This article discusses the use of pulse-echo ultrasonic testing for the inspection of adhesive bonds between thin metal sheets (0.8 mm). The method is based on the measurement of the reflection coefficient at the metal/adhesive interface. After describing briefly the physical aspects of the phenomenon, an index is defined to detect defective zones of the joint (both for the lack of adhesive and for insufficient adhesion); the influence of the experimental variables (transducer frequency, coupling medium and contact force) on the measurement is discussed. The analysis shows that coupling medium and transducer frequency do not influence the results and that the method is robust with respect to the variation of contact force. By means of a control experiment it is shown that the statistical distribution of the index corresponding to good and defective adhesion zones are sufficiently separate to be distinguished. Finally, a procedure based on the statistical theory of decisions is proposed to evaluate the integrity of the joint under test.

Ultrasonic testing of adhesive bonds of thin metal sheets

In this paper, a noncontact ultrasonic testing method with using a small electromagnetic acoustic transducer (EMAT) in resonant mode is proposed for metal-based adhesive debonding inspection. The small EMAT with a new magnet configuration and a small coil is developed firstly to improve the spatial resolution and detecting ability for minute defects. Different from conventional pulse echo method using short pulse excitation for debonding inspection, a new method based on power spectrum estimation of the resonant ultrasonic signal with long burst excitation is applied to increase the signal-to-noise ratio (SNR) and detecting ability of the small EMAT. Finally, ultrasonic C-scan images of specimens with various adhesive bonding are obtained to demonstrate the capability of this technique.

Adhesive debonding inspection with a small EMAT in resonant mode

Application of a higher order crossing feature to non-destructive evaluation: a sample demonstration of sensitivity to the condition of adhesive joints

A new time-domain ultrasonic NDE technique is reported in this paper for the measurement of the thickness (given the wavespeed) or wavespeed (given the thickness) of thin plates; by thin we mean the thickness is less than the wavelength. By introducing a retrieve function the incident field can be reconstructed from the transmitted (reflected) field by a two-term (three-term) summation. A systematic sensitivity analysis of this technique has been carried out. The new technique has been used to measure the thickness or the wavespeed of aluminum plates with thickness ranging from 0.089 to 6.426 mm using (low frequency) 1-MHz transducers: 0.014≤(thickness/wavelength)≤1.0. The measurement error was found to be about 1% for thickness and 3% for the wavespeed.

A new technique for time-domain ultrasonic NDE of thin plates

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Infrared thermography and ultrasonic inspection of adhesive bonded structures, overview and validity

A novel method for evaluating nondestructively the quality of bonded assemblies is presented. It is shown that a combination of HOC (higher order crossing) analysis and another frequency domain parameter can distinguish between different surface treatments, as well as predicting cohesive or adhesive failure of bonds. It is believed that this method will open the way to predict the durability of bonded structures at the manufacturing or inservice stages.

E. Segal

Papers

A novel method of processing pulse echo data in adhesive bond inspection

Acoustic Resonance Methods for Measuring Dynamic Elastic Modulus of Adhesive Bonds