Acoustic method for estimating mechanical properties of a material and apparatus therefor

Abstract

A method for estimating the time varying mechanical properties of a material comprising: propagating acoustic waves through a sample of material; measuring signals corresponding to acoustic waves after they propagate in the sample; and characterised by: comparing attributes from the measured signals with attributes from corresponding signals provided by a calculating model, the calculating model accounting for at least initially estimated acoustic properties of the material; using the differences between the measured and calculated signals to update the initially estimated acoustic properties of the material in the calculating model; processing N iterations of the comparison and updating either until the differences between measured and calculated signals are within a given tolerance factor or when the number i reaches a prescribed maximum; and calculating the time varying mechanical properties of the material from the final iterated acoustic properties. Also described is an apparatus for implementing the method.

We claim:  
     
         1 . A method for estimating the time varying mechanical properties of a material comprising: 
 propagating acoustic waves through a sample ( 3 ) of said material;    measuring signals corresponding to said acoustic waves after they have propagated in said sample ( 3 ); and    calculating the time varying mechanical properties of said material; the method characterised by: 
 comparing attributes from said measured signals with attributes from corresponding signals provided by a calculating model, said calculating model accounting for at least initially estimated acoustic properties of said material;  
 using the differences between said measured and calculated signals to update said initially estimated acoustic properties of said material in the calculating model;  
 processing (N) iterations of said comparisons and updatings until a given calculating stop; and  
 calculating the time varying mechanical properties of said material from said final iterated acoustic properties.  
   
     
     
         2 . A method according to  claim 1 , wherein the acoustic waves are both compressional and shear waves.  
     
     
         3 . A method according to  claim 1 , wherein the given final stop is when the differences between measured and calculated signals are within a given tolerance factor.  
     
     
         4 . A method according to  claim 1 , wherein the given final stop is when the iteration number (N) reaches a prescribed maximum.  
     
     
         5 . A method according to  claim 1 , wherein the attributes of the measured and calculated signals are the transit times of the acoustic waves and amplitudes of said signals.  
     
     
         6 . A method according to  claim 5 , wherein the acoustic properties of the material comprise the velocity of the acoustic waves through the sample ( 3 ) of said material.  
     
     
         7 . A method according to  claim 1 , wherein the propagation and measuring means comprise ultrasonic compressional transducers, among which a transmitter ( 1 ) and a receiver ( 2 ).  
     
     
         8 . A method according to  claim 7 , further comprising the steps of immersing the transmitter ( 1 ) and the receiver ( 2 ) in a medium ( 4 ) and placing the sample ( 3 ) of material between said transmitter and said receiver.  
     
     
         9 . A method according to  claim 8 , wherein at the i th  iteration (where i≦N), the value of the velocity for the acoustic waves through the sample ( 3 ) of material is updated according to:  
       
         
           
             
               
                 
                   V 
                   l 
                   
                     i 
                     + 
                     1 
                   
                 
                 = 
                 
                   
                     V 
                     l 
                     i 
                   
                   + 
                   
                     δ 
                     
                       V 
                       l 
                     
                   
                 
               
               ; 
               
                   
               
                
               
                 
                   where 
                    
                   
                       
                   
                    
                   
                     δ 
                     
                       V 
                       l 
                     
                   
                 
                 = 
                 
                   
                     - 
                     
                       
                         
                           t 
                           l 
                           ref 
                         
                         - 
                         
                           t 
                           l 
                         
                       
                       
                         
                           t 
                           l 
                           ref 
                         
                         - 
                         
                           
                             ( 
                             
                               d 
                               - 
                               h 
                             
                             ) 
                           
                           / 
                           
                             V 
                             f 
                           
                         
                       
                     
                   
                    
                   
                     V 
                     l 
                     i 
                   
                 
               
             
           
           
           
               
           
         
       
       where l means either the shear (S) or compressional (P) waves, t l   ref  is the transit time of the waves in the measured signals, t l  is the corresponding transit time of the calculated signal with a certain value V l  for the wave velocity, d is the distance between the propagating and the measuring means, h is the sample thickness, and V f  is the sound speed in the medium.  
     
     
         10 . A method according to  claim 9 , wherein the iteration stops when the relative difference between two successive iterated values of the velocity for the acoustic waves through the sample ( 3 ) of material is within a prescribed tolerance factor ε V     l    such as:  
       
         
           
             
               
                  
                 
                   
                     δ 
                     
                       V 
                       l 
                     
                   
                   
                     V 
                     l 
                   
                 
                  
               
               < 
               
                 
                   ɛ 
                   
                     V 
                     l 
                   
                 
                 . 
               
