## 2 Materials and Methods
### Machine-Made Parts and Experimental Data Acquisition  
A set of 29 machine-made connecting rods manufactured in a workshop was characterized experimentally. Five _reference parts_ were used, which are indicated with the letters A to E. These parts were machine-made, following the geometrically mean part, which means that the identical mean geometry was set as the nominal geometry. Additionally, two dozen _validation parts_ were used. These parts were labeled with consecutive numbers from 1 to 24. The nominal geometric dimensions of the validation parts were set with random numbers, considering the basic population and its value ranges. For the parts no. 1 to 12, the normally distributed basic population was imputed directly. For the parts no. 13 to 24, a uniform distribution was selected instead, whereby the interval borders are the mean value plus or minus three times the standard deviation of the basic population for each geometric parameter. Due to the limited amount of machine-made parts, the uniform distribution was chosen to describe many parts that are more likely to have values near the borders of the essential value ranges instead of being more centered with a higher probability. However, the part geometries could also come from the basic population, because all of the dimensions are inside the typical value ranges of the basic population. Figure 7: Eigenfrequency distribution for connecting rods depending on quality state and amount of geometric part-to-part variations regarding Mode-16. For some parts, a single dimension was manually set to the mean value plus or minus four times the standard deviation. This should achieve dimensions slightly outside the usual ranges. It is noted that all of the nominal geometric values were rounded to integer multiples of 0.05 mm for better handling. Finally, the parts were machined from a single batch of rolled aluminum alloy, according to the defined geometric dimensions. Using a single material batch should ensure a nearly constant material behavior. Additionally, a consistent production strategy was agreed with the manufacturer to reflect a reproducible series process. Through-holes representing structural defects were drilled into 14 of the 24 parts, using a conventional pillar-drilling machine and enlarged stepwise. We selected the parts no. 1 to 7 and no. 13 to 19 for the defect implementation. The process started with a defect diameter of 1 mm and resulted in a final size of 3 mm, with steps of 0.5 mm. The defect position was individual for each part. The defects were placed manually over the parts to achieve a uniform defect distribution, considering the limited number of parts and taking into account that the application example does not assume any preferred defect positions. Symmetry was taken into account to avoid similar positions due to the limited amount of parts. Figure 8 shows the chosen defect position depending on the part number. The actual positions can vary up to 1 mm in the lateral directions compared to the sketch, because the positioning was completed by hand and not by machine. In addition, the figure includes photos of two of the machine-made parts with their final 3 mm defect size. The machine-made parts were characterized experimentally as briefly described in our previous publication. For example, we recognized a homogenous but slightly anisotropic material behavior linked with the rolling structure of the material and discovered, with the help of radiography, that the parts featured no macroscopic volume defects. During the measurements, the temperature was measured and the temperature effects on the quantitative measurement data were computationally corrected to the best possible accuracy. Before implementing the through-holes, all of the connecting rods were measured geometrically, because each actual geometry differs inevitably from the nominal dimensions due to production tolerances. A coordinate measuring machine was used for this. The measured data were then applied to our geometry model.