Infrared thermography on adhesive joints

M. Schweikert, Sommersemester 2020

Introduction

The infrared thermography (IRT) is used to detect different kinds of flaws in adhesive joints. Therefore different methods of excitation are employed. These methods have to be chosen well-considered depending on the used materials, the geometry of the joint and the expected defects.

• 1 Introduction
• 2 Fundamentals of the infrared thermography
• 3 Fundamentals and types of adhesive joints
• 4 Possible failures to be detected/materials to be evaluated
• 5 Active Thermography – methods of excitation – for evaluation of adhesive joints
  • 5.1 Lock in excitation
  • 5.2 Pulse excitation
  • 5.3 Ultrasonic excitation
  • 5.4 Inductive excitation
• 6 Conclusion
  • 6.1 Table of suitable methods for different defect scenarios
  • 6.2 Fitting into the chain with other NDT methods
• 7 Linked articles
• 8 References

Fundamentals of the Infrared thermography

The fundamentals of the IRT are described in the main article.

Fundamentals and types of adhesive joints

The basics of adhesive bonds are shown in this article: Basics of adhesive bonds

Possible failures to be detected/ materials to be evaluated

The possible defects of adhesive joints are described in this article: Failure mechanisms and defects

Adherend materials to be tested are metals like steel or aluminum, composites as CFRP or GFRP as well as plastic materials.

Active Thermography – methods of excitation – for evaluation of adhesive joints

Lock in excitation

Physical principle

The Lock in thermography excites the surface of the tested part by sinusoidal heating by light source (i.e. halogen spotlight). A detailed description of this method can be found in its main article.

Strengths and limitations

The principle of the optical excited thermography methods is based on the thermal contrasts that the defects generate in the sample part. Thus, the higher the thermal contrast effected by a defect, the better it can be detected and (quantitative) identified. If the thermal contrast of a defect is very small, as it is for example considering kissing bonds or small closed cracks, the defect may not be detected.[3]

Also, the overall geometry of the adhesive joint is relevant for the effective use of optical excited thermography. Is the glued surface not planar but for example cylindric, it takes much more effort to apply thermography as it has to be done from many different angles.

The aptitude of this method to detect different types of defects is shown in the table 1.

The Lock in thermography is also able to evaluate additional characteristics of the test piece like coating thickness, the effects induced in

bonded structures by substrate surface treatments, and the effects of crosslinking in polymers.[2]

Pulse excitation

Physical principle

In pulse thermography a short heat impulse i.e. generated by a flashlight excites a thermal waves in the sample. A detailed description of this method can be found in its main article.

Strengths and limitations

For the basics refer to the strengths and limitations of the lock in method.

The aptitude of this method to detect different types of defects is shown in the table 1.

Ultrasonic excitation

Physical principle

Fig 1: Scheme of the set-up for ultrasinc thermography (Source: M. Schweikert)

Ultrasonic thermography (also known as thermosonics, sonic IR or vibro-thermography) uses powerful ultrasonic vibrations in the test piece the cause frictional heating at crack surfaces. These ultrasonic pulses are induced by a plastic welding horn, which is being pressed onto the surface of the test piece,as shown in Figure 1. Typically, pulses of high acoustic power (between 1 kW and 80 kW) in the 15–50 kHz frequency range are applied in a duration from 30 to 200 ms. This excitation method offers very good conditions for IRT, because the heat in the test piece is mainly generated by the “clapping” motion and friction (“rubbing”) of closed cracks. [1]

Strengths and limitations

The ultrasonic thermography is well fitted for the detection of critical discontinuities such as vertical micro-crack and delaminations in metallic as well as in nonmetallic or mixed structures. It can also be used for complicated geometry. For example, deeper delaminations can be detected, even if they are shadowed by shallower defects. A big advantage of the ultrasonic excitation compared to the optical methods is, that the heat does not travel from the surface to a defect and back to the surface. It is generated directly at the defect and therefore has only half the way to travel though the material, which reduces the needed timespan in testing. [4]

Considering difficulties and drawbacks, the following points have to be mentioned. At the moment the ultrasonic IRT is not well automated and often needs manually placing and orienting. There are also some difficulties with bad repeatability and low accuracy when quantitative parameters should be determined.  Increasing the contact force of the plastic horn in order to be able to use stronger aucoustic pulses might cause surface damages in fragile parts. [4]

The aptitude of this method to detect different types of defects is shown in the table 1.

Inductive excitation

Physical principle

Fig 2: Scheme of the set-up for induction thermography (Source: M. Schweikert)

A sample set-up of the induction thermography is shown in Figure 2.

