Abstract:
Systems and methods for analyzing structural test data are disclosed. In one embodiment, a method includes applying a sequence of loads to a test article, receiving raw test data indicative of the applied loads from at least one sensor operatively associated with the test article, receiving predicted test data indicative of the predicted loads on the test article, filtering out invalid test data, cycle counting to pair loads in the test data, performing a first fatigue damage computation based on the raw test data, performing a second fatigue damage computation based on the predicted test data, and comparing the first and second fatigue damage computations. The filtering, cycle counting, and performing of the first and second fatigue damage computations, and the comparison of the first and second fatigue damage computations, may be performed simultaneously using a spreadsheet program.

Description:
FIELD OF THE INVENTION 
   This invention relates to structural testing, and, more specifically, to methods and systems for accurately and efficiently analyzing real-time structural test data. 
   BACKGROUND OF THE INVENTION 
   Due to the time and expense associated with full-scale structural fatigue testing, test laboratories are typically under pressure to reduce their operating expenses and to produce results more quickly. An increase in the cycling rate of a full-scale fatigue test may lead directly to lower laboratory costs and may provide test results sooner. For example, in the aircraft industry, significant potential savings may be realized in retrofit and fleet repair costs if structural fatigue test results can be provided more quickly. 
   Cycling rate may be a function of many parameters. One parameter that is particularly critical is the accuracy of the applied test loads. Typically there is a required spectrum severity that must be maintained. As the cycle rate increases, there is a potential for the loads to be applied with less accuracy, reducing (or increasing) the severity of the applied spectrum. A current process to evaluate the tested spectrum severity involves applying a significant portion of the spectrum while recording the load feedback and strain gage output. These data are then evaluated, which may take days or weeks before results are known. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to methods and systems for analyzing structural test data. Methods and systems in accordance with the present invention may advantageously reduce the time and expense associated with providing accurate structural fatigue test results. 
   In one embodiment, a method of analyzing structural test data includes applying a sequence of loads to a test article, and receiving raw test data indicative of the applied loads from at least one sensor operatively associated with the test article. The method further includes receiving predicted test data indicative of a predicted load on the test article, performing a first fatigue damage computation based on the raw test data, and performing a second fatigue damage computation based on the predicted test data. The method also includes comparing the first and second fatigue damage computations. The performing of the first and second fatigue damage computations, and the comparison of the first and second fatigue damage computations, may be performed simultaneously using a spreadsheet program. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred and alternate embodiments of the present invention are described in detail below with reference to the following drawings. 
       FIG. 1  is a flowchart of a method for performing a structural test in accordance with an embodiment of the invention; 
       FIG. 2  is an embodiment of a test setup input screen of the method of  FIG. 1 ; 
       FIG. 3  is an embodiment of a control point input screen of the method of  FIG. 1 ; 
       FIG. 4  is an embodiment of a tested actuator loads screen of the method of  FIG. 1 ; 
       FIG. 5  shows the tested actuator loads screen of  FIG. 4  after processing of test data files; and 
       FIG. 6  is a representative plot of damage versus time for comparing the actual and predicted fatigue damages of the method of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   The present invention relates to methods and systems for structural testing. Many specific details of certain embodiments of the invention are set forth in the following description and in  FIGS. 1–6  to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description. 
   In general, methods and systems in accordance with the present invention enable test personnel to evaluate the effects of applied test load variances, and to make necessary adjustments, in real-time while conducting a test. Data generated by a test article are collected and then processed using methods and systems in accordance with the invention to assess the spectrum severity. Thus, embodiments of the present invention may advantageously provide substantially real-time analysis results during a test, thereby reducing the time and expense associated with providing accurate structural fatigue test results. 
