Keywords: structural health monitoring, magnetic methods, construction monitoring, civil engineering, strain measurements.

Contemporary infrastructure elements are becoming increasingly complex technical systems. At the same time, the use of computers revolutionised the process of analysis which nowadays covers not only the durability of structures but also their energy efficiency and environmental impact. The complexity and diversity of civil engineering structures imposes requirements on the measuring system, which are difficult to be satisfied by a single sensor. As a solution, complex monitoring systems, composed of different types of sensors are being developed [1].

It is worth noticing that the research involving steel as well as pre-stressed concrete structures focuses mainly on the detection of the occurrence of faults and defects at the earliest possible stage of their development [2]. The weaknesses of existing methods belonging to this group includes their usage for local measurements only, difficulty in maintaining diligence of measurements during the research, and above all lack of the clear possibility of assessing the threat that the discovered defect poses for the analysed structure.

Passive magnetic methods rely on ferromagnetic material interaction with the earth's magnetic field. They are based on the observation that there exists a relation between stress and degree of magnetization [3]. Tracking of the qualitative changes of the eigenmagnetic field of an object could provide the information as to whether the object has come into the zone of plastic strain which is dangerous for structures.

From the point of view of the relevance of diagnostic information, one of the basic problems related to the proactive operation strategy is the adoption of the relevant diagnostic and prognostic models. In this paper the task of the estimation of the remaining useful life of a device (RUL) is formulated for the possibility of the occurrence of different defects with random values of periods of initiation and propagation. It is based on the assumption that the possibility of the detection of the defect initiation occurs with a certain probability and that the probability of defect initiation and propagation for the assumed threshold value can be defined. The assumptions apply to each of the possible defect mode. Additionally the effect of the dependence of various factors on the mean system lifetime is considered.

In the last part of the paper some examples were presented.

1

A. Galezia, S. Gontarz, M. Jasinski, J. Maczak, S. Radkowski, J. Senko, "Distributed system for monitoring the large scale infrastructure structures based on analysis of changes of its static and dynamic properties", presented at the Second International Workshop on Smart Diagnostics of Structures, Krakow, Poland, 2012. doi:10.4028/www.scientific.net/KEM.518.106

2

S. Radkowski, K. Szczurowski, "Use of vibroacoustic signals for diagnosis of prestressed structures", Eksploatacja i Niezawodnosc - Maintenance and Reliability, 14(1), 84-91, 2012.

3

S. Gontarz, S. Radkowski, "Impact of Various Factors on Relationships Between Stress and Eigen Magnetic Field in a Steel Specimen", Magnetics, IEEE Transactions on, 48(3), 1143-1154, Mar. 2012. doi:10.1109/TMAG.2011.2170845

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