Fibreglass and NDT

Fibreglass and NDT

25 May 2019 Off By Facto Edizioni

/ by Aldo Avancini /

Radiography, ultrasound and thermography: Aldo Avancini briefly explains the characteristics of these non-destructive test methods and their limits.

Ing. Aldo Avancini / Proposta Srl

I have already covered composite materials and non-destructive testing (NDT) in previous articles. Here I look at these two topics again by examining non-destructive testing of composite materials, focusing in particular on certain test methods and the possibility of their realistic application to the production of fibreglass components. I use the term “realistic”, even though I know that such test methods can be used, based on the consideration that ride manufacturers are by definition an “industry”, that is, they implement and apply processes and technologies developed in other areas yet in their industrial form, which makes the equipment, training and costs acceptable for an optimal, repeatable and objectively assessable result. 

The first NDT approach that is theoretically applicable to the production of fibreglass components is radiography, which may be the classic type – currently implemented using electronic devices for both measurement and exposure – or performed using tomographic techniques. The difference lies in the fact that a classic X-ray ‘compresses’ the image onto a plane (the plate), while this does not occur with tomographic radiography, which maintains the images on a series of differentiated planes.

This type of testing substantially allows differences to be detected in density, linked to both the resin and the positioning of the fibres; in other words, breaks, displacements, misalignments, erroneous or unwanted overlapping, while detection of any incorporated air depends both on its dimensions and its positioning with respect to the X-ray beam. The effectiveness of this test method can be enhanced by using specific contrast liquids.

The complexity of the instruments and the professionalism needed even just to read the images are quite obvious, two aspects that are difficult to include in routine or high numbers of tests.

A second testing process is based on the use of ultrasound; this differs substantially from the methods and instrumentation used in normal ultrasound testing. The main problems relate to the positioning of the probe (or pairs of probes in specific situations) and in the coupling product, while the reading is complicated by the fact that either comparative analysis is conducted (i.e. with reference to a known sample of a defect) or of the many available testing technologies, the one that is deemed best for the presumed defect in the component is chosen. The above considerations are as intuitive as is the technology.

Finally, I should also mention the possibility of thermographic testing, using various procedures applied in different ways with clearly defined limits. A “transient” system will in practice only identify a certain distribution of fibres (specifically carbon) in the material; while another system called “pulsed” allows the identification of surface defects, and so on.

So it’s impossible to know what to do, isn’t it? Not exactly. I can cite my personal experience in which, working on a series of products by a leading ride manufacturer that provided products produced at different times by different operators, a limited number of samples were taken and subjected to NDT (the types described so far), showing low variation in the primary mechanical characteristics evaluated in the samples tested. Of course such uncertainty requires the application of significant safety factors, perhaps with unnecessary weights and costs, but today in the absence of technologies that perhaps will soon be available on production lines, guest safety comes before any other consideration.

Taken from Games&Parks Industry May 2019, page 84

Ing. Aldo Avancini /  Proposta Srl /