tailieunhanh - Volume 10 - Materials Characterization Part 8

Tham khảo tài liệu 'volume 10 - materials characterization part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chill Columnar zone zone Central Columnar Chill zone zone zone Mold wall Mold wall Fig. 10 Sketch of grains in a typical cast ingot. Optical macroscopic examination of a fracture surface may reveal features that will help establish the failure process example Fig. 11 illustrates a fracture in a railroad rail. The relatively smooth region in the photograph represents growth because of cyclic or fatigue loading. The dark spot or fish eye in the center of the reflective area represeni fracture-initiation site. The remainder of the surface failed by overload. This macroscopic observation of the failed shows that the failure was initiated because of the dark-appearing defect in the rail. Fig. 11 Transverse fracture of an AISI 1075 steel railroad rail. Fracture nucleus dark area near top of railhead initiated a fatigue crack large light area around nucleus . The macroscopic nature of many fractures is such that the fracture origin is easily recognizable. Brittle or low-duel fractures have characteristic V-shaped markings on the fracture surface Fig. 12 known as chevrons or herrinc marks. The tip of the V generally points to the origin of the failure. The origin of fatigue failures can also be isola using macroscopic examination. For example Fig. 13 shows a failure of a steel housing tube initiated in four reg Each initiation region is observable in the macrographs as shown by the four arrows. The position of the crack fror various times during the failure process is also visible as the so-called beach marks that are initially fairly concentr the origin. The major problem with optical macroscopic or microscopic examination of fracture surfaces is the technic inability to obtain favorable focus over the entire surface if the magnification exceeds 5 to 100 Therefore SEM h become a standard metallographic tool in failure analysis. Fig. 12 Example of how fracture surface features can point to the failure origin a Fractograph of a high-velocity fracture in steel plate showing