Non-destructive testing of adhesive bonds

Jana Dostal, Februar 2019

In German industries almost 2.4 million metric tons of adhesives, such as sealing adhesives and cementitious construction adhesives, and nearly 1 billion square meters of carrier-bound adhesives (tapes and adhesive foil) are produced every year. Those adhesives are used in industrial applications and subject to high-quality requirements. In order to check the quality of a bond, non-destructive testing methods are used. There are various methods available for non-destructive testing of adhesive bonds, which method to be applied is depending on the types of errors that are of interest ^[1].

Motivation

Nowadays, joining with adhesive bonds is common in many industries including, the food industry, the health industry, electronic engineering, household applications, crafts, and industrial production. By using adhesives, it is possible to combine different materials, creating a larger scope for design, material selection, and producing technics. In addition, there is no material damage (e.g. holes), which makes the preservation of material properties, adaptability, and tolerance compensation feasible. Finally, it is possible to connect smallest, but large-scale parts as well ^[2]. Work samples, examined during production, are usually tested destructively. However, this causes scrap costs, and the tests cannot be performed during the manufacturing process and maintenance. Occasionally, non-destructive methods are used. Thus, quality features, such as the minimum width of the adhesive bond, the percentage of joined surface, and potential defects in the adhesive bonds, can be tested. Non-destructively tested connections have the advantage that they can be used repeatedly since they can perform during production and early damage detection is possible. In addition, maintenance is possible due to the non-destructive process. Nevertheless, non-destructive testing methods face some challenges; it should be applicable in complex geometries and all stages of the manufacturing process and maintenance. Moreover, often there is only one-sided access, and the inspector for non-destructive testing must be qualified and certified according to DIN EN 473 ^[3].

Basics of adhesive bonds

According to DIN EN 923, an adhesive is a non-metallic material that bonds together adhering parts of different or same material through adhesion and cohesion. Adhesive bonds consist of two joining partners and the intermediate adhesive layer. At the boundary phase, contact to the adhesive leads to interaction of the molecules of the adhesive layer with the molecules of the adherend (adhesion). For best bounding, the adhesive must be liquid during the joining process. The internal strength of a material (cohesion) is possible through physical processes or chemical reactions ^[4]. Figure 1 shows the schematic structure of such an adhesive joint.#

Failure mechanism and defects ^[5][6]

Failure mechanism

Irregular wetting of the surface

• Surface contamination by fat, oils and dusts
• Insufficiently formed adhesion
• Low contact
• Cavities, pores, bubbles
• Seam interruption, lack of adhesive volume, fillets
• Geometric error at position, adhesive surface, or in the vicinity
• Moisture condensation through temperature difference
• Oozed Softener components
• Too high adhesive viscosity

Insufficient adhesion properties and adhesion breakage

• Adhesive detachment
• Insufficiently formed adhesion: exceeding the pot life, contamination, proceeding temperature too low
• Insufficiently cured adhesive: Wrong condition of adhesion, sticky surface
• Underground production errors: Fracture, stress crack in the adherend, insufficient preparation and coating
• Corrosion of the adherend on the adhesive surface, or close to it
• Fogging with and without penetration in adherend

Insufficient cohesive strength

• Coherent crack in the adhesive:Cut, mechanic fault, shrinkage
• Insufficiently cured adhesive: mixing errors, adhesive errors, overlays, mechanical failure during curing
• Module and maximum stress deformation of the adhesive beyond permissible limits: Curing at too high temperature, post-crosslinking or crystallization
• Permanent stress- and deformation state of the adhesive layer due to creep: With plastically deformed parts: bend, dent; With elastically deformed parts: mechanical stress
• Damage: Structural damage, swelling, dissolving, stress fracture

Defects

Figure 2 shows defects of adhesive bonds.

Non-destructive testing

Visual testing

In visual inspections, the adhesive bond to be examined is visually checked by a human. This way, errors, such as a difference in colour and shine, mechanical defects, large surface cracks and in transparent materials, as well as blisters and cavities, can be detected.

The advantage of this method is the fast, simple and cost-effective analysis. A disadvantage is the limited accuracy of the human eye. In non-transparent material, visual testing is limited to the surface of the material ^{[7] [8]}.

Example: testing the adhesive bonds of glass panes in the automotive industry ^[9].

Leak-Test

The leak-test is a method to detect manufacturing errors and defects that allow a leakage of liquid ^[10]. Usually, trough Measuring the flow rate of a gas, a leak or defect can be localised. Nowadays, those tests are strictly defined through a lot of standardising organisations, such as the requirements of ASTM (American Society for Testing and Materials), SAE (Society of Automotive Engineers), and internal OEM (Original Equipment Manufacturer) specifications.

There are several methods to perform such a test. The most common technique is to seal the part (overpressure) and place it in a pool of water. Leak points then cause blistering. These gas blisters can be detected using ultrasonic sensors. By evaluating the transit time measured, the defect can be localised. Another possibility is to generate a pressure difference between the object and the detection device. The pressure of the gas increases or decreases on the side of the object, compared to the side of the detection device. If there is a leak, a constant gas flow from the high-pressure side to the low-pressure side can be detected. For the most part the test gases used are helium (high reliability, as it does not enter into chemical bonds as inert gas), sulphur hexafluoride, or hydrogen.

