Variation Of Mechanical Characteristics Of Polyurethane Foam: Effect Of .

Materials 2019, 12, 26723 of 20characteristics are seen as contributing to the load support performance by maintaining the geometrywith relative densities inside the material. Furthermore, as shown in Figure 1, the mechanical behaviorof rigid PUF composed of segments indicates that it can sensitively react to external environmentalconditions unlike other homogeneous materials.There are usually two issues to be prevented from an engineering design perspective. Becauseof the inability to consider a combination of factors affecting each other in the environment, thereare states in which failure occurs without the maximum load, and excessive allowable capacity isapplied to the demand. In any case, it is necessary to understand the material fracture characteristicsaccurately, to develop a safe and reliable structure. It can also be applied to situations in which space islimited by adjacent structures in bulk units, not just materials or installations, or where forces are notuniformly distributed across the entire area of the material, i.e., concentrated loads. Thus, the criteriafor mechanical characteristics for actual working loads assume that the global displacement of thematerial used is constrained when it occurs, meaning that the environment, such as the confining effect,should be considered as an incidental consideration [41,42].However, the existing standards regarding how to evaluate the mechanical behavior of rigidPUF do not specifically reflect the surrounding physical environment. There is limited research onconditions that can be easily exposed to the effects of surrounding structures, in contrast to thoseconsidered only for specific external variables, such as temperature. However, these restraint conditionsneed to be addressed in terms of research, as they are overlooked in comparison to their actual impact.Based on the recognition of the association of these complex factors, the purpose of this study wasto perform a mechanical performance evaluation by adding a jig installed on the side of a rigid PUF.These restraint attempts were intended to answer basic and important questions in terms of materialbehavior for reliable bulk structure design in extreme environments by applying and reviewing newmethods that are not presented via conventional experimental methods.2. Experiment2.1. Experimental OverviewThe types of loads applied to a structure widely vary from a static form, resulting from thesimple cargo mass itself, to a dynamic form, resulting from impact. Therefore, the risk of damage isdetermined depending on the design perspective. The circumstance in which unexpected impulsiveloads are applied is mainly characterized by a kinetic energy, governed by the weight and speed at theinstant of impact. In most cases, a certain portion of the kinetic energy remaining after the impact isdissipated as strain energy. Generally, this dissipated strain energy acts as an external factor that causesdeformation along with structural damage. This corresponds to the material ductility and stabilityand is directly related to the load-bearing function [43]. Figure 2 shows international test standardsfor assessing the mechanical properties of rigid PUF from critical hazards. The dimensions of the testspecimen required for each test method are summarized in Table 1.2.1.1. Tensile TestA tensile test was performed according to the ASTM D 1623 standard. The recommendeddimensions of the test specimen are shown in Figure 2a. The standard speed of testing was such thatbreakage occurred in 3–6 min. The rate of crosshead movement was 1.3 mm/min for each 25.4 mm oftest section gauge length. The load at the moment of breaking was presented in kN units, divided intothe original cross-sectional area, and the tensile strength was calculated. The tensile modulus wasmeasured using a set of extensometers.2.1.2. Compressive TestThis test was performed according to the ASTM D 1621 standard. As shown in Figure 2b, aload was applied in the direction of foaming of the test specimen with a minimum cross-section of

