# Notch Fatigue Life Research Based on Critical Distance Theory
## 3 Notched Specimen Fatigue Life Test
### Experimental Design  
The test specimens were designed and produced in accordance with the standard specification of \"Metallic materials fatigue testing axial force controlled method\" (GB/ T3075-2021) [37], and three arc-shaped notched plate specimens with notch radii of 10, 20, and 30 mm, respectively, are shown in Figure 3, and the dimensional parameters of the specimens are shown in Table 2. To ensure consistent force flow at the kerf, the width of the center position of the three types of specimens was controlled to be 40 mm.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Materials** & \(\mathbf{\sigma_{b}}\) & \(\mathbf{\varepsilon_{y}}\) & \(\mathbf{\sigma_{y}}\) & \(\mathbf{\varepsilon_{b}}\) & \(\mathbf{E}\) & \(\mathbf{A}\) & \(\mathbf{Z}\) & \\  & **MPa** & & **MPa** & & **MPa** & **\%** & **\%** & **\%** & **\(\mathbf{v}\)** \\ \hline Q345 qd steel plate & 515.9 & 0.134 & 347.5 & 0.0015 & 204,153 & 30 & 72.54 & 0.3 \\ \hline \hline \end{tabular}  
* where \(\sigma_{b}\) is the tensile strength; \(\varepsilon_{y}\) is the strain corresponding to the tensile strength; \(\sigma_{y}\) is the yield strength; \(\varepsilon_{b}\) is the strain corresponding to the yield strength; \(E\) is the [PERSON]’s modulus; \(A\) is the elongation at break; \(Z\) is the shrinkage at section; \(v\) is the Poisson’s ratio.  
\end{table}
Table 1: Mechanical properties of materials.  
Figure 3: Standard specimen.  
According to the \"Structural adhesives guidelines for the surface preparation of metals and plastics prior to adhesive bonding\" GB/T21526-2008 [38] specification put forward inside the surface modification recommendations and plastics before bonding of structural adhesives, relevant surface modification is required before strain gauges are applied to metal surfaces. Integrating the material properties and the test situation, this paper adopts the physical method suggested in the specification in grinding for the relevant treatment, and its specific abrasion steps are as follows:  
1. Firstly, polish off the protective coating and impurities of the specimen with a grinding machine to make the specimen show a metallic luster;
2. Dry grinding with water-grit sandpaper (grit range 45 \(\upmu\)m-106 \(\upmu\)m), firstly, 45\({}^{\circ}\) forward to the position of the strain gauges, until uniform light scratches appear on the surface of the test piece;
3. Continue dry grinding perpendicular to the previous direction until all the light scratches in the forward 45\({}^{\circ}\) direction have been removed;
4. Sand in a circular direction (diameter \(\leq\) 100 mm) until all the scratches produced in the first two steps are ground off and the surface is uniform;
5. Clean the debris. Use a clean lint-free cotton cloth dipped in alcohol to scrub the test piece until no contamination occurs after the cotton cloth is scrubbed.  
Starting from the fatigue hot spot, multiple strain gauges are arranged along the direction of the highest stress gradient. The strain gauges sg1-sg4 are arranged on the same side of the specimen surface, and sg5-sg8 are symmetrically arranged on the other side of the specimen surface. As shown in Figure 4. Among them, sg1 (sg5) and sg4 (sg8) are symmetrically arranged at 5 mm from the free edge of the left and right cuts, and sg2 (sg6) and sg3 (sg7) are symmetrically arranged at 15 mm from the free edge of the left and right cuts. The size of the resistive strain gauge sensitive grid is 1 mm \(\times\) 2 mm.