37,233 research outputs found
Investigation of manufacturing effects by strength assessment, NDI and guided waves based SHM in composite plates reinforced with bonded stringers
No full textStatic behavior of a disbonded stringer in a co-infused stiffened panel
No full textStructural Health Monitoring deals mainly with sensorized structures where sensors can be secondary bonded on metallic or composite structural elements. Aerospace structural design must account for Damage Tolerance (DT) of structures. To accomplish the airworthiness, a flawed structure is required to stand the design load without any growth and, eventually, repaired. For metallic materials, the damage tolerance approaches are well-established and rely on the evaluation (theoretically and experimentally) of crack propagation velocity. For composite structures the damage-tolerance design is more challenging as the failures that may occur are of different type, most of the times hidden inside the structure and can grow up to a critical size before the conventional inspection techniques detect them. Within the DT approach one of the showstoppers for the full implementation of adhesive bonds in composites (i.e. stringer-skin connections for stiffened plates) are the airworthiness certification requirements for composite aircraft structures as presented within the FAA Advisory Circular 20-107B. In that document the general methods for substantiating the limit load capacity of any bonded stiffener, the failure of which would result in catastrophic loss of the airplane, are prescribed. Among the suggested methods, the only one really permitting to achieve the optimal bonding efficiency without the addiction of disbond stoppers (i.e. rivets), is a “repeatable and reliable non-destructive inspection techniques ensuring the strength of each joint”. That assumption implies the implementation of a reliable SHM system capable of monitoring the extent of an eventual disbond until it reaches a critical dimension at limit load. This paper will present the preliminary results of a research activity where the authors apply static loads to a stiffened plate made of a skin and a bonded stringer (co-infused) where a disbond “starter” has been included during manufacturing. The plate has been sensorized with a strain gauge system to detect the disbonding evolution during load application, in order to verify the effectiveness within a DT approach
Simulation of waves propagation into composites thin shells by FEM methodologies for training of deep neural networks aimed at damage reconstruction
No full textStructural Health Monitoring (SHM) deals mainly with structures instrumented by secondary bonded or embedded sensors that, acting as both signal generators and receivers, are able to “interrogate” the structure about its “health status”. Sensorised structures appear promising for reducing the maintenance costs and the weight of aerospace composite structures, without any reduction of the safety level required. Much effort has been spent during last years on signal analysis techniques in order to extract from signals provided by the sensors networks many parameters, metrics, and images correlated to damages existence, location and extensions. As in many other technological fields, like medical image diagnostics, deep learning techniques in general and artificial neural networks in particular can be a very powerful instrument for damage patterns reconstruction and selection provided that a sufficient and consistent amount of data related to healthy and damaged configuration of the item under test are available. Within this work explicit finite element analysis has been employed to simulate waves propagation within composite plates with and without delaminations due to impacts. The numerical results have been previously validated with analytical solutions and experimental signals then have been used to populate the data sets necessary for deep learning. This paper will present the preliminary results achieved by the authors
CO-INFUSED AND SECONDARY BONDED COMPOSITE STIFFENED PANELS LOADED IN COMPRESSION: NUMERICAL ANALYSES AND EXPERIMENTAL TESTS IN LINEAR AND POST-BUCKLING REGIMES
No full textAdhesive junctions or co-infusion of skin and stiffeners represent efficient manufacturing processes for aircrafts composites stiffened panels leading to weight saving, although they have not been widely adopted yet due to certification issues and the lack of well-established design tools and procedures.
