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

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

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

Modification of creep and low cycle fatigue behaviour induced by welding

In this work, the mechanical properties of Waspaloy superalloy have been evaluated in case of welded repaired material and compared to base material. Test program considered flat specimens on base and TIG welded material subjected to static, low-cycle fatigue and creep test at different temperatures. Results of uniaxial tensile tests showed that the presence of welded material in the gage length specimen does not have a relevant influence on yield strength and UTS. However, elongation at failure of TIG material was reduced with respect to the base material. Moreover, low-cycle fatigue properties have been determined carrying out tests at different temperature (room temperature RT and 538°C) in both base and TIG welded material. Welded material showed an increase of the data scatter and lower fatigue strength, which was anyway not excessive in comparison with base material. During test, all the hysteresis cycles were recorded in order to evaluate the trend of elastic modulus and hysteresis area against the number of cycles. A clear correlation between hysteresis and fatigue life was found. Finally, creep test carried out on a limited number of specimens allowed establishing some changes about the creep rate and time to failure of base and welded material. TIG welded specimen showed a lower time to reach a fixed strain or failure when a low stress level is applied. In all cases, creep behaviour of welded material is characterized by the absence of the tertiary creep

Vision-based Wheel Sinkage Estimation for Rough-Terrain Mobile Robots

For mobile robots driving across soft soils, such as sand, loose dirt, or snow, it is critical that the dynamic effects occurring at the wheel-terrain interface be taken into account. One of the most prevalent of these effects is wheel sinkage. Wheels can sink in soft soils to depths sufficient to prohibit further motion, leading to danger of entrapment with consequent mission failure. This paper presents an algorithm for visual estimation of wheel sinkage in deformable terrain. We call it the Visual Sinkage Estimation (VSE) method. It assumes the presence of a monocular camera mounted on the wheel assembly, with a field of view containing the wheel–terrain interface. An artificial pattern, composed of concentric circumferences equally spaced apart on a white background, is attached to the wheel side in order to determine the contact angle with the terrain, following an edge detection strategy. The paper also introduces an analytical model for wheel sinkage in soft, deformable terrain based on terramechanics. In order to validate the VSE module, several tests were, first, performed on a single-wheel test bed, under different operating conditions including non-flat terrains, variable lighting conditions, and terrain with and without rocks. Successively, the effectiveness of the proposed approach in real context was proved, employing an all-terrain rover traveling on a sandy beach

Mechanical Characterization of single crystal capacitor discharge welded bars ”,

In the present work the static behavior of Single Crystal Nickel-base superalloy CMSX-4 has been analyzed, performing mechanical and microstructural characterization, necessary to establish in detail the superalloy behaviour at the expected service stresses. Tests have been performed both at room and working temperature, comparing base material properties with CD welded and Laser Clad cylindrical bars. The microstructure has been successively analyzed with optic microscope and scanning electron microscope (SEM) observations. The micrographic study concerned base material and as-welded specimens; this way the microstructural modifications and the Heat Affected Zones have been studied as well as the interaction between the weld material layers and parent metal. The mechanical and microstructural characterization for “as welded” and clad material is needed because of the requirements to develop advanced technologies for the repair of damaged blades, particularly in the case of blisks, constituted of blading and rotor in one piece

Confronto numerico-sperimentale per il Controllo termografico di Laminati GFRP

Lo scopo di questo lavoro è quello di identificare le potenzialità e i limiti della tecnica termografica pulsata nell’individuazione e caratterizzazione delle difettosità su GFRP, confrontando i risultati sperimentali con quelli di analisi numeriche agli elementi finiti che riproducono le stesse indagini. A partire dai risultati sperimentali ottenuti, che permettono di determinare caratteristiche calorimetriche del difetto in base alla risposta termica in superficie, viene simulato con modelli numerici appositi agli elementi finiti il comportamento termico del difetto in relazione ai parametri di conducibilità, allo spessore del composito e alla profondità ed estensione del difetto. Il metodo consente di implementare in maniera rapida differenti modelli numerici e caratterizzare numerose tipologie di difetti, stabilendo delle correlazioni iniziali fra posizione dello stesso e risposta termica del modello di partenza