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A system for detecting the onset of structural failure in a structural element subject to a mechanical load comprises a metering array, a signal processor, and an output processor. The metering array measures physical quantities associated with the structural element. The signal processor transforms

A system for detecting the onset of structural failure in a structural element subject to a mechanical load comprises a metering array, a signal processor, and an output processor. The metering array measures physical quantities associated with the structural element. The signal processor transforms the measured physical quantities into a series of sample mode spectra, and the output processor generates output as a function of the series of sample mode spectra. Methods are also disclosed for detecting the onset of failure in a structural element subject to a mechanical load, for structural testing, and for structural health monitoring.

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The invention claimed is: 1. A system for detecting onset of structural failure in a structural element subject to a mechanical load, the system comprising: a metering away positioned for providing measurements of physical quantities associated with the structural element; a signal processor for tr

The invention claimed is: 1. A system for detecting onset of structural failure in a structural element subject to a mechanical load, the system comprising: a metering away positioned for providing measurements of physical quantities associated with the structural element; a signal processor for transforming the measurements into a series of sample mode spectra characterizing a natural oscillation frequency of the structural element; and an output processor for generating an output indicating the onset of structural failure as a function of a shift in the natural oscillation frequency near a critical point of the structural element, wherein the shift in the natural oscillation frequency comprises a shift toward zero frequency near the critical point. 2. The system of claim 1, wherein the structural element comprises at least one of a beam, post, pipe, wall, pressure vessel, vane, blade, or housing, and the natural oscillation frequency is subsonic. 3. The system of claim 2, wherein the structural element is a composite structure, comprising a plurality of structural elements. 4. The system of claim 2, wherein the mechanical load comprises at least one of a compressive, stress, strain, tension, torsion or pressure load, and wherein the shift in the natural oscillation frequency near the critical point depends upon the mechanical load. 5. The system of claim 1, wherein the metering away includes at least one of a position sensor, velocity sensor, accelerometer, angular sensor, stress gauge, strain gauge, subsonic sensor, audio sensor, ultrasonic sensor, laser vibrometer, optical sensor, temperature sensor, or pressure sensor. 6. The system of claim 1, wherein the natural oscillation frequency is a lowest natural frequency of oscillation for the structural element. 7. The system of claim 1, wherein the signal processor transforms the measurements into the series of sample mode spectra via a fast Fourier transform. 8. The system of claim 1, wherein the shift in the natural oscillation frequency comprises a divergence from a loaded frequency curve near the critical point. 9. The system of claim 8, wherein the divergence is with respect to an analytical model of the loaded frequency curve. 10. The system of claim 1, wherein the output characterizes the natural oscillation frequency with respect to time and the output comprises a derivative of the series of sample mode spectra. 11. The system of claim 1, additionally comprising a driving force element mechanically coupled to the structural element. 12. The system of claim 1, additionally comprising a load controller coupled to the mechanical load. 13. The system of claim 12, wherein the system comprises a portion of a structural testing apparatus. 14. The system of claim 1, wherein the system comprises a portion of a structural health monitor. 15. A method for detecting onset of structural failure in a structural element subject to a mechanical load, the method comprising: measuring physical quantities associated with the structural element; transforming the measured physical quantities into a series of sample mode spectra characterizing a subsonic natural frequency of oscillation of the structural element; and generating an alarm as a function of a variation in the series of sample mode spectra near a critical point of the structural element, wherein the variation in the series of sample mode spectra comprises a shift in the subsonic natural frequency of oscillation toward zero frequency near the critical point. 16. The method of claim 15, wherein the measured physical quantities are representative of at least one of position, velocity, acceleration, angle, stress, strain, tension, torsion, vibrational frequency, temperature or pressure. 17. The method of claim 15, wherein the measured physical quantities are transformed via one of a fast Fourier transform or a wavelet transform. 18. The method of claim 15, wherein the series of sample mode spectra characterize a fundamental mode of oscillation of the structural element and a higher-order mode of oscillation of the structural element. 19. The method of claim 15, wherein the variation in the series of sample mode spectra comprises an approach to an unbounded period of oscillation near the critical point. 20. The method of claim 19, wherein the variation in the series of sample mode spectra comprises an approach to an unbounded slope in frequency near the critical point. 21. The method of claim 19, wherein the variation in the series of sample mode spectra comprises a variation with respect to a series of baseline spectra. 22. The method of claim 19, wherein the variation in the series of sample mode spectra comprises a derivative of the series of sample mode spectra. 23. A method for structural testing of a structural element subject to a mechanical load, the method comprising: measuring physical quantities associated with the structural element; transforming some of the measured physical quantities into a series of sample mode spectra characterizing a natural frequency of a fundamental mode of oscillation of the structural element; generating an output as a function of a variation in the series of sample mode spectra near a critical point of the structural element, wherein the shift in the series of sample mode spectra comprises a shift in the natural frequency of the fundamental mode of oscillation toward zero frequency near the critical point; and controlling the mechanical load to detect onset of structural failure in the structural element, as a function of the output. 24. The method of claim 23, wherein the mechanical load comprises at least one of a compressive, stress, strain, tension, torsion, or pressure load, and wherein the shift in the natural oscillation frequency near the critical point depends upon the mechanical load. 25. The method of claim 23, wherein transforming some of the measured physical quantities additionally comprises transforming some of the measured physical quantities into a series of baseline spectra characterizing the fundamental mode of oscillation. 26. The method of claim 25, additionally comprising: adjusting a set of sampling characteristics according to the baseline spectra, in order to provide increased sensitivity to the onset of structural failure in the structural element. 27. The method of claim 26, wherein the set of sampling characteristics comprises at least one of scale sensitivity, sampling period, integration time, or transformation window. 28. The method of claim 23, wherein the method is performed as part of a non-destructive testing process in which controlling the mechanical load is limited as a function of the output, in order to prevent structural failure of the structural element. 29. The method of claim 23, wherein the method is performed as part of a destructive testing process in which the output characterizes the onset and progress of structural failure in the structural element. 30. The method of claim 23, wherein the method is performed as part of a periodic inspection program. 31. The method of claim 30, wherein the structural element is a structural element of an aircraft. 32. The method of claim 23, wherein the output comprises an alarm based on the variation in the series of sample mode spectra. 33. The method of claim 32, wherein the alarm comprises at least one of an audible, visual, or electronic alarm. 34. The method of claim 32, wherein the alarm comprises an electronic signal to limit controlling the mechanical load. 35. A method for structural health monitoring, the method comprising: measuring physical quantities associated with a composite structural element; transforming the measured physical quantities into a series of sample mode spectra characterizing a natural frequency of oscillation of the composite structural element; and generating an output as a function of the series of sample mode spectra, wherein the output comprises an alarm based upon an alarm-generating function of the sample mode spectra near a critical point of the composite structural element, and wherein the alarm-generating function comprises a shift in the natural frequency of oscillation toward zero frequency near the critical point. 36. The method of claim 35, wherein the alarm-generating function comprises a derivative of the natural frequency of oscillation near the critical point. 37. The method of claim 35, additionally comprising: calibrating the structural health monitoring method by adjusting a sampling characteristic using calibration data, in order to provide increased sensitivity to onset of structural failure in the composite structural element. 38. The method of claim 35, wherein the alarm-generating function comprises a divergence of the natural frequency of oscillation from a loaded frequency curve near the critical point.