Stiffness-sensitive structures

Composite materials are ideal for structural applications where high strength-to-weight and stiffness-to-weight ratios are required. Aircraft and spacecraft are typical weight-sensitive structures in which composite materials are cost-effective. When the full advantages of composite materials are utilized, both aircraft and spacecraft will be designed in a manner much different from the present. 

A number of sensitivity studies have been performed which investigate the effects of changing piping wall thickness, secondary support stiffness, and structure material and temperature on the ASME primary and primary plus secondary stress limits discussed above. A summary of the cases run is listed In Table 9-32. The primary stresses are only evaluated for the wall thickness study, because the primary stresses are not dependent on the other parameters studied. For all of the results tables, the minimum, average, and maximum stress values refer to the minimum, average, and maximum stress Intensity values at all of the element nodes evaluated in the model. It is presented in this form to better comprehend the distribution of stress values. 

TA2 mode ([IlO] polarization) visibly depends on the influence of the electrons on the lattice vibrations (see Fig. 7). Without V2, the stiffness of the TAi mode is enlarged. Therefore, we can conclude that this TA2 mode is very sensitive to changes in the electronic structure and measures the stability of the B2 phase. This may be a hint of the temperature or stress dependence of the Cu-Zn system in the martensitic region (Zn < 42 at%) close to the stoichiometric concentration CuZn... 

Whereas the tensile strength was not a sensitive function of the monomer structure, the tensile modulus (Young s Modulus) was clearly related to the monomer structure. This is expected since the tensile modulus is a measure of the polymer s resistance to deformation and is related to the "stiffness" of a polymeric material. The highest tensile modulus (22,000 kg/cm2,2.2 GPa) was measured for poly(BPA iminocarbonate). Replacement of BPA by Dat-Tyr-Hex reduced the tensile modulus significantly. This observation can possibly be attributed to the presence of the long hexyl ester pendent chain in Dat-Tyr-Hex. Generally, the polyiminocarbonates were somewhat "stiffer" than the corresponding polycarbonates. Thus, the tensile moduli of poly(Dat-Tyr-Hex iminocarbonate) and poly(Dat-Tyr-Hex carbonate) were 16,300 kg/cm2 (1.6 GPa) and 13,900 kg/cm2 (1.3 GPa) respectively. 

All of the likely conformations of cellobiose, cellulose, and xylan are explored systematically assuming the ring conformations and IC-D-O-IC-4 ) angle for each pair of residues to be fixed and derivable from known crystal structures. The absolute van der Waals energies, but not the relative energies of different conformations, are sensitive to the choice of energy functions and atomic coordinates. The results lead to possible explanations of the known conformational stiffness of cellulose and Its solubility properties in alkali. The characteristics of xylan conformations are compared with cellulose. 

For a part to exhibit structural stiffness, flexural moduli should be above 2000 N/mm2 (290,000 psi). Notched Izod impact values should be determined at different thicknesses. Some plastics exhibit different notch sensitivities. For example, PC, 3.2 mm thick, has a notched Izod impact of 800 J/m (15 ftlbf/in.) which drops to 100 J/m (1.9 fflbf/in.) at 6.4-mm thickness. On the other hand, one bisphenol A phthalate-based polyarylate resin maintains a 250-J/m (4.7-ftibf/in.) notched Izod impact at both thicknesses. Toughness depends on the structure of the part under consideration as well as the plastic employed to make the part. Mechanical properties, like electrical properties, are also subject to thermal and water-content changes. 

Concerning properties, SFM has become a unique technique in probing local adhesion, friction and elastic response of various materials. This is based on the ability to measure forces as small as picoNewtons and probe areas well below 100 nm. The peculiar sensitivity of the force probe to different types of static and dynamic interactions provides a great number of contrast mechanisms which can map the surface structure regarding the chemical composition and physical properties. However, in most SFM measurements the interpretation of the surface maps remain to be very intricate, mostly because of the concurrent contribution of different forces into the net force. The progress in this field relies on new developments in technique which would allow to measure the properties like stiffness, adhesion, friction and viscosity, separately. 

Another study used 13C NMR to examine the thermal ageing of NR with both peroxide and conventional and EV sulfur vulcanised samples [59]. The samples were heat aged at 70 °C in air with 13C NMR recorded after 3 and 21 days. The vulcanisate structures vulcanised by peroxide and the EV sulfur system showed low sensitivity to ageing effects while peaks in the conventional sulfur system almost disappeared after 21 days of ageing. In addition, the main chain isoprene carbon peaks showed considerable broadening due to increasing stiffness of the network. 

Epoxies are especially reliable when used with epoxy-based composites because they have similar chemical characteristics and physical properties. Room temperature curing adhesives are often used to bond large composite structures to eliminate expensive fixtur-ing tools and curing equipment required of higher-temperature cure adhesives. However, room temperature epoxies require long cure times, so they are not suitable for large, highspeed production runs. Some of the lower-temperature composite materials are sensitive to the heat required to cure many epoxies. Epoxies are too stiff and brittle to use with flexible composites. 

The double helix is a relatively stiff and elongated molecule. Consequently, a solution of DNA has a high viscosity. If such a solution is heated to 95°C, the viscosity drops markedly, reflecting a collapse of the double-helical structure. This is known as denaturation and is accompanied by separation of the duplex into its single strands, which are fairly flexible. Denaturation and renaturation provide valuable information on important properties of the DNA obtained from various sources. Denaturation also provides the basis for very precise and sensitive approaches to the identification of specific sequences in both DNA and RNA. This has been central to the rapid developments in molecular genetics. 

Acetal translucent crystalline polymer is one of the stiffest TPs available. It provides excellent hardness and heat resistance, even in the presence of solvents and alkalies. Its low moisture sensitivity and good electrical properties permit direct competition with die-cast metal in a variety of applications. In addition, acetal has extremely high creep resistance and low permeability. Acetal is also available as a copolymer (Hoechst Celanese Corp. s Celcon) for improved processability. The homopolymer (DuPont s Delrin) has a very low coefficient of friction and its resistance to abrasion is second only to nylon 6/6. Acetals are frequently blended with fibers such as glass or fluorocarbon to enhance stiffness and friction properties. Acetal is not particularly weather-resistant, but grades are available with UV stabilizers for improved outdoor performance. Acetal, whether homopolymer or copolymer, is not used to any significant degree in forming structural foams. 

The fatigue behaviour of the woven fabric shows that this textile structure is sensitive to fatigue loading. This sensitivity is confirmed by testing thermoplastic matrix systems (polyamide 12, PA 12) reinforced with a glass fabric. A strong stiffness reduction, with a pronounced creep deformation was observed under tension-tension fatigue [24]. 

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