Acoustic emission to model the fatigue behaviour of quasi-isotropic carbon-epoxy laminate composites

As a consequence of the heterogeneity of composite materials, initial defec ts are difficult to be eliminated completely. To guarantee the structural s afety of these materials it is necessary to investigate the effects of thei r defects under service conditions. When a composite material is loaded, di fferent types of damage will occur: matrix cracking, delamination fibre/mat rix debonding and fibre failure. Some of these damage types will initiate e ven at a very low load level, without causing overall failure of the compos ite part. However, during further loading the initial damage will grow and create other types of damage. The fatigue damage accumulation in fibre rein forced composite materials can be evaluated by different non-destructive te sting techniques. The most commonly used NDT techniques to dare have been t he ultrasonic C-scan and the X-ray radiography for detecting primarily inte rnal delamination and matrix cracking respectively. Nevertheless, an interr uption of the fatigue loading and removal of the specimen from the test fix ture is usually required, which may affect the fatigue results. Acoustic emission monitoring during fatigue loading of composite laminates offers many advantages for detecting fatigue damage and damage growth. Acou stic emission technique is the only one which can possibly detect all diffe rent damage modes. However there are some drawbacks: Firstly, the large amo unt of non-filtered data produced in long life fatigue tests may create dat a storage problems. Secondly, friction generated emission has to be disting uished from emission generated by new damage. Finally, complex damage proce ss in composites has caused difficulties in correlating the features of AE (peak amplitude, event duration, event energy...) with different failure me chanisms. Studies have been done indicating that different failure mechanis ms can generate a wide range of AE signal amplitudes. Many authors supporte d that fibre failure events have the highest amplitude. Others, have shown that matrix cracking can account for higher amplitude events. Therefore, am plitude distributions do not give a clear picture of the different damage m echanisms in the material. However if event amplitudes are coupled with the load of occurrence of the event using correlation plots of load versus amp litude it is possible to extract a relation between these events and failur e mechanisms activated at that load. In this study, the cumulative AE counts have been used to quantify the fati gue damage development in quasi-isotropic CFRP laminates and a model based on AE data has been formulated.