   More specifically,  FIG. 1  is a flowchart of a method  100  for performing a structural test in accordance with an embodiment of the present invention. In the embodiment, the method  100  begins with a test setup at a block  102 . In one embodiment, the test setup (block  102 ) is performed via a menu or spreadsheet-formatted input screen  200 , as shown in  FIG. 2 . During the test setup, a user may define the following parameters: 
   Test Name  202 —a brief identifier for the test to be conducted; 
   Test Description  204 —a description of the test to be conducted; 
   Test Life  206 —a duration of the subject test (e.g., in flight hours); 
   Test Folder Path  208 —a path name to a folder for storing test data; 
   Test Loads Folder Path  210 —a path name to a folder for storing loads data; 
   Test History Folder Path  212 —a path name to a folder for storing processed test data for subsequent analysis by an analysis engine; 
   Test Initialization File  214 —a filename of an initialization file created by the method  100  that will be supplied to a crack initiation engine (described below); 
   Test Materials Database File  216 —a filename of a file containing a list of materials and the material properties required to perform a crack initiation analysis; 
   Test Predicted Loads File  218 —a filename of a file containing a set of predicted loads for the subject test; 
   Test Spectrum Load Lines per Block  220 —a number of load events that will be repeated as a block throughout the test; 
   Test Spectrum Flight Hours per Block  222 —a number of equivalent flight hours for each block of the test spectrum; 
   Test Control Points  224 —a number of test control points determined by the method  100  (not input by the user), as described more fully below; 
   Test Data Files Processed  226 —a number of test data files processed to date determined by the method  100  (not input by the user); 
   Test Data File Type  228 —an expected format of the resultant test data files; and 
   Flight Hours at Test Start  230 —a number of equivalent test flight hours at a start of the test, which is usually “0”, but which may be greater then zero to allow for the data processing to start subsequent to the start testing. 
   The input screen  200  includes navigation buttons  232  that permit a user to edit the inputs described above, and to return to one or more other portions of the method  100 . In one particular embodiment, the input screen  200  is a spreadsheet screen generated using the EXCEL program commercially available from the Microsoft Corp. of Redmond, Wash., and includes a plurality of worksheet tabs  234  that enable the user to move swiftly between various portions of the method  100 . 
   As further shown in  FIG. 1 , the method  100  further includes defining control points at a block  104 . The control points are those specific locations on a test article where crack initiation analyses and comparisons will be performed. In one embodiment, the control point definition (block  104 ) is performed via a control point input screen  300 , as shown in  FIG. 3 . During the control point definition, the user may define the following parameters: 
   Control Point No.  302 —a numeric identifier (e.g., an integer) for a control point; 
   Name  304 —a text identifier (or nickname) for each control point; 
   Description  306 —a text description (e.g., a detailed description) for each control point; 
   Material  308 —a material definition for the control point; 
   Fstress  310 —a factor used in a control point equation to calculate an analytic fatigue stress from the analytical fatigue load output by Equation  316 . This factor is used by a crack initiation engine, as described more fully below; 
   Kt  312 —a stress concentration factor used by the crack initiation engine to calculate fatigue damage for the control point; 
   Kc/Kt  314 —a factor that accounts for the fact that a “compression” load path may be different from a “tension” load path, the factor being required by the crack initiation engine to calculate fatigue damage for the control point; 
   Equation  316 —an equation for determining an analytical fatigue load from a plurality of test measurands; actual test measurand names or “nicknames” may be used as variable inputs. Test measurand names or nicknames (i.e., non-constant entries) may be enclosed in brackets, for example Analytical Fatigue Load=[WRBM]*1.00; 
   UpperBound  318 —an upper bound of the equation  316  used to identify erroneous data in a Test Loads file; and 
   LowerBound  320 —a lower bound of the equation  316  used to identify erroneous data in a Test Loads file. 
   Again, the control point input screen  300  may include navigation buttons  322  that permit a user to edit the inputs described above, and to return to one or more other portions of the method  100 , and a plurality of worksheet tabs  324  that enable the user to move swiftly between various portions of the method  100 . 
