Significant Effect Of Microwave Curing On Tensile Strength Of Carbon .
Volume 24, Number 3 - July 2008 through September 2008Significant Effect of MicrowaveCuring on Tensile Strength ofCarbon Fiber CompositesBy Dr. Brian B. Balzer and Dr. Jeff McNabbPeer-Refereed ArticleApplied PapersKEYWORD SEARCHComposite MaterialsManufacturingMaterials & ProcessesMaterials TestingPlastics / PolymersThe Official Electronic Publication of the National Association of Industrial Technology www.nait.org 2008
Journal of Industrial Technology Volume 24, Number 3 July 2008 through September 2008 www.nait.orgSignificant Effect ofMicrowave Curing onTensile Strength of CarbonFiber CompositesBy Dr. Brian B. Balzer and Dr. Jeff McNabbDr. Brian Balzer is a research scientist and technical consultant in the automotive and aerospace industries. Aspresident of BEI Management Consulting / SAI Global,he provides project management in manufacturingprocess improvement, lean implementation, continuousimprovement and operations management. Brian’sprojects have included on-site operations improvement,lean Six Sigma course design and implementation,manufacturing excellence and operations assessmentsin Europe and North America. Previously, Brian wasVice President of Quality and Technical Services atSpartanburg Industries where he managed engineersfor advanced product planning and process control ofstamping and welded component assemblies to originalequipment manufacturers in the automotive industry.Dr. Balzer was awarded his PhD in Technology Management specializing in Manufacturing Systems from theSchool of Graduate Studies at Indiana StateUniversity on December 2007.Dr. Jeff McNabb is an associate professor of Manufacturing Systems Technology in the College of Technologyat Indiana State University. He currently teaches automation related classes at the undergraduate level andmanufacturing systems classes at both the master’s anddoctoral level. His recent accomplishments include anomination to the “Who’s Who Among America’s Teachers”, funding for the research and development of a PLCbased training simulator, and funding for implementingthe Project Lead the Way curriculum in many schoolsnear Indiana State University. He is the author of the“Rule of Thumb Measuring System” workbook used bymany Technology Education programs. Dr. McNabb isthe recent past President of the NAIT research division,and is also currently serving as the Associate Dean ofthe College of Technology at Indiana State University.AbstractThe traditional process for curingcarbon fiber (CF) composites hasbeen the autoclave system. A reviewof recent research indicates curing CFcomposites in a microwave oven hasthe potential for reducing processingtime. The problem statement of theresearch study was that the impact ofa microwave curing process on tensilestrength (maximum tensile stress) ofselected CF composite specimens wasunknown. The research study describesthe statistical procedure and analysisof data to answer the specific questionfor the experimental trials: What is thesignificant effect on the tensile strengthof cured CF composite samples dueto the variables of the autoclave andmicrowave curing process cycle timeand temperature? ASTM Internationalstandard test method designation D5083 – 02 was used for testing tensilestrength of reinforced carbon fiberplastics using straight-sided specimens.Data was obtained for evaluating theeffects of process cycle time and temperature on tensile strength of the CFcomposite specimens. The result wasthat curing time of the autoclave systemand microwave process had significanteffects on the tensile strength of CFcomposite specimens. The CF composite specimens from the microwaveprocess showed lower tensile strengththan the autoclave specimens due togreater void content.IntroductionIn 2005, U.S. EPA fuel economyregulations were increased to 27.5 mpgfor passenger vehicles (EnvironmentalProtection Agency [EPA], 2005). “Ofcritical importance will be the extentto which more than 200 million light2vehicles on U.S. highways . . . becomemore fuel efficient as vehicle buyerschoose the lower fuel costs of lighter orhybrid vehicles” (Greenspan, 2005).One approach being followed by OEMautomakers is to design vehicles thatconsume less fuel by using new materials like carbon fiber (CF) composites(Aronson, 1999). In aerospace, corporate jets are being produced usinglightweight material to reduce weightand increase range (Sutton, 1998;Dornheim & Meacham, 2005).The traditional method for curing CFcomposite material is the autoclave system with cycle time ranging as long as8 – 10 hours and produces known tensile strength (Ashley, 1997, Dornheim& Meacham, 2005). The manufacturingprocess of carbon-fiber composites hasthe highest potential for the reductionof cycle time (Feher & Thumm, 2004).A faster processing method is themicrowave system. According to Feherand Thumm, the microwave system is abetter system with the potential benefitof reducing processing time (2004).The potential benefits of using the microwave system for curing CF composites include the reduction of processingtime, energy consumption and loweroperating costs.The effects on the tensile strength(maximum tensile stress) of cured CFcomposite material are unknown whenusing a microwave curing system. Tensile strength is defined as the maximumtensile stress of reinforced thermosetting plastics (ASTM, 2002). Therefore,the problem statement of this researchstudy was that the impact of themicrowave curing process on tensilestrength of selected CF composites was
Journal of Industrial Technology Volume 24, Number 3 July 2008 through September 2008 www.nait.orgunknown. Experimental trials werecompleted to determine the effect.Review of LiteratureConventional fabrication of carbonfiber composites is a slow, laborintensive process (Morey, 2007). Theconventional process for curing carbonfiber composites utilizes an autoclave.Autoclave composite manufacturingbegins by heating resin until it is liquefied. In the curing process, the liquefiedresin impregnates a fabric form called aprepreg matrix (prepreg).A specific number of prepregs are thenstacked, or laid-up, to the requiredthickness. The laid-up prepregs areplaced onto tooling that will formit into the desired shape. A plasticmembrane is applied as a cover overthe prepreg matrix and an adhesive isused to seal it to the tooling. The sealedprepreg and tooling are placed into thechamber of the autoclave for curing.A vacuum is applied to evacuate airfrom the plastic membrane forcing theprepreg matrix against the tooling wallsto form its final shape.Morey (2007) points out that a moreinnovative approach is the resin transfermolding (RTM) process. In RTMprocessing, a single three-dimensionalfabric preform is woven in the shape ofthe finished part and placed in a mold.A vacuum is induced prior to injectingthe resin into the mold to draw the resinthrough all of the spaces and voids ofthe preform. The final part can then becured in the mold itself, or it can betransferred into an autoclave for finalcuring.The aerospace industry has been theleader in utilizing an advanced RTMprocess for mass producing carbonfiber components (Ashley, 1997). Thetraditional curing process of RTM is theautoclave system, however, innovativemethods are being applied to reducecost (Morey, 2007). As the entireRTM preform assembly is heated, theresin is injected, then cured under highpressure. The preparation, lay-up andde-molding is a highly manual process.A vacuum assisted process (VAP) canFigure 1. – Components of a microwave system.reduce the manual labor process (Feherand Filsinger, 2005).In an autoclave system, carbon fibermatrix or preform woven material isplaced in a vacuum bag and mold. Theautoclave processing method utilizesa vacuum and pressure level. Unlikean autoclave, the curing process in amicrowave oven is accomplished bythe distribution of magnetic-fields frommicrowave energy (Orbzut, 2006).The basic components of a typicalmicrowave system are shown in Figure1. Power is supplied to the magnetroncreating an electromagnetic (EM) fieldthat is distributed by the waveguidesystem into the applicator. Temperaturesensors are located at the magnetrongenerator and air inlets to monitor thetemperature and prevent overheating.To prevent damage to the magnetroncaused by reflective microwaves, industrial microwaves use a circulator todeflect electromagnetic waves into anabsorber (Chan & Reader, 2000).There are two basic components topropagating electromagnetic waves: (a)electric field (Ε), volts per meter (V/m),and (b) magnetic field (Η), amperesper