Materials 2019, 12, 26724 of 2025 cm2 and a maximum of 230 cm2 . The test specimen placed in the center between the two parallelplates wasat REVIEWa rate possibly up to 10% of its original height per minute until the heightofMaterials2019,compressed12, x FOR PEER4 of 20the specimen was reduced to 85% deformation. The stress at the yield point if yield occurred beforedeformation,or, inor,theabsenceof sucha yield,thethestress10% deformation,in theabsenceof sucha yield,stressatat10%10%deformationdeformationisis thethe compressivestrength.strength. TheThe modulusmodulus ofof elasticityelasticity waswas determineddetermined byby thethe straightstraight portionportion below the proportionallimit of the stress-strain curve.Shear TestTest2.1.3. ShearAs shownshown in Figure 2c, a test was performed in the vertical directions of the panel specimensAsaccording totoASTMASTMCC273.273.TheThetesttestspecimenshada thicknessequalto thethickness,a widthaccordingspecimenshada thicknessequalto thecorecorethickness,a esthethickness.Thetestspeedwassettobeless than 50 mm, and a length not less than 12 times the thickness. The test speed was set to be a valueavalueat whichthe ndedrecommendedstandardstandard headatwhichthe specimenwas wasbrokendownwithin3–6The ultimateultimate corecore shearshear strengthstrength was calculatedcalculated by dividing thedisplacement rate was 0.50 mm/min. Themaximum recorded force on the specimen in the cross-section as detaileddetailed inin TableTable 1.1.(a)(b)(c)FigureFigure 2.2. standards:(a) tensile, (b) compression, and (c) shear test methods.Table 1.1.Dimensionsof theaccordingto the evaluationmethod of mechanicalTableDimensionsof testthe specimentest specimenaccordingto the evaluationmethod of performance of rigid polyurethane foam STM D 1623)(ASTM D 1623)Compressive testCompressive(ASTMD 1621) test(ASTM D 1621)Shear testSheartest(ASTMC 273)(ASTM C erDiameterCross-sectionCross-sectionRadius of curvatureRadius of cknessThicknessLengthLengthWidthWidthmm InchesInchesmm(in) (in)NoteNote25.4 0.5 in25.41 1 0.5 in28.728.71.13 1.130.13 0.1321 in- 0.471 in211.918 to the center line.11.90.4718 to the center line.25.41Less than width or diameter25.41 Less than widthor diameter 4 in2 , 6 in2 4 in2, 6 in2 core specimen-- core 12specimentimes thickness-- 12 times thickness 2 in 2 in2.2. Material PropertiesRigid PUF that has excellent adhesion between components is required to be evaluated in termsof mechanical performance similar to other structural materials. Insulation structures with rigid PUFare exposed to tensile, compression, and shear stress depending on the characteristics of theapplication environment. This material is subjected to the type of load that is usually pushed down.

Materials 2019, 12, 26725 of 202.2. Material PropertiesRigid PUF that has excellent adhesion between components is required to be evaluated in terms ofmechanical performance similar to other structural materials. Insulation structures with rigid PUF areexposed to tensile, compression, and shear stress depending on the characteristics of the applicationenvironment. This material is subjected to the type of load that is usually pushed down. In particular,because the compressive strength including Young’s modulus is a perfect value for a foam material, theimportance of the performance evaluation considering tensile or shear loads is relatively reduced [8].In environments under tensile or shear loads, some restrictions may exist, but they do not have asignificant effect when considering the direction of the loading components applied to the material.In the case of a shear test, it is difficult to identify a pure shear situation for the specimen because ofvarious factors (facesheets, adhesives, precures or bonds, etc.) and, therefore, it is not preferred as amethod to determine the impact of constraints.What makes shock loading different from normal compression loads is that it has an unexpectedeffect on the breaking characteristics of the material according to the time and period of the impactenergy transferred. Although the sum of the impact quantities is similar when a large-sized loadis applied in a relatively short period of time (or a small-sized load operates over a long period oftime), the damage mode that occurs in the materials is quite different. In addition, when shocks areconcentrated on a portion of a cross-section of a structure, they can be interpreted as quasi-staticthrough the binding effect produced by other surrounding structures that are not directly exposed tothe force [44].Under a compressive load applied with quasi-static strain, a rigid PUF with a closed-cell structuretypically exhibits behavioral characteristics such as those shown in Figure 3. As the relative densityof the cell’s internal structure changes because of the constant action of external forces, it graduallyconstitutes nonlinearity as the elastic region. The fracture phenomenon that appears in the rigid PUFbeyond the yield point is characterized by solid and gas phases inside the closed-cell structure [45].Assuming that the load is critically applied through the plastic section, the gas phase excluding thesolid phase is compressible. The closed-cell volume fraction contained in the foam material, c , isdefined as follows:Vc,(1) c VPwhere Vc is the volume of the solid phase such as the cell wall in the foam except the gas phase. andVP is the total volume of the foam. The collapse of the cell due to compression deformation can reducethe value of Vp , but there is no significant change in Vc unless some parts of the specimen fall apart.Therefore, the total density of the foam, ρ, can be written as follows:ρ ρc c ρg (1 c ),(2)where ρc is the density of the solid fraction of the strut, and ρg is the density of the gas phase.The equation means that, for a given ρc , ρ depends on the relative value of c irrespective of ρg . Asplastic deformation occurs, the 1 c of the right term converges to zero and the ρg (1 c ) becomesnegligible relative to ρc c ; thus, it can be expressed as ρ ρc c . Notably, the value of c occupiesa large proportion of ρ as the deformation of the foam progresses [46–48]. This notation is used todetermine the material strength performance as follows:σelρ CEsρs!3,(3)where ρc is the density of the solid fraction of the strut, ρg is the density of the gas phase, σel is theelastic collapse stress in a closed-cell material, Es is the Young’s modulus of the cell wall, and C is thematerial constant. It can be seen that the relative density of the foam, which is artificially changed inresponse to external conditions, is an important factor involved in strength performance [49–51].

