Cyclic loading

Rehabilitation of Pipelines Using Fiber-reinforced Polymer (FRP) Composites 

Failure is typically initiated as a crack in the polymer matrix and rarely occurs in the hber matrix. The number of load cycles required to initiate the development of a crack is called the initiation time. The time to failure after a crack is initiated is usually much less than the initiation time, such that when a crack does develop, the FRP is in close proximity to failure and only a few more cycles may be all that is required to begin crack propagation. 

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Standard procedures that are used for testing of construction materials are based on square pulse actions or their various combinations. For example, small cyclic loads are used for forecast of durability and failure of materials. It is possible to apply analytical description of various types of loads as IN actions in time and frequency domains and use them as analytical deterministic models. Noise N(t) action as a rule is represented by stochastic model. 

The cyclic loading applied to hull structure will eventually lead to fatigue cracking. 

The use of fatigue data and crack length measurements to predict the remaining service life of a stmcture under cyclic loading is possibly the most common application of fracture mechanics for performance prediction. In complex stmctures the growth of cracks is routinely monitored at intervals, and from data about crack growth rates and the applied loadings at that point in the stmcture, a decision is made about whether the stmcmre can continue to operate safely until the next scheduled inspection. 

Fig. 5. Typical acoustic emission (AE) waves observed at different zones and during fracture of cyclic loading of PMMA. The amplitude is drawn on...

Data on fatigue crack propagation are gathered by cyclically loading specimens containing a sharp crack like that shown in Fig. 15.7. We define... 

In the case of a pressure vessel subjected to cyclic loading (as here) cracks can grow by fatigue and a vessel initially passed as safe may subsequently become unsafe due to this crack growth. The probable extent of crack growth can be determined by making fatigue tests on pre-cracked pieces of steel of the same type as that used in the pressure vessel, and the safe vessel lifetime can be estimated by the method illustrated in Case Studv 3. 

The engine could still be run for demonstration purposes. Suppose that you are called in to assess its safety. We will suppose that a crack 2 cm deep has been found in the connecting rod - a cast-iron rod, 21 feet long, with a section of 0.04 m. Will the crack grow under the cyclic loads to which the connecting rod is subjected And what is the likely life of the structure ... 

A component is made of a steel for which K. = 54 MN m. Non-destructive testing by ultrasonic methods shows that the component contains cracks of up to 2a = 0.2 mm in length. Laboratory tests show that the crack-growth rate under cyclic loading is given by... 

Component stress—during the back flow and forward reversals, all liic components involved with the gas propulsion are loaded diui unloaded, placing blading in a cyclic loading mode. 

G. Neighbour, B. McEnaney and M. Philips. Acoustic emission responses from cyclic loading of a nuclear graphite. Carbon, 1992, 30(3), 359 363. 

During cyclic loading, any cracks in the material will propagate until they reach this critical size. If the article is to have an endurance of at least 10 cycles then equation (2.119) may be used to determine the size of the smallest flaw which can be present in the material before cycling commences. 

Pipe, valve, and fittings tables must specify which size of each class of pipe is threaded, flanged, or socket welded. ANSI B31.3 provides no specific guidance except that it suggests that threads be avoided where corrosion, severe erosion, or cyclic loading is anticipated. 

Material Properties. Materials possess various mechanical and chemical properties, and, therefore, it is possible to select materials appropriate for severe corrosion conditions. For example, if the equipment is under cyclic loading, a material with high fatigue strength is desired. Similarly, it is desirable to have corrosion-resistant materials for the corrosive environments. There are several sources for obtaining information on materials properties. Some are listed in Table 4-173. 

Ward et a/."" have shown that, under cyclical loading, the oxidation rate of steels is similar to that under unstressed isothermal conditions, provided the fatigue stress is below the stress required to exceed the scale failure strain. If, however, the failure strain is exceeded, the oxidation rate is accelerated due to repetitive scale failure, and linear kinetics are observed. 

McIntyre, P., Hydrogen-steel interactions during cyclic loading , Proc. of UK/USSR Seminar on Corrosion Fatigue of Metals, Lvov, USSR, 19-22 May 1980, The Metals Society pp. 62-73 (1983)... 

Fig. 2-33 Example of strain behavior under various intermittent and cyclic loads.

Under a repeated applied cyclic load, fatigue cracks begin somewhere in the product and extend during the cycling. Eventually the crack will expand to such an extent that... 

Cyclic loading significantly reduces the amount of allowable stress a material can withstand. If data are not available on the endurance limit of a material being considered for use, a percentage of its tensile strength can... 

In addition, it should exhibit a fairly high hysteresis level that would have the effect of dissipating the sharp mechanical impulse loads as heat. The material will develop heat due to the stress under cyclical load. Materials used are the elastomeric plastics used in the products or as a coating on products. 

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