PSI - Issue 37

Dmitrijs Serdjuks et al. / Procedia Structural Integrity 37 (2022) 555–562 Dmitrijs Serdjuks et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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The aim of current investigation was to determine a structural joint ’ s stiffness as a function of the vibration parameters of the structure. A non-model vibration analysis method of coaxial accelerations correlation was used. For this, it was necessary to provide a measuring device with a description of its components and the main stages of application of the proposed method. A stand was also developed and tested from steel beams with a semi-rigid connection of variable stiffness was also developed and tested. 2. Methods for health monitoring of structural joints The application of non-destructive testing (NDT) methods to assessing the quality of structural joints is largely determined by the critical elements of the joints that perform the connecting function. For example, in welded joints, the main determinants of the quality of the joint are the quality and defect-freeness of the weld itself. In bolted joints, the quality of the assembly is determined by the quality of the bolts and the degree of their tightening, whereas in adhesive joints, such an indicator is the quality of adhesion of the elements being fastened. Respectfully, NDT methods in these cases are specific to key elements examined. So welded joints are checked by ultrasonic testing in a pulse echo mode using either a single-channel device, a phase array system, or the time-of-flight diffraction method (Ahmad et al. (2018)). For NDT of adhesive joints, the echo-pulse method applying the analysis of ringing tones in multiple reflections and the propagation of Lamb waves in plates were found suitable (Zeighami et al. (2009), Heller et al. (2000)). At the same time, there is a need for holistic methods that allow an integral assessment of the functionality of a structural joint by a non-destructive way without specific addressing to its structural elements. Such methods, traditionally attributed to the field of structural health monitoring (SHM) use mechanical impact on the joint as a whole and recording the responses of the reactions from its structural elements. In this case, the impact levels can be close to those at the operating conditions, including extremal ones, or be significantly lower, taking as an assumption the linear behavior of the system. According to the way of exposure, the methods are subdivided into shock and vibration modes using a short pulse impact or a continuous oscillation, correspondingly, and referring to dynamic testing modalities both (Hanly (2016)). Both shock and vibration testing are historically acknowledged methods with standardized guidelines for the measurement and evaluation of the effects on structures (ISO 4866 (2010)), and dynamic testing procedures for investigation of building structures such as bridges (ISO 14963 (2003)). Dynamic characteristics of structural systems highly depend on joint properties. Due to the complexity of joints, it is highly difficult to describe dynamic behavior of joints with analytical models only. Reliable models are generally obtained using experimental measurements. Frequency response function (FRF) of structures containing a set of resonant peaks of natural frequencies, damping and mode shape can be obtained analytically or experimentally allowing the calculation of the stiffness of the structure with certain limitations (Tol et al. (2012)). All types of accelerometers, single and multi-axes electromechanical sensors, transforming energy of motion into an electrical output are used to measure responses on shock and vibration. The advent of low-cost MEMS accelerometers and open-source electronic platforms, such as Arduino, have facilitated the design of low-cost systems suitable for modal identification and measuring natural frequencies and damping ratios in building structures, including multi degree of freedom (DOF) systems (Varanis et al. (2018)). Dynamic behavior of big structures, such as high-rise buildings, assessing its horizontal and vertical displacements and fluctuations is proposed to monitor by differential global positioning systems (GPS) with a reference to a stationary GPS station [A9]. The experimental technique used to determine the dynamic characteristics of buildings is based on records of low intensity oscillations of the building produced by various natural factors, such as micro seismic agitation motions, city traffic, wind etc. (Park et al. (2018)). In another option of vibration testing of building structures, reasonably accurate evaluation can be frequently obtained by a simple technique such as floor response at a single point due to heel drop compared by its testing effect with thorough modal analysis of multiple synchronous measurements with a transducers network (Dobre et al. (2017)). Application of multi-sensor accelerometer network, particularly two tri-axial accelerometers allowed for the enhancement of damage indicator when the structure is perturbed sensors, laying the basis for the creation of an IoT system for SHM (Nguyen (2020)). It was shown that multi-axis test was more representative of a true operational environment. Thus 6 DOF testing system improved the traditional 1 DOF approach by a more representative stress state in a test article during mechanical shock and vibration (Muttillo et al. (2020)). To monitor the 6-DOFstructural