Study of the effect of liquid shim on single-lap joint using 3D Digital Image Correlation

Composite Structures, Volume 96, February 2013, Pages 216-225.
J.X. Dhôte, A.J. Comer, W.F. Stanley, T.M. Young.

 

Irish Centre for Composites Research (IComp), Materials and Surface Science Institute (MSSI), University of Limerick, Ireland.

 

Abstract

The influence of a liquid-shim layer in place at the interface between the two laminates of single-lap joints used in primary aircraft structures have been investigated in this study. To this end, experiments were performed to evaluate the complex structural behaviour under tensile loading of single-lap bolted joints. Two cases were considered: with and without a shim layer. Three-dimensional Digital Image Correlation (DIC) has been used to provide the complete surface strain fields and to analyse the out-of-plane phenomena of the joints, and successfully helped to identify the effect of a liquid-shim layer. It has been shown that the introduction of a shim layer magnifies secondary and tertiary bending and the out-of-plane deformations of the joint and modifies the strain distribution, potentially leading to higher tensile strains in the laminates.

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Additional information

Aircraft structures are not monolithic by design and thus require individual components to be joined and assembled into sub-structures, for example: composite wing panels to aluminium ribs. At the interface between the two parts of these joints, a layer of liquid shim – an epoxy-based material – can be used to fill gaps resulting from manufacturing tolerances between the faying surfaces, and allows a better transfer of load in order to prevent damage. The influence of this liquid-shim layer at the interface between two composite laminates of single-lap joints used in primary aircraft structures have been investigated in this study. To this end, experiments were performed to evaluate the complex structural behaviour under tensile loading of single-lap bolted joints: both composite to composite and hybrid composite to aluminium joints. For each joint type, two cases were considered: with and without a shim layer. Three-dimensional Digital Image Correlation (DIC) has been used to provide the complete surface strain fields and to analyse the out-of-plane deformation of the joints, and successfully helped to identify the effect of a liquid-shim layer. Digital Image Correlation (DIC) effectively tracks the movement of a surface pattern during a test or experiment. DIC therefore allows the determination of displacement and strain fields of a test specimen under load. Correlation is done by analysing the displacement of the patterns within discretised subsets or facets of the whole image. The user acquires a series of images during a mechanical testing experiment, with the first image normally corresponding to the zero load condition. The use of two CCD cameras allows the stereoscopic imaging of the loaded specimens, in this case bolted joints, thus allowing a 3D analysis. In contrast to traditional instrumentation such as extensometers and strain gauges, 3D DIC is a non-contact method and provides a full-field strain analysis over relatively large areas. A conclusion of this extensive experimental investigation is that the introduction of a shim layer magnifies secondary and tertiary bending and the out-of plane deformations of the joint and modifies the strain distribution, potentially leading to higher tensile strains in the laminates and thus these joints must be carefully designed.

 

Figure Legend

Experimental set-up (on the right hand side of the picture) showing one of the two CCD cameras of the 3D DIC system located in front of the single-lap joint under investigation. The joint was tested in a hydraulic tensile testing machine and the area under investigation was covered with a speckled surface pattern. On the left hand side, two full-field contour plots, obtained by 3D DIC, are presented i.e. longitudinal strain and the out-of-plane displacement of the upper substrate. Both plots indicate the extent of secondary, or out-of-plane, bending experienced by the joint under tensile load.

 

Study of the effect of liquid shim on single-lap joint using 3D Digital Image Correlation

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