Cooling buckling

Plastic products are often constrained from freely expanding or contracting by rigidly attaching them to another structure made of a material (plastic, metal, etc.) with a lower coefficient of linear thermal expansion. When such composite structures are heated, the plastic component is placed in a state of compression and may buckle, etc. When such composite structures are cooled, the plastic component is placed in a state of tension, which may cause the material to yield or crack. The precise level of stress in the plastic depends on the relative compliance of the component to which it is attached, and on assembly stress. 

This paper describes application of mathematical modeling to three specific problems warpage of layered composite panels, stress relaxation during a post-forming cooling, and buckling of a plastic column. Information provided here is focused on identification of basic physical mechanisms and their incorporation into the models. Mathematical details and systematic analysis of these models can be found in references to the paper. 

In designing axi-symmetric shell structures such as large-type cooling towers, it is necessary to predict the vibration responses to various external forces. The authors describe the linear vibration response analysis of axi-symmetric shell structures by the finite element method. They also analyze geometric nonlinear (large deflection) vibration which poses a problem in thin shell structures causes dynamic buckling in cooling towers. They present examples of numerical calculation and study the validity of this method. 11 refs, cited. 

Buckling of Cooling-Tower Shells Bifurcation Results Cole, Peter P. Abel, John F. Billington, David P. 

The paper examines the behavior of natural draft cooling tower wind pressure. Buckling loads of the towers of different meridional curvatures and shell thicknesses are computed and compared. The results show that an increase in stiffness of the structure with an increase in meridional curvature and changes of buckling load caused by changes in shdll thickness is approximately proportional. 10 refs, cited. 

Experimentally the situation was unclear because of STM experiments showing apparently symmetric dimers on an almost defect-free terrace of Si(001)(2xl), with tilting only near steps (Wiesendanger et al., 1990). However, recent temperature-dependent STM work has shown that on cooling to 120 K, the number of buckled dimers increases (Wolkow, 1992). It seems likely that the bistability of the asymmetric dimer results in flipping between... 

Welds of dissimilar metals result in (1) Warping, buckling, and/or excessive residual stresses caused by different thermal expansion coefQcients. (2) Hard spots in heat-affected zones and fusion zones are caused by formation of hard intennetallic compounds such as carbides. The heating/cooling cycle can harden HAZs and can cause a sensitized HAZ in nonstabilized alloys. 

N203 passed into an ice-cooled soln. of tributylacetamide (prepn. s. 127) in glacial acetic acid, allowed to stand at room temp, overnight, and heated 2 hrs. on the steam bath — tributylacetic acid. Y 80%.— The reaction can also be performed with butyl nitrite. (N. Sperber, D. Papa, and E. Schwenk, Am. Soc. 70, 3091 (1948) s. a. F. J. Buckle, R. Heap, and B. C. Saunders, Soc. 1949, 912.)... 

The characteristic size of single voids observed on different specimens ranged from 3 up to about 30 p,m-. jLarge voids were found, for example, on the specimen shown in Figs. 7c and d. The specimen exhibited also a few spontaneous spallations. If one inserts the thermal expansion mismatch stress of 2.7 GPa for cooling from 1223 K to RT into the lhs of formula (10), a value of 15 p,m is obtained for the critical radius where the scale buckles (h = 1.1 xm). This value is compatible with the observed size of the largest voids and explains therefore the appearance of a few spontaneous spallations. 

Figure 5.36 Scanning electron micrographs of (a) the surface and (b) the cross-section of an Fe-Cr-Al alloy which was oxidized at 1000 °C and cooled to room temperature. Buckling of the alumina scale is evident.

Injection mouldings are usually thin walled to minimise the cooling part of the cycle time. Explain why they are more likely to fail under a compressive load by viscoelastic buckling, than by uniaxial compressive yielding. 

Eigure 7.14 shows that, at ultimate failure, the buckling load of the noncooled specimen approached the applied load of 145 kN after 43 min, which represents an underestimation of the measured time-to-failure (49 min) of 12%. The buckling load of the water-cooled specimens approached a value of 1007 kN, as shown in Figure 7.15, which represents the buckling load of a specimen that completely lost one face sheet. At the end of the longer experiment (120 min), however, this critical load was still exceeded by almost 70%. 

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