## 4 Applications
### Applications in the Automotive Industry
#### 4.3.2 Surface-Hardened Parts and Machined Parts
For years, 3 MA has been used for the nondestructive detection and characterization of grinding burns. Such applications of 3 MA have been developed in cooperation with industrial partners [40]. As part of a research project (FVA No. 453) with research partner FZG from Technical University of Munich, the relationships between the load-bearing capacity of gearwheels and the hardness (Hd) and residual stress (RS) in the gear wheel tooth flanks that are influenced by grinding were examined [73]. On the basis of the 3 MA technique, the gearwheels were nondestructively tested, so that a tested gearwheel could be systematically examined in running tests with regard to its tooth flank load capacity and with regard to the change of the material properties in the tooth flanks. A specially adapted and miniaturized 3 MA probe was developed and then integrated into a multi-axis manipulation system (see Figure 24). This so-called \"gearwheel scanner\" allows for the tooth flanks to be scanned in meander-shaped surface scans. For calibration, the 3 MA measuring parameters were recorded on undamaged gear wheels with different degrees of grinding damage. Subsequently, the gradients of Hd and RS in these gearwheels were measured with the reference methods (Vickers hardness and X-ray diffractometry). On this database, the 3 MA system was calibrated to hardness (Hd) at the material depths of 0.1, 0.2, 0.3, 1.0, and 3.0 mm and to the residual stress (RS) at the depths of 0.00, 0.01, 0.02, 0.04, and 0.06 mm (see Figure 25). The results show remarkably good correlations between 3 MA parameters and Hd and RS; the RMSE value for the Hd calibration was 3 HV and for the RS calibration 10 MPa, respectively. Figure 23: Results of the calibration to hardening depth: (a) case-hardening depth (CHD) and (b) surface-hardening depth (SHD). The application of the calibrated 3 MA system on gearwheels with different degrees of grinding damage is shown in Figure 26. Table 10 shows the meaning of the marking codes of the curves in the diagrams of Figure 26.  
Figure 26: Results of measurements with the calibrated 3 MA in gearwheels showing different degrees of grinding damage with designations FA0-II. FE3-II: (**a**) hardness profiles and (**b**) residual stress. Figure 24: 3 MA gearwheel scanner system. Figure 25: Results of the calibration to depth-dependent material properties: (**a**) hardness at depths of 0.1, 0.2, 0.3, 1.0, and 3.0 mm and (**b**) residual stress at depths of 0, 0.01, 0.02, 0.04, and 0.06 mm. Because the application of 3 MA is completely nondestructive, such measurements of hardness and residual-stress profiles could be repeated during the stress tests of the grinded gearwheels, as is often as desired. These investigations allowed for detailed analysis of the relation between the extent of grinding damage and the behavior of the gearwheels during the stress tests. Characteristic features in the Hd and RS profiles that are relevant to the durability of the gearwheel could be identified, allowing for the load-bearing capacity (pitting fatigue strength) of grinded gearwheels to be predicted based on measurements with 3 MA. The determination of depth-profiles of hardness (Hd) and residual stress (RS) is useful, not only for gearwheels, but also for other elaborately processed components, such as valve springs. The production process of these high-performance parts includes several steps that result in a final product with complicated Hd and RS depth-profiles. These can be nondestructively determined with the 3 MA \"valve spring scanner\", which is shown in Figure 27a. As shown in Figure 27b, H can be determined at material depths of 0.25, 0.50, 1.00, and 2.00 mm and RS at depths of 0.00, 0.05, 0.10, 0.20, and 0.30 mm.