System and related method for detecting aircraft structural damage

Abstract

A system for detecting damage to an aircraft structure and an associated method include an aircraft structure and a structural health monitoring (SHM) system for monitoring the health of the aircraft structure independently of temperature. The SHM system includes an actuator coupled to the aircraft structure and configured to generate a reference vibration signal having a reference amplitude that propagates through the aircraft structure at a first time, and the actuator generates a comparison vibration signal having a comparison amplitude that propagates through the aircraft structure at a second time after the first time. The comparison amplitude represents damage caused by the aircraft structure between the first time and the second time. The SHM system includes a sensor coupled to the aircraft structure that receives the reference vibration signal and the comparison vibration signal, and also includes a processor that is configured to calculate a gain damage index as a function of the reference amplitude divided by the comparison amplitude.

Description

Technical Field

[0001] The field of the present disclosure relates generally to structural health monitoring (SHM), and more particularly to systems and methods for temperature-insensitive damage detection. Background Art

[0002] At least some SHM systems detect damage in a structure, for example, by exciting a piezoelectric transducer or actuator coupled to the structure to generate a vibration signal or wave, and then receiving the vibration signal using another piezoelectric transducer coupled to the structure at a different location. Damage present in the structure in the path of the propagating wave affects the characteristics of the propagating wave, such as, for example, amplitude and phase. Therefore, the amplitude and phase of the received vibration signal (comparison signal) are compared with the amplitude and phase of the previously received vibration signal (reference signal), so that damage that occurred during the duration between the reference signal and the comparison signal can be detected. In addition, this comparison enables the detection of damage at a location along the path of the propagating wave in the structure.

[0003] The propagation of waves through the structure is also affected by environmental parameters, including, for example, the ambient temperature of the structure in which the SHM system operates. Typically, the amplitude of the vibration signal propagating through the structure increases at warmer temperatures and decreases at cooler temperatures. The ambient temperature can further change the phase of the propagating waves or cause them to shift. Such environmental effects can mask damage in the structure, i.e., false negatives, or at least interfere with its detection. Conversely, environmental effects can produce false positive damage detections.

[0004] At least some SHM systems utilize algorithms or other signal processing to quantify the damage in a given structure to be tested and mitigate the effects of environmental disturbances such as temperature disturbances. The quantification of damage is referred to as a damage index (DI). This quantification enables regular monitoring of the structural health of a given structure over time. One such algorithm for calculating the DI is to calculate the root mean square (RMS) of the "error" signal, or in other words, to calculate the RMS of the difference between the comparison signal and the reference signal. There are various methods for mitigating environmental disturbances. For example, one method is to collect reference signals at various temperatures and select appropriate reference signals to compare with the comparison signal. However, this method is time-consuming and requires coolers, ovens, and other environmental temperature control devices to produce various temperatures within a temperature range, such as from 13 degrees Fahrenheit to 120 degrees Fahrenheit. Another example method is to use a calculated correlation between the comparison signal and the reference signal, which mitigates at least some amplitude disturbances, but can mask the detection of damage. Yet another exemplary method is to measure and correct the phase shift caused by environmental disturbances, which can be found in U.S. Patent No. 8,892,384. However, there is a need for a SHM system that generates a DI that mitigates the effects of temperature disturbances while reducing false positives and false negatives.

[0005] This background technology section is intended to introduce the reader to various aspects of technology that may be relevant to the present disclosure, which is described and / or claimed below. It is believed that this discussion helps to provide the reader with background information to facilitate a better understanding of various aspects of the present disclosure. Therefore, it should be understood that these statements should be read from this perspective, and not as admissions of prior art. Summary of the invention

[0006] One aspect relates to a system for detecting damage in an aircraft structure. The system includes an aircraft structure and a structural health monitoring (SHM) system having an actuator, a sensor, and a processor. The actuator is coupled to the aircraft structure and is configured to generate a reference vibration signal having a reference amplitude that propagates through the aircraft structure at a first time. The actuator is also configured to generate a comparison vibration signal having a comparison amplitude that propagates through the aircraft structure at a second time after the first time. The comparison amplitude represents damage caused to the aircraft structure between the first time and the second time. The sensor is coupled to the aircraft structure and is configured to receive the reference vibration signal and the comparison vibration signal. The processor is configured to calculate a gain damage index as a function of the reference amplitude divided by the comparison amplitude. The gain damage index enables the SHM system to monitor the structural health of the aircraft structure independently of temperature.

[0007] Another aspect is a method of detecting damage in a structure. The method includes retrieving a reference vibration signal of the structure from a memory, wherein the reference vibration signal was previously collected. The method includes exciting a first transducer coupled to the structure to generate a comparison vibration signal propagating through the structure. The method includes receiving the comparison vibration signal at a second transducer coupled to the structure. The method includes executing a gain damage index algorithm using the reference vibration signal and the comparison vibration signal to calculate a gain damage index as a function of an amplitude of the reference vibration signal multiplied by an amplitude of the comparison vibration signal divided by the square of the amplitude of the comparison vibration signal. The method includes identifying damage in the structure when the gain damage index is positive.

