Abstract:
A device for generating an acoustic stress wave in metal structures in real-time. A cylindrical housing includes a chamber, a piezostack actuator inside the chamber, and a support ring for relieving strain. The actuator has a high tensile strength block extending therefrom for contacting the surface. A pulse wave generator drives the actuator in accordance with a predetermined frequency to cause microindentation and generate an acoustic stress wave in the surface. The stress wave is transmitted to a signal transformer for producing a signal for use in systems for interpreting the signal. In a preferred embodiment, the high tensile block has a ceramic tip for engagement with the surface. The preferred piezostack actuator includes a poled element.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for monitoring helicopter rotor heads to identify fatigue and stress corrosion cracks. More particularly the invention relates to a real-time technique for performing condition-based maintenance and structure health assessment. The Government may have rights in this invention pursuant to Contract No. N00167-94-C-0064, awarded by the Department of the Navy. 
    
    
     BACKGROUND OF THE INVENTION 
     Real-time monitoring of fatigue and stress corrosion cracks in U.S. Navy helicopter rotor head systems is needed to improve operational safety and permit “condition-based” maintenance and increase operational life. Rotor acoustic monitoring systems have been developed to demonstrate the detection of rotor head fatigue cracks and to investigate the early warning of propagating fatigue cracks. As an example of one helicopter hub, the H-46 Navy helicopter has a fully articulated rotor hub where each rotor blade has three distinct bearings, or degrees of freedom, commonly called hinges, allowing movement the lead/lag direction (fore/aft), offset flapping (up/down) and feathering thrust directions. These three hinges are designed to incorporate metal ball race bearings or elastomeric bearings made of synthetic rubber to minimize rotor head vibration effects. Several factors cause fatigue cracks to initiate and propagate in rotor heads, including: aerodynamic loading forces which cause chord-wise and flap-wise bending; severe corrosion and temperature extreme environments; extended fatigue cycling; and maintenance-induced damage. Rotor head components experience significant fatigue cracks in highly loaded locations such as the main rotor hub, connecting link, pitch shaft, pitch housing, lead/lag dampers, blade fittings, and drive shafts. These cracks can cause catastrophic failure, leading to loss of aircraft and life, and can degrade mode of operation by providing minimal control capability. 
     Several non-destructive and indirect inspection techniques exist to detect the presence of rotor head cracking, but each method has one or more significant technical limitations. These techniques include visual inspection, tap testing, ultrasound tests, eddy current, x-ray, and magnetic particle tests. Also used as an indirect monitoring approach are blade imbalance and power train monitoring. Visual inspection is most commonly used to check surface conditions such as cracks in main hub assemblies or general surface corrosion, but does little to detect in situ cracks hidden within the bearing retainer area of the connecting link or pitch shaft. Each method has two significant drawbacks, namely: (1) a lack of ability to perform “condition-based” maintenance, requiring manpower to disassemble the rotor assembly and increasing operational costs, limiting in-flight usage, and raising the vulnerability to create maintenance induced faults; and (2) lack of real-time in-flight detection capability, to provide early warning indications of crack initiation or propagation faults. The aerodynamic loads incurred during flight cause the crack initiation and propagation. No existing approach can provide this means of detection. 
     One system which has been effective in direct monitoring rotor health is set forth in my co-pending patent application entitled REMOTE SELF-POWERED STRUCTURE MONITOR, filed Jul. 24, 1996, and having Ser. No. 08/690,263, now U.S. Pat. No. 6,076,405. In this system, non-repetitive high frequency acoustic emission events from stress wave acoustic emission energy coming from the beginning of structural cracks is converted into electrical signals and processed to provide data, including an alarm. However, in order to demonstrate the efficacy of this self-powered structure monitor, it has now been determined that simulated fatigue cracks should be generated for testing and evaluating purposes. Various methods for generating simulated high frequency stress-wave acoustic fatigue having characteristics consistent with an actual structural fatigue crack have been considered. It is necessary to replicate waveform shape and amplitude as well as frequency content, and it must do so in an environment similar to helicopter rotor head operation conditions. 
     Accordingly, it would be of great advantage in the art if a device could be provided to generate simulated fatigue cracks in operating rotor hubs and the like. 
     It would be another great advance in the art if the simulated fatigue cracks could be generated by the device in a remote location, particularly when the structure is being flown or otherwise operated. 
