Tham khảo tài liệu 'asm metals handbook - desk edition (asm_ 1998) episode 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Stereo Views Stereo-pair photographs of fracture surfaces provide a means of viewing the fracture contours in simulated three dimensions. The basic technique for preparing such photographs entails taking two pictures of a subject area the second from an angle slightly different from the first. The photographs are then examined under a visual condition in which for example the viewer s left eye focuses on the first picture and his right eye focuses on the second. Stereo viewers are available and in most cases are necessary to ease the task of viewing stereo pairs. The effect is to convince the brain that the eyes are indeed seeing a three-dimensional scene. If the angular displacement between the two pictures is appropriate an included angle of 12 or 14 is desirable the illusion is very vivid. Stereo images by light microscopy have been used only to a limited extent because of the restricted depth of field. They provide a useful means of studying fractures at magnifications generally not greater than 200x. Stereo-pair photographs can be taken using a single-lens camera if it is provided with a mount that will pivot about a horizontal axis through the subject. After the subject is properly aligned beneath the camera with the mount vertical the camera is swung to an angle of 6 to 7 from vertical and an exposure is made. The second exposure is made with the camera at the same angle as for the first exposure but swung to the other side of the vertical or zero position. With the stereomicroscope shown in Fig. 3 it is possible to observe the voids that are a part of ductile dimpled fracture and in brittle fractures cleavage facets and some of the river marks can be discerned. It is most useful for preliminary examination of fracture surfaces leaving final documentation of fine details for scanning electron microscopy. Fig. 3 Optical stereomicroscope with camera box partly removed for direct viewing. The light source is two 40 watt incandescent tubes above the specimen. An
