Fusion-based damage diagnostics for stiffened composite panels

Abstract

Conducting damage diagnostics on stiffened panels is commonly performed using a single SHM technique. However, each SHM technique has both its strengths and limitations. Rather than straining the expansion of single SHM techniques going beyond their intrinsic capacities, these strengths and limitations should instead be considered in their application. In this work, we propose a novel fusion-based methodology between data from two SHM techniques in order to surpass the capabilities of a single SHM technique. The aim is to show that by considering data fusion, a synergy can be obtained, resulting in a comprehensive damage assessment, not possible using a single SHM technique. For this purpose, three single-stiffener carbon–epoxy panels were subjected to fatigue compression after impact tests. Two SHM techniques monitored damage growth under the applied fatigue loads: acoustic emission and distributed fiber optic strain sensing. Four acoustic emission sensors were placed on each panel, thereby allowing for damage detection, localization, type identification (delamination), and severity assessment. The optical fibers were adhered to the stiffener feet’ surface, and its strain measurements were used for damage detection, disbond localization, damage type identification (stiffness degradation and disbond growth), and severity assessment. Different fusion techniques are presented in order to integrate the acoustic emission and strain data. For damage detection and severity assessment, a hybrid health indicator is obtained by feature-level fusion while a complementary and cooperative fusion of the diagnostic results is developed for damage localization and type identification. We show that damage growth can be monitored up until final failure, thereby performing a simultaneous damage assessment on all four SHM levels. In this manner, we demonstrate that by proposing a fusion-based approach toward SHM of composite structures, the intrinsic capacity of each SHM technique can be utilized, leading to synergistic effects for damage diagnostics.

On the Challenges of Upscaling Damage Monitoring Methodologies for Stiffened Composite Aircraft Panels

“…The change in buckling pattern with now multiple half waves present in the multi-stiffener panel will affect the strain measurements and correspondingly the algorithms based on those measurements. A strain-based health indicator relying on certain trends in the data under damage, such as the ones presented in [6,8,9], may be affected when mode I and II are both predominantly present along the same stiffener foot. Consequently, it necessitates an adaption in its methodology to handle the simultaneous presence of both modes as well as a new inclusion of an approach on how to deal with the transfer-region between both modes.…”

“…Whereas for the single-stiffener panel, the predominant mode along a stiffener foot is constant as being either mode I (opening) or mode II (in-plane shear), the presence of multiple half waves in the multi-stiffener panel causes the predominant mode to shift between mode I and mode II longitudinally along each stiffener foot. From previous studies [6,7], we know that the disbond is more likely to propagate under mode I and in the case of the single-stiffener panel, this means that the disbond can grow freely in longitudinal direction. For the multi-stiffener panel, this is no longer the case causing the propagation of the disbond to change, for example by declining or having the disbond propagate to the other stiffener foot where mode I is observed.…”

“…The changes are not only observed in disbond propagation between the single-and multi-stiffener panel, but also in the effect of damage on the structural integrity of the panels. For the single-stiffener panel, an impact damage located near the skin-stiffener bond where mode I is dominant may lead to final failure of the panel under fatigue loads in an FCAI test [6,7]. However, for the multi-stiffener panel, an equally sized impact damage located in a similar area under comparable loading conditions may not lead to a loss in load-bearing capacity in an FCAI test.…”

Health Monitoring of Aerospace Structures Utilizing Novel Health Indicators Extracted from Complex Strain and Acoustic Emission Data

“…Despite the low sensitivity of regular FBG sensors in AE recording [20,21], these specialized phase-shifted FBGs successfully identified damage modes in three-point bending experiments, and the results were comparable to those of regular AE sensors. Broer et al [22,23] recently proposed a methodology of fusing two SHM techniques, namely AEs and strain sensing. The idea of fusing results from several sensors was proposed in order to take advantage of the strengths of both SHM techniques, and thus obtain more effective damage diagnostics of deteriorating composite panels.…”

“…These features are usually referred to as health indicators (HIs). The quality of the HIs' evolution over operational time largely determines the effectiveness of diagnostic systems [22] and affects the performance of the prognostics methodologies [24]. As we discussed previously in [25], HIs to be potentially used for diagnostic and prognostic tasks should possess the properties of monotonicity, prognosability, and trendability.…”