Corrosion assisted fatigue occurs when corrosion damage changes the surface

Corrosion assisted fatigue occurs when corrosion

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Corrosion assisted fatigue occurs when corrosion damage changes the surface texture and significantly increases the local stresses acting on a pump component due to notch sensitivity. When this happens, fatigue cracking of the component is likely. In some cases, crack propagation can be influenced by oxidation, which can mask the features of the fatigue mechanism. Corrosion oxides, which form along the face of the crack, can produce a wedging effect, which mechanically is able to increase the local tensile forces acting on the crack tip, and thus, increase the rate of crack propagation. Fretting or wear can also produce sites where fatigue cracking can initiate. In addition, sharp radii and defects on the material surface, such as in the case of porosity and poor machining, can act as stress concentrations in susceptible materials [11]. Fatigue can be seen in its three stage progression from (1) crack initiation, which is sometimes associated with pre-existing defects, (2) crack propagation, and (3) final failure. The applied stress level, geometry, flaw size, and mechanical properties determine the existence and extent of these stages of fatigue. These three stages of fatigue cracking can be observed on the fracture surface, provided that there is no secondary damage that masks the characteristic appearance. The bands that result from the fatigue are often referred to as “clamshell markings”, “crack arrest lines”, or “beach marks” and reflect different periods of the crack growth. “Ratchet lines”, which are the joining of two different crack fronts on different planes, are observed in cracks originating from multiple origins. These types of fatigue fractures are normally associated with rotating components. Once the cause of the fatigue cracking has been identified, corrective actions can be taken to remedy the problem. These corrective actions may include: using fatigue resistant materials, modifying the design of the pump, treating the affected surface, or using more highly corrosion resistant materials [11].
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9 4 OTHER MODES OF FAILURE There are some modes of failure that fall neither under the categories of hydraulic or mechanical problems. These modes of failure are sometimes structural, such as in the case of erosion and corrosion, or are a result of a multitude of different sources, such as in the case of excessive power consumption. This section will cover the causes and solutions, if any, of these types of problems. 4.1 Erosion Erosion on pumps take on one of five different forms: cavitation erosion (see 2.1), adhesive wear, fretting, abrasive wear, or erosion by solid particle impingement [11]. Adhesive wear is a result of material to material contact. This is the primary cause of material loss when handling fluids that contain no solids in the stream. Surfaces of parts inside the pump are able to have material-to-material contact, producing surface disruptions, material grooving, transferral of material, and possibly, galling. Two important characteristics that should be considered for materials that come into contact are their adhesive wear traits and their galling threshold.
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