During the second reporting period of the HEGEL project, a framework of activities, including both experimental testing and numerical modelling, was undertaken.
An extensive experimental fatigue testing programme was performed by University of Twente, involving frequency, temperature (up to 50°C), humidity (dry and fully saturated) and strain levels as main parameters. Frequencies higher than 100Hz were considered during testing as this allowed to determine and assess how self-heating phenomena affected the fatigue life of the specimens. The experimental setup consisted of an electromagnetic shaker, instrumented with a single point Laser Doppler Vibrometer (LDV) to measure the vibration response (velocity), an accelerometer to record the base excitation and a thermal camera to capture self-heating caused by the vibration. The material considered during the experimental testing was T700-M21 from Hexcel. The specimen fatigue life was monitored by tracing the response phase of the specimen, instead of the resonance frequency. A major benefit in using this methodology is the higher sensitivity of the ‘phase’ parameters to small changes in the structural integrity, enabling identification of ‘critical events’ at early stages of the fatigue life. In addition, the low level of constraints associated with the test setup, compared to that of conventional tests, enables to replicate more closely real case scenarios of HCF loading conditions (e.g. free vibration caused in engine environment). Furthermore, the implemented methodology allows to significantly reduce the testing time compared to HCF conventional test procedures. The experimental activities were supported by NLR, through physical analysis and non-destructive inspections and fatigue data analysis.
Modelling activities were also performed by TWI and concerned the definition and implementation of two different modelling frameworks. One modelling framework is suitable for simulating conventional/standardised fatigue tests (often static or quasi-static), where a combination of relatively high level of constrains imposed by the test setup (e.g. tensile, three point bending, four point bending) and load conditions (e.g. low frequencies) allows neglecting dynamics effects. The mentioned modelling framework could cover the study of both damage initiation (based on continuum damage mechanics approach) and crack growth (based on fracture mechanics principles – e.g. VCCT technique). A second modelling framework was also investigated and aimed to replicate fatigue problems where the dynamic nature of the tests can no longer be neglected (the fatigue response is affected by the individual modes). This second modelling framework involved an iterative process of a sequence of different type of analyses, such as modal analysis, steady state dynamic and VCCT static analyses. The framework can be particularly suitable for fatigue assessments where the load envelope triggers dynamic responses of the test specimen, which are far more challenging to be replicated in a conventional/standardised mechanical testing environment. The modelling frameworks were verified against the experimental tests.
As a result of the activities described above, HEGEL provided the aerospace community with ‘know-how’ and a framework of fatigue prediction methodologies, which can facilitate the design process of composite structures subject to HCF and fatigue in general. The topic and findings from the project were disseminated via different routes, including conferences (AeroMat 2018, Chemnitzer seminar at Fraunohfer 2018, 7th ECCOMAS Thematic Conference on the Mechanical Response of Composites, Thermal Analysis Conference (TAC) 2019, International Modal Analysis Conference 2020), proceedings, newsletter and press releases.
