Design for Fatigue

Regardless of whether a material is metallic or ceramic, design to overcome fatigue failure and to increase resistance to cyclic deformation is essential. However, for the time being, this task seems almost impossible. As indicated 

In this equation, SpL is the fatigue limit. To-date, the author does not know if these relations hold for ceramic materials and whether similar relations exist between monotonic stress and fatigue strength for all ceramics. 

Attempts have been made to relate fatigue properties to static parameters, based on the similarity between monotonic and fatigue mechanisms, which control cyclic straining and plastic flow. This is somewhat problematic, because most ceramics are brittle and may be observed in their plastic state only at elevated temperatures. It would be more practical to discover a relation between static-yield stress and fatigue strength, since the yield stress in brittle materials coincides more or less with the fracture strength. 

To summarize the requirements for fatigue design, the following is a list of some of the many factors that influence fatigue life and must be taken into account during the design process  

Anderson TL (1991) Fracture mechanics fundamentals and applications. CRC Press, Boca Raton 

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When a vessel is subject to repeated loading that could cause failure by the development of a progressive fracture, the vessel is in cyclic service. ASME Code, Section VIII, Division 2, has established specific criteria for determining when a vessel must be designed for fatigue. 

P(2) The fundamental requirements of design for fatigue shall he as specified ... 

P(4) Where none of the above conditions applies, the design for fatigue shall he checked at the ultimate limit state. For n stress cycles the following criteria shall he met at all points in the FRP composite components subject to fatigue loading ... 

When designing for fatigue, the designer should pay particular attention to areas where stress concentrations are likely to be present. These may occur at connections, re-entrant corners and points of acute change of direction. 

Fatigue properties in bending are most appropriate for copper aHoys as these are often used as spring contact components in beUows and electrical switches and coimectors. These articles are usuaHy designed for acceptable service Hves at a moderate to high number of stress cycles. 

Part AD This part contains requirements for the design of vessels. The rules of Division 2 are based on the maximum-shear theoiy of failure for stress failure and yielding. Higher stresses are permitted when wind or earthquake loads are considered. Any rules for determining the need for fatigue analysis are given here. 

The toughness of wood is important in design for exactly the same reasons that that of steel is it determines whether a structure (a frame building, a pit prop, the mast of a yacht) will fail suddenly and unexpectedly by the propagation of a fast crack. In a steel structure the initial crack is that of a defective weld, or is formed by corrosion or fatigue in a wooden structure the initial defect may be a knot, or a saw cut, or cell damage caused by severe mishandling. 

The first type of bonded design for this application was the beaded doubler panel (Fig. 28). This design was fairly successful at addressing the problems with simple riveted structure but had two primary drawbacks. The area under the beads remained a single thickness sheet and was still prone to fatigue. Reducing the unbonded areas under the beads was not a solution because it reduced the overall stiffness of the panel. Secondly, tooling for these panels was complex and not very robust. Autoclave pressure applied to the beaded areas of the doubler would cause them to collapse, so thick frames were fabricated with cutouts for the beads to protect them. A rubber layer bonded to the surface of the frames... 

A principle of application relative to internals is to obtain as many 180° or (second preference) 90° turns of the gas flow after it enters the drum as reasonably possible. Plat plate pieces, baffles, etc., welded to the side or nozzles are not recommended, because the vibration forces tend to fatigue and crack the welds. Baffle plates have been known to break loose on one face and ratde around in the drum. This can be potentially dangerous. The vessel itself should be designed for rugged service. 

Butyl rubber - This material generally had the least endurance in fatigue tests, but it may be adequate for some cardiovascular applications. Advantages include less sensitivity to stress concentrators than Pellethane, a very low permeability to fluids, a moderate creep resistance and widespread availability at low cost. Disadvantages include a relatively low fatigue resistance compared to the elastomers specifically designed for these applications. The rubber tested was not designed for medical applications and had standard rubber additives and modifiers that were cytotoxic unless the material was extracted after manufacture. 

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). 

Nonmetallics such as plastics are finding increasing use in place of metals and alloys. These materials are also subject to failure due to fatigue and other types, such as impact, chemical attack, pressure, fire and joint failures. The main types of failure along with the causes and recommended design for joints are given in Table 1.20 and Figure 1.55. 

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