Categories and classifications of cracks in steel pipe metal

Cracks come in various forms, but to analyze specific problems, we must first find the crack source, and then conduct a specific analysis based on the crack source and crack shape, such as transgranular cracks, fatigue cracks, confluence cracks, grain boundary cracks, thermal cracks, etc. Fatigue cracks, quenching cracks, cracks, etc. Cracks are classified according to different angles, as follows:

1. According to the degree of deformation before fracture: plastic and brittle cracks. Pure plastic and pure brittle cracks are less common and are usually a mixture of the two.

The characteristics of ductile fracture are as follows:

① There are wine cup-shaped dimples on the fracture surface;

② The fracture surface is cup-cone-shaped, with the cone bottom perpendicular to the principal stress, the cone surface parallel to the maximum shear stress, and at an angle of 45 degrees to the principal stress.

③The fracture surface is fibrous and gray.

The characteristics of brittle fracture are as follows:

①The fracture is perpendicular to the normal stress;

②The fracture surface is flush and bright;

③The fracture has herringbone or radioactive patterns;

The herringbone pattern is a significant macroscopic feature of brittle fracture.

1. The formation process of herringbone cracks is as follows:

Under the action of external force, the sample first undergoes plastic deformation in the weak link, forming micro-cracks, and then expands into a half-moon-shaped, small-scale fiber area. When the crack reaches the critical size, it rapidly expands unstablely. First, a three-dimensional stress zone is generated not far from the front end of the crack, nucleates in this zone, and then expands into an elliptical cake-shaped internal crack. Under the action of external force, The inner crack grows rapidly and intersects with the front crack, causing the crack to advance. Then a new crack front, a new three-dimensional stress zone, and a new internal crack are formed; these cracks grow up at different times in turn, merge and continuously form new internal cracks, and continue to expand forward to form a parabolic crack front.

2. Classification according to the crack propagation path: intergranular, transgranular, and mixed crystal.

3. Classification according to crack mechanism: cleavage fracture and shear fracture; usually cleavage fracture is a brittle fracture, but it also has large plastic deformation;

4. Classification according to force type: static load fracture (tensile fracture, torsion fracture), impact fracture, and fatigue fracture.

5. Classification according to the medium environment: low temperature cold brittle fracture, static load lag fracture, stress corrosion fracture, hydrogen embrittlement fracture, etc.