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fracture and Brittle fracture. These two modes are easily recognized when they occur in exclusion, but fractures in metal often have mixed morphology and it is aptly called mixed mode.

      The mechanisms that initiate the fracture are Shear fracture, Cleavage fracture, and Intergranular fracture.

      Only shear mechanism produces ductile fracture.

      It may be noted that like modes discussed above, the failure mechanism also have no exclusivity.

      Irrespective of the fracture being ductile or brittle, the fracture process is viewed as having two principal steps.

      1 1. Crack initiation and

      2 2. Crack propagation

      The knowledge of these two steps is essential as there is noticeable difference in the amount of energy required to execute the two steps. The relative levels of energy required for initiation and for propagation determine the course of events, which will occur when the metal is subjected to stress.

      There are several aspects to the fracture mechanics that tie-in with the subject of metal ductility and toughness but this short discussion is not planned for the detailed information on those aspects hence these are not discussed, but listed below are fracture mechanic topics that are directly related to assessing the toughness of material. The list is provided to raise awareness to these important factors that help determine the performance of metal under various stress conditions including in low temperature conditions.

       • Effects of axiality of stress,

       • Crack arrest theory,

       • Stress intensity representation,

       • Stress gradient,

       • Rate of Strain,

       • Effect of Cyclic Stress

       • Fatigue Crack,

       • Crack Propagation, (KIc = σ √πa)

       • Griffith’s theory of fracture mechanics,

       • Irwin’s K = √E x G,

       • Crack Surface Displacement Mode,

       • Crack Tip Opening Displacement (CTOD), (BS 5762-1979 and BS 7448 Part -I and Part II)

       • R-Curve Test methods

       • J- Integral Test method

       • Linier-Elastic Fracture Mechanics (LEFM) (ASTM E 399),

       • Elastic-Plastic Fracture Mechanics (EPFM),

       • Nil Ductility Temperature (NDT)

      Three conditions significantly influence the toughness behavior of a metal. These are listed below.

      1 1. The rate of straining,

      2 2. The nature of the load, (the imposed load is uniaxial or multiaxial.)

      3 3. The temperature of the metal.

      Weld metals are easily subject to these conditions. Hence the critical welds are subject to toughness testing. The toughness tends to decrease if the rate of straining is raised, or temperature is reduced, or stresses are changed from uniaxial to multiaxial.

The safety of ductile metal structure is often ensured by keeping the designed stress below the material’s yield strength. This is the fails safe approach to design. The more specific approach is to conduct stress analysis to assess that the nominal stresses are below the yield strength of the metal. However, there are metals in design conditions that may fail below the yield strength, such fractures are classified as brittle fracture. These fractures can occur from the effect of critical flaw size, in welds or base metal, often these are planner defects, and they are altered in any significant way the stress distribution and they are often neglected in the stress analysis.

      The lateral restraint in a structure are often cause of brittle fracture, a discontinuity in a weld in a restraint condition can greatly reduce the ductility, leading to the brittle fracture.

      For many classes of structures, that may include, ships, bridges, pressure vessels and structures that lie in the environment such as the seismic zone, or subject to cyclic stresses – like risers (SCRs) in offshore construction. A correlation has been established between material’s performances to its notch toughness test values. These values relate to both the base metals well as welds.

      The designer often assume the weld as a flawless solid lump of metal in the given shape, but that is not true. While a practical restriction can be placed on acceptable type and size of these flaws through inspection, their existence and their impact cannot be totally eliminated. Welded joints always contain some discontinuities, the challenge is to find a way to determine the type and extent of acceptable discontinuities. While the conventional test methods for the toughness cannot fully resolve this challenge, the application of concept of Fracture Mechanics, comes handy in such situation, and ensures the safety of the structure. This approach permits the direct estimation of allowable flaw sizes, and geometries in the operating conditions.

      In the critical design conditions like cyclic stresses, the allowable flaw size, and orientation and its location within the weld are assessed. This is a fracture mechanics approach and is often referred as Engineering Critical Evaluation of ECA in many codes and specifications.

      2.1.4 Low Temperature Properties

      Lowering the temperature of metal profoundly affects fracture behavior, particularly of metals that have bcc structure. Strength, ductility, toughness and other properties are changed in all metals when they are exposed to temperature near absolute zero. The properties of metals at very low temperatures are of more than casual interest, because pipeline, welded pressure equipment and vessels are expected to operate satisfactorily at levels below room temperatures. For example, moderate sub-zero temperatures are imposed on equipment for de-waxing petroleum and for storage of liquefied fuel gases and pipelines. Much lower temperatures are involved in cryogenic services, metal temperature –100^oC (–150 ^oF) and below. The cryogenic service may involve storage of liquefied industrial gases like oxygen and nitrogen. Down near the very bottom of the temperature scale, there is a real challenge for metals that are used in the construction of equipment for producing and containing liquid hydrogen and liquid helium, because these elements in liquefied form are increasingly important in new technologies. Helium in liquefied form is only slightly above absolute zero, which is 1 Kelvin (–273.16^oC or –459.69^oF).

      Absolute zero (1^oK) is the theoretical temperature at which matter has no kinetic energy, and atoms no longer exhibit motion. Man has yet to cool any material to absolute zero, so it is not known how metals would behave when cooled to this boundary condition.

However, metal components have been brought to the temperatures very close to absolute zero, hence it presents a special challenge to metals and welded components as they would be required to serve in this extremely low temperature.

      On cooling below room temperature every metal will reach a temperature where the kinetic energy will be reduced to nil. The atoms of the element will move closer and the lattice parameters would become smaller. All these changes would affect the mechanical properties of the metal.

      With above information on the physics of metal in mind, let us review the behavior of an un-notched specimen without flaws. It may be pointed out that in real life, there is no material without flaw, every material has some flaw in it, and hence the assumption to a material without flaw is more of hypothetical in nature. It is the flaw that has to be considered as initiator of the material behavior in the given environment.

      Consider the graph in Figure 2.2 below, the material is ductile until a very low temperature, point A, where Y.S. equals the UTS of the material (σ_o = σ_u). Point A represents the NDT temperature for a flaw-free material. The

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