Before we will have a closer look for some of the decisions we made for the presented ontology we should emphasize that the ontology is not supposed to be a complete set of definitions for axiomatizing the domain of materials. First, the materials groups and single materials were chosen according to our specific application. Secondly, we represented only a small part of it in Ontolingua (cf. appendix A,B, and C) since Ontolingua will not be our final representation language [Bac93]. However, for the benefit of reusing existing ontologies and for comparing it with others we choose to represent a small subset of it in Ontolingua. Finally, we have not decided yet how to represent the composites (see below).
We regard our ontology of materials also as a test of how already existing ontologies can be used. For that purpose we adopted the ontology of ENGINEERING MATHEMATICS [GO94]. We enhanced the ontology STANDARD UNITS AND DIMENSIONS to capture our specific dimensions. Additionally, we added the possibility to define the physical properties of a materials group via an interval of values. Also mechanical and thermal quantities were defined, representing a small subset of imaginable ones, that might be important for classifying materials. The two ontologies THERMAL QUANTITIES and MECHANICAL QUANTITIES on top of the previously defined ontologies SCALAR QUANTITIES and VECTOR QUANTITIES capture them. It turned out that the preexisting ontologies are powerful enough to provide a basis for our needs. The result is shown in appendix A.
Vice versa the presented ontology will also serve as a testbed of how it can support the structuring of already existing knowledge--bases on materials that are tailored for a specific application in the sense of [Mar93]. For that purpose we are currently investigating the already existing knowledge base of the VEGA project [BBK94]. This knowledge base covers knowledge about the recyclability of glass fiber reinforced plastics with a thermoplastic matrix.