Comparative Vacuum Monitoring: a New Method of In-Situ

Comparative Vacuum Monitoring (CVM) offers a novel method for in-situ, real-time monitoring of crack initiation and/or propagation. CVM is a measure of the differential pressure between fine galleries containing a low vacuum alternating with galleries at atmosphere in a simple manifold. If no flaw is present, the vacuum will remain at a stable level. However, if a flaw develops, air will flow through the passage created from the atmosphere to the vacuum galleries. Sensors may either take the form of self-adhesive polymer “pads” or may form part of the test component. A transducer measures the fluid flow between the galleries; the rate of flow an indication of flaw size. CVM has been developed for use in both a laboratory environment or on operating equipment. The laboratory testing equipment has rapidly gained acceptance in several testing facilities within the aviation industry; often used to control a fatigue testing program due to its proven reliability and built-in fail-safe properties. A portable unit, with its own data logger, has recently been developed to allow monitoring of components in operating equipment where access to critical components is difficult, time consuming and costly. INTRODUCTION CVM has been developed by Structural Monitoring Systems, with the original patents being granted in 1995. The sensors and hardware have been continually improved, with a strong research and development program over the last decade. The hardware now consists of a rugged digital system, with sensor manufacture based on accurate CAD based design and tooling. The CVM system consists of three primary components; a sensor, a fluid flow meter and a stable source of low vacuum. For the laboratory environment, up to sixteen fluid flow meters (SIM8b) may be connected to a single vacuum source (Kvac4); the portable system (PM3) combining both the vacuum source and fluid flow meter in a single, self-contained unit with a data logger. The methodology utilises a direct measurement of the test surface; the test surface becomes one face of the vacuum/atmosphere manifold, effectively becoming part of the sensor. This has the advantage that no proxy medium (generally an adhesive) exists between the measuring device and the test component, resulting in a high reliability [1,4] and built in fail-safe mechanisms. Figure 1 SMS sensor mounted on specimen. An increase in ∆P results when a crack intersects adjacent channels maintained at vacuum and atmospheric pressure. A variety of sensor types have been developed. These include self-adhesive elastomeric sensors for the measurement of surface crack initiation or propagation and sensors integral within structure, for example between the faying surfaces of a lap joint. The sensors are produced from a variety of * Corresponding Author Email: m.wishaw@smsystems.com. au materials to suit the environmental conditions and can have a sensitivity down to 250 μm with an accuracy of better than 10 μm (4%). The sensors are inert and may be left in-situ on a test object for real-time or periodic monitoring. Figure 2 The SMS Laboratory Kit including a Kvac 4 (vacuum source), a SIM 8b (fluid flow meter) and a sensor. The self-adhesive sensors are manufactured from an elastomeric polymer, allowing the sensor to conform to the surface contours, including peened surfaces [2,3,4] and complex curves. Surface preparation is similar to degreasing for a strain gauge application. The sensing face of the sensor has an pressure sensitive adhesive layer applied at manufacture, allowing the sensor to be installed in much the same way as a self-adhesive label and without the requirement to heat cure, which may effect the material properties of the component under test. The elastomeric properties of the sensor material (~7%) allow the sensor to conform to any strains applied to the test component without affecting its ability to detect crack initiation – the sensors have performed equally under all loading conditions applied to date. Further, the sensors do not suffer the restrictions of wire crack gauges and the like, which have a high probability of failure if a bending moment is repeatedly applied to the sensor. The CVM system will give a false positive reading in the unlikely event of a sensor disbond, allowing the test program to automatically shut down before a catastrophic failure of the specimen can occur. CRACK INITIATION MEASUREMENTS CVM has been developed primarily as a tool to detect crack initiation. Once a sensor has been installed, a base line reading is made. Generally, the differential pressure between the reference vacuum (Kvac) and the sensor will be approximately 0 Pa. However, if there is a known existing flaw beneath the sensor, or the permeability of the test material is high, the fluid flow meter (SIM) will measure a non-zero base value. If this non-zero value is constant, it will not affect the ability of the CVM system to detect an increase in total crack length. When a vacuum gallery is breached by a crack, molecules of air will begin to flow through the path created by the crack. The SIM contains an impedance to the flow of air, with the fluid flow being measured as the difference in pressure between the sensor input and the reference vacuum level. Once the system has reached an equilibrium flow rate, the volume of air passing through the crack is equal to the volume of air passing through the SIM; the measured differential pressure will become constant at a higher value. In this way the SIM is very sensitive to any changes in the total crack size. The CVM method is unable to differentiate between a single large crack and several smaller flaws, however is sensitive to any increase in the total crack length. The sensitivity of the sensor is determined by the gallery wall thickness, determining the shortest crack that may be detected. A test program was conducted by DSTO in 1996 [2,3,4] to determine if CVM and Holographic Laser Interferometry (HLI) could be used to detect cracks on peened surfaces. An additional complication to the test program was the specimen type generally fails from a corner crack, so the NDI methods had to be applicable to these corner regions. Figure 3 shows the CVM sensor, as setup on the specimen, and the galleries in the sensor after the specimen was broken. The gallery wall thickness is 250μm. Due to a limited number of specimens the HLI was used post CVM detection. Figure 3 Picture of CVM system on a peened specimen surface. The capillary wall thickness is 250μm. Two coupons were used, the first specimen to generate a CVM output vs. crack depth curve and the second to confirm the result. The specimen failed from a corner crack, with the progress of the crack throughout the test determined by quantitative fractography. Figure 4 shows the CVM response with number of programs, as well as the crack growth behaviour. Figure 4 suggests once the initial atmosphere/vacuum breach has occurred, that CVM will respond to crack growth. 1 Defence Science and Technology Organisation, Engines and Airframes Division, Melbourne, Victoria, Australia