Biomedical Engineering Reference
In-Depth Information
FGM are often considered in thermomechanical applications, such as ceramic-metal
composite structures where a higher ceramic concentration at the surface exposed to
an elevated temperature or heat flux may be beneficial. As most biological applications
serve in a narrow temperature range, we intentionally exclude thermal FGM problems
from this chapter. The chapter introduces the concept of FGM, demonstrates a mathe-
matical approach to their analysis, and presents a review of representative modern
studies. Finally, we illustrate an example where grading the facings of a sandwich panel
results in an impressive improvement in its stability and vibration characteristics.
2.2
Introduction to FGM
FGM were first suggested in Japan in the mid-1980s for thermal barrier coatings in
aerospace applications. Since their inception, these materials have experienced a
rapid development and found many applications in various fields of engineering.
There are a number of reviews outlining the development of FGM, including topics
[ 1 , 2 ], review articles [ 3 ], and proceedings of conferences dedicated to the field of
FGM [ 4 , 5 ]. The areas where FGM offer potential improvements and advantages in
engineering applications include a reduction of in-plane and transverse through-the-
thickness stresses, prevention or reduction of delamination tendencies in laminated
or sandwich structures, improved residual stress distribution, enhanced thermal
properties, higher fracture toughness, and reduced stress intensity factors.
Typical FGM architectures employ a variation of volume fractions of constituent
materials in one or several directions. For example, a one-dimensional variation of
mineral and metal phases can be employed to improve fracture toughness of
prosthesis joints [ 6 ] and improve their mechanical strength, while retaining neces-
sary biocompatibility as schematically shown in Fig. 2.1 [ 7 ]. Another example of a
one-dimensional functional grading is found in sandwich panels where a function-
ally graded core (e.g., [ 8 ]) or a graded facing-core interface can improve fracture
characteristics, and in particular, prevent debonding between the facings and core.
Besides a through-the-thickness variation of material volume fractions, the
grading and corresponding property tailoring can be achieved through an in-surface
variable volume fraction and through a coordinate-dependent orientation and sizing
of fibers in a composite material. This type of grading is essentially biomimetic; for
example, collagen fibers vary their orientation across the tendon-to-bone insertion
site as is discussed below. Bamboo is an interesting example of a natural FGM
where both the number and shape of fibers vary from the inner to outer periphery,
i.e., the fibers at the outer periphery have a nearly circular cross section and a higher
density, while their counterparts in the inner section have an ellipsoidal cross
section and a lower volume fraction [ 9 ]. Besides grading the volume fraction
and/or shape of constituent materials, FGM include materials with graded porosity
that can be applied in such dissimilar components as hard tissue implants and diesel
engine filters [ 10 ].
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