Determination of fracture toughness for carbon fiber reinforced plastics free of the crack initiator using the acoustic microscopy

Abstract

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.