Strength of Composite Joints in Structural Components and Articles

Abstract

Currently, the composite materials (CM) are widely used in the aerospace technology and mechanical engineering where the key parameters characterizing structural components and articles are related to their weight characteristics as well as their performance under high temperatures. For the experimental investigation of composite-based threaded items, the rod-based 3Dreinforced carbon-carbon composite material (CCCM) was chosen. The subject of the research was the metric thread. The test samples were cut of the aforesaid material along one of the reinforcement direction. The following thread sizes were analyzed: М24×1.5; М24×2; М24×3. Dependence of the thread strength on the number of thread turns was determined within the range of 1 to 10 thread turns for М24×2 thread size. The remaining test samples were used to obtain the relationship between the thread load bearing capacity and the thread pitch. The obtained dependence of the thread load bearing capacity on the number of thread turns showed the following: a) An increment in the thread load bearing capacity decreases with increasing number of thread turns. However, this effect is less pronounced than that observed for the metal ‘screw/nut’ pair. b) With the CCCM material under study, it proved to be impossible to test configurations having only 1 or 2 thread turns. c) The wide confidence range (CR) of the failure load can be explained by the fact that the material under study features an apparent structural heterogeneity, with a different probability of hitting a unit cell of the matrix and filler of the material. Nevertheless, the confidence range is stable, thus indicating indirectly the possibility of using this test method for further study of composite-based threaded items. There is an explicit correlation between the thread load bearing capacity and the thread pitch. Increase in the thread strength with increasing thread pitch depends on the relationship between the pitch size and the characteristic size of the unit cell of the material. The test research has shown that it is possible to make CCCM-based screw joints designed for operation under high temperatures as an alternative for metal screw joints, provided that the key parameters such as the thread profile, pitch, diameter and the number of thread turns are properly chosen.