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

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

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

Characterization of fatigue damage in long fiber epoxy composite laminates

A study of damage characterization of a GFRC laminate is resented here. Forty fatigue tests were executed and S–N curves traced. Two parameters were chosen to monitor damage evolution during each test: stiffness and dissipated energy per cycle. Moreover, the presence of three zones in graphs of rocessed data can be observed and it is evident that the most important structural transformations take place only in the very final part of life. Adopting a continuum mechanics approach, the degradation through the whole life in composite is evaluated and it is shown that the two parameters are strictly related to damage state of composite material. A method for predicting the remaining life in a GFRC is here proposed

Mechanical and technological analysis of AISI 304 butt joints welded with capacitor discharge process

In the present work, the capacitor discharge welding process (CDW) applied on AISI 304 circular bars was studied. The CDW process is essentially an electrical resistance welding technology, realized through current pulses of high intensity and discharged by large capacitors; the process allows to reduce stress concentration effects at the weld toe, obtaining thin welds and achieve good material integrity. CDW process characteristics lead to conceive the idea to investigate on the interaction between the weld technological aspects and the related mechanical properties. In this research activity, 150 cylindrical specimens with bore diameter 6 mm and different igniter dimensions were machined in AISI 304 austenitic stainless steel; a special equipment was designed to clamp specimens and to assure perfect electrical continuity. The main CDW welding parameters (energy input P, applied forces and igniter dimensions) were studied in order to optimise the welding process. The static and fatigue properties were finally analysed for the welded bars and the results were correlated to process parameters; mechanical tests give good results with respect to base metal if the proper welding parameters are used, despite the fact a brittle character was observed for the welded joints

Fatigue damage in notched GFR composites with thermal and digital image measurements

Complex time dependent damage phenomena are characteristic of Composites under fatigue; this is due to material structural heterogeneity and fibre/matrix interactions. Fatigue life is mainly determined by the progressive damage evolution, difficult to be modeled with numerical tools or mathematical frames. Experimental methods are to be applied to achieve correct information about the evolving fatigue damage and the authors propose a methodology based on Digital and Thermal Image analyses, to measure on notched GFRC specimens the dissipative sources in terms of temperature and deformation based data; starting from previous works, the measurements procedures were improved for these techniques, in order to obtain useful results with notched composites and capable to detect different damage states on specimen surface

Méthodes modernes pour le dessin en fatigue des éléments structurels soudés

LA THESE EST ADRESSEE POUR APPROFONDIR DIFFERENTS PROBLEMATIQUES DANS LE DOMAINE DU DESSIN EN FATIGUE DES JOINTS SOUDES. A TELLE FIN, ON RETROUVE DES RECHERCHES SUR LES METHODES ACTUELLEMENT UTILISES ET SUR LES CODES POUR LE BIDIMENSIONNEMENT ET LA VERIFICATION DES ELEMENTS SOUDES EN ACIER. UNE VASTE SERIE D'ESSAIS EXPERIMENTAUX ET TESTS DE FATIGUE A ETE PREVUE POUR DES TYPOLOGIES DE SOUDURES DIFFERENTES (EN PARTICULIER CELLES EN T) POUR VERIFIER ET METTRE AU POINT LES ASPECTS RELATIFS AUX NOUVELLES METHODOLOGIES DE CALCUL. DANS LA PREMIERE PARTIE DU TRAVAIL ON OFFRE LES NOTIONS SUR LESQUELLES ON APPUIE MAINTENANT SOIT LES PROCEDURES DE DESSIN FONCTIONNANTES, SOIT CELLES PLUS INNOVATRICES EN PHASE DE INVESTIGATION. DANS LA DEUXIEME ON DONNE L'ETAT D'ART A PROPOS DE LA CLASSIFICATION, RESISTANCE ET ANALYSE DES STRUCTURES SOUDEES DANS UN POINT DE VUE PRODUCTIF ET DE DIMENSIONNEMENT. DE PLUS, ON PRESENTE LES CODES QUI MARCHENT EN EUROPE, LES PROPOSITIONS LES PLUS SIMPLES ET ACTUELLES ET LES DIRECTIVES POUR LES TESTS. SUCCESSIVEMENT, DANS LA TROISIEME PARTIE, ON DECRIT LES JOINTS ANALYSES SELON LA GEOMETRIE, LES MATERIAUX ET LE TYPE DE CHARGEMENT ; EN AUTRE ON ETUDIE LES MODELES NUMERIQUES POUR L'ANALYSE DU CHAMP DES POUR EVALUER LES ET L'INFLUENCE DU MONTAGE ET DES CARACTERISTIQUES CYCLIQUES DU MATERIAU. LA QUATRIEME SECTION REGARDE LES PROCEDES DE CALCUL NUMERIQUE-ANALYTIQUE, L'APPLICATION DES METHODES PROPOSEES ET LA VERIFICATION AVEC LES RELATIVES CONFRONTATIONS. DE LA COTE DES CONCEPTS, ON ILLUSTRE ET ON ECLAIRE LES DOUTES VIS A VIS DEUX NOUVELLES METHODES DE DESSIN DES ELEMENTS COMPLEXES SOUDES : LA METHODE DE LA DEFORMATION LOCALE ET LA METHODE VOLUMETRIQUE. SI APPAREMMENT LES DEUX METHODES SONT TOUT A FAIT DIFFERENTES, DU FAIT QUE LE PREMIER EST DE CARACTERE EXPERIMENTAL ET ON SE BASE SUR L'ELABORATION LOCALE EN PLUSIEURS POINTS DE MESURE POUR ETRE LIEE A LA VIE EN FATIGUE, LE DEUXIEME EST DE TYPE NUMERIQUE ET IL S'AGIT D'ETUDIER LES CONTRAINTES PLASTIQUES DANS UN VOLUME DE CONTROLE D'ENDOMMAGEMENT AU BOUT D'ENTAILLE, ON PEUT IDENTIFIER PLUSIEURS SIMILITUDES ENVERS LES RESPECTIVES POINTS DE FORCE. ON OBTIENT DES COURBES DE REFERENCE EN FATIGUE VALABLES EN SENS PLUS GRANDE, C'EST A DIRE INDEPENDANTES DE LA GEOMETRIE, DE LA CLASSEUR, DU CHARGEMENT ET, DANS UNE CERTAINE FACON, DU TYPE D'ACIER. EN DEFINITIVE, IL A ETE CONSIDERE NECESSAIRE D'ANALYSER TOUS CES ARGUMENTS ET D'EXECUTER PLUSIEURS ESSAIS NUMERIQUES ET EXPERIMENTAUX POUR DEVELOPPER LES CONNAISSANCES THEORIQUES ET PRATIQUES DANS LE DOMAINE DE RECHERCHE CONSIDERE.METZ-SCD (574632105) / SudocSudocFranceF