     Yet another advantage would be if the device would permit an automated real-time technique for injecting acoustic fault phenomena into structures and machinery of interest to perform condition-based maintenance and health assessment. 
     Other advantages will appear hereinafter. 
     SUMMARY OF THE INVENTION 
     It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, the present invention comprises use of a microindentation technique, where a small localized region of the material of interest is loaded rapidly to inject an automated real-time acoustic fault phenomenon into the structure. 
     A technique comprises the generation of an acoustic stress wave in metal structures in real-time. A housing is mounted on a surface of the structure being monitored. Inside the housing, a piezostack actuator having an end extending therefrom for contacting said surface is activated by a signal from a pulse wave generator with a power amplifier to drive the actuator in accordance with a predetermined frequency. When so driven, the end creates microindentations in the surface to generate an acoustic stress wave at a desired frequency. 
     The acoustic stress wave is transmitted to one or more signal transformers for producing a signal from said stress wave which is suitable for evaluation during inspection and detection of defects in the structure. Not only is the present invention suitable for inspection and maintenance of aircraft for both fixed wing and helicopter rotating parts, it can be used for any metal or other material capable of transmitting an acoustic stress wave generated by microindentation. The effects of acoustic response to joint load variation is shown below. In addition, the correlation of mechanical loads to machinery performance, as shown herein, permits evaluation of machinery performance and health. Bearing wear, loss of lubrication, corrosion, cracking from aging or excessive use, and other defects can be determined. Railcars and surface transportation are two other areas where the invention is applicable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the invention, reference is hereby made to the drawings, in which: 
     FIG. 1 is a schematic view of the preferred embodiment of the invention shown in use on a helicopter rotor head; 
     FIG. 2 is a sectioned view of one component of the invention shown in FIG. 1; 
     FIG. 3 is a perspective view of another component of the invention shown in FIG. 1; 
     FIGS. 4 a ,  4   b  and  4   c  are three schematic illustrations of the component shown in FIG. 3, under various applied voltages; 
     FIG. 5 is a schematic illustration of the principal of operation of the present invention, with an enlarged portion in the area designated with the circle  5   a;    
     FIG. 6 is an enlarged, partially sectioned, side elevational view of the invention shown in FIG. 1, as mounted on a surface; 
     FIG. 7 is a schematic view of a test apparatus demonstrating the efficacy of the present invention; 
     FIGS. 8 and 9 present test data from the apparatus in FIG. 8; 
     FIGS. 10 and 11 present enlarged test data the apparatus in FIG. 8; 
     FIG. 12 presents data showing details of a test; 
     FIG. 13 is a sectioned view of another application of the present invention; 
     FIG. 14 is a schematic illustration of the application of FIG. 13; 
     FIG. 15 is a schematic illustration of data collection for the preferred embodiment of this invention; and 
     FIG. 16 is a schematic circuit diagram of the FET shown in FIG.  7  and is a crack generator drive circuit producing the cycles of operation for the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawings, FIG. 1 illustrates a simplified view of the present invention as implemented for a helicopter rotor head, such as the CH-46 Sea Knight helicopter,  11  generally. Mounted on a rotor head component called the vertical hinge pin cap  13  and removed for illustration purposes by line  15  is a piezo actuator  17 , shown in greater detail below. Location of the piezo actuator  17  in the hinge pin cap  13  simulates the effect of a vertical hinge pin bore crack, one such crack occurring at the vertical hinge pin joint. At least one fatal flight accident has been caused by failure of this part. 
     A pulse waveform generator  19  provides a four-voltage level drive circuit via amplifier  21  with a +/−200 volt output for actuation of actuator  17 . The actuator  17  is an off-the-shelf piezostack actuator made by Burleigh, Inc., designated as a PZT pusher, model no. PZ-100. It is presently used to align and position optical components such as mirrors and gratings in optics research to within a five micron displacement. 
     FIG. 2 illustrates a cross section of the Burleigh product, where the actuator  17  generally includes a stainless steel housing  23  having a base  25  for supporting the piezostack  27  at one end, with a hard ceramic tip  29  extending out from housing  23 . Piezostack  27 , as seen in FIG. 3, extends and contracts when voltage is applied in relationship to the width and length of the piezoelectric tube which forms piezostack  27 . Voltage input  31  and output  33  provide the electrical energy that the piezoelectric material responds to in a mechanical movement, shown more clearly in FIGS. 4 a ,  4   b  and  4   c.    
