Cyclic stressing

The SPATE system detects the infrared flux resulting from the minute temperature changes in a cyclically stressed structure or component. 

The research activity here presented has been carried out at the N.D.T. laboratory of l.S.P.E.S.L. (National Institute for Occupational Safety and Prevention) and it is aimed at the set up of the Stress Pattern Analysis by Measuring Thermal Emission technique [I] applied to pressure vessels. Basically, the SPATE system detects the infrared flux emitted from points resulting from the minute temperature changes in a cyclically stressed structure or component. 

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. 

To avoid operating difficulties, the torsional critical frequencies of the combined engine and driven equipment should be calculated or measured to assure that operating speeds are removed from these criticals or that vibration dampers are provided or that the equipment is designed for the resulting cyclic stresses. 

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. 

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

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. 

Static stress A stress whose magnitude remains at a constant value. Contrast with cyclic stress. 

Here Acto is the cyclic stress range for failure in Nf cycles under zero mean stress, and Acr m is the same thing for a mean stress of a .) Goodman s Rule is empirical, and does not always work - then tests simulating service conditions must be carried out, and the results used for the final design. But preliminary designs are usually based on this rule. 

The cyclic stress intensity AK increases with time (at constant load) because the crack grows in tension. It is found that the crack growth per cycle, da/dN, increases with AK in the way shown in Fig. 15.8. 

A certain grade of PMMA has a K value of 1.6 MN m and it is known that under cyclic stresses, cracks grow at a rate given by (2 x 10 A/( ). If the intrinsic defects in the material are 50 mm long, how many hours will the material last if it is subjected to a stress cycle of 0 to 10 MN/m at a frequency of 1 Hz. 

To initiate a chemical relaxation it is necessary to perturb the system from its initial equilibrium position. This is done by applying a forcing function, which is an appropriate experimental stress to which the system responds with a shift in equilibrium configuration. Forcing functions can be transient (a sudden, essentially discontinuous Jolt ) or periodic (a cyclic stress of constant frequency). 

As von Nimitz points out, only cyclic stresses are directly related to failure probability. These stresses are often produced by pulsations in the fluid system, by mechanical vibrations produced by the mechanical movement of certain equipment components, and as a result of the fluid pulsations. Figure 13-8 lists the sequence of events that leads to most failures of equipment and piping. 

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. 

Mechanical aspects the presence of tensile or cyclic stresses. 

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). 

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