Acoustic Emission in Composites under Static & Dynamic Loads We have developed an technique for acoustic emission (AE) waveform analysis in advanced structural composites. We first studied the relationship between the surface response and microfracture modes in composite laminates to establish a theoretical background for waveform analysis of AE signals. Lamb waves produced by arbitrary internal sources in unidirectional and cross-ply composite laminates are also investigated. A mechanistic model of the acoustic emission process was then developed to unbounded unidirectional materials, and then extended to multilayered angle ply graphite epoxy composites. The response of a homogeneous, transversely isotropic plate with the axis of symmetry parallel to the free surfaces is studied for a variety of localized sources inside the plate, such as signal force, double force without moment, single couple and double couple. We conducted laboratory experiments to validate the theoretical models. Correlation was made between the received acoustic waveform on the surfaces of a composite plate to the micro-failure mechanisms inside the plate and monitoring. The results of this research is useful in developing practical nondestructive testing tools to monitor damage initiation and evolution in composite structures.

Force Representation for Matrix Cracking Comparison between Theoretical and Experimental Waveform for Matrix Cracking Instron with AE Instrumentation Fracture Wave Detector used for Detecting Acoustic Emission

Typical Waveform and Spectrum of a Type II Signal

KC-135 Fuselage Panametric Wedge Transducer and Digital Wave Transducers on the Lap-Joint of the KC-135 Fuselage.

Guided Wave Propagation in Lap-Joints The objective of this study is to develop a nondestructive evaluation technique using ultrasonic guided waves for the detection of hidden defects inside component parts of aging aircraft and other structures. We considered the theoretical problem of the propagating a guided waves across a lap-joint with hidden defects to provide a scientific basis for the application of the technique. The problem is rather complex and analytical solutions do not exist. Therefore, we employed Global Local Finite Element Method (GLFEM), a hybrid method, to carry out the needed calculations. We investigated transmission and reflection of guided elastic waves propagating across a defect in a plate. Both 2-D and 3-D models are used to simulate various types of defects. We also performed experiments on lap-jointed plates and aged fuselage parts to verify the applicability of our theoretical models to practical problems.

Material and Defect Characterization using Leaky Lamb Waves (LLW) We use this relatively new ultrasonic nondestructive technique we developed in collaboration with Dr. Yoseph Bar-Cohen of JPL for materials and defects characterization in plate like specimens. Using an inversion algorithm, we deduced material properties from experimentally acquired dispersion curve of leaky guided (Lamb) waves. The time history of multiply reflected bulk wave within composite specimens are acquired in the experiment. At higher frequencies, the method yields the influence of individual layers while at lower frequencies, the calculation gives the global laminate property. In unidirectional composites, we were able to determine all 5 elastic constants and damping properties by this pitch catch experiment with the Leaky Lamb Wave setup. In a variety of specimens, we have obtained excellent agreement between our experimental data and the theoretical model. It has been demonstrated as an effective tool for determining property degradation in fire damaged composites, and in adhesive bonded joints.

Leaky Lamb Wave Set-Up at JPL An On-Line Journal Article in JNDT Theoretically Calculated and Experimentally Acquired Leaky Lamb Wave Dispersion Curves

Modified 9/98