Adhesive bonding is a joining technique in which the adhesive takes the place of a mechanical fastener. It has many advantages over mechanical fasteners, including: homogenous stress distribution, full automation capability, lighter finished goods, strong and complex structure design, capability of joining two distinct materials, electrolytic and corrosion protection properties, and high fatigue resistance. In order to understand where adhesive bonding can be used, evaluation methods, such as non-destructive testing (NDT), need to be understood.

NDT refers to evaluation techniques that allow observers to collect information about an object without damaging it. It also may be referred to as non-destructive examination or evaluation (NDE) or non-destructive inspection (NDI). In practical applications, NDT can refer to several activities, including inspection tools, inspection methods, or the entire field of non-destructive inspections. NDT test methods do not necessarily require specialized instruments. In adhesive applications, NDT is most commonly used to evaluate composites and bonded joints.

Conventional NDT methods are often unable to assess the performance of an adhesive bond. Literature concludes that, although defects can clearly be detected in adhesive composite joints, information is unavailable regarding the quality of the adhesion, its strength, or its properties.

“NDT refers to evaluation techniques that allow observers to collect information about an object without damaging it.”

Destructive vs Non-Destructive Testing (NDT)

Many common adhesive testing methods are considered destructive testing methods, meaning the test alters, damages or destroys the materials tested and renders them irreparable.

The motives for NDT include:

• Safety – NDT is an attractive technique because most NDT techniques are harmless to conduct.

• Investment – NDT is more enticing than destructive testing because it allows the material being tested to survive intact. This saves money and resources, and it allows for greater use of adhesive bonding compared to mechanical bonding.

• Effectiveness – NDT methods allow for detailed and rapid evaluation of assets, which can be crucial for ensuring continued safety and performance on a job site.

• Accuracy – NDT methods have been proven to be accurate and predictable.

Common NDT Methods

Here are some of the most common NDT techniques used in adhesive bond evaluation.

Visual Testing

Visual testing (VT) is the act of collecting visual data on the status of a material. VT is the most basic way to examine a material or object without altering it in any way. It can be done with the naked eye or with a tool, such as a fast speed camera.

Ultrasonic Testing

Ultrasonic or ultrasound testing (UT) is the process of transmitting high-frequency sound waves into a material to identify defects or imperfections on the surface of the material. UT is the most commonly used adhesive bonding NDT method and falls into two categories: compression wave (out-of-plane measurement) or shear wave (in-plane measurement), with many specialized technologies available in these categories. Shear waves tend to be used more frequently since adhesive bonds often rely on shear stress rather than compressive stress.

The limitation of this NDT is that the technology may not be able to detect changes in adhesive bond strength. The ultrasonic waves can still yield information regarding the structure and elastic features at the adhesive bond site.

Acoustic Emission Testing

Acoustic emission (AE) testing uses acoustic emissions to identify possible defects and imperfections in a material by using bursts of acoustic energy. Intensity, location, and arrival time can be studied to expose information about possible defects within the material.

Leak Testing

Leak testing (LT) refers to the process of studying leaks in a vessel or structure using measurements taken with a pressure gauge, soap-bubble tests, or electronic listening devices to identify its defects.

Digital Shearography Testing (DST)

Digital shearography testing (DST) measures compressive (in-plane) and shear (out-of-plane) waves, by observing the electromagnetic waves between an object and the inspected surface. This technique is sensitive to a few nanometers and can be disrupted by environmental conditions, such as heat.

NDT CODES AND STANDARDS

The most common organizations for the creation of NDT standards and codes include:

• American Society for Testing and Materials (ASTM)

• American Society for Nondestructive Testing (ASNT)

• French Committee for Non-destructive Testing Studies (COFREND)

• Canadian Standards Association (CSA Group)

• Canadian General Standards Board (CGSB)      

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