meter (A/m) (Ulaby, 2004). Twoelectrostatic charges E spaced apart ina vacuum are known as permittivityin free space, ε0. Two current loops H3spaced apart in a vacuum are describedas permeability in free space, µ0. Theprimary component to heating material in a microwave is the electric fieldof the electromagnetic wave (Akhtar,Feher & Thumm, 2006; Orbzut, 2006).If the space between the electrostaticcharged particles is filled with dielectricmaterial, the mechanical force betweencharges is increased by a factor knownas relative permittivity of the material,ε′ (Chan & Reader, 2000).Although the electric field strength(V/d, volts per meter distance) overa rectangular conductive material isrelatively constant (Ulaby, 2004), thegeometric discontinuity of the edgesand sharp corners increases the electricfield strength, causing surface chargesto accumulate (Meredith, 1998). Thisaccumulation of electrostatic chargesin the corners causes certain material to arc during microwave heating.This is one reason why material at thecorners and edges heats faster (Pearce,2005). Figure 2 shows the geometricdiscontinuity.Using a microwave oven in the manufacturing process of carbon-fibercomposites has the highest potentialfor reduction of cycle time and cost.As stated by Feher and Thumm (2004),“The highest potential for cost reduction is to be found [in] the manufactur-
Journal of Industrial Technologying process which implies substantiallong-time and high-energy consumption, as well as a low degree of automation” (p. 73). Potential benefits of usingthe microwave technology for fabrication of carbon fiber composites caninclude “high heating rates – reductionof processing time; savings on energyconsumption; [and] ‘clean’ heatingtechnology” (Feher & Thumm, 2004,p. 73). Volume 24, Number 3 July 2008 through September 2008 www.nait.orgFigure 2. – Electromagnetic waves and geometric discontinuity.MethodThe selection of the statistic, includingthe error rate, the statistical procedureand analysis of data was used to answerthe specific question for the experimental research study: What is the effectof the autoclave and microwave curingprocess cycle times and temperatureson the tensile strength of cured CFcomposite samples? By completingexperimental trials of CF compositesamples using the commercial microwave curing system, analytical data wasobtained for evaluating the effects ofcycle time and temperature on tensilestrength of the CF composite specimens.Statement of HypothesisThere were two process variables usedas independent variables (IV) and onedependent variable (DV): The typeof curing process (autoclave / microwave) was an IV with two factors: (a)cycle time, Factor A, was an IV, (b)temperature, Factor B, was an IV; andCF tensile strength (maximum tensilestress) was a DV. Factor A, cycle time,had two levels: (a) 180 minutes for theautoclave and (b) 30 minutes for themicrowave. Factor B had two levels:(a) 250 ºF temperature and (b) 356 ºFtemperature. The following states thehypothesis:mean measurement of microwave curing process temperatures, 250 & 356degrees Fahrenheit, on the effects of CFmaximum tensile stress.The independent variable subjects werecategorical factors with two levels forFactor A, cycle time, and two levels forFactor B, temperature. The dependentvariable subject was an interval factorand random in nature, being representative of the population. The main effectswere identified.Curing time Factor A Level 1 was 180minutes for the autoclave and Factor ALevel 2 was 30 minutes for the microwave. Temperature was Factor B Level1 for 250 degrees Fahrenheit and FactorB Level 2 for 356 degrees Fahrenheit.The autoclave had only Factor B Level1 for 250 degrees Fahrenheit. The microwave had Factor B Level 1 for 250degrees Fahrenheit and Factor B Level2 for 356 degrees Fahrenheit.Statement of ProceduresThe research study used two different systems to cure the CF compositematerials: autoclave and 2.45-GHz microwave oven. The equipment used forthe autoclave study was the Herculesautoclave. The equipment used for themicrowave study was the Amana 3 kWcommercial 2.45 GHz microwave oven,as shown in Figure 3 (Amana, 2001). Adigital temperature data recorder withfiber-optic probe was used for temperature sensing. Two universal tension testmachines were used in the researchstudy. Each machine was calibratedprior to testing. Experimental trialswere completed using the CF prepregvariable temperature matrix productcode VTM 264/CF0300 provided byAdvanced Composites Group, Inc. Itis a 2 X 2 woven CF pre-impregnatedresin and precursor matrix.Figure 3. 