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WELDS-STATIC AND FATIGUE STRENGTH-III WELD -STATIC AND FATIGUE STRENGTH -III

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WELDS-STATIC AND FATIGUE STRENGTH-III WELD -STATIC AND FATIGUE STRENGTH -III
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   WELDS-STATIC AND FATIGUE STRENGTH-III Version II 32 -1 WELD - STATIC AND FATIGUE STRENGTH -III 1.0   INTRODUCTION A component or a structure, which can withstand a single application of load, may fracture if the same load is applied a large number of times. This type of failure is classified as fatigue fracture. Thus, fatigue failure can be defined as the number of cycles or the time taken to attain a pre-defined failure criterion. A more precise definition of fatigue is given as the process of progressive localised permanent structural change occurring in a material subjected to conditions, which produce fluctuating stresses and strains at some points and which may culminate in cracks or complete rupture after sufficient number of fluctuations. Hence, fatigue phenomenon is experienced by structures, which are subjected to moving loads, such as bridges and crane girders, or structures subjected to cyclic loads such as offshore platform structures and machinery supporting structures. Fatigue as a failure mechanism was identified initially in the rolling stocks and tracks of railways and subsequently in railway bridges. Recently, fatigue problem has been experienced in highway bridges, with a few failures of bridges. In many instances, due to timely repair measures, catastrophic collapse of structures due to fatigue has been avoided. The lower the stress ranges i.e., the difference between the alternating maximum and minimum stresses, the larger the number of cycles the structure can withstand before the occurrence of fracture. In the case of fatigue fracture of engineering structures, the following are the two main types of fatigue loading.    High–cycle low–stress fatigue.    Low–cycle high–stress fatigue. In a typical high-cycle fatigue problem, the endurance limit of the material after millions of cycles of load application is of concern, whereas in low-cycle fatigue, fracture before 10 5   cycles is the consideration. In high-cycle fatigue problems, the critical portion of the structure is subjected to frequent repeated loads, such as welded tubular joints in steel offshore platform structures subjected to wave loading. In such areas, several million (100 million) cycles are achieved during the lifetime of the offshore structure (about 25 years). An example of low fatigue fracture is the hull structure of a ship. When structural components are exposed to corrosion environment, such as seawater, the synergistic effect of corrosion and fatigue, known as corrosion fatigue becomes a serious problem. © Copyright reserved 32   WELDS-STATIC AND FATIGUE STRENGTH-III Version II 32 -2 Since fatigue failure evaluation is influenced by a number of uncertainties, an accurate  prediction of fatigue life is difficult even for a very simple detail. Fatigue failure  prediction is difficult in structural components due to the following uncertain features:    The effect of environment in which the structure is functioning.    Difficulty in accurate calculation of internal stresses developed due to external forces at critical locations in the structure    The time to failure of the structure. Two basic approaches for fatigue life assessment of structural components are: 1) the S – N method and 2) the method of fracture mechanics. The S – N method of life  prediction is based on empirically derived relationships between applied stress ranges ( S  ) and number of cycles of load application (  N  ). The fracture mechanics approach takes into account the crack growth rate of an existing defect as it propagates under the cyclic loading. In this chapter, topics such as characteristics of fatigue, methods of evaluation of fatigue life, improvement of fatigue resistance, fatigue-resistant design etc. have been covered. 2.0 CHARACTERISTICS OF FATIGUE FRACTURE Structural materials undergo mechanical changes when subjected to cyclic stresses and trigger many engineering failures due to fatigue. Poor design and fabrication are the  prime reasons for the failures. A fatigue failure occurs as a result of various mechanisms, which take place in three stages during the life of a structure. As a result of cyclic loading a microscopic defect initiates, then propagates in a gradual manner, resulting finally in an unstable fracture. Cracking srcinates mostly on the surface at a point of stress concentration – a hole, notch, keyway, scratch, weld bead, sharp fillet etc. Crack initiation may occur, occasionally, at an interior point such as a defect in a weld. In most welded steel structures the crack initiation phase does not exist as crack–like weld defects are invariably present in them. Thus the fatigue life of a connection containing welds is entirely due to crack growth. Final failure usually occurs in a tension region when the reduced section is no more sufficient to carry the peak load. Many repetitions of the stresses - of the order of millions – may be required for complete rupture. Similarly, the time required for final collapse may be short or many years in some cases. The maximum stress at the fracture location would be well below the value obtained under static loading. It is very difficult to detect a fatigue crack even up to the  point of failure. Since there is very little plastic deformation around the crack, there is no evidence of the presence of crack, repeated through large deformation. A small crack initiated grows slowly with the repetition of stress cycles. A fatigue crack is said to be transgranular i.e., its grows within grains rather than along the grain  boundaries. As the crack propagates, the cross-section reduces and the stress on the reduced cross section increases. As a result, there will be an increase in the rate of crack  propagation. The final rupture occurs when the remaining area is no longer sufficient to support the applied load.   