ndt thermographic techniques on cfrp structural components for aeronautical application

Abstract This paper describes the application of active pulsed Thermography (PT) as a Non-Destructive Test (NDT) method for the investigation of CFRP aeronautical components. The analyzed specimens include T-shaped stringers, previously monitored by ultrasonic analysis, and laminated flat plates with internal production defects. Several set-up tests allowed to identify optimal configurations for the defect detection, according to specimen geometry and defect location. A custom post-processing algorithm has been developed to improve thermographic data for more precise defect characterization, whilst a successive full-field contrast mapping allows to achieve a reliable defect distribution map and a better definition on larger areas. Detection of defects was studied with a specific thermal contrast evaluation, with a suitable choice of undamaged reference area during the transient cooling phase. The influence of heating time and experimental set-up on the thermal contrast results has also been studied; moreover, the ability of thermographic technique to detect real small production defects with accuracy and reliability is verified for CFRP aeronautical components

Evaluation of the Stress State in Aluminium Foam Sandwiches

In this paper a discussion about the determination of the stress state corresponding to the application of four-points bending load on a sandwich panel having a core made of closed cell aluminium foam is reported. An analytical model based on laminated plate classical theory is compared to a more complex FEM model, considering the effect of geometric parameters of panels, like core and plate thickness, and of loading mode, like span length. The results show the difficulties to define a reliable model to calculate stress state in this kind of composite material

10/08/1996 - EIU Homecoming Features Movie Theme.pdf

During the high cycle fatigue of aluminium alloys, an energy dissipation occurs. This dissipation is hard to be estimated because of the high diffusivity of such alloys and the importance of the thermoelasticity effects in comparison with others standard metallic materials (e.g., steels). Nevertheless the study of the energy balance gives valuable information about the nature of deformation mechanisms facilitating the construction of constitutive models associated with the microplasticity and damage of the aluminium alloy. In this work, the different energies involved in the energy balance were deduced from two complementary imaging techniques. The dissipation and thermoelastic sources were derived from an infrared thermography system, while the deformation energy was estimated from a digital image correlation system. Three tests with various loading blocks were carried out and a comparison between deformation and dissipation energies was systematically performed. (C) 2009 Elsevier Ltd. All rights reserved

fatigue damage on cfrp plates under bending by thermographic and ut analysis aided with fem dic prediction

Abstract In this article, a combination of numerical and experimental methodology for delamination evolution analysis on unidirectional CFRP elements under fatigue is suggested. Fiber-reinforced composite structures exhibit continuous damage accumulation with degradation of effective mechanical properties during cyclic HCF loads. Since advanced composites applications are allowed after extensive experimental certification tests, proposed work is based on experimental procedures to better detect and predict damage initiation and growth, monitoring static displacements and strains under 4-points bending by digital image correlation and measuring compliance variation under fatigue. In addition, non-destructive investigation of composite plates is conducted during cycling tests by using infrared thermography and ultrasonic measurements to detect damage location and validate FEM predictions. Experimental results are analyzed and compared, employing also digital image correlation technique; in addition, thermographic and ultrasonic monitoring inspection with Matlab elaborated measurements are implemented to check results for fatigue damage analysis of same specimens

Characterization of steel welded joints with hybrid projection and capacitor discharge welding (CDW) processes

This work studies the improved hybrid Capacitor Discharge Welding process (CDW), based on projection welding principles applied to stainless steel AISI 304. The innovative idea is to modify the igniting points geometry on the section to be welded and optimize the weld characteristics in order to guide the local fusion processes more uniformly on the whole area and enhance the weld properties. Eight different profile geometries for the contact surfaces have been realized in order to evaluate the geometry influence on joints quality and according to process parameters influence. The mechanical behavior of the welds has been verified with static characterization at room temperature and fatigue tests for welded samples with the better observed microstructure

real time monitoring of damage evolution by nonlinear ultrasonic technique

Abstract In this work, the ultrasound technique was used to monitor the damage of material subjected to fatigue loads. Prediction of structural damage is critical for safe and reliable operation of engineered complex systems. In these measurements, conventional ultrasonic probes (transmitter and receiver) were stably fixed to the tested samples with steel brackets, in order to eliminate ever possible variability associated with the coupling of probes. The transmitted and received ultrasonic signals were recorded and analyzed using a digital oscilloscope. The data were converted into the frequency domain using an algorithm developed in Matlab based on Fast Fourier Transform (FFT) for received signal in dependence of the applied stress level and the accumulated fatigue damage was deeply studied in order to recognize quantitative effects, suitable for an experimental prediction of the integrity of the material. The acquired data were compared with the reference signal, at the beginning of the fatigue tests. Particular care has been paid to UT signal attenuation and to the study of the frequency spectrum as the number of load cycles varies. The applied experimental technique has proved efficient for detecting damage induced by mechanical stress

optimization and comparison of ultrasonic techniques for ndt control of composite material elements

