FRACTURE MECHANICS-KINDS OF FAILURE: Some people suffer from hypertension and are susceptible to heart attacks. Many, unfortunately, suffer from diabetes and take special precautions to avoid or delay its destructive effects. On an average, a person in our society is vulnerable to one or two of such diseases as cancer, arthritis, asthma, hepatitis, gastritis, tuberculosis, etc. and is conscious about it. Similarly, a component in a structure may be susceptible to one, two or more kinds of failure. For example, under given load conditions a roller bearing is most likely to fail through fatigue of its rollers after a certain number of rotations. We should know the different conditions that can cause the failure of structural component. Some of the common causes of failure are: 1) Yielding 2) Deflection beyond a certain stage. 3) Buckling 4) Fatigue 5) Fracture 6) Creep 7) Environmental degradation 8) Resonance 9) Impact 10) Wear A component is designed so as to avoid yielding of the worst loaded point (critical point).Safety against yield failure is considered to be the basic requirement of any design and is taught in all courses on ‘strength of materials’ at undergraduate level. Although stress tensor is quite complex with six independent components, criteria like MISES or TRESCA are adopted to obtain a scalar number (E.G maximum shear stress in TRESCA criterion).The scalar number is then compared with a limiting value which is determined through experiments. In simple cases, the worst loaded point may be easily being identified, whereas it may be difficult to find where exactly the worst loaded point is in a general case. These days excellent computer packages based on finite element analysis are available. In a finite element simulation, the body of the component can be divided into different colors; for example red color shows the portion where the stresses are maximum, green color shows the lowest stresses and other colors for intermediate stresses. However it has been found that often a structural component fails even when a worst loaded point is well within the yield stress. Thus we conclude that the design of worst component based entirely on avoiding yielding is not adequate in certain cases. The component may be susceptible to crack growth. All engineering material deform on loading. A structural component may be deformed to such an extent that its performance is affected. Certain plastic has been developed to posses high strength, but they may not be suitable for many structural components because of their low stiffness, i.e., about 1% of steels stiffness. If the wings of an airplane are made of a polymer, they may drop down so much that the tip of the wings would touch the ground. In fact, many components of conventional engineering materials (steel, aluminum and the like) are designed to meet constraints of deflection, although the stress of the worst loaded point is considerably lower than yield stress. These kinds of components are thus susceptible to failure through deflection beyond a certain value. If crack grows in a component, it stiffness decreases and the deformation may exceed that allowable limit. A thin member under a compressive load or a thin tube under torsion or lateral compression may be susceptible to buckling, and the design should be checked against likely failure through buckling. Many components of the modern industrial world are subjected to fluctuating loads and consequently may fail through fatigue. In fact, failure through fatigue is so common that more than 80% failures are caused by fluctuating loads. Many investigators are currently working for the development of this field. However convenient and effective methods to control fatigue failures are still not adequately developed. A critical structural component should be regularly checked to detect fatigue cracks through non destructive tests. This has led to the development of excellent methods of crack identification, such as ultrasonic crack detection-RAY and radiation filming, detecting through monitoring acoustic emission, magnetic flux method, decoration of surface cracks through dye penetration, etc. Fracture mechanics is based on the implicit assumption that there exists a crack in a work component. The crack may be manmade such as a hole, a notch, a slot, a re-entrant corner, etc.The crack may exist within a component due to manufacturing defects like slag inclusion, cracks in a weldment or heat affected zones due to uneven cooling and presence of foreign particles. A dangerous crack may be nucleated and grown during the service of the component (fatigue generation crack nucleation of notches due to environmental dissolution). Fracture mechanics deals with the question is a know crack likely to grow under a certain given loading condition. Fracture mechanics is also applied to crack growth under fatigue loading. Initially, the fluctuating load nucleates a crack, which then grows slowly and finally the crack growth rate per cycle picks up speed. Thereafter comes the stage when the crack length is long enough to be considered critical for a catastrophic fracture failure. About 50-60 years ago, when accurate analysis for predicting the growth of a crack was not available, a reasonably high factor of safety was chosen to account for unforeseen factors. A large part of this ambiguity has been cleared with the development of fracture mechanics and understanding the causes and effects of fatigue failure. This now enables a designer to use a much lower factor of safety, thus reducing cost of such structural components. Simultaneously, the weights of these components are reduced and their reliability is enhanced. Other ways of failure (creep, environmental degradation, wear, etc) are also as important and must be looked into and analyzed, so that the component may not develop snags on their account. However, one thing should also be borne in mind, which a component is usually not likely to fail through more than two or three ways. Therefore, susceptibility should be kept in mind at the time of designing a component. PRESSURE SCIENCE RESEARCH & DEVELOPMENT GROUP
Posted on: Thu, 24 Oct 2013 06:12:22 +0000
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