[0008] Another aspect includes a structural health monitoring (SHM) system having a plurality of transducers, a data acquisition (DAQ) circuit, and a processor. The plurality of transducers are configured to be distributed over an area of ​​a structure and coupled to the structure. The plurality of transducers include a first transmitting transducer and at least one sensor transducer. The DAQ circuit is coupled to the plurality of transducers and configured to actuate the first transmitting transducer to generate a reference vibration signal propagating through the structure at a first time. The DAQ circuit is configured to receive the reference vibration signal via at least one sensor transducer. The DAQ circuit is configured to actuate the first transmitting transducer to generate a comparison vibration signal propagating through the structure at a second time after the first time. The DAQ circuit is configured to receive the comparison vibration signal via at least one sensor transducer. The processor is configured to calculate a corresponding gain damage index of at least one receiving transducer as a function of the amplitude of the reference vibration signal divided by the amplitude of the comparison vibration signal, the gain damage index enabling the SHM system to monitor the structural health of the aircraft structure independently of temperature.

[0009] There are various improvements on the features mentioned in the above aspects. Other features may also be included in the above aspects. These improvements and additional features may exist alone or in any combination. For example, the various features discussed below with respect to any illustrated embodiment may be incorporated into any of the above aspects alone or in any combination. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a schematic block diagram of an example SHM system;

[0011] Figure 2A is a graph of an example reference vibration signal from a SHM system;

[0012] Figure 2B Graphs comparing vibration signals for examples from a SHM system;

[0013] Figure 2C For respectively from Figure 2A and 2B A graph of an example error signal of a reference vibration signal and a comparison vibration signal shown in;

[0014] Figure 3A and 3B a series of graphs of example reference vibration signals and example comparative vibration signals collected at various temperatures; and

[0015] Figure 4 A flow chart of an example method for detecting damage in a structure.

[0016] Although specific features of various embodiments may be shown in some drawings and not in others, this is for convenience only. Any feature of any drawing may be referenced and / or claimed in combination with any feature of any other drawing.

[0017] Unless otherwise indicated, the drawings provided herein are intended to illustrate features of the embodiments of the present disclosure. It is believed that these features are applicable to various systems including one or more embodiments of the present disclosure. Therefore, the drawings are not intended to include all conventional features known to those of ordinary skill in the art for implementing the embodiments disclosed herein. DETAILED DESCRIPTION

[0018] Embodiments of the described system include a structural health monitoring (SHM) system that calculates a gain damage index (DI) that is capable of detecting damage in a structure (such as, for example, an aircraft structure) independently of temperature. The SHM system described herein includes a plurality of transducers distributed over a structural area and bonded to a surface of the structure. Each transducer is configured to be excited to generate a vibration signal that propagates through the structure and is then received at each of the other transducers in the plurality of transducers. When excited, the transducer is referred to as an actuator or transmitter. Therefore, the other transducers in the plurality of transducers are referred to as sensors or receivers. The actuator transducer first generates a reference vibration signal that propagates through the structure and is received by at least one sensor transducer, collected by a data acquisition (DAQ) circuit, and stored in a memory. Then, the actuator transducer later generates a comparison vibration signal that propagates through the structure and is received by the sensor transducer and collected by the DAQ circuit. Then, a processor calculates a gain DI as a function of the amplitude of the reference vibration signal divided by the amplitude of the comparison vibration signal. More specifically, the processor calculates respective gains DI for a plurality of sensor transducers, each gain DI being calculated as a function of the amplitude of a reference vibration signal multiplied by the amplitude of a comparison vibration signal divided by the square of the amplitude of the comparison vibration signal. When the gain DI is positive, damage that occurred during a duration between the time the reference vibration signal was generated and the time the comparison vibration signal was generated is identified. A positive gain generally refers to a gain greater than 1. In certain embodiments, the gain DI is normalized to zero by subtracting 1 from the gain value, such that a "positive gain" instead refers to a gain value greater than zero. Each gain DI value represents a quantification of the health state of a structure (e.g., a metallic structure or a composite structure) along a path between an actuator transducer and a sensor transducer for that gain DI value. Thus, the quantification enables periodic monitoring of the structural health of the structure over time.

[0019] Figure 1Schematic diagram of an exemplary SHM system 100 for detecting damage (such as cracks or delamination) in a structure 102. The SHM system 100 includes a plurality of transducers 104 bonded to a surface of the structure 102. The transducers 104 are distributed over an area of ​​the structure 102 and are arranged, for example, in an array 106 or any other suitable arrangement for testing the structure 102. For example, for an aircraft structure, the transducers 104 may be arranged so that they are concentrated on a high stress area of ​​a given aircraft structure, which may be composed of, for example, a composite material or a metallic material having multiple layers, or a stack. The transducers 104 may include a ceramic piezoelectric transducer, such as, for example, a lead zirconate titanate (PZT) transducer. When a voltage is applied to one of the transducers 104 bonded to the structure 102, the transducer is actuated and generates a vibration or vibration signal that propagates through the structure 102. The resulting waveform exhibits some amplitude, phase, and frequency. In the embodiments of the SHM system 100 described herein, the transducer 104 generally generates a vibration signal having a frequency in the range of about 200 kHz to about 450 kHz. However, in alternative embodiments, transducers 104 that generate vibration signals outside of this range may be utilized and are within the scope of the present disclosure.

