Bio-inspired, Flexible Structures and Materials - Biotechnologies and Biomimetics for Civil Engineering - page 273

E t
2 r M ; Rk

r min ¼

ð 3 Þ

Previous work by Ashby has provided very helpful tools in the selection of
adequate materials for a given design task (Ashby 2005 ). The 'Ashby diagrams'
open up a property-space by comparing multiple material classes to each other as
well as identifying the property range within the individual classes. Providing
design guidelines to define specific 'search regions' additionally facilitates the
practical use of these diagrams. Figure 12.1 follows Ashby's approach and lists a
selection of common building materials. Here, the materials are plotted on a graph
with logarithmic scale and are brought into context based on their ratio of flexural
strength to stiffness. The range of the axis is chosen to include the material classes
investigated. Focusing on the context of building structures, the values in
Table 12.1 are taken from the Eurocodes DIN EN 1993-1995 and 1999, DIN
1052:2004-08, and DIN 17221, as well as from Knippers et al. ( 2011a , b , p. 77)
and Gas et al. ( 1985 ).
Based on this diagram, adequate materials for static bending-active structures
should offer a ratio of r M,Rk /E [ 2.5 (with r M,Rk [MPa] and E [GPa]). When it
comes to elastic-kinetic structures and large-scale compliant mechanisms, the
additional requirements for fatigue control further limit the permissible permanent
elastic stress; therefore, a ratio of r M,Rk /E [ 10 is needed. This is indicated by the
design guideline with the inclination r M,Rk /E. Moreover, the diagram shows
clearly that Fibre-Reinforced Polymers (FRP) and certain types of timber and high
strength metals are particularly appropriate materials for the use in bending-active
structures. When it comes to elastic-kinetic structures and large-scale compliant
mechanisms, FRPs mostly fulfil these requirements.
In addition to this short overview, it is critical to consider the material's long-
term behaviour. For static bending-active structures, this means paying particular
attention to time-dependent deformation (creep). The effects differ in the various
materials and are significantly higher in timber than in FRP, for example. The
creeping of a material needs to be considered because it can lower the pre-stress in
the structure, which dependent on the design of the structure can be more or less
relevant and may negatively affect the system's integrity. If the pre-stress is not
playing a decisive role for the structure's stiffness, materials such as timber may be
chosen. Regarding elastic-kinetic structures, the most important long-term
behaviour that needs to be checked is a material's fatigue behaviour. Only if a
chosen material has a sufficient fatigue life it can be guaranteed that the compliant
mechanism can undergo high and cyclical loading. For the designer, however, it is
difficult to assess if the mechanism can perform according to the prescribed
functions because the motion of its deflecting members is limited by their strength,
which is a property that is not easily readable from the outside. Therefore, precise
knowledge of the material properties and the desired motion range is required
when designing an elastic-kinetic structure.