   After the test setup is performed (block  102 ) and the control points are defined (block  104 ), data processing may start in conjunction with the test. As shown in  FIG. 1 , the method  100  further includes defining tested actuator loads at a block  106 .  FIG. 4  is an embodiment of a tested actuator loads screen  400  of the method of  FIG. 1 . New test data files may be loaded by selecting a load data files command button  402 . These test data files may represent the unprocessed test data gathered by the test sensors during applications of loads to the test article. These unprocessed test data may represent raw, unprocessed electrical signal outputs from a variety of different sensors types (e.g., strain gauges, transducers, thermocouples, etc.). When the load data files command button  402  is selected by user, the test loads folder  210  specified in the setup input screen  200  ( FIG. 2 ) will be searched for new test data files. Each selected test data file is assigned a file identifier  404 , a date  406 , a test loads file name  408 , a specified control point  410  for analysis, and a test status  410 . Prior to being processed by the method  100 , the new files are tagged as having the test status  412  of “pending”. 
   Files loaded and tagged with the “pending” test status  412  may be processed by clicking a process data command button  414 . Upon selection of the processed data command button  414 , the method  100  may process all of the “pending” files in a background or batch mode. During processing, portions of the tested actuator loads screen  400  may be continuously updated to provide the results of the data processing. Also during the processing, inputs provided by the user during the test setup (block  102 ) may be employed. For example, at a block  108  ( FIG. 1 ), the method  100  receives target actuator loads contained in the Test Predicted Loads File  218  specified by the user during the test setup ( FIG. 2 ). 
   For example,  FIG. 5  shows the tested actuator loads screen  400  of  FIG. 4  after processing of the test data files  408 . As shown in  FIG. 5 , after the test data files  408  are processed, the tested actuator loads screen  400  provides a flight hours indicator  416  indicating a simulated number of flight hours represented by the test data, a cumulative flight hours indicator  418  indicating a cue at a number of flight hours are presented by the test data (maybe same as the flight hours indicator  416 ), a predicted test result  420  (e.g., based on analytical predictions, empirical predictions, etc.) for comparison with the actual test results, and an actual test result  422  computed from the raw test data (e.g., transducer outputs, strain gauge outputs, etc.). 
   The manner in which the actual test results  422  are computed will now be described. Referring again to  FIG. 1 , the method  100  proceeds to a crack initiation engine at a block  110  to predict crack initiation damage. First, at a block  112 , the crack initiation engine (block  110 ) includes a spectrum generator that performs a cycle counting in which an effective pairing of load peaks and valleys is performed. The spectrum generator filters the sequence of actual test results such that each test point produces a slope change when compared to the previous two test points. Any load points that do not produce a slope change are eliminated from the actual test data. Any load points eliminated from the actual test data are also eliminated from the predicted test data. This filtering process is performed simultaneously for each control point. While filtering, the spectrum generator also identifies invalid actual test results. Invalid results include spikes and dropouts. When the actual test results exceed the bounds  318  and  320  or deviate from the predicted test results by more than 50% and the actual test results are greater than 10% of the upper bound  318 , the results are eliminated from both the actual test results and the predicted test results. Actual and predicted test results that are output by the filtering portion of the spectrum generate are immediately used in a cycle counting process which pairs load peaks and valleys. In one particular embodiment, the cycle counting process operates by the generally-known rules of rainflow cycle counting. The cycle counting is performed simultaneously on all control points. When the cycle counting process identifies a paired peak and valley, at a block  114 , a control point spectra portion performs a calculation of notch stress and strain based on the applied loads and the elastic KT. In one embodiment, for example, the crack initiation engine  110  implements the nominally elastic Neuber&#39;s Equation to determine notch stress and strain. The nominally elastic Neuber&#39;s Equation is presented below as Equation (1) in its basic form:
 