[0020] The transducers 104 are coupled to the DAQ circuit 108 via a wiring harness 110. The DAQ circuit 108 may include a plurality of analog input / output channels for applying or measuring electrical potentials on the transducers 104. For example, the measured electrical potentials may be converted to digital values ​​by one or more analog-to-digital converters within the DAQ circuit 108. The DAQ circuit 108 controls the excitation of the transducers 104 to generate vibration signals and receives vibration signals through the sensor transducers 104. For example, more specifically, the DAQ circuit 108 selects the actuator transducer 104a to operate as a transmitter, while the other transducers 104 operate as receivers, or are referred to as sensor transducers 104b and 104c, for example. The DAQ circuit 108 applies a voltage to the actuator transducer 104a, which causes the generation of a reference vibration signal, which propagates through the structure 102 along paths 112 and 114, for example, leading to the sensor transducers 104b and 104c, respectively. Sensor transducers 104b and 104c receive the reference vibration signal, thereby generating a potential or voltage at the terminals of the device. The voltage present over time at a given sensor transducer represents the reference vibration signal of that sensor transducer, and more specifically, represents the reference vibration signal of the corresponding path between the actuator transducer and the sensor transducer, such as, for example, paths 112 and 114 between actuator transducer 104a and sensor transducers 104b and 104c, respectively.

[0021] The SHM system 100 includes a processor 116 and a memory 118. The processor 116 executes a series of computer executable instructions, program codes, or software stored in a memory, such as the memory 118. By executing such program codes, the processor 116 is configured to perform various steps of calculating the gain DI. For example, when executing the program codes stored in the memory 118, the processor 116 is configured to receive a reference vibration signal from the DAQ circuit 108. The processor 116 is also configured to write the reference vibration signal to the memory 118. In some embodiments, the processor 116 may process the reference vibration signal before writing to the memory 118. In an alternative embodiment, the reference vibration signal may be written directly from the DAQ circuit 108 to the memory 118.

[0022] DAQ circuit 108 controls transducer 104 so that a reference vibration signal is generated and collected at a first time, for example, when the structure is new or otherwise known to be undamaged. DAQ circuit 108 controls transducer 104 to generate a comparison vibration signal at a second time after the first time. SHM system 100 can be used periodically to test the structural health of structure 102. For example, the duration between collecting reference vibration signals and comparison vibration signals can be one or more years of operation. Alternatively, the duration can be much shorter, for example, on the order of days, weeks, or months. SHM system 100 is able to detect damage independently of the duration between collections or "sweeps". Such detection is limited to the duration between the collection of reference vibration signals and comparison vibration signals. The duration is typically selected based on expected wear and fatigue over the life cycle of structure 102. In some embodiments, transducer 104 remains in place, coupled to structure 102, for the duration between collections. In other embodiments, transducer 104 is coupled to structure 102 for collecting reference vibration signals and thereafter removed for normal operation or use of structure 102. Then, the sensor 104 is again coupled to the structure 102 at a later time to collect the comparison vibration signal. Generally, the transducer 104 should be placed in approximately the same position for collecting the reference vibration signal and the comparison vibration signal.

[0023] Figure 2A For SHM systems such as Figure 1 2 and an example reference vibration signal 202 for a given structure to be tested. The reference vibration signal 202 is plotted as an amplitude 204 over time 206. The amplitude 204 is expressed in volts, and the reference vibration signal 202 ranges from about 1 volt to about -1 volt over a duration of about 25 microseconds. The reference vibration signal 202 has a frequency of about 200 kilohertz (or about a 5 microsecond period).

[0024] Reference again Figure 1, to collect the comparative vibration signal, the DAQ circuit 108 selects the actuator transducer 104a to operate as a transmitter, while the other transducers 104 operate as receivers, or, for example, sensor transducers 104b and 104c. The DAQ circuit 108 applies a voltage to the actuator transducer 104a, which causes the generation of a comparative vibration signal, which propagates through the structure 102 along, for example, paths 112 and 114 to the sensor transducers 104b and 104c, respectively. The sensor transducers 104b and 104c receive the comparative vibration signal, thereby generating a potential or voltage at the terminals of the device. The voltage present over time on a given sensor transducer represents the comparative vibration signal of the sensor transducer, and more specifically, represents the comparative vibration signal of the corresponding path between the actuator transducer and the sensor transducer, such as, for example, paths 112 and 114 between the actuator transducer 104a and the sensor transducers 104b and 104c, respectively.

[0025] Figure 2B For SHM systems such as Figure 1 2 and a graph of an example comparative vibration signal 208 for a given structure to be tested, for the SHM system 100 shown in FIG. 2 . The comparative vibration signal 208 is plotted as an amplitude 204 over a time 206. The amplitude 204 is expressed in volts, and the comparative vibration signal 208 ranges from about 1.1 volts to about -1.1 volts over a duration of about 25 microseconds. The comparative vibration signal 208 has a frequency of about 200 kilohertz (or about a 5 microsecond period). It is noted that the amplitude and phase of the comparative vibration signal 208 are relative to the Figure 2A The reference vibration signal 202 shown in FIG. 1 is distorted.

[0026] Reference again Figure 1 , damage that occurs within the structure 102 during the duration between collections is represented in the comparison vibration signal as changes in the amplitude and phase of the comparison vibration signal relative to the corresponding reference vibration signal. In addition, changes in amplitude and phase may also be caused by differences in ambient temperature when the reference vibration signal is collected and when the comparison vibration signal is collected.

[0027] Figure 2C For respectively from Figure 2A and 2B2 and 208. The error signal 210 is plotted as an amplitude 204 over time 206. The amplitude 204 is expressed in volts, and the error signal 210 ranges from about 0.25 volts to about -0.25 volts over a duration of about 25 microseconds. The error signal 210 represents a rough DI, where larger amplitudes generally indicate larger damage. It is noted that the error signal 210 does not take into account, for example, temperature changes between the collected reference vibration signal 202 and the comparative vibration signal 208.

