Designing Weldments - Группа авторов

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of metals does affect some of the mechanical properties leading to metals failures. Optical • Color,• Reflectivity. Nuclear • Radiation obsorbtivity,• Nuclear cross section,• Wavelength of characteristic X-rays

      The following is the discussion on these material properties.

      2.1.1 Structure Insensitive Properties

      These are well stablished and defined properties of a metal. These properties are standard from one piece of metal to another, from the engineering aspect they do not change. These properties are verifiable and can be tested for verification. These can be calculated, rationalized by consideration of the chemical compositions and crystallographic structure of metal.

      2.1.2 Structure Sensitive Properties

      Now we briefly discuss these properties as they apply to metals in engineering application.

      2.1.3 Mechanical Properties

      Mechanical properties of metals make them useful for engineering applications. These properties make them strong, playable, to form shape and still retain their strength. Metals possess a combination of properties like toughness, strength and ductility that vary from metal to metal this variation allows the choice of specific metal for specific needs of the structure. These properties of some metals like steel, and aluminum can be altered and improved to make them more suitable for specific objectives.

      Through a combination of both alloy selection and heat treatment gives design engineers a selection of mechanical properties in metals to choose from. During the fabrication process too, the applied heat, joining methods like welding and brazing choice of filler metal for welding all affect metal’s mechanical properties. Some of these properties are counter to each other; that is, if you increase one property the other may be lowered and vice versa. This leads to some compromises in selection process. This brings in the importance of fully knowing the properties of metals. To know the specific properties of metal in given condition and during its formation during fabrication it is essential to test and know exact properties of the material that is being used for design purpose.

      In the following paragraphs a brief introduction to some of the mechanical properties is discussed.

       2.1.3.1 Modulus of Elasticity

      The ability of a metal to resist stretching (stain) under the stress is defined by the ratio of the two. This is called the Modulus of Elasticity and indicated by letter E. This is a constant value for specific metal. The Table 2.2 below gives Modulus of Elasticity values of some of the common engineering metals.

      where;

      Table 2.2 Modulus of elasticity of common engineering metals.

Metal Modulus of elasticity, psi
Aluminum 9.0 × 106
Beryllium 42.0
Columbium 15.0
Copper 16.0
Iron 28.5
Lead 2.0
Molybdenum 46.0
Nickel 30.0
Steel, (Carbon and alloy steels) 29.0
Tantalum 27.0
Titanium 16.8
Tungsten 59.0

      The elastic modulus is a structure sensitive property, (see Table 2.1) is not changed by metal’s gain size, cleanliness, by significant alloying, or by heat treatment. However, modulus of elasticity decreases with increasing temperatures, and the rate of change is not same for all metals.

      The modulus indicates that, how much a beam would deflect elastically under the load, or a bar would elastically stretch, when loaded. In welding engineering the modulus is frequently used to determine the level of stress created in a piece of metal when it is forced to stretch elastically for a specific amount. In this case the stress (σ) can be determined by multiplying the strain (ϵ) by the modulus of elasticity (E) which is a constant for the given metal.

       2.1.3.2 Tensile Strength

      By far the most often used property is the metals’ ability to sustain the load while it is put under tensile strain. During testing, it is determined by the sustained load at which the test specimen breaks or the metal has lost its elasticity and entered in the Plastic state and deformed. This value is divided by the cross-section of the specimen being tested to obtain the Ultimate Tensile Strength (UTS) of the metal under test. The Figure 2.1 below is the tensile test graph of typical mild steel, it indicates the key points of mechanical behavior during the testing.

      Figure 2.1 A typical strain and stress diagram, describing 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

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