Fiber-Reinforced Composites

Overview of Composites

Composites consist of two or more combined materials. Combining materials results in very specific material properties, such as stiffness and strength in specific directions while keeping weight to a minimum, thus enabling the development of new applications. 

Composites are used in many new products. Airplanes such as the Airbus A380 and A350, or the Boeing 787 are current examples in civil aviation in which a high percentage of carbon-fiber composites are used. BMW's I3 and I8 automobiles have vehicle chassis made entirely of GFRP materials. They are so light that two people can carry one. Race cars have been using fiber composites for quite some time. The blades of large wind turbines are built with a variety of composites: unidirectional fiber composites absorb the centrifugal forces, the outer surfaces are made of multidirectional fiber composites, and the overall structure has a sandwich design. Composites are also used in medical engineering, for example, in prostheses, in the construction industry as multifaceted materials for bridges, and in facade engineering.

In composites materials, fibers are embedded in a component of the composites known as the matrix. This creates a fiber-matrix system. The fibers can run in one or several defined directions and have preferred directions.

Laminates consist of different numbers of layers, on top of the other. Materials in which there are three layers, two of which are identical external layers, are known as sandwich compounds. 

Sandwiches are used in lightweight construction. The core, which is located between the two external layers, can be made of foamed plastic or a honeycomb structure. The latter is known as a honeycomb compound.
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Definition of Composites

A variety of composites are used in technical applications such as

  • Glass fiber-reinforced plastic (GFRP)
  • Carbon fiber reinforced plastic (CFRP)
  • Aramide fiber reinforced plastic (AFRP)
  • Natural fiber reinforced plastic (NFRP)

Fiber composites consist of fibers that are filaments or staple fibers, for example, roving fabrics, and as a matrix, ensure bond strength.  

The characteristic profile, along with the selection of fiber and matrix material, is essentially determined by the orientation of the fibers in the textile fabric. A distinction is made between unidirectional and multidirectional laminates in testing technology. 

Materials testing usually involves individual load scenarios on standard-defined specimens. Since the characteristics are heavily dependent on the direction, the various loading types are applied with different specimen sampling, for example in parallel or perpendicular to the fiber direction.  

In addition to the international standards (ISO), these tests are described in various national and regional standards (ASTM, EN, and DIN), as well as in company standards (Airbus AITM, and Boeing BSS). This results in a scope of more than 170 standards 'describing approximately 20 generic test methods.

The testing of components, structural sections, and complete structures applies loads that mirror those occurring in real-world applications. Strength, energy consumption (crash), material fatigue, and service life evaluations are the focus.

Due to directional and shear sensitivity of the fibers, test loads must be applied precisely in the intended direction. The axial error is described as misalignment and is subject to narrow limits. To measure the misalignment, Zwick uses special measuring devices, which correspond to the shape and dimension of the specimen. The testing machine's test axes are aligned with a mechanical alignment fixture.

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Modular Testing System for Composites

Large testing labs with correspondingly high throughput rates use several different large testing machines for the individual test methods to minimize the time and costs involved in rebuild. The standardized test methods can be divided into the following force ranges: 

  • Forces up to 1 kN: flexure tests, fracture toughness energy, tensile tests on single filaments
  • Forces up to 10 kN: shear tests, for example, IPS, ILSS, and V-notch, tensile tests on filament strands, UD 90° tensile tests, tensile tests in the thickness direction
  • Forces up to 100 kN: UD 0° tensile test, MD tensile tests for smaller laminate thicknesses, compression tests to SO, ASTM and EN standards, notch compression tests, bearing pressure tests
  • Forces over 100 kN: tensile and compression tests to Airbus standards with corresponding laminate thicknesses, compression after impact
If throughput rates are not high or consistent enough that an investment in multiple testing machines makes sense, an alternative option is to equip a single testing machine so that it is possible to perform as many test methods as possible with the least amount of rebuild effort. Zwick has developed a modular testing machine concept for electromechanical and servohyrdaulic testing machines to address these different needs. The advantage of this modular system is clear: all fixtures and tools, extensometers, software, any protective panes, and the temperature chamber are modular and designed to work together. Furthermore, this system is also ready for the next generation, since all components can be retrofitted as well.
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