Testing composites
Composites consist of two or more materials that are bonded together. This combination of materials results in very special material properties, such as stiffness and strength in specific directions while keeping weight to a minimum and opening the doors for new areas of application.
Fiber composite materials are finding new uses in many different products, which in turn increases the significance of composites testing. Aircraft, including the Airbus A380 and A350, or the Boeing 787 are current examples from the field of civil aviation in which a high percentage of carbon-fiber composites are used. The chassis of BMW's I3 and I8 automobiles are made entirely of carbon fiber reinforced plastic (CFRP) materials that are so light that they can be carried by two people. In race cars, fiber composites have been a standard material for quite some time. Various types of composites are also used in the blades of larger wind turbines. Unidirectional fiber composites absorb the centrifugal forces, the outer surfaces are made of multidirectional fiber composites, and the overall structure is designed as a core composite. Composites are also used in the medical industry, for example, in prostheses, as well as in the construction industry as multifaceted materials for bridges, and in facade engineering.
A topic of high interest in the aerospace field is storage of liquid hydrogen at cryogenic temperatures. The objective of the material test is to identify characteristics and discover new insights into the behavior of the material at very low temperatures.
Testing composites Types of composites Composite testing requirements Modular testing system Customer projects Composites brochure Cryogenic composites test
Types of composites
- In fiber composites, fibers are embedded in one of the components of the composite material known as the matrix, resulting in a fiber-matrix system. The fibers can run in one or several specific directions and have preferred directions.
- Sandwich constructions consist of different numbers of superimposed layers. A special case is three layers, two of which are identical outer layers, which is called a sandwich composite.
- Sandwich core materials are used in lightweight construction. The core, which is located between the two outer layers, can be made of foamed plastic or a honeycomb structure. The latter is known as a honeycomb composite.
Composite testing requirements
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, ZwickRoell 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.
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, energy release rates, 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 tests, MD tensile tests for lesser laminate thicknesses, compression tests to ISO, 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.
ZwickRoell has developed a modular testing machine concept for electromechanical and servohydraulic 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. The fact that every component can be retrofitted, also makes this system future-proof..
