Mehran Tehrani Marwan Al-Haik 1 e-mail: alhaik@vt.edu Department of Engineering Science and Mechanics, Virginia Tech, Blacksburg, VA 24061 Hamid Garmestani School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332 Dongsheng Li Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, MSIN K7-90, Richland, WA 99352 Effect of Moderate Magnetic Annealing on the Microstructure, Quasi-Static, and Viscoelastic Mechanical Behavior of a Structural Epoxy In this study, the effect of moderate magnetic fields on the microstructure of a structural epoxy system was investigated. The changes in the microstructure have been quantita- tively investigated using wide angle X-ray diffraction (WAXD) and pole figure analysis. The mechanical properties (modulus, hardness, and strain rate sensitivity parameter) of the epoxy system annealed in the magnetic field were probed with the aid of instru- mented nanoindentation, and the results are compared to the reference epoxy sample. To further examine the creep response of the magnetically annealed and reference sam- ples, short 45 min duration creep tests were carried out. An equivalent to the macroscale creep compliance was calculated using the aforementioned nanocreep data. Using the continuous contact compliance (CCC) analysis, the phase lag angle, tan (d), between the displacement and applied force in an oscillatory nanoindentation test was measured for both neat and magnetically annealed systems through which the effect of low mag- netic fields on the viscoelastic properties of the epoxy was invoked. The comparison of the creep strain rate sensitivity parameter, A/d(0), from short term(80 s), creep tests and the creep compliance J(t) from the long term (2700 s) creep tests with the tan (d) sug- gests that former parameter is a more useful comparative creep parameter than the creep compliance. The results of this investigation reveal that for the epoxy system cured under low magnetic fields both the quasi-static and viscoelastic mechanical properties have been improved. [DOI: 10.1115/1.4005406] 1 Introduction Due to their excellent mechanical properties, compared to ther- moplastics, epoxies are used extensively as matrices in the fabri- cation of fiber reinforced composites both at the microscale using microscale carbon fibers [1] and nanoscale using carbon nano- tubes [2,3]. The mechanical and physical properties of epoxies are strongly influenced not only by the morphological structure of the epoxies [4] but also by their molecular deformation and orienta- tion of chains [5]. Improving these properties can be achieved by manipulating the spatial arrangement of the molecules at the nanoscale via self-organization of the chains [6] or by introducing alignment via an external field [7]. Chain alignment can be induced by flow (shear) field’s meth- ods, such as extrusion [8] and injection molding [9]. Both of these techniques produce morphologies that comprise an oriented shell of the polymer with a relatively disoriented inner core. This uneven shell-core orientation is attributed to rheological and interfacial properties of the constituent components (such as shear viscosity, fluid elasticity, interfacial tension, and blend composition) and processing variables (such as temperature and shear rate) [10]. Since the use of shear flow fields poses several limitations on the types of epoxies that can be processed via this route, several investigations explored the use of alternative fields such as electri- cal and magnetic fields or even a combination of both [11–13]. The principle of using both electrical and magnetic fields relies on orienting liquid crystalline monomers in an external field and sub- sequently locking the orientation by polymerization and curing, either thermally, chemically, or photochemically. Obviously, the macroscopic ordering might be irreversibly fixed by the chemical reaction, resulting in texture generated anisotropy in the physical and mechanical properties [14]. Shiota and Ober [15] investigated the curing of liquid crystalline epoxies under applied ac electric fields. The investigators observed that the extent of polymerization has strongly influenced the molecular response to the ac electric fields and that the final net- works could be aligned only perpendicular to the electric field. The oriented network possessed high orientation as well as high transla- tional order. Controlling the orientation of organic molecules with electric fields plays an important role in liquid crystal displays. Magnetic field induced-alignment of polymeric materials has been the focus of numerous investigations [16–20]. Unlike the flow fields, magnetic fields effective strength does not decrease in the center of the sample, and thus the formation of the core-shell morphology is unlikely. Polymeric materials can reorient inside a magnetic field due to diamagnetic anisotropy of the constituent chemical units, pro- vided that the magnetic anisotropy and/or the size of the mole- cules is sufficiently large [7]. The energy that the chemical unit (monomer or chain) gains through the interaction with an external magnetic field depends on the orientation of the unit relative to the magnetic field, and hence the unit tends to align in a direction that would minimize its energy [21]. The tendency of a molecular unit to align along the field direction is suppressed by the thermal agitation, i.e., if the energy reduction due to alignment cannot compensate the energetic penalty arising from the expenditure of thermal energy. This is the case for nonliquid crystalline polymers in melts and solutions. 1 Corresponding author. Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received March 10, 2011; final manuscript received July 29, 2011; published online December 14, 2011. Assoc. Edi- tor: Hussein Zbib. Journal of Engineering Materials and Technology JANUARY 2012, Vol. 134 / 010907-1 Copyright V C 2012 by ASME Downloaded 23 Oct 2012 to 128.173.167.237. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm