The impact of holes on the reduction of the strength of composite specimens with different laying of fibers

The methods of fastening structural elements made of fiber composites to other structural elements are almost the same as those used for metals. With the most common bolting, a hole is drilled in the element, i.e., a part of the material is removed, thus resulting in a stress concentration near the hole. Certain difficulties can thus arise in designing and calculating of bolted joints of metal elements of critical structures. The use of composites is associated with more serious problems. In this case, both the structural element and the material for this element are manufactured simultaneously. Subsequent machining is quite undesirable, since it violates the integrity of the element and, consequently, the bearing capacity of the entire structure. The efficiency of using composites depends on the technology of manufacturing the material of the element by 90%. We present the results of experimental and theoretical study of the effect of holes made by different technologies on the strength and bearing capacity of fiberglass specimens with different fiber laying patterns. The stress concentration coefficient is compared with the strength reduction coefficients for the holes made by drilling and by expanding the fibers without their damage. The results of tensile tests of specially made composite specimens with different reinforcement structures: [0], [0/90], [0/±45/90] and with central holes are presented. The strength reduction factors calculated from the results of the experiment turned out to be significantly lower than the stress concentration factors. The reasons for this effect are considered and an estimate of the characteristic curvature radius formed after «blunting» of a hole in a composite specimen is given. The obtained results of the study made it possible to draw conclusions about the effectiveness of the technology for hole production in composite samples without breaking fibers, as well as about the effect of the number of fiber families on the strength reduction coefficient near the holes.

1. Timoshenko S. P. History of the strength of materials. — McGraw-Hill, 1953. — 452 p.

2. Kolosov G. V. On one application of the complex variable theory to a flat problem of the elastic theory. Doctoral thesis. — Yur’ev, 1909. — 207 p. [in Russian].

3. Inglis C. E. Stresses in a plate due to the presence of cracks and sharp corners / SPIE Milestone Series. 1913. Vol. 137. P. 3 – 17.

4. Neuber H. Kerbspannungslehre. Theorie der spannungskonzentration genaue berechnung der festigkeit. — Berlin, Heidelberg: Springer, 2001. — 326 p. DOI:10.1007/978-3-642-56793-3

5. Polilov A. N. Mechanisms of stress concentration reduction in fiber composites / J. Appl. Mech. Tech. Phys. 2014. Vol. 55. N 1. P. 154 – 163.

6. Polilov A. N., Tatus’ N. A. Strength biomechanics of fibrous composites. — Moscow: Fizmatlit, 2018. — 328 p. [in Russian].

7. Malakhov A. V., Polilov A. N. Design algorithm of rational fiber trajectories in arbitrarily loaded composite plate / Journal of Machinery Manufacture and Reliability. 2017. Vol. 46. N 5. P. 479 – 487. DOI:10.3103/S1052618817050090

8. Khaja A., Rowlands R. Stresses associated with multiple holes whose individual stresses interact / J. Strain Anal. Eng. Design. 2017. N 52(3). P. 162 – 176. DOI:10.1177/0309324717690282

9. Zhu Y., Liu J., Liu D., Xu H., Hui D. Fiber path optimization based on a family of curves in composite laminate with a center hole / Composites Part B: Engineering. 2017. Vol. 111. P. 91 – 102. DOI:10.1016/j.compositesb.2016.11.051

10. Malakhov A. V., Polilov A. N. Design of composite structures reinforced curvilinear fibres using FEM / Composites Part A. Appl. Sci. Manufact. 2016. Vol. 87. P. 23 – 28. DOI:10.1016/j.compositesa.2016.04.005

11. Mori Y., Matsuzaki R., Kumekawa N. Variable thickness design for composite materials using curvilinear fiber paths / Composite Struct. 2021. Vol. 263. 113723. DOI:10.1016/j.compstruct.2021.113723

12. Pan Z. Z., Zhang L. W., Liew K. M. Adaptive surrogate-based harmony search algorithm for design optimization of variable stiffness composite materials / Computer Meth. Appl. Mech. Eng. 2021. Vol. 379. 113754. DOI:10.1016/j.cma.2021.113754

13. Balla V. M., Kate K. H., Satyavolu J., Singh P., Ganesh J., Tadimeti D. Additive manufacturing of natural fiber reinforced polymer composites: Processing and prospects / Composites Part B: Engineering. 2019. Vol. 174. 106956. DOI:10.1016/j.compositesb.2019.106956

14. Brooks H., Molony S. Design and manufactured parts with three dimensional continuous fibre reinforcement / Mater. Design. 2016. Vol. 90. P. 276 – 283. DOI:10.1016/J.MATDES.2015.10.123

15. Scott M., Dell’Anno G., Clegg H. Effect of process parameters on the geometry of composite parts reinforced by through-the-thickness tufting / Appl. Compos. Mater. 2018. N 25. P. 785 – 796. DOI:10.1007/s10443-018-9710-4

16. Dickson A. N., Barry J. N., McDonnell K. A., Dowling D. P. Fabrication of continuous carbon, glass and kevlar fibre reinforced polymer composites using additive manufacturing / Additive Manufact. 2017. Vol. 16. P. 146 – 152. DOI:10.1016/J.ADDMA.2017.06.004

17. Roth S., Pracisnore F., Countandin S., Fleischer J. A new approach for modelling the fibre path in bolted joints of continuous fibre reinforced composites / Composite Struct. 2020. Vol. 243. 112184. DOI:10.1016/j.compstruct.2020.112184

18. Almeida J. H. S., Bittrich L., Spickenheuer A. Improving the open-hole tension characteristics with variable-axial composite laminates: Optimization, progressive damage modeling and experimental observations / Composites Sci. Technol. 2020. Vol. 185. 107889. DOI:10.1016/j.compscitech.2019.107889

19. Hou Z., Tian X., Zhang J., Li D. 3D printed continuous fibre reinforced composite corrugated structure / Composite Struct. 2018. Vol. 184. P. 1005 – 1010. DOI:10.1016/j.compstruct.2017.10.080

20. Xu J. Y., Lin T. Y., Chen M., Davim J. P. Machining responses of high-strength carbon/epoxy composites using diamond-coated brad spur drills / Mater. Manufact. Proc. 2021. Vol. 36. N 6. P. 722 – 729. DOI:10.1080/10426914.2020.1854475

21. Lin M. J., Tsai K. H., Hwan C. L. Strength prediction for composite plates with an inclined elliptical hole / Mech. Compos. Mater. 2020. N 56. P. 619 – 628. DOI:10.1007/s11029-020-09908-z

22. Komkov M. A., Bolotin Yu. Z., Vasil’eva T. V. Determination of holes forming parameters in uncured woven composite by puncturing with sharpened indenter / Inz. Zh. Nauka Innov. 2017. N 9(69). P. 11. DOI:10.18698/2308-6033-2017-9-1678

23. Akhmedshin E. Kh., Polilov A. N., Tatus’ N. A. Holes manufacturing technology influence on the strength of fibrous composites / IOP Conf. Ser.: Mater. Sci. Eng. 2020. Vol. 747. 012096. DOI:10.1088/1757-899X/747/1/012096

24. Mileiko S. T., Suleimanov F. K. Model of a macrocrack in a composite / Mech. Compos. Mater. 1981. N 17. P. 276 – 280. DOI:10.1007/BF00605067