Cyclic Stress Fatigue

Crushing surfaces Ejected parts/fragments Fatigue/cyclic stresses Flexure... 

Corrosion fatigue is a type of failure (cracking) which occurs when a metal component is subjected to cyclic stress in a corrosive medium. In many cases, relatively mild environments (e.g., atmospheric moisture) can greatly enhance fatigue cracking without producing visible corrosion. 

Several theories have been proposed to explain the corrosion-fatigue phenomena. One is that cyclic stressing causes repeated rupture of protective coatings. Corrosion-fatigue cracks propagate as the coating is successively reformed and ruptured along a plane. 

No common industrial metal is immune to corrosion fatigue since some reduction of the metal s resistance to cyclic stressing is observed if the metal is corroded, even mildly, by the environment in which the stressing occurs. Corrosion fatigue produces fine-to-broad cracks with little or no branching. They are typically filled with dense corrosion product. The cracks may occur singly but commonly appear as families of parallel cracks (Fig. 10.2). They are frequently associated with pits, grooves, or other forms of stress concentrators. Like other forms of... 

Perhaps the most important stress factor affecting corrosion fatigue is the frequency of the cyclic stress. Since corrosion is an essential component of the failure mechanism and since corrosion processes typically require time for the interaction between the metal and its environment, the corrosion-fatigue life of a metal depends on the frequency of the cyclic stress. Relatively low-stress frequencies permit adequate time for corrosion to occur high-stress frequencies may not allow sufficient time for the corrosion processes necessary for corrosion... 

Alter the environment to render it less eorrosive. This approach may be as simple as maintaining clean metal surfaces. It is well known that the chemistry of the environment beneath deposits can become substantially different than that of the bulk environment. This difference can lead to localized, underdeposit corrosion (see Chap. 4, Underdeposit Corrosion ). The pit sites produced may then induce corrosion fatigue when cyclic stresses are present. The specific steps taken to reduce corrosivity vary with the metal under consideration. In general, appropriate adjustments to pH and reduction or elimination of aggressive ions should be considered. 

Intergranular corrosion-fatigue cracks in copper may he difficult to differentiate from stress-corrosion cracking. The longitudinal orientation of the cracks revealed that the cyclic stresses were induced by fluctuations in internal pressure. 

The drill collars and particularly their connections are also exposed to cyclic stresses. Subsequently, these are susceptible to fatigue damage, but the changes that may influence failure arc more quickly discovered. 

Cumulative fatigue damage The total of fatigue damage caused by repeated cyclic stresses. 

The simultaneous action of cyclic stress alternating tensile and compressive and corrosive attack is known as corrosion fatigue. Corrosive attack can be in the form of pitting. These pits function as notches, acting as stress risers and initiate cracks. Once a crack is formed, the probability of pipe failure is enhanced by further corrosion as corrosion is accelerated by action of stress. The tip of the crack deep within the fracture, the area under the greatest stress, is anodic to the wider part of the crack. As corrosion progresses, the metal at the tip of the crack goes into the solution, the crack deepens and eventually penetrates the wall of the tube. 

Stainless steels are subject to fatigue failure under dry conditions as are all metallic materials, having distinct fatigue limits where level is dependent on steel type and heat treatment. The limits can be depressed by the simultaneous action of a corrodent, the degree depending upon the nature of the corrodent. Under severe conditions the limit can be displaced to very low values and it is customary to describe resistance by an endurance limit, that is the cyclic stress to give rupture at a specific number of cycles when in contact with a specific corrodent. Some comparative data are in Table 3.25. 

Stress below the proof stress does not normally affect corrosion rates. Cyclic stresses in combination with a corrosive environment (corrosion fatigue) can produce failure at below the ordinary fatigue limit. Alloys susceptible to intergranular attack may corrode faster when stressed (see Section 8.5). 

Barsom, J. M., Effect of cyclic stress waveform on corrosion fatigue crack propagation... 

Undue static or cyclic stressing and other features which give rise to stress concentrations should be avoided as these may lead to premature failure by stress-corrosion cracking or corrosion fatigue. 

It is often difficult to conduct laboratory tests in which both the environmental and stressing conditions approximate to those encountered in service. This applies particularly to the corrosive conditions, since it is necessary to find a means of applying cyclic stresses that will also permit maintenance around the stressed areas of a corrosive environment in which the factors that influence the initiation and growth of corrosion fatigue cracks may be controlled. Among these factors are electrolyte species and concentration, temperature, pressure, pH, flow rate, dissolved oxygen content and potential (free corrosion potential or applied). 

Plastics are susceptible to brittle crack-growth fractures as a result of cyclic stresses in much the same way as metals. In addition, because of their high damping and low thermal conductivity, plastics are prone to thermal softening if the cyclic stress or cyclic rate is high. Examples of the TPs with the best fatigue resistance include PP and ethylene-propylene copolymers. 

The term corrosion fatigue is used to describe the premature failure of materials in corrosive environments caused by cyclic stresses. Even mildly corrosive conditions can markedly reduce the fatigue life of a component. Unlike stress corrosion cracking, corrosion fatigue can occur in any corrosive environment and does not depend on a specific combination of corrosive substance and metal. Materials with a high resistance to corrosion must be specified for critical components subjected to cyclic stresses. 

A detailed fatigue analysis is required if any of these conditions is likely to occur to any significant extent. Fatigue failure will occur during the service life of the vessel if the endurance limit (number of cycles for failure) at the particular value of the cyclic stress is exceeded. The codes and standards should be consulted to determine when a detailed fatigue analysis must be undertaken. 

Dynamic properties are taken to mean the results from mechanical tests in which the plastic is subjected to a deformation pattern from which the cyclic stress-strain behaviour is calculated. These do not include cyclic tests in which the main objective is to fatigue the material. 

Cyclic fatigue measurements require the specimen to be subjected to cyclic stress or strain of a higher amplitude than that employed for the simple dynamic test just described. The deformation must be of sufficient intensity to bring about specimen failure after a certain number of cycles, N. The value of stress leading to failure for a given N is typically 20% to 40% of the static tensile strength. 

The first work reported in this area was an investigation of isotactic polypropylene which was fatigued at a rate of 10 hz with an elongation of one to five percent. The spectra showed no shifts in frequency but reversible intensity changes were observed. The data were interpreted in terms of the presence of a smectic-like structure as a result of the cyclic stress 304i 305>. 

Property Summary Nylons are recommended for general-purpose gears and other mechanical components. Acetals for maximum fatigue life, for highly accurate parts, or exposure to extremely humid conditions. Phenolic-fabric laminates for low-cost, thin stamped gears or parts. Polycarbonates for intermittent, very high impacts (not recommended for applications involving repeated cyclical stress). TFE-filled acetals for heavy-duty applications. 

The problem- points in high-pressure machines are certainly the dynamic shaft- and piston-or plunger seals. Seals create special problems when they slide rapidly under high-pressure and if the fluid being sealed has no lubricating actions. Cyclic stress creates additionally fatigue problems in the seal. 

The need to ensure that the stresses in piping systems meet the appropriate code requirements and the concern that cyclic stresses resulting from events such as periodic heating and cooling of the piping may lead to fatigue failures, make accurate evaluation of the stresses and strains in piping systems a necessity. 

Corrosion fatigue, resulting from the combined effects of corrosion and cyclic stresses. Racks of Ihis type are characteristically Iranscrystalline. 

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