In opening cracks for examination, care must be exercised to prevent damage, primarily mechanical, to the surface of the fracture. This can usually be accomplished if opening is done in such a way that the two surfaces of the fracture are moved in opposite directions, normal to the fracture plane. Generally, a saw cut can be made from the back of the fractured part to a point near the tip of the crack, using extreme care to avoid actually reaching the tip of the crack. This saw cut will reduce the amount of solid metal that must be broken. The final breaking of the specimen can be done in By clamping the two sides of the fractured part in a tensile-testing machine, if the shape permits, and pulling By placing the specimen in a vise and bending one half away from the other by striking it with a hammer in a manner that will avoid damage to the surfaces of the crack By gripping the halves of the fracture in pliers or vise grips and bending or pulling them apart Cooling the part with liquid nitrogen often reduces the force and plastic deformation necessary to fracture the part Fortunately, there is little confusion during subsequent examination as to which part of the fracture surface was obtained in opening the crack. It is recommended that both the crack separation and the visible crack length be measured prior to opening. The analyst may have to use dye penetrant or other nondestructive evaluation technique to actually see the length of a tightly closed crack. Often, the amount of strain that occurred in the specimen can be determined from a measurement of the separation between the adjacent halves of a fracture. This should be done before preparation for opening a secondary crack has begun. The lengths of cracks may also be important for analyses of fatigue fractures or for consideration for the application of fracture mechanics Macroscopic Examination of Fracture Surfaces One very important part of any failure analysis is the macroscopic examination of fracture surfaces. Performed at magnifications from 1 to 50 or 100x, it may be conducted by the unaided eye, a hand lens or magnifier, a low-power stereoscopic microscope, or a SEM. Macroscopic photography of up to 20x magnification requires a high-quality camera and special lenses; alternatively, a large magnifying glass may be used to enlarge a specific area in the photo, such as a crack or other small detail. A metallographic microscope with macroobjectives and lights may be used for somewhat higher magnifications. However, depth of field becomes extremely limited with light optics. For much greater depth of field, a SEM may be used for low-magnification photography as well as higher-magnification work. Stereo or three dimensional photographs may also be made to reveal the topographic features of a fracture or other surface Frequently, a specimen may be too large or too heavy for the stage of the metallograph or the chamber of a SEM, and cutting or sectioning the specimen may be difficult or not allowed because of legal limitations at the time. In these instances, excellent results can be achieved by examining and-where appropriate-photographing replicas made by the method for cleaning fractures(see discussion under "Cleaning"). These replicas can be coated with a thin layer(about 20 nm, or 2 x 10 m, thick)of vacuum-deposited gold or aluminum to improve their reflectivity, or they may be shadowed at an angle to increase the contrast of fine detail. The replicas may be examined by incident-light or transmitted-light microscopy. Because they are electrically conductive, the coated replicas may also be examined with a SEM The amount of information that can be obtained from examination of a fracture surface at low-power magnification is extensive. A careful scan of the exterior surface of the part in the area adjacent to the fracture is required to determine whether specific stress raisers are present of a type that could have initiated the fracture. If any marks possess sha reentrant angles, they constitute sites of stress concentration, a frequent cause of crack initiation. In this situation, obvious remedy is more careful handling procedures and better inspection. Tool marks are another source of stress concentration. A fillet that has too small a radius, even though the surface of the fillet may be an excellent example of high-grade machining, is a recognized initiation site for fatigue cracks. Sharp-bottomed tool marks can initiate fatigue fractures even though the general contour of the area has a generous radius The shape, size, and cross section of a specimen or structural component can have a large effect on both the macroscopic and the microscopic appearance of the fracture surface, especially when pronounced stress raisers are present. Holes, corners, notches, machining marks, and, most of all, preexisting cracks actively influence fracture appearance Pronounced stress raisers are more likely to be contained in a large part than in a small part, because large parts have greater volume and surface area The orientation of the fracture surfaces must be consistent with the proposed mode of failure and the known loads on the failed part. Failure in monotonic tension produces a flat(square) fracture normal (perpendicular)to the maximum tensile stress and frequently a slant(shear) fracture at about 45. This 45 slant fracture is often called a"shear lip. Many fractures are flat at the center, but surrounded by a picture frame of slant fracture. An example of this behavior is to be found in the familiar cup-and-cone fracture of a round tensile test bar Thefileisdownloadedfromwww.bzfxw.comIn opening cracks for