Designing Weldments. Группа авторов

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СКАЧАТЬ yet fully understood. Only one assessment of toughness can be made with some reasonable accuracy from ordinary tensile test is that the metal displays either ductile or brittle behavior. From that it can be assumed that the metal displaying little ductility is not likely to display a ductile failure if stressed beyond its limits. The failure in this case would be brittle.

      The temperature of metal is found to have profound influence on the brittle/ductile behavior. The influence of higher temperature on metal behavior is considerable. The rise in temperature is often associated with increased ductility and corresponding lowering of the yield strength. The rupture at elevated temperatures is often Intergranular, and little or no deformation of the fractured surface may have occurred. As temperature is lowered below room temperature, the propensity to brittle fracture increases.

      Before we proceed further on the subject let us take note of some terminology that we would use in this discussion. ASTM E 616 defines some of the terminology associated with Fracture Mechanics and Testing. The following definitions are taken from ASTM E 616, it is recommended that latest version of these referenced specifications is referred for more accurate use.

       • The term fracture is strictly defined as irregular surface that forms when metal is broken into separate parts. If the fracture has propagated to only part way in the metal and metal is still in one piece, it is called crack.

       • A Crack is defined as two coincident free surfaces in a metal that join along a common front called the crack tip, which is usually very sharp.

       • The term fracture is used when the separation in metal occurs at relatively low temperature and metal ductility and toughness performance is chief topic.

       • The term rupture is more associated with the discussion of metal separation at elevated temperatures.

      As pointed out above, basically there are two types of fracture that occur in metals: Ductile 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, СКАЧАТЬ