST fabric can increasetwo fold when comparing the selvadgeedge to the middle of the prepreg. The frictional values due to the linear relationship to opennessfurther substantiatethis claim. Subsequent evaluation into Supplier (A)‘s impregnation processindicates variation acrossthe width of the prepreg (note: proprietary agreementswith supplier (A) do not allow further elaboration involving their process).It should be pointed out that the NT fabric is more uniform acrossthe width of the prepreg.As a result of the above evaluation, sampling for the friction tests , are now taken from the same location on the prepreg. Round robin friction verification tests are currently in-progressto verify standarddeviation deltas. Supplier (B) indicated a somewhat similiar relationship acrossthe
width of their prepreg.Further work needsto be accomplishedin this areain order to determinethe supplier processmechanismthat causesthis variation. The degreeof opennessand friction test apparatuswill be usedby Boeing commercial to verify processoptimization at both supplier (A&B).
Conclusions The friction test apparatusdevelopedby the PCL and optimized by Boeing’s MR&D organizationhas been successfully used to demonstratethat fiber type is the predominatefactor contributing to core movement and ultimately crush in honeycombcomposite structures.Supporting quantitative data indicates that the probable causeof core crush in structuresthat use carbon fabric prepregsis the combination of (NT) 3K carbon fibers with either a low or medium flow resin system. In addition, supporting data indicates that NT fiber results in reduced degreeof openness.Which in turn createsa slip plane due to excesssurface resin, resulting in: 1) reducedply friction, 2) ply movement,and 3) ultimately core crush. The discriminator panel has also demonstratedthat by introducing ST fiber into the structure you can minimize and eliminate core crush. Boeing’s Fabrication Division has successfullyeliminated core crush and additional stabilizing methodsby introducing ST fiber fabric prepregsinto two fly away structural parts with core heights in excessof two inches. In addition, Boeing of W innepeg has demonstratedthat high frictional resistant fiberglass prepregshave successfullyeliminated core crush and the need for stabilization techniques in fly away parts with a core thickness less than 1.2 inches in height. Future work will involve evaluating the successof thesefindings on core height restrictions, as well as determining which composite part designswill benefit from this work.
Acknowledgments The authorsexpresstheir appreciationto Rudy Braun and Mark W ilhelm of the Boeing Commercial Airplane company for helpful discussions.In addition, we expressappreciationto the individual contributors from suppliers (A&B).We especially want to thank Steve Moffitt of Boeing Commercial Airplane for all the lab testing.
References and Bibliography 1) D.J. Renn, T. Tulleau, and J.C. Seferis, “Composite Honeycomb Core Crush In Relation To Internal PressureMeasurement,”pg 4,6,9, Journal of Advanced Materials, (1995). 2) C.J. Martin, J. C. Seferis and M.A. Wilhelm, “ Frictional Resistanceof ThermosetPrepregsand Its Influence on Honeycomb Composite Processing,”pg 3-6, Composites,(1996). 3) Boeing Company, 1999,Boeing Material Specification, Epoxy PreimpregnatedGraphite tapesand Woven Fabrics-350°F cure, BMS 8-256. 4) Boeing Company, 1999, Boeing Material Specification, Glass Fabric PreimpregnatedEpoxy Resin low temperatureCuring, BMS S-79.
List of Figures 1) Complex Viscosity of Supplier (A&B) 2) Frictional Resistanceof BMS 8-79 Fiberglass 3) % Core Crush as a Function of Friction of Supplier (B) 4) Fiber Forms vs Spreadability 5) Frictional Load vs Weave Openness 6) Weave Analysis of NT and ST Fiber 7) Friction and OpennessVariation
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