Bulk Microstructure and Local Elastic Properties of Laminate Composites Studied by the Microacoustical Technique
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Bulk Microstructure and Local Elastic Properties of Laminate Composites Studied by the Microacoustical Technique. Vadim M. Levin1), Songping Liu2), Enming Guo2) 1) Lab of Acoustic Microscopy, Institute of Biochemical Physics, Russian Academy of Sciences, 4 Kosygin St, Moscow, 119991, Russia; E-mail: [email protected] 2) NDT&E Center for Composites, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing, 100024, People’s Republic of China; E-mail: [email protected]
ABSTRACT The focused ultrasonic beam technique has been employed for imaging of bulk microstructure of fiber-reinforced composites, for measuring local elastic properties and mapping their distribution over the composite body. Ultrashort probe pulses (operation frequency of 50 MHz) provide resolution of 100 µm. The technique has been employed for nondestructive layer-bylayer imaging of microstructure of CFR laminate composites. Series of acoustic images in planes parallel to the specimen face (C-scans) or perpendicular to it (B-scans) enable to reconstruct of bulk microstructure of compound unidirectional CFR laminates. Different types microdefects, including failures in interply adhesion, buckling of single prepreg plies, internal defoliations and disbonds, have been observed. The method has been applied to measure local values of elastic wave velocities in laminates within area of 100 - 150 µm diameter. Quality of acoustic images depends on kind and density of fiber packing and on depth of position of the imaging plane. Experience shows that specimens 2-10 mm thick are appropriate for visualization of their bulk microstructure and measuring local values of sonic velocities. The technique is promising for NDE of different composites, not only for carbon-based ones. INTRODUCTION Ultrasonic techniques are widely employed for studying mechanical properties and evaluating structure of laminate carbon fiber-reinforced (CFR) composites. Propagation of elastic waves through a composite medium dramatically depends on their frequency. The frequency range 1÷10 MHz is traditionally used for ultrasonic NDT of CFR composites. At these frequency the wavelength is essentially larger than typical sizes of all structural elements of laminates. The material is perceived by ultrasound as a homogeneous structured medium. Occurrence of laminate fibrous mesostructure results in expressed elastic anisotropy of CFR laminate. Ultrasonic measurements of elastic anisotropy [1], ultrasonic attenuation and background scattering [2] are employed for characterization of composite microstructure in a specimen as a whole. At high frequency (50 MHz and higher) the ultrasonic wavelength becomes smaller than characteristic sizes of the most of structural components. At 50 MHz wavelength (≈ 60 µm) is smaller than thickness of a single prepreg ply (≈ 120 µm), width of fiber bundles or diameter of fiber threads. For elastic waves CFR composites can be treated as a system of numerous ordered reflectors. Echo signals generated by large-sized structural units (p
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