Acoustic inspection methods

Parallel to tliese tests, CETIM has informed the administration which establish regulations for pressure vessels of the advantage of the acoustic emission method for the inspection in-service. 

Insofar as Ultrasonic Array probes have come onto the market from several years and are now moving from prototype stages into industrial tools for on-site inspections, methods and tools for acoustic characterization is becoming a real concern. Furthermore, the lack of standards, either national or European, enhances the needs for guidelines proposal. 

The utility of the acoustic emission method is also illustrated in the inspection and failure rates of FRP tank given below ... 

Although there is no standard test method for acoustic microscopy. Method 2030, Ultrasonic Inspection of Die Attach of MIL-STD-883F is used to nondestructively detect unbonded regions and voids in IC die-attachment materials. Parts are rejected if a single void larger than 15% of the contact area is present (>10% at a comer) or more than 70% of any given quadrant is detached. 

General corrosion control, prevention, and monitoring should be planned for from the onset. To detect and monitor corrosion, various methods must be used, while corrosion protection would be done by the addition of inhibition and use of protection coatings [5]. Visual examination, laser methods, the replication microscopy liquid penetration testing method, magnetic particles testing, the eddy current inspection method, acoustic emission technique, thermal methods of inspection, and nondestructive methods are the various methods to be adopted when monitoring pipeline corrosion [5]. 

To the extent practicable, the pressure retaining components shonld be visually examined while the system is operating under the test pressnre and temperature conditions. The test pressure and temperature should be maintained for a sufficient period before the examination to ensure that all possible leakages can be identified. The accessibility of components to be visually examined should be considered (for example, with regard to the possible need for removal of insulation). Acoustic emission methods may be used as part of such inspections. 

Summary. Nondestructive inspection methods vary from the simple straightedge to sophisticated acoustic emission, and the method selected will depend upon DOT or TC regulations and the nature of the inspection required. 

Acoustic leak detection methods such as illustrated in Fig. 8.4 may find already broken or damaged pipes [7]. However, corrosion monitoring possibly may identify areas where corrosion activity on pipes is likely while the remote field effect can inspect pipes to find damage before they fail. A complete diagnostic program is likely to use all these inspection methods. 

Nestleroth et al. [15], Segal et al. [16] considered some established novel signal processing schemes to assist in adhesive bond inspection. Sinclair et al. [17] and Filimonov [18] employed acoustic resonance methods for dynamic elastic modulus measurements in adhesively bonded structures. Yost and Cantrell [19], Achenbach and Parikh [20] and Nagy et al. [21] considered a nonlinear response of bonded structures to estimate material characteristics. In Achenbach and Parikh [20], failure was preceded by nonlinear behavior of thin boundary layers at the interfaces. Billson and Hutchins [22] considered lasers and EMATS in bond investigations. It was shown that this non-contact technique was reasonable when compared to that obtained by conventional piezoelectric transducers. Ince et al. [23] also characterized bonds with laser-generated ultrasound and through-transmission measurements. 

However, it appears that none of the non-destructive tests currently employed directly correlate with any critical failure property. Most industrial test techniques such as through-transmission and pulse-echo ultrasonics, sonic vibration techniques. X-ray radiography, thermal inspection methods, holography, liquid penetrants, etc. basically attempt to find defects in the joint. Such defects may arise from several sources. Some defects arise from porosity, cracks or voids in the adhesive layer or at the interface and are typically filled with air they will simply be referred to as voids in the present discussions. However, during the service life of the joint such voids may fill with water which makes them far more difficult to detect since, for example, water has a much higher acoustic, impedance than air. Also, zero-volume voids, or debonds, may occur when the adhesive and substrate are in contact but no... 

There have been numerous efforts to inspect specimens by ultrasonic reflectivity (or pulse-echo) measurements. In these inspections ultrasonic reflectivity is often used to observe changes in the acoustical impedance, and from this observation to localize defects in the specimen. However, the term defect is related to any discontinuity within the specimen and, consequently, more information is needed than only ultrasonic reflectivity to define the discontinuity as a defect. This information may be provided by three-dimensional ultrasonic reflection tomography and a priori knowledge about the specimen (e.g., the specimen fabrication process, its design, the intended purpose and the material). A more comprehensive review of defect characterization and related nondestructive evaluation (NDE) methods is provided elsewhere [1]. 

This paper intends to give, through different examples, guide-lines for characterization of array probes. We discuss, particularly, beam pattern measurement methods and raise the question whether it is useful to achieve a full characterization of all beams steered by the probe or to limit the characterization to a minimum set of acoustic configurations. An automatic bench for full characterization of tube inspection probes is described. 

The new instrument introduced for inspection of multi-layer structures from polymeric and composite metals and materials in air-space industry and this is acoustic flaw detector AD-64M. The principle of its operation based on impedance and free vibration methods with further spectral processing of the obtained signal. 

Magnesium anodes are usually built into the object to be protected through isolating sockets or holes (see Fig. 20-6) and joined to them with cables. They must be readily accessible and easily exchangeable for convenient control [4]. The directions for use have to indicate the necessity for an inspection after 2 years of operation. During operation, control can be exercised by electrical measurements (current, resistance). In addition, acoustic and optical methods exist to determine the amount of anode consumption [4]. The life of the anodes is usually more than 5 years (see Section 6.6). 

Acoustic Emission Inspection Quite useful for locating defects in fiberglass and other composite materials. Once the location of a defect is determined, other NDE methods can help determine its severity. 

Applying a combination of NDT methods is advisable in many cases. A useful guiding principle is to choose NDT methods that yield complementary information, eg, based on different probe types (Table 2). One example is acoustic emission monitoring with high sensitivity for potentially critical areas, followed by high spatial resolution inspection of such areas by, eg, ultrasonic or X-ray methods to quantitatively locate and size the indication. 

Once a coated substrate has been placed into service it is important to periodically inspect the film for defects. It is just as importeuit to know whether or not there has been any deterioration in the bond strength between the film and the substrate. Methods to accomplish this have been developed based on acoustic or thermo-optical principles. 

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