Mechanics and Physics of Functionally Graded Materials
Glaucio H. Paulino, Associate Professor
Burton and Erma Lewis Faculty Scholar
Department of Civil and Environmental Engineering
University of Illinois at Urbana-Champaign
Date: Tuesday, November 6, 2001
Time: 3:30 p.m.
Place: W183 Nebraska Hall
Functionally graded technology refers to techniques to functionally grade the mechanical, electronic, chemical, optical, and/or biological material properties. This presentation focuses on the effects of introducing compositional gradients in engineered materials from a thermo-mechanical point of view with special emphasis on fracture mechanics.
For metal/ceramic functionally graded materials (FGMs), cracks originating in the brittle side should be prevented from catastrophic failure by increasing fracture toughness as the material increases in metal content. These properties have been demonstrated in a titanium/titanium monoboride (Ti/TiB) FGM for which an average resistance curve (R-curve) has been obtained. Fracture testing of ductile/brittle FGMs single edge notched beams (SENB) with the starting notch in the ceramic-rich region requires the initiation of a sharp short crack at a specified desired location in the brittle region. Thus, a novel technique is presented to generate sharp cracks in metal/ceramic FGMs by reverse 4-point bending.
An investigation is made of FGMs that are elastically homogeneous but thermally nonhomogeneous. A transient thermal stress analysis for an edge crack in an FGM is performed. A multilayered material model is used to solve the temperature field. By using the Laplace transform and an asymptotic analysis, an analytical first order temperature solution is obtained. Thermal stress intensity factors are obtained for ceramic/ceramic FGMs such as TiC/SiC and MoSi_2/SiC.
Modeling of crack problems in ceramic/ceramic FGMs using gradient elasticity is also addressed in this presentation. The crack boundary value problem is solved by means of the Fourier transform and hypersingular integral equations of the Fredholm type. The solution naturally leads to a cusping crack. Examples comparing the results of various types of material microstructure (described by characteristic lengths) and gradation (described by moduli functions) are provided.
