Application of High-resolution Acoustic Imaging for Adhesive Bond Evaluation

The quality of adhesive bond joints is characterized using acoustic methods and discussed in this paper. Acoustical images of basic types of defects in models and real industrial samples are represented. The material variety includes metal (steel, aluminium alloys, magnesium) bonded with epoxy-based structural adhesive. Three-dimensional data were obtained using an acoustic scanning microscope in the frequency range of 10-250 MHz. Additional digital signal processing was applied to reduce interfering reflections inside the jointed structure. Classification of defects is given according to their detectability. The advantages and main limitations of the acoustical imaging method are discussed. The acoustical data are proved by comparing them with destructive testing results. The discussed results assigned for quantitative estimation of quality of the adhesive joint to create requirements for development of specialized ultrasonic equipment. Introduction: Adhesive bonding has been used for many years for construction of aircraft and in the automotive industry. The utilization of adhesive bond joints in automotive and aerospace assemblies is increasing as the selection of materials used to build these assemblies broadens. The use of adhesive bonding in combination with traditional joints (spot welds and rivets) gives significant cost-effective advantages, especially in automotive production. Some of the advantages of using adhesives include the following: uniform distribution of bond stress, ability to join dissimilar substrates and fill gaps, high bond strength, and higher fatigue resistance than mechanically fastened structures, corrosion protection, and invisible bonding. The necessity for quality assessment of the joints, preferably non-destructive techniques, increases with development of adhesive bonding technology [1]. Due to industry’s increasing use of adhesive technology as an alternative to mechanical fastening, nondestructive characterization of the adhesive joints has received attention in the past decades. NDE of the interfacial region of the adhesive joint has significant practical importance as by indirection indicates adhesion strength and can reveal appearance of the defects on the interface during the production process and after environmental influence. A number of methods are used for evaluation of the microstructure of the adhesive joints as well: visual inspection; light microscopy, electron transmission and scanning microscopy; X-rays, and IR-radiometry, based on the recording variations in heat absorption or emission. The method of pulse-echo scanning acoustical microscopy is still the most reliable for non-destructive examination of the quality of adhesive joints. Using of frequencies of 3.5 – 100 MHz gives a resolution of up to 20 μm with the affordable depth of sound penetration in most of materials. Visualization of the results in the form of Band C-scans provides direct information about the nature, structure and spatial distribution of the defects. The problem discussed in this paper regards the ultrasonic evaluation of the adhesive defects in the thin metal sheets joined using a layer of epoxy-based adhesive. Results and Discussion: Specimens for the experiments consist of two steel sheets with a thickness of 0.8-2 mm, joined by a 0.3 mm layer of the industrial structural epoxy adhesive. Images have been obtained on the Sonix HS-1000 and Tessonics AM-1103 acoustical microscopes. Both microscopes are able to produce three dimensional data with the capability for following data processing (Fig.1.). Fig. 1. Three-dimensional acoustical data. Conventional ultrasound instruments employing the reflection of pulsed acoustic wave gives an A-scan in the form of damping series of the oscillations (Fig. 2). The pulse travels through the material which is under investigation and then the amplitude and position of the pulse are analyzed. The first and second pulses (signals 1 and 2) represent reflections from upper and lower surfaces of the metal sheet. The operational frequency should be high enough for the separation of these signals, and we found that the optimal frequency range for metal sheets with thickness of 0.8 – 2.0 mm is 10 – 50 MHz. Mapping of the second pulse’s amplitude (C-scan) gives us an image of the interface conditions, voids and disbonding areas. This imaging method is the most sensitive to the changes in acoustic parameters of the adhesive