[0028] Reference again Figure 1 , for each path between the transducers 104 (e.g., paths 112 and 114), the processor 116 retrieves a reference vibration signal from the memory 118 and receives a corresponding comparison vibration signal from the DAQ circuit 108. The processor 116 is configured to calculate a gain DI as a function of the amplitude of the reference vibration signal (reference amplitude) divided by the amplitude of the comparison vibration signal (comparison amplitude). The gain DI represents the "gain" required to match the amplitude of the comparison vibration signal to the amplitude of the reference signal. According to typical "gain" notation, a gain greater than 1 is referred to as a "positive" gain because applying such a gain increases the amplitude of the resulting signal. Similarly, a gain less than 1 is referred to as a "negative" gain because applying such a gain decreases the amplitude of the resulting signal. Under this notation, a "positive" gain DI indicates that the comparison amplitude is less than the reference amplitude, and also indicates that damage has occurred in the structure 102 along the path between the transducers (e.g., path 112 or 114). Typically, the damage suppresses the amplitude of the wave as it propagates through the structure 102. Likewise, the temperature at which the comparison vibration signal is collected affects the amplitude and phase of the comparison vibration signal compared to the temperature at which the reference vibration signal is collected.

[0029] Figure 3A and 3B To use SHM systems (such as Figure 11 and 3. A series of graphs 302, 304, 306, 308, 310, and 312 of example reference signals and example comparison signals collected at various temperatures for a given structure under test using the SHM system 100 shown in FIG. 1 . Note that the reference and comparison signals are collected at approximately the same time without any interfering wear, fatigue, or other damage to the structure under test. Thus, the graphs 302, 304, 306, 308, 310, and 312 illustrate the effects of temperature variations between collecting a reference vibration signal and collecting a comparison signal, represented as a voltage 314 over time 316. Graph 302 illustrates a reference vibration signal 318 collected at 64 degrees Fahrenheit (F), or about room temperature, and a comparison vibration signal 320 collected at -9 degrees Fahrenheit. The cooler temperature at which the comparison vibration signal 320 is collected reflects at least a reduction in the amplitude of the comparison vibration signal 320 relative to the amplitude of the reference vibration signal 318. The comparison vibration signal 320 also exhibits some negative phase shift when compared to the reference vibration signal 318 .

[0030] Graph 304 shows a reference vibration signal 318 collected at 64 degrees Fahrenheit, or approximately room temperature, and a comparison vibration signal 322 collected at 26 degrees Fahrenheit. The cooler temperature at which comparison vibration signal 322 was collected reflects at least a reduction in the amplitude of comparison vibration signal 322 relative to the amplitude of reference vibration signal 318, although to a lesser extent than comparison vibration signal 320 shown in graph 302. Comparison vibration signal 322 also exhibits some negative phase shift when compared to reference vibration signal 318, although again to a lesser extent than comparison vibration signal 320 shown in graph 302.

[0031] Graph 306 shows a reference vibration signal 318 collected at 64 degrees Fahrenheit, or approximately room temperature, and a comparison vibration signal 324 also collected at 64 degrees Fahrenheit. The equal temperatures render reference vibration signal 318 and comparison vibration signal 324 indistinguishable in graph 306 .

[0032] Graph 308 shows a reference vibration signal 318 collected at 64 degrees Fahrenheit, or approximately room temperature, and a comparison vibration signal 326 collected at 80 degrees Fahrenheit. The higher temperature at which comparison vibration signal 326 was collected reflects at least an increase in the amplitude of comparison vibration signal 326 relative to the amplitude of reference vibration signal 318. Comparison vibration signal 326 also exhibits some small positive phase shift when compared to reference vibration signal 318.

[0033] Graph 310 shows a reference vibration signal 318 collected at 64 degrees Fahrenheit, or approximately room temperature, and a comparison vibration signal 328 collected at 100 degrees Fahrenheit. The higher temperature at which comparison vibration signal 328 was collected reflects at least an increase in the amplitude of comparison vibration signal 328 relative to the amplitude of reference vibration signal 318, although to a greater extent than comparison vibration signal 326 shown in graph 308. Comparison vibration signal 328 also exhibits some positive phase shift when compared to reference vibration signal 318, although again to a greater extent than comparison vibration signal 326 shown in graph 308.

[0034] Graph 312 shows a reference vibration signal 318 collected at 64 degrees Fahrenheit, or about room temperature, and a comparison vibration signal 330 collected at 120 degrees Fahrenheit. The higher temperature at which the comparison vibration signal 330 was collected reflects at least an increase in the amplitude of the comparison vibration signal 330 relative to the amplitude of the reference vibration signal 318, although to a greater extent than the comparison vibration signals 326 and 328 shown in graphs 308 and 310, respectively. The comparison vibration signal 330 also exhibits a positive phase shift when compared to the reference vibration signal 318, although again to a greater extent than the comparison vibration signals 326 and 328 shown in graphs 308 and 310, respectively.