2.45-GHz commercial microwave oven and temperature data recorder.Null hypothesis 1 (H01). µA1 µA2,states that there is no difference in themean measurement of autoclave curingprocess cycle time, 180 minutes, andmicrowave curing process cycle time,30 minutes, on the effects of CF maximum tensile stress.Null hypothesis 2 (H02). µB1 µB2,states that there is no difference in the4
Journal of Industrial Technology Volume 24, Number 3 July 2008 through September 2008 www.nait.orgFigure 4 shows the slab of prepreg matrix prior to curing. Preparation of thespecimen was based on the AdvancedComposites Group ([ACG], 2006) andThomas and Kardos (1994) method formaterial preparation. There were eightlayers in the slab to achieve a minimum2 mm (0.079 in) thickness. Each cornerof the slab was trimmed, as shown inFigure 4.The ASTM International Standardknown as D 5083 – 02 standard testmethod for testing tensile strength ofreinforced composite plastic was usedas the reference guideline for preparingtest specimens. The preferred specimen size was an overall length: 250mm (9.843 in); width: 25 mm /- .5mm (0.984 in /- .020 in); and, thickness: between 2 mm and 14 mm (0.079in and 0.551 in) (ASTM, 2002). Aftercuring the slab, each specimen was cutinto the correct rectangular dimensionsize, as shown in Figure 5.For the research experimental trials,there were three trial runs with 30 CFcomposite specimens from each type ofcuring process, for a total of 90 specimens: (a) one trial run from the autoclave at 250 ºF, (b) one trial run fromthe microwave at 250 ºF, (c) and oneFigure 4. – Trimmed slab with minimum2 mm (0.079 in) thickness.Figure 5. Cured carbon fiber composite specimens cut into 2.58 cmwidth and 25.4 cm length.trial run from the microwave process at356 ºF.The research procedure used (a) preheat (ramp-up) cycle and (b) cure(soak) cycle. The ramp-up time forthe autoclave was 2 hours and for themicrowave the time was 8 minutes. Thecure cycle time in the autoclave was 1hour and in the microwave the time was22 minutes. For this research study, theamount of time to complete the combined pre-heat and cure cycles equaledthe total curing cycle time.Table 1 shows the microwave powersetting and cycle time. Based on preliminary trials of required power outputto fully cure CF composite specimens,the microwave power settings used toreach and sustain 250 ºF were as follows: (a) Setting 5 for 8 minutes, (b)Setting 0 for 4 minutes, (c) Setting 1for 13 minutes, and (d) Setting 2 for 9minutes. The power settingsnecessary to reach and sustain 356 ºFwere: (a) Setting 5 for 9 minutes, (b)Setting 3 for 5 minutes, (c) Setting 1for 6 minutes, and (d) Setting 2 for 10minutes. The total power cycle timewas 30 minutes for both temperatures.Power settings of the microwave ovenwere pre-set by the manufacturer.Power Setting 5 is 50% of the availablepower of 3 kW, or 1.5 kW. Power Setting 3 is 30% (.9 kW), Setting 2 is 20%(.6 kW), and Setting 1 is 10% (.3 kW).Calibration of the microwave oven wasachieved by using known data valuesfor (a) specific heat and rate of temperature rise of water, (b) specific heat forthe temperature rise and power densityof water, and (c) time to boil water.Standard rules of thumb were followedfor achieving specific heat and powerdensity, 600 W – 800 W: (a) 4 minutesto boil 1 cup of water, (b) 6 minutes toTable 1.Microwave Power Settings and DurationTemp./powerCuring cycleStage1234Power settingWatts (kW)51.50010.320.6Duration (min.)841391234Power settingWatts (kW)51.530.910.320.6Duration (min.)95610250 ºF356 ºFStage5