             
           
           
           
               
           
         
       
     
     
         11 . A method according to  claim 1 , wherein the time varying mechanical properties of the material are the Young's modulus, the shear modulus and the Poisson's ratio.  
     
     
         12 . A method according to  claim 1 , wherein the acoustic properties of the material comprise attenuation of the acoustic waves through the sample of material.  
     
     
         13 . A method according to  claim 12 , wherein at the i th  iteration (i≦N), the value of the attenuation for the acoustic wave is updated according to:  
       
         
           
             
               
                 
                   α 
                   l 
                   
                     i 
                     + 
                     1 
                   
                 
                 = 
                 
                   
                     α 
                     l 
                     i 
                   
                   + 
                   
                     δ 
                     
                       α 
                       l 
                     
                   
                 
               
               ; 
               
                   
               
                
               
                 
                   where 
                    
                   
                       
                   
                    
                   
                     δ 
                     
                       α 
                       l 
                     
                   
                 
                 = 
                 
                   
                     - 
                     
                       
                         
                           A 
                           l 
                           ref 
                         
                         - 
                         
                           A 
                           l 
                         
                       
                       
                         A 
                         l 
                         ref 
                       
                     
                   
                    
                   
                     α 
                     l 
                     i 
                   
                 
               
             
           
           
           
               
           
         
       
       where l means either the shear (S) or compressional (P) waves, A l   ref  is the amplitude of the acoustic waves in the measured signals and A l  is the corresponding amplitude of the calculated signal with a certain value for the attenuation α l .  
     
     
         14 . A method according to  claim 13 , wherein the iteration stops when the relative difference between two successive iterated values of the attenuation for the acoustic wave is within a prescribed tolerance factor ε V     l    a such as:  
       
         
           
             
               
                  
                 
                   
                     δ 
                     
                       α 
                       l 
                     
                   
                   
                     α 
                     l 
                   
                 
                  
               
               < 
               
                 
                   ɛ 
                   
                     α 
                     l 
                   
                 
                 . 
               
             
           
           
           
               
           
         
       
     
     
         15 . A method according to  claim 1 , wherein the calculating model accounts for characteristics of the propagating and measuring means.  
     
     
         16 . A method according to  claim 15 , wherein the characteristics of the propagating and measuring means comprise the radiation and the reception properties of said propagating and measuring means and the estimation of the transfer function of the electronic devices included in said means.  
     
     
         17 . A method according to  claim 1 , wherein the calculating model accounts for environmental parameters such as temperature and pressure.  
     
     
         18 . A method according to  claim 18 , wherein the material is a settable material.  
     
     
         19 . An apparatus for propagating and measuring acoustic waves through a sample ( 3 ) of material, the mechanical properties of which vary with time, said apparatus comprising: 
 at least one transmitter ( 1 ) propagating said acoustic waves towards at least one receiver ( 2 ), said receiver detecting said acoustic waves after they have passed through said sample; and means for calculating properties of the sample based on the detected acoustic waves; characterised in that the apparatus comprises:    means for varying the incidence angle (θ) of the acoustic waves on the sample of the material; and    means for: 
 comparing attributes from said detected signals with attributes from corresponding signals provided by a calculating model, said calculating model accounting for at least initially estimated acoustic properties of said material;  
 using the differences between said measured and calculated signals to update said initially estimated acoustic properties of said material in the calculating model;  
 processing (N) iterations of said comparisons and updatings until a given calculating stop; and  
 calculating the time varying mechanical properties of said material from said final iterated acoustic properties.  
   
     
     
         20 . An apparatus according to  claim 19 , wherein the sample of material is rotational such that its surface makes different incidence angles (θ) with respect to the transmitter and receiver sensitivity line.  
     
     
         21 . An apparatus according to  claim 20 , wherein the transmitter ( 1 ) and receiver ( 2 ) alignment is controlled via an external mechanism.  
     
     
         22 . An apparatus according to  claim 21 , wherein the receiver translates along a line parallel to the surface of the sample of material.  
     
     
         23 . An apparatus according to  claim 20 , further comprising multiple pairs of transmitters and receivers.  
     
     
         24 . An apparatus according to  claim 23 , wherein the transmitters and receivers are aligned from each part of the sample of material such that each transmitter-receiver pair sensitivity line makes a different incidence angle (θ) with respect to said sample.  
     
     
         25 . An apparatus according to  claim 23 , wherein the number of pairs is such that several incidence angles (θ) from zero to a maximum angle are covered, said maximum angle being preferably superior to 30°.  
     
     
         26 . An apparatus according to  claim 19 , wherein the transmitter or the receiver comprises an array of transducers that respectively each emits or detects acoustic waves that are time spaced from one transducer to another.