History shows a need for accurate, reliable, in-situ monitoring of the structural integrity of aircraft, bridges, pipelines, and buildings.
Once tested, a structure is assumed to be safe until the next inspection. However, much can occur to the structure before the next periodic test. Aside from this uncertainty, manual testing is a reoccurring, labor-intensive procedure that is subject to human error and misinterpretation.
Early techniques developed to assist manual inspection included visual & magnetic testing and dye penetration. These techniques were focused on surface defects only. Subsequent techniques such as x-ray, eddy current, and ultrasonic technologies could detect internal flaws in materials.
The need to monitor structures became known as Structural Health Monitoring, and it is in this category that Structural Monitoring Systems Comparative Vacuum Monitoring (CVM™) technology is the forerunner.
The aviation industry provides a great array of applications and programs for the full commercialization of the CVM™ technology platform. As such, SMS has undertaken and completed multiple civilian programs to gain full certification for the use of CVM™ from both regulators and key aerospace OEMs.
Looking ahead, CVM™ will aim to be the routinely accepted method for performing periodic, continual maintenance on all aircraft types worldwide.
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CVM™ (Comparative Vacuum Monitoring) offers a novel method for in-situ, monitoring of crack initiation and/or propagation. CVM™ technology uses the principle that a vacuum maintained within a small volume is extremely sensitive to leakage. Essentially the CVM™ technology provides a measure of the differential pressure between alternating channels containing air at a partial vacuum pressure and channels containing air at atmospheric channels in a simple manifold.
The CVM™ principle relies on placing a sensor onto the surface of a component where damage is expected to occur. The sensor contains a manifold of fine channels that are open to the surface. Once the sensor has been installed on the surface, the channels form closed “galleries” to which a vacuum can be applied. It is important to note that the surface of the component forms part of the sensor system, with the crack itself providing the leakage path for air into the vacuum galleries.
If no flaw is present, the vacuum will remain at a stable level. When a crack develops, it forms a leakage path between the atmospheric and vacuum galleries, producing a measurable change in the vacuum level. This change is detected by the CVM™ system (PM200 device).
The PM200 is a microprocessor-controlled handheld instrument powered internally from a rechargeable Lithium-Ion battery capable of interrogating groups of hard-to-access sensors from a suitable access point. The PM200 stores the location of inspection and interrogation pass / fail data for download by USB connection.
The sensitivity of the sensor is governed by the gallery spacing and the rate of airflow, this provides an indication of the size of the developing flaw. Sensors may either take the form of self-adhesive polymer sensors or may form part of the component. Since the sensor physics is based on pressure measurements, there is no electrical excitation involved.
The CVM™ sensor is manufactured from multiple layers of FEP sheets, where 2 to 8 sheets are laminated together with an acrylic pressure-sensitive adhesive, the same adhesive is on the bottom layer and facilitates the adhesion to the aircraft.
The sensor galleries are continuous through the sensor, allowing the PM200 to perform a self-check of the gallery condition. As well, CVM™ technology is inherently fail-safe, such that damage or leaks within the system are either detected as false positives or are large enough to indicate a system failure.