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The structure of samples is studied using a pulsed acoustic microscope combined with a specially designed miniature testing machine. The features of such experiments are considered. The possibility of measuring the deformation of a sample during mechanical tests via nondestructive acoustic microscopy is demonstrated.

The fracture toughness which reflects change in the elastic deformation energy of the structural element with an increase in the crack area per unit at the onset of straining is one of the crack resistance parameters of carbon fiber plastics (CFRPs). When studying the fracture toughness, the position of the crack front is determined: both the initial one and that obtained as a result of crack growth. Currently existing test standards (STO TsAGI, ASTM D7905) determine the viscosity by the shear mode GIIc using the samples with a crack initiator. However, the method does not reflect the real conditions of crack initiation in CFRPs structures and can lead to a decrease in the accuracy of determining the load of crack initiation. A new technique of the fracture viscosity determination free of the standard delamination initiator has been developed in TsAGI. We present the results of developing the proposed methodology. The GIIc values were determined for a shear crack under three-point bending conditions after wedging. To determine the position and shape of the crack front, as well as to assess the dynamics of its propagation under subsequent loads, we used ultrasonic methods — ultrasonic flaw detection (ultrasonic NDT) and acoustic microscopy instead of the standard visual observation of the crack boundaries from the end surface of the samples. It is shown that acoustic microscopy at a frequency of 50 MHz provides determination of the crack front position in CFRP samples at a depth of 3.0 – 3.5 mm with a high resolution about 100 μm. The features of the crack growth under shear conditions are discussed. The results of the study show that high accuracy of acoustic microscopy in comparison with traditional ultrasonic NDT diagnostics is strongly sought for determining the shape of the cracks, as well as for analyzing the dynamics of crack growth and revealing the mechanisms of interlayer crack propagation in a composite material.

Nondestructive means of pulsed acoustic microscopy are used to visualize and assess the bulk microstructure of carbon fiber reinforced composites. Irreversible changes in the composite structure under the influence of external mechanical and climatic factors are studied, and the dynamics of the accumulation and growth of microscopic defects leading to destruction is studied. Ultrasonic explorations are conducted at frequencies of 50–100 MHz. It is shown that scattered (diffracted) radiation participates in image formation; this makes it possible to detect small cavities (detachment of reinforcing fibers), clusters of microscopic defects, and inclined extended cracks oriented along the fibers’ packing. These cracks are precursors to the brittle fracture of a composite, and their visualization is difficult with standard ultrasonic methods.

Strength characteristics of carbon fiber reinforced plastics (CFRPs) are investigated by nondestructive means as microstructural changes in a material’s bulk under external mechanical loads. CFRP microstructure is studied experimentally via pulsed ultrasonic microscopy at the level of mechanical deformation resulting in degradation of a material’s properties. The process of composite deformation is studied by means of stepped stretching. Acoustic emissions are used to identify the stage preceding final destruction (the accumulation of microcracks, fibers breaking, and delamination) as an indicator of a material’s degradation. Pulse acoustic microscopy is used to observe the accumulation of microcracks in individual layers of a material. To study the behavior of a CFRP microstructure upon mechanical loading, tensile stress was applied to samples with cross-ply packing of fibers (0°, 90°) and (45°, −45°). It is shown that the brittle fracturing of reinforcing fibers is typical of CFRPs with fiber orientation (0°, 90°), and is accompanied by growing areas of stress concentration and a rise in of acoustic emission activity, with a subsequent increase in the signal energy and the formation of extensive interlaminar delamination. Acoustic emission shows a low level of activity for CFRP samples with fiber orientation (45°, −45°), which is accompanied by the formation of structural microdefects that are clearly visible in acoustic images.

The paper shows the experience of using scanning pulsed acoustic microscopy (SIAM) to solve practical problems of quality control of polymer composite materials (PCM), to identify flaws and in-service damage, and to study changes in their internal microstructure under external influences. Acoustic images of structural defects and damages typical for carbon fiber reinforced composites are presented.

Non-destructive inspection of carbon-fiber-reinforced (CFR) composites applied in aerospace industry attracts a wide attention. In the paper, high frequency focused ultrasound (50-100 MHz) has been applied to study the bulk microstructure of the CFR material and mechanisms of its destruction under the mechanical loading. It has been shown impulse acoustic microscopy provides detecting the areas of adhesion loss at millimeter and micron level. Behavior of the CFR laminate structure fabricated by prepreg or infusion technology has been investigated under the tensile and impact loading.