Abstract:
A system and methods with which changes in microstructure properties such as grain size, grain elongation, texture, and porosity of materials can be determined and monitored over time to assess conditions such as stress and defects. An example system includes a number of ultrasonic transducers configured to transmit ultrasonic waves towards a target region on a specimen, a voltage source configured to excite the first and second ultrasonic transducers, and a processor configured to determine one or more properties of the specimen.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. application Ser. No. 12/984,291, filed Jan. 4, 2011, entitled “System and Methods to Determine and Monitor Changes in Microstructural Properties,” which is a continuation application of U.S. application Ser. No. 12/079,925, filed Mar. 28, 2008, entitled “System and Methods to Determine and Monitor Changes in Microstructural Properties,” which claims priority to U.S. Provisional Application No. 60/920,991, filed Mar. 30, 2007, entitled “Detection System and Methods,” and which is a continuation-in-part application of U.S. application Ser. No. 11/724,025, filed Mar. 14, 2007, entitled “Systems and Methods to Determine and Monitor Changes in Rail Conditions Over Time,” which claims priority to U.S. Provisional Application No. 60/782,608, filed Mar. 15, 2006, entitled “Systems and Methods for Monitoring Longitudinal Stress in Rail,” all of which are incorporated herein by reference in their entirety for all purposes. 
     
    
       [0002]    This invention was made with government support under DRFR53-04-G00011 awarded by the Federal Railroad Administration and DE-FG02-01ER45890 awarded by the Department of Energy. The government has certain rights in the invention. 
     
    
     TECHNICAL FIELD 
       [0003]    The present invention relates generally to a system and methods with which changes in microstructural properties such as grain size, grain elongation, texture, and porosity of materials can be determined and monitored over time to assess conditions such as stress and defects. 
       BACKGROUND 
       [0004]    For purposes of this application, the present invention is discussed in reference to polycrystalline materials, but the present invention is applicable to any heterogeneous material such as paracrystalline materials. A polycrystalline material is a material that is made of microstructure comprising many smaller crystallites, or grains, with varying orientation. The variation in direction of the grains, known as texture, can be random or directed depending on growth and processing conditions. The grains also vary in size, deformation (elongation), and void spaces between grains, or porosity. 
         [0005]    A polycrystalline material includes almost all common metals and many ceramics. A polycrystalline material is a structure of a solid, for example, steel or brass, that when cooled form liquid crystals from differing points within the material. 
         [0006]    One example of a polycrystalline material is steel. For exemplary purposes, the present invention is discussed in reference to steel in the form of railroad rail, but the present invention is applicable to any material in any form or size or shape for which material properties are desired to be determined and monitored over time such as to assess conditions of stress and defects. 
         [0007]    Rail is used on railways, otherwise known as railroads, which guide trains without the need for steering. As shown in  FIG. 1 , rail tracks  20  typically consist of two parallel rails  22 ,  23 . Rails are typically made from steel, which can carry heavier loads than other materials. Rails  22 ,  23  are laid upon cross ties  24  that are embedded in ballast  26 . Cross ties  24 , also known as sleepers, ensure the proper distance, or gauge, between the rails  22 ,  23 . Cross ties  24  also distribute the load, or force, on the rails  22 ,  23  over the ballast  26 . Plates  28  are positioned on top of cross ties  24  to receive rails  22 ,  23 . The rails  22 ,  23  are then fastened to the cross ties  24  by a fastener  30 , for example, with rail spikes, lag screws, bolts, or clips. The fastener  30  is driven through the plate  28  and into the cross tie  24 . 
         [0008]    Shown in  FIGS. 2 and 3  is a representative rail  22 . Rail  22  consists of rail sections  22 ′,  22 ″. Rail sections  22 ′,  22 ″ can be aligned and secured together by joint bars  32  ( FIG. 2 ) or welding  34  ( FIG. 3 ). Most modern railways use welding to align secure rail sections, known as continuous welded rail (“CWR”), to form one continuous rail that may be several miles long. In this form of track, the rails are welded together such as by thermite reaction or flash butt welding. 
         [0009]    Longitudinal stress is a problem over large regions of rail track. Stress is a measure of force per unit area, typically expressed in pound-force per square inch (psi). The term “longitudinal” means “along the major (or long) axis” as opposed to “latitudinal” which means “along the width”, transverse, or across. 