Journal of Industrial Technologyboil 2 cups of water, and (c) 10 minutesto boil 4 cups of water (Meredith, 1998;Dodson, 2001).Selection of Statistic / Error RateThe primary statistical procedure usedwas two-way analysis of variance(ANOVA). The general linear model(GLM) in SPSS 14.0 was used foranalyzing data. The SPSS 14.0 software program includes an additionalcolumn labeled Sig. for significance(Norusis, 2005). Several assumptionswere made for the two-way ANOVAstatistical procedure in the study: (a)the dependent variable was measuredon an interval scale; (b) samples wererandomly selected from the populationand randomly assigned to groups; (c)there was homogeneity of variance;and, (d) the error rate selected was .05Type I error, (α .05).ResultsThe descriptive statistics for the dependent variable are shown in Table 2.The mean value for maximum tensilestress for the autoclave curing time andtemperature was 110.5 ksi.The mean value for the microwavecuring time at the two temperatures of250 ºF and 356 ºF were the values of67.2 ksi and 57.6 ksi, respectively. Thestandard deviation for the autoclavesystem mean measurement result was3.82. The microwave process standarddeviations were 3.14 at 250 ºF and 7.33for the higher temperature of 356 ºF.Statistics AnalysisUsing the two-way ANOVA based onthe SPSS 14.0 univariate GLM, the testof the between-subjects effects wasrun for the main effects as shown inTable 3. Curing time was statisticallysignificant, F(1, 87) 1079.88, p .00.Temperature was statistically significant, F(1, 87) 53.08, p .00. Usinga test of significance reference tablefrom Mendenhall and Reinmuth (1978,p. 711, the F-critical value was 6.3 at 1degree of freedom and alpha equals .05,which was the 95% confidence level.The F-observed value for curing timeof 1079.88 with 1 degree of freedom,and temperature of 53.08 with 1 degree Volume 24, Number 3 July 2008 through September 2008 www.nait.orgTable 2. Descriptive Statistics for the Dependent Variable: Maximum StressCuring timeTemperatureMean (ksi)SDNLevel 1 (Autoclave)Level 1 (250 ºF)110.53.8230Total110.53.8230Level 1 (250 ºF)67.23.1430Level 2 (356 ºF)57.67.3330Total62.47.3860Level 2(Microwave)Note. Dependent variable: maximum stress.Table 3. Test of Between-Subjects Effects for Main EffectsSourceType III sum ofsquaresdfMean squareFSig.Corrected 3785.516326.07.00Curing ed Total49756.789Note. Dependent variable: maximum stress.of freedom were greater than the Fcritical value, 6.3. Based on analysis ofthe F-statistic values, curing time andtemperature were significant at the 95%confidence level.Results from HypothesisThe determination for rejection of aspecific hypothesis was based on thesignificance level for the respectivefactor. When the F-observed value isgreater than the F-critical value for agiven variable, the difference betweenthe variables is stated to be significantand the null hypothesis is rejected.The difference in the mean measurement of curing time was statisticallysignificant, F(1, 87) 1079.88, p .00. Therefore, the first null hypothesisof H01: µA1 µA2 was rejected. Thealternative hypothesis of HA1: µA1 µA2, states that there is a difference inthe mean measurement of curing timeon the effects of CF maximum tensilestress. The mean measurement results6for tensile strength of CF specimensfrom the autoclave system were largervalues than the results from the microwave process.The difference in the mean measurement of temperature was statisticallysignificant, F(1, 87) 53.08, p .00.The second null hypothesis of H02: µB1 µB2 was rejected. The alternativehypothesis of HA2: µB1 µB2, states thatthere is a difference in the mean measurement of temperature on the effectsof CF maximum tensile stress. The tensile strength results from the microwaveprocess at 250 ºF were larger valuesthan the results at 356 ºF.Results of Fiber Matrix DensityTable 4 shows the fiber matrix densityof autoclave specimens. The resultsshowed variation in the thickness andfiber matrix density of CF specimensfrom the three different trials. Themean value for thickness of the CFspecimens from the autoclave system
Journal of Industrial Technologywas .22 cm (2.23 mm) and fiber matrixdensity was 1.96 g/cm3. The meanvalue for thickness of the CF specimensfrom the microwave process at 250 ºFwas .28 cm (2.77 mm) and fiber matrixdensity was 1.30 g/cm3. The meanvalue for thickness of the CF specimensfrom the microwave process at 356 ºFwas .30 cm (3.01 mm) and fiber matrixdensity was 1.12 g/cm3.Figure 6 shows the comparison of themeasurement for maximum stress forCF composite specimens from themicrowave process at the temperaturesof 250 ºF and 356 ºF. When comparingthe mean measurement data result fromthe microwave at 250 ºF with the meanmeasurement data result of the microwave at the higher temperature of 356ºF, shows a higher maximum tensilestrength in favor of the lower temperature setting. Volume 24, Number 3 July 2008 through September 2008 www.nait.orgTable 4. Fiber Matrix Density of SpecimensAutoclave (250 ºF)Microwave (250 ºF)Microwave (356 ºF)Thickness .28. 