Non-Destructive Evaluation

Courtesy The ChemQuest Group

This material was extracted from ChemQuest's North American Adhesive Bonding Professional Training Course. To learn more and to register for the next course, click here.

Conventional test methods, such as overlap shear test, compression shear and cyclic fatigue all result in the destruction of the joint. Such test methods are not suited for in-process control of adhesive joints. Test methods that do not destroy the bonded part are needed. A number of non-destructive test (NDT) methods are available, but their use is currently limited to a few industries.

NDT methods include:

Visual Inspection

The principle behind the Visual Inspection Technique is, quite simply, to inspect the bonded component with the naked eye. Often this process is standardized. Many leading aerospace companies, such as EADS Airbus, Bombardier, and Boeing, have established test plans in place.

One of the shortcomings of this technique is the fact that it is limited by the resolution of the human eye, and limited to visible external portions of the bonded component (unless the user has “X-ray” vision!).

The types of defects which are more commonly discovered using this technique include: condition of the surface of the bonded component (i.e. are such flaws as scratches, imprints, cracks, corrosion present?) as well as external portions of the bonded joint which may be contaminated by corrosion, blisters, cracks and voids.

The Tap Test

The Tap Test involves tapping the bonded component with a coin or small hammer (e.g. Boeing Hammer). The sound produced differs between bonded areas which are considered intact (a good bond) vs. defects.

One of the shortcomings of this technique is that the human ear is limited in terms of how fine a resolution it can pick up.

The types of defects which are more commonly discovered using this technique include those present near the surface. However, it is often difficult to quantify the size or position of them.

Applications which commonly use this technique include: metal-to-metal bonded joints; voids, delaminations, and disbonds (where the adhesive is not uniformly applied across the bond line). Several prominent aerospace companies such as Airbus and Boeing have developed standardized approaches.

Ultrasonic Testing

The Ultrasonic Test technique involves an ultrasonic transducer and transmitter. The ultrasonic pulse which is generated is expressed in terms of burst length or cycles, most typically ranging from 2 to 10 MHz. The ultrasound wave propagates through the bond joint and reaches defects and other non-homogeneous characteristics. The transmitted wave undergoes attenuation and, sometimes, extinction. The ultrasound receiver will detect transmission and reflection of the wave. The intensity of the wave received correlates with defect size and specific adhesion properties. The time of flight (travel time) correlates with the depth of a defect. The surface of the bonded component is scanned with a test head to asses the position of the defects. A coupling agent, such as water, honey, or oil (viscous medium) is employed.