Fatigue testing for composites

Composites made of a polymeric matrix reinforced with fibres have played an important role in reducing the environmental impact of numerous sectors. They reduce the weight of many structural parts and systems while imparting excellent mechanical properties.

The increasing prevalence of FRPs in safety-critical components highlights the need for advanced and highly accurate testing and failure criteria. The EU-funded project MIFACRIT laid the foundations for a methodology toolbox of accelerated multi-axial fatigue testing with highly accurate failure criteria. The focus was on one-ply and multi-ply sandwich or stacked composites for the aerospace industry.

Multi-axial fatigue loading is, as its name implies, loading along more than one of the three axes of the Cartesian coordinate system. Until recently, most discussions of cyclic loading assumed uni-axial loading. However, in real life, many systems such as rotating shafts and numerous automotive and aircraft components experience a multi-axial state of cyclic stress.

MIFACRIT addressed this situation for the case of FRPs. Its great strength lies in the interplay between extensive experimentation and advanced numerical simulation exploiting methods from fracture and damage mechanics.

Material tests integrated viscoelastic characterisation during constant strain and stress tests during cyclic loading. Both temperature and frequency parameters were varied. The two-stage mathematical modelling accounts for both local and global descriptions. Simulation results were compared to experimental ones in a reiterative way to fine-tune the models. Threshold values for the various criteria were determined.

MIFACRIT developed the procedure to capture elastic properties and failure and damage properties of FRPs by evaluating mechanical effects within the microstructure over various loading conditions. Further optimisation will enhance the reliability of the assessed properties, ensuring accurate prediction of the lifetimes of FRPs under multi-axial load conditions. This in turn will enable more rapid production of high-quality, safety-critical structural components.