[0035] Reference again Figure 1 , and more specifically, the gain DI is calculated as a function of the reference amplitude multiplied by the comparison amplitude and divided by the square of the comparison amplitude (which produces the "instantaneous" gain DI). The reference vibration signal and the comparison vibration signal are usually collected as amplitude samples over a period of time called a time series. The gain DI of the reference time series and the comparison time series is represented by the following algorithm, each time series has N samples:

[0036]

[0037] Where Ref represents a reference time series, Comp represents a comparison time series, and n is a coefficient of the time series. At least some SHM systems use the gain calculation shown in Equation 1 as a preprocessing step for calculating DI. For example, some SHM systems utilize a mean square error (MSE) DI, which utilizes a gain calculation similar to that shown in Equation 1 as an intermediate step, rather than DI itself. Referring again to the gain DI, typically, a "positive" gain DI indicates the presence of damage. The location of the damage in the structure is along a given path between the actuator transducer and the sensor transducer. A "negative" gain DI indicates that there is no damage along the given path. A negative gain DI is typically associated with an increase in temperature when the comparison vibration signal is collected compared to the temperature (e.g., room temperature) at which the reference vibration signal is collected, because damage to the structure 102 typically does not result in an increase in the amplitude of the comparison vibration signal. In contrast, a positive gain DI can be associated with a decrease in temperature at which the comparison vibration signal is collected and damage that occurs along the path between the transducers 104. However, the colder temperature affects the amplitude of the comparison vibration signal less than the damage, thereby enabling the effects to be distinguished. Therefore, the gain DI is more capable of detecting with greater certainty whether damage, such as cracks or delamination, is present in the structure under test. In contrast, DIs such as MSE and correlation coefficient do not consider the direction or sign of the amplitude difference between the reference vibration signal and the comparison vibration signal.

[0038] To make the gain DI evaluation more intuitive, in some embodiments, the gain DI is normalized to zero:

[0039] Gain DI = Gain′-1 Equation 2

[0040] Thus, a "positive" gain DI corresponds to a gain value greater than zero, whereas a "negative" gain DI corresponds to a gain value less than zero.

[0041] In some embodiments, the processor 116 is also configured to measure and remove the phase shift in the comparison vibration signal caused by the temperature change between the time when the reference vibration signal is collected and the time when the comparison vibration signal is collected. This phase shift correction enables the reference vibration signal and the comparison vibration signal to be compared accurately in a timely manner. For example, when the comparison vibration signal is out of phase with the reference vibration signal, the amplitude of a given peak in the comparison vibration signal will deviate in time from the corresponding peak in the reference vibration signal. Typically, as described in U.S. Patent No. 8,892,384, temperature produces a gradually increasing phase shift over time, where the phase shift grows or "stretches" linearly over time, especially in homogeneous structures such as metals. Inhomogeneous structures, such as composite materials, tend to produce nonlinear growth of phase shift over time. This stretch is sometimes referred to as a phase shift measured by time delay. Damage tends to produce a phase shift that grows nonlinearly over time. Therefore, before the processor 116 calculates the gain DI according to Equation 1 above, the phase shift in the comparison vibration signal caused by temperature is approximated and corrected.

[0042] More specifically, in order to correct the phase shift in the comparison vibration signal due to temperature, the processor 116 divides the comparison vibration signal and the reference vibration signal into multiple time windows, or samples the comparison vibration signal and the reference vibration signal in other ways. Then, the processor 116 calculates the cross-correlation between the comparison vibration signal and the reference vibration signal for each time window. The cross-correlation is determined by calculating the amount of time shift required to maximally correlate the comparison vibration signal and the reference vibration signal within a given time window. Then, a weighted regression is performed on the time to estimate the time delay, where the weight is based on the relative amount of signal energy from the reference vibration signal within each time window. The weighted regression using the time window energy as a weighting function maximizes the effectiveness of phase shift compensation by focusing on the most important part of the signal. The weighted regression produces the relationship between time and time shift as a quadratic or higher order equation. This relationship then enables the processor 116 to correct the phase shift in the comparison vibration signal.

[0043] Figure 4 To use SHM systems (such as Figure 1 1. A flow chart of an example method 400 for detecting damage in a structure using an SHM system 100 (shown in FIG. 1 ). At a first time, the SHM system 100 excites, for example, the actuator transducer 104a to generate a reference vibration signal that propagates through the structure 102 and is received, for example, at the sensor transducers 104b and 104c. The collected reference vibration signal is then stored in the memory 118 until a subsequent test with the SHM system 100 is needed. The previously collected reference vibration signal is retrieved 410 from the memory. The actuator transducer 104a is again excited 420 to generate a comparison vibration signal that propagates through the structure 102. The comparison vibration signal is then received 430 at the sensor transducers 104b and 104c. The processor 116 receives the reference vibration signal and the comparison vibration signal for each path 112 and 114 and executes 440 a gain DI algorithm (such as the algorithm shown in Equation 1) using the corresponding reference vibration signal and comparison vibration signal pairs to calculate the gain DI. Damage along, for example, paths 112 and 114 is identified 450 when the corresponding gain DI for that path is positive.

[0044] Example technical effects of the methods, systems, and apparatus described herein include at least one of: (a) mitigating the effects of temperature on the amplitude and phase of a comparison vibration signal relative to a reference vibration signal; (b) detecting damage in a structure under test independent of temperature changes between collecting the reference vibration signal and collecting the comparison vibration signal; (c) improving false positive and false negative rates for damage detection in structures such as metals and composites; and (d) reducing the time required to collect a reference vibration signal by eliminating the need to collect the reference vibration signal at more than one temperature.

[0045] Furthermore, the present disclosure includes embodiments according to the following clauses:

[0046] Clause 1: A system for detecting damage to an aircraft structure, the system comprising: an aircraft structure; and a structural health monitoring (SHM) system configured to monitor the structural health of the aircraft structure, the SHM system comprising: an actuator coupled to the aircraft structure and configured to: generate a reference vibration signal having a reference amplitude and propagating through the aircraft structure at a first time, and generate a comparison vibration signal having a comparison amplitude and propagating through the aircraft structure at a second time after the first time, the comparison amplitude representing damage caused by the aircraft structure between the first time and the second time; a sensor coupled to the aircraft structure and configured to receive the reference vibration signal and the comparison vibration signal; and a processor configured to calculate a gain damage index as a function of the reference amplitude divided by the comparison amplitude, the gain damage index enabling the SHM system to monitor the structural health of the aircraft structure independently of temperature.