         [0010]    Longitudinal rail stress (“LRS”) is usually related to rail contractions and expansions due to changes in temperature. Longitudinal rail stress leads to failure, which is loss of load-carrying capacity. Examples of failure include, for example, buckling and fracture. Rail experiences tensile stress in cold temperatures, which can lead to fracture or separation of a rail into two or more pieces. In hot temperatures rail experiences compression stress, which can lead to buckling or warping. Tensile stress is a stress state causing expansion (increase in volume) whereas compression stress is a stress state causing compaction (decrease in volume). It should be noted that a zero stress state is when the material does not experience any stress. Failures, among other things, cause derailments and service disruption. 
         [0011]    The ability to measure longitudinal rail stress is a primary challenge in the railway industry. The presence of large regions of rail track reduces the ability of rail to expand and contract easily due to daily and seasonal temperature changes. Thus, high longitudinal stresses can develop, which, in turn, leads to possible failure. 
         [0012]    In the United States, from years 2001-2003, there were over 98 derailments associated with track buckling. Damage estimates for these derailments exceed $37 million. In addition, over 900 additional incidents associated with rail stress were reported. LRS is an on-going major difficulty for railroads. 
         [0013]    There has been extensive research to develop a non-destructive method to measure LRS. Current techniques include strain gauges (e.g., available from Salient Systems) and rail uplift (e.g., the VERSE system by Vortok, Inc.). There are downfalls to these current techniques. Strain gauges only provide measurements related to stress in a local, or confined area. Additionally, strain gauges present difficulty in determining the zero stress state. Measurement by rail uplift is costly and requires a section of rail to be detached from the ties. Techniques, such as these single-point measurements, make it difficult to obtain measurements on large regions of rail track. Besides steel, a variety of other polycrystalline materials may need to be assessed to determine and monitor microstructural properties over time. 
         [0014]    Traditional ultrasonic inspection methods include stress induced displacement, angle of incidence, differential pulse transit, pulse count, and signal relativity. With induced displacement, an ultrasound wave is introduced into the material using a transducer at a specified angle of incidence. The signal is received by an array of sensors at a predetermined spacing a distance from the transmitter after passing through and reflected from the bottom surface of the material. The spacing between the transmitter and receiving sensors is modified by a hydraulic servo-controller to maximize the signal at the center receiving sensor. Material height is measured independently in order to quantify the travel distance of the incident wave. 
         [0015]    Angle of incidence introduces a longitudinal wave into the top surface of the material. The refraction path through the material and reflected from the bottom surface is measured by sensors. The angle of the transmitting transducer is adjusted to maintain a constant signal at the receiving sensors. This change in angle is used to determine the stress states of the material. 
         [0016]    Differential pulse transit uses a pair of pulse trains coupled into the material. The time difference measured in the equivalent pulses in the pair of pulse trains is then related to the stress state of the material. The baseline travel time is based on measurements on stress free material. Differences in the travel time are indications of compressive or tensile stress. 
         [0017]    The pulse count method introduces a pulse train into the material. The pulses are spaced such that the stress states will cause them to overlap or spread in time. The number of pulses is counted to extract the stress state provided the pulse separation is appropriately chosen. 
         [0018]    Two successive sinusoidal waves are introduced into the material with signal relativity. As the waves propagate, their spacing in time changes based upon the stress state. This spacing is determined by quantifying the amplitude of the received signal relative to the incident waves. 
         [0019]    Problems with these methods are that they all require introduction of the ultrasonic wave through the top surface of the material and reflection of the incident waves from the bottom surface of the material without considering the microstructural properties of the material. 
         [0020]    An improved ultrasonic inspection system and methods are needed for any and all types of materials regardless of size and shape to assess microstructure properties of the material. Determining and monitoring material properties of microstructure over time may lead to specific types of processing of these materials in order to reduce or eliminate stress or defects in the material. For example, a specific sequence of a heat treatment process, such as annealing or sintering, may be utilized to alleviate significant alterations of microstructure during processing. 
