Acoustic emission energy as a fatigue damage parameter for CFRP composites

Carbon fibre reinforced plastic composite laminates have been tested under static and fatigue loading with acoustic emission (AE) as the main damage monitoring method. AE energy results were found to correlate with damage indicated by ultrasonic C-scanning and microscopic inspection in terms of type, location and accumulation of damage and to provide indications of effective failure states before catastrophic failure. AE energy recorded along the gauge length indicated important critical intra and inter laminar damage occurring as early as 27% of ultimate tensile strength (UTS), typically 0.3% ultimate strain, for laminates with 90° layers. Considering these thresholds of significant AE as effective intermediate failure states suggests that the arbitrary choice of fatigue stress levels at a high percentage of UTS should be re-considered in order to avoid the destruction of relevant damage states in the first cycles of fatigue loading. Using relatively low stress levels as guided by the effective failure levels of static tests, blocked fatigue cycle tests provided AE energy data which could be extracted for each block type to reveal fatigue damage accumulation trends and effective fatigue failure states. It is therefore concluded that AE energy provides a valid and useful damage parameter and effective failure criteria for fatigue life prediction, accounting for the material’s current damage state, its response to variations in loading and effective intermediate failures.