Secondary structural materials

Sodium corrosion effect on the primary and secondary structural material (CrlSNilOTi stainless steel) was studied by taking steel samples (Fig. 1) after 49 000 hours to 210 000 hours operation in sodium at the temperatures from 300 to 450°C. 

The secondary structural materials are materials that if the structure fails can only cause local damage that can be repaired, such as secondary wall panels for a steel framed building in a modular construction. These panels, which are aesthetically pleasing, are light to handle and are low in maintenance, are SWP with FRP or rigid metallic skins on the face and have a polymeric foam core, usually of EPS or PU. The load on these panels is mainly the pressure induced by wind. 

Since there is no clear choice for structural material, a number of candidate materials would need to be identified for both the primary and the secondary structures. Materials that can be used for multiple functions should be sought. For example, titanium can be used as a primary structure element, as a fastener, and as a moderately thermal isolating material due to it low conductivity compared to other metals. However, titanium would not be a good choice for an application requiring good thermal conduction, such as a heat spreader. Thus, a number of different materials will need to be identified and tested in order to assure that sufficient options are available for the different challenges. 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

Table 1 contains the metal-to-metal engineering property requirements for Boeing Material Specification (BMS) 5-101, a structural film adhesive for metal to metal and honeycomb sandwich use in areas with normal temperature exposure. The requirements are dominated by shear strength tests. Shear strength is the most critical engineering property for structural adhesives, at least for the simplistic joint analysis that is commonly used for metal-to-metal secondary structure on commercial aircraft. Adhesive Joints are purposefully loaded primarily in shear as opposed to tension or peel modes as adhesives are typically stronger in shear than in Mode I (load normal to the plane of the bond) loading. 

In most cases, pyrazino[l,2- ]pyrazines have been synthesized as highly saturated derivatives with the aim of preparing conformationally restricted compounds which mimic the secondary structure of reverse-turn regions of peptides and proteins. The saturated pyrazino[l,2- ]pyrazine 241 was synthesized from readily available starting materials, the key steps being the preparation of the keto amide 239 and subsequent tandem cyclizations from [6+0] atom fragments (Scheme 42) <20000L301>. 

More recently, Ingram et al. and Wright et al. independently tried to develop new polymer hosts with secondary structures similar to that of a liquid crystalline state, so that ion transport could occur with a higher degree of freedom in the highly oriented environments and become at least partially decoupled from the polymer segmental relaxations. Ion conductivities approaching liquidlike values have been obtained on the condition that the liquid crystalline state could be maintained. However, the incorporation of these novel polymer materials in electrochemical devices remains to be tested. 

The term secondary structure is used to describe the molecular shape or conformation of a molecule. The most important factor in determining the secondary structure of a material is its precise structure. For proteins, it is the amino acid sequence. Hydrogen bonding is also an important factor in determining the secondary structures of natural materials and those synthetic materials that can hydrogen-bond. In fact, for proteins, secondary structures are... 

Both synthetic and natural polymers have superstructures that influence or dictate the properties of the material. Many of these primary, secondary, tertiary, and quaternary structures are influenced in a similar manner. Thus, the primary structure is a driving force for the secondary structure. Allowed and preferred primary and secondary bondings influence structure. For most natural and synthetic polymers, hydrophobic and hydrophilic domains tend to cluster. Thus, most helical structures will have either a hydrophobic or hydrophilic inner core with the opposite outer core resulting from a balance between secondary and primary bonding factors and steric and bond angle constraints. Nature has used these differences in domain character to create the world around us. 

The two major secondary structures found in nature— the helix and the sheet—are also major secondary structures found in synthetic polymers. The helix takes advantage of both the formation of intermolecular secondary bonding and relief of steric constraints. Some materials utilize a combination of helix and sheet structures such as wool, which consists of helical protein chains connected to give a pleated sheet. 

Chain folding depends on the primary and secondary structures of materials. Thus, the particular atomic composition of a chain dictates, under equilibrium conditions, its tertiary and quaternary structures. 

---