PSI - Issue 2_A

Tarpani J.R. et al. / Procedia Structural Integrity 2 (2016) 136–143 Tarpani et al. / Structural Integrity Procedia 00 (2016) 000–000

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and immobilize the fracture site while healing takes place. More recently, Kumar et al . (2012) highlighted the use with that purpose of thermoplastic matrices reinforced with high performance continuous fibers, such as glass and carbon. The question then comes up whether carbon fibres, due to their intrinsically high electrical conductivity and electromagnetic interference shielding properties, will act as a barrier against incoming or outgoing emissions of electromagnetic frequencies (i.e., EMI effect), therefore hampering or even preventing NMR imaging, since it relies in short pulses of radiofrequency radiation. 1.5 Nuclear Magnetic Resonance Imaging (NMRI) Both proton and neutron nuclear particles have intrinsic angular momentum (p) and intrinsic magnetic moment (µ), so that they behave as infinitely small magnets with a north pole and an opposing south pole. For this reason, both p and µ are vectors with magnitudes and directions in space. The magnitude of the angular momentum, and thus the magnitude of the magnetic moment, is characterized by the nuclear spin (I), and both the proton and the neutron have I = ½. The net magnetic moment of an atomic nucleus is simply the sum of the magnetic moments of all the protons and neutrons in the nucleus. It turns out that an atomic nucleus with a net magnetic moment behaves as well like a little magnet, with the angular momentum vector having rotary movement around its own axis, which is called motion of precession or Larmor precession (Noll, 2001; Rothwell, 1985; Rothwell et al ., 1984). As reported by Marble et al. (2009), Laplante et al. (2005) and Callaghan (1991), this behaviour is fundamental to the development of the NMR phenomenon. In regard to the sensitiveness of atomic species to the NMR effect, one must recall the relationship between the magnetic moment (µ) and the angular momentum (p), as follows: µ =  * p (1) The proportionality constant ߛ is called the magnetogyric ratio. Each atomic nucleous has the unique value of  , which has units of s -1 .T, where T = Tesla, the unit of magnetic field strength. Hydrogen has  = 26.8 s -1 .T, the highest one known, rendering this chemical element the most sensitive one to the NMR effect. Therefore, it is not surprising that NMRI is successfully applied particularly to the examination of soft tissues of human body. To make it possible to imaging an element, substance or chemical compound by NMR (i.e., NMRI) an interaction is required between an externally applied magnetic field and the atomic nuclei having non-zero magnetic moment (Mazzola, 2009; Haacke et al ., 1999). Thus, in an NMR experiment the object to be "imaged" is placed under the external action of a strong static magnetic field. A superconducting magnet that remains continuously active normally provides the static magnetic field (B 0 ) and, accordingly, the magnetic moments associated with the nuclear spin of the target object tend to align parallel or anti-parallel to B 0 , according to the state energy of protons and/or neutrons. When submitted to a second oscillating external magnetic field, called RF (radiofrequency), the spins absorb energy when hit and excite a resonant frequency. A coil standing nearby the analyzed object can detect the magnetization derived from the precessional motion of the nuclear spin, which will decay over time. In agreement with Ishida (2009), the signal captured by the coil positioned perpendicular to the magnetization plan is called FID - Free Induction Decay. The present work aimed at evaluating the potential of NMRI technique to characterize in vitro conditions two different classes of fracture, specifically translaminar and delaminaton, created deliberately and in a controlled manner in continuous carbon fibre-reinforced solid laminates with respectively two polymer matrices, namely thermosetting epoxy resin and thermoplastic PPS polymer, which were kept immersed in a simulated body fluid (SBF) for a long time after being damaged. The main goal of this study was to collect compelling evidences on the viability of future use of NMRI for real-time, in vivo determination of the structural integrity state of human orthopaedic implants made with these non-magnetic polymer matrix fibrous composites, thus making feasible the prediction of their remaining lifetime and, eventually, the extension of their service lifetime.