Abstract
Ultrasound has been used to inspect composite laminates since their invention but only recently has the response from the internal plies themselves been considered of interest. This paper uses modeling techniques to make sense of the fluctuating and interfering reflections from the resin layers between plies, providing clues to the underlying inhomogeneities in the structure. It shows how the analytic signal, analyzed in terms of instantaneous amplitude, phase, and frequency, allows 3-D characterization of the microstructure. It is found that, under certain conditions, the phase becomes locked to the interfaces between plies and that the first and last plies have characteristically different instantaneous frequencies. This allows the thin resin layers between plies to be tracked through various features and anomalies found in real composite components (ply drops, tape gaps, tape overlaps, and out-of-plane wrinkles), giving crucial information about conformance to design of as-manufactured components. Other types of defects such as delaminations are also considered. Supporting evidence is provided from experimental ultrasonic data acquired from real composite specimens and compared with X-ray computed tomography images and microsections.
Highlights
I N THE composites manufacturing environment, the nondestructive testing (NDT) profession is increasingly criticized for being unable to provide sufficient information about the defects it detects and the conformance of as-manufactured components to the design [1], [2]
The aim of this paper is to demonstrate that characteristic signatures from microstructural variations that exist in the reflected analytic signals from composite laminates and that an automated process can be used for mapping these variations
The aim of this paper was to develop an automated process for characterizing composite laminates based on characteristic signatures of microstructural features
Summary
I N THE composites manufacturing environment, the nondestructive testing (NDT) profession is increasingly criticized for being unable to provide sufficient information about the defects it detects and the conformance of as-manufactured components to the design [1], [2]. An ultrasonic model has been implemented in MATLAB and used to simulate the pulse-echo response of a composite laminate. It is one of a number of ways of obtaining the exact solution to a 1-D wave-propagation problem and is based on the recursive analytical multilayer model developed in [22], which had been benchmarked against various other models and experiment; it is described more fully in [34]. The Hashin mixture rules for long-fiber composites [35] are applied to determine effective-medium properties for the composite plies based on a fiber volume fraction which is allowed to vary to compensate for ply-thickness variations, such as in wrinkled regions. Two scenarios are used for embedding the composite laminate either: 1) in water as for immersion scanning or 2) in a matched homogeneous, isotropic medium with the same effective-medium bulk properties as a carbon fiber composite with a 60% fiber volume fraction, to view the ply responses without any interference from the front and back surfaces
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