WELDS-STATIC AND FATIGUE STRENGTH-III Version II 32 -3 The above features of fracture due to fatigue can be seen on the fracture surface. The fracture surface may be either crystalline or fibrous depending upon whether the fracture is brittle or ductile. In the close neighbourhood of the crack’s srcin, the fractured surface has a smooth, silky appearance, which is produced by rubbing of the surface as the crack  propagates (Fig.1). The smooth region grows progressively into a rougher texture as the distance from the origin increases. An examination of this surface would reveal the  presence of concentric rings or beach markings around the fracture nucleus and radial lines emanating from it. 3.0 THE MECHANISM OF FATIGUE Presently there is no rational theory available for fatigue failure prediction relating stresses and material properties. This is due to the complex mechanism involved in the fatigue process. The mechanism of fatigue is explained briefly in the following. Due to stress concentration effects, the stress in a localised region in a structural element may attain the value needed for plastic flow. The nominal stress or the stress without concentration effects may be below the proportional limit. At this stage, slip might occur in an unfavorably oriented crystallographic plane due to excessive shear stress on the  Fig.1 Fractured surface of a specimen   WELDS-STATIC AND FATIGUE STRENGTH-III Version II 32 -4  plane. This might be a fine slip i.e., a slip of order 10 -6 mm  below adjacent region of the crystal [Fig.2 ( a )]. The reversal of stress at this time might partially set right the disorientation. Repetition of the stress cycle and the resulting back and forth slip on closely spaced parallel planes will cause slip band to develop [Fig.2 ( b )]. This would form a notch. A microscopic crack may form because of the stress raising effect of the notch or the notch itself becomes deeper. Once the crack has formed, the process is further intensified. 4.0 FACTORS INFLUENCING FATIGUE BEHAVIOR The fatigue behavior of various types of structures, members and connections is affected  by a large number of factors, many of which may produce interrelated effects. The  parameters that influence fatigue behavior are: stress range, material, stress concentration, rate of cyclic loading, residual stresses, size, geometry, environment, temperature, and previous stress history. These are explained briefly in the following. 4.1 Stress range The most important factor governing the rate of crack growth is the stress range in the vicinity of the crack tip. Hence in a fatigue design, the stress concentration effect has to  be reduced and the stress range has to be realistically estimated. The importance of stress concentration is brought out in detail in the following section. 4.2 Stress concentration The geometry and the consequent stress concentration have a large impact on fatigue lives of structural members and their connections with other members. Stress distribution is generally different from that adopted in design mainly due to stress concentration. Such  points of stress concentration under cyclic loading undergo reduction in strength, often leading to fracture. The importance of stress concentration is illustrated in Fig.3. In Fig.3 ( a ), typical detail of a connection in an industrial roof structure is shown. Here the design is mainly governed by static strength. The local stress at the connection could be 5 to 10 times the average stress calculated by the simple theory for static design. The structure,  by way of local yielding, accommodates safely the discontinuities and stress concentration. Fig.3 (b) shows a fatigue-sensitive bridge structure connection detail. 5*10  –6   mm (a) Fine slip 10  -  min (b) Coarse bond produced by alternating slip  Fig. 2 Fatigue mechanism   WELDS-STATIC AND FATIGUE STRENGTH-III Version II 32 -5  Here, stress concentration has to be reduced by careful design. Detailed design considering both primary axial stress and secondary bending stress has to be performed. Weld profiling should be smoothened to minimize local discontinuities. It is appropriate now to consider three levels of stress concentration. 4.2.1 Structural action In the static analysis of a structure, elastic analysis is carried out based on compatibility concepts. The relative deformation between neighboring elements is often ignored. These local deformations develop additional strains and stresses. Secondary members, whose effects are often ignored in the static analysis, develop stresses due to the relative deformation in neighbouring elements. These additional stresses cause stress concentration. In the roof connection shown in Fig.3 ( a ), a simple static design would ignore the incompatibility that is caused by the restraint to the end rotation of the individual elements. The resulting bending stresses may be of similar magnitude as axial stresses in a truss with larger size sections. Therefore, in a fatigue design these bending stresses must be considered.  4.2.2 Macroscopic stress concentration This type of stress concentration arises due to geometric interruptions to stress flow. The smooth flow of stress trajectories are modified due to changes in cross section, notches, holes and other discontinuities. Fig. 4 shows a discontinuous structure with the attendant stress concentration effect.  Fig. 3 Typical connection details (a) Industrial roof structure (b) Bridge structure  Fig. 4 Bending stresses in a discontinuous beam (a) Beam arrangement (b) Stress flow at change in direction
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