**1. Introduction**

Structural condition monitoring has been a subject of great interest for many leading research groups for last several decades. It has also become more and more popular among engineers, who put a lot of effort into the reduction of maintenance costs [1]. As technical data for further analysis, various structural parameters and their changes have been considered in the case of damage detection algorithms [2,3]. This includes structural parameters obtained from: modal analysis [4–9], impedance measurements [10,11], ultrasonic inspection [12–15], wave propagation [16–30], and many others [31–35]. A simple graph is presented in Figure 1 to illustrate in numerals the most popular parameters used in damage detection algorithms. One can see that modal analysis and wave propagation are *the leaders* among many scientific and industrial interests, and the number of publications devoted to these two subjects is relatively large in comparison to ultrasonic inspection and impedance based algorithms. In the authors' opinion, the simplicity of measurements, equipment availability, and sensitivity to structural changes are the reasons for this *popularity*. Nowadays, numerical calculations based on customised numerical/computational models are ideal alternatives to actual measurements. According to the literature, numerical modelling may be considered as one of the most important aspects of structural condition monitoring [36,37]. In the following figure, Figure 2, a summary of the number of publications devoted to numerical modelling of one-dimensional structural elements, devoted to modelling of wave propagation phenomena, and finally devoted to the use of various numerical models for SHM (structural health monitoring) purposes, has been presented. In every case the number of publications increases, which is definitely a sign that the subject of this paper is of interest to the scientific society.

**Figure 1.** Graphic summary of the number of publications, related with structural parameters used for structural health monitoring (SHM) systems. Web of Science (26 March 2020).

**Figure 2.** Graphic summary of the number of publications, related with modelling of 1D elements. Web of Science (26 March 2020).

The purpose of this article is to draw the attention of the research community to some aspects of numerical modelling that may have a significant impact on the correctness of the results of numerical computations; i.e., aspects that may lead to significant errors and misinterpretation of results. The aim of the article is also to draw attention to the problems of numerical nature, which are immanent to modelling. The authors illustrate this in the case of simple one-dimensional structural elements (rods and beams) for various discretisation methods known from the literature.
