Magnetic flaw detector system for reciprocating pairs of leakage field detectors with means for adjusting the spacing between each pair of detectors

Abstract

A non-destructive testing system for detecting longitudinal and transverse defects, simultaneously, in metallic tubes or pipes in which a plurality of essentially punctiform magnetic sensing elements are disposed along a generally straight line adjacent the workpiece surface. A mechanical oscillator reciprocates the sensing elements along a line parallel to that of the sensing elements disposition. Adjacent elements are electrically connected in opposition to form differential probe pairs, the line connecting the adjacent probes to form the differential pairs being inclined at 45 degrees to the line of reciprocation. In addition to providing for the simultaneous detection of longitudinal and transverse defects, the system disclosed can resolve the signals from ID and OD defects to a degree heretofore unobtainable.

What is claimed is: 
     
       1. Apparatus for detecting defects in the walls of metal tubing, comprising: means for generating a magnetic field within the body of said tubing whereby a stray field is produced lying radially outwardly of said tubing adjacent defects in said tubing;   a plurality of paired sets of magnetic field sensing devices arranged in spaced relation to the outer surface of said tubing, the devices of each pair being electrically connected in opposition and positioned relative to each other and the test pieces such that a straight line connecting the devices of each pair would be oriented at an acute angle with respect to the tubing longitudinal axis;   means for adjusting the spacing between paired sets of devices;   means for producing relative helicoidal movement between the tubing and the sensing devices; and   means for reciprocating the field sensing devices generally longitudinally of the tubing during relative movement.   
     
     
       2. Apparatus as in claim 1, in which the devices of each pair are separated from one another at a distance substantially equal to the sum of the tubing wall thickness and the spacing of the devices from the outer tubing wall. 
     
     
       3. Apparatus as in claim 1, in which the pairs of devices are arranged in paired sets extending consecutively along a line parallel to the tube axis and with the line between devices of each pair being oriented angularly with respect to the tubing axis at a common angle of substantially 45°; and the means for reciprocating connected to move all of the sensing devices as a single unit. 
     
     
       4. Apparatus as in claim 1, in which the devices of each paired set are arranged in mutually spaced relation, and the field sensing devices are reciprocated longitudinally of the tubing an amount bearing a significant ratio to the spacing of the paired devices from each other. 
     
     
       5. Apparatus for the nondestructive testing of metal tubes to locate longitudinal and transverse defects, comprising: means for moving a tube to be tested along a helicoidal path;   a carriage mounted upon the outer surface of said tube;   reciprocating means mounted on said carriage;   at least one pair of mutually spaced magnetic field sensing elements carried by the reciprocating means and disposed in slightly spaced relation to the outer surface of said tube, means for adjusting and spacing between said elements, said elements being arranged in a line extending generally at 45° to the tube axis, and said elements being connected in electrical opposition to form a differential pair whereby the signal obtained from the pair of elements corresponds to the difference in the defect characteristics of the two regions being sensed by the elements; and   electric circuit means connected to receive signals from said sensing elements and including a first channel responsive to the frequency characteristic of a tube outer surface defect and a second channel responsive to the frequency characteristics of signals caused by defects other than on the tube outer surface.   
     
     
       6. Apparatus as in claim 5, in which means are provided on said carriage for selectively changing the arrangement of said elements from 45° to 135°. 
     
     
       7. Apparatus as in claim 5, in which the sensing elements are spaced from each other a distance equal substantially to the sum of the tube wall thickness and the spacing of the elements from the outer surface of the tube. 
     
     
       8. Apparatus for the nondestructive testing of metal tubes to locate longitudinal and transverse defects, comprising: means for moving a tube to be tested along a helicoidal path;   a carriage mounted upon the outer surface of said tube;   reciprocating means mounted on said carriage;   at least one pair of mutually spaced magnetic field sensing elements carried by the reciprocating means and disposed in slightly spaced relation to the outer surface of said tube, means for adjusting the spacing between said elements, said elements being arranged in a line extending generally at 45° to the tube axis, and said elements being connected in electrical opposition to form a differential pair whereby the signal obtained from the pair of elements corresponds to the difference in the defect characteristics of the two regions being sensed by the elements; and   electric circuit means connected to receive signals from said sensing elements.