Thermal Expansion and Stresses

CTE values were collected from various sources. Ranges of values or different values are due to differences in test methods, test conditions or purity of the sample.  

Hybrid chemistry paste, stress absorbing, electrically conductive, very low moisture absorption Ablebond 2000 Ablestik 59 65 200 

Electrically conductive, polyimide adhesive for high bond strength at elevated temperatures Ablebond 71-1 Ablestik 240 41 - 

Epoxy paste, electrically conductive, high-temperature stable Ablebond 84-lLMISR Ablestik 120 40 150 

---

The model correctly simulates the qualitative behavior exhibited by the EOCN. Stress retention upon cooling after both the 125°C and 200°C cures is noted, with a decreased Ea when more fully cured. A compressive stress which relaxes out is observed on heating the partially cured resin to 200°C. Both cool-down curves exhibit curvature near the cure temperatures due to the dependency of modulus, thermal expansion, and stress relaxation on temperature, Tg, and crosslink density. 

Fligher temperatures result in increased thermal expansion and stress on plated holes as a result. 

In the derivation of equations 24—26 (60) it is assumed that the cylinder is made of a material which is isotropic and initially stress-free, the temperature does not vary along the length of the cylinder, and that the effect of temperature on the coefficient of thermal expansion and Young s modulus maybe neglected. Furthermore, it is assumed that the temperatures everywhere in the cylinder are low enough for there to be no relaxation of the stresses as a result of creep. 

At very high and very low temperatures, material selection becomes an important design issue. At low temperatures, the material must have sufficient toughness to preclude transition of the tank material to a brittle state. At high temperatures, corrosion is accelerated, and thermal expansion and thermal stresses of the material occur. 

Acceptable comprehensive methods of analysis are analytical, model-test, and chart methods, which evaluate for the entire piping system under consideration the forces, moments, and stresses caused by bending and torsion from a simultaneous consideration of terminal and intermediate restraints to thermal expansion and include all external movements transmitted under thermal change to the piping by its terminal and intermediate attachments. Correction factors, as provided by the details of these rules, must be applied for the stress intensification of curved pipe and branch connections and may be applied for the increased flexibihty of such component parts. 

Stresses may be applied and/or residual. Examples of applied stresses are those arising from thermal expansion and contraction, pressure, and service loads. Applied stresses may be continuous or intermittent. Examples of residual stresses are those arising from welding, fabrication, and installation. The importance of residual stresses in SCC should not be underestimated. Residual stresses may... 

The cyclic stresses responsible for this failure were apparently bending stresses associated with cyclic thermal expansion and contraction. 

The orientation of the cracks indicates that cyclic bending stresses or cyclic axial stresses generated by thermal expansion and contraction provided the responsible stresses. The large number of crack initiation sites and the tightness of the cracks indicate high-level stresses. 

Stresses from welding result principally from the effects of differential thermal expansion and contraction arising from the large temperature difference between the weld bead and the relatively cold adjacent base metal. Shrinkage of the weld metal during solidification can also induce high residual stresses. Unless these residual stresses are removed, they remain an intrinsic condition of the weldment apart from any applied stresses imposed as a result of equipment operation. 

Obviously, the designer must take thermal expansion and contraction into account if critical dimensions and clearances are to be maintained during use where material is in a restricted design. Less obvious is the fact that products may develop high stresses when they are constrained from freely expanding or contracting in response to temperature changes. These temperature-induced stresses can cause material failure. 

When materials with different coefficients of linear thermal expansion (CLTE) are bolted, riveted, bonded, crimped, pressed, welded, or fastened together by any method that prevents relative movement between the products, there is the potential for thermal stress. Most plastics, such as the unfilled commodity TPs, may have ten times the expansion rates of many nonplastic materials. However there are plastics with practically no expansion. Details are reviewed in Chapter 2, THERMAL EXPANSION AND CONTRACTION. 

Piping joints shall be selected to suit the piping material, with consideration of joint tightness and mechanical strength under expected service and test conditions of pressure, temperature, and external loading. Layout of piping should, insofar as possible, minimize stress on joints, giving special consideration to stresses due to thermal expansion and operation of valves (particularly a valve at a free end). 

In addition to the design requirements for pressure, weight, and other loadings, hydrogen piping systems subject to thermal expansion and contraction or to similar movements imposed by other sources shall be designed in accordance with the requirements for the evaluation and analysis of flexibility and stresses specified herein. 

Once the temperature and degree of cure can be predicted throughout the material during cure, the nonmechanical loadings (thermal expansion and chemical shrinkage) can be obtained. With this knowledge in hand the residual stresses can then be analyzed. 

---