     FIG. 4 b  illustrates the poling axis of piezoelectric stack  27 , which axis is, of course, aligned along the length of piezostack  27  for movement of ceramic tip  29  of FIG.  2 . FIG. 4 a  illustrates how the piezostack  27  is subjected to a compressive force when the output voltage  33  is the same polarity as piezostack  27 . Similarly, FIG. 4 c  illustrates how the piezostack  27  is subjected to a tensile force when the output voltage  33  is the opposite polarity as piezostack  27 . Rapid alternating of the voltage, as described below, will cause similar rapid movement of the ceramic tip  29  into and out of any surface in which tip  29  is in contact with, to create the acoustic patterns in accordance with the invention. 
     FIG. 5 illustrates the principal of operation of piezostack actuator  27  to generate an applied force below actuator tip  29 , when the piezostack  27  is extended, as described above. The localized area on surface  37  is deflected at  39 , causing a small change in the surface area. As the piezostack  27  is actuated and extends tip  29 , it creates a deflection at the surface  37 , then, when tip  29  is rapidly released by movement of piezostack  27  in the opposite direction, a high frequency stress wave acoustic signature  41  is created which propagates in all directions from tip  29  as a simulated event. The amplitude of the event is directly related mathematically to the change in surface area at the point  39  of impact of tip  29 . 
     FIG. 6 illustrates a simplified cross-sectional view of the rotor mount package of the present invention for use with the vertical hinge pin cover used in test flights using the RAMS flight test as described in my co-pending application entitled REMOTE SELF-POWERED STRUCTURE MONITOR, filed Jul. 24, 1996, and having Ser. No. 08/690,263. Piezostack  27  is mounted on top of hinge pin cover  61  in a support housing  63  which captures the piezostack  27  and provides both mechanical and positioning support, as well as environmental protection. A right angle cable connector  65  is attached on top of housing  63  to provide a command signal via coaxial cable  67  to actuate piezostack  27  and cause ceramic tip  29  to move as described above. The pin cover  61  includes a cone shaped wave guide  69  for transmission of the acoustic stress wave to the appropriate detection and processing equipment for the system in which the present invention is being used. 
     Drive electronics for the present invention may take a variety of configurations. A pulse generator  71  shown in FIG. 7 incorporates a high speed field-effect transistor (FET)  73  which operates as a switch to generate a bipolar drive signal on line  75 . In this configuration, the bipolar drive signal has four selectable output voltages. Used in evaluation of the present invention, as shown in FIG. 7, were output voltages of 100 volts, 48 volts, 20 volts, and 10 volts, which generates unique acoustic waveform signatures, each with an amplitude of 8 to 10 millivolts, 2 to 3 millivolts, 1 millivolt and 0.5 millivolts, respectively. These wave forms and their respective amplitudes correspond with fatigue cracks of different lengths, with the 0.5 millivolt output approximating a fatigue crack of 0.020 inches in length. The piezostack actuator  77  is excited by the applied voltage generated by opening and closing FET switch  73 , which electrically grounds the piezostack actuator  77 , discharging the capacitance of the piezostack actuator  77  on half of the switching cycle and charging piezostack on the other half of the cycle through a dedicated pull-up resistor, not shown in FIG.  7 . 
     The test arrangement shown in FIG. 7 was used to evaluate the drive electronics and crack generator hardware of the present invention. Actual tests were performed on the vertical hinge pin cap of a retired or scrapped H- 46  helicopter rotor head. A wide-bandwidth 5 MHz Harasonics acoustic transducer  79  was mounted on the H-47 rotor cap next to a prototype of the present invention of the type in FIG. 7, to characterize the acoustic response of the piezostack actuator  77 . The output of the pulse generator or piezostack actuator  77  was connected to a 1 GHz Lecroy model 9354 digital storage oscilloscope  81 . The output of the acoustic transducer  79  was amplified with a 40 dB preamplifier prior to being sampled by the oscilloscope  81 . The acoustic wave forms generated by the test are shown in FIGS. 8 and 9, with a unique set of acoustic waveform signatures. The two wave forms correspond with the microindentation properties of the crack generator  77 , an unique acoustic event for the falling edge of the pulse waveform which corresponds with the lifting off action of the piezostack tip. The rising edge corresponds with the impacting action of the tip. Each waveform has an unique signature, defined by amplitude, duration, and frequency content, with the lift off action of the piezostack creates an acoustic event having slightly higher spectral content. 
