Toward to an Automatic Crack Size Estimation with iFEM for Structural Health Monitoring

Abstract

The inverse Finite Element Method (iFEM) is a model-based technique to compute the displacement (and then the strain) field of a structure from strain measurements and a geometrical discretization of the same. Different literature works exploit the error between the numerically reconstructed strains and the experimental measurements to perform damage identification in a Structural Health Monitoring framework. However, only damage detection and localization are performed, without attempting a proper damage size estimation. The latter could be based on machine learning techniques, however, an a priori definition of the damage conditions would be required. To overcome these limitations, the present work proposes a new approach in which the damage is systematically introduced in the iFEM model to minimize its discrepancy with respect to the physical structure. This is performed with a maximum likelihood estimation framework, where the most accurate damage scenario is selected among a series of different models. The proposed approach is experimentally verified on an aluminum plate subjected to fatigue crack propagation, which enables the creation of a Digital-Twin of the structure itself. The strain field fed to the iFEM routine is experimentally measured with an Optical Backscatter Reflectometry fiber and the methodology is validated with independent observations of lasers and the Digital Image Correlation.

Debonding quantification in adhesive bonded joints by the inverse finite element method

“…The two sensor networks may share sensors [53] or, to better highlight the presence of damage, have no sensors in common [57,58].…”

“…A second damage diagnosis approach with iFEM has been recently introduced by Oboe et al in [57,58] further exploiting the concept of compatibility between the real structure and its iFEM model. The key idea is to introduce the damage in the iFEM model to restore the compatibility between the model and the physical, damaged, structure.…”

“…Several damage scenarios are generated off-line according to the expected damage mechanisms and stored in a database, although they might be generated in near real time according to the detected damage propagation, as demonstrated in [57,58]. Afterwards, for each measured sample, the iFEM full field displacement and strain field are computed through the input sensor network for all the models, being the iFEM computationally inexpensive for real-time applications.…”

“…This paper extends the anomaly index-based strategy initially developed for anomaly identification [53] and expands the methodology initially developed for cracks and presented in [57,58] to estimate the debonding length on cracked leap shear (CLS) specimens representative of adhesively bonded repair patches: it is thus one of the few methods to provide a load-independent estimate of the debonding entity and the first application of the iFEM to the shape sensing of an adhesive bonded joint. The approach based on model selection demonstrates adequate performances and significant scientific advancement with respect to the current state of the art.…”

“…The iFEM, being load-independent, has been successfully exploited to perform damage detection [42,53] and a formulation taking into account a mixture of boundary conditions has been recently developed [54] to model non-ideal boundary conditions. Latest developments have introduced physics-based pre-extrapolation [55] and uncertainty in the iFEM [56]; Oboe et al in [57,58] applied the iFEM to estimate the crack size in a model-updating framework where multiple damages are modeled within the iFEM and the most likely model of the structure is selected according to a maximum likelihood estimate (MLE), demonstrating the potential for iFEM models to be integrated in digital-twins.…”