kf Evaluation in GFRP Composites by Thermography

Since the presence of a notch in a mechanical component causes a reduction in the fatigue strength, it is important to know the kf value for a given notch geometry and material. This parameter is fundamental in the fatigue design of aeronautical components that are mainly made of composites. kf is available in the literature for numerous types of notch but only for traditional materials such as metals. This paper presents a new practice, based on thermographic data, for the determination of the fatigue notch coefficient kf in composite notched specimens. The innovative aspect of this study is therefore to propose the application on composite materials of a new thermographic procedure to determine kf for several notch geometries: circular, U and V soft and severe notches. It was calculated, for each type of notch, as the ratio between the fatigue limits obtained on the cold and hot zone corresponding to the smooth and notched specimen, respectively. Consequently, this research activity provides, for the first time, a little database of kf for two particular typologies of composite materials showing a fast way to collect further values for different laminates and notch geometries

CFRP Fatigue Damage Detection by Thermal Methods

none3In this work, the fatigue damage of CFRP uniaxial composite specimens were studied using thermal methods to determine the fatigue behavior. The aim was to evaluate the fatigue damage as a function of the number of cycles. Consequently, the damage process was studied in terms of a global indicator, considering the stiffness decay, and in terms of local parameters, considering the evolution of temperature maps acquired during the fatigue tests. A direct correlation between the damage index, corresponding to 90% of the fatigue life, and the temperature variation of the most stressed area was found. Another parameter taken into consideration was the heating rate during the application of the first thousands cycles. This parameter was proportional to the stress amplitude, making it a useful parameter since it refers to the initial part of the specimen fatigue life.Marta De Giorgi; Riccardo Nobile; Fania PalanoDE GIORGI, Marta; Nobile, Riccardo; Palano, Fani

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

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

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

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

Studio di giunti in acciaio AISI 304 saldati con tecnica ibrida CDW a proiezione

L’obiettivo del lavoro è lo studio del processo ibrido di saldatura di testa a scarica capacitiva CDW a proiezione [1], applicato a provini cilindrici in acciaio inossidabile austenitico AISI 304. L’idea innovativa è stata quella di creare più contatti distribuiti in diversi punti della sezione da saldare in modo da innescare il processo di fusione in maniera più uniforme sull’intera area; si sperimenta così una versione ibrida del processo CDW a proiezione: CDW Multipoint. Sono stati realizzati 8 profili speciali delle superfici di contatto aventi differenti geometrie, in modo tale da valutare sia l’influenza della geometria che quella dei parametri di saldatura sulla resistenza dei giunti. E’ stato quindi analizzato il comportamento meccanico dei provini saldati, effettuando una caratterizzazione statica a temperatura ambiente ed a fatica per i giunti più significativi. E’ stata inoltre osservata la microstruttura di campioni di metallo saldato mediante indagini eseguite al microscopio ottico

Comportamento a Fatica di pannelli sandwich in Schiuma di alluminio

Il comportamento a fatica di diverse tipologie di pannelli sandwich è stato determinato eseguendo delle prove di fatica a flessione a quattro punti. Le curve di resistenza a fatica sono espresse in funzione dell’ampiezza di sollecitazione nella schiuma, calcolata con un semplice modello analitico basato sulla teoria classica dei laminati. I parametri costitutivi del core di schiuma sono stati, ove possibile, determinati sperimentalmente mediante prove di compressione uniassiale. Nonostante l’elevata dispersione dei dati di fatica, causata dalla intrinseca disomogeneità della schiuma, gli andamenti ottenuti sono coerenti e permettono di fornire utili indicazioni sull’influenza degli spessori del core e delle pelli sulla resistenza a fatic

ND small defects detection of GFRP laminates using pulsed thermography

Three symmetrical composite panels reinforced with fiberglass (unidirectional E type fiberglass and epoxy resin) were realized through hand lay‐up and artificial inter‐laminar delaminations of various geometry were introduced at different depths. Several thermal stimulation procedures with different ways to consider the defects in the specimens enable to optimize the thermographic technique, by solving some initial problems related to the experimental set -up and issues. Above all, the minimum requirements for defect which can be revealed trough pulsed thermography have been established, according to present experience