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Dr. Choi and his assistant in the lab.

ESTABLISHED IN 1975, the Korea Research Institute of Standards and Science (KRISS) has been providing a solid infrastructure for the growth of national science technology. As a representative institution for measurement and standards, KRISS has been playing a pivotal role in terms of quality improvement in a wide range of application areas, including aerospace, the heavy chemical industry, the semi-conductor market, ship and vehicle construction. In addition, KRISS has been contributing to improving the quality of national measurement standards in Korea up to standards of the developed countries through technical support for small and medium-sized companies.

Today, KRISS is working on the development of standards for non-destructive testing by means of thermal imaging. KRISS is now acting as chairman and leader of the international standard for non-destructive and infrared imaging testing (ISO/ TC135/SC8). The organization also has decision-making authority and as such, KRISS is developing international thermal imaging test standards for metal and composite materials that are widely used for wind power generator installations.

Wind power generator blades

A wind power generator blade converts wind into electric power. It usually has a length up of to one hundred metres, weighing up to dozens of tons depending on the generating capacity. Wind power generator blades are manufactured with composite materials and are lightweight and solid. However, they are continuously subject to significant amounts of stress during the manufacturing and testing process, which results in cracks. This is the reason why Europe – which introduced wind power earlier than Korea – has witnessed multiple examples of blades braking down during operation.

Fig 1. A wind power generator blade.

In the light of these problems, KRISS has started with research of non-destructive test standards for wind turbine blades. On the one hand, KRISS is hoping to prevent negligence accidents with wind power generators installed in the Daegwallyeong area, on the west coast, and on Jeju Island. On the other hand, the organization wants to actively support the research into international safety standards, an achievement the industry is eagerly waiting for.

Traditional blade testing methods

Various methods exist for testing blades and detecting defects in the blades, including direct contact and ultrasonic testing. However, each test has its weakness. As for the ultrasonic method, it is difficult to easily identify defects due to the disparity of the sound impedance from composite materials. Furthermore, the ultrasonic method will only detect local areas, which means that a significant amount of effort and time is required to examine one blade that is usually more than one hundred metres long.

Fig 2a & 2b. Debonding detected on the blade.

A visual test or direct contact test includes a manual check of the blade for cracks. This method is frequently used alongside the ultrasonic detection, but is also limited. The biggest drawback of a visual test is that it is impossible to detect fine cracks inside the blade in advance.

Thermal imaging for blade testing

A non-destructive test based on active infrared thermal imaging technology can be considered a good alternative. Compared to other methods, thermal imaging reduces the amount of effort and time for scanning large-scale objects and direct contact with objects is not required. In addition, thermal imaging technology is easy to operate. To perform their tests, KRISS opted for FLIR’s high-resolution thermal imaging camera.

Compared to ultrasonic testing, a thermal imaging test identifies anomalies in advance by detecting heterogeneity in the object displayed as temperature differences. This makes it possible to repair or replace a wind power generator blade at in an early stage. In addition, thermal imaging is a non-contact inspection method that allows KRISS to detect defects instantaneously on site, without having to disassemble the blade.

Dr. Man-Yong Choi, Vice President of the Korean Society for Non-Destructive Testing is currently leading the development of international standardization of non-destructive test technology. They use thermal imaging cameras for the detection of defects in composite materials including wind power generator blades.

– We are currently performing research using a FLIR thermal imaging camera in order to develop international standards and now after two years of effort, we are witnessing desirable results. The high-resolution camera has particular functions that are required for infrared imaging testing, such as high sensitivity and high speed thermal imaging, Dr. Man-Yong Choi commented.

Passive infrared thermal imaging is a technology that manually receives the energy emitted from the test object and interprets it, while active infrared thermal imaging detects the abnormal movement of energy emitted from an object after applying energy on it by using halogen lamp, flash lamp, ultrasonic forcing, induced current or a heating gun.

Fig. 4. Delamination detected on the tail of the blade.

Fig 5a & 5b. Inclusion detected on the tail of blade

Developing the standard

KRISS is now quantifying the tests with the wind power generator blade and proceeding research to develop a standardized defect identifying method and test system. These proceedings started 5 years ago and were finalized in 2013. In order to set the standard for non-destructive infrared imaging testing, KRISS needs to establish standards on the test methods, tools, and qualification of personnel. This is the quantification task of the non-destructive test.

KRISS has started comparative tests between the ultrasonic and thermal imaging methods by creating artificial defect specimen of Glass Fibre Reinforced Polymer (GFRP) materials. First results show that thermal imaging is significantly reducing the amount of testing time and effort and that thermal imaging cameras provide very reliable detection results.

Ruud Heijsman,

Flir

ruud.heijsman@flir.com