Abstract:
A wireless, non-powered fatigue monitoring sensor uses a piezoelectric element that is attached to the surface of or embedded within a structure to be monitored. When subjected to stress over time, the material properties of the element change reliably and permanently. These properties are used to determine the fatigue history of the structure. The monitoring device requires no power for monitoring and is nondestructively queried to determine the stress history using wireless means such as radio frequency technology.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates in general to monitoring stress in a structure and, in particular, to an improved system, method, and apparatus for a wireless, non-powered stress history and fatigue monitoring of a structure.  
         [0003]     2. Description of the Related Art  
         [0004]     Many structures, especially in the aerospace industry, must be monitored over time for overload and/or fatigue damage. Typically, this is done in one of two ways. First, a structure may be statistically evaluated for stress or fatigue. With this method the exact number of cycles is not known, but it does offer a prediction of what is typical for a certain amount of running time or flights. Second, active strain gages and/or accelerometers, for example, may be used to count cycles. Unfortunately, the first method is approximate, while the second method is costly and not feasible for most in-service applications due to wiring and power requirements. Some practitioners have proposed “power harvesting” for such applications whereby energy is harvested from vibrations, solar, RF, thermal, or other power sources. Although that solution would eliminate the power wiring requirements or batteries, the proposition is very challenging for continuous operation while a structure is in service.  
         [0005]     Examples of externally powered prior art solutions include U.S. Pat. No. 4,433,581 to Scott, which describes strain gages that are powered by conventional means. U.S. Pat. No. 5,520,055 to Fussinger shows a device that has notches that break upon a sufficient number of cycles of fatigue. U.S. Pat. No. 5,531,123 to Henkel uses a fatigue monitor that must be physically removed from the underlying structure and then inspected by viewing striations formed in the monitor due to the fatigue. U.S. Pat. No. 6,014,896 to Schoess discloses a piezoelectric device that emits an acoustic wave and is battery powered. U.S. Pat. No. 6,928,881 to Brennan discloses a power detector with strain gages and a memory device with data storage. Finally, U.S. Patent Application No. 20040078662 to Hamel describes the energy harvesting concept for wireless sensors. Although each of these designs is workable, an improved solution for monitoring the fatigue of components while they are in service and can remain in service after inspection would be desirable.  
       SUMMARY OF THE INVENTION  
       [0006]     One embodiment of a system, method, and apparatus for a wireless, non-powered, non-destructive fatigue monitoring sensor is disclosed. The present invention comprises, for example, a piezoelectric element that is attached to the surface of or embedded within a structure to be monitored. When subjected to stress over time, the electromechanical properties of the element change reliably and permanently. The change in these properties is then used to determine the fatigue history of the structure. The monitoring device requires no power for monitoring and is queried to determine the stress history using wireless means such as radio frequency (RF) technology. Moreover, the monitoring device is not required to be removed from the underlying structure.  
         [0007]     In one embodiment, the sensor element is bonded to the surface of the structure being monitored. Before service, the electromechanical behavior of the element is calibrated. This behavior may be measured by, for example, its piezoelectric properties, components and/or magnitude of the admittance or impedance, or by its piezoelectric coupling. Once recorded, the structure is put into service and subjected to cycles of stress. When loaded, the electromechanical properties of the monitoring device degrade, thus changing its measured characteristics (e.g., impedance, admittance, etc.). High stresses produce a large change and provide a measure of overload. Time at elevated stress also degrades the properties and provides a measure of fatigue.  
         [0008]     After a period of service, the material is queried to measure its new electromechanical properties by a remotely powered device, such as an RF circuit. The change in these properties is related to the stress history of the monitoring element and, therefore, the host structure. With the stress history known, a prediction of residual life is determined. In addition, multiple piezoelectric elements that are tuned to respond to different stress levels can be attached to the host structure to provide better stress history fidelity.  
         [0009]     The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     So that the manner in which the features and advantages of the invention, as well as others which will become apparent are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only an embodiment of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.  