[0047] Clause 2: The system of clause 1, wherein the gain damage index indicates the presence or absence of cracks in the aircraft structure.

[0048] Clause 3: The system of clause 1, wherein the aircraft structure comprises a composite structure having a plurality of layups.

[0049] Clause 4: The system of clause 1, wherein the actuator comprises a lead zirconate titanate (PZT) transducer.

[0050] Clause 5: The system of clause 1, wherein the processor is further configured to remove a phase shift in the comparison vibration signal caused by a temperature change between the first time and the second time.

[0051] Clause 6: The system of clause 1, wherein the processor is further configured to calculate the gain impairment index as a function of the reference amplitude multiplied by the comparison amplitude divided by the comparison amplitude squared.

[0052] Clause 7: The system of clause 6, wherein a positive gain impairment index indicates the presence of impairment.

[0053] Item 8: A method for detecting damage in a structure, the method comprising: retrieving a reference vibration signal of the structure from a memory, wherein the reference vibration signal was previously collected; exciting a first transducer coupled to the structure to generate a comparison vibration signal that propagates through the structure; receiving the comparison vibration signal at a second transducer coupled to the structure; executing a gain damage index algorithm using the reference vibration signal and the comparison vibration signal to calculate a gain damage index as a function of the amplitude of the reference vibration signal multiplied by the amplitude of the comparison vibration signal divided by the square of the amplitude of the comparison vibration signal; and identifying damage in the structure when the gain damage index is positive.

[0054] Clause 9: The method according to Clause 8 also includes: detecting a phase shift in the comparison vibration signal caused by a change in temperature when the comparison vibration signal is received relative to a change in temperature when the reference vibration signal was previously collected; and removing the phase shift from the comparison vibration signal before executing the gain damage index algorithm.

[0055] Clause 10: A method according to clause 9, wherein detecting and removing the phase shift includes: dividing the comparison vibration signal and the reference vibration signal into multiple time windows; calculating the cross-correlation between the comparison vibration signal and the reference vibration signal in each of the multiple time windows by calculating the amount of time shift required to maximally correlate the comparison vibration signal and the reference vibration signal; performing weighted regression to estimate the time shift as a function of time; and correcting the phase shift as a function of time in the comparison vibration signal.

[0056] Clause 11: The method according to clause 8 further includes: exciting a third transducer to generate the reference vibration signal, the third transducer being coupled to the structure at the approximate location of the first transducer and before being coupled to the first transducer; receiving the reference vibration signal at a fourth transducer at the approximate location of the second transducer and before being coupled to the second transducer; and storing the reference vibration signal in the memory.

[0057] Clause 12: The method of clause 11, wherein a first temperature at which the fourth transducer receives the reference vibration signal differs from a second temperature at which the second transducer receives the comparison vibration signal by at least 20 degrees Fahrenheit.

[0058] Clause 13: The method of clause 8, wherein identifying damage in the structure comprises detecting cracks in the structure.

[0059] Clause 14: A structural health monitoring (SHM) system comprising: a plurality of transducers configured to be distributed over an area of ​​a structure and coupled to the structure, the plurality of transducers including a first transmitting transducer and at least one sensor transducer; a data acquisition (DAQ) circuit coupled to the plurality of transducers and configured to: actuate the first transmitting transducer to generate a reference vibration signal propagating through the structure at a first time; receive the reference vibration signal via the at least one sensor transducer; actuate the first transmitting transducer to generate a comparison vibration signal propagating through the structure at a second time after the first time; and receive the comparison vibration signal via the at least one sensor transducer; and a processor configured to calculate a corresponding gain impairment index for the at least one sensor transducer as a function of an amplitude of the reference vibration signal divided by an amplitude of the comparison vibration signal, the corresponding gain impairment index enabling the SHM system to monitor the structural health of the structure independently of temperature.

[0060] Clause 15: The SHM system of clause 14, wherein the processor is further configured to: perform a gain impairment index algorithm using the reference vibration signal and the comparison vibration signal to calculate a corresponding gain impairment index for the at least one sensor transducer as a function of the amplitude of the reference vibration signal multiplied by the amplitude of the comparison vibration signal divided by the square of the amplitude of the comparison vibration signal; and identify damage in the structure when at least one corresponding gain impairment index is positive.

[0061] Clause 16: The SHM system of clause 14, wherein the DAQ circuit comprises a plurality of analog input / output channels corresponding to the plurality of transducers and comprises an analog-to-digital converter.

[0062] Clause 17: The SHM system of clause 14, wherein the processor is further configured to: detect a phase shift in the comparison vibration signal caused by a change in temperature when the comparison vibration signal is received relative to a temperature when the reference vibration signal was previously collected; and remove the phase shift from the comparison vibration signal before calculating the corresponding gain corruption index.

[0063] Clause 18: The SHM system of clause 17, wherein the processor is further configured to detect a linearly progressive phase shift between the reference vibration signal and the comparison vibration signal.

[0064] Clause 19: The SHM system of clause 14, wherein the processor is further configured to, when at least one of the corresponding gain corruption indices is negative, identify an amplitude difference of the comparison vibration signal relative to the reference vibration signal caused by a change in temperature when the comparison vibration signal is received relative to a temperature when the reference vibration signal was previously collected.

