According to the basic principle of magnetic particle flaw detection (the geometric discontinuity of the ferromagnetic material workpiece and the inhomogeneity of the material force the part of the magnetization field inside the workpiece to escape, attract nearby magnetic powder, and form obvious magnetic marks), the longitudinal magnetization can detect the circumference (horizontal) The longitudinal defects can be found by circumferential magnetization, because the intermittent defects perpendicular to the magnetization field are most likely to cause the leakage field, while the defects parallel to the magnetization field cannot.
But in fact, there are still abnormal phenomena that run counter to this general rule. For example, the side edge of the square steel absorbs magnetic powder during longitudinal magnetization, and the magnetic powder accumulates on the bottom edge of the circular shaft during circumferential magnetization. Sometimes longitudinal cracks can be found in longitudinal magnetization. Circumferential cracks can also be detected.
Why the above-mentioned abnormal phenomena occur, the author has already explained the causes of the first two abnormal phenomena, and now I will find the answers to the latter two difficult problems.
1. Theoretical basis
The magnetic charge excited by the longitudinal magnetization of the square steel will be uniformly distributed on each ridgeline of the square steel, and the magnetic charge line density σml (Wb/m) on each ridgeline is (Figure 1a)
In the formula
2l, 2w, h—the length, width and height of the cuboid, m Xm—the magnetic susceptibility of the material, which is a dimensionless pure number
μ0—vacuum permeability
H0—Magnetizing field strength, A/m
C — a function related to the magnetization field strength, the size of the square steel and the position of the field point, which can be regarded as a constant in the roughest approximation
After parallelepiped and triangular prism are longitudinally magnetized, the magnetic charge will be evenly distributed on each ridgeline (Fig. 1b,c), and satisfy the following formula
In the formula, σmα—the magnetic charge line density on the bottom edge and the magnetization field at an angle of α, Wb/m
(Figure 1 Magnetization of cuboids, parallelepipeds and triangular prisms)
2. Magnetic charge distribution on the transverse groove during longitudinal magnetization
Assuming that the square steel with transverse grooves (Fig. 2a) is divided into three parts as shown in Fig. 2b, the magnetic charge excited on each edge of the latter when H0 is magnetized is shown in Fig. 2b.
Obviously, since the polarity of the magnetic charge on the same ridge line is opposite when it is divided into two parts, there is actually no net magnetic charge along the ridge line, so the magnetic charge distribution along each edge of the square steel with transverse grooves after longitudinal magnetization It should be as shown in Figure 2a.
(Figure 2 Magnetic charge distribution on square steel with transverse grooves after longitudinal magnetization)
3. Magnetic charge distribution on the longitudinal groove during longitudinal magnetization
It can be proved from the lateral repulsion characteristics of the magnetic dipole chain and the minimum magnetic free energy principle of the magnetic charge system that the positive and negative magnetic charges can only be distributed on the convex ridge line of the workpiece parallel to the magnetization field (the A ridge line in Figure 3). Segments gather and do not appear on the corresponding concave ridge (B ridge in Figure 3).
Therefore, whether the longitudinal grooves on the workpiece are through (Fig. 4a) or closed (Fig. 4b), when the longitudinal magnetization is in a stable state, their top ribs (protruding ribs parallel to the magnetization field) sometimes accumulate magnetic charges, And its bottom edge (concave edge) has no magnetic charge (Fig. 4c, d).
Figure 4a, b is the initial moment of magnetization; c, d is the steady state of magnetization.
(Figure 3 Convex ridges and concave ridges parallel to the magnetization field on the workpiece)
(Figure 4 Magnetic charge distribution on the longitudinal groove of the workpiece during longitudinal magnetization)
4. Abnormal magnetic marks on longitudinal grooves during longitudinal magnetization
It is not difficult to see from Figure 4c, d that in the stable state of longitudinal magnetization, the excited positive and negative line magnetic charges sometimes remain on the top edge (convex edge) of the longitudinal groove on the surface of the workpiece, and they must be able to absorb Magnetic powder particles that form abnormal magnetic marks on longitudinal grooves.
5. Abnormal magnetic marks on the circumferential groove during circumferential magnetization
According to the above analysis, when magnetizing in the circumferential direction, evenly distributed positive and negative magnetic charges sometimes gather on the top edge (convex edge) of the circumferential (horizontal) groove on the surface of the workpiece (Figure 5), and they can naturally absorb magnetic powder. Particles, forming abnormal magnetic marks of circumferential (transverse) grooves.
6 Conclusion
(Figure 5 Magnetic charge distribution on the circumferential groove of the workpiece during circumferential magnetization)
Since the stable spatial distribution of the magnetic charge will make the interaction energy take the lowest value, so when certain conditions are met, there will be positive and negative line magnetic charges evenly distributed on the ridge line of the open crack parallel to the magnetization field, and they will adsorb the magnetic powder particles. , forming abnormal magnetic marks.
Therefore, longitudinal magnetization can sometimes find longitudinal cracks, and circumferential magnetization may also detect circumferential (transverse) defects.
In fact, this abnormal phenomenon does not violate the basic principle of magnetic particle inspection, but the reason for its occurrence has not been understood at first.
