Microstructrural aspects

It is on a microscopic scale that the characteristics of ductile deformation and fracture really become unique. It is necessary to examine both the changes that are visible with a light microscope and those that can be examined only with an electron microscope at magnifications unattainable by a light microscope.

Note that the major crack is near the center of the reduced section and that there is a smaller crack close to it nearer the center. Many smaller cracks have joined together to form the major cracks.

There is also tremendous vertical (axial) elongation of the original equiaxed (nondirectional) grains of this hot rolled steel. Grain distortion of this kind is characteristic of ductile deformation and fracture and is frequently observed in examination of damaged and failed parts, provided the plane examined is in the direction of deformation, as in this photograph.

The process of microvoid coalescence continues to form larger cracks which have approximately half of each cavity on each side of the fracture surface as a tiny cup or "dimple." The actual fracture surface of a ductile metal, therefore, is essentially nothing but a mass of dimples - or half-voids - which usually can be seen only with the aid of an electron microscope: this is termed a "dimpled rupture" fracture surface.
Examination of these dimples is exceedingly useful in studying fractures, because the dimples are extremely sensitive to the direction of the stresses that formed them.
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