What is Structural Health Monitoring (SHM)? Distributed Fiber Sensors (DOFS)?

Hello everyone!

So, from my last publication you got to know me little bit and the process of how I ended up in this project entitled: Development of optical fibre distributed sensing for SHM of bridges and large scale structures. 

But, what is in fact SHM and DOFS?

Well, everything has a lifetime period, from living organisms to inanimate objects. Deterioration and decay is a constant all around us and engineering structures are not an exception. In the United States alone, over 11% of the nation’s 607 380 bridges are structurally deficient and the cost to repair these deficient bridges is estimated to be $76 billion (ASCE 2013). Well maintained civil infrastructure can substantially increase a country’s competitiveness in a global economy and enhance resilience to adverse circumstances. Therefore, a structure, especially in the present days, must be able to reliably produce information regarding the alterations in its structural health condition and communicate it to the responsible operators and decision makers both in time and either automatically or on-demand in order to decrease these costs. 

The control and monitoring of the aging process of civil engineering structures is of extreme importance for their quality and safety. Furthermore, there are different external events that can induce damage to a structure. The process of employing a damage identification strategy for engineering and aerospace infrastructures is referred to as structural health monitoring (SHM). The early detection of structural malfunctions allows the increase of the service life-time of the structure at the same time that decreases the maintenance costs associated with every infrastructure and the economic losses related with repair/reconstruction in the case of structural failure being critical for the emergence of sustainable civil and environmental engineering.

The act of damage identification has been around probably, in a qualitative manner, since modern man has used tools. Notwithstanding, SHM has been recently a fast-developing area in aerospace and engineering disciplines especially in the civil engineering field. The innovation in the SHM technologies as well as the development of the large‑scale SHM systems has been a great subject of interest within the engineering and academic communities over the last two decades. However, despite its great potential, SHM has not been applied in large scale and in a systematic manner to civil infrastructures. One significant reason for this is the deficit of reliable and affordable generic monitoring solutions. 

Currently, evaluations of buildings, bridges, dams, tunnels and other vital infrastructures are usually carried out by engineers trained in visual inspection, which sometimes can be inaccurate due to differences in their personal experience with safety condition assessment. In order to improve the inspection accuracy and efficiency, optical fiber sensors (OFS) are one of the fastest growing and most promising researched topic, due to their features of durability, stability, small size and insensitivity to external electromagnetic perturbations, which makes them ideal for the long-term health assessment of built environment. 

Different kinds of sensors, embedded or attached to the structure, can be used in SHM systems but only those based on fiber technology provide the ability to accomplish integrated, quasi-distributed, and truly distributed measurements on or even inside the structure, along extensive lengths. Standard monitoring practice is normally based on the choice of a limited and relatively small number of points that are supposed to be illustrative of the structural behavior. For a large scale structure, the number of point sensors needed to generate complete strain information can grow rapidly. Discrete short‑gauge sensors provide useful and interesting data of the structure related with local behavior but might omit important information in locations where degradation occurs but that not is instrumented. 

Distributed optical fiber sensors (DOFS) offer an advantage over point sensors for global strain measurements. The thousands of sensing points that the DOFS provides enables mapping of strain distributions in two or even three dimensions. Thus, real measurements can be used to reveal the global behavior of a structure rather than extrapolation from a few point measurements. A truly distributed optical sensor is expected to measure temperature, strain and vibration data at any point along an entire fiber trough light scattering. The great challenge has been to develop these sensors in a way that they can achieve appropriate sensitivity and spatial resolution. Fortunately, great advances have been made in the last decades in order to improve this area.

Scattering is at the origin of DOFS and it can be defined, in a simple way, as the interaction between the light and an optical medium. Three different scattering processes may occur in a DOFS, namely: Raman, Brillouin and Rayleigh scattering. Distributed fiber optic sensors can depend on different techniques and principles. For different SHM applications, different DOFSs sensors can be developed and so different techniques are applied.

Nevertheless, the DOFS that have been mostly applied in civil engineering SHM applications are based on the following techniques: Brillouin optical time domain reflectometer (BOTDR), Brillouin optical time domain analysis (BOTDA) and Rayleigh based optical frequency reflectometry that is better know by the optical backscattered reflectometer (OBR) designation.

BOTDR based sensors have been the most studied and applied measuring systems in civil structures SHM due to their extended measurement range potential that makes them very useful for the application on large structures, such as dams, pipelines, tunnels and long span bridges. Notwithstanding, some applications require a better spatial resolution than the one provided by these sensors. The BOTDA sensing technique, through the application of advanced and complex algorithms, can address this point but in the process increases the price of this technology. OBR technique (Rayleigh OFDR) offers a more cost-effective way of achieving high spatial resolution limiting, nonetheless, the sensing range to 70 meters.

In the next, publication , I will present some recent applications of these sensors on civil engineering structures, including some developments carried out by our research group at UPC. As a preview, I share with you a photo of me on one of those carried experiments :)

More updates next month, stay tuned!

Greetings from Barcelona!