         [0021]    There is a demand, therefore, for an improved ultrasonic inspection method that is reliable, practical, and cost effective with which changes in microstructural properties can be determined and monitored over time, including conditions related to stress and defects. The present invention satisfies that demand. 
       SUMMARY 
       [0022]    The present invention determines and monitors microstructural properties of materials. In one embodiment, the present invention is an ultrasonic inspection system and methods utilizing ultrasonic wave speed. In another embodiment, the present invention is an ultrasonic inspection system and methods utilizing scatter of an ultrasonic wave. Utilizing scatter of an ultrasonic wave eliminates exploitation of the subsurface longitudinal wave which requires an angle of incidence. Scatter is a general physical process whereby propagating waves are forced to deviate from a straight trajectory because of non-uniformities in the material through which it passes. 
         [0023]    According to the present invention, an improved ultrasonic inspection system and methods utilize scatter of an ultrasonic wave to determine and monitor changes in material properties, such as changes in microstructure grain size, grain elongation, texture, and porosity. Microstructural properties of materials can be determined and monitored over time to assess conditions such as stress and defects. The present invention determines and monitors microstructural properties of materials of any size and shape such as planar, cylindrical, and spherical. 
         [0024]    For purposes of this application, the present invention is discussed in reference to rail tracks on railways, but the present invention is applicable to any structure, including geological structures. For example, the present invention can determine and monitor changes in conditions of buildings, bridges, fault lines for predicting earthquakes, and land mass for prospecting oil. 
         [0025]    In one embodiment, the present invention is directed to a system and methods with which changes in microstructure properties can be determined and monitored over time using scatter of an ultrasonic wave. A transducer holder is positioned on a specimen. For purposes of this application, the term “specimen” is any heterogeneous material for which conditions such as stress or defects are desired to be determined and monitored. A transducer holder includes a top surface and a bottom surface in which a plurality of guides are created. The plurality of guides extends from the top surface to the bottom surface of the transducer holder. Each guide is angularly positioned within the transducer holder with respect to the top surface. A transducer is positioned within each guide and a voltage source excites an ultrasonic wave to propagate through the specimen. The voltage source may include, for example, a signal generator device or a laser. A laser generates heat creating an ultrasonic sound wave, whereas one type of signal generator device generates electromagnetic waves coupled into ultrasonic waves. In embodiments that use a laser to generate a signal, the laser is fired at the material, thereby generating heat and an ultrasonic wave which may be received by a laser interferometer. As a result, the ultrasonic wave is scattered within the specimen. Each transducer receives a signal from the scattered ultrasonic wave and a digital signal processor digitizes the signals. A pulse-echo technique is appropriate when using the same transducer to send and receive an ultrasonic wave for embodiments exploiting scatter of an ultrasonic wave. 
         [0026]    In a specific embodiment, rail conditions can be determined and monitored over time using scatter from an ultrasonic wave. In the broadest form, the present invention includes a transducer holder, a voltage source, an energy conversion device, an electronic test device, a database, a computing device, and a navigation device. 
         [0027]    A voltage source, such as a signal generator device, excites a pulse from a transducer that is ultimately useful to non-destructively assess material conditions in rail. One embodiment of a signal generator device is a pulser-receiver. A pulser-receiver includes a pulser that generates pulses, such as electrical signals, and thereby ultrasonic sound waves, and a receiver to receive them. 
         [0028]    The signal generator device introduces a signal into the rail. The angle at which the signal is introduced for rail steel is between a range of 0 degrees and 33 degrees. An energy conversion device converts signals from one form to another. One such type of energy conversion device is a transducer, which includes such types as electromagnetic, electrochemical, electromechanical, electroacoustic, photoelectric, electrostatic, or thermoelectric. Transducers typically communicate from a transmitting transducer to a receiving transducer. 
         [0029]    One embodiment of the invention includes a system and methods wherein the energy conversion device is securable to the rail track of a railway. Another embodiment of the invention includes a system and methods wherein the energy conversion device is securable to a coupling device, such as an applicator. The applicator is any homogeneous material that allows the energy conversion device to introduce the signal at a specific angle to propagate into the rail. 
