Thermal expansion stress

As mentioned above, when the oxygen partial pressure is reduced, the lattice expands as CC+ is oxidized to Cr3+. If this expansion is too large, just as with thermal expansion, stresses can be generated and lead to cracking. Increasing the dopant... 

Many properties of adhesive bonds are influenced by the Tg or mobility of the molecular chain structure, as shown in Fig. 3.16. When chain segments can move easily, such as when the temperature exceeds the Tg, they can deform under impact or assume new alignments under mechanical or thermal expansion stresses. This movement spreads the applied energy over a greater number of atoms and thus gives the bond a better chance to resist stress. Brittleness is, therefore, reduced and flexibility is increased. 

Liou et al. [13] have measured the effect of absorbed moisture on the thermal expansion coefficients of carbon fibre epoxy laminates. Moisture lowers the transverse coefficient, but has little effect on Oi at room temperature. Failure in some epoxy laminates used to make multilayer PCBs has been traced to high z-axis thermal expansion stresses on marginal plated-through holes. Moisture is a factor, and complete drying can sometimes eliminate the problem [14]. 

In particular, Gernot et al. [134] used a composite Pd-based MR, whose stmcture consists of a three layers stacking. This membrane structure lowers the thermal expansion stresses between the membrane and the support as well as the quantity of noble metal active layer. At 600°C and for 500 h of work, as best... 

THERMAL-EXPANSION STRESSES IN REINFORCED PLASTICS. RESEARCH PAPER NO. 1745. 

Since the programming is the same for the free recovery specimens and fully constrained specimens, the focus will be on step 4 of the thermomechanical cycles. The stress-temperature behavior under a fully constrained recovery condition is shown in Figure 3.19 for the two programming stresses (47 kPa and 263 kPa). The recovery stress-time behavior of the foam programmed at 47kPa pre-stress is also highlighted by the inset in Figure 3.19. The recovery stress comes from two parts thermal expansion stress and entropically stored stress or back stress. Since this is a 1-D fully constrained recovery, the thermal stress can be calculated as... 

From Figure 3.50, the stress comes from two parts during recovery thermal expansion stress and entropieaUy stored stress (memorized stress). When the temperature is below Tg, that is, the... 

Graphitization of a carbon-carbon composite has a greater effect on the matrix or fiber and occurs mainly in the matrix as a sheath, some 1-3 pm thick. This thermal expansion stress is termed stress graphitization [34]. Jortner depicts other forms of orientations (Figure 14.3) that have been observed [35]. 

Computed stresses are based on test thickness at test temperature. Since water pressure is a short-term condition, the allowable stresses for structural parts such as supports are frequently increased by a factor of 1.2. The upper limit of stress in the vessel shell during a hydrotest of pressure parts is not specified by the Code. However, it is a good engineering practice to limit the maximum membrane stress in any part of the vessel during a hydrotest to 80 percent of the yield strength. Thermal expansion stresses and local mechanical stresses will be absent and need not be considered. 

As mentioned above, the temperature distribution in the blank is homogenized at Tg, meaning that the temperature of the surface is raised to 675 °C. During this phase it is essential that no zones of the blank that have reached temperatures below 740 °C are raised to temperatures above 800 °C again. Nuclei are produced below 740 °C and crystals can grow on them when the blank is reheated (especially above 800 °C). A partially crystallized zone results, which is different from the other material with respect to its thermal expansion coefficient. Because of this difference in thermal expansion, stresses which can lead to rupture of the glass can form during the cooldown to room temperature. 

There are two basic loads that are imposed on the lining, the gravity load and the thermal expansion load. The thermal expansion stresses are usually several orders of magnitude greater than the gravity load. 

Plastics expand at high temperatures and contract at low temperatures to different amounts, depending on their coefficients of thermal expansion. After bonding, part movement is restricted, and the two materials must expand or contract to the same extent. If the plastics have different coefficients of thermal expansion, stresses are produced, which lower the strength of the joint. Coefficients of thermal expansion can be decreased by adding fillers or reinforcements or by... 

The actual piping will have more supports than what. is modeled. The resulting stress ranges in this analysis are likely un-conservative because adding additional piping supports will likely increase the thermal expansion stresses. This does not change the conclusions made in this in this analysis. 

Figure 10.20 The effect of substrate temperature on film stress. Note that the thermal expansion mismatch stress can be either compressive or tensile while the growth stress is generally tensile at low temperatures. If the thermal expansion stress is tensile then the total stress is the sum of the two curves shown. If the thermal expansion stress is compressive then the total stress is the difference of the two curves.

The differential thermal expansion contribution to film stress is straightforward to understand. When a film is deposited it typically has only the growth stresses described above. However, if the deposition temperature is significantly different from room temperature (usually higher) then when heating is terminated after growth and the sample cools, the film usually contracts at a different rate than does the substrate. Because the substrate is usually very much stiffer than the film this differential thermal expansion stress is accumulated in the film. 

---