Fokker bond tester

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]. 

Fokker Bond Tester. An ultrasonic inspection technique commonly used for aircraft structures is based on ultrasonic spectroscopy [2]. Commercially available instruments (bond testers) used for this test operate on the principle of mechanical resonance in a multi-layer structure. A piezoelectric probe shown in Figure 3b, excited by a variable frequency sine signal is placed on the surface of the inspected structure. A frequency spectrum in the range of some tens of kHz to several MHz is acquired by the instrument, see Figure 3a. 

Figure 3 Experimental set-up for bond inspeelion (a) and probe for the Fokker Bond Tester (b).

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. 

One of the oldest and best known ultrasonic testing systems for NDT is the Fokker Bond Tester. This method uses a sweep frequency resonance method of ultrasonic inspection. Some degree of quantitative analysis is claimed with the Fokker Bond Tester in the aircraft industry. 

Ultrasonic properties Ultrasonic resonance method, Fokker Bond Tester A—>C 50,51... 

Resonance testers A number of devices are available. In Europe, the Fokker bond tester is commonly used. This was developed by the Fokker Aircraft Company in order to... 

Fig. 1. Comparison of destructive shear strength tests on laminates with the predicted strengths using a Fokker bond tester. A series of laminates is represented where the adherend thickness t varies as follows ( ), t = 0.6 mm (O), t = 0.8 mm (x), f = 1.0 mm (-h), f = 1.2 mm (A), t = 1.5 mm. To construct this standard plot, 120 specimens of Dural 2024-T3 were used 95% of all results are within the range 0.36 kg mm . (From R J Schliekelmann, Non-destructive testing of adhesively bonded joints, in Adhesion, Fundamentals and Practice, McClaren, London, 1966)...

Figure 1 shows an example of the degree of correlation that can be achieved between strengths predicted by the Fokker bond tester and those determined destructively for a bonded laminate. Figure 2 shows a similar correlation for bonded honeycomb structures. 

Figure 6.34 The typical relationship between single lap-shear strength and response from the Fokker Bond Tester A-scale . With strengths increasing from zero up to 50% of the maximum, the resonance peak moves to the left further increases in the quality of the joints brings the peak back to the far right-hand side of the oscilloscope, whereupon it continues to move to the left as the strength increases to a maximum [138].

A different principle is used in ultrasonic resonance testing. Here, thickness vibrations are induced in the adherend/adhesive/adherend sandwich in which the two adherends eire considered to act as rigid masses while the adhesive is an almost massless spring. If the type and thickness of the metal sheets is constant, then the resonant frequency is a function of the thickness and bulk modulus of the adhesive. The well-known Fokker Bond Tester (Mark II) overcomes the difficulty of the many variables in the ultrasonic resonance test by coupling the joint to a system of well-defined resonance characteristics. Changes in the frequencies of the joint-transducer system are correlated with known defects and a calibration curve obtained. The calibration curves can then be used to establish acceptance limits which can be (and are extensively) used for quality control in production. 

Guyott CCH, Cawley P, Adams RD (1986) The nondestructive testing of adhesively bonded structure a review. J Adhesion 20 129-159 Guyott CCH, Cawley P, Adams RD (1987) Use of the Fokker bond tester on joints with varying adhesive thickness. Proc IMechE 201(B1) 41—49 Hagemaier DJ (1985) Adhesive bonding of aluminium alloys. Marcel Dekker, New York Hart-Smith LJ, Thrall EW (1985) Adhesive bonding of aluminium alloys. Marcel Dekker, New York, pp 241—335... 

---