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Theoretical and experimental evaluations of strain field modification induced by flaws in loaded composite structures

Periodic Reporting for period 2 - MODIFLAW (Theoretical and experimental evaluations of strain field modification induced by flaws in loaded composite structures)

Reporting period: 2021-05-01 to 2022-09-30

MODIFLAW project is part of Clean Sky 2 initiative. MODIFLAW project aims at becoming a part of R-IADP. Outputs of MODIFLAW project will cover the definition, design, manufacturing, assembling and on-ground testing phases for full-scale structural Outer Wing Box (WP 3.1) and Fuselage and Passenger Cabin demonstrators (WP 3.2) representative of a Regional Aircraft.
The ability of a structure to maintain strength and stiffness throughout its service life is referred to as its durability. A structure must have adequate durability when subjected to expected service loads and environmental spectra to prevent excessive maintenance, repair, or modification costs over the service life. Thus, durability is primarily an economic consideration. New and improved technologies positively impact the durability, contributing to a reduction in operating costs through lower fuel burn, reduced maintenance costs, reduced navigation costs, and reduced airport fees as a result of structural weight savings due to innovative aircraft configurations and the use of lighter materials. In order to apply new and improved technologies to regional aircraft, a significant structural weight reduction, manufacturing recurring cost reduction, maintenance improvement, and the implementation of new eco-compatible materials and processes need to be assured. The MODIFLAW project aimed to contribute to this goal in two ways: (a) by enabling a more efficient and cost-effective manage and maintain system through the application of enhanced SHM software and a system for damage detection and monitoring of an outer wing box and fuselage; and (b) by enhancing the use of novel SHM strategies through experimental testing, numerical simulation, and virtual testing of an outer wing box and fuselage that offer reduced maintenance costs. To conclude, the SHM software was benchmarked using different flaw detection and mapping during structure loading which was directly affected by the two applied methodologies for deformation mapping due to the presence of flaws. Developed numerical models enabled a better understanding of flaws and provided tools for the prediction of flaw evolution. Various tools and methodologies for virtual modelling of different flaws in composite structure, including morphology were developed.
The technical work began with extensive FE modelling to design proper insert definition that was put in the composite plates during manufacturing based on the test plan. Two types of damage defined - manufacturing damage - impact damage). These were simulated and compared with pristine condition. The damage type, size, shape, lay-up position and loading condition were studied in comparison of pristine condition. FE analysis of shear test specimen and element test specimen with inserts were done. The impact damage morphology was under optimisation on the base of first impact tests. For tension, compression and bending specimens, three standard ASTM tests were numerically simulated. Non-standard test was simulated for the shear panel and for the proposed typical aeroplane sub-structure. Static, linear FEM analysis was executed for tension, bending and shear load, linear buckling analysis. However, these analyses were completed with the real impact simulations by the dynamic, non-linear, explicit FEM. Generally, for all tests for both lay-up configuration, the strain modification was more important for the delamination near the surface (near the top or the bottom) than for the delamination deep in the structure. Consequently, the flaw near the surface is much easily detectable than the flaw deep in the structure.
Specimen testing part started with fabrication of composite plates by TM based on the approved test plan. The plates were manufactured and were delivered to VZLU. Specimens were machined from the panels and evaluated by ultrasonic NDT by VZLU. Barely visible impact damage was made using drop a weight machine after specimen extraction to certain specimens. Experiments started by performing of compression tests including strain gages and DIC measurements. Tension tests were done including strain gages and DIC measurements. Two strain gage networks were proposed by VZLU to evaluate changes of the strain field near the flaws. Generally, effect of impact is evident up to 34 mm from impact. None or very low effect was visible on the last strain gage 48 mm far.
Element testing started with fabrication of a beam by TM based on the approved test plan. The beam was delivered to VZLU, machined to proper dimensions, and NDT tested. Element testing started with element tests that were done including a network of strain gage sensors and DIC evaluation. Impact of he element was done afterwards. Testing in compression was done for both samples. Deformation was measured by a network of strain gages and by digital image correlation to evaluate the buckling shape.
Virtual testing started with definition of the subcomponent and the component – flat keel panel for strain analysis during compression loading and curved panel for bending testing. VZLU defined building of FE model of part of real structure including typical structure parts such as skin, stringers, and ribs. Various morphologies of the flaws were modelled to study the relative strain field modification. The location of the flaws were modelled in two locations from skin side. Four cases were analysed. Based on the component definition, the flaws were modelled in two panel locations: at mid bay (flaw no. 1), and on stringer skin coupling internal side (flaw no. 2).
These results were disseminated at the ICEAF, EAN and IWPDF conferences with arciticals and in journals. A functional sample of test equipment for shear testing using a frame was prepared for exploitation. TM benchmarked its SHM software that is ready for exploitation.
The SHM software was benchmarked using extensive modelling analyses verified by complex experimental results. Thorough the integration of the benchmarked SHM software technology into the Outer Wing Box, structural weight reduction, manufacturing and assembling recurring cost reduction are expected. Novel methodologies for stress mapping with direct consequence to ability of various flaw detection and mapping during structure loading were published. Numerical models that were developed allowed for a better understanding of defects and gave tools for predicting the evolution of flaws. These verified models were published in two articles in a journal with high impact factor within the first quartile. The advanced modelling with high confidence can be the benefit for a regional aircraft design that must be built with significantly less structural weight, with better maintenance based on exact flaw analyses, and with the use of innovative eco-friendly materials.