Ultrasonic characterization of the fiber-matrix interfacial bond in aerospace composites.

The properties of advanced composites rely on the quality of the fiber-matrix bonding. Service-induced damage results in deterioration of bonding quality, seriously compromising the load-bearing capacity of the structure. While traditional methods to assess bonding are destructive, herein a nondestructive methodology based on shear wave reflection is numerically investigated. Reflection relies on the bonding quality and results in discernable changes in the received waveform. The key element is the "interphase" model material with varying stiffness. The study is an example of how computational methods enhance the understanding of delicate features concerning the nondestructive evaluation of materials used in advanced structures.

Damage accumulation in cyclically-loaded glass-ceramic matrix composites monitored by acoustic emission.

Barium osumilite (BMAS) ceramic matrix composites reinforced with SiC-Tyranno fibers are tested in a cyclic loading protocol. Broadband acoustic emission (AE) sensors are used for monitoring the occurrence of different possible damage mechanisms. Improved use of AE indices is proposed by excluding low-severity signals based on waveform parameters, rather than only threshold criteria. The application of such improvements enhances the accuracy of the indices as accumulated damage descriptors. RA-value, duration, and signal energy follow the extension cycles indicating moments of maximum or minimum strain, while the frequency content of the AE signals proves very sensitive to the pull-out mechanism.

Rupture of anterior cruciate ligament monitored by acoustic emission.

The scope of this study is to relate the acoustic emission (AE) during rupture of human soft tissue (anterior cruciate ligament, ACL) to the mechanisms leading to its failure. The cumulative AE activity highlights the onset of serious damage, while other parameters, show repeatable tendencies, being well correlated with the tissue's mechanical behavior. The frequency content of AE signals increases throughout the experiment, while other indices characterize between different modes of failure. Results of this preliminary study show that AE can shed light into the failure process of this tissue, and provide useful data on the ACL reconstruction.

Acoustic emission monitoring of degradation of cross ply laminates.

The scope of this study is to relate the acoustic activity of damage in composites to the failure mechanisms associated with these materials. Cross ply fiber reinforced composites were subjected to tensile loading with recording of their acoustic activity. Acoustic emission (AE) parameters were employed to monitor the transition of the damage mechanism from transverse cracking (mode I) to delamination (mode II). Wave propagation measurements in between loading steps revealed an increase in the relative amplitude of the propagated wave, which was attributed to the development of delamination that confined the wave to the top longitudinal plies of the composite.

Experimental study of focused ultrasonic beams reflected at a fluid-solid interface in the neighborhood of the Rayleigh angle.

A technique for measuring the lateral and axial displacements of the focal point of an acoustic focused beam reflected at a water-aluminum interface in the neighborhood of the Rayleigh angle incidence is presented together with a qualitative representation of other nonspecular phenomena, based on the cartography of the reflected beam. A study of the feasibility of the experiment and the choice of the transducer is carried out. The results obtained are in agreement with theoretical predictions.