Out-of-plane pull-through performance and failure mechanisms of composite material fastening structures considering temperature effects

Alternating high and low temperatures represent a typical operational environment for fastening structures (e.g., bolted structures) in the aerospace field, which have a pronounced/significant influence on the mechanical performance of bolted structures. In order to explore the impact of temperature variations on the pull-through mechanical performance of different bolts, an out-of-plane pull-through experiment was conducted on carbon-fiber-reinforced bismaleimide resin composites. Additionally, a specialized pull-through experiment fixture was developed for high/low temperature conditions. Using acoustic emission (AE) techniques, optical microscopy, and scanning electron microscopy (SEM), a multidimensional characterization of pull-through failure mechanisms was conducted, revealing the influence of temperature environments and bolts on the pull-through experiment failure mechanisms of composite materials. The findings reveal a correlation between temperature variations and the pull-through strength of differing bolts in composite materials. Specifically, as temperatures rise, the pull-through strength of protruding head fasteners demonstrates a gradual decline. However, the pull-through strength of countersink fasteners exhibits an initial increase followed by a subsequent decrease. The temperature exerts an influence on the damage patterns during the pull-through process of fastening structures. Observations in elevated temperature environments reveal a river pattern of matrix failure, confirming the existence of matrix plastic deformation in the process of interlaminar crack propagation at high temperatures. This provides a plausible explanation for the observed phenomenon of a decrease in pull-through strength of countersink structures with increasing temperature.