         [0030]    Another embodiment of the invention includes a system and methods in which the energy conversion device is securable to the wheels of a railway car to implement a “rolling” system. A “rolling” system allows the present invention to become mobile, thereby allowing rail conditions to be determined and monitored over large regions of rail track. In this embodiment, a fluid-filled roller is used. The rollers can further house the energy conversion device, such as a transducer. The energy conversion device is positioned within the roller such that it introduces the signal into the rail at the desired angle. It is further contemplated that a “rolling” system can be integrated with other rail measurement techniques, or with defect detection vehicles such as those used by Sperry Rail Service or Herzog Services, for example. 
         [0031]    An electronic test device captures data, such as voltage, current, ultrasonic wave information, temperature, date, time, position, or any measurement just to name a few. Such equipment may include an infrared temperature detector, Global Positioning System (“GPS”), voltmeter, ohmmeter, ammeter, power supply, signal generator, pulse generator, oscilloscope, and frequency counter, for example. Ultrasonic wave information can include scatter, speed, amplitude, and wavelength. 
         [0032]    A computer system is used to calculate and store data. The computer system may be remote from, or integrated with, the ultrasonic inspection system. The computer system allows for real-time data analysis. A computer system is a machine for manipulating data according to a list of instructions. For example, a computer can be a laptop computer, handheld device, or personal digital assistant. 
         [0033]    With embodiments using scatter of an ultrasonic wave, a computer processor calculates a spatial variance value from the measured signals received by the transducer. A computer database analyzes the value by comparing the calculated spatial variance to a theoretical spatial variance value to assess changes in microstructure properties. The theoretical spatial variance value is stored in the database as a first set of data. The database may further store the calculated spatial variance values as a second set of data. It is contemplated that the first set of data or second set of data is historical data taken over time at the same location on the specimen. Various calculations can be performed on the first set of data and second set of data, such as an average of one set of data or a comparison between both sets of data. 
         [0034]    With embodiments using speed of an ultrasonic wave, a computer processor uses an autocorrelation component to calculate the wave speed from the measured signals received by the transducer. 
         [0035]    The present invention also includes a database for the storage of a grouping of data. A grouping of data can include one or more sets of data. One or more sets of data can be compared with other one or more sets of data, as well as utilized for various calculations. For example, a first set of data can be compared with a second set of data. Likewise, data can be computed and analyzed, for example, to determine the stress state or defects in a specimen. The database can be retained on a computer used to conduct much of the analyses or retained on a separate computer or computing device, or even an on-board or integrated computer system. 
         [0036]    Data includes, for example, location measurement such as from a Global Positioning System (“GPS”), wave speed, temperature, and the grain size, grain elongation, texture, and porosity of materials. It is further contemplated that baseline data can be established for comparison with the grouping of data. The baseline data can be, for example, “stress-free” or “zero” measurements. If baseline data is not established, one grouping of data can be compared to another grouping of data. The database may also include acoustoelastic constants, which are properties of a material that correlate changes in wave speed to changes in stress or defects. 
         [0037]    In one example, the improved system and methods of the present invention permit changes in rail conditions, most specifically longitudinal rail stress, to be assessed and monitored over time dynamically and nondestructively. One embodiment of the system includes a signal generator device that generates a signal that is transmitted to an energy conversion device. The energy conversion device converts the signal to a sound wave that propagates through the rail and is returned to the energy conversion device. The navigation device determines position of the sound wave at specific time intervals. A navigation device is a device with position or location capability, such as a Global Positioning System (“GPS”). An electronic test device captures this data and stores the data to a database. 
         [0038]    In embodiments using scatter, the computing device processes the data pertaining to microstructural properties such as grain size, grain elongation, texture and porosity, which govern the scatter of the ultrasonic wave. The ultrasonic wave is associated with longitudinal and shear wave scattering manifested through spatial variance. 
         [0039]    In embodiments using wave speed, the computing device processes the data pertaining to position of the sound wave at specific time intervals to compute wave speed. The computer system analyzes waves, such as longitudinal, shear and Lamb waves. The wave speed at specific intervals of time as a function of position is also stored in the database for comparison to previous or subsequent data to determine and monitor changes in rail conditions. 
         [0040]    According to the present invention, increasing wave speeds indicates an increase in longitudinal rail stress potentially leading to rail breaks while decreasing wave speeds indicates a decrease in longitudinal rail stress potentially leading to rail buckling. 
