In the current climate it can be disheartening for the composites industry when focusing on the downturn in the aerospace sector. However, the industry has witnessed the development of composite technologies spin-off into industries as diverse as automotive, oil & gas and even the medical sector. It is the ability of those working in the sector to adapt to difficult situations that will allow the industry to thrive and become more resilient throughout and beyond the Covid-19 crisis.
Global economic conditions and market forces have embraced the adoption of fibre-reinforced composite materials for designing modern airframes. The unique combination of improved fuel efficiency and serviceability is the major driving factor for the application of these engineered materials in airframes. Major airframe makers Airbus and Boeing have played a key role in the advancement of composite materials to handle design and service challenges. Composites are well recognised for weight reduction applications in the aerospace industry. Their strength and stiffness can be changed with directional loading. They are suitable for use in areas vulnerable to corrosion as well as in high fatigue load applications. Their potential for customisation to obtain specific properties is a major factor driving the advancement of innovative composite materials.
Significance of testing
It is necessary to test fibre-reinforced composites in order to support design and quality management programmes. Characterising the unique properties offered by composites through physical testing is critical to ensure their compliance with required specifications. It can be difficult to measure material properties of composites due to fibre orientation when compared with plastics and metals. Moreover, composite materials do not exhibit isotropic behaviour, which further increases the complexity. Hence, composites demonstrate diverse material properties and failure modes in different aircraft and it is necessary to characterise them in multiple planes in order to better understand their material behaviour.
The availability of a wide range of standards and test procedures further complicates the testing of composite materials. At present, there are over 150 standards available that outline the physical testing of fibre-reinforced composites. Besides national and international series of standards such as DIN, EN, ISO & ASTM, there are aircraft industry-specific standards designed by Airbus, Boeing and NASA.
Specific testing methods
Flexural and compressive properties must be tested independently as it is not possible to predict them based on tensile properties. There are many different techniques available for measuring shear properties, enabling the characterisation of properties in different shear directions. Non-ambient conditions such as humidity and temperature need to be considered for fatigue tests, which are critical for aerospace structural applications. Besides flexure, compression and tensile tests, there are many different specific tests which are employed for the evaluation of composites. Compression-after-impact (CAI) testing is utilised for evaluating the tolerance of a composite material to the damage caused by a bird strike or due to contact with other foreign objects during flight. Fibre composites are also subjected to tests such as open-hole compression; plain, open-hole and filled-hole tensile; end and end-loading compression; and shear. Thus, the three normal stresses are characterised in a nine-component stress tensor. The six shear stresses of this tensor may be determined by specific test techniques such as the lap shear test, the V-notch shear test, the ±45° in-plane shear test, and the short beam shear test for materials qualification.
Versatile testing solutions
To meet the different requirements, testing laboratories and companies previously used many different testing machine configurations, some of which were highly complicated. Highly efficient modular testing solutions are available with optional temperature chambers to accommodate non-ambient testing in the range of -70°C to +250°C. Such versatile solutions can perform a multitude of tests, ranging from lap-shear tests to determination of interlaminar shear strength (ILSS) to V-notched shear tests.
In addition to facilitating the measurement of properties of the entire composite, compression testing also provides data on fibre strength. Inducing compression deformation until the material fails and without buckling is a challenge. During the compression test, the specimen is placed between two support plates that are engineered to prevent buckling. Specimen grips without bonded tabs are employed for precise axial loading of the composite within the measurement range in order to measure compression modulus. Accessing the test specimen can become difficult due to the use of fixtures in compression tests. This, in turn, increases the complexity of strain measurement.
The challenges involved in performing these tests often lead to excessive specimen flexure. To resolve this issue, a hydraulic composites compression fixture (HCCF) is available, which radically simplifies the clamping procedure and eliminates wedge movement during the test.
While test fixtures accommodate the contact between test device and specimen, software provides the operator with access to test sequences, assessment, data storage and logging. Pre-configured test programs minimise operator involvement and ensure test repeatability.
A serious consideration with composite materials, particularly in the aerospace sector, is the ability to withstand crack propagation. Fracture toughness testing is becoming an area of increasing interest, as composite materials are notoriously weak in this area. Recent developments such as 3D weaving and ‘z pinning’ aim to address this weakness, however a testing solution must be available to assess their benefits.
ZwickRoell understand this importance and as such, have developed the most accurate, reliable and repeatable system currently available. The system, which has recently been supplied to the Nadcap accredited composites testing laboratory R-TECH Materials, in South Wales, includes a camera which tracks the crack growth and provides the user with a side by side video of the test data and crack growth, which can be replayed in slow motion to ensure that the crack growth measured by the user is as accurate possible.