Materials 2019, 12, 2672Materials 2019, 12, x FOR PEER REVIEW6 of 206 of 20FigureMechanical behavioralFigure 3.3. Mechanicalbehavioral characteristicscharacteristics ofof rigidrigid PUFPUF underunder aa compressedcompressed load.load.The compressive test, through consideration of influential factors, determined that a quasi-static2.3. Experimental Preparationspeed was relevant to reflect the environmental impact from surrounding structures. Therefore, it isThereweretypesmethodof the rigidPUF specimensused inthis study:purestudypolyurethanefoamexpectedthatthe twoproposedof mechanicalperformanceevaluationin thiscan ethanefoam(RPUF).ThepurePUFandRPUFidentify the behavioral tendency of rigid PUF with or without a restraint.specimens were manufactured by adding a foaming agent to polyol and isocyanate, followed by2.3.ExperimentalPreparationmixingand blowingusing a homogenizer. Both the pure PUF and the RPUF are classified as the turesurethaneduring the foamingThere were two typesof the rigid PUFspecimensusedandin thisstudy:bondspure materialsisthattheglassfibersareaddedduring(pure PUF) and glass-fiber-reinforced polyurethane foam (RPUF). The pure PUF and RPUF specimensmanufacturingin thefibersagentdecreasethe insulationperformancetheweremanufacturedby latter.addingThesea foamingto polyoland isocyanate,followedbutby increasemixing efore,RPUFwasusedforcontrolpurposestoblowing using a homogenizer. Both the pure PUF and the RPUF are classified as the same polymericdeterminevalidity of the restraintin thisstudy.Tablethe2 liststhe specimenfoamwith thethree-dimensionalnetworkconditionsstructures proposedand mensionswerecommonlyselectedintheformofacubeof50mm 50mm 50The difference between the two materials is that the glass fibers are added during tandard.latter. These fibers decrease the insulation performance but increase the strength performance againsta compressive load. Therefore, RPUF was used for control purposes to determine the validity of theTable 2. Test specimen properties. RPUF—reinforced polyurethane foam.restraint conditions proposed in this study. Table 2 lists the specimen properties; the dimensions werecommonly selected inMaterialthe form ofDimensiona cube of 50mm Mass50 mmmm according(mm)(g) 50Density(g/cm3) to the compressiontest standard.Pure PUF12.630.1150 50 50RPUF15.880.13Table 2. Test specimen properties. RPUF—reinforced polyurethane foam.3)Figure 4 is aMaterialmimetic diagramDimensionshowing (mm)an overviewMassof thisTheexperimentalset(g) experiment.Density(g/cmup consisted of aPureuniversalKyoungsung TestingPUF testing machine (UTM, KSU-5M,12.630.11Machine CO., LTD.,50 compressive50 50Anyang-si, Korea)RPUFfor the conventionaltest anda restraining jig 0.13installed at the central15.88point where testing was performed. The custom-built jig for the test method proposed in this studywas Figuremade ofsteel(SUS showing316) to preventdamagecausedby brittlenessin the cryogenic4 isstainlessa mimeticdiagraman overviewof thisexperiment.The temperaturechamber.consisted of a universal testing machine (UTM, KSU-5M, Kyoungsung Testing Machine CO., LTD.,Anyang-si, Korea) for the conventional compressive test and a restraining jig installed at the centralpoint where testing was performed. The custom-built jig for the test method proposed in this study wasmade of stainless steel (SUS 316) to prevent damage caused by brittleness in the cryogenic environmentcreated through the low-temperature chamber.

