Sealants thermal stresses

DIN 52 451 1983 Testing of building B sealants determination of the change in volume after thermal stress dipping and weighing method. 

The glass fabric reinforcement serves another function in that it lowers the overall coefficient of linear thermal contraction to a point where thermal stresses at the sealant-substrate interface are reduced to a minimum. 

Determining the appropriate as-dispensed thickness and width of the sealant and developing an application process that consistently achieves this target. Thermal stress analysis can be used to determine optimal as-formed seal thickness for transient and steady-state operation. To retain this seal thickness over the lifetime of the stack, a small concentration of insoluble, monosize particulate can be incorporated into the sealant to achieve a minimum stand-off distance, for example, between the cell and window frame. 

The solid oxide fuel cell is highly influenced by the temperature variation during operation, constantly suffering from thermal stress, which requires excellent compatibility between the thermal expansion coefficients of the cell components. The cylindrical shape of tubular SOFC contributes significantly to minimize the difference in the coefficients of thermal expansion, thus avoiding the formation of cracks and delamination. This model also makes unnecessary the use of sealant gases. On the other hand, the efficiency is impaired, since the path made by the electric current is increased, causing ohmic losses (Minh, 1993). 

The joint region in compliant seals can be plastically deformed above room temperatures, which lessens the thermal stresses caused due to CTE mismatch. This category of sealants includes metallic components and hence is electrically conducting. In addition to this, the cell bowing and non-uniformities in the gas distribution are potential problems. Brazing and bonded compliant seals fall in this category and are discussed in Sects. 5.3.2.1 and 5.3.2.2. 

Basically, there are two major considerations when one is formulating or selecting adhesives or sealants for low-temperature applications. The first is the effect of the low temperature on the bulk properties of the polymer, and the second is the effect of thermal cycling and resulting internal stresses on the joint interface. 

The adhesive s thermal conductivity is important in minimizing transient stresses during cooling. This is why thinner bond thickness and adhesives or sealants with higher levels of thermal conductivity generally have better cryogenic properties. 

C), severe problems are encountered with bagging materials and sealants, and autoclave malfunctions are frequent. In addition, surface degradation of the adherend, residual stresses in the bondline, and thermal and/or oxidative degradation of the adhesives are typical of the detrimental effects of processing at high temperatures. 

Loading can be either load (or stress) controlled, displacement (or strain) controlled, or something in between. Examples include aerodynamic loads on an aircraft (see Aerospace applications), which tend to be load controlled, and the displacement of a sealant between relatively stiff adherends, which is displacement controlled. Because average adhesive strain, in its simplest form, is defined as displacement divided by bond thickness, strains and resulting stresses are higher in thin bondlines subjected to displacement-controlled loading scenarios. Joints loaded in such a manner often perform better with thicker bondlines. Displacement-controlled situations include thermal expansion/shrinkage of adherends, mismatched adherend expansion, and attachments bonded to pressure vessels or other adherends that are stressed. 

Thermosets also benefit from the foam structure, as evidenced by improved thermal insulation, sound dampening and mechanical stress absorption responses to temperature changes or impact. Hollow spheres with an already set volume are normally used, that is, pre-expanded microspheres. The reason is that the curing reactions often interfere with any expansion before a sufficient volume increase has been obtained. Hollow organic spheres are found in products such as sealants, adhesives, putties, pipes, cultured marble, body fillers, model-making materials, and pastes [2, 3, 19]. Common suitable matrix materials are epoxies, PUR, and polyesters. 

In the same year, Weil et al. [13] introduced a sealing concept for planar SOFCs. The finite element method (FEM) was used to aid in scaling up a bonded compliant sealant design to a 120 x 120 mm component. The stresses of the cell, foil, brazes, and frame were calculated and compared with experimental fracture and yield stress results. A quarter symmetrical model was used. The commercial software AN SYS was utilized. The tensile stress of the component was predicted, considering thermal cycling from elevated temperature to room temperature. The materials used were mentioned, but no properties were given. Regarding the structural analysis boundary conditions and the failure criteria employed, material models were not depicted. 

Weil and Koeppel [15] reported a study in 2008 involving bonded comphant sealants. The thermally induced stress-strain behavior of the cell, sealant, and frame components was investigated using the 3D FEM. ANSYS was used for the calculations. The assembly was heated and cooled uniformly. A one-quarter section was modeled due to the fourfold symmetry of the component. The cell was treated computationally as a single material, considering the anode properties. 

A bilinear elastic-plastic constitutive model was employed for the interconnector plate. Material properties obtained from the literature were used. The sealant was treated as elastic. Isothermal conditions were set. They found that some bowing occurred during coohng, and that the stress is transferred to the sealing foil and soft silver braze that were used in the study. The component stresses were found to be lower in a second thermal cycle compared with the first cycle. 

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