Structural Health Monitoring

Reasons for Usage

Constructs like wind turbines, ships, bridges and others are dynamically stressed. Hence, the knowledge about structural health is of great importance. This area of expertise of fatigue deals with the prevention of damages through the regular fluctuations of occurring stress.

The difference between fatigue and statics lies in the regular frequency of stress, instead of singular strains like a monster wave. It is assumed that ships are stressed about 50 million times in their life time. Wind turbines have eben more of these so-called load circles. Partly due to the unforeseen raise of the number of trucks load circles are increasing for bridges resulting in a premature maintenance or demolition. Such harmful vibrations are produced by the intended purpose and the present environmental conditions.

The impact of the influencing forces varies. Storms with stronger waves occur comparatively rarely. Characteristic of stress resulting from environmental conditions are small to medium-sized load circles. These occur while long weather periods with up to medium-sized waves.

A single load circle owing to a smaller wave contributes not very much to the damage of a construct. Accumulated, however, small waves can prove dangerous to a construct’s structural health. Emerging damages usually manifest in the form of tiny cracks which are likely to appear where forces are redirected. Welding seams, general form changes and crossovers to support structures are usually problematic areas. If no suited steps are taken these cracks can grow continuously and expand until single components or the whole construct is in danger. This phenomenon was not recently discovered since there are many examples showcasing this problem.

Approach and Field of Application

It is state-of-the-art that calculations regarding the structural health are performed in the design phase. In accordance with these calculations constructs are put into operation. After that regular inspections occur, maintenance is provided and components are exchanged.

This type of monitoring is well-tried and has been time-testes for many decades. However, it is not based on updated calculations of the used life time but rather on empirical insights. Maintenance and repairs for innovative constructs or a different service profile are therefore difficult to estimate. This is the case for offshore wind turbines or novel specially made ships where no long-standing experience is available.

As a result, complications can arise. These are mostly controlled by technical measures which can be very expensive. A failure of an offshore wind turbine may result in a total breakdown for months because repairs cannot be ad mistered due to the weather conditions. These are usual cases for the structural health monitoring.

However, even with well-known constructs and operating conditions the use of the structural health monitoring can have its advantages. Reasons are commonly demands for higher safety standards and economic efficiency. Further development of existing constructs is more cost-effective than manufacturing and operating their predecessors. This results in advances in the maintenance process without lowering the reliability. By the means of the structural health monitoring the service induced material fatigue can be overseen continuously.

To accomplish this, it is necessary to conduct regular and specified checks to analyse the dynamic stress. Also, these results have to be processed in a timely manner in order to calculate the existing damage. On this basis prognoses regarding the remaining service time of the construct and its components can be put forward. On top of that it is possible to ascertain the optimal point of time for maintenance or answer questions about the safe and efficient operation of existing constructs beyond their nominal service time.

State of Research

This procedure is recognised by experts and is already in use. It is relevant for the practical use that a reliable analysis can be done with as less checks as possible. The data transfer, the data processing, its evaluation and application to suited calculation models are also important parts of the method. The current state of the development of the structural health monitoring has not matured enough to be used in a broad practical operation.

At this point RELIABLES offshore comes into play. Its goal is to improve the application maturity of the structural health monitoring. Also tests regarding the fatigue of materials can be conducted for regional enterprises within the scope of the Crossborder Vibration and Fatigue Testcenter.