Ultrasonic resonance testing

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. 

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 are considered to act as rigid masses while the adhesive is an almost massless spring. If the type and thickness of the metal sheets are constant, then the resonant frequency is a function of the thickness and bulk modulus of the adhesive. 

Aerospace struetwes made of composite. As part of the evaluation of the developed ultrasonic spectroscopy system the NSC software was tested on ultrasonic resonance spectra from composite panel samples. Spectra were collected with four different types of damages, and from flawless samples. The damages included a small cut in one of the carbon fiber... 

We have presented a neural network based spectrum classifier (NSC) aimed at ultrasonic resonance spectroscopy. The ultrasonic spectroscopy and the NSC has been evaluated in many industrial applications, such as concrete inspection, testing of aerospace composite structures, ball bearings, and aircraft multi-layer structures. The latter application has been presented in some detail. 

Delaminations reduce the time required for the reflected signal to arrive at the surface in pulse-echo setups (also shown in Fig. 2.6) which have both the transmitting and detecting transducers on the same side of the test sample. The same method can distinguish between flaws at different depths. A slightly different ultrasonic resonance method utilizes the fact that delaminations reduce the sample stiffness and hence produce a resonant frequency shift. 

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. 

Before considering particular test methods, it is useful to survey the principles and terms used in dynamic testing. There are basically two classes of dynamic motion, free vibration in which the test piece is set into oscillation and the amplitude allowed to decay due to damping in the system, and forced vibration in which the oscillation is maintained by external means. These are illustrated in Figure 9.1 together with a subdivision of forced vibration in which the test piece is subjected to a series of half-cycles. The two classes could be sub-divided in a number of ways, for example forced vibration machines may operate at resonance or away from resonance. Wave propagation (e.g. ultrasonics) is a form of forced vibration method and rebound resilience is a simple unforced method consisting of one half-cycle. The most common type of free vibration apparatus is the torsion pendulum. 

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. 

The resonance frequency technique has been used for determining the adiabatic Young s moduli in dependence on test temperatures up to 1000 °C. The shear moduli were measured by the pulse-echo ultrasonic technique. The bulk moduli were determined by synchrotron radiation diffraction. The temperature-dependent Young s and shear moduli are plotted in fig. 7. 

High-frequency transformers and resonant devices These include various transducers, sonar devices, ultrasonic cleaners, ultrasonic welders, filters, transformers, delay lines, and test-heads for nondestructive testing. Such devices operate at working frequencies ranging from kilohertz to megahertz. 

In the brewing industry, there is no ideal foam-control method currently in use, although ultrasonics have been tested to suppress foaming in fermenting vessels. The ultrasound generated by ceramic piezoelectric transducers has the unusual property that when an electric voltage is applied across their crystalline ceramic structure, deformation occurs, so that an alternating current thus produces a sinusoidal vibration. The resonance frequency depends on the physical dimensions of the... 

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