The Challenge of Attaching Dissimilar Materials - Structural Interfaces and Attachments in Biology - page 14

A possible enhancement of the concept of functionally graded interfaces
between dissimilar materials suggested here is an in-plane graded interface or an
interface graded in two or three dimensions. In-plane grading in such interfaces
may be designed to deflect or arrest a crack.
It is noted that fracture referred to in the above-referenced studies is concerned
with cracks propagating in continuous materials and interfaces. The problem is
different if the material is discontinuous, e.g., it consists of a family of fibers
interconnected by discontinuous “bridges.” For example, evidence exists that
collagen fibers in tendons are discontinuous, requiring connections between fibers
for load transfer [ 25 ]. These connections may involve chemical cross-links between
molecules, bridging proteins between fibers (e.g., decorin proteoglycans), or min-
eral platelets large enough to cross from one fiber to another. Such a model
complicates the analysis of failure; classical fracture analysis approaches in this
case should be replaced with a nano- or micro-mechanical stress analyses that trace
the degradation of connections between adjacent fibrils.

1.4 Conclusions

An outline of typical attachments of dissimilar materials in engineering and biology
was presented, demonstrating a remarkable diversity of attachment mechanisms.
One general conclusion that can be made from the studies discussed above relates to
the advantages of graded interfaces between dissimilar materials. Interfacial
gradings in engineering and biology reduce local stress concentrations, enhance
the toughness of the attachment, and reduce fracture trends. When designing an
optimum interface between two materials, one should be aware of material
limitations. Mechanical properties should be derived from mechanical analyses
based on the distribution and geometry of constituent materials forming the inter-
face. This understanding is necessary in order to translate theoretical attachment
designs to practical use. These analyses must also consider multiscale modeling
approaches, as many of the attachment systems are hierarchical in nature, from the
nano- to the millimeter scales. A second observation related to functional grading at
the interface between dissimilar materials is that the design criteria should satisfy
several requirements, including strength, fracture, toughness, and stiffness. This
may require prioritization of mechanical property behavior (e.g., sacrificing
strength for improved toughness). Further study of biological attachments may
provide biomimetic lessons for medical and engineering applications. For example,
the advantages of a compliant zone between the attachment of a compliant
orthotropic material with a stiff isotropic material would not have been realized
without careful experimental and modeling studies of tendon, ligament, and

meniscal attachments to bone.