Memoirs of the Faculty of Engineering, Kyushu University, Vol.66, No.1, March 200655Study of Cryogenic Mechanical Strength and Fracture Behavior of Adhesivesfor CFRP Tanks of Reusable Launch VehiclesbyTakayuki SHIMODA*, Jianmei HE** and Sigeru ASO***(Received February 20, 2006)AbstractBonding strengths of three types of adhesives were evaluated at differenttemperatures. The effects of temperature and adhesive thickness on the strengthwere clarified from the results. It concluded that EA9394 cured at roomtemperature is the overall best performing adhesive for CFRP applications atcryogenic temperatures. The initial failure mode of double-lap specimens wasalso discussed from the results by strain/stress analysis. And also the cryogenicfracture performance of bonded structure between CFRP using AF163-2K isexamined. The tensile Mode-I and shear Mode-II fracture strengths ofCFRP/AF163-2K bond joints are evaluated at room temperature and cryogenictemperature. The results indicate that both cryogenic Mode-I and Mode-IIfracture strengths of CFRP/AF163-2K bond joints dropped considerably. Themismatch of coefficients of linear expansion between CFRP and AF163-2K isconsidered to be the primary reason. On the other hand, feasibilityinvestigations of CFRP composite as applied to unlined CFRP cryogenic tankwas conducted. Reflecting the results of the previous two phase tests on smallprototype filament-wound tank conducted at room temperature and cryogenictemperature, the improved third phase of FW tank test was conducted and theresult show the feasibility of CFRP tank and identified the problem to beresolved.Keywords: Filament winding, CFRP composite, Propellant tank, Adhesive,Cryogenic, Tensile Strength, Shear Strength, FEM1.IntroductionIn the development of low-weight CFRP (carbon fiber reinforced plastics) liquid-propellanttank systems and CFRP structure for the realization of Reusable Launch Vehicles, not only the******Associate Senior Engineer, Japan Aerospace Exploration AgencyAssociate Senior Researcher, Japan Aerospace Exploration AgencyProfessor, Department of Aeronautics and Astronautics
56T. SHIMODA, J. HE and S. ASOcryogenic performance of adhesive bonds but also the cryogenic performance of adhesive itselfbecomes very important from viewpoints of the safety and durability. This is because the liquidpropellants like liquid hydrogen (LH2) and liquid oxygen (LOX) have to be stored at very lowtemperatures: below –196 C for LH2 and near LHe temperature at -269 C for LOX. So far not somany studies on the cryogenic performances of adhesive bonds and of the adhesive itself have beendone yet, especially for lower temperature CFRP applications.In 1991 Goeders and Perry1) investigated 24 types of adhesives for bonding between twoadherend materials of Peek/IM-6, titanium and IM-7/8551-7(Graphite/Epoxy). They recommendedusing FM300 and EA9394 adhesives for metallic bonding as a result of their cryogenicinvestigations. A NASA contractor report by David E. Glass2) in Analytical Services & Materials,Inc., Hampton, Virginia, concludes that the HT435, the 340oF cure adhesive, was the superioradhesive at room temperature. The report also pointed out that EA9394, Crest3170 and PR1664 allworked for bonding stainless steel down to liquid helium temperature at -269 C. Another report ofthe work performed by Materials Research & Engineering, Inc. for McDonnell Douglas SpaceSystems, Co.3) indicated that EA9394 is the overall best performing adhesive for metallicapplications in the lower temperature range and has the usable temperature range between -253 Cand 204 C. Almost all of these investigations concluded the good results of adhesive bond strengthfor metal material applications at very lower temperatures.In this study, three types of adhesive materials, FM300, HT435 and EA9394, wereexperimentally investigated through the double-lap tensile specimen as applied to two types ofCFRP adherend materials, which we called the first test series. From the experimental results of thefirst test series, HT435 and EA9394 adhesives both showed relatively high cryogenic performancewhen bonded with different CFRP adherends. Because HT435 is an adhesive cured at a hightemperature 170 C (443K), it’s not desired for CFRP application. Then the room temperature cureadhesive EA9394 is chosen as the most promising adhesive material for CFRP cryogenicapplications in our study. And also, finite element modeling of the double-lap tensile specimen wasexecuted to evaluate the bond thickness, bending effects on the strength measurements of adhesivebonds and to examine the failure modes of the double-lap tensile specimens.On the other hand, controlling the cryogenic tensile Mode-I and shear Mode-II fractureperformance of adhesively bonded joints between composite materials is very important for thesafety and durability of low-weight composite tanks of liquefied propellants for launch vehiclesdue to the cryogenic temperatures. The substantial thermal stress-strain caused by the mismatch ofthermal expansion ratios between adhesives and composites can lead to fracture strengthdeterioration of multiphase bonded structures, particularly at very low temperatures such as thoseused in cryogenics.The cryogenic tensile Mode-I and shear Mode-II fracture behaviors of structures adhesivelybonded between CFRP laminates using structural adhesive film AF163-2K were examined in thisstudy. The structural adhesive film (AF163-2K) offers advantages, including superior resistance tohigh-moisture environments before and after curing, high fracture toughness and peel strength.Lastly, the feasibility investigations4)5)6)7)8) of CFRP composite as applied to unlined CFRPcryogenic propellant tank for reusable vehicle systems are described. Two phases of pressurizationtests on small prototype filament-wound (FW) tanks with 300mm diameter were conducted fromroom temperature to liquid nitrogen temperature in our previous studies. Based on the resultsreflection of the above-mentioned first two phases of pressurization tests, the third phase FW tankwas manufactured and tested to resolve the remaining problem.
