various elements of tensile test.
2.1.3.3 Yield Strength
The yield strength of a metal is the load at which the metal transits from being elastic to plastic. This load is reached at a point called yield point, however it also transits and peaks at one point, where metal exhibits total plasticity and YIELDS to the applied load. Both these points are shown in the Figure 2.1 above.
Note that there is a line at 0.2 percent offset, this value of the yield is often the engineering yield value that is used for design calculations.
2.1.3.4 Fatigue Strength
When a metal structure is subject to repeat (a cyclic load) loading, the metal is subject to specifically more stringent conditions. The cyclic loading fatigues the metal structure and reduces the life of the structure, and ultimately fractures and fail. Fatigue strength is an important mechanical property to know about the metal and welds if cyclic loading is one of the demands of the designed structure. Metal’s ability to sustain cyclic loading, for longer time of its design life is the Fatigue strength of that metal. Fatigue strength is a measures of load versus the time.
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
