Stress, allowable thermal

Division 2. With the advent of higher design pressures the ASME recognized the need for alternative rules permitting thinner walls with adequate safety factors. Division 2 provides for these alternative rules it is more restrictive in both materials and methods of analysis, but it makes use of higher allowable stresses than does Division 1. The maximum allowable stresses were increased from one-fourth to one-third of the ultimate tensile stress or two-thkds of the yield stress, whichever is least for materials at any temperature. Division 2 requkes an analysis of combined stress, stress concentration factors, fatigue stresses, and thermal stress. The same type of materials are covered as in Division 1. 

Stack material and architecture combinations that allow for effective sealing and reduction in life-limiting thermal stresses during thermal cycles. 

In most cases, the cells are either in the form of tubes (open or closed-ended) or flat plates. Although the cells based on flat plates are more compact, the stacks require high-temperature seals, they are more sensitive to thermal stress and thermal cycling, and they are more difficult to pressurize. The tubular designs are more robust, allow the use of room-temperature seals, and exhibit a good thermal cycling capability. 

To achieve compact design in order to meet speed requirements (especially at high-lead count), fine spacing of leads, e.g., 2-mil wide leads on 4-mil centers, or less will be required. The delicate nature of such leads and their close spacing will impose severe constraints on the amount of allowable thermally induced stress (or strain) generated by an encapsulant or by a package due to differences in the TCE. Similarly, substrates will need to maintain dimensional stability, dielectric constant values, planarity, etc., to much finer tolerances than in the past. 

The high resistance of metal materials to thermal stress allows the application of reactive adhesives, which cure at elevated temperatures and show high bond strengths (up to 40 MPa). 

Partially, the necessary test methods have already been described in detail in Chapter 3, partially they are explained in the relevant technical regulations. One further calorimeter shall be mentioned here. A specific problem related to drying substances is the determination of the allowable thermal stress. The uncertainties inherent to the deduction of critical temperature limits with the help of screening tests have been discussed in detail at the relevant points. 

Elastic Stress Analysis - Thermal Stress Ratcheting Assessment. This section will evaluate the allowable limit on the secondary stress range from cyclic thermal loading. 

A rotational viscometer connected to a recorder is used. After the sample is loaded and allowed to come to mechanical and thermal equiUbtium, the viscometer is turned on and the rotational speed is increased in steps, starting from the lowest speed. The resultant shear stress is recorded with time. On each speed change the shear stress reaches a maximum value and then decreases exponentially toward an equiUbrium level. The peak shear stress, which is obtained by extrapolating the curve to zero time, and the equiUbrium shear stress are indicative of the viscosity—shear behavior of unsheared and sheared material, respectively. The stress-decay curves are indicative of the time-dependent behavior. A rate constant for the relaxation process can be deterrnined at each shear rate. In addition, zero-time and equiUbrium shear stress values can be used to constmct a hysteresis loop that is similar to that shown in Figure 5, but unlike that plot, is independent of acceleration and time of shear. 

The stress—relaxation process is governed by a number of different molecular motions. To resolve them, the thermally stimulated creep (TSCr) method was developed, which consists of the following steps. (/) The specimen is subjected to a given stress at a temperature T for a time /, both chosen to allow complete orientation of the mobile units that one wishes to consider. (2) The temperature is then lowered to Tq T, where any molecular motion is completely hindered then the stress is removed. (3) The specimen is subsequendy heated at a controlled rate. The mobile units reorient according to the available relaxation modes. The strain, its time derivative, and the temperature are recorded versus time. By mnning a series of experiments at different orientation temperatures and plotting the time derivative of the strain rate observed on heating versus the temperature, various relaxational processes are revealed as peaks (243). 

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