- The primary work of the fracture analysis of the part is to find out the initial fracture part from the fractured piece, and then find the fracture source to develop the fracture analysis step by step, and analyze the cause of the fracture and the fracture property. The source of the crack can be found according to the following characteristics.
(1) T-type method: If there are two intersecting cracks on one part to form a "T" shape, as shown in Figure 2-1, under normal circumstances, the crack A is the first crack. Because the crack generated on the same part cannot propagate through the original crack, the crack propagation direction is parallel to the A crack. The crack source is located at a or b, and the crack in the source region is wider and deeper. In the figure, A is the main crack, B is the secondary crack, and a or b is the crack source.
Figure 2-1 T-type method to identify the main crack(2) Bifurcation method: Many bifurcations are often generated during the fracture process of the part. Generally, the direction of crack bifurcation is the direction of crack propagation, and the opposite direction of expansion points to the location of the crack source. On the main crack, the main crack is generally wide and long. See Figure 2-2.
Figure 2-2 Schematic diagram of the bifurcation methodThe T-type method and the bifurcation method are usually used to determine the main crack and crack source of the brittle fracture.
(3) Deformation method: The part of ductile fracture is broken into several pieces after deformation in the process of fracture. The part with large deformation after the pieces are assembled is the main crack, and the crack source is on the fracture formed by the main crack, see Figure 2-3. In the figure, A is the main crack, and B and c are the secondary cracks.(4) Parts that are broken by environmental factors such as stress corrosion, hydrogen embrittlement, corrosion fatigue, creep, etc., an oxide film or a corrosion layer due to oxidation or corrosion of the fracture. The source of the crack is at the surface or subsurface of the most severely corroded or oxidized part. The reason is that the length of this part is affected by environmental factors.(5) Determine the crack source according to the gap size after the fracture is split: the two parts of the fracture are joined together, and the large crack is the source of the crack. See Figure 2-4.
Figure 2-3 Schematic diagram of the deformation method Figure 2-4 Contact crack size to determine the crack source(6) If there are radiation stripes on the fracture, the convergence of the radiation fringes is the source of the crack. See Figure 2-5. If there is a human mark on the fracture, the tip of the herringbone points to the crack source when there is no stress concentration. When there is stress concentration, the herringbone tip is reversely directed to the crack source. See Figure 2-6 and Figure 2-7.(7) If there is a rough fibrous area on the fracture, the source of the crack is generally located in the center of the fiber zone.(8) If there is a fatigue arc on the fracture, the source of the crack is located in the smooth zone where the fatigue arc converges. If there is a step for the multi-source fatigue fracture, Figure 2-8.
Figure 2-5 Radiation stripe convergence is the source of the crack(a) the fracture of the circular member; (b) the fracture of the rectangular member
Figure 2-6 Herringbone tip pointing to the crack source Figure 2-7 Herringbone tip reverse pointing crack source
Figure 2-8 Fatigue fracture source area
In the analysis of fractures, specific problems should be analyzed in detail, and it must not be hardened.
(9) When the high-pressure vessel is exploded, the final fracture zone is sawtooth, as shown in Figure 2-9.
Figure 2-9 Schematic diagram of the explosion of the high pressure vessel
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