[0065] Clause 20: The SHM system of clause 14, wherein the processor is further configured to identify a location of damage in the structure based on the corresponding gain damage index being positive.

[0066] Some embodiments involve the use of one or more electronic processing or computing devices. As used herein, the terms "processor" and "computer" and related terms (e.g., "processing device," "computing device," and "controller") are not limited to those integrated circuits known in the art as computers, but broadly refer to processors, processing devices, controllers, general-purpose central processing units (CPUs), graphics processing units (GPUs), microcontrollers, microcomputers, programmable logic controllers (PLCs), reduced instruction set computers (RISC) processors, field programmable gate arrays (FPGAs), digital signal processing (DSP) devices, application-specific integrated circuits (ASICs), and other programmable circuits or processing devices capable of performing the functions described herein, and these terms are used interchangeably herein. The above is only an example, and is therefore not intended to limit the definition or meaning of the terms processor, processing device, and related terms in any way.

[0067] In the embodiments described herein, the memory may include, but is not limited to, non-transitory computer-readable media, such as flash memory, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), and non-volatile RAM (NVRAM). As used herein, the term "non-transitory computer-readable medium" is intended to represent any tangible computer-readable medium, including but not limited to non-transitory computer storage devices, including but not limited to volatile and non-volatile media and removable and non-removable media, such as firmware, physical and virtual storage, CD-ROM, DVD, and any other digital source, such as a network or the Internet, and digital means that have not yet been developed, with the only exception of transient propagation signals. Alternatively, a floppy disk, a compact disk-read-only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disk (DVD), or any other computer-based device implemented in any method or technology for short-term and long-term storage of information, such as computer-readable instructions, data structures, program modules and submodules, or other data, may also be used. Thus, the methods described herein may be encoded as executable instructions implemented in a non-transitory computer-readable medium, such as "software" and "firmware". Furthermore, as used herein, the terms "software" and "firmware" are interchangeable and include any computer program stored in a memory for execution by a personal computer, workstation, client, and server. When executed by a processor, these instructions cause the processor to perform at least a portion of the methods described herein.

[0068] Moreover, in the embodiments described herein, the additional input channels may be, but are not limited to, computer peripherals associated with an operator interface such as a mouse and keyboard. Alternatively, other computer peripherals may also be used, which may include, for example, but are not limited to, a scanner. Furthermore, in the embodiments described herein, the additional output channels may include, but are not limited to, an operator interface monitor.

[0069] The systems and methods described herein are not limited to the specific embodiments described herein, but rather, components of the systems and / or steps of the methods may be utilized independently and separately from other components and / or steps described herein.

[0070] Although specific features of various embodiments of the present disclosure may be shown in some drawings and not in other drawings, this is only for convenience. According to the principles of the present disclosure, any feature of the drawings may be referenced and / or claimed in combination with any feature of any other drawings.

[0071] As used herein, an element or step listed in the singular and beginning with the word "a" or "an" should be understood as not excluding a plurality of elements or steps, unless such exclusion is explicitly stated. In addition, reference to "one embodiment" or "example embodiment" of the present disclosure is not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the described features.

[0072] This written description uses examples to disclose various embodiments, including the best mode, to enable any person skilled in the art to implement those embodiments, including making and using any device or system and performing any combined methods. The patentable scope is defined by the claims and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements that are insubstantially different from the literal language of the claims.

Claims

1. A system for detecting aircraft structural damage, the system comprising: an aircraft structure; and a structural health monitoring system configured to monitor a structural health condition of the aircraft structure, the structural health monitoring system comprising: an actuator coupled to the aircraft structure and configured to: generate a reference vibration signal having a reference amplitude that propagates through the aircraft structure at a first time, and generate a comparison vibration signal having a comparison amplitude that propagates through the aircraft structure at a second time after the first time, the comparison amplitude being indicative of damage caused by the aircraft structure between the first time and the second time; a sensor coupled to the aircraft structure and configured to receive the reference vibration signal and the comparison vibration signal, the reference vibration signal and the comparison vibration signal propagating through the aircraft structure along a path between the actuator and the sensor; as well as a processor configured to calculate, for the path, a gain impairment index as a function of the reference amplitude multiplied by the comparison amplitude divided by the square of the comparison amplitude, the gain impairment index enabling the structural health monitoring system to monitor a structural health of the aircraft structure independent of temperature, The reference vibration signal and the comparison vibration signal are collected as amplitude samples over a period of time called a time series, and the gain corruption index of the reference time series and the comparison time series is represented by the following algorithm, each of the reference time series and the comparison time series having N samples: Where Ref represents the reference time series, Comp represents the comparison time series, and n is the coefficient of the time series.

2. The system according to claim 1, wherein: The gain damage index indicates the presence or absence of cracks in the aircraft structure.

3. The system according to claim 1, wherein: The aircraft structure includes a composite structure having a plurality of laminates.

4. The system according to claim 1, wherein: The actuator includes a lead zirconate titanate transducer.

5. The system according to claim 1, wherein: The processor is further configured to remove a phase shift in the comparative vibration signal caused by a temperature change between the first time and the second time.

