## 3 Experiments, Results and Discussion
### Effects of Magnetic Particle Concentration on Magnetic Particle Indication Images
When the magnetic particle concentration in the suspensions was in the range of 15-50 mL/L, the background gray scale value increased gradually and reached the threshold of 248, the maximum value in Table 2. It should be noted that the contrast ratio value increased first and then decreased in the magnetic particle concentration range of 1-15 mL/L. The contrast ratio reached the peak when the concentration of the magnetic particle suspension was 3 mL/L and then started to decrease with the increase of the magnetic particle concentration. When the concentration of the magnetic particle suspension reached 10 mL/L, the contrast ratio tended to be consistent. The conditions for automatic defect recognition (ADR) are different from but similar to that for manual MPI. In this study, the main focus is on the three characteristic parameters, namely, indication gray scale, background gray scale, and contrast ratio. For an ADR process, these three parameters can be easily set and adjusted so that to produce a satisfactory image in a manual MPI, the image quality is largely dependent on the operators' experience. Therefore, by using these quantitative parameters, ADR can offer better image quality. Figure 10: (**a**) Magnetic particle indication gray scale and background gray scale curves and (**b**) Contrast ratio curve. Therefore, the optimal magnetic particle concentration was found to be in the range of 3-4 mL/L. Moreover, when the magnetic particle concentration in the theoretical model increased, the number of particles in the critical distance increased monotonically. A critical value was noticed when the magnetic field strength of the workpiece reached 2400 A/m, therefore, the influences of kinetic effects, the change of the background magnetic field after stacking, and the change of the number of particles adsorbed in the range within the adsorption limit distance should be explored. In real conditions, a too high or too low magnetic of a particle concentration is not conducive to obtaining the best magnetic particle indication images. When the magnetic particle suspension concentration was too low (<3 mL/L), very few magnetic particles were available to form an effective magnetic particle indication image in the cracked area, on the other hand, when the magnetic particle suspension concentration was too high (>4 mL/L), due to the continuous increase of the magnetic particle density, the magnetic particle suspension was transformed from the mobile fluid state with suspended magnetic particles \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Concentration** & **1.00** & **2.00** & **3.00** & **4.00** & **5.00** & **6.00** \\ \hline
**Gray scale** & & & & & & \\
**image** & & & & & & \\
**Indication** & 47 & 98 & 133 & 175 & 195 & 208 \\
**Background** & 4 & 8 & 11 & 13 & 21 & 26 \\
**gray scale** & 36 & 90 & 139 & 129 & 98 & 87 \\
**Contrast** & **7.00** & **8.00** & **9.00** & **10.00** & **15.00** & **20.00** \\ \hline
**Gray scale** & & & & & & \\
**image** & & & & & & \\
**Indication** & 213 & 226 & 232 & 247 & 255 & 255 \\
**gray scale** & 45 & 56 & 68 & 78 & 197 & 210 \\
**Contrast** & 81 & 65 & 47 & 32 & 29 & 20 \\ \hline
**Concentration** & **25.00** & **40.00** & **50.00** & & & \\
**mL/L** & & & & & & \\
**Gray scale** & & & & & & \\
**image** & & & & & & \\
**Indication** & 255 & 255 & 255 & \\
**gray scale** & 222 & 246 & 248 & & & \\
**Contrast** & 14 & 9 & 2 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Evaluation parameters under different magnetic particle concentrations. to the slurry state with, consequently, too many particles of extremely poor fluidity. The magnetic field leakage at the crack no longer effectively affected the movement and accumulation of magnetic particles to form the indication; thus, the signal-to-noise ratio of the image was too low.