Materials used in the fabrication of wind turbine blades are increasingly required to operate at higher efficiencies and withstand greater loads as we push the boundaries of engineering. The load variability and the number of load cycles are far beyond those typically experienced by other structures used in bridge building, shipbuilding and aviation. The advent of innovative materials is driven by the cost of wind turbine design. Raw materials are required in large quantities because the average size of the blade is growing heavier and longer in order to optimize return on investment.
Materials for Wind Turbine Blades
Proven fiberglass composite fabrication techniques are used to produce blades in the 40-50 m range, while advanced composite materials are being specified for fabricating larger blades of 73.5 m. Carbon-fiber reinforced laminates deliver very high strength-to- weight ratio and stiffness, while carbon fiber- reinforced load-bearing spars are able to increase stiffness and decrease weight. The scope of these benefits expands as blade size grows.
Significance of Analyzing Material Properties
It is necessary to know the physical, chemical, mechanical and fracture-mechanical properties of the materials to optimize the design and fabrication of rotor blades for advanced wind turbines. Determining the aptness of materials for wind turbine rotor blades by conducting material behavior tests is one of the key elements of applied materials development. Determining the age behavior of materials under various conditions, such as UV light, salt, moisture and other offshore-specific environmental factors, is also highly significant.
Testing and assessing composite materials for use in wind turbine blades can reduce the economics of technology development and can help evaluate the performance of the composite materials, according to Gurit’s technical program manager, Kevin Cadd. Gurit, based in Switzerland, manufactures wind turbine materials and components. It operates testing laboratories in the UK, China and Canada, with the capability to perform most of the mechanical tests and a variety of analytical tests.
In accordance with Gurit, typical analyses for wind energy materials include determination of reactivity by means of differential scanning calorimetry (DSC) and characterization of visco-elastic properties using resin rheology tests. Dynamic mechanical analysis (DMA) is typically used to measure thermal properties of cured laminates, while their mechanical properties are measured by means of G1C, G2C, interlaminar shear, in-plane shear, tensile, and compressive tests. Moreover, compressive, tensile, cleavage, lap shear and K1C tests are used to measure the mechanical properties of cured adhesives and resins, indicated Gurit. A chain of analyses may be used to assess structural core mechanical properties through many different tensile, compressive and shear tests as key elements.
Solutions from Zwick
Zwick/Roell has developed a variety of test fixtures to perform a wide range of tests in the characterization of the mechanical properties of fiber-reinforced composites. A test fixture designed by Zwick for supporting Interlaminar Shear Strength (ILSS) testing is depicted in Figure 1.
Figure 1. Zwick’s test fixture designed specifically for the ILSS testing of fiber-reinforced composites.
One of the key challenges in the material analysis of high performance composites is the suite of international standards applied to the process. Customer specifications are currently focusing on aerospace or automotive standards that require a level of capability other than the basic qualification data. The most difficult element of any product development may indeed be the determination of underlying customer specifications. Gurit employs a dedicated Composite Processing Team to gain insights into the usage of materials by the customers in order to replicate and account for the same during test coupon production.
Key values need to be considered for designing the mechanical behavior of carbon fiber reinforced materials utilized in wind turbine applications include fiber orientation and the fiber-matrix connection. Various tests on fiber orientation need to be performed to obtain a complete characterization of mechanical properties.
Zwick supplies a host of equipment for static and dynamic characterization of fiber-reinforced composites used in the development and fabrication of wind turbines. The company’s new Allround-Line testing system, shown in Figure 2, is designed to meet the unique specifications of composites testing. This new system allows customers to conduct over 20 different types of tests as outlined in more than 100 standards in a single instrument.
Figure 2. Zwick’s new Allround-Line system for testing of fiber-reinforced composite specimens is compatible with 13 different test fixtures and enables testing to more than 100 standards.
Carbon fiber-composites are subjected to many different tests, including end and end-loading compression, shear, open-hole, plain, and filled-hole tensile in addition to open-hole compression. This characterizes the three normal stresses in a nine-component stress tensor, whose six shear stresses can be experienced in specific test procedures such as the short beam shear test, lap shear test, the -notch shear test and the ±45º in-plane shear test.
The new Allround-Line system for composites testing provides customers the ability to streamline characterization routines and decrease overall testing time. The new solution is offered in 100 kN and 250 kN lengths and is compatible with 13 different test fixtures and supportive to both ambient and non- ambient testing specifications.
For over 150 years the name of Zwick Roell has stood for outstanding technical expertise, innovation, quality and reliability in materials and component testing. Our customers’ confidence in us is reflected in our position as world- leaders in static testing and the significant growth we are experiencing in fatigue strength testing systems. The figures tell the same story: in the 2011 the company achieved incoming orders of €185m.
With innovative product development, a comprehensive range and worldwide service, this family concern supplies tailor-made solutions for the most demanding research and development and quality assurance requirements in over 20 industries. With over 1100 employees, a production facility in Ulm, Germany, additional facilities in America and Asia plus agencies in 56 countries worldwide, the Zwick brand name guarantees the highest product and service quality.
This information has been sourced, reviewed and adapted from materials provided by Zwick.
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