"Induction thermography or pulsed eddy current thermography uses electromagnetic pulses to excite eddy currents in electrically conductive materials. An inductor is positioned at a certain distance from a surface to be tested for surface defects like cracks. In the test object, eddy currents are induced that form closed loops in the material. The currents have to circumvent the cracks. This causes local changes of the electrical current densities in the material. The eddy currents generate heat by resistive losses. The heat can be detected by an infrared camera observing the surface. Around cracks, the current density is changed, e.g., increased at the crack tips. As heat generation is proportional to the square of the current density, the cracks will form a characteristic thermal pattern on the surface which has a specific time dependence that reveals the position and the orientation of the crack." [6] In contrast to optical excitation methods, induction thermography has both an electromagnetic and a thermal aspect. This means that the defect interacts with both the current and the heat flow.

For an estimation of the maximum depth a defect can be detected in, the skin depth has to be considered. This effect is described in the article of eddy current testing.

Strengths and limitations

It is necessary that at least one of the used materials is conductive. So, either the adherends must be conductive (i.e. metal or CFRP) or it is also possible to take adhesive into account, which is made conductive by adding metal particles to its mixture. Besides from that, this excitation method works best for ferromagnetic materials like ferritic steel. [5]

Besides from cracks in the bonded metal parts, thermal contrast is needed to detect a defect in the adhesive just like for the optical methods. The higher the contrast, the better the detection. [7]

The aptitude of this method to detect different types of defects is shown in the table 1.

Conclusion and Outlook

Table of suitable methods for different defect scenarios

Table 1: Aptitude of excitation methods for different defects (Source: M. Schweikert)

Fitting into the chain with other NDT methods

The IRT is well suited for a 100% - evaluation of all produced parts when it is integrated in the production line, since it is fast, contactless, open to automating and can give clean documented results.

Shearography is an interesting competitor of the IRT when evaluating adhesive joints between sheet metal plates. These to methods can also be used in addition to each other in order to increase the achieved information from the evaluated product. Refer to [8].

If IRT detects some irregularities, but it is unsure, whether they are decisive, NDT methods with a higher resolution (and higher effort in cost and time) can be applied. This is especially interesting for very expansive parts or if the production process should be improved. In this case, ultrasonic testing or computer tomography are often focused.

Linked articles

Infrared Thermography

Infrarot-Thermographie an Kohlefaserverbundwerkstoffen

Non-destructive testing of adhesive bonds

Ultrasound (Overview)

Fundamentals of eddy current testing

References

[1]          Ciampa, F., Mahmoodi, P., Pinto, F., Meo, M.: Recent Advances in Active Infrared Thermography for Non-Destructive Testing of Aerospace Components. Sensors 2018. www.mdpi.com/journal/sensors

[2]          Meloa, C., Carlomagno, G. M.: Application of infrared thermography to adhesion science. J. Adhesion Sci. Technol., Vol. 20, No. 7(2006), pp. 589–632.

[3]          Tighe, R. C., Dulieu-Barton, J. M., Quinn, S.: Identification of kissing defects in adhesive bonds using infrared thermography. International Journal of Adhesion & Adhesives 64 (2016), p. 168–178.

[4]          Umar, M. Z., Vavilov, V., Abdullah, H., Ariffin, A. K.: Ultrasonic Infrared Thermography in Non-Destructive Testing: A Review. Russian Journal of Nondestructive Testing, 2016, Vol. 52, No. 4, p. 212–219.

[5]          Balaji, L., Balasubramaniam, K., Krishnamurthy, C. V.: Induction thermography for non-destructive evaluation of adhesive bonds. AIP Conference Proceedings 1511, 579 (2013).

[6]          Netzelmann, U.: Induction Thermography of Surface Defects. In: Handbook of Advanced Nondestructive Evaluation. Springer publ., (2019), p. 1497-1520.

[7]          Srajbr, C., Thiemann, C., Zäh, M. et al.: Induction-Excited Thermography — a Method to Visualize Defects in Semi-Structural Adhesive Bonds of Car Body Structures. Weld World 56, 126–132 (2012). https://doi-org.eaccess.ub.tum.de/10.1007/BF03321343

[8]          Kryukov, I., Thiede, H. & Böhm, S.: Quality assurance for structural adhesively bonded joints by eddy current shearography. Weld World 61, 581–588 (2017). https://doi-org.eaccess.ub.tum.de/10.1007/s40194-017-0436-y