 K   T   2   =K   σ   ×K   ε ,  (1)
 
where:
 
   K T  is the elastic stress concentration factor; 
   K σ  is the plastic stress concentration factor, and 
   K ε  is the plastic strain concentration factor. 
   Next, a crack initiation analysis is performed at a block  116 . In one embodiment, the crack initiation analysis uses an Equivalent Strain Equation, a correction of the calculated notch strain for load cycle stress ratio (R) effects. In one particular embodiment, the crack initiation engine  110  implements the Smith, Watson, and Topper equation to analytically account for mean stress effects. The Smith, Watson, and Topper equation is: 
   
     
       
         
           
             
               
                 
                   
                     
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   At a block  118 , a summation of the calculated damage and the failure criteria is then computed. In one particular embodiment, the crack initiation engine  110  uses the Palmgren and Miner&#39;s Rule to sum the damage associated with continued load cycling. Palmgren and Miner&#39;s Rule, states: 
   
     
       
         
           
             
               
                 
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   In one particular embodiment, crack initiation is assumed to occur when the total damage is equal to 1.0. Crack initiation failure is typically assumed to be the development of a 0.01-inch flaw, although in various embodiments, other crack initiation failure thresholds may be employed. 
   As shown in  FIG. 1 , the calculation of fatigue damage (block  118 ) is performed by the crack initiation engine (block  110 ) using both the tested actuator loads (block  106 ) and the target actuator loads (block  108 ). As shown in  FIG. 5 , these fatigue damage calculations are output to the user as the actual test data  412  in the predicted test data  420 , respectively, for comparison purposes. The ratio  422  ( FIG. 5 ) is the actual test data  412  over the predicted test data  420 . 
   Finally, the method  100  includes comparison of the fatigue damage based on the actual test data  412  with the fatigue damage based on the predicted test data  420  at a block  120 . For example,  FIG. 6  is a representative plot  600  of damage versus time for comparing the actual and predicted fatigue damages (block  120 ). As shown in  FIG. 6 , a set of actual damage data  602  is compared with a set of predicted damage data  604 . 
   Because the method  100  may be performed in substantially real-time during a structural test, the actual damage data  602  may be rapidly compared with the predicted damage data  604  which may be useful for validation of the test data, and may provide an immediate indication of how variances in the applied actuator loads are affecting the severity of the test (i.e., undertesting or overtesting). In a presently preferred embodiment, the method  100  utilizes a spreadsheet program (e.g., Microsoft EXCEL) that enables test set parameters ( FIG. 2 ) to be rapidly changed, and the results of such changes may be immediately determined and assessed by viewing the computation results (block  120 ). In some embodiments, various portions of the method  100  shown in  FIG. 1  are performed simultaneously for all control points (sensors) on the test article. In one specific embodiment, for example, the crack initiation portion  110  of the method  100  is performed simultaneously for all control points. 
   Embodiments of methods and systems in accordance with the present invention may provide significant advantages over the prior art. In conventional structural testing, the massive quantities of data necessary to evaluate spectrum severity of applied to full-scale test article were collected and recorded during a structural test by data acquisition system, reformatted, and transferred to a separate computing system for analysis and interpretation by structural engineers. During the analysis interpretation, the structural engineers evaluate whether the test data were valid, and if so, would then calculate the spectrum severity. The structural engineers would analyze each control point independently, essentially repeating the same task several times. If the test data were not valid, appropriate corrections were made and the structural testing would be repeated, requiring considerable time and expense. However, methods and systems in accordance for the present invention enable analysis of the structural test data in real-time during a test. Based on the comparison at block  120 , the test engineers may make appropriate adjustments to the applied actuator loads or to other variables involved in the test, or may be assured that the test setup is providing valid and accurate test results. Embodiments of the present invention provide the capability to calculate and compare crack initiation damage real-time, enabling test personnel to evaluate the effects of applied test load variances and make necessary adjustments while conducting a test. Thus, methods and systems in accordance with the present invention are efficient, inexpensive, and robust, and advantageously reduce the time and expense associated with providing accurate structural fatigue test results. 
   While preferred and alternate embodiments of the invention have been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of these preferred and alternate embodiments. Instead, the invention should be determined entirely by reference to the claims that follow.