         [0041]    The present invention has an objective of providing a system and methods to determine and monitor changes in microstructural properties such as grain size, grain elongation, texture, and porosity of materials to assess conditions such as stress and defects. 
         [0042]    The present invention has another objective of providing a system and methods to determine and monitor changes in material microstructure such as rail conditions, including conditions related to stress and defects. 
         [0043]    Another object of the present invention is to exploit ultrasonic waves at high frequencies, such as frequencies greater than 10 Megahertz, although any frequency is contemplated. 
         [0044]    Another object of the present invention is to measure rail stress over large regions of rail track to mitigate stress-related issues, such as fractures and buckling. 
         [0045]    The present invention increases rail track safety by predicting failures before they occur. 
         [0046]    Another object of the present invention is to provide a system and methods for rail track maintenance. 
         [0047]    While current technology is focused on single-position measurements, the present invention provides multiple position measurements of stress in rail. 
         [0048]    Another object of the present invention is to provide a database for mass storage of data. The database can be accessed for analysis of the data including various calculations to determine and monitor changes in rail conditions over time. 
         [0049]    Another object of the present invention is to utilize a navigation system to accurately determine position of the failure. 
         [0050]    These and other advantages, as well as the invention itself, will become apparent in the details of construction and operation as more fully described and claimed below. Moreover, it should be appreciated that several aspects of the invention can be used in other applications where monitoring of stress would be desirable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0051]      FIG. 1  illustrates rail tracks; 
           [0052]      FIG. 2  illustrates rail tracks aligned and secured together by joint bars; 
           [0053]      FIG. 3  illustrates rail tracks aligned and secured together by welding; 
           [0054]      FIG. 4  is a block diagram for determining and monitoring microstructural properties utilizing ultrasonic wave speed according to the present invention; 
           [0055]      FIG. 5  is a block diagram for determining and monitoring microstructural properties utilizing scatter of an ultrasonic wave according to the present invention; 
           [0056]      FIG. 6  is a block diagram of a general computer system according to the present invention; 
           [0057]      FIG. 7  illustrates an embodiment of the present invention, a transducer holder for use with a planar specimen; 
           [0058]      FIG. 8  illustrates an embodiment of the present invention, a transducer holder for use with a cylindrical specimen; 
           [0059]      FIG. 9  illustrates a system for determining and monitoring stress in rail according to the present invention; and 
           [0060]      FIG. 10  illustrates the measurements taken from the system of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0061]    The present invention will now be described in detail with reference to certain embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention and how it may be applied. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well-known process steps and/or structures have not been described in detail to prevent unnecessarily obscuring the present invention. 
         [0062]    An embodiment utilizing ultrasonic wave speed of the system and methods of the present invention are illustrated as a block diagram  100  in  FIG. 4 . In this embodiment, a pulser-receiver  110  generates an electrical signal that is transmitted  113  to a transducer  122 . The transducer  122  converts the electrical signal to an ultrasonic wave  115  that propagates through the rail  180  to a receiving transducer  124 . The GPS  130  determines position of the sound wave  115  at specific time intervals. An oscilloscope  140  captures measurements of data transmitted  117 , such as ultrasonic wave information, temperature, date, time, and position, and provides the data via transmission  119  to the computer  150  for processing. The computer  150  can further include a database  160  for storage of the data. 
         [0063]    In another embodiment, a laser is used to generate a signal by firing the laser at a rail, thereby generating heat and an ultrasonic wave which may be picked up by a receiving transducer  124 . 
         [0064]    The computer  150  may include an autocorrelation component for embodiments of the present invention that utilize wave speed to correlate changes in wave speed to changes in stress or defects. An autocorrelation component assists in calculating the travel time of the ultrasonic wave. The travel time is then used to calculate the ultrasonic wave speed. If the initial electrical signal generated from the transducer  122  includes a set of voltages V i  at times t i , then the autocorrelation formula is defined as: 
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         [0065]    Maxima in the vector r determines the travel times, otherwise referred to herein as the speed, of the ultrasonic wave. The travel times are dictated by the peak(s) of the ultrasonic wave (see  FIG. 10 ). This data is stored in the database  160  and used for comparison with other measurements (baseline, past or subsequent). 
