Browsing by Subject "Acoustic emission testing"

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Acoustic emission signature analysis of failure mechanisms in fiber reinforced plastic structures
(2002) Ativitavas, Nat; Fowler, Timothy John; Fowler, David W.
The objective of the research program was to develop reliable pattern recognition and neural network analysis methods to determine the failure mechanism signatures in fiber reinforced plastic structures from acoustic emission (AE) data. The AE database was collected from a range of test specimens. Visual inspection and observation with a scanning electron microscope were performed to identify failure mechanisms in the specimens at various load levels. It was found that different types of specimen and structural loading yielded different types of failure. The failure mechanisms of interest were matrix cracking, debonding, delamination, and fiber breakage. Two method of analysis were used to determine the AE signatures. The first was visual AE pattern recognition. This analysis used a comparison of dissimilarities among AE correlation plots of data from different specimens. The results showed several AE signatures. The analysis also explains the correlation of material properties to failure mechanism evolution. The second analysis method was the use of neural networks to perform AE pattern recognition. The neural networks were trained using AE data in order to perform two tasks: determine the failure mechanisms and to assess the damage severity. The performance of the networks was found to be excellent for the first task and promising for the second task. The neural network was also applied to additional AE data from full-scale and coupon tests. By comparing the results from the network with visually observed damage, the network results are shown to be very reliable in determining failure mechanisms.
Detection of transverse cracking in a hybrid composite laminate using acoustic emission
(2003-12) Jong, Hwai-jiang, 1962-; Schapery, Richard Allan; Ravi-Chandar, K.
Transverse cracking detection in a uniaxially-loaded symmetric cross-ply hybrid laminate containing 0◦ IM7/8552 carbon/epoxy and a very thin 90◦ S2/8552 glass/epoxy layer is studied using the acoustic emission (AE) technique. By conducting modal-based AE experiments and analysis, we investigate some parameters that can be used as the waveform signatures to identify transverse crack growth in the hybrid laminate. Wave dispersion relations of the hybrid laminate are established, and a comparison with those from a material homogenization model based on the equivalent stiffness is made. It is found that material homogenization is not accurate for predicting wave dispersion in the hybrid laminate. Wave dispersion for a homogeneous IM7/8552 unidirectional plate is also constructed. Cut-off frequencies belonging to various wave modes are discussed concerning their significance in interpreting AE signals. The wave attenuation behaviors that exist in the hybrid laminate and in the homogeneous IM7/8552 plate are compared and discussed using the finite element method (FEM). The use of singular elements dealing with the high strain gradient near the crack tip is addressed for convergence purposes. It is shown by the FEM results and demonstrated in the AE experiments that wave attenuation in the cross-ply hybrid laminate is much stronger than in the plain IM7/8552 plate. A simple calibration method for the AE sensors is discussed. Some important aspects in conducting an AE experiment, such as the sensor averaging effect and sensor frequency response range, are addressed. A new source location method based on the waveform’s first peak search and the associated primary frequency content is proposed. The accuracy of the source location method is verified by pencil-lead break experiments. The so-called symmetric energy fraction (SEF) of the AE signals in conjunction with the finite element analysis result in identification of the transverse cracking event. Lastly, a material failure kinetics-based characterization of the transverse cracking process is proposed in terms of the unloading forcing function on the transverse crack face. Finite element results based on this loading are compared to the AE signals.