Magnetic field characterization of stresses and properties in materials

Abstract

Described are methods for monitoring of stresses and other material properties. These methods use measurements of effective electrical properties, such as magnetic permeability and electrical conductivity, to infer the state of the test material, such as the stress, temperature, or overload condition. The sensors, which can be single element sensors or sensor arrays, can be used to periodically inspect selected locations, mounted to the test material, or scanned over the test material to generate two-dimensional images of the material properties. Magnetic field or eddy current based inductive and giant magnetoresistive sensors may be used on magnetizable and/or conducting materials, while capacitive sensors can be used for dielectric materials. Methods are also described for the use of state-sensitive layers to determine the state of materials of interest. These methods allow the weight of articles, such as aircraft, to be determined.

What is claimed is:  
     
         1 . A method for monitoring of material properties, said method comprising: 
 disposing a sensor having at least two linear primary conductor segments for imposing a magnetic field when driven by a time varying electrical current and at least one sense element oriented parallel to the primary conductor segments near a material being processed, the sensor and material being exposed to the same process conditions;    connecting the at least one sense element to dedicated electrical circuitry in an impedance measurement instrument;    and converting each sense element response into an effective property.    
     
     
         2 . The method as claimed in  claim 1  wherein the at least one sense element is a rectangular absolute sensing coil.  
     
     
         3 . The method as claimed in  claim 1  wherein sense element connections include a nearby pair of conductors to compensate for the connections' effect on the measured response of each sense element.  
     
     
         4 . The method as claimed in  claim 1  wherein the process conditions include exposure to elevated temperatures.  
     
     
         5 . The method as claimed in  claim 1  wherein the effective property is electrical conductivity.  
     
     
         6 . The method as claimed in  claim 1  wherein the effective property is lift-off.  
     
     
         7 . The method as claimed in  claim 1  wherein measurements are performed at multiple excitation frequencies.  
     
     
         8 . The method as claimed in  claim 1  wherein the sensor does not make contact with the material surface.  
     
     
         9 . The method as claimed in  claim 1  wherein the sensor does not make contact with the article surface.  
     
     
         10 . A method for stress monitoring of materials, said method comprising: 
 disposing a sensor having at least two parallel drive conductors with at least one sense element on a substrate proximate to a surface of a test material;    passing a time-varying electric current through the drive conductor to create a magnetic field;    measuring the response from the at least one sense element to determine an electrical property, with the response measured in more than one sensor orientation; and    correlating this electrical property with stress.    
     
     
         11 . The method as claimed in  claim 10  wherein the response is measured in two orientations.  
     
     
         12 . The method as claimed in  claim 11  wherein the orientations are mutually perpendicular.  
     
     
         13 . The method as claimed in  claim 11  wherein the orientations correspond to the maximum and minimum principal stresses.  
     
     
         14 . The method as claimed in  claim 10  wherein the substrate is flexible.  
     
     
         15 . The method as claimed in  claim 14  wherein the substrate further comprises a foam backing.  
     
     
         16 . The method as claimed in  claim 14  wherein the substrate further comprises a rigid backing that conforms to the shape of the test material.  
     
     
         17 . The method as claimed in  claim 10  wherein the sensor has a plurality of sense elements.  
     
     
         18 . The method as claimed in  claim 17  wherein the sense elements are aligned in a direction perpendicular to the direction of sensitivity for the sensor.  
     
     
         19 . The method as claimed in  claim 17  wherein the sensor is scanned over the surface of the material.  
     
     
         20 . The method as claimed in  claim 19  wherein the sensor is scanned in two mutually perpendicular orientations.  
     
     
         21 . The method as claimed in  claim 20  wherein the orientations correspond to the maximum and minimum principal stresses.  
     
     
         22 . The method as claimed in  claim 10  further comprising at least two sensors mounted at different locations on the test material.  
     
     
         23 . The method as claimed in  claim 22  wherein the sensors have different orientations.  
     
     
         24 . The method as claimed in  claim 22  wherein the orientations are mutually perpendicular.  
     
     
         25 . The method as claimed in  claim 22  wherein the orientations correspond to the maximum and minimum principal stresses.  
     
     
         26 . The method as claimed in  claim 10  wherein the response is measured at multiple frequencies.  
     
     
         27 . The method as claimed in  claim 10  wherein the effective property is magnetic permeability.  
     
     
         28 . A method for inspection of magnetizable materials, said method comprising: 
 disposing a sensor having at least two parallel drive conductors with at least one sense element on a substrate proximate to a surface of a test material at more than one location;    passing a time-varying electric current through the drive conductor to create a magnetic field;    measuring the response from each sense element to determine magnetic permeability; and, correlating this magnetic permeability with an overload effect.    
     
     
         29 . The method as claimed in  claim 28  wherein the response is the permeability distribution over the material surface.  
     
     
         30 . The method as claimed in  claim 28  wherein the response is measured for different sensor orientations.  
     
     
         31 . The method as claimed in  claim 28  wherein the response is measured at selected locations on the material.  
     
     
         32 . The method as claimed in  claim 28  wherein the sensor has a plurality of sense elements.  
     
     
         33 . The method as claimed in  claim 32  wherein the sense elements are aligned in a direction perpendicular to the direction of sensitivity for the sensor.  
     
