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Title:	Investigation of the response of a ferrofluid in a surface crack under an applied magnetic field
Author:	Musterni Girbau, Gerard
Other authors:	Rojas Gregorio, José Ignacio
Abstract:	Non-destructive testing (NDT) is of paramount importance in both the aerospace and general markets. Nowadays, several NDT techniques are available and widely recognised for inspection services. However, wide research is being done in the NDT field to achieve better, cheaper and more efficient techniques. In this research, an assessment of an innovative NDT tehcnique termed Magnetic Early Defect Detection (MEDD) is made using a commercial software package. The results are compared with experimental and analytical results obtained in previous works. The ultimate goal is to verify the feasibility and sensitivity of this innovative NDT technique, and to check if it can be exploited commercially. One of the most relevant benefits of MEDD is the applicability in a wide range of materials used in aerospace applications such as aluminium alloys and composites like Carbon Fibre Reinforced Polymers and Glass Fibre Reinforced Polymers. In the proposed technique, under an applied DC magnetic field, crack detection would be based on local variations of magnetic flux density due to a ferrofluid accumulated in the crack. Instead, under an applied AC magnetic field, detection could be based on the phase lag of the ferrofluid magnetization (and thus the phase lag of the magnetic field close to the crack) respect to the applied field. Thus, two cases have been studied: (i) the response of the ferrofluid under an external DC magnetic field and (ii) the response under an AC magnetic field. For the first case, the numerical simulations led to induced magnetic field results of the same order of magnitude as those obtained experimentally. These signals could be detected using common magnetometers. However, a calibration process was followed for the relative permeability of the ferrofluid (which was not consistently characterized). The simulations show also that a cleaning process should be performed to wipe the excess ferrofluid from the surface to achieve an unmistakable signal. This validation ensures that no-high-end technology must be used and therefore makes DC MEDD a cheap, easy and efficient NDT technique. For the AC case, a phase lag is introduced because the particles are not able to follow the rapid change of the magnetic field. This phase lag is frequency dependent and increases with it. Even though several approaches to solve the problem were done, none of them ended up with a valid solution due to the limitations encountered in the software. AC MEDD has a huge potential in high-demanding applications where the sensitivity and accuracy are of utmost importance and where the price is not the main concern.
Abstract:	Els assajos no destructius son de vital import ´ ancia tant en el sector aeroespacial com en ` el mercat en general. A l’actualitat, hi ha moltes tecniques no destructives disponibles que ` son´ ampliament reconegudes com a serveis d’inspecci ` o. No obstant, s’est ´ a duent a terme ` una amplia recerca dins el camp dels assajos no destructius per aconseguir t ` ecniques ` mes econ ´ omiques i m ` es eficients que les que actualment estan disponibles. En aquesta ´ investigacio, s’avalua una t ´ ecnica no destructiva innovadora denominada ` Magnetic Early Defect Detection (MEDD) mitjanc¸ant un paquet de programari comercial. Els resultats s’han comparat amb els resultats teorics i experimentals obtinguts en treballs anteriors. ` L’objectiu final es comprovar la viabilitat i la sensibilitat d’aquesta nova t ´ ecnica i comprovar ` si pot ser explotada comercialment. Un dels beneficis mes importants d’aquesta t ´ ecnica ` es l’aplicabilitat en una ´ amplia gamma de materials que s ` on molt utilitzats en el sector ´ aeroespacial com els aliatges d’alumini i els composits com els ` Carbon Fibre Reinforced Polymers i els Glass Fibre Reinforced Polymers. Segons la tecnica proposada, sota un camp magn ` etic generat amb corrent continu, la ` deteccio d’esquerdes es basa en variacions locals de la densitat de flux magn ´ etic a causa ` de l’acumulacio de ferrofluid a l’esquerda. Pel cas de camps magn ´ etics generats amb ` corrent altern, la deteccio es basa en el retard de fase de la magnetitzaci ´ o del ferrofluid (i ´ per tant el retard del camp magnetic prop de l’esquerda) respecte el camp extern aplicat. ` Per tant, s’han estudiat dues situacions: (i) la resposta del ferrofluid en un camp magnetic ` generat amb corrent continu i (ii) la resposta sota un camp magnetic generat amb corrent ` altern. Per al primer cas, les simulacions ens mostren que la variacio del camp magn ´ etic ` es del mateix ordre de magnitud que els resultats obtinguts experimentalment. De totes ´ maneres, es va fer un proces de calibraci ´ o per la permeabilitat relativa del ferrofluid ja ´ que era l’unic valor que no estava caracteritzat correctament. Les simulacions tamb ´ e ens ´ mostren que despres d’aplicar el ferrofluid sobre el material a analitzar, s’ha de netejar el ´ ferrofluid restant que queda per aconseguir resultats inequ´ıvocs. Aquesta validacio asse- ´ gura que per utilitzar la tecnica MEDD amb corrent continu no es necessita instrumentaci ` o´ d’altes prestacions i que per tant aquesta tecnica ` es econ ´ omica, f ` acil i eficient. ` Per al cas de corrent altern, degut a que les nanopart´ıcules magnetiques no poden seguir ` els canvis del camp magnetic es crea un retard. Aquest retard dep ` en de la freq ` u¨encia, ` augmentant amb ella. Tot i que s’han analitzat multiples aproximacions per intentar resol- ´ dre el problema, no s’ha arribat a una solucio v ´ alida degut a les limitacions del programari. ` La tecnica MEDD amb corrent altern t ` e un gran potencial en aplicacions molt exigents on ´ la sensibilitat i la precisio de les mesures sigui d’alta import ´ ancia i la q ` uesti ¨ o econ ´ omica ` quedi en segon pla.
Subject(s):	-Àrees temàtiques de la UPC::Enginyeria dels materials -Nondestructive testing -Aluminum alloys -Magnetic fluids -Magnetic fields -Crack -Magnetic particle -Magnetic field -Ferrofluid -Aluminium alloy -Composite -NDT -NDE -Assaig de materials -Alumini -- Aliatges
Rights:	Attribution-NonCommercial-ShareAlike 3.0 Spain http://creativecommons.org/licenses/by-nc-sa/3.0/es/
Document type:	Bachelor Thesis
Published by:	Universitat Politècnica de Catalunya
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