Disbond

The performance of the classifier has been verified using a number of practical applications, such as civil engineering [3], inspection of aerospace composite structures, ball bearings and aircraft multi-layer structures. Here we present shortly some results, focusing on detection of disbonds in adhesively joint multi-layer aerospace structures using Fokker Bond Tester resonance instrument, details can be found in [1]. 

During the inspection of an unknown object its surface is scanned by the probe and ultrasonic spectra are acquired for many discrete points. Disbond detection is performed by the operator looking at some simple features of the acquired spectra, such as center frequency and amplitude of the highest peak in a pre-selected frequency range. This means that the operator has to perform spectrum classification based on primitive features extracted by the instrument. 

A number of real objects with artificially made disbonds were tested using the Fokker Bond Tester and spectra were stored in a PC for the classification. One of the objects, "Lower wing skin is shown in Figure 4. As can be seen, the positions and sizes of flaws are marked. The same marks were also drawn on the actual objects to facilitate measurements. 

The NSC was trained using labeled data acquired during inspection of objects with known defects. Examples of spectra for the object Lower wing skin are shown in Figure 5, the spectra measured for the flawless structures for different number of layers in the upper panel, the spectra corresponding 100% and 50% disbonds in the middle and lower panel, respectively. The size of the disbonds is given as a percent of active surface of the probe used for the test. 

Classifier structures resulting from the training were verified in a blind test. To evaluate the reliability and performance of the NSC it was subjected to a blind test using unknown data containing spectra measured for various sizes and locations of the disbonds (from 50% to over 100% of the probe size). 

Generally, it has been shown that the NSC is capable of detecting very small disbonds (larger than 25% of the sensor area) and correctly identifying their position in the structure (identifying the defect joint). 

Today the coin-tap test is a widely used technique on wind turbine rotor blades for inspection of thin GFRP laminates for disbonded and delaminated areas. However, since the sensitivity of this technique depends not only on the operator but also on the thickness of the inspected component, the coin-tap testing technique is most sensitive to defects positioned near the surface of the laminate. Therefore, there has been an increasing demand for alternative non-destmctive testing techniques which is less operator dependent and also more sensitive to delaminations and disbonded areas situated beyond thicker GFRP-laminates. 

Shell and tube Rolled ends at tube sheet Open welds at tube sheet Beneath deposits Water box gaskets Bolt holes, nuts, washers Baffle openings Disbonded water box linings... 

Resilient materials such as rubber and some plastics may be useful in certain applications, especially under conditions of low cavitation intensities. However, such materials are subject to disbondment at the metal and elastomer interface at high cavitation intensities, even if the exposure is brief. 

Water pipe length 2 to 4 m, 1.3 m deep soil pipe length 12 m, 1 m covering. The disbonding is clearly time dependent. 

The production of OH ions according to Eq. (2-17) or (2-19) in pores or damaged areas is responsible for cathodic disbonding [9,10], where the necessary high concentration of OH ions is only possible if counter-ions are present. These include alkali ions, NH and Disbonding due to the presence of Ca ions is... 

Fig. 5-8 Total adhesion loss of a 500-/xm-thick coating of EP (liquid lacquer), 0.2 M NaCI, galvanostatic = -1.5 /tA nrr, 5 years at 25"C. Left coating with a pin pore loss of adhesion due to cathodic disbonding. Right pore-free coating loss of adhesion due to electro-osmotic transport of H O. In both cases the loose coating was removed at the end of the experiment.

The disbonding rate decreases with time [35], which can be attributed to the consumption of OH" ions by reaction with adhesive groups. This consumption is obviously partly compensated for by the formation of OH" ions through oxygen reduction these permeate inward from the outer surface of the coating. If this permeation is hindered by an aluminum foil gas seal, the disbonding rate falls off... 

Fig. 5-9 Effect of KOH concentration on disbonding depth steel pipe with PE coating on fusion adhesive, free corrosion, 10 days, 25°C, defect 1 cm diameter.

Fig. 5-10 Effect of NaCl concentration on disbonding depth on coated pipes,...

Fig. 5-11 Effect of potential on disbonding depth on coated pipes, 0.1 A/Na SO, 370 days, 25°C, size of artificial holiday,...

A steel surface can be wetted and corroded by permeation of corrosive agents (see Section 5.2.2) and by cathodic disbonding (see Section 5.2.1.5). In all cases the corrosion rates are negligibly small (see Table 5-2). In the case of cathodic... 

The pulse-echo and resonance impedance techniques are capable of determining which layer contains a disbond, given an appropriate standard to compare against. Inspection standards are bond assemblies made to simulate specific areas of a part with disbonds purposefully placed in them. By comparing the signal response of a suspected void to that of a known disbond in the standard, the... 

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