301.125.221.89.281.16.301.15Mean .221.96.281.30.301.12No.DiscussionThere are several impacts of the microwave curing process on maximumtensile stress of CF composite specimens. One impact of the microwavecuring process was that a 30 - minutecuring cycle time has been validatedto fully cure CF composite specimenscompared to 180 minutes in the autoclave system. Another impact was thatcomparing the results of just the microwave process showed that using a 250ºF temperature in the microwave curingprocess provided better tensile strengthresults than a 356 ºF temperature.Lastly, fiber matrix density is lowerfrom the microwave curing processcompared with the results of autoclave.The following discussion reviews theconclusions about the results of theexperimental trials.ConclusionsFirst, the results showed that the maximum tensile stress was higher for CFcomposite specimens from the autoclave system compared to the CF composite specimens from the microwaveprocess. The results showed the meanmeasurement of the maximum stressfor the autoclave CF composite specimens was 110.5 ksi. The mean measurement of the maximum stress for theFigure 6. Comparison of maximum tensile stress.Actual data measurements are plotted.microwave CF composite specimensat the same temperature was 67.2 ksi.The difference in mean measurement ofmaximum tensile stress was 43.3 ksi.Second, the results showed that themean measurement difference of temperatures was significant. The microwave process at the higher maximumtemperature of 356 ºF had the lowestmean measurement for maximumtensile stress of 57.6 ksi. The microwaveprocess at the lower temperature of 250ºF had a mean measurement of maximum tensile stress of 67.2 ksi, whichis a difference of 9.6 ksi. The standarddeviation for the lower microwaveprocess temperature specimens was 3.147compared to the standard deviation forhigher microwave process temperaturespecimens at 7.33. A smaller standarddeviation indicates that the data distribution is narrow. A narrower distribution ofdata suggests that the lower temperaturemicrowave process has a higher probability of producing specimens withhigher tensile strength, when comparedto specimens from the microwave process at a higher temperature.Third, arcing occurred on the outeredges and corners of the CF prepregslab in the microwave process. Arcingcaused the loss of vacuum by burninga hole in the vacuum bag. In this study,electrostatic charges accumulated at the
Journal of Industrial Technologycorners and exposed edges of the CFprepreg slab which resulted in arcing(Meredith, 1998). As a result of this occurrence, it was necessary to eliminatethe resin absorbing cloth and vacuumbag from the microwave curing processprocedure. Figure 7 shows the autoclave and microwave specimens. Lackof vacuum in the microwave processallowed greater expansion of moistureand evaporation of resin. The result waslarger voids in the microwave CF composite specimens, Larger voids increasedthe thickness and reduced the fibermatrix density and tensile strength of themicrowave CF composite specimens.Lastly, curing time for the autoclavesystem was 180 minutes compared to30 minutes in the microwave process.Although the maximum tensile stressresults were lower, the microwavecuring process was 83% faster than theautoclave system.Acknowledgements: Dr. Guo-PingZhang, Technical Advisor, Indiana StateUniversity; and, Dr. David Stienstra,Technical Advisor, Rose-Hulman Institute of Technology.ReferencesAdvanced Composites Group [ACG],(2006). Variable temperature moulding prepreg systems. AdvancedComposites Group website. Retrieved November 16, 2006 d Composites Group [ACG],(2006). Advanced CompositesGroup training course: Materialpreparation. Advanced CompositesGroup website. Retrieved November25, 2006 from http://www.advancedcompositesgroup.com/Amana Commercial Products Division(2001). TRC30S 3000 watts commercial microwave oven: specifications. Amana Commercial ProductsDivision website. Retrieved August29, 2006 from http://www.amanacommercial.com/American Society for Testing andMaterials (ASTM), (2002). D 5083-02: Standard test method for tensileproperties of reinforced thermoset- Volume 24, Number 3 July 2008 through September 2008 www.nait.orgFigure 7. Specimens’ avg. thickness - left: autoclave (2.23 mm);right: microwave (2.77 mm).ting plastics using straight-sidedspecimens. West Conshohocken,PA: ASTM International. RetrievedOctober 14, 2006 from: http://www.astm.org/ANOVA terms. Retrieved July 13, 2004from http://www.texasoft.com/winkanov.htmlAronson, R. B. (1999). Materials forthe next-generation vehicle. Manufacturing Engineering. 