Study of Cryogenic Mechanical Strength and Fracture Behavior of Adhesives for CFRP Tanks of RLVs2.57Mechanical Characteristics Comparison between FM300, HT435 and EA93942.1 The first test series for the three adhesives2.1.1Test configurationsIn the first two test series, two kinds of CFRP flat samples (IM7-UD for first test series andT300-UD for second test series) were used as adherend materials respectively. Three types ofadhesives, FM300, HT435 and EA9394, were investigated as applied to these CFRP adherends. Asknown FM300 and HT435 are commercial epoxy film adhesives with fixed bond thickness at0.2mm, while EA9394 is a two-part structural paste adhesive cured at room temperature and thebond thickness need to be controlled as desired. The double-lap tensile specimen shape and size forbond strength measurements in the first two test series is shown in Fig. 1. For both test series, theCFRP adherend thickness was fixed at 2.0mm.In the first test series with IM7-UD adherends, the strength measurements of adhesive bondswere measured at room temperature and -150 C of cryogenic temperature to assess the strengthperformance of adhesive bonds at LH2 stored temperature. Liquid nitrogen was utilized to cooldown the test environment at -150 C for cryogenic temperature measurements. For EA9394adhesive specimens, the bond thickness was uniformly controlled at 0.1mm.In the second test series with T300-UD adherends, additional strength measurements were alsoexecuted at liquid helium temperature of -269 C to assess the strength performance of adhesivebonds at LOX stored temperature. In this test series, threads with 0.1mm diameter were used forcontrolling the bond thickness of EA9394 specimens to minimize the unevenness. Because theliquid helium is very difficult to be stored, special experimental equipment for liquid heliumevaluation was introduced as shown in Fig. 2. The specimen length for all adhesives in the secondtest series was changed as shown in Fig. 1 to fit the size requirement for cryostat shown in Fig. 2.Fig. 1 Double-lap specimen in the first two test series.Fig. 2 Cryogenic experiment.The helium test equipment includes the test vacuum chamber (cryostat) with the test specimenloaded inside the Instron machine; the liquid helium container and a helium gas pump to push theliquid helium into the cryostat. The liquid helium will begin to be stored inside the cryostat afterthe vacuum chamber is cooled down to the liquid helium temperature at -269 C. The double-laptensile tests were started after the vacuum chamber was full-filled with the liquid helium. In theliquid helium testing, the pressure of helium gas pump was controlled during the cooling process ofthe cryostat in order to minimize the waste of liquid helium and reduce cost.