6. The system according to claim 1, wherein: A positive gain damage index indicates the presence of damage.

7. A method (400) of detecting damage in a structure by means of a structural health monitoring system, the structural health monitoring system comprising a memory and a first transducer and a second transducer coupled to the structure, the method comprising: Step (410): retrieving a reference vibration signal of the structure from the memory, wherein the reference vibration signal is previously collected and has a reference amplitude; Step (420): exciting the first transducer coupled to the structure to generate a comparative vibration signal propagating through the structure, the comparative vibration signal having a comparative amplitude; Step (430): receiving the comparative vibration signal at the second transducer coupled to the structure, the comparative vibration signal propagating along a path between the first transducer and the second transducer; Step (440): performing a gain impairment index algorithm using the reference vibration signal and the comparison vibration signal to calculate a gain impairment index as a function of the amplitude of the reference amplitude multiplied by the comparison amplitude divided by the square of the comparison amplitude, the reference vibration signal and the comparison vibration signal being collected as amplitude samples over a period of time referred to as a time series, the gain impairment index of the reference time series and the comparison time series being represented by the following algorithm, each of the reference time series and the comparison time series having N samples: Where Ref represents the reference time series, Comp represents the comparison time series, and n is the coefficient of the time series; and Step (450): When the gain damage index is positive, identifying damage in the structure.

8. The method (400) according to claim 7, further comprising: detecting a phase shift in the comparison vibration signal due to a change in temperature when the comparison vibration signal was received in step (430) relative to a temperature when the reference vibration signal was previously collected; as well as The phase shift is removed from the comparison vibration signal prior to executing the gain corruption index algorithm of step (440).

9. The method (400) of claim 8, detecting and removing the phase shift comprises: dividing the comparison vibration signal and the reference vibration signal into a plurality of time windows; calculating a cross-correlation between the comparison vibration signal and the reference vibration signal within each of the plurality of time windows by calculating an amount of time shift required to maximally correlate the comparison vibration signal and the reference vibration signal; performing weighted regression to estimate time shift as a function of time; as well as The phase shift as a function of time in the comparison vibration signal is corrected.

10. The method (400) according to claim 7, further comprising: exciting a third transducer of the structural health monitoring system to generate the reference vibration signal, the third transducer being coupled to the structure at an approximate location of the first transducer and prior to coupling to the first transducer; receiving the reference vibration signal at a fourth transducer of the structural health monitoring system, the fourth transducer being engaged to the structure at an approximate location of the second transducer and prior to engaging the second transducer; as well as The reference vibration signal is stored in the memory.

11. The method (400) according to claim 10, wherein: A first temperature at which the fourth transducer receives the reference vibration signal differs from a second temperature at which the second transducer receives the comparison vibration signal by at least 20 degrees Fahrenheit.

12. The method (400) of claim 7, wherein: Identifying damage in the structure in step (450) includes detecting cracks in the structure.

13. A structural health monitoring system comprising: a plurality of transducers configured to be distributed over an area of ​​a structure and coupled to the structure, the plurality of transducers including a first transmitting transducer and at least one sensor transducer; a data acquisition circuit coupled to the plurality of transducers and configured to: actuate the first transmitting transducer to generate a reference vibration signal propagating through the structure at a first time; receiving the reference vibration signal via the at least one sensor transducer; actuating the first transmitting transducer to generate a comparison vibration signal that propagates through the structure at a second time after the first time; and receiving the comparison vibration signal via the at least one sensor transducer, the reference vibration signal and the comparison vibration signal propagating through the aircraft structure along a path between the first transmitting transducer and the at least one sensor transducer; as well as a processor configured to calculate, for the path, a respective gain impairment index for the at least one sensor transducer as a function of an amplitude of the reference vibration signal multiplied by an amplitude of the comparison vibration signal divided by an amplitude squared of the comparison vibration signal, the respective gain impairment index enabling the structural health monitoring system to monitor a structural health of the structure independent of temperature, The reference vibration signal and the comparison vibration signal are collected as amplitude samples over a period of time called a time series, and the gain corruption index of the reference time series and the comparison time series is represented by the following algorithm, each of the reference time series and the comparison time series having N samples: Where Ref represents the reference time series, Comp represents the comparison time series, and n is the coefficient of the time series.

14. The structural health monitoring system according to claim 13, wherein: The processor is further configured to: performing a gain impairment index algorithm using the reference vibration signal and the comparison vibration signal to calculate the respective gain impairment index of the at least one sensor transducer as a function of an amplitude of the reference vibration signal multiplied by an amplitude of the comparison vibration signal divided by an amplitude squared of the comparison vibration signal; and When at least one of the corresponding gain impairment indices is positive, damage in the structure is identified.

15. The structural health monitoring system according to claim 13, wherein: The data acquisition circuit includes a plurality of analog input / output channels corresponding to the plurality of transducers and includes an analog-to-digital converter.

16. The structural health monitoring system of claim 13, wherein: The processor is further configured to: detecting a phase shift in the comparison vibration signal due to a change in temperature when the comparison vibration signal is received relative to a temperature when the reference vibration signal was previously collected; and The phase shift is removed from the comparison vibration signal before calculating the corresponding gain impairment index.

17. The structural health monitoring system of claim 16, wherein: The processor is further configured to detect a linearly progressive phase shift between the reference vibration signal and the comparison vibration signal.

18. The structural health monitoring system of claim 13, wherein: The processor is further configured to, when at least one of the corresponding gain corruption indices is negative, identify an amplitude difference of the comparison vibration signal relative to the reference vibration signal due to a change in temperature when the comparison vibration signal is received relative to a temperature when the reference vibration signal was previously collected.

19. The structural health monitoring system of claim 13, wherein: The processor is further configured to identify a location of a damage in the structure based on the corresponding gain damage index being positive.

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