**Piotr Nazarko \*,† and Leonard Ziemia ´nski †**

Department of Structural Mechanics, Faculty of Civil and Environmental Engineering and Architecture,

Rzeszow University of Technology, Powstancow Warszawy 12, 35-959 Rzeszow, Poland; ziele@prz.edu.pl

**\*** Correspondence: pnazarko@prz.edu.pl; Tel.: +48-178-651-621

† These authors contributed equally to this work.

Received: 30 June 2020; Accepted: 11 August 2020; Published: 14 August 2020

**Abstract:** There is a group of measurement techniques that can be used in the task of force identification in steel bolts. In this paper, the potential of elastic wave propagation signals was studied for possible application in force monitoring systems. A series of laboratory tests was carried out on flange connections subjected to static tensile tests. Each one contained six screws of the same diameter. Four bolts were equipped with washer load cells. Alternatively, selected bolts were equipped with piezoelectric transducers (actuator and sensor) in order to measure the elastic wave signals. Principal components analysis, time of arrival, and neural network compression were used for dimensionality reduction of the measured signals. Examples of the obtained results with respect to the studied connections show that the tension forces in bolts can be estimated with relatively good accuracy.

**Keywords:** elastic waves; signal processing; neural networks; force prediction, flange connection; static test
