SUPERCONDUCTING ELECTROMAGNETIC SENSOR FOR NONDESTRUCTIVE EVALUATION OF STRESS IN METALS

WE PROPOSE TO DEVELOP A NON-CONTACTING ELECTROMAGNETIC SYSTEM TO MEASURE RESIDUAL STRESS IN FERROMAGNETIC ARMOR PLATE. WE USE A SUPERCONDUCTIVE (SQUID) MAGNETIC GRADIOMETER IN A NOVEL CONFIGURATION AS THE SENSOR. IT IS COMBINED WITH A MAGNETIZING SOURCE CONSISTING OF A NESTED SET OF SUPERCONDUCTING COILS. THE PROPOSED SYSTEM CAN MEASURE A NUMBER OF MAGNETIC AND ELECTRICAL CHARACTERISTICS SIMULTANEOUSLY. COMBINED MEASUREMENT OF THE PARAMETERS GIVES RESIDUAL STRESS. THE SUPERCONDUCTIVE SYSTEM OFFERS GREAT IMPROVEMENTS IN SENSITIVITY AND SPATIAL RESOLUTION OVER CONVENTIONAL MAGNETIC AND ELECTROMAGNETIC NDE SYSTEMS. FURTHERMORE, ITS UNMATCHED SENSITIVITY AT FREQUENCIES OF FEW HERTZ EXTENDS THE PENETRATION DEPTH BY NEARLY TWO ORDERS OF MAGNITUDE OVER THAT OF CONVENTIONAL SYSTEMS. CONSEQUENTLY, IT PROVIDES A UNIQUE TOOL FOR TESTING THICK ARMOR PLATE. A PROTOTYPE SYSTEM TO BE FABRICATED AND TESTED IN PHASE II WORK USES LIQUID HELIUM TEMPERATURE (4K) SUPERCONDUCTORS. NONETHELESS, THE NEW CLASS OF LIQUID NITROGEN TEMPERATURE (77K) SUPERCONDUCTORS RECENTLY DISCOVERED OFFERS THE PROMISE OF A CONVENIENT AND ECONOMICAL SYSTEM.