     FIGS. 10 and 11 illustrate a detailed view of each simulated acoustic crack event, the lift off action of the piezostack creates an acoustic event having a peak-to-peak amplitude of 1.5 millivolts. This dual mode operation of the crack generator of the present invention, creating two distinct but similar acoustic wave forms with complimentary amplitudes and spectral frequency content is a unique feature of this invention. The two signature wave forms provided the benefits of more robust signal detection and redundancy in measurement during actual RAMS flight tests, potentially minimizing false alarms. 
     Other subassembly fatigue tests were performed on a complete H-46 rotor arm assembly, including a pitch shaft, connecting link, pitch housing, and blade attach fitting. Simulated centrifugal load forces in excess of 60,000 pounds were introduced via a universal joint. Flap-wise and chord-wise excitation was introduced via two hydraulic RAMS at the respective ends of the fatigue tester. A device according to the present invention was mounted on the top of the vertical hinge pin joint in the same location as described with respect to FIG.  7 . The tests accomplished the following objectives: (1) the acoustic energy response of the rotor subassembly under actual fatigue load conditions was assessed; (2) the acoustic energy properties of the rotor head assembly, including transmissibility from one rotor component to another, signal attenuation, background noise effects and their impact on signal detection and recovery during flight tests were characterized; and (3) the performance of the present invention device was evaluated. Examples of these tests are shown in FIG. 12, which summarizes a significant lesson verified by the tests, in that fatigue crack acoustic content can be transmitted through an operational joint. FIG. 14 highlights two sets of plots. The left plot has the command signal issued to the crack generator as the top trace, followed by the acoustic reference signal trace (AE transducer located next to the crack simulator, denoted as transducer number  5 ) and transducers nos.  2  and  3 . The signal traces for transducer no.  2  corresponds with a transducer mounted on the pitch shaft component while transducer no.  3  corresponds with the pitch housing component. The response detected at transducer no.  2  indicates the acoustic energy can be successfully coupled through the vertical hinge pin joint with only 20 dB of signal attenuation. The transducer no.  3  response indicates that acoustic crack energy can be coupled and transmitted even farther away, through the vertical hinge pin and the roller bearing joint of the pitch housing which provides pitch control to the rotor head with only a signal loss of 28 dB. The set of plots on the right-hand side of FIG. 12 provides a more detailed view of the test results. 
     The device of the present invention may also be used for reliable inspection and detection of hidden and inaccessible damage in aging structures such as aircraft skins, bridges, and the like. Corrosion, particularly when there is loss of structural integrity, is a major concern. FIG. 13 illustrates the use of an acoustic energy transmitter  117  mounted in the aircraft skin  119  to project acoustic energy into the lap joint toward the damaged area. One or more acoustic receivers would detect and capture the transmitted energy and examine transmitted verses reflected energy to determine material loss and location of the hidden structural damage. FIG. 14 illustrates this concept. 
     FIG. 15 illustrates an automatic acoustic data collection which highlights the timing pattern during operation of the invention. The transducer cycles are shown operating at one second, where the pinger cycles are also shown. The pinger drive is on for 10 ms over a time of 25 ms, with a rise time less than or equal to 200 ns. FIG. 15 illustrates the time allowed to allow the simulated crack energy to propagate through the rotor head structure after it is injected, and time to collect background noise. 
     FIG. 16 illustrates schematically the drive electronics for the present invention in its preferred embodiment. In FIG. 16, the schematic summarizes a design incorporating a high speed field-effect transistor operating as a switch to generate a bipolar drive signal. As noted with FIG. 7, the FET  73  operates as a switch to generate a bipolar drive signal on line  75 . To get the proper logic levels and sequences, the device functions effectively with the circuit in FIG. 16. A drive signal  161  enters IC drive  162  from a general drive device, not shown, to drive the FET  163 . Decoder  164  feeds four opto solid state switches  165  connected in series to get the desired voltages of 10 volts, 20 volts, 48 volts and 100 volts, which are then sent to the piezo stack or pinger via junction  166  and line  167 . All of the elements shown in FIG. 16 are off-the-shelf circuit elements. 
     While particular embodiments of the present invention have been illustrated and described, it is not intended to limit the invention, except as defined by the following claims.