Fiber-reinforced composites (composites) are made of two or more materials and are used in many areas because they have a number of advantages, such as lighter weight and high rigidity and strength compared to other conventional materials. This also applies to the aerospace sector, where fibre-reinforced composites already account for a considerable proportion of the materials used. New composite materials are regularly developed and tested with the aim of further saving resources and energy.
Especially in the aviation sector, where very high safety standards apply, a new material has to pass a series of tests before it can be used. For the testing of a new fiber composite material, a renowned aviation company has chosen Hegewald & Peschke testing technology.
For this purpose, a universal testing machine of the type Inspekt 250 kN from Hegewald & Peschke and a temperature chamber (-70°C to 250°C) are used, which is capable of simulating elevated or reduced ambient temperatures as they occur during flight operations. The complexity of the various material properties to be determined requires the use of special fixtures.
For the determination of the longitudinal elongation of tensile tests in the temperature chamber, a 250 kN wedge screw tightener is used. These tensile tests are recorded with video extensometers for optimum test evaluation. Pressure tests are also carried out in the temperature chamber using a conical wedge pressure tester in accordance with ASTM standard D 6641 and ISO 14126 (fiber-reinforced plastics - determination of the pressure properties in the laminate layer). Two pressure plates with a diameter of 200 mm each serve as contact area. This is a compression test with combined force application. Part of the force is applied via the specimen clamping and part via the specimen ends. This test method is used in particular when fibre-reinforced composites are to be tested under higher loads.
With a 3- and 4-point flexure device, bending tests are carried out according to ASTM standard C 393 at roomtemperature on sandwich panels for the interior fittings of aircraft. Sandwich panels are a special form of layered composite materials that are typically composed of three layers, with the outer layers being thin and stiff top layers and primarily absorbing tensile and compressive forces, whereas the inner core layer is made of low-density, thicker and capable of transmitting shear forces. The bending strength and shear properties of the sandwich composite are determined by bending the test specimen.
In addition, the longitudinal and transverse strain is determined by tensile tests. For example, a V-notch device is used to test fabrics in accordance with ASTM standard D 7078. The shear deformation is introduced via the upper and lower clamping clamps of the test tool, which are moved in the tensile direction during the test. This results in a shear stress concentration due to the notch in the specimen, which eventually leads to fracture of the specimen.