Thermal NDE enhanced by 3D numerical modeling applied to works of art

摘要

The international literature frequently presents optical techniques suitablefor a massive testing of valuable paintings. The still open problem seems tofit perfectly the potentialities of IR thermography. Unfortunately, resultsare not fully satisfying, mainly due to the limited reliability and the lackof a standardised procedure.Nevertheless, a review of the more recent algorithms dedicated at theThermal Non Destructive Testing and Evaluation shows the enormous progressof this method and the new features of modern equipment.However, even if it is a matter of direct observation in some fields, asaerospace, this progress is not found in works of art. The main reasons arestrict bonds and unevenness typical of this application field. Analysingdifferent reports of specialised journal, a completely new deal seemsmature.Nowadays, mathematical modelling of the involved thermal problem supports aquantitative and precise testing and evaluation of extended fresco surfaces.The state of the art indicates direct use of simulation in optimising thetesting procedure. Furthermore, the solution of the direct problem allowssetting up the function needed for the characterisation of defects. In thispaper a totally new approach is presented, where thermal modelling allowsautomating the data processing, improving results.For several years, we have been studying peculiarities of detecting defectsin real historical frescoes in situ, using also realistic specimens atcontrolled lab environment. References and theoretical bases are given ondetecting defects that are located at different depths, having differentsizes. All achieved results are reported together with limiting factors,mainly due to the painted surface.Up today, in Thermal Non-Destructive Testing, all existing algorithms ofdata processing do not take into account 3D heat diffusion phenomena. Inparticular, lateral heat conduction generates false alarms and artifactswhen data are processed by means of algorithms implementing a simplified 1Dmodel. In this study, an adaptive procedure is proposed to dynamicallyprocess experimental data. According to it, artificial image sequencesgiving the thermal evolution of sound materials are computed, starting frominitial experimental conditions. The usefulness of this approach isdemonstrated especially for the characterization of defects buried onfresco, where surface clutter is especially significant.A fresco has been modeled with a multi-layer slab. The numerical schemeimplemented has enabled to simulate processes of dynamic heating.Experimental results are reported on plaster specimens containing a fewartificial defects at different depths and tested with alternative opticalNdT method. A tomography of the fresco is obtained under fast transientthermal state.