Applied Composite Materials (v.17, #3)

Experimental Investigation of Dynamically Loaded Bolted Joints in Carbon Fibre Composite Structures by Garth M. Pearce; Alastair F. Johnson; Rodney S. Thomson; Donald W. Kelly (271-291).
This paper reports on recent experimental work to investigate the response of bolted carbon fibre composite joints and structures when subjected to constant dynamic loading rates between 0.1 m/s and 10 m/s. Single fastener joints were tested in both the bearing (shear) and pull-through (normal) loading directions. It was found that the joints exhibited only minor loading rate dependence when loaded in the pull-through direction but there was a significant change in failure mode when the joints were loaded in bearing at or above 1 m/s. Below 1 m/s loading rate the failure mode consisted of initial bolt bearing followed by bolt failure. At a loading rate of 1 m/s and above the bolt failed in a ‘tearing’ mode that absorbed significantly more energy than the low rate tests. A simple composite structure was created to investigate the effect of loading rate on a more complex joint arrangement. The structure was loaded in two different modes and at constant dynamic loading rates between 0.1 m/s and 10 m/s. For the structure investigated and the loading modes considered, only minor loading rate effects were observed, even when the dominant contribution to joint loads came from bearing. It was observed that the load realignment present in the structural tests allowed the joints to fail in a mode that was not bearing dominant, and hence the loading rate sensitivity was not expressed.
Keywords: Carbon fibre composites; Polymer matrix composites; Bolted joints; Dynamic loading; Rate effects

While it is true that honeycomb core can trap moisture, it is a myth that widespread moisture ingression is an inevitable outcome when honeycomb sandwich structures are exposed to real world environments. It is also a myth that when moisture ingression occurs, progressive weight gain and strength loss are inevitable outcomes. Using a rebuttal of a 2004 honeycomb-critical paper as the focal point, this paper summarizes multiple sources which indicate both that moisture ingression is preventable and, even if it does occur, that proper material choices can prevent severe consequences. These claims are further supported by results from in-plane compression testing of typical (0.064 g/cm3 aramid-paper/phenolic core with 0.116 cm carbon/epoxy faces) composite honeycomb sandwich specimens after impacting, water submersion, and extensive thermal cycling.
Keywords: Honeycomb; Sandwich structures; Composites; Moisture ingression; Water trapping; Environmental exposure

An approach to estimate interface shear stress of ceramic matrix composites during fatigue loading has been developed in this paper. By adopting a shear-lag model which includes the matrix shear deformation in the bonded region and friction in the debonded region, the matrix crack space and interface debonding length are obtained by matrix statistical cracking model and fracture mechanics interface debonding criterion. Based on the damage mechanisms of fiber sliding relative to matrix in the interface debonded region upon unloading and subsequent reloading, the unloading counter slip length and reloading new slip length are determined by the fracture mechanics method. The hysteresis loops of four different cases have been derived. The hysteresis loss energy for the strain energy lost per volume during corresponding cycle is formulated in terms of interface shear stress. By comparing the experimental hysteresis loss energy with computational values, the interface shear stress corresponding to different cycles can then be derived. The theoretical results have been compared with experimental data of three different ceramic composites.
Keywords: Ceramic matrix composites; Fatigue; Hysteresis loops; Interface shear stress

Numerical Investigation of Dynamically Loaded Bolted Joints in Carbon Fibre Composite Structures by Garth M. Pearce; Alastair F. Johnson; Rodney S. Thomson; Donald W. Kelly (329-346).
This paper presents quasi-static and dynamic modelling of bolted composite structures using the explicit finite element code PAM-CRASH. User controlled point link (PLINK) elements were investigated for modelling the bolted composite joints used in the structures. Simulation results were compared with quasi-static and dynamic structural testing reported previously. Two loading configurations were considered. It was shown that the PLINK element modelling approach agreed well with the experimental results for both loading configurations and for one case offered significant improvements over other simplified bolt modelling methods. A stacked shell modelling approach was used to model the interlaminar delamination damage present in the ball-loaded impact mode. The overall response of the structure was significantly improved by the addition of these energy absorbing interfaces.
Keywords: Carbon fibre composites; Polymer matrix composites; Bolted joints; Impact modelling; Explicit finite element analysis