123(2), 94102.Ashley, S. (1997). Carbon compositesfly high. Mechanical Engineering.119(9), 66-69.Chan, T. V., & Reader, H. C. (2000).Understanding microwave heatingcavities. Norwood, MA: ArtechHouse, Inc.Dodson, Carolyn (2001). Basic principles of using a home microwaveoven. In A. K. Datta & R. C. Anantheswaran (Eds.), Handbook ofmicrowave technology for food applications (pp. 339-352). New York:Marcel Dekker, Inc.Dornheim, M. A., & Mecham, M.(2005, January 17). From dream tohardware. Aviation Week & SpaceTechnology. 162(3), 398–399.F statistic terms. Retrieved July 13,2004 from , L., & Filsinger, J. (2005). Newapproaches for the application ofmicrowave heating for processingcomposite materials in the aeronautic industry. Proceedings of theInternational Microwave PowerInstitute 39th Annual MicrowaveSymposium, Seattle, WA, 39, 50-52.8Feher, L., & Thumm, M. (2004). Microwave innovation for industrialcomposite fabrication – the HEPHAISTOS technology. IEEE. 32(1),73-79.Greenspan, A. (2005, April). Remarksby Chairman Alan Greenspan onenergy. Speech before the National Petrochemical and Refiners Association Conference, SanAntonio, TX. Retrieved March 16,2006 from: 005/20050405/default.htmMendenhall, W., & Reinmuth, J. E.(1978). Statistics for managementand economics. (3rd ed.). North Scituate, MA: Duxbury Press.Meredith, R. (1998). Engineers’handbook of industrial microwaveheating. London: The Institution ofElectrical Engineers.Morey, B. (2007, April). ProcessesReduce Composite Costs. Manufacturing Engineering, 138(4), AT6AT11. Retrieved April 1, 2008, fromResearch Library database. (Document ID: 1264640811).Norusis, M. J. (2005). SPSS 14.0 Statistical procedures companion. UpperSaddle River, NJ: Prentice Hall.Orbzut, J. (2006). Coaxial line reflection method for dielectric permittivity of thin film samples at microwavefrequencies: numerical and experimental analysis. Proceedings of the40th Annual Microwave Symposium– IMPI, Boston, 40, 85-88.Pearce, J. (2005). Introduction tothe physics of electromagnetics.Presentation from Fundamentals of
Journal of Industrial Technologymicrowave science: An IMPI shortcourse. Proceedings of the 39th Annual Microwave Symposium – IMPI,Seattle, WA.Sutton, O. (1998, October). Premierset to change the bizjet mindset. INTERAVIA. 53(624), 32-33. RetrievedMarch 23, 2005 from the ProQuestdatabase. Volume 24, Number 3 July 2008 through September 2008Thomas, M. M., Kardos, J. L., B.Joseph, (1994). Shrinking horizonmodel predictive control appliedto autoclave curing of compositelaminate materials. Proceedings ofthe American Control Conference,(1)29, 505–509.9 www.nait.orgUlaby, F. T. (2004). Fundamentals ofapplied electromagnetics 2004 media edition. Upper Saddle River, NJ:Pearson Prentice Hall.U.S. Environmental Protection Agency(2005). Title 2 emissions regulations. U.S. Environmental Protection Agency website. RetrievedMarch 16, 2005 from http://www.epa.gov/regulations/
samples using the commercial micro-wave curing system, analytical data was obtained for evaluating the effects of cycle time and temperature on tensile strength of the CF composite speci-mens. Statement of Hypothesis There were two process variables used as independent variables (IV) and one dependent variable (DV): The type of curing process .
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Rail Transportation and Engineering Center (RailTEC) 205 N Mathews Ave. Urbana, IL, United States 61801. ABSTRACT . Previous research has focused on the effect of rail cant on rail wear and wheel/rail interaction, indicating that a steeper rail cant results in increased wear on rails and wheels. However, no research has investigated the effect .