58T. SHIMODA, J. HE and S. ASO2.1.2Test resultsThe bond strengths of three types of adhesive materials as applied to two different CFRPadherend materials under different temperatures including cryogenics are experimentally obtainedfirstly. The results of bond strength are shown in Fig. 3 and in Fig. 4 for the first two test series,respectively. ２５ １５０ 40.0Fracture Stress(MPa)Fracture Stress (MPa)35.030.025.020.015.010.05.00.025 -150 -269 FM300HT A9394Fig. 3 Bond strength results of the first test series. Fig. 4 Bond strength results of the second test series.From these two figures, one can see firstly that the strength results of FM300 adhesive bond atsecond test series is almost half of the results from the first test series, which implies the bondstrength of FM300 is very sensitive to the CFRP adherends. For HT435 and EA9394 adhesives thefirst two test series show relatively the same results of bond strength for different CFRP adherends.It is seen that the bond strength of HT435 and EA9394 adhesive does not get affected by the CFRPadherend like FM300 does.Secondly one can see that both HT435 and EA9394 adhesive bonds show relatively higherperformance at cryogenic temperatures as applied for different CFRP adherends. Because HT435 isan adhesive cured at a higher temperature about 170 C (443K), EA9394 adhesive cured at roomtemperature might be then chosen here as the more promising adhesive material for CFRPcryogenic application. More detail mechanical behaviors of EA9394, such as tensile and shearelastic modulus, tensile and shear strength etc., need to be clarified with different bond thickness atdifferent temperatures including the cryogenic temperatures.2.1.3DiscussionsIn the first two test series, the bond strength results of all the adhesives studied here, especiallyfor EA9394 adhesive, showed large deviations at different temperatures. Different failure behaviorsat the failed surfaces are also observed from the double-lap specimens of different adhesives atdifferent temperatures. To explain these results, the effects of adhesive bond thickness on the bondstrength measurements are first discussed by comparing the strength result of EA9394 to thestrength results of FM300 and HT435 adhesive bonds. And the failure behavior of double-laptensile specimen around the adhesive/adherend bond surfaces is also discussed through theinspection of the failed surfaces.Bond thickness effects on strength measurement: Generally speaking, the bond strength is verysensitive to all the adhesive bond thickness. Compared to commercial film adhesive FM300 andHT435, EA9394 is a two-part structural paste adhesive and it’s difficult to control the bondthickness, which means un-uniform bond thickness distribution might be easily appeared in thelongitudinal and width directions of EA9394 double-lap specimen and the un-uniformity will causebending and/or torsions which will effect the bond strength measurements. In the first test series, noattention was paid in the bond thickness of EA9394 and in the second test series we tried to control
Study of Cryogenic Mechanical Strength and Fracture Behavior of Adhesives for CFRP Tanks of RLVs59the EA9394 adhesive bond thickness at 0.1mm by using of threads with 0.1mm diameter. ForFM300 and HT435 adhesive bonds, relatively small unevenness in the strength measurementresults was thought coincidence with relatively uniform bond thickness because of these filmadhesives.Failure surface inspection for CFRPspecimen: Because larger unevenness occurredin the strength results of the first two test seriesespecially for EA9394 adhesive bond at roomtemperature, a typical failure surface pairs forEA9394 double-lap specimen at roomtemperature is shown in Fig. 5.From this figure we can see thatasymmetry of the failed surface pairs isEA9394observed for EA9394 double-lap specimen,which is not desired due to the symmetry of(5a)(5b)double-lap specimen. The asymmetry wasFig. 5 Failure surfaces of EA9394 specimen.mainly considered as the results of possiblebending and/or torsion.On one of the failed surface pair (6a), part of EA9394 adhesive bond was left on the same twosides of the bond surface, which implies that the failure initiation occurred inside EA9394 adhesivebond layer, instead of the interface between adhesive and adherend. On the other failed surface pair(6b), EA9394 adhesive is partially peeled off from one side and bonded to another side of the bondsurface, which means the failure initiation occurred from the interface between EA9394 and CFRPadherend. The same phenomena can hardly be found from the results of EA9394 at cryogenictemperatures.2.2 The second test series for EA9394 adhesive2.2.1Test configurationsBased on the bond strength results of the first two test series described in the previous chapter,HT435 and EA9394 adhesives showed relatively higher performance of bond strength as applied totwo kinds of CFRP adherends at both room temperature and cryogenic temperatures. Because ofthe necessary higher temperature condition at 170 C for cure processing of HT435, EA9394 ischosen as the most promising adhesive material for CFRP cryogenic application in our study. Moredetail investigations to understand EA9394’s mechanical behavior like elastic modulus, Poisson’sratio, tensile and shear strength etc at different temperatures need to be done.As the third test series in this study, the shear strength of EA9394 adhesive itself was evaluatedthrough ASTM standard D3528-969). The double-lap tensile specimen shape and size are based onthe ASTM standard and also designed to fit the cryostat for liquid helium testing as shown in Fig. 6.Two types of EA9394 double-lapT1AT2specimen with different bondBT1thicknesses (0.1mm and 0.2mm) aremanufactured by Japan Mold System,CDC 20.0Inc. The aluminum (AL/5020)25.025.043.543.5Ladherend material is used following87.0 L137.0 Lthe recommendation of ASTMstandard. In this series of tests, theT1: 1.0 mm; T2: 2.0 mm; L: 12.7 mmtesting temperature is also changedFig. 6 Double-lap specimen based on ASTM standard.
