Method and apparatus for measuring strain using a luminescent photoelastic coating

Abstract

A method and apparatus for measuring strain on a surface of a substrate utilizes a substrate surface coated with at least one coating layer. The coating layer provides both luminescence and photoelasticity. The coating layer is illuminated with excitation light, wherein longer wavelength light is emitted having a polarization dependent upon stress or strain in the coating. At least one characteristic of the emitted light is measured, and strain (if present) on the substrate is determined from the measured characteristic.

1 . A method for measuring strain, comprising the steps of: 
 providing a substrate surface coated with at least one coating layer, said coating layer providing both luminescence and photoelasticity;    illuminating said coating layer with excitation light, wherein longer wavelength light is emitted having a polarization dependent upon stress or strain in said coating layer;    measuring at least one characteristic of said emitted light, and determining strain on said substrate surface from said characteristic.    
     
     
         2 . The method of  claim 1 , said coating layer includes at least one luminophore for providing said luminescence.  
     
     
         3 . The method of  claim 2 , wherein said luminophore is a polarization preserving material.  
     
     
         4 . The method of  claim 1 , wherein said coating layer is polarization generating, wherein said excitation light is non-polarized light.  
     
     
         5 . The method of  claim 1 , wherein said excitation light comprises polarized light.  
     
     
         6 . The method of  claim 5 , wherein said polarized light comprises elliptically polarized light.  
     
     
         7 . The method of  claim 1 , wherein said characteristic comprises at least one selected from the group consisting of the maximum principal strain, the minimum principal strain and the maximum shear strain on said substrate surface.  
     
     
         8 . The method of  claim 1 , wherein said characteristic comprises directions of maximum principal strain and the minimum principal strain on said substrate surface.  
     
     
         9 . The method of  claim 1 , wherein said coating layer consists of only a single layer.  
     
     
         10 . The method of  claim 1 , wherein said at least one coating layer comprises at least two layers, said at least two layers comprising a luminescent layer disposed on said substrate surface and a photoelastic layer disposed on said luminescent layer.  
     
     
         11 . The method of  claim 10 , wherein said luminescent layer includes a first luminophore and said photoelastic coating includes a second luminophore, wherein said first and second luminophore provide different emission wavelengths.  
     
     
         12 . The method of  claim 11 , wherein an emission wavelength of said first luminophore corresponds to an absorption spectrum of said second luminophore.  
     
     
         13 . The method of  claim 1 , further comprising the step of optical filtering to selectively pass said higher wavelength light and reject said excitation light.  
     
     
         14 . The method of  claim 1 , further comprising the steps of providing said strain on said substrate surface to an analytical model and updating said analytical model based on differences between said strain on said substrate surface and strain data generated by said analytical model.  
     
     
         15 . The method of  claim 1 , wherein said illuminating step comprises a process comprising oblique incidence, said determining step providing individual values for maximum principal strain and minimum principal strain on said substrate exclusively using said method.  
     
     
         16 . The method of  claim 1 , further comprising the step of scanning said excitation light across said substrate surface, wherein regions of high strain are identified.  
     
     
         17 . An apparatus for measuring strain, comprising: 
 an excitation light source for illuminating a surface of a substrate, said substrate including a coating which provides both luminescence and photoelasticity;    a detector for measuring light emitted by said substrate surface responsive to said excitation light, said emitted light being at a longer wavelength and having a polarization modified as compared to said excitation light based upon stress or strain on said coating, and    a computer for processing to determine strain on said substrate surface from said emitted light.    
     
     
         18 . The apparatus of  claim 17 , wherein said excitation light source provides polarized light.  
     
     
         19 . The apparatus of  claim 17 , wherein said computer provides at least one selected from the group consisting of the maximum principal strain, the minimum principal strain and the maximum shear strain on said substrate surface.  
     
     
         20 . The apparatus of  claim 17 , wherein said wherein said computer provides directions of maximum principal strain and the minimum principal strain on said substrate surface.  
     
     
         21 - 40 . (canceled)