         [0011]      FIG. 1  illustrates plots of a time-dependent property of a material over different stress ranges and mechanical cycles;  
         [0012]      FIG. 2  illustrates plots of a time-dependent admittance property of a material after various mechanical cycles at constant stress;  
         [0013]      FIG. 3  is a schematic side view of one embodiment of a device mounted to a structure and is constructed in accordance with the present invention;  
         [0014]      FIG. 4  is a schematic side view of another embodiment of a device mounted to a structure and is constructed in accordance with the present invention; and  
         [0015]      FIG. 5  is one embodiment of a method constructed in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     Piezoelectric materials have time-dependent properties that may be employed for use by monitoring them as degradation mechanisms. For example, with respect to aging, there is a loss in mobility of the domain walls as they become pinned by structural inhomogeneities (e.g., pores and cracks) and charge carriers (e.g., vacancies and dopants). Above a certain field strength, domain wall movement can occur. Degradation starts above this threshold and increases with field due to more extensive domain wall motion. With regard to stress level, the degradation also occurs in a similar fashion as that under high field strength as the domain walls are unable to respond quickly at higher frequencies. Thus, the aging rate is less than at lower frequencies. Depoling involves the net polarization direction of the material with respect to the poled direction being lost, because the domains “switch” (i.e., change their orientation) to become more random. Depoling is affected by temperature, stress, and field. In addition, microcracking and failure of electrode/ceramic interface may be evaluated as a degradation mechanism.  
         [0017]     For example,  FIG. 1  depicts various plots that illustrate a time-dependent, piezoelectric property of a material over different stress ranges and mechanical cycles. The material selected may comprise a piezoelectric (e.g., ferroelectric) material. At a stress level plot  11  at 13 MPa, almost negligible degradation of the material property is observed. The same is true for stress level plot  13  at 25 MPa. However, at plots  15 ,  17  at 64 MPa and 128 MPa, respectively, a significant degradation of the property is observed.  
         [0018]     Another example of the degradation characteristic is depicted in  FIG. 2 . A series of plots  21 - 26  illustrate the time-dependent admittance property of a material as a function of frequency after various mechanical cycles at constant stress. At zero cycles, the baseline response of the material is captured by plot  21 . As the number of cycles at constant amplitude stress in the material accumulates, the peak admittance of the sensor element degrades as depicted by plots  22 - 26 . As presented in the figure legend, the piezoelectric coupling ratio “k” also degrades as a function of the number of cycles. Either of these quantities can be related to the fatigue history of the structural element.  
         [0019]     Referring now to  FIG. 3 , one embodiment of an apparatus and system for analyzing structural stress and/or monitoring fatigue history of a structure is shown. A structure  31  such as, for example, a wing skin, a spar, a rib, an engine cowling, etc., is monitored over time to assess its on-going service life and determine whether it is in need of repair or replacement. A device  33  having an element  35  is mounted to the structure  31 . In the embodiment shown, the structure  31  forms a portion of a component  37  (e.g., an aircraft). In this version, the device  33  is mounted inside the component  37  such that the device  33  (and element  35 ) are physically inaccessible from an exterior of the component  37 .  
         [0020]     The element  35  of device  33  may comprise, for example, a piezoelectric element, and has a material property that degrades over time due to stress cycling of the structure  31  during service operation of the structure  31 . The property may comprise an electromechanically coupled property such as impedance, admittance, piezo strain coefficients, piezo coupling, and complex portions of the impedance or admittance. The element  35  may comprise, for example, a thin film, a sheet, a beam, or a rod, and may be, for example, bonded to the structure  31  or embedded inside the structure  31 .  