         [0066]    The computer utilizes the autocorrelation formula to calculate the wave speed of the ultrasonic sound wave. The wave speed is calculated by dividing transducer separation distance by the travel time of the sound wave. This wave speed data, along with other data such as temperature, date, time, and position of the sound wave at specific intervals determined by the navigation device, are stored onto a database  160 . 
         [0067]      FIG. 5  is a block diagram  200  for determining and monitoring microstructural properties utilizing scatter of an ultrasonic wave according to the present invention. In this embodiment, a voltage source  210  generates an electrical signal that is transmitted  213  to excite transducer  222 . The transducer  222  converts the electrical signal to an ultrasonic wave  215  that propagates through the specimen  280 . The ultrasonic wave  215  is also received by transducer  222  utilizing a pulse-echo technique. It is further contemplated that a GPS  230  may determine position of the ultrasonic wave  215  at specific time intervals. A digital signal processor  240 , for example, an oscilloscope, captures transmitted  217  data from the ultrasonic wave such as grain size, grain elongation, texture, and porosity. Temperature may be measured independently using, for example, an infrared temperature detector. The digital signal processor  240  provides the data via transmission  219  to the computer  250  for processing. Numerous signals are used to calculate a spatial variance value. The spatial variance is calculated to determine changes in the microstructure. 
         [0068]    The spatial variance of the signals is calculated by first determining the spatial average: 
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         [0069]    where M is the number of positions and V(t) is the measured signal at position i. The spatial variance is defined as: 
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         [0070]    and is determined on a computer or other signal processing board. This spatial variance represents a measure of the microstructure state in the specimen. Changes in the microstructure are determined by examining how the theoretical spatial variance differs from the measured value used to determine the stress state in the sample. 
         [0071]    The computer  250  can further include a database  260  for storage of the data. 
         [0072]    The data stored within the database  260  includes grain size, grain elongation, texture, and porosity at specific intervals of time as a function of position. Data also includes grain size, grain elongation, texture, and porosity which can be determined from changes in wave speed. This data is compared to a grouping of data stored within the database  260  to determine and monitor changes in the condition of the specimen  280 . 
         [0073]      FIG. 6  is a block diagram showing the structure of a general computer system  150  according to the present invention. The system  150  includes a central processing unit (CPU)  151 , a read-only memory (ROM)  152 , a random access memory (RAM)  153 , a processor  155 , and a database  160 , all interconnected by a system bus  154 . The database  160  serves as a storage device and may further include data  161 . 
         [0074]      FIG. 7  illustrates an embodiment of the present invention, transducer holder  320 , for use with a planar specimen of a polycrystalline material. This embodiment is designed for measurements on planar or flat specimens such as plates, beams, and rails to name a few. The transducer holder  320  is made from any homogeneous material, such as plexiglass, and includes a top surface  321  and a bottom surface  322 . The transducer holder  320  has at least two guides  350 , wherein the guide extends through the top surface  321  and bottom surface  322  of the transducer holder  320 . The guides  350  are oriented at specific angles between and including zero to thirty degrees with respect to the top surface  321 . As shown in  FIG. 7 , the guides  350  are cylindrical through-holes  351 , although any sized or shaped guides are contemplated. 
         [0075]    The guides  350  act as channels for placement of the transducers  122  (or  222 ). A voltage source (not shown) excites the transducers  122  (or  222 ) to propagate ultrasonic waves. Ultrasonic waves travel through a coupling medium, such as air, water, glycerine, or any viscous fluid in the specimen. Each transducer  122  (or  222 ) then receives a signal after the wave returns. The signal is then digitized and placed in a pulse-echo technique. A pulse-echo technique is appropriate when using the same transducer  122  (or  222 ) to send and receive an ultrasonic wave. Numerous signals are used to calculate on a spatial average value. Spatial averaging is calculated to determine changes in the microstructure of a specimen. It is desirable to collect the numerous signals by moving the transducer holder  320  to various positions on the specimen. Typically each position is at least 0.5 mm away from the other positions and at least 20 positions are needed to have a relatively smooth result. 
