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

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 G_IIc 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 G_IIc 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.

1. Pettit D. E., Lauraitis K. N., Cox J. M. Advanced residual strength degradation rate modeling for advanced composite structures / AFWAL-TR-79-3095. Vol. I. Task I: preliminary screening, 1979.

2. Loss of Rudder in Flight Air Transat Airbus A310-308 C-GPAT, Miami, Florida, 90 nm S, 6 March 2005, Transportation Safety Board of Canada / Report Number A05F0047, 2005.

3. Schoen J., Nyman T., Blom A., Ansell H. A numerical and experimental investigation of delamination behavior in the DCB specimen / Composite Science and Technology. 2000. Vol. 60. N 2. P. 173 – 184.

4. Perez Carlos L., Davidson Barry D. Evaluation of Precracking Methods for the End-Notched Flexure Test / AIAA Journal. 2007. Vol. 45. N 11. P. 2603 – 2611.

5. Frossard G., Cugnoni J., Gmür T., Botsis J. Ply thickness dependence of the intralaminar fracture in thin-ply carbonepoxy laminates / Composites Part A. 2018. N 109. P. 95 – 104.

6. Jones R., Kinloch A. J., Hu W. Cyclic-fatigue crack growth in composite and adhesively bonded structures: The FAA slow crack growth approach to certification and the problem of similitude / International Journal of Fatigue. 2016. Vol. 88. N 10. P. 8.

7. Yakovlev N. O., Gulyaev A. N., Lashov O. A. Crack resistance of layered polymer composite materials (overview) / Trudy VIAM. 2016. N 40. P. 106 – 114 [in Russian].

8. Krylov V. D., Yakovlev N. O., Kurganova Yu. A., Lashov O. A. Interlaminar fracture toughness of structural polymeric composite materials] / Aviats. Mater. Tekhnol. 2016. N 1(40). P. 79 – 85 [in Russian].

9. Yakovlev N. O., Gulyaev A. N., Krylov V. D., Shurtakov S. V. Microstructure and properties of structural composite materials tested for static interlayer crack resistance / Nov. Materialoved. Nauka Tekhn. 2016. N 19. P. 65 – 74 [in Russian].

10. Funaria M. F., Grecoa F., Lonettia P., Lucianob R., Penna R. An interface approach based on moving mesh and cohesive modeling in Z-pinned composite laminates / Composites Part B. 2018. N 135. P. 207 – 217.

11. Krueger R. Computational fracture mechanics for composites. State of the art and challenges. Presented at the NAFEMS Nordic Seminar: Prediction and Modelling of Failure Using FEA. — Copenhagen/Roskilde, Denmark, 2006.

12. Pascoe J. A., Alderliesten R. C., Benedictus R. Methods for the prediction of fatigue delamination growth in composites and adhesive bonds — A critical review / Engineering Fracture Mechanics. 2013. Vol. 112 – 113. P. 72 – 96.

13. Xie J., Waas A. M., Rassaianc M. Estimating the process zone length of fracture tests used in characterizing composites / International Journal of Solids and Structures. 2016. N 100 – 101. P. 111 – 126.

14. Clay S. How ready are progressive damage analysis tool? / Composites World. July 2017. Vol. 3. N 7. P. 8 – 10.

15. ASTM D7905/D7905M-14. Standard Test Method for Determination of the Mode II Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites, ASTM International, West Conshohocken, PA, 2014.

16. State Standard GOST 33685–2015. Method for determining the specific work of delamination under shear conditions GIIc. Polymer composites. — Moscow: Standartinform, 2016 [in Russian].

17. Zakutailov K. V., Levin V. M., Petronyuk Yu. S. High-resolution ultrasonic ultrasound methods: Microstructure visualization and diagnostics of elastic properties of modern materials (Review) / Inorganic Materials. 2010. Vol. 46. N 15. P. 1655.

18. Petronyuk Y., Morokov E., Levin V., Ryzhova T., Chernov A., Sherbakov V., Shanygin A. Study of failure mechanisms of CFRP under mechanical load by impulse acoustic microscopy / Polymer Engineering Science. 2017. N 57. P. 703 – 708.

19. Morokov E. S., Levin V. M. Spatial Resolution of Acoustic Microscopy in the Visualization of Interfaces inside a Solid / Acoustical Physics. 2019. N. 65. P. 165 – 170.