A disadvantage of this test is the dependence on pressure difference: With larger pressure difference, the gas flow increases the same on the proper side as it does on the defect one. The advantage is that the leakages can be determined qualitatively and quantitatively. Moreover, the test is sensitive to distinguish between big and small leakages ^{[11] [12]}.

Tap-test

The tap test involves tapping the bonded component with a coin or a small hammer. This creates a local vibration. The generated sound is different in bonded areas that are flawless (good bond), compared to defective areas ^[13]. Using a microphone, the vibrations are recorded.

The advantage is that the position and size of a defect are determinable. However, the test is limited to defects near the surface, and the depth of the defect cannot be determined through the measurement data ^[14].

Example: testing of metal-to-metal bonded joints, several aerospace companies have developed standardized approaches ^[15].

Ultrasonic testing

Ultrasound

Analysis of adhesive defects

For ultrasonic testing of adhesive defects in adhesive bonds, the ultrasonic echo method can be used, which only requires a one-sided access. A transmitter is coupled with an agent (water, oils, and pies). This sends an ultrasonic pulse into the workpiece. The waves of the pulse are continuously moving into the component, until they encounter an impedance difference. Because of the impedance difference, part of the impulse is reflected and returned to the object surface, where they are registered by the receiver ^[16].

Normally, air and material have a different impedance. Therefore, air inclusions (e.g. cracks and cavities) can be detected easily. In addition, knowing the runtime, the test can also provide information about the depth of the defect and the thickness of the bond ^[16].

A disadvantage is that defects, that are smaller than the wave length of the ultrasonic signal, cannot be recognised, because the wave passes them. Therefore, those defects cannot be measured. Furthermore, near-surface errors cannot be detected when the transmitter and the receiver are one device, therefore no reflections can be detected during the time interval when the transmitter switches to receiver ^[17].

Analysis of cohesive defects

Ultraschall-Durchschallung

Cohesive defects and their position can be determined via ultrasonic transmission. In ultrasonic transmission measurement, an ultrasonic signal is emitted to one side of the test object and recorded on the other side. Knowing the distance between transmitter and receiver and measuring the time between emitting and receiving the signal, the speed of the wave can be determined. With knowing the speed and determine the change of transmission signal, the cohesive defects can be localized ^{[17] [18]}

Air-coupled ultrasonic

Air-coupled ultrasound: operating principle and applications

Air-coupled ultrasonic testing is a special method of the ultrasonic testing. In contrast to the other methods, air is used as the coupling agent ^[19].

Example: testing of bonded folded seams joints in the automotive industry ^[20]

Acoustic emission testing

In the acoustic emission test, the sudden change in the structure of a material by overstraining is used for the emission of sound. For this purpose, an external load is applied to the object, leading to a stress concentration. These stress concentrations cause microcracks in the material long before the material fails. This creates a momentary movement impact, the so-called sound emission event. It results in the spread of an elastic wave, which is converted, through a piezoelectric transducer, into an analogue electrical signal, which then can be evaluated ^[21].

One advantage is that a defect in the full volume of the object can be detected with a small number of sensors. In addition, structures can be tested under operating conditions, so that even the smallest indication of damage can be detected. A localisation of the source of a sound emission signal is possible, and with further signal analysis, the types of sources can be distinguished. The disadvantages are that this testing methode is not entirely non-destructive and that static defects cannot be identified ^[21].

Example: testing of Composite sandwich structures (e.g. carbon fibre reinforced composite-to-metal adhesive bonded substrates) ^[22].

Radiography and computed tomography

X-ray radiation is used in imaging processes that show inhomogeneities, material flaws, and thickness changes in matter. ^[18]. In this case, high-energy electromagnetic waves (ionizing radiation) from the X-ray source pass through the object and are recorded on the other side of the object. Since X-rays are greatly attenuated by matter, a contrast can be made visible. However, this method has the disadvantage that some materials (eg. steel) absorb most of radiation, so defects are difficult to find.

Example: testing of adhesive bonding in wood ^[23].

Shearography

Shearography

This method is based on detecting relative phase differences of coherent electromagnetic waves between the object and the detector. For this purpose, the object is illuminated with coherent laser light, which causes diffuse reflections of the light on the surface of the object. This results in microscopic interference waves. A statistical overlay of many interference waves with different alignments leads to speckle formation: Correlation stripes are created ^[24]. Then two different load conditions are compared.

Through a qualitative evaluation of the Shearogramm, the position of strain concentrations can be determined. These provide comprehensive information of defects on and below the surface ^[25].

Example: inspection of aircraft structure ^[26].

Infrared thermography

Infrarot-Thermographie an Kohlefaserverbundwerkstoffen

In infrared thermography the given fact is used, that each body emits above absolute zero thermal radiation ^[27]. In infrared thermografic testing, the object emits electromagnetic heat waves inside the infrared range. Through a lens system, these are projected onto electromagnetic image sensors. The sensors convert the temperature information into electrical signals. Since materials at the same temperature emit the same waves in some cases an external stimulation is necessary to make flaws visible, e.g. in testing adhesive bonds (Lock-In Thermography).

The advantages of this method are, that the measurements are contact-free, so that immediate imaging and image sections of different sizes, can be tested integrally. However, a decrease of the resolution is to be considered by enlarging the image size. On the other hand, the low penetration depth and the possible influence of stray radiation and relative humidity, are among the disadvantages ^[17].

Examples: control of water inclusion in the horizotal stabilizer, for detachments in the area of outer skin and for faults in the rudder of aircrafts ^[28].

Conclusion

Literature

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