Expansion joints fatigue

The thin-wall bellows element should be designed for membrane stresses to conform to code-allowable stresses. The sum of membrane and secondary bending stresses should not exceed 1.5 times the yield stress in order to prevent the collapse of the corrugations caused by pressure. Bellows subjected to external pressure can be analyzed in a manner similar to a cylinder, utilizing an equivalent moment of inertia. The fatigue life can be estimated based on the sum of deflections and pressure stresses as compared to S/N curves based on bellows test data or using the curves in B31.3 Appendix X, Metal Bellows Expansion Joints. Formulas for the stress analysis of bellows are available in the Expansion Joints Manufacturing Association (EJMA) Standards (37). 

Inconel alloy 625 —Low-cycle fatigue resistance has been increased from 70-80.000 to 110-120,000 psi at 10 cycles. This has been achieved through grain size control and improved product cleanliness. Major applications are bellows and expansion joints. 

When selecting a bellows valve, it is important to pay some special attention that the material selection is in accordance with the process conditions. Some SRV manufacturers use as standard bellow material INCONEL alloy 625LCF-UNS N06625 (ASME SB0443). This material is not perfect either but, compared to simple stainless steel, has an enhanced resistance to mechanical fatigue and sour gases it is commonly used in refinery FCC systems for expansion joints. 

Stability, fluid compatibility and fatigue resistance also are associated with wear resistance. Vibration and pulsation in mechanical systems, or pipelines are common in chemical process industries and can lead to leakages of corrosive fluid. The vibration fatigue is an unavoidable factor which is further aggravated by improper installation of the moving system. Leakages due to these pulsations can be prevented by the installation of flexible rubber expansion joints. 

Bellows can vibrate both from internal fluid flow, and externally imposed mechanical vibrations. At high flow, velocities and flow induced resonance produces bellows fatigue. Multiple bellows are less susceptible to vibration failures because of the damping effect of the interplay friction. The benefits of multiple rubber expansion joints are given below ... 

The x-y CTE for standard FR-4 at 14 to 20 ppm/°C is higher than for ceramic or sihcon. The resulting thermal expansion mismatch between an ML-PWB and the attached devices can lead to solder joint fatigue failures when the system undergoes multiple heat cycles during power-up and power-down. Packages with comphant leads can accommodate the CTE mismatch and so can be used with standard ML-PWB material systems. 

What is important for this space truss problem depends on which of the various technical issues influence the design. Is stiffness an issue Is strength an issue If so, why Is buckling an issue Can fatigue be a problem Or corrosion Thermal expansion or joints Those factors are listed in Figure 7-23. 

Data for thermal movement of various bitumens and felts and for composite membranes have been given (1). These describe the development of a thermal shock factor based on strength factors and the linear thermal expansion coefficient. Tensile and flexural fatigue tests on roofing membranes were taken at 21 and 18°C, and performance criteria were recommended. A study of four types of fluid-applied roofing membranes under cyclic conditions showed that they could not withstand movements of <1.0 mm over joints. The limitations of present test methods for new roofing materials, such as prefabricated polymeric and elastomeric sheets and liquid-applied membranes, have also been described (1). For evaluation, both laboratory and field work are needed. 

Thermal fatigue in solder joints occurs because of the thermal expansion (CTE) mismatch between the PCB and the component interconnected by the solder joint (see Fig. 57.12). 

Interconnect failures due to thermal fatigue of solder joints can be reduced by closely matching the x-y plane thermal expansion properties of the substrate to at-risk components. Large leadless ceramic components that are used because of their hermeticity pose a particular risk. Possible approaches include altering the laminate reinforcement material, adding constraining metal cores or planes, and switching to a ceramic substrate. The first two approaches are discussed here. A more extensive discussion of these options can be found in Ref. 33. 

Miniaturisation and higher component density, which are resulting in the use of surface mounted leadless components, produce problems of heat dissipation and mismatch of thermal expansion of component package and base board that may produce fatigue stress in soldered joints and potential failure. 

Boots are typically blow molded from thermoplastic elastomers, CVJ boots from TEEE (COPE) and TPV, and shock absorber boots from TPU(E) such as Desmopan. Thermoplastic etherester elastomers and thermoplastic esterester elastomers TEEE (COPE), Arnitel, and Hytrel are blow molded into transmission and wheel CVJ boots for front-wheel drive (FWD) and rack-and-pinion boots. CVJ boots keep grease inside the joints and keep out dirt, grease, salt, and water. The TEEE CVJ drive axle boots have a balance of flexibility and stiffness, fatigue resistance to compression and expansion strains, chemical resistance, and meet accordion design requirements between -40 to 212°F (-40 to 100°C). 3- ... 

Cracking. Due to the temperature fluctuation caused by the circuit power on/off cycles, ICA interconnects have to sustain cyclic stresses from thermal expansion mismatch between the substrate and component, and thermo-mechanical fatigue cracking is considered as one of the primary failure mechanisms. Based on temperature cycling tests and cross-section observations, the fatigue cracking behavior of ICA joints of... 

The term thermal mechanical fatigue (TMF) was introduced to describe this temperature cycle plus thermal expansion mismatch degradation mode in surface mount solder joints. This phase emphasizes that the source of the cyclic deformation as being temperature variations rather than applied cyclic loads (e.g., vibration), thereby distinguishing TMF from mechanical fatigue. 

The only SM components with obvious fatigue failures after more than 6700 cycles of 0-100°C, or 5000 cycles of —55 to + 125°C were LCCCs and 1206 chip resistors. No leaded SM devices exhibited failures. There were no unexpectedly early or catastrophic chip carrier or passive component failures. Those failures that occurred followed the same component order as observed for eutectic Sn-Pb. The ranking of alloys relative to eutectic Sn-Pb varied with thermal cycle and component type. Each solder alloy is able to withstand different amounts, types, and rates of loading, which are dependent upon the different coefficients of thermal expansion (CTE) and mechanical properties of the board, components, and alloys, and upon solder joint configurations. 

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