Abstract This work contains an overview of innovative procedures related to the optimization of non-destructive control ultrasonic techniques for defect investigation on composite plates. The inspection procedure improvement allows developing ideal ultrasonic setup and methods, giving the operator appropriate criteria and guidelines in terms of equipment, material and control procedures. Ultrasonic inspections are conducted on different GFRP laminates with artificial defects; tests are improved using special parts designed for probe positioning and contact conditions on inspected components. The data processing of UT procedures allows comparing detection sensitivity of different probe frequencies and plate material behavior. Contact ultrasonic method presents best results for GFRP plates using 1 MHz Olympus A103S probe, detecting small defects with maximum signal amplitudes. Finally, a statistical study is performed for repeatability demonstration of UT inspections

damage investigation of aeronautical cfrp laminates under bearing tests

Abstract The use of mechanical fasteners is still main assembling method for CFRP sub-structures in aircrafts and helicopters. However, this type of joint introduces complex stress field in the hole surroundings producing failures risk. In this work, a progressive damage 3D model of the riveted joint has been implemented to predict the residual strength and compliance after first damage signs and reproduce the final failure of composite joints under tensile test. Two 3D FEM Models were used and results are compared to experimental tests. The junction stiffness under load was evaluated and preliminary analysis shows both the coefficient of friction and preload induce not significant alteration of the composite joint behaviour. On refined model, a 3D Hashin-type failure criterion was used to analyze damage of matrix and fibers, beginning in the hole where contact conditions with the bolt pin and head are more severe and is evenly distributed in various plies, leading to final rupture of second or third one. Successively, delaminations were introduced with cohesive model, since is considered to produce reliable results, because the initial compression collapse in critical layers in contact with rivet is proved to affect shear and compression load transfer to other layers, giving rise to localized internal delaminations, propagating successively in width

CDW aluminium joints welding and optimisation with NDT/mechanical testing

This work investigates the possibility to apply the improved hybrid capacitor discharge welding (CDW) process, based on projection welding principles, to aluminium alloy Al 5754. The CDW process is an electrical resistance welding technology, realised with high-intensity current pulses discharged by large capacitors. The innovative aspect is the effective possibility to weld aluminium alloys with CDW process and improve the mechanical weld characteristics and the presence of defects as a function of the technological parameters; intrinsic CDW process characteristics need to be investigated on the basis of interaction between the technological and geometrical aspects and the related mechanical properties, in order to improve welding shape and reduce defect size. In order to optimise the process, visual and ultrasonic inspections of the most significant welded joints were performed, and residual stress values were checked; in addition, high-cycle fatigue tests after room temperature tensile tests were executed to optimise the weldments

Verifica a fatica dei giunti saldati sulla base di misure di deformazione locale

Lo studio della resistenza a fatica delle giunzioni saldate rappresenta un campo di prova molto interessante dal punto di vista scientifico, con ricadute pratiche altrettanto importanti. Si tratta in generale di fornire dei metodi di verifica e progetto deterministici, come è proprio dell’ingegneria, per revedere il comportamento meccanico di un materiale che, quando è sotto forma di saldatura, cambia le sue proprietà meccaniche e microstrutturali rispetto al materiale base, si dispone in una geometria locale del cordone estremamente variabile e non definibile a priori, è infine affetto da campi di tensione residua  non proprio trascurabili. In questo lavoro, partendo da una breve panoramica sui principali indicatori e metodi che sono stati utilizzati nel corso degli anni per la stima dello stato di sollecitazione in un giunto saldato e della vita residua a fatica, si presenta l’approccio basato sulla misura della deformazione locale che è stato seguito da diversi autori nel corso degli anni, evidenziandone i vantaggi ma anche le limitazioni rilevate attraverso le numerose attività sperimentali direttamente eseguite