Airworthiness requirements for composite structures pose major challenges to the certification of adhesively bonded or co-infused stiffened structures. FAA Advisory Circular 20-107B prescribes the methods for substantiating the limit load capacity of any bonded stiffener, the failure of which would result in catastrophic loss of the airplane. Most of today design approaches lead to stiffer and heavier structures if compared to letting the compressed skins to work in post-buckling until failure. In order to exploit the full structural potentiality of this type of structures under compressive loads new design approaches, mostly based on Finite Element Modelling, have to be developed and validated with experimental results. In this context, the joining technique of the stringers to the skin has a particular importance, barely influencing the linear behavior of a stiffened plate until its first instability load, but being responsible for relevant differences in the ultimate failure load. Within this paper compressed stiffened plates obtained by different manufacturing processes have been modelled with approaches of increasing complexity, from “classical” FE models to predict the first buckling load, to post-buckling analyses up to more refined techniques including the behavior of the skin-stiffener interface. These latter, based on the use of cohesive elements, allow to account for interface properties due to different manufacturing processes. A critical analysis of the numerical and corresponding experimental results as well as a comparison with the expected nominal structural performances will be presented
MULTI-PARAMETERS SELECTION AND STATISTICAL PATHS ANALYSIS IMPLEMENTATION IN A GUIDED WAVES BASED SHM SYSTEMS FOR RELIABLE IDENTIFICATION AND LOCALISATION OF DAMAGES IN COMPOSITES STIFFENED PLATES
No full textStructural Health Monitoring deals mainly with structures instrumented by secondary bonded or embedded sensors that, acting as both signal generators and receivers, are able to “interrogate” the structure about its “health status”. This innovative approach to the damage analysis is particularly promising for reducing the maintenance costs and eventually the weight of aerospace composite structures, without any reduction of the safety level required. These structures are currently designed and employed with significant reduction of the pristine material allowables to take in account of certain failure mechanisms that frequently bring to relatively small hidden damages called Barely Visible Damages, consisting among others in delaminations and/or debondings and being detectable only by specific instruments operated by trained personnel. It has been proved that the propagation of guided waves is affected by the presence of such type of damages, but their effective identification and localization depends on the accurate “tuning” of the wave characteristic (frequency, amplitude, velocity, mode) as well as on the proper selection of the best parameter of the specific wave mode selected and data analysis algorithm. This paper presents a review of the methodologies implemented and technologies tested by the authors within the latest years within two European funded research project as well as an overview of next futures research activities aimed at the implementation of guided waves based SHM system on a real aircraft
Co-infused and secondary bonded composite stiffened panels in compression: Numerical and experimental strength assessment combined with NDI and guided waves based SHM
No full textAdhesive junctions or co-infusion of skin and stiffeners represent efficient manufacturing processes for aircrafts composites stiffened panels leading to weight saving, although they have not been widely adopted yet due to certification issues and the lack of well-established design tools and procedures. Airworthiness requirements for composite structures pose major challenges to the certification of adhesively bonded or co-infused stiffened structures. FAA Advisory Circular 20-107B prescribes the methods for substantiating the limit load capacity of any bonded stiffener, the failure of which would result in catastrophic loss of the airplane. Today, composites primary structures that work mostly under compressive loads are designed following the no-buckling criteria up to Ultimate Load. Such a design approach leads to stiffer and heavier structures if compared to letting the compressed skins work in post-buckling until failure. In order to exploit the full structural potentiality of this type of structures under compressive loads new design approaches, mostly based on Finite Element Modelling, have to be developed and validated with experimental results to correctly predict the nonlinear mechanisms of load absorption beyond skin buckling onset. Furthermore state-of-the art Non-Destructive-Techniques and Structural Health Monitoring Systems can be employed for a continuous monitoring of the joints health status. In this context, the joining technique of the stringers to the skin has a particular importance; indeed, although different joining processes barely influence the linear behavior of a stiffened plate until its first instability load, they are responsible for relevant differences in the ultimate failure load. This paper presents numerical and experimental activities carried out to study the behavior of compressed stiffened plates obtained by different manufacturing processes as well as monitoring techniques of the health status of the panels by classical NDT and guided waves based SHM systems. The numerical problem has been modelled with approaches of increasing complexity, from “classical” FE models to predict the first buckling load, to post-buckling analyses up to more refined techniques including the behavior of the skin-stiffener interface
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