service that may have initiated the failure, or if any recent repairs or overhauls had been carried out and why. In addition, it is also necessary to inquire whether or not the failure was an isolated example or if others have occurred, either in the component under consideration or in another of a similar design. In the routine examination of a brittle fracture, it is important to know if, at the time of the accident or failure, the prevailing temperature was low, and/or if some measure of shock loading was involved. When dealing with failures of crankshafts or other shafts, it is generally desirable to ascertain the conditions of the bearings and whether any misalignment existed, either within the machine concerned or between the driving and driven components In an analysis where multiple components and structures are involved, it is essential that the position of each piece be documented before any of the pieces are touched or moved Such recording usually requires extensive photography, the preparation of suitable sketches, and the taking and tabulation of appropriate measurements of the pieces Next, it may be necessary to take an inventory to determine if all of the pieces or fragments are present at the site of the accident. For example, an investigation of an aircraft accident involves the development of a considerable inventory including listing the number of engines, flaps, landing gear, and the various parts of the fuselage and wings. It is essential to establish whether all the primary parts of the aircraft were aboard at the time that it crashed Providing an inventory although painstaking, is often invaluable. An experienced investigator determined the cause of a complex aircraft accident when he observed that a portion of one wing tip was missing from the main impact site. This fragment was subsequently located several miles back on the flight path of the aircraft. The fragment provided evidence of a fatigue failure and was the first component separated from the aircraft, thus accounting for the crash The most common problem encountered in examining wreckage involves the establishment of the sequence of fractures to determine the origin of the initial failure. Usually, the direction of crack growth can be detected from marks on a fracture surface, such as V-shaped chevron marks. The typical sequence of fractures is shown in Fig. 1(b), where A and B represent fractures that intersect at about 90. Here the sequence of fractures is clearly discernible from crack branching Obviously, fracture A must have occurred prior to fracture b because the presence of fracture a served to arrest cracking at fracture B. This method of sequencing is called the T-junction procedure and is an important technique in wreckage PROPAG森ToN LOCATION OF CRACK。RGN P Subsequent fracture. A fracture. B Fig. 1 General features to locate origin from crack paths (a)branching and(b) sequencing of cracking by the T-junction procedure, where fracture a precedes and arrests fracture B Provided the fragments are not permitted to contact each other, it is also helpful to carefully fit together the fragments of broken components which, when assembled and photographed, may indicate the sequence in which fractures occurred Figure 2 shows a lug that was part of a pin-joint assembly; failure occurred when the pin broke out of the lug. With the broken pieces of the lug fitted together, it is apparent from the deformation that fracture a must have preceded fractures B Thefileisdownloadedfromwww.bzfxw.comservice that may have initiated the failure, or if any recent repairs or overhauls had been carried out and why. In addition, it is also necessary to inquire whether or not the failure was an isolated example or if others have occurred, either in the component under consideration or in another of a similar design. In the routine examination of a brittle fracture, it is important to know if, at the time of the accident or failure, the prevailing temperature was low, and/or if some measure of shock loading was involved. When dealing with failures of crankshafts or other shafts, it is generally desirable to ascertain the conditions of the bearings and whether any misalignment existed, either within the machine concerned or between the driving and driven components. In an analysis where multiple components and structures are involved, it is essential that the position of each piece be documented before any of the pieces are touched or moved. Such recording usually requires extensive photography, the preparation of suitable sketches, and the taking and tabulation of appropriate measurements of the pieces. Next, it may be necessary to take an inventory to determine if all of the pieces or fragments are present at the site of the accident. For example, an investigation of an aircraft accident involves the development of a considerable inventory, including listing the number of engines, flaps, landing gear, and the various parts of the fuselage and wings. It is essential to establish whether all the primary parts of the aircraft were aboard at the time that it crashed. Providing an inventory, although painstaking, is often invaluable. An experienced investigator determined the cause of a complex aircraft accident when he observed that a portion of one wing tip was missing from the main impact site. This fragment was subsequently located several miles back on the flight path of the aircraft. The fragment provided evidence of a fatigue failure and was the first component separated from the aircraft, thus accounting for the crash. The most common problem encountered in examining wreckage involves the establishment of the sequence of fractures to determine the origin of the initial failure. Usually, the direction of crack growth can be detected from marks on a fracture surface, such as V-shaped chevron marks. The typical sequence of fractures is shown in Fig. 1(b), where A and B represent fractures that intersect at about 90°. Here the sequence of fractures is clearly discernible from crack branching. Obviously, fracture A must have occurred prior to fracture B because the presence of fracture A served to arrest cracking at fracture B. This method of sequencing is called the T-junction procedure and is an important technique in wreckage analysis. Fig. 1 General features to locate origin from crack paths (a) branching and (b) sequencing of cracking by the T-junction procedure, where fracture A precedes and arrests fracture B Provided the fragments are not permitted to contact each other, it is also helpful to carefully fit together the fragments of broken components which, when assembled and photographed, may indicate the sequence in which fractures occurred. Figure 2 shows a lug that was part of a pin-joint assembly; failure occurred when the pin broke out of the lug. With the broken pieces of the lug fitted together, it is apparent from the deformation that fracture A must have preceded fractures B The file is downloaded from www.bzfxw.com