Simulation-Driven and On-line Condition Monitoring with Applications to Aerospace

Periodic Reporting for period 1 - SiMAero (Simulation-Driven and On-line Condition Monitoring with Applications to Aerospace)

Periodo di rendicontazione: 2018-10-01 al 2020-09-30

In current aircraft maintenance procedures, damage is monitored by means of frequently scheduled inspections, which might often require dismantling of the entire assembly in order to reach specific parts of the structure. The latter leads in increased operational costs, while may not warn of sudden or cumulative appearance of damage. On the other hand, recent technological advancements allow for acquisition of data from the structure in operation at low cost and with sufficient accuracy. Furthermore, advanced modelling techniques can accurately reproduce the response of complex components and structures, in both damaged and healthy conditions, thus complementing the information obtained through measurements.
The current project aims at combining the aforementioned tools, i.e. sensing technologies and modelling techniques, to enable the development of constant and continuous monitoring tools that can complement standard maintenance procedures. In order to investigate different possibilities, two application scenarios are considered:

Detection of damage in advanced stages in operating conditions.
This scenario involves the detection of cracks of larger size under unknown operating loads using vibration measurements, such as accelerations and strains. In this case, two challenges are posed with respect to the use of models. The first one is related to computational time, which has to be low enough to allow for online application. The second one stems from the fact that the exact operational loads are not known.

Detection of damage in early stages in testing conditions.
This scenario involves the detection of cracks of small size using guided waves, generated and measured by piezoelectric transducers. Then, the damage detection problem can be formulated as an inverse problem involving the repeated solution models for different crack locations and sizes. As in the previous scenario, computational time becomes also an issue, while flexibility with respect to the representation of damage is also required.

Standard and extended finite element methods (FEM/XFEM) have been employed to simulate cracked solids and shells. Cracks were either explicitly meshed in the case of standard FEM, or represented by appropriate enrichment functions in the case of XFEM.
In a first attempt to combine numerical models with data-driven methods, a two step crack detection scheme was developed. The first step relied on a damage index based on mode shape curvatures, a quantity typically used in the SHM literature to provide a rough initial damage localization. In the second step, a numerical model for cracked plates, based on the XFEM, was employed along with an evolutionary optimization algorithm to refine the initial localization and eventually yield an accurate estimate for the crack location, size and orientation. At this stage, operational loads were considered known, while the full order XFEM models were used.
Subsequently, a method for parametrizing models of cracked solids with respect to the damage location and extend was developed, relying on mesh morphing. This in turn, enabled the use of state of the art model order reduction (MOR) techniques to construct parametric reduced order models (pROMs), providing similar accuracy to their full order counterparts, over a wide array of damaged configurations, at a fraction of their numerical cost.
With efficient and accurate parametric ROMs available, two alternative approaches for damage detection were developed. In the first, damage indices that rely on output only quantities, and thus do not require information regarding the applied loading were employed. Then optimization was employed to minimize the difference between the measured values of the damage indices and the ones predicted by the pROMs, leading to the detection of damage.
In the second approach, the time history of the loading as well as the damage location and extent are simultaneously estimated, in an online manner, through the use of particle filters. More specifically, a hierarchical Bayesian filter was developed, the input load was estimated at a first level, while, at a second level, multiple models were generated, selected and updated using evolutionary operations, to yield an estimate for the damage extent and location.

Exploitation and dissemination of the results.
The results of the project were presented in three international conferences, two workshops and a seminar. Additional presentations were planned, however several conferences have been cancelled due to COVID-19. One paper has been published with results from the project, with three more under review and one in preparation.

The main scientific contributions of the project can be summarized in the following points:
- The parametrization of ROMs for cracked solids and shells has been enabled by the developed methods.
- The fast detection of cracks in shells and solids, in the absence of information regarding the time history of the applied loading is rendered possible through the proposed crack detection schemes.
- The simultaneous estimation of the system input as well as the parameters describing the location and extent of damage is achieved by the proposed ROM-particle filter combination. This approach has the added advantage of providing an estimation for the full response of the system, thus providing information even for unmeasured locations.
-A paradigm has been set for the incorporation of ROMs within an SHM framework, as well as for their combination to data-driven methods.
In terms of potential impact, the work conducted constitutes a first step towards the more widespread use of numerical models in condition monitoring. Harnessing the full potential of models in this field could lead to more advanced, continuous monitoring tools that will in turn allow the safer and more cost effective operation of aircraft structures.

Graphical Abstract of the Project

Periodic Reporting for period 1 - SiMAero (Simulation-Driven and On-line Condition Monitoring with Applications to Aerospace)

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