         [0076]      FIG. 8  illustrates an embodiment of the present invention, transducer holder  330 , for use with a cylindrical specimen of a polycrystalline material. This embodiment is designed for measurements on cylindrical, or curved, specimens such as packaging or a pressure vessel. The transducer holder  330  is made from any homogeneous material, such as plexiglass, and includes a top surface  331  and a bottom surface  332 . Again, transducer holder  330  is placed on the specimen for which microstructure properties are desired. The transducer holder  330  has at least two guides  350 , wherein the guide extends through the top surface  331  and bottom surface  332  of the transducer holder  330 . The guides  350  are oriented at specific angles between and including zero to thirty degrees with respect to the top surface  321 . As shown in  FIG. 8 , the guides  350  are cylindrical through-holes  352  although any sized or shaped guides are contemplated. 
         [0077]    Transducers  122  (or  222 ) are placed within the guides  350  and a voltage source (not shown) excites the transducers  122  (or  222 ) to propagate ultrasonic waves. Each transducer  122  (or  222 ) then receives a signal after the wave returns, which is then digitized. 
         [0078]      FIG. 9  illustrates an embodiment of a system  500  for determining and monitoring stress in rails according to the present invention. A transducer  122  and receiving transducer  124  are sized and shaped such that each may be positioned on a surface of the rail  25  such as a side surface  25 A, or top surface  25 B of a rail  25  through a coupling device  27 , such as applicator. In embodiments that use scatter of an ultrasonic wave, the transducer  122  receives the ultrasound wave scatter without the need for transducer  124 . 
         [0079]    The coupling device  27  may be in the form of a wedge or other shape to permit easy adherence to the rail surface  25 A,  25 B. The coupling device  27  is preferably formed of a material to facilitate the transmission of the ultrasonic wave by the transducer  122  and the receptor of the ultrasonic wave by the receiving transducer  124 . Acrylic is one of the many materials that may be used for this purpose. Other embodiments of the system and methods utilize the positioning of the transducers  122 ,  124  on the wheels of a railway car. 
         [0080]    With reference to  FIG. 9 , embodiments that use scatter of an ultrasonic wave include a voltage source  110  that sends a voltage signal to the transducer  122 . The transducer  122  converts the voltage signal to an ultrasonic sound wave that propagates through the rail  25  and is received by transducer  124 . The transducer  122  amplifies and digitizes the sound wave into signals. The signals received from the transducer  122  may be acquired such as with an oscilloscope  140  and conveyed to a database, for example, within a laptop or equivalent computer (not shown). The database is used for data analysis of the signals. The computer utilizes an autocorrelation formula to calculate the travel time of the sound wave. The wave speed is then calculated by dividing transducer separation distance by the travel time of the sound wave. 
         [0081]    This wave speed data, along with other data such as temperature, date, time, and position of the sound wave at specific intervals determined by the navigation device, are stored in a database. Again, in embodiments that use ultrasonic wave scatter, the computer calculates a spatial variance value. This spatial variance data, along with other data such as grain size, grain elongation, texture, and porosity, are stored into the database. Data such as temperature can be taken by the transducers  122 ,  124  on the rail  25 . Likewise, the navigation device (not shown) can take the position data at the location where the temperature data is taken. 
         [0082]    The database  160  may store the data at specific intervals of time as a function of position. The database  160  can be on the computer  150  or on a separate computer. 
         [0083]    The computer  150  compares data of the database  160 . A first set of data can be compared to other sets of data. The first set of data can be one data point, a plurality of data points, a base line or control data points. A second set of data points can be one data point or a plurality of data points for comparison with the first set of data points. The comparison between data points determines abnormalities or changes, if any, between the data over time. The database would store theoretical spatial variance values as well as historical values measured at the same location for comparison. 
         [0084]    According to the present invention, a first set of data points, such as spatial variance, is compared to a second set of data points. A comparison resulting in an increase in wave speeds indicates an increase in longitudinal rail stress potentially leading to rail breaks while a comparison resulting in a decrease in wave speeds indicates a decrease in longitudinal rail stress potentially leading to rail buckling. 
         [0085]    While endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicants claim protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon. While the apparatus and method herein disclosed forms a preferred embodiment of this invention, this invention is not limited to that specific apparatus and method, and changes can be made therein without departing from the scope of this invention, which is defined in the appended claims. 
         [0086]    Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.