Flight mission. Testing, testing
Dr Natalia Becerra, Technical Director of Polymers and Composites at Exova, on how advanced composites and materials fatigue testing could help the aerospace sector meet ever changing performance and efficiency targets.
Environmental regulations in aerospace are starting to push the boundaries on lowering specific fuel consumption and increasing component durability. In addition, the key drivers for manufacturers, such as minimising manufacture and operating costs, and improving aircraft performance while reducing environmental impact remain. This has led to unprecedented opportunities for the application of advanced composite materials in the aerospace industry.
The increase in the use of composites has also necessitated development of innovative testing solutions to address customer specifications. Materials may vary from base metal alloys to highly engineered reinforced composites with test temperatures ranging from 80°C to more than 1,000°C in metals, and 80°C to 150°C with different humidities for composites.
Reproducing in-service conditions in a laboratory environment is not a simple task. Permutations of testing configurations for advanced materials must take into consideration the types of resins, fibres, orientations and processes, which may seem almost endless, combined with the loading cases and environmental conditions. These loading cases may include compression, tensile, flexural, interlaminar shear, inplane shear, bearing strength, fracture toughness and damage tolerances, (such as compression after impact and fatigue life), environmental conditions, which can be ambient, sub-ambient, elevated temperature and wet conditioned.
Historically, the behaviour of metals in fatigue is well understood and international standards are widely used to control testing. However, the use of advanced materials has created the need to better understand material behaviour in order to develop adequate life prediction in line with service conditions. Furthermore, the repair of composite materials requires knowledge of the effects on the component life after any repair has been undertaken.
Composite materials exhibit complex failure mechanisms under fatigue loading because of the anisotropic characteristics in their strength and stiffness. Fatigue causes extensive damage throughout the specimen volume, leading to failure from general degradation of the material instead of a predominant single crack, as with metals.
The extent of fatigue damage in composite materials is often characterised by running a defined number of fatigue cycles under specific constant amplitude loading conditions, and then testing the residual strength or stiffness. Any decrease in the residual material properties relative to a control test piece that has not been previously fatigue tested is attributed to the accumulation of fatigue damage.
Post-test analysis of specimen compliance can also be used as an indicator of fatigue damage in composites – as fatigue damage accumulates, the specimen compliance increases and stiffness decreases. This manifests itself as an increase in the displacement of the test frame that is essential to reach the required test load. NDT methods can be used to intermittently assess the extent of fatigue damage during tests. For metallic components, the assessment of the failure mode is also critical. The investigation of failures on test samples and in-service failures provides knowledge of material behaviour and enables companies to redesign and improve their processes to avoid these issues in the future.
Testing in the aerospace industry helps to shape product development, ensuring improved safety and performance and, in many cases, contributes directly to the improvement of specifications and legislation. A recent example of this is a research collaboration between Exova, Airbus and NPL to address the lack of certified standard calibration materials for dynamic mechanical analysis testing in the industry.
Controlling the glass transition in a polymeric material is critical to assure its mechanical performance. Typically, the glass transition temperatures of aircraft components are controlled within ±1°C, but current calibrants are not certified and have been known to vary by up to 10°C. The industry must constantly adapt testing standards and technologies to keep up with stringent specifications, qualifications, changing trends, pricing and government legislation.
New materials and manufacturing methods present unique challenges in material characterisation and life predictions. But the sector is now calling for a move beyond a building-block approach to design and certification, towards highly technical computer modelling and largescale component testing. This is where testing methods will truly play a critical role in achieving the most demanding standards of quality, safety and performance.