Thermal/Mechanical Response and Damage Growth in Polymeric Composites at Cryogenic Temperatures

AIAA-2002-1416THERMAL/MECHANICAL RESPONSE AND DAMAGE GROWTH INPOLYMERIC COMPOSITES AT CRYOGENIC TEMPERATURES 1KaJcen S. Whitley t and Thomas S. Gates*NASA Langley Research CenterHampton, VA 23681AbstractIn order to increase the reliability of the nextgeneration of space transportation systems, themechanical behavior of polymeric matrix composite(PMC) materials at cryogenic temperatures must beinvestigated. This paper presents experimental data onthe residual mechanical properties of a carbon fiberpolymeric composite, IM7/PETI-5 both before and afteraging at cryogenic temperatures. Tension modulus andstrength were measured at room temperature, -196°C,and -269°C on five different specimen ply lay-ups,[0] 12, [90112, [-+4513s, [-+2513s and [45,903,-45,03,-45,903,45]. Specimens were preconditioned with oneset of coupons being isothermally aged for 555 hours at-184°C in an unloaded state. Another set ofcorresponding coupons were mounted in constantdisplacement fixtures such that a constant uniaxial strainwas applied to the specimens for 555 hours at -184°C.The measured lamina level properties indicated thatcryogenic temperatures have an appreciable influenceon behavior, and residual stress calculations based onlamination theory showed that the transverse tensile plystresses could be quite high for cryogenic testtemperatures. Microscopic examination of the surfacemorphology showed evidence of degradation along theexposed edges of the material due to aging at cryogenictemperatures.IntroductionThe National Aeronautics and Space Administration(NASA) has recently initiated the Space LaunchInitiative (SLI) program that will, in part, advance somet Aerospace Engineer, Mechanics and DurabilityBranch.* Senior Materials Research Engineer, Mechanicsand Durability Branch. Associate Fellow, AIAA.1 Copyright 2001 by the American Institute ofAeronautics and Astronautics, Inc. No copyright isasserted in the United States under Title 17, U.S. Code.The U.S. Government has a royalty-free license toexercise all rights under the copyright claimed hereinfor Government Purposes. All other rights axe reservedby the copyright owner.of the key technologies required for the next generationof launch vehicles. This next generation of spacetransportation systems may require both reusablelaunch vehicles (RLV's) and expendable launchvehicles (ELV's) to satisfy mission requirements. Oneof the key technologies identified for RLV's andELV's has been the reduction in structural weightthrough the use of advanced materials andmanufacturing methods. This reduction in structuralweight must be tempered against the increaseddemands on performance, damage tolerance, andlifetime durability.One potential source for structural weight reductionis the replacement of traditional metallic cryogenic fueltanks with polymeric matrix composite (PMC) tanks.The interest in design of polymeric composite,cryogenic-fuel tanks for launch vehicles goes backseveral years and includes research associated with theNational Aerospace Plane (NASP), single-stage-to-orbit (SSTO) vehicles [1]-[3], and other launch vehicleapplications [4]. For internal and external tanks, designof the tank may take the form of externally stiffenedshells of PMC material or thin-walled sandwich shellconstructed with lightweight core and PMC facesheets.Regardless of the design, the PMC-based tanks will berequired to safely carry pressure and flight loads andoperate over temperatures that may range from -250°Cto +120°C. From a durability perspective, the primaryperformance criteria of the PMC material is to retainmechanical properties within allowable limits over thelife-time of the tank while minimizing loss ofcryogenic fuel due to permeation or leakage throughthe tank wall.Aside from their use in space launch vehicles, therehave been very few applications of PMC's as structuralmaterials in cryogenic environments. Consequently, asearch of the available literature provides limitedexperimental data on the mechanical properties ofpolymers or PMC's operating at cryogenictemperatures. For amorphous and crystalline polymericmaterials, Perepechko [5] provides details on a numberof non-mechanical properties including thermalexpansion, thermal conductivity, as well as viscoelasticor dynamical mechanical properties. From thesestudies, it was clear that many of the polymerproperties are not linear with respect to temperatureAmerican Institute of Aeronautics and Astronautics