Basic Manufacturing Process - Ch.2.2

Q.3    Explain the following in brief :
(a) Elastic strength     (b) Plastic strength     (c) Factor of safety.
Ans.    (a) Elastic Strength: -

        It is the value of strength which corresponds to transition from elastic range to plastic range. To determine elastic strength, certain stress values called ‘Ideal stress’ are used in engineering practices. Proportional limit and elastic limit are such stresses.
(b) Plastic Strength: -
    It is the value of strength which corresponds to plastic range and fracture (rupture). Ultimate strength is the most popular of these plastic range stress values.
(c) Factor of Safety: -
    In general sense, factor of safety is the ratio of ultimate strength of the material to its working stress.
                                      
Working Stress: -
    The greatest value of stress to which a material is subjected to as a machine part of structure, is called working stress. This stress should be below elastic limit of the material.
                               
Q.4    Classify the engineering material.
Ans.   
          


Q.5     What do you mean by fracture. Explain it briefly.
Related Questions-
Q.      Differentiate between ductile fracture and brittle fracture.                      (AKTU - 2011 - 12)
Q.    Write short note on brittle fractures.                                                          (AKTU - 2012 - 13)    

Q.    Briefly discuss the characteristics of a ductile fracture.                           (AKTU - 2012 - 13)
Ans.    Fracture: -

        A fracture is the (local) separation of an object or material into two, or more, pieces under the action of stress.
    The word fracture is often applied to bones of living creatures, or to crystals or crystalline materials, such as gemstones or metal. Sometimes, in crystalline materials, individual crystals fracture without the body actually separating into two or more pieces. Depending on the substance which is fractured, a fracture reduces strength (most substances) or inhibits transmission of light (optical crystals). Fracture can be classified into two categories.
Brittle fracture: -                                                                                              (AKTU - 2010 - 11)
                  


    In brittle fracture, no apparent plastic deformation takes place before fracture. In brittle crystalline materials, fracture can occur by cleavage as the result of tensile stress acting normal to crystallographic planes with low bonding (cleavage planes). In amorphous solids, by contrast, the lack of a crystalline structure results in a conchoidal fracture, with cracks proceeding normal to the applied tension.   
Recently, scientists have discovered supersonic fracture, the phenomenon of crack motion faster than the speed of sound in a material. This phenomenon was recently also verified by experiment of fracture in rubber-like materials.
Ductile Fracture: -                                                                                             (AKTU - 2010 - 11)
    In ductile fracture, extensive plastic deformation takes place before fracture. Many ductile metals, especially materials with high purity, can sustain very large deformation of 50–100% or more strain before fracture under favorable loading condition and environmental condition. The strain at which the fracture happens is controlled by the purity of the materials. At room temperature, pure iron can undergo deformation up to 100% strain before breaking, while cast iron or high-carbon steels can barely sustain 3% of strain.
    Because ductile rupture involves a high degree of plastic deformation, the fracture behavior of a propagating crack as modeled above changes fundamentally. Some of the energy from stress concentrations at the crack tips is dissipated by plastic deformation before the crack actually propagates.
    The basic steps sample of smallest cross-sectional area, void formation, void coalescence (also known as crack formation), crack propagation, and failure, often resulting in a cup-and-cone shaped failure surface.



                                                                       

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