Comparative Performance of Threaded Inserts in Fiber-Reinforced Composite Laminates under Static and Fatigue Pull-Out Loading

Abstract

Due to the weak interlaminar strength of fiber-reinforced composite laminates, directly tapping threads into the laminate is generally not recommended. In monolithic composite laminates where through-bolting or sandwich-panel potting is not feasible, threaded inserts are commonly used to achieve reliable fastening and maintain structural integrity. Various insert types are commercially available, and selecting the appropriate configuration is critical for achieving robust joint performance. To compare the performance of different insert concepts, seven configurations were evaluated: a direct-threaded laminate, barbed inserts, screw-to-expand inserts, tapping inserts, tapping inserts with adhesive, inserts embedded during laminate fabrication, and embedded metal blocks tapped after cure. Static pull-out tests showed that press-fit type inserts (barbed and screw-to-expand) showed significantly lower strength, with the embedded metal block performing the worst overall. Among all types, the embedded insert demonstrated the highest pull-out capacity. Flexural pull-out fatigue tests were then conducted on the threaded and embedded inserts to assess durability under repeated loading. A custom flexural pull-out fatigue fixture was developed, and the results showed that the tapping insert with adhesive provided the greatest fatigue resistance. These findings offer practical guidance on the selection and use of threaded inserts in composite laminates, highlighting the strengths and limitations of each insert family and informing design choices for improved structural reliability and long-term performance.