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Non-Destructive Evaluation of Sandwich Composite Structures

Introduction

Interest in the use of composite structures is growing throughout various industries. Composite structures have many valuable advantages, such as high strength, low weight, insulating properties and design versatility. Furthermore, when joining composite structures, the welding processes are omitted. In principle, composite sandwich structures have two outer surfaces with thin sheets of strong and stiff material separated by a thick layer of a core material, such as a foam or balsa.

Like any other engineering structures, sandwich composite structures need to be inspected using non-destructive evaluation (NDE) techniques; however, these types of structures are challenging to inspect. The energy that is applied to these components for the investigation is very highly attenuated, due to absorption and scattering. Therefore, correct interpretation of the received energy and sizing is a very challenging task.

Objectives

The objective of this study was to deploy ultrasonic testing (UT) techniques to detect artificially induced flaws inside the composite structure (Figure 1). The flaws were manufactured as side pockets and flat bottom hole, located on various depths in the structure.

 

Figure 1. Sandwich structure containing E-glass vinyl and glass fibres in directions 0/45/90/-45deg.
Figure 1. Sandwich structure containing E-glass vinyl and glass fibres in directions 0/45/90/-45deg.

Methodology

The inspection was performed in an ultrasonic immersion tank, whereby the specimens were immersed in water, as shown in Figure 2. The ultrasonic wave generated by the UT probe (Figure 2c) passed through the water and propagated inside of the specimen (Figure 2c).  Any obstacles inside of the specimens reflected the wave. By analysing the reflected waves, it was possible to build a 2-dimensional image of the reflections, which is called a C-scan. The obtained results are shown in Figure 3.

Using a standard flaw detector like Sonatest Sitescan also allowed detection of the flaws as it can be seen in Figure 4.

Further trials were conducted using a phased array system with a probe that contained 32 elements, which focused the energy on the set-up depths. The phased array was not as successful as the conventional ultrasonic testing.

Conclusions

Sandwich composite structures with glass fibres can be successfully inspected using ultrasonic techniques. Where possible, the testing should be conducted in an immersion tank, which gives the best coupling conditions. If this is not possible, a standard flaw detector can also be deployed.

Phased array units have many significant advantages over conventional ultrasonic testing units. However, in the case of composite structures, the voltage that is applied to the transducers dictates the strength of the mechanical wave that is delivered to the composite. In the case of standard ultrasonic testing units, the voltage is five times higher than a phased array system. The stronger the ultrasonic mechanical wave, the better it can propagate inside the acoustically challenging material and the interpretation of the achieved results is more reliable.

 

Acknowledgement

This case study presents work done in the project, ‘Engineering, production and life‐cycle management for the complete construction of large‐length FIBRE‐based SHIPs (FIBRESHIP)’ that has received funding from the European Union's Horizon 2020 under grant agreement No 723360.

Figure 2. Immersion set up. A: system applied. B: principles of usage. C. 5MHz immersion probe selected.
Figure 2. Immersion set up. A: system applied. B: principles of usage. C. 5MHz immersion probe selected.
Figure 3. C-scan displays from the composite structure.
Figure 3. C-scan displays from the composite structure.
Figure 4. A-scan results from the flaw detector.
Figure 4. A-scan results from the flaw detector.
Avatar Grzegorz Ptaszek Project Leader, Non-Destructive Testing (NDT)

Grzegorz (Greg) Ptaszek joined TWI in November 2013.  He is a doctor in mechanical engineering with 24 years’ experience in non-destructive evaluation.  Grzegorz has built up his knowledge and skills in a range of different roles including working in academia for Imperial College London, at a power generation plant, for a steel manufacturer and for a titanium manufacturer.  Grzegorz has also studied financial engineering and is the co-owner of a pump rotor patent.  In his current role in the Non-Destructive Testing team, Grzegorz works on a variety of projects employing various techniques such as non-linear ultrasonics, phased array, eddy current and computed tomography, and he also has extensive knowledge of thermography.

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