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Designing Weldments. Группа авторов
Читать онлайн.Название Designing Weldments
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isbn 9781119865827
Автор произведения Группа авторов
Издательство John Wiley & Sons Limited
The fatigue fracture develops as each successive applied load advances the tip of the crack further to a point where the metal has no more ability to resist fracture. This type of crack progression is called stable growth of the crack. The rate of advance of the crack increases with the time and applied load, and the growth becomes unstable, and sudden failure occurs. It is important to know that any crack growth can occur only under cyclic loading in combination with residual tensile stresses in the metal itself, if there is a compressive stress, at the crack tip the crack growth will stop.
The stress that the metal can endure without fracture successively decreases as the number of cycles increases. Each successive stress cycle reduces the metal’s ability to sustain further.
For steel the fatigue strength is almost constant beyond about two million cycles. Several million more cycles are needed for steel to cause reduction in steel’s fatigue strength. Therefore, the Fatigue Limit becomes maximum stress or a range of stress which metal will be able to bear for infinite number of cycles without fracture, this is called the Endurance Limit. This leads us to the definition of the Fatigue Life. Fatigue life is the number of cycles of stress that the metal can sustain in the stipulated conditions.
Several tables are published that list various metal’s endurance limits. These are developed using a polished round test bars that are tested in air. These values may be useful in certain calculations for parent metal that may closely meet the test conditions. However, these endurance-limit values are not applicable to welds, and weld and metals that are in service in very specific environment. One of them very specific to welds is that welds have very abrupt changes in cross section, geometry, and also metallurgical make up, HAZ that has coarse grains, is one of them. Also welds contain some very specific discontinuities for example lack of fusion, lack of penetration, undercuts, etc., that is not found in the parent metal.
The life cycle of a welded structure is dependent on the welds that it contains. And the number of repeated variations of tensile or alternating between the tensile and compressive stresses that the weld goes through. These stresses are initially within the elastic range, and as long as it remains within that elastic range the structure is deemed safe, however if there are any stress concentrating anomaly in the weld (or even in the structure itself) will change that equilibrium, and subsequently and over the time they transfer to plastic stage and that leads to eventual failure through that weld and structure anomaly. When we say the structural detail or weld geometry, we include aspects like, type of joint, type of weld, surface finish, and structural details, all these are capable of amplifying the stress to the tip of the mechanical notches.
When designing welded built-up members, or welded connections for fatigue loading, the local codes and standards must be followed for its safe operation. In the absence of such guidance of code and standards full finite element analysis should be conducted for the design to be safe. Structural construction contain significant amount of residual stress, these residual stress are enhanced in a welded structure. It is understood that residual stress themselves do not cycle and cause fatigue stress, but they augment or detract from applied stress. The augmentation or detraction is dependent on the signs (positive or negative) of the residual stress. What we derive from the just preceding is statement? That we can reduce the impact of the residual stress y inducing compressive stresses, and that can be achieved by either or a combination of following
1 (1) Welding in sequence,
2 (2) Localized heat treatment
Both the above methods can develop compressive stress on welds, reducing or at least not augment the effect of cyclic loading.
2.1.3.5 Ductility
Ductility is defined as the amount of plastic deformation that metal undergoes in resisting the fracture under stress. This is a structure sensitive property and is affected by the chemical composition.
From the above Figure 2.1 we see that the material shows some level of ductility, the stress range between the limit of proportionality and ultimate tensile strength (UTS) defines the metals ductility. This varies from metal to metal. The amount of plastic deformation that the metal or the weld undergoes all through the fracture as shows in the stress and strain diagram above is the measure of metal or weld metals ductility. The Stress and strain plot do not actual characteristics of ductility but shows a relative value as a comparative number for the metal in identical condition. The plasticity of the metal is the deformation during the yielding process, as shown in the Figure 2.1.
Ductility is a structure sensitive property; hence it is affected by the test conditions, that may include any or a combination of several of the following.
• Test temperature,
• Shape and size of the test specimen,
• Rate of straining,
• Metallurgical structure of the metal,
• The surface condition of the specimen.
The ductility values obtained through the testing is used only as an indicator of materials ductility. For design purpose the precise values obtained through the testing is seldom used. Most structures are designed to operate much below the yield strength of the material; thus, the metal is rarely tested (in real operation) for its ductility. Often metals are tested to determine some degree of ductility and Toughness through Impact testing at given temperature. The subject of impact testing spans through the low temperature ductility of metals , this is discussed in much detail further in the chapter, however this is the place to discuss various approaches to impact test a metal.
2.1.3.6 Elastic Limit
As we know that metals have elastic property, this elastic behavior reaches a limit at a level of stress called elastic limit. The term Elastic limit is more a definition than an exact indicator of the stress level that indicates the limit. The stress level of course varies for each metal, it is structure sensitive, and also depends on the rate at which the metal is strained.
Elastic Limit is the upper bound of the stress where when the stress is released the metal will return to its original dimensions. On the other side of this limit, a permanent deformation in the metal would result.
The Figure 2.1 above shows the typical stress and strain diagram. The stress σ is plotted on the Y axis and the resulting strain ϵ is plotted on the X axis.
As the metal under test is stressed, the strain on the X axis increases. To a point this increase is proportional to the increasing stress, up to this point the load is proportionally balanced by the strain, if the stress is removed at or below this point the metal will show no permanent stress - damage. Further loading deviates from the proportionality however the metal remains elastic to the point where elastic stain is reached, this means that further from this point the metal is not elastic, and it will not revert back to its original form or shape – a permanent damage has taken place. The stress below this limit allows the metal to revert back to its original shape and size if the stress is removed. This is the point where the limit of metal’s elasticity is reached.
2.1.3.7 Impact Strength
Impact properties are related to the toughness of the material. It establishes the material’s inherent ability to resist fracture on application of sudden and high impact. In subsequent paragraphs more details is discussed on this very important structure sensitive property of metals.
Impact properties are determined by tests. These tests are conducted to assess metals ductility and toughness. These two properties in combination define metal’s ability to resist fracture on sudden impact.