Experimental investigation of the thermal properti

Authors: Craig A Steeves Chris Mercer Emilio Antinucci Ming Y He Anthony G Evans

Publish Date: 2009/03/17

Volume: 5, Issue: 2, Pages: 195-202

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Abstract

Composite bimaterial lattice structures which possess both low tailorable thermal expansion and nearly optimal stiffness have been proposed for applications which require high structural stiffness in environments which include large temperature fluctuations such as the surfaces of highspeed aerospace vehicles An experimental validation of the thermal properties of these lattices when they are constructed of practical materials with easily manufactured bonded joints is contained herein Bonded lattices comprising aluminum and titanium alloys have been manufactured with pressfit dovetail joints and tested in a variety of thermal environments Results for equilibrium heating rapid transient heating and thermal cycling leading to shakedown are presented and shown to be consistent with theoretically and numerically attained resultsIn structural systems that experience large temperature changes and thermal gradients the associated strains are a significant impediment to successful design and implementation The large thermal strains result in excessive geometric changes and substantial thermal stresses when hightemperature components are connected to lower temperature structures with smaller thermal strains The consequences include failure by yielding fracture or fatigue as well as the formation of gaps that require sealing and extreme forces on attachments to other structuresMultimaterial lattices have been proposed as a solution to these issues By combining two or more materials with empty space composite lattice structures can be manufactured to have a thermal expansion coefficient which is tailored to the application and which can be zero or even negative Lakes 1996 Sigmund and Torquato 1996 Gibiansky and Torquato 1997 and Jefferson 2006 have all proposed lattice structures with these properties However the Lakes and Jefferson lattices are both bendingdominated structures see Deshpande et al 2001 and are consequently very compliant while the structures proposed by Torquato and his collaborators are biaxially but not uniaxially stiff but very difficult to manufacture More recently Grima et al 2007 demonstrate a lattice material which is stiff but has highly anisotropic thermal propertiesA key aspect of this lattice design is its high overall stiffness compared to other lattices with low thermal expansion The lattice described here is stretchingdominated see Deshpande 2001 and hence is approximately one order of magnitude stiffer than comparable bendingdominated structures Gibiansky and Torquato 1997 developed a theoretical bound for the biaxial stiffness for bimaterial lattices which have zero thermal expansion The structural performance of this lattice lies close to the theoretical bound and hence the lattice is nearly optimally stiff see Steeves et al 2007a for detailsThese lattices can be constructed using many material options including hightemperature refractory alloys These form the basis for application of the concept to the hot exterior faces of a thermal protection panel which combines bending stiffness with thermal resistance while reducing overall mass Steeves et al 2007b In such panels the stationary nodes Fig 1 allow the lattice to be connected to cool vehicle structures without generating thermal stresses The results in Fig 2 have been derived for a pinjointed lattice and verified experimentally for such a system In practice because of manufacturing limitations the lattices are bonded at the nodes Such bonding increases the effective lattice CTE from the ideal values for pinned systems The focus of the present article is on the behavior of a system with bonded nodes

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