## 2 Formulation of the Problem  
The proposed procedure enables the detection of anomaly occurrence in pretensioned bolts of steel connections, based on Digital Image Correlation (DIC) in-plane measurements. The basis for assessing the condition of the connectors is observation of changes in the selected area neighboring the investigated node, during harmonic vibrations of the system. Many parameters were taken into account, e.g., the vertical and horizontal displacements of selected points, the linear and rotational displacements of segments, as well as the varying variants of plane figures. The aim of the procedure is not only the detection of changes in connections, but also the determination of their types and the location of loosened bolts. For the purpose of better visualization, the consecutive steps of the proposed procedure are shown in Figure 1.  
The applied measurement technique--Digital Image Correlation--is a vision-based measurement method enabling contactless registration of the observed sample shape and dimensions, as well as the full field displacement measurements within the observed area of the tested sample. On the basis of the data collected using DIC, it is possible to determine the deformations and accelerations of points in the observed area and the material constants: Young's modulus or Poisson's ratio [24]. The method is used for both static and dynamic tests [25]. It may be applied to analyze long-lasting processes (e.g., fatigue tests [26]), as well as fast-changing phenomena (e.g., vibrations and/or cracking). The main idea of DIC is to track a digitally registered random pattern placed on the tested surface; the pattern may be either natural or created by a DIC operator before the measurements. The tested surface changes its shape during loading, and so does the pattern on it; thanks to algorithms based on the correlation of photo sequences taken during the sample deformation process, it is possible to determine the location of selected (or even all) points in the consecutive photos (and in the consecutive steps of the deformation phase) [27]. One camera enables the determination of in-plane displacements; the use of a two-camera system allows for the observation of displacements of a plane sample in 3D. DIC is a rapidly growing measuring method [28]. In recent years, some new multi-camera DIC systems that allow for the observation of samples from many sides simultaneously (e.g., a cylinder observed from all around) have been developed. These systems require at least four cameras for recording changes on the edge of the two planes, or observing two opposite sides of the sample. The increase in the number of cameras allows for the observation of objects with increasingly complex shapes [29]. Unfortunately, the number of cameras used during the measurement determines the level of complication of the task. DIC has found application in both destructive and non-destructive testing, on both macro and micro scales. [PERSON] et al. [30] studied deformations of a road and pavement under a heavy truck load, while [PERSON] and [PERSON] [31] used 3D DIC for microscopic examination of biocompatible materials. [PERSON] et al. [32] proposed the use of DIC in a hybrid system of simultaneous measurement of temperature distribution and displacement of engineering objects. The ability to measure displacements in all three directions was used in dynamic tests of aircraft plating components [33], as well as for inventory and monitoring of the \"Monte Seco\" railway tunnel [34] and for the measurement of cracks in reinforced concrete beams [35]. In [36], the authors present the use of DIC as a method enabling verification of the anchoring of composite samples in thw jaws of testing machines; the obtained results were additionally used to determine the properties of tested materials and for the validation of numerical models. Validation of numerical models using the data obtained from DIC was also presented in [37; 38].