# Investigating Bayesian Parameter Identification Using Non-Standard Laboratory Specimens
## 4 Test Design and Experimental Setup
### Experimental Setup  
Experimental investigations were conducted in the Structures Laboratory of FCEAG at the University of Split. Multiple measurement systems were used to obtain precise data on the physical properties of the test specimens (see Figure 6). The samples were fabricated from 6060 T4 aluminum alloy stock using a CNC milling machine, in accordance with the ISO 6892 standard. Additionally, some of the specimen was notched to induce stress concentration areas, as illustrated in Figure 3. Dimensional accuracy was verified prior to every test. Specimens were loaded using a Shimadzu AGX-V 250 kN Universal Testing Machine (UTM), Shimadzu, Singapore. Strains were measured with an SIE extensometer and a crossbar positional sensor; both devices are classified as accuracy class 0.5. Forces were recorded using a load cell (nominal capacity 250 kN, accuracy class 0.5). Testing was performed under displacement control of the upper jaw at a constant speed of 2 mm/min, chosen to match the strain rate from ISO 6892. The termination condition was defined as a 10% drop in subsequent force, and data were sampled at 100 Hz. To determine Poisson's ratio, additional sensors and measurement systems were utilized. Longitudinal and transverse strains were measured using LVDT sensors (HBM WA-L, 0-10 mm nominal range). A custom made 3D-printed jig, manufactured from ABS plastic on a BambuLab X1 Carbon desktop 3D printer, was used to hold the sensors orthogonally and maintain attachment to the sample during loading. The testing regime for measuring Poisson's ratio was limited to the elastic deformation region and cycled five times before data collection to ensure the positional stability of all components in the measurement chain. The LVDT sensors were connected to an HBM Quantum MX840B universal amplifier, which acquired data independently of the UTM. To synchronize the two data sources (the UTM and the MX840B), analog signals representing crossbar displacement and force were recorded by the MX840B. Figure 5: Sobol sensitivity (single\(-\)notched specimen): Partial variances of force due to changes in elastoplastic and fracture parameters (white area represents mixed modes influence). In addition to the mechanical measurements, an optical measurement system was employed. An industrial camera (2560 x 1440 pixels) was positioned with its imaging plane parallel to the specimen plane and configured to capture an image every 0.5 s. It was synchronized with the data acquisition software so that crossbar displacement and force values were recorded for each frame. The camera used a varifocal lens (5-50 mm) with a maximum aperture of f/1.6. Captured images were undistorted to remove radial and tangential distortions using camera parameters obtained through a checkerboard calibration procedure; this step was repeated for every experimental setup to account for the variable zoom setting. Camera parameter determination and image undistortion were carried out in MATLAB (version 9.13.0, R2022b, The MathWorks Inc.: Natick, MA, USA, 2022). A random speckle pattern was applied to the samples to facilitate Digital Image Correlation (DIC). The DIC analysis was performed using the open-source software [64], implemented in the MATLAB environment [65]. To assess the accuracy of the DIC system, a calibration run was carried out. Before each tensile test, the specimen was translated through the camera's field of view using the UTM crossbar. The sample was held only by the upper jaw, leaving the lower end free. The position of the sample was recorded using the MX840B amplifier. Analysis of the calibration run revealed displacement accuracy and apparent strain values related to lens irregularities, specimen preparation, and DIC limitations. The root-mean-square error in displacement was 0.1 mm, and the strain noise was below \(1\times 10^{-3}\), primarily due to camera limitations (and to a lesser extent, specimen preparation and inherent DIC constraints).