Increasing reliability of forensic analysis while research on destroyed relief marking with magneto-optical devices

Abstract

While research on destroyed relief marking of metal objects using the magneto-optical method, visualization of (invisible) fields of internal stress in the VIN plate area is performed and then a forensic analysis of obtained instrumental data is carried out (indirect organoleptic observation of the visualization results); thus, forensic analysis reliability of results directly depends on sensitivity of instruments and informativeness of instrumental data. The main quantitative characteristic in this case is probability of correct signal recognition  (contours of marking signs) against the background of noise (structural noise of investigated surface and the noise of the visualization method itself) determined by the signal-to-noise ratio. This article presents results of a comparative experimental assessment of signal-to-noise ratio and probability of correct signal recognition while restoringthe destroyed relief markings for two complexes of magneto-optical imaging – models of 2006 and 2018. This article purpose is a quantitative and qualitative comparative assessment of results of visualization of internal stresses in areas of completely removed relief marking of metal objects. The results of successful practical research obtained by forensic experts from different countries make it possible to assess effectiveness and prospects of using the magneto-optical imaging method. In a new modification of the magneto-optical complex: signal level is 4.35 dB higher (contrast of reconstructed marking signs); 2.71 dB lower noise level (surface relief/texture and magnetic copying noise);• probability of correct character recognition is P> 0.995 (increased by 14.9%). Technical improvements in implementation of magneto-optical visualization method made it possible to expand the range of materials for research objects(magnetic and electrically conductive materials were investigated). The high efficiency of method for restoring marking is illustrated by results of forensic examinations for materials with a low level of residual stresses (aluminum alloy, low-carbon steel) which  chemical etching method did not give results for. The use of new modification allows examining the rust layer, up to cases of corrosion to the entire depth of marks. Considering non-destructive nature of magneto-optical researches, possibility of their repeated repetition without losing  object properties, this method (in accordance with the order of application of types of studies) deserves more attention for application.