     
         34 . The method as claimed in  claim 32  wherein the sensor is scanned over the surface of the material.  
     
     
         35 . The method as claimed in  claim 34  wherein the sensor is scanned with different orientations.  
     
     
         36 . The method as claimed in  claim 32  wherein the sensor is mounted to the surface of the material.  
     
     
         37 . The method as claimed in  claim 36  further comprising at least one additional sensor mounted in a different orientation than the first sensor.  
     
     
         38 . A method for determining weight of an article, said method comprising: 
 disposing a sensor array comprising at least one linear drive conductor with a plurality of sense elements proximate to a surface of a magnetizable test material that transfers the load from the article;    passing a time-varying electric current through the drive conductor to create a magnetic field;    measuring the response from each sense element to determine magnetic permeability; and, correlating this magnetic permeability with the article weight.    
     
     
         39 . The method as claimed in  claim 38  wherein the article is an aircraft.  
     
     
         40 . The method as claimed in  claim 38  wherein the properties are monitored at selected locations on the article.  
     
     
         41 . The method as claimed in  claim 38  wherein the sensor is scanned over the surface of the article.  
     
     
         42 . The method as claimed in  claim 38  wherein the sensor is mounted to the surface of the article.  
     
     
         43 . The method as claimed in  claim 38  wherein the sensor is oriented to measure the response parallel to the maximum principal stresses.  
     
     
         44 . A method for monitoring the state of an article, said method comprising: 
 affixing a state-sensitive material to the article;    the material having an electrical property that varies with article state;    measuring said electrical property with at least one sensor; and    relating said electrical property to the state of the article.    
     
     
         45 . The method as claimed in  claim 44  wherein the state is stress.  
     
     
         46 . The method as claimed in  claim 44  wherein the state is temperature.  
     
     
         47 . The method as claimed in  claim 44  wherein the state is an overload condition.  
     
     
         48 . The method as claimed in  claim 47  wherein the condition is a thermal overload.  
     
     
         49 . The method as claimed in  claim 47  wherein the condition is a mechanical overload.  
     
     
         50 . The method as claimed in  claim 44  wherein the state is accumulated fatigue damage.  
     
     
         51 . The method as claimed in  claim 44  wherein the state is the presence of a crack within the article.  
     
     
         52 . The method as claimed in  claim 44  further comprising embedding the sensor within a layer of the article.  
     
     
         53 . The method as claimed in  claim 44  wherein the state-sensitive material is divided into multiple strips.  
     
     
         54 . The method as claimed in  claim 53  wherein the strips have different orientations with depth in the article.  
     
     
         55 . The method as claimed in  claim 44  wherein the sensor is an eddy current sensor.  
     
     
         56 . The method as claimed in  claim 55  wherein the sensor further comprises separate layers for the drive conductors that create a magnetic field when driven by a time-varying current and the sense elements that respond to the magnetic field.  
     
     
         57 . The method as claimed in  claim 56  wherein the drive and sense conductors are placed in different layers of the article.  
     
     
         58 . The method as claimed in  claim 44  wherein the sensor is protected by a durable material.  
     
     
         59 . The method as claimed in  claim 57  wherein the durable material is a ceramic.  
     
     
         60 . The method as claimed in  claim 57  wherein the durable material is a stainless steel.  
     
     
         61 . The method as claimed in  claim 44  wherein the sensor is an eddy current sensor array.  
     
     
         62 . The method as claimed in  claim 61  wherein the sensor array is mounted to the surface of the article.  
     
     
         63 . The method as claimed in  claim 61  wherein the sensor array is scanned over the surface of the article.  
     
     
         64 . The method as claimed in  claim 44  wherein the electrical property is magnetic permeability.  
     
     
         65 . The method as claimed in  claim 44  wherein the electrical property is electrical conductivity.  
     
     
         66 . The method as claimed in  claim 44  wherein the sensor is a dielectric sensor.  
     
     
         67 . The method as claimed in  claim 44  wherein the sensor is a giant magnetoresistive sensor.  
     
     
         68 . A method for remotely monitoring the state of a article, said method comprising: 
 disposing a sensor proximate to an article having a hidden material that has an electrical property that varies with article state;    measuring said electrical property with at least one sensor; and    relating said electrical property to the state of the article.    
     
     
         69 . The method as claimed in  claim 68  wherein the state is stress.  
     
     
         70 . The method as claimed in  claim 68  wherein the state is temperature.  
     
     
         71 . The method as claimed in  claim 68  wherein the sensor is an eddy current sensor.  
     
     
         72 . The method as claimed in  claim 68  wherein the sensor is an eddy current sensor array.  
     
     
         73 . The method as claimed in  claim 72  wherein the sensor array is mounted to the surface of the article.  
     
     
         74 . The method as claimed in  claim 72  wherein the sensor array is scanned over the surface of the article.  
     
     
         75 . The method as claimed in  claim 68  wherein the electrical property is magnetic permeability.  
     
     
         76 . The method as claimed in  claim 68  wherein the electrical property is electrical conductivity.  
     
     
         77 . The method as claimed in  claim 68  wherein the sensor is a giant magnetoresistive sensor.