60T. SHIMODA, J. HE and S. ASOfrom room temperature to two of the cryogenic temperatures: -150 C near LH2 stored temperatureand LHe temperature –at 269 C near LOX stored temperature.Shear Strength (MPa)20.0RT-150 -269 16.012.08.04.00.0ASTM StandardFracture Shear Stress (MPa)2.2.2Test ResultsAs one part of more detail studies, investigations on the shear strength of EA9394 adhesive arealso reported as the third test series of this study here. Two type of EA9394 double-lap specimenwith different bond thicknesses (0.1mm and 0.2mm) are manufactured from Japan Mold System,Inc. Because experimental measurements are still now under way, only the shear strength results of0.1mm EA9394 adhesive bond were reported here as shown in Fig. 7 and the comparison of 0.1mmthick EA9394 adhesive bond strength of all the test series at different temperature is then shown inFig. 8.40.0RT-150 -269 32.024.016.08.00.0AliminumT300-UDIM7-UDFig. 7 Shear strength results of 0.1mm EA9394 adhesive. Fig. 8 Comparison of EA9394 bond strength.2.2.3DiscussionsFirstly from Fig. 7 one can see that the shear strength of EA9394 adhesive doesn’t change verymuch at different temperatures. This is different from the bond strength results of first two testseries. The uncertainty of the strength results at this test series is also relatively smaller than at thefirst two test series for EA9394 adhesives. Secondly by comparing the results of EA9394 as appliedfor different adherend materials shown in Fig. 8, one can also conclude that the bond strength ofEA9394 cryogenic temperatures show relatively stable results.3.Parametric Shear Strength Study with Different Thickness for EA93943.1 Parametric shear strength testsTo evaluate the effect of thickness, shear strength of EA9394 adhesive were measured fromroom temperature to cryogenic temperature based on ASTM standards.At first the shear strengths of EA9394 adhesive were evaluated with four types of bondthickness (0.1mm, 0.2mm, 0.5mm and 1.0mm) based on ASTM standard D3528-969) (Double-laptensile test) from room temperature to cryogenic temperatures. The double-lap tensile specimenwith shape and size as shown in Fig. 6 are introduced following ASTM standard and also designedto fit the cryostat for liquid helium testing. Aluminum (AL/5020) is used as adherend materialfollowing the recommendation of ASTM standard.Figure 9 shows the failure loads obtained from above-mentioned EA9394 double-lap tensiletest at different temperatures. First of all one can see that the shear strength of EA9394 adhesivedecreased with the bond thickness increased. This might be considered because of the qualityinferior caused by the thicker bond. Secondly form these results we can see that the cryogenic shear
Study of Cryogenic Mechanical Strength and Fracture Behavior of Adhesives for CFRP Tanks of RLVs61strength of EA9394 adhesive almost doesn’t change so much compared with the room temperatureresults, and for thicker bond the cryogenic shear strength becomes higher.Shear Strength Testing10000t 0.1mm9000t 0.2mmt 0.5mmt 1.0mmFailure Load (N)800070006000-160 5000-160 4000LHe3000-160 -160 LHe20 20 20 20 200010000Fig. 9 Shear Strength of Bulk EA9394 Adhesive at Different Temperatures.3.2 Finite element model analysisThe effects of bond thickness and bending on the bond strength measurements are investigatedthrough the stress analysis to validate the double-lap tensile specimen for shear strengthcharacterization.The finite element analysis (FEA) is employed using the commercial soft package ANSYS toexamine the effects of bond thickness and bending on the bond strength measurements, whichmight be induced from the misalignment of double-lap specimen during experiments. 2D planestrain approach is introduced and the full specimen is modeled as shown in Fig. 10 because of theasymmetric loading-boundary conditions of the double-lap tensile test. x yFig. 10 Finite element modelingof double-lap specimen.Fig. 11 Equivalent stress distribution.The necessary input material properties are shown in Table 1 and the model sizes are used asthe same as experimental specimens of the third test series based on the ASTM standard. The finiteelement mesh is refined near the adhesive/adherend interfaces due to the existence of stressconcentrations and free-edge effects. The EA9394 adhesive bond thickness is changed from0.05mm to 0.3mm for parametric investigations. Displacement in thickness direction (Dy) wasapplied to assess the possible bending effects on the stress distributions of double-lap tensilespecimen.Figure 11 shows the typical equivalent stress distribution around the bond area of double-laptensile specimen without any bending. From here we can see that stress concentration occurred atthe interfaces between adhesives and adherend faces and these stress concentration may effect onthe initiation failure mode.Because different maximum stress component corresponding to different initi