Acoustic fault injection tool

Details Acoustic fault injection tool Acoustic fault injection tool

1999-07-23

2001-07-24

Williams, Hezron (Department: 2856)

Measuring and testing

Vibration

Resonance, frequency, or amplitude study

C073S012090, C073S012120, C073S802000

Reexamination Certificate

active

06263737

ABSTRACT:

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.


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