Materials 2019, 12, 2672Materials 2019, 12, x FOR PEER REVIEW7 of 207 of 20Figure 4. Mimetic diagram of test specimen and equipment.Figure 4. Mimetic diagram of test specimen and equipment.2.4. Experimental Scenarios2.4. Experimental ScenariosThe experimental scenarios of this study are shown in Table 3. Restraint conditions were set asexperimentalvariables tovalidateoftheproposedexperimentalmethod.The compressiveloadappliedThe experimentalscenariosthisstudy areshown in Table3. Restraintconditionswereset asperpendicularto the foamingdirectionof pure PUFand RPUFmethod.was the Thedisplacementforce.Then,theexperimental variablesto validatethe it of theloadwas setto be approximately5 kN,withthethevariationup to 85%the testperpendicularto thefoamingdirectionof pure PUF htto loaddeterminethetooverallfracture sectionof therigidaccordingstandardupper limitof thewas setbe approximately5 kN,withthe PUFvariationup toto85%of the ISOtest844[52]. Inheightthis study,quasi-staticwas performedandtheloadPUFspeed,i.e., the tostrainrate, wasspecimento determinethe analysisoverall fracturesection oftherigidaccordingstandardISOapplieds 1 referringto theanalysisspecificationand previousdata speed,[32–34].i.e.,Thetemperature844 [52].atIn0.0017this study,quasi-staticwas performedandstudythe loadthestrain rate, C) 1conditionswereatdividedcases: toroom(25andC) andcryogenictemperature( 163 Thewas applied0.0017 intos dydata [32–34].consideringenvironmentthe use intoof insulation.In roomthe caseof cryogenictemperature,the testtemperaturetheconditionswerefordividedtwo cases:temperature(25 C) and cryogenic C through the incoming liquid nitrogen controlled outside the chamber.specimenwasexposedto 163temperature ( 163 C) considering the environment for the use of insulation. In the case of cryogenicThetest was conductedafter a pre-coolingfor approximately2 h, satisfyingthe thermalequilibriumtemperature,the test specimenwas exposedto 163 C throughthe incomingliquidnitrogenstateof the outsidespecimentochamber.reduce thedeviationthe results afteraccordingto the exposuretime. For morecontrolledtheThetest wasofconducteda pre-coolingfor approximately2 h,precisemeasurements,experimentswereper casethedeviationstandard. of the resultssatisfyingthe thermalfiveequilibriumstateof repeatedthe specimento basedreduceontheaccording to the exposure time. For more precise measurements, five experiments were repeated perTable 3. Compressive test scenario.case based on the standard.ConventionalMaterialMaterialPure PUFRPUFPure PUFRPUFTemperatureTable 3. Compressive test scenario.( C)Strain Rate(s 1 )ConventionalRoomCryogenic(1 h)Temperature ( C)(s 1)25 163 Strain Rate0.0017Room Cryogenic (1 h) 25 163 0.0017 3. Results and Discussion3.1. Shape Structure AnalysisRestraintTemperature ( C)Strain Rate(s 1 )RestraintRoomCryogenic(1 h)Temperature ( C)Strain Rate(s 1)25 1630.0017Room Cryogenic (1 h) 25 163 0.0017

Materials 2019, 12, x FOR PEER REVIEW8 of 20Materials2019, 12, 26723.1.1. Conventional8 of 20Compressive TestFigure 5 shows the shape of the rigid PUF specimen ((a) pure PUF and (b) RPUF) afterperformingstaticcompression in accordance with the conventional test standard. In the existing tests,3.Results andDiscussionthe results for the two specimen types were observed to expand deformation on the sidewall as a3.1.Shape StructureAnalysiscompressionforce wasapplied because no interference factors were considered in the vicinity of thetest specimen from external conditions. As shown in a previous study, as the compression3.1.1. Conventional Compressive Testdeformation to the plastic section progressed, it was found that the crack advanced on the sidewall5 shows theshape ofoftherigid PUF specimen((a) pure(b) RPUF)aftercellperformingof theFiguretest specimenregardlesstemperature[53]. The reasonforPUFthisandfailureis that thestructurestaticin hasaccordancewith the conventionaltest standard.In the existingtests, theunderresultsainsidecompressionthe rigid PUFa compressibilitythat can reducea certain portionof the volumefortwospecimentypes weredeformationon limitthe sidewallas a compressionloadthe[54].Whenthe uniaxialload observedcontinues toto expandwork beyondthe elastico

for assessing the mechanical properties of rigid PUF from critical hazards. The dimensions of the test specimen required for each test method are summarized in Table1. 2.1.1. Tensile Test A tensile test was performed according to the ASTM D 1623 standard. The recommended dimensions of the test specimen are shown in Figure2a.