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Effective Technique for CFRP Coating Thickness Measurement

14-04-2021

Carbon Fibre Reinforced Plastic (CFRP) is most commonly used in OEM manufacturing of structural components of an aircraft such as wings, fuselage, ducts, panels, and fan blades or high-end automobiles including racing models. It produces significantly lightweight components as compared to steel and offers high strength and durability and adds a smoother and sleeker touch to the appearance of the parts. Additionally, CFRP composite materials are also preferred for their non-corrosiveness and high fatigue resistance. 

The CFRP composites are painted on the exterior side to either obtain the desired colour and texture or for Ultraviolet (UV) rays protection. However, it is crucial for manufacturers to know the thickness of paint applied to a CFRP or metal substrate. This is mainly because too much paint can lead to increased weight and stiffness, which leads to increased cost of raw materials and rework. On the other hand, too little paint could lead to premature ageing or can degrade the aesthetic quality of the final product. Therefore, this paint thickness has to be controlled in the process in close tolerances to prevent any of the above-mentioned problems. 

Traditional Approach of CFRP Coating Thickness Measurement 

Traditionally, Eddy Current or Ultrasonic measurement techniques have been employed for measuring paint thickness on metallic substrates. However, these techniques are not technically feasible for measuring thickness on CFRP substrates as the applied coating is generally thin and it has low electrical conductivity and diverse dielectric properties. Furthermore, these substrates are anisotropic in nature, which implies that the properties of the material would be dependent on its direction along different axes. 

Recommended Alternative to Traditional Approach 

A novel measurement method based on Microwave Technology provides reliable measurement on the dielectric and non-conducting materials such as CFRP. This technique not only overcomes the above limitations but also offers opportunities to simultaneously evaluate the thicknesses of multiple coating layers with micrometer precision. 

Microwaves are electromagnetic radiation with frequencies between 300 MHz and 300 GHz . The Microwave sensing technology has been highly adapted in both metallic and non-metallic materials as microwaves can freely penetrate these materials. The microwave sensor utilises electromagnetic radiation and extends the microwave from the transmitter which radiates into the device under test. The reflected signal is separated from the transmitted signal transferred to the receiver, and there evaluated with respect to magnitude and phase.

Portable Coating Thickness Gauge for Non-Destructive Measurement

Bestech supplies the new microwave sensors from MicroEpsilon, FSC series coating thickness gauge, which is ideal for non-contact measurement of coating thickness in the aviation and defence industry. These gauges offer reliable measurement on advanced materials such as carbon fiber-reinforced plastics (CFRP), CFRP with metallic lightning protection and metals. The measurement is non-intrusive and it doesn’t impact the target surface, i.e. the coating or paint and the substrate. In the case of multi-layer coatings, the total thickness of insulating layers is always measured up to the first conductive material layer. The substrates can be isotropic and anisotropic and have a medium or high electrical conductivity. The measurement can be performed on any metallic substrate with the same signal quality, regardless of its electric and magnetic properties.

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