Material condition assessment with eddy current sensors

Abstract

Eddy current sensors and sensor arrays are used for process quality and material condition assessment of conducting materials. In an embodiment, changes in spatially registered high resolution images taken before and after cold work processing reflect the quality of the process, such as intensity and coverage. These images also permit the suppression or removal of local outlier variations. Anisotropy in a material property, such as magnetic permeability or electrical conductivity, can be intentionally introduced and used to assess material condition resulting from an operation, such as a cold work or heat treatment. The anisotropy is determined by sensors that provide directional property measurements. The sensor directionality arises from constructs that use a linear conducting drive segment to impose the magnetic field in a test material. Maintaining the orientation of this drive segment, and associated sense elements, relative to a material edge provides enhanced sensitivity for crack detection at edges.

1 . A method for assessing cold work process quality of a test material comprising: 
 a) placing an eddy current sensor proximate to a surface of the test material prior to cold working;    b) measuring a sensor response at a plurality of registered positions along the surface;    c) combining the sensor response with position information to form a spatial response in at least one dimension;    d) cold working the material and repeating steps a) and c); and    e) comparing responses obtained before and after cold working at registered positions to assess cold work quality.    
   
   
       2 . The method as claimed in  claim 1 , step d) further comprising: 
 maintaining the same spatial registration for the spatial response before and after cold working.    
   
   
       3 . The method as claimed in  claim 1  wherein the cold work process is shot peening and the response is a two-dimensional image of a property.  
   
   
       4 . The method as claimed in  claim 1  wherein the cold work quality is measured in terms of coverage.  
   
   
       5 . The method as claimed in  claim 1  wherein the cold work quality is measured in a manner correlated with an alternate scale for cold work intensity such as residual stress or Almen intensity.  
   
   
       6 . The method as claimed in  claim 4  wherein the cold work quality is measured in terms of uniformity of cold work intensity.  
   
   
       7 . The method as claimed in  claim 1  wherein the test material is a nickel alloy.  
   
   
       8 . The method as claimed in  claim 1  wherein the sensor measurement is performed at a single excitation frequency.  
   
   
       9 . The method as claimed in  claim 1  wherein the sensor measurement is performed at multiple excitation frequencies.  
   
   
       10 . The method as claimed in  claim 1  wherein the sensor response corrects for roughness variation.  
   
   
       11 . The method as claimed in  claim 1  further comprising converting the sensor response to a property value using a physics based model.  
   
   
       12 . The method as claimed in  claim 11  where the conversion is made using a precomputed database of sensor responses at one or more excitation frequencies.  
   
   
       13 . The method as claimed in  claim 1  wherein the sensor is a flexible array that can conform to the complex surface geometries.  
   
   
       14 . The method as claimed in  claim 13  wherein the test material is an engine component and the responses are two-dimensional images of a property related to cold work quality.  
   
   
       15 . The method as claimed in  claim 1  wherein local outlier sensor responses are suppressed or removed so that an average sensor response without the outlier values can be recorded.  
   
   
       16 . A method for assessing the quality of an operation comprising: 
 a) performing a preconditioning action to introduce a measurable anisotropy in a material;    b) measuring the anisotropy using a sensor;    c) thereafter, performing an operation on the material;    d) after performing the operation, measuring the anisotropy using the same approach as in step b);    e) comparing the anisotropy measurement before and after the operation to assess the quality of the operation.    
   
   
       17 . The method as claimed in  claim 16  wherein the preconditioning action is a mechanical overload.  
   
   
       18 . The method as claimed in  claim 16  wherein the operation is shot peening and the material is titanium.  
   
   
       19 . A method for monitoring a material condition comprising: 
 a) performing a preconditioning action to introduce a measurable anisotropy in a material;    b) measuring the anisotropy using a sensor;    c) measuring the anisotropy using the same approach as in step b) at one or more later times; and    d) using a change in the anisotropy to identify a change in material condition.    
   
   
       20 . The method as claimed in  claim 19  wherein the change in material condition is caused by thermal exposure and the change in the sensor response is used to determine that said thermal exposure was above a prescribed level.  
   
   
       21 . The method as claimed in  claim 20  wherein the material is a nickel alloy with a temperature value of 650° C. and further comprises duration of at least 48 hours.  
   
   
       22 . The method as claimed in  claim 19  wherein the sensor is an eddy current sensor capable of measuring a direction dependent electrical conductivity.  
   
   
       23 . A method for detecting a crack near a material edge comprising: 
 disposing an eddy current sensor proximate to the edge of a test material, the sensor having a linear conducting segment to impose a field in the test material when driven by a time varying current, the linear segment oriented at an angle to the edge, and a sense element providing an output related to the imposed field;    measuring a sensor response at several positions along the edge with the orientation of the linear segment maintained relative to the edge; and    using the sensor response to determine the presence of a crack near the edge.    
   
   
       24 . The method as claimed in  claim 23  wherein the field is a magnetic field.  
   
   
       25 . The method as claimed in  claim 23  wherein the time varying current is an electric current.  
   
   
       26 . The method as claimed in  claim 23  wherein the linear segment is perpendicular to the edge.  
   
   
       27 . The method as claimed in  claim 23  wherein a sense element is located over the edge so that it is partly covering the test material and partly in air, off of the test material.  
   
   
       28 . The method as claimed in  claim 23  wherein the sensor has an array of sense elements oriented parallel to the linear conducting segment.  
   
   
       29 . The method as claimed in  claim 23  further comprising the use of a library of signature responses where a sensor response as a function of position along the edge is stored in the form of a crack signature and is used to construct an appropriate response signature for filtering the sensor response to detect a crack when the material is from a component.