examination, care must be exercised to prevent damage, primarily mechanical, to the surface of the fracture. This can usually be accomplished if opening is done in such a way that the two surfaces of the fracture are moved in opposite directions, normal to the fracture plane. Generally, a saw cut can be made from the back of the fractured part to a point near the tip of the crack, using extreme care to avoid actually reaching the tip of the crack. This saw cut will reduce the amount of solid metal that must be broken. The final breaking of the specimen can be done in several ways: · By clamping the two sides of the fractured part in a tensile-testing machine, if the shape permits, and pulling · By placing the specimen in a vise and bending one half away from the other by striking it with a hammer in a manner that will avoid damage to the surfaces of the crack · By gripping the halves of the fracture in pliers or vise grips and bending or pulling them apart Cooling the part with liquid nitrogen often reduces the force and plastic deformation necessary to fracture the part. Fortunately, there is little confusion during subsequent examination as to which part of the fracture surface was obtained in opening the crack. It is recommended that both the crack separation and the visible crack length be measured prior to opening. The analyst may have to use dye penetrant or other nondestructive evaluation technique to actually see the length of a tightly closed crack. Often, the amount of strain that occurred in the specimen can be determined from a measurement of the separation between the adjacent halves of a fracture. This should be done before preparation for opening a secondary crack has begun. The lengths of cracks may also be important for analyses of fatigue fractures or for consideration for the application of fracture mechanics. Macroscopic Examination of Fracture Surfaces One very important part of any failure analysis is the macroscopic examination of fracture surfaces. Performed at magnifications from 1 to 50 or 100×, it may be conducted by the unaided eye, a hand lens or magnifier, a low-power stereoscopic microscope, or a SEM. Macroscopic photography of up to 20× magnification requires a high-quality camera and special lenses; alternatively, a large magnifying glass may be used to enlarge a specific area in the photo, such as a crack or other small detail. A metallographic microscope with macroobjectives and lights may be used for somewhat higher magnifications. However, depth of field becomes extremely limited with light optics. For much greater depth of field, a SEM may be used for low-magnification photography as well as higher-magnification work. Stereo or three￾dimensional photographs may also be made to reveal the topographic features of a fracture or other surface. Frequently, a specimen may be too large or too heavy for the stage of the metallograph or the chamber of a SEM, and cutting or sectioning the specimen may be difficult or not allowed because of legal limitations at the time. In these instances, excellent results can be achieved by examining and—where appropriate—photographing replicas made by the method for cleaning fractures (see discussion under “Cleaning”). These replicas can be coated with a thin layer (about 20 nm, or 2 × 10-8 m, thick) of vacuum-deposited gold or aluminum to improve their reflectivity, or they may be shadowed at an angle to increase the contrast of fine detail. The replicas may be examined by incident-light or transmitted-light microscopy. Because they are electrically conductive, the coated replicas may also be examined with a SEM. The amount of information that can be obtained from examination of a fracture surface at low-power magnification is extensive. A careful scan of the exterior surface of the part in the area adjacent to the fracture is required to determine whether specific stress raisers are present of a type that could have initiated the fracture. If any marks possess sharp reentrant angles, they constitute sites of stress concentration, a frequent cause of crack initiation. In this situation, the obvious remedy is more careful handling procedures and better inspection. Tool marks are another source of stress concentration. A fillet that has too small a radius, even though the surface of the fillet may be an excellent example of high-grade machining, is a recognized initiation site for fatigue cracks. Sharp-bottomed tool marks can initiate fatigue fractures even though the general contour of the area has a generous radius. The shape, size, and cross section of a specimen or structural component can have a large effect on both the macroscopic and the microscopic appearance of the fracture surface, especially when pronounced stress raisers are present. Holes, corners, notches, machining marks, and, most of all, preexisting cracks actively influence fracture appearance. Pronounced stress raisers are more likely to be contained in a large part than in a small part, because large parts have greater volume and surface area. The orientation of the fracture surfaces must be consistent with the proposed mode of failure and the known loads on the failed part. Failure in monotonic tension produces a flat (square) fracture normal (perpendicular) to the maximum tensile stress and frequently a slant (shear) fracture at about 45°. This 45° slant fracture is often called a “shear lip.” Many fractures are flat at the center, but surrounded by a “picture frame” of slant fracture. An example of this behavior is to be found in the familiar cup-and-cone fracture of a round tensile test bar. The file is downloaded from www.bzfxw.com