Fig.4 Fracture of a steel tube (a) Fracture surface at approximately actual size, showing point of crack initiation(at arrow), chevron and fanlike marks, and development of shear ips.(b)Fracture-origin area at 5x; note that fracture nuclei differ in texture from the main fracture surface Microscopic Examination of Fracture Surfaces Microscopic examination of fracture surfaces is typically done with a SEM. A SEM has the advantage over light microscopy because of the large depth of field and very high magnifications attainable, typically 5000 to 10,000x. In addition, SEMs are often equipped with microanalytical capabilities, for example, energy-dispersive x-ray spectroscopes (EDS). Chemical analysis can be helpful in confirming the chemistry of microstructural features that may be confused with fracture features The primary limitation of SEM analysis is sample size. A SEM analysis must be conducted in a vacuum so the sample must be put into a chamber that typically holds a sample less than 20 cm( 8 in. )in diameter. Although there are some fracture surface features that are commonly associated with particular failure modes, the novice failure analyst must be very careful in fractographic analyses. Some of the more classic examples of fracture surface topography that indicates a fracture mode are Dimpled rupture typical of overstress failures of ductile metals and alloys Cleavage facets, typical of transgranular brittle fracture of body-centered cubic and hexagonal close-packed metals and alloys Brittle intergranular fracture typical of temper-embrittled steel, where fracture is due to segregation of an embrittling species to grain boundaries(such as oxygen in iron or nickel), due to intergranular stress-corrosion cracking, or due to hydrogen embrittlemer Thefileisdownloadedfromwww.bzfxw.comFig. 4 Fracture of a steel tube. (a) Fracture surface at approximately actual size, showing point of crack initiation (at arrow), chevron and fanlike marks, and development of shear lips. (b) Fracture-origin area at 5×; note that fracture nuclei differ in texture from the main fracture surface. Microscopic Examination of Fracture Surfaces Microscopic examination of fracture surfaces is typically done with a SEM. A SEM has the advantage over light microscopy because of the large depth of field and very high magnifications attainable, typically 5000 to 10,000×. In addition, SEMs are often equipped with microanalytical capabilities, for example, energy-dispersive x-ray spectroscopes (EDS). Chemical analysis can be helpful in confirming the chemistry of microstructural features that may be confused with fracture features. The primary limitation of SEM analysis is sample size. A SEM analysis must be conducted in a vacuum so the sample must be put into a chamber that typically holds a sample less than 20 cm (8 in.) in diameter. Although there are some fracture surface features that are commonly associated with particular failure modes, the novice failure analyst must be very careful in fractographic analyses. Some of the more classic examples of fracture surface topography that indicates a fracture mode are: · Dimpled rupture typical of overstress failures of ductile metals and alloys · Cleavage facets, typical of transgranular brittle fracture of body-centered cubic and hexagonal close-packed metals and alloys · Brittle intergranular fracture typical of temper-embrittled steel, where fracture is due to segregation of an embrittling species to grain boundaries (such as oxygen in iron or nickel), due to intergranular stress-corrosion cracking, or due to hydrogen embrittlement The file is downloaded from www.bzfxw.com