Thermal Expansion Differences

One of the most common causes of internal stress is due to the difference in the thermal expansion coefficients of the adhesive and the adherend. These stresses must especially be considered when the adhesive solidifies at a temperature that is different from the normal temperature that it will be exposed to in service. 

FIGURE 3.8 Journal bearing application. Outer cylinder (stainless steel) is bonded to inner cylinder (polyamide-imide) with an epoxy adhesive. Exposure to low temperatures causes significant stress on bond due to differences in coefficient of thermal expansion. 

FIGURE 3.9 Plot of tensile shear strength of an aluminum joint bonded with an elevated-temperature curing epoxy adhesive as a function of test temperature. 

There are several possible solutions to the expansion mismatch problem. One is to use a resilient adhesive that deforms with the substrate during temperature change. The penalty in this case is possible creep of the adhesives, and highly deformable adhesives usually have low cohesive strength. Another approach is to adjust the thermal expansion coefficient of the adhesive to a value that is nearer to that of the substrate. This is generally accomplished by selection of a different adhesive or by formulating the adhesive with specific fillers to tailor the thermal expansion. A third possible solution is to coat one or both substrates with a primer. This substance can provide either resiliency at the interface or an intermediate thermal expansion coefficient that will help reduce the overall stress in the joint. 

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Thermal expansion differences exist between the tooth and the polymer as well as between the polymer and the filler. The tooth has a thermal expansion coefficient of 11 x 10-6/°C while conventional filled composites are 2-4 times greater [63, 252], Stresses arise as a result of these differences, and a breakdown between the junction of the restoration and the cavity margin may result. The breakdown leads to subsequent leakage of oral fluids down the resulting marginal gap and the potential for further decay. Ideal materials would have nearly identical thermal expansion of resin, filler, and tooth structure. Presently, the coefficients of thermal expansion in dental restorative resins are controlled and reduced by the amount and size of the ceramic filler particles in the resin. The microfilled composites with the lower filler loading have greater coefficient of thermal expansions that can be 5-7 times that of tooth structure. Acrylic resin systems without ceramic filler have coefficients of thermal expansion that are 9 times that of tooth structure [202-204, 253],... 

It is worth pointing out that carbon fibre itself has anisotropic thermal expansion properties, and therefore this mismatch between the carbon fibres and the a-sialon matrix should be considered in both the radial and axial directions when carbon fibres are unidirectionally aligned in the composite. The thermal stress caused by thermal expansion differences between the carbon fibres and the matrix in the radial (cr) and axial (oa) directions can be estimated from the formulae (Chawla, 1993 Kerans and Parthasarathy, 1991) ... 

Gas tightness over a long period of time is real challenge in high-temperature electrolysis. The presence of both metallic and ceramic materials is largely responsible for the difficulties. The thermal expansion differences and the use of brittle materials such as ceramics led to new sealing solutions. Here, a new metallic seal is proposed. It presents a C-shape with two components a strong inner part,... 

Thermal expansion differences between the adhesive and the substrate (mainly associated with adhesives that cure at temperatures different from their normal service temperatures)... 

Less shrinkage and internal stress resulting from thermal expansion differences... 

Other opportunities for stress concentration in bonded joints that may be aggravated by low-temperature service include trapped gases or volatiles evolved during bonding, residual stresses in adherends as a result of the release of bonding pressure, and elevated-temperature cure (i.e., shrinkage and thermal expansion differences). 

Note that the residual stress aM — 0 on the elastic properties becomes homogeneous (Ef = Em = EL). While connections between the residual stresses and constituent properties are rigorous, experimental determination is still necessary, because ft is not readily predictable. In general, ft includes terms associated with the thermal expansion difference, ay— am, as well as volume changes that occur either upon crystallization or during phase transformations. For CVI systems, intrinsic stresses may also be present. 

Most experimental results are consistent with the misfit strain predicted from the thermal expansion difference, am — atf, and the cooling range from the processing temperature. Examples for SiC/CAS and SiC/SiC are given in Table 1.3. 

Refractory flaws can also be caused by the frequency of temperature cycling, the rate of temperature drop, and the amount of temperature drop, which can lead to thermal shock or structural flaws from thermal expansion differences. Where monolithic linings are used, anchors attach the refractory material to the shell. Those anchors can experience failure from mechanical stresses, metal fatigue, or corrosion, leading to gaps between the refractory shell and the lining. Any gap impacts heat transfer and can initiate other types of refractory failure, such as slag corrosion because of heat buildup at those sites. 

The stress resulting in a glass-metal seal is related to both the thermal expansion difference and the temperature at which the viscosity of the glass is such that stresses start to form during cooling (setting temperature). Both depend on the cooling rate. Therefore,... 

The results of the shrinkage measurements, shown in Figures 4 and 5, also show the retardation of sintering in oxygen. Up to 300°C there is an apparent expansion of the samples due to the thermal expansion difference between Ba2YCu307.x and alumina. The... 

Hardness and Tensile Properties. Recall that an increased hardness was observed in the HAZ. This trend was opposite to that found in precipitation-hardened and/or cold-worked aluminum alloys, which exhibit a large drop in hardness in the HAZ due to overaging or recrystallization, respectively. The exact reason for the increase in HAZ hardness here is not known however, it may have resulted from cold working of the HAZ during FSW and/or from straining during cooling due to coefficient of thermal expansion differences between the a and 3... 

The analyses for structural adequacy Identified that the thermal expansion or wind deflection of photovoltaic modules can result In the development of mechanical stresses In the encapsulated solar cells sufficient to cause cell breakage. The thermal stresses are developed from differences In the thermal expansion properties of the load carrying panel, and the solar cells. However, the analysis Interestingly Identlfed that the solar cell stresses from either thermal expansion differences or wind deflection can be reduced by Increasing the thickness t of the pottant, or by using pottants with lower Young s Modulus E. 

The pottant Is the soft, elastomeric, vibration-damping material that Immediately surrounds both sides of fragile solar cell wafers and their electrical contacts and Interconnects. It protects the cells from stresses due to thermal expansion differences and external Impact. It Isolates them electrically and helps protect their metallic contacts and Interconnects from corrosion. 

The plant starts up by heat entering from the primary pump and the system temperature rises to 350°C from the cold shutdown state. Under this condition, all parts of the system, including the recirculation line in the water system, are uniformly heated. Then, a neutron absorber at the center of the core is withdrawn. At temperatures below 350°C, the neutron absorber cannot be withdrawn by the self-connected mechanism using the thermal expansion difference between the stainless steel and Cr-Mo steel (Fig. 14). After withdrawal of the neutron absorber, the reflector is lifted up by the hydraulic system to reach critical condition at 350 C. A ficzy control system is employed for this approach and a fully automatic operation circuit is provided because no malfunction causes severe reactivity insertion as described previously. 

One common characteristic of all C/SiC composites is their distinct anisotropy in the mechanical as well as thermophysical properties. Considerable lower values of the tensile strength and the strain to failure have to be considered for an appropriate design if the load direction and the fibre alignment are not congment. As the carbon fibres show a different physical behaviour in longitudinal and radial direction, the composite s properties like thermal conductivity and coefficient of thermal expansion differ widely with respect to the in-plane or transverse direction. 

Different thermomechanical microvalves use different mechanisms to control the valve although all of them are based on the deformation of the material due to thermal expansion. For example, a thermal expansion microvalve utilizes the thermal expansion of a single material to open or close the flow, a bimetallic microvalve utilizes the thermal expansion difference between two materials which are bond together to open or close the flow, and a thermally driven SMA... 

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