With qualified testing processes to the serial use of fibre-reinforced composites

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Helicopters and aircraft are only allowed to take off once they’ve passed a number of tests. In particular, this applies to the high-load components made from fibre-reinforced composites used to build them. That’s why high standards are used for the non-destructive testing – commonly abbreviated as NDT – of GFRP and CFRP components. Testing methods such as ultrasound, thermography, shearography, X-ray and computed tomography are in particularly high demand, both in the development, production optimisation and quality assurance of composites and in the repair of composite structures. The individual testing methods will be on display as well at COMPOSITES EUROPE, which will take place in Stuttgart from 6 to 8 November.

Besides aviation, all the other sectors where GFRP and CFRP are deployed also require reliable tests for these specialised materials. The automotive industry, in particular, is tasked with qualifying testing processes that facilitate complete material characterisation and error tracing in a non-destructive and reasonably time- and cost-effective way.

Detecting hidden defects with lock-in thermography

In Germany, numerous scientific institutes and research institutions are working on the development of suitable methods. One of them is the Fraunhofer Institute for Manufacturing Engineering and Automation (Fraunhofer IPA), whose “Machine Vision and Signal Processing” department has developed a thermographic system for evaluating CFRP and GFRP. The system has been constructed on a modular basis. Different IR cameras and excitation units can be mounted regardless of the geometry of the component requiring inspection. The analysis method is based on the principle of lock-in thermography. Hidden structures or defects alter the course of temperature distribution over time. This enables them to be detected by means of phase images generated by the sensitive lock-in thermography system.

The results of the thermographic assessments are analysed with the aid of diverse in-house image-processing algorithms to automatically obtain IO/NIO information. Any areas containing defects are shown in colour in the resulting image. The system developed at IPA is suitable for rapid analysis and supplies sound qualitative information. There is also the option of evaluating the ROI on further sensory analyses (e.g. CT), thus reducing the amount of time and money required for quality control.

Complete analysis in 60 seconds

This analysis method makes it possible to examine real components in order to detect missing layers, inclusions or adhesions as well as material placement errors. Impact damage such as external and internal spreading of cracks and/or delamination as well as perforation damage can be detected, as well. Moreover, fibre displacement, displacement of threads or seams, the absence of a seam, and pores and cavities can also be recognised. Shifts in position and the formation of waves on the inside (undulation) as well as missing resin and/or the absence of curing (dry spots) are also detectable.

Magdeburg-Stendal University of Applied Sciences, FI Test- und Messtechnik GmbH and IFC Composite GmbH have jointly created a system for the microwave-based testing of leaf springs made from GFRP. These components are used to make the Mercedes Sprinter and VW Crafter, among other vehicles. The development was aimed at facilitating a complete analysis in the production line for components with dimensions of approximately 1400 mm x 70 mm x 30 mm in about 60 seconds, plus data post-processing, evaluation by the operator and archiving of the data. A 24 GHz testing system is deployed for this task.

A need for the development of additional composites testing methods

The leaf springs are scanned by a transmitter array with ultra-low, safe output power. The transmission signal is received by a receiver array on the back of the leaf springs before being processed further. In order to examine the entire component, 30 individual channels are sequentially activated electronically in the lateral direction with transmitting/receiving pairs of antennas; in the longitudinal direction, the rigidly connected transmitting/receiving arrays are moved across the leaf spring. Subsequent to the scan run, the signal is processed and the scan dimensionally rendered to enable analysis by the operator. Beyond testing leaf springs, the developers see potential applications for microwave testing with rotor blades of wind turbines and adhesions of GFRP pipes.

These applications show that the composites industry must develop its own testing methods to add to the spectrum of tests available from the plastics processing sector. One example is the DAkkS-accredited testing laboratory of Süddeutsche Kunststoff-Zentrum (SKZ), where thermoplastic as well as fibre-reinforced plastics are tested. This includes methods such as Barcol hardness testing, bending and tensile tests of laminates, and assays of textile-glass and mineral content.

The Chair of Carbon Composites at the Technical University of Munich also deploys a wide range of the latest testing methods and facilities with the goal of developing experimental methods to study the material behaviour of fibre-reinforced plastic. Among them are thermal analysis and rheology to determine the physical and mechanical properties of matrix systems and FRP; examinations of the drapeability and permeability of unidirectional and semi-finished textile fibre materials; and the static and highly dynamic (impact, crash) determination of material behaviour on the component, individual-layer and laminate level.