Measuring and testing – Vibration – Resonance – frequency – or amplitude study
Reexamination Certificate
2002-02-20
2003-10-28
Raevis, Robert (Department: 2856)
Measuring and testing
Vibration
Resonance, frequency, or amplitude study
Reexamination Certificate
active
06637266
ABSTRACT:
FIELD OF THE INVENTION
The following invention is generally related to instrumentalities and methodologies for the non-destructive inspection, and especially for testing and evaluation of aircraft components.
BACKGROUND OF THE INVENTION
Recent tragedies in aircraft transportation have caused concern over the ability of airlines to evaluate the airworthiness of aircraft within their respective fleets. As airframes age, the characteristics of the materials that constitute the airframe components change due to the stresses and strains associated with flights and landings. The material goes beyond the point of elasticity (the point the material returns to its original condition) and into the point of plasticizing or worse, beyond to failure. As a result, inspections and testing are conducted on aircraft components periodically during the aircraft's component life cycle as are mandated by governing bodies and based largely on empirical evidence.
Current commercial industry inspection and repair methods are inefficient, costly and not standardized. Their inspection and repair procedures and processes have changed little in the past 20 or 30 years and have not solved the “Aging Aircraft” safety problems. Inspection of aircraft components are historically limited to the “Tap Test,” visual inspection, and Eddy Current analysis. Standardized technical repairs are nonexistent. Commercial safety integrity is continually compromised by not determining the extent of aircraft structure corrosion and fatigue.
Unfortunately, manned inspection is still the state of the art. Inspection timetables are developed and updated primarily as a function of anecdotal evidence, all too frequently based on airline catastrophes.
Inspections and testing are bificurated into two areas: destructive testing and nondestructive inspection (NDI), nondestructive testing (NDT) or nondestructive evaluation (NDE). The area of destructive testing, as the name implies, requires the aircraft component under scrutiny to be destroyed in order to determine the quality of that aircraft component. This can result in a costly endeavor because the aircraft component is destroyed even though it passed the test procedure. It is, therefore, no longer available for use. Frequently, where destructive testing is done on samples (e.g. coupons) and not on actual components, the destructive test may or may not be reflective of the forces that the actual component could or would withstand within the flight envelope of the aircraft.
On the other hand, NDI, NDT or NDE have the obvious advantage of being applicable to actual aircraft components in their actual environment. Several important methods of NDI, NDT or NDE that are performed in a laboratory setting are listed and summarized below.
Radiography. This is a general term for the inspection of a material by subjecting it to penetrating irradiation. X-rays are the most familiar type of radiation used in this technique, although good damage detection has been done using neutron radiation. Most materials used in aircraft component manufacturing are readily acceptable to X-rays. In some instances, an opaque penetrant is needed to detect many defects. Real-time X-rays are starting to be used to permit viewing the area of scrutiny while doing the procedure. Some improvement in resolution has been achieved by using a stereovision technique where the X-rays are emitted from dual devices which are offset by about 15°. When viewed together, these dual images give a three-dimensional view of the material. Still, the accuracy of X-rays is generally no better than +10% void content. Neutrons (N-ray), however, can detect void contents in the +1% range. The difficulty is the obvious problem with safety and radiation sources. addition to the normal use to detect internal flaws in the metals and composite structures, X-rays and neutrons can detect misalignment of honeycomb cores after
Ultrasonics. This is most common method for detecting flaws in composite materials. The method is performed by scanning the material with ultrasonic energy while monitoring the reflected energy for attenuation (diminishing) of the signal. The detection of the flaws is somewhat frequency-dependent and the frequency range and scanning method most often employed is called C-scan. In this method, water is used as a coupling agent between the sending device and the sample. Therefore, the sample is either immersed in water or water is sprayed between the signal the signal transmitter and the sample. This method is effective in detecting defects even in thick samples, and may be used to provide a thickness profile. C-scan accuracies can be in the +1% range for void content. A slightly modified method called L-scan can detect stiffness of the sample by using the wave speed, but this method requires that the sample density be known.
Acousto-ultrasonics. This analysis method is similar to ultrasound except that separate sensors are used to send the signal and other sensors are used to receive the signal. Both sensors are, however, located on the same side of the sample so a reflected signal is detected. This method is more quantitative and portable than standard ultrasound.
Acoustic emission. In this method, the sounds emitted by a sample are detected as the sample is subjected to a stress. The stress can be mechanical, but need not be. In actual practice, in fact, thermal stresses are the most commonly employed. Quantitative interpretation is not yet possible except for well-documented and simple shapes (such as cylindrical pressure vessels).
Thermography. This method, which is sometimes called IR thermography, detects differences in the relative temperatures of the surface and, because these temperature differences are affected by internal flaws, can indicate the location of those flaws. If the internal flaws are small or far removed from the surface, however, they may not be detected. Two modes of operation are possible-active and passive. In the active mode, the sample is subjected to a stress (usually mechanical and often vibrational) and then the emitted heat is detected. In the passive mode, the sample is externally heated and the thermal gradients are detected.
Optical holography. The use of laser photography to give three-dimensional pictures is called holography. This method can detect flaws in samples by employing a double-image method where two pictures are taken with an induced stress in the sample between the times of the pictures. This method has had limited acceptance because of the need to isolate the camera and sample from vibrations. Phase locking may eliminate this problem. The stresses that are imposed on the sample are usually thermal. If a microwave source of stress is used, moisture content of the sample can be detected. For composite material, this method is especially useful for detecting debonds in thick honeycomb and foam sandwich constructions. A related method is called shearography. In this method, a laser is used with the same double exposure technique as in holography with a stress applied between exposures. However, in this case an image-shearing camera is used in which signals from the two images are superimposed to give interference and thereby reveal the strains in the samples. Because strains are detected, the size of the pattern can give an indication of the stresses concentrated in the area and, therefore, a quantitative appraisal of the severity defect is possible. This attribute, plus the greater mobility of this method over holography, and the ability to stress with mechanical, thermal, and other methods, has given this method wide acceptance since its introduction.
Even though there are a wealth of diagnostic tools, there is a need to provide systems and principled processes to execute NDI, NDT and NDE of aircraft and their constituent components to take advantage of the methods briefly described above in order to better characterize the material properties of materials used in the manufacturing of aircraft components. The present invention fulfills this need outside
Kreten Bernhard
Raevis Robert
LandOfFree
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