Material and component testing

Fraunhofer IWES has more than ten years of expertise with fiber-composites and rotor blades and offers customized material certificates in addition to standard tests. A particular focus is placed on the specific production methods of fiber-composite test specimens and the implementation and evaluation of testing methods. These range from a rough material screening through to a complete characterization. Depending on the problem, materials such as non-woven fabrics, resins, and foams are investigated as to their suitability for rotor blades. Material samples are manufactured according to the customer specifications.

Experts in the area of fatigue test

Since rotor blades in operation are subjected to enormous loads, fatigue tests play a key role here. The institute is accredited in accordance with DIN EN ISO 17025:2005 and employs the latest equipment and efficient test methods. The servo-hydraulic testing machines are equipped with particularly stiff and precise clamping tools which have been developed especially for the testing of fiber-composite test specimens under dynamic load. All activities on the test bench are recorded with the most up-to-date camera technology. This facilitates the complete and detailed documentation of the damage progression and makes it easier to understand the formation of cracks.

State-of-the-art facilities

The parallel simulation of climatic and mechanical loads is conducted in a climate chamber, which can be cooled to minus 40 degrees Celsius, conforms with standards (DIN / ISO), can be controlled with high precision, and is reproducible. Moreover, Fraunhofer IWES has an analytical laboratory for the investigation of fundamental physical material properties. These include the glass-transition temperature and the density of material samples, for example. The range of services offered also includes a large number of non-destructive tests. They provide accurate information on the damage progression during a mechanical test.

Equipment

• Universal testing machines for static and dynamic testing with maximum force of between 25 and 2500 kN
• Servo-hydraulic tension-compression-torsion testing machine for biaxial testing
• Climate chamber for parallel mechanical and climatic load simulation (-45° / 130° / humidity / UV)
• Composite Lab
  · RTM
  · Vacuum infusion
  · Heating bench
  · Precision specimen manufacture
• Differential Scanning Calorimetry (DSC)
• Dynamic Mechanical Analysis (DMA)
• Heat Deflection Temperature (HDT)

The expertise at Fraunhofer IWES lies particularly in the testing of extremely critical and structurally relevant areas such as bonded joints, ply drops, spar caps, and blade trailing edges. One focus of the work is the testing of spar caps to web bond lines on which a special combination of shear and axial loads acts. In fatigue tests, different adhesives can be compared and their respective failure thresholds and the permissible thicknesses of their bonded joints determined.

Flexible infrastructure

Producers of materials, blade manufacturers, and other industrial customers benefit in particular from the flexible infrastructure at Fraunhofer IWES, which is tailored towards individual customer requirements. In addition to ten years of experience in component and structural testing, the institute offers a state-of-the-art testing environment which includes an air-conditioned testing laboratory.

Non-destructive testing

The tests are accompanied by non-destructive examinations. Here, Fraunhofer IWES uses a combination of thermography, ultrasonic tests, and acoustic emission. Our experts also have outstanding expertise in the production of test specimens which are manufactured according to the wishes of our customers.

Equipment

• 1 & 2.5 MN servo-hydraulic test machine
  · 150 mm stroke (±3.5 mm at 2 Hz)
  · Max. specimen length: 3,000 mm
  · Flexible test set-ups by T-notch table
  
• Strong floor and strong wall
  · Dimensions: 12 x 3 m
  · 3,500 kNm bending moment
  · Hydraulic cylinders (25 to 200 kN and up to 800 mm displacement)
  
• Proven shutdown strategies
  
• NDT to accompany tests

The rotor blade tips of a wind turbine reach speeds in excess of 300 km/h in full load operation. At this speed, raindrops act on the surface like sandpaper. Even slight damage causes the surfaces to become rough at certain points, which reduces the aerodynamic efficency and thus affects the economic efficiency and the service life of the whole turbine.

Rain erosion test bench

The parts which are very seriously affected such as the leading edge of the wind turbine blade are therefore equipped with special protection systems, for example films or coatings – known as Leading Edge Protection (LEP). The problem: there is still no protection system which can survive rain, hail, temperature fluctuations, UV light, and humidity over the whole service life of a turbine. Hence, Fraunhofer IWES has been operating a rain erosion test bench since 2015 to better understand damage processes and derive effective protection measures. On the rain erosion test bench, substrates with different coatings are tested under a variety of conditions. The number and size of the drops, the point in time and frequency of the impacts, the temperature, and the UV irradiation can be controlled very precisely.

Development of damage models

A droplet impact system that can detect the position and the impact energy of the droplet strikes is currently under development. The complete test bench will be reproduced in a CFD simulation and the damage events will be documented with a laser-based inspection system and a high-speed camera. Also documented are the topology of the samples, and damage in the micrometer range. Developing a suitable material and damage model makes it easier to understand the processes on the material level. The results assist suppliers providing material for the rotor blade production to achieve optimum adaptation of their products to the practical demands. In 2018, the erosion test bench was expanded such that ice accretion tests can be carried out with a high degree of reliability

Equipment

• Max. speed: 550 km/h
  
• Temperatures: 4°C to 40°C
  
• Variable drop size: 1.5 to 5.5 mm