         [0021]     A sensor  41  is coupled to the element  35  for detecting a condition of the property of the element  35 . No portion of device  33 , including the sensor  41  and the element  35  have an internal source of power. Alternatively, an internal power source, such as a battery, may be provided depending on the application. In one embodiment, the sensor  41  comprises an impedance analyzer that scans a frequency range with respect to the element  35 , an antenna  43  for enhancing communication with a detector  45 , and a microprocessor  47  for controlling the sensor  41 . The detector  45  provides wireless power to and wirelessly communicates with the sensor  41  to detect the condition of the property of the element  35  and thereby ascertain a level of fatigue and prediction of residual life of the structure  31 . As loads are applied over time to the structure  31 , stress and strain are generated. In effect, the load history of the structure  31  also is monitored. In the embodiment shown, the detector  45  is non-invasive, non-destructive, and remote relative to the element  35  and the sensor  41 . For example, the sensor  41  may be inductively powered by the detector  45  and use radio frequency to communicate with the sensor  41 .  
         [0022]     Referring now to  FIG. 4 , another embodiment of a device  51  constructed in accordance with the present invention is shown. For simplicity other components, such as the sensor, antenna, and microprocessor, are not shown. In this version, the element comprises a plurality of elements  53 ,  55 . Although only two elements are shown, many more elements may be used depending on the application. Each element  53 ,  55  has a different physical property than the others to increase a fidelity and sensitivity to fatigue cycle prediction of the structure  57 .  
         [0023]     In the embodiment shown, a first fixture  59  is secured to the structure  57 . A second fixture  61  is mounted to the structure  57 , spaced apart from the first fixture  59  and displaceable relative to the first fixture  59 . The elements  53 ,  55  extend between the first and second fixtures  59 ,  61 . As structure  57  (and, thus, device  51 ) experience stress during operational service of structure  57 , the elements  53 ,  55  and second fixture  61  are displaced in length (see arrows  63 ). One of the elements  53 ,  55  is sensitive to low stresses, and another one of the elements  53 ,  55  is sensitive to high stresses. For example, element  53  may have a fatigue life of 0 to 10,000 cycles, and element  55  may have a fatigue life of 50,000 cycles. In this way the fidelity of the measurements monitored by the detector (described above) is improved. Device  51  and its system otherwise operates in the same manner and has the same advantages as described above for device  33  and its system.  
         [0024]     The present invention also comprises a method of monitoring structural stress. In one embodiment ( FIG. 5 ), the method begins as indicated at step  101 , and comprises mounting a device (e.g., with a unique radio frequency identifier) to a structure (step  103 ); querying the device for a baseline condition of the device (step  105 ); saving the baseline condition (step  107 ); placing the structure in service such that the structure and device experience stress cycles (step  109 ); querying the device after a period of service (step  111 ); determining whether the device and, therefore, the structure have an acceptable fatigue history (step  113 ); continuing service of the structure if the fatigue history is acceptable (step  115 ); and discontinuing service of the structure if the fatigue history is unacceptable (step  117 ); before ending as indicated at step  119 .  
         [0025]     Alternatively, step  103  may comprise bonding the device to a surface of the structure or embedding the device inside the structure. Steps  105 ,  111  may be performed wirelessly, non-intrusively, non-destructively, and remotely, and may be performed without providing the device with a power source such that wireless power is provided to the device from an external source. Step  109  may comprise degrading a property of the device over time due to stress cycling of the structure during service.  
         [0026]     In addition, the method may further comprise providing the device with a piezoelectric element and monitoring a property of the piezoelectric element; inductively powering the device and using radio frequency to communicate with the device; and/or increasing a fidelity and sensitivity to fatigue cycle prediction of the structure via different physical properties of the device. Moreover, step  117  may comprise repairing or rebuilding the structure.  
         [0027]     The present invention has several advantages, including no requirement for power to record the stress history of the host structure. The query to determine fatigue of the structure is externally powered and has an external control circuit so that the structure is wirelessly investigated for measurements and communications by using, for example, RF power harvesting to power the circuit that measures and transmits the new electromechanical properties of the monitoring element. In addition, the monitoring element may be scaled down to micro-electromechanical systems (MEMS) for minimal intrusiveness. The multiple elements embodiment may be used to tune the device to respond to different stress histories of the host structure while making the device sensitive to a larger range of stresses and cycles.  
         [0028]     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.