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Composites analysis

Case Study

Glass Reinforced Plastic (GRP) and Aluminium Bolted T-joints

The UK's National Physical Laboratory (NPL) used LUSAS Composite to undertake a parametric study of two types of bolted T-joints under various static loading conditions. The effect of varying section properties such as web and base plate thicknesses, flange radii and bolt positions was investigated for both yield and fatigue assessments and for verification with laboratory testing. This on-going work forms part of the Measurements for Material Systems (MMS) Programme funded by the UK's Department of Trade and Industry.

Overview

Bolted T-joints have a variety of mechanical, civil, aerospace and marine applications where they are expected to sustain static or cyclic fatigue loads for considerable periods of time with no adverse effect on their load bearing capacities. However, under dynamic fluctuating or constant loads, joints have sometimes been seen to fail at stress levels much lower than the strength of the joint under static loading. To address this issue NPL is investigating which parameters are of direct concern when designing a bolted T-Joint. The eventual aim is to produce a working guide stating the importance of those parameters with respect to general joint performance.

Aluminium bolted T-joint

Bolted T-Joint Types

Glass Reinforced Plastic (GRP) and aluminium bolted joints were chosen for investigation. Both were of similar make-up, differing only in their nominal thickness. The GRP joint components consisted of a combination of 0.64mm thick unidirectional plies and 0.4mm thick biaxial woven plies to make up a nominal 12.16mm plate thickness. Aluminium joints were 15mm thick and of 2014-T6 aluminium alloy. In both cases a pair of radiused flanges are connected to web and base plates using 12mm diameter bolts positioned centrally on each flat region of the flange.

Modelling

3D models of both T-joint types were created in LUSAS Composite and a direct tension, a lateral and an inclined load case applied to each model. For the direct tension loadcase, and because of symmetry, only a half-model needed to be defined. Whilst modelling of individual laminate plies is possible in LUSAS, NPL decided to simplify models by carrying out global modelling of the GRP layers using an orthotropic solid material model. with properties obtained from its in-house CoDA software. A nonlinear material model was used for the aluminum T-joint. To model the clamping force of the torqued bolts a prestress force was applied to the shank of each bolt prior to loading the model with the chosen applied loading.

Correct modelling of the contacting surfaces of components is the key to analyses of this type so extensive use was made of the LUSAS slideline facility. Slidelines automatically take care of any frictional contact between contacting components. In all, 42 sets of slidelines covered the various contact situations that would occur in the analysis of the full T-joint model as used in the lateral and inclined loading analyses.

Slidelines were used to model:

  • Initial contact between web, base and flange plates.

  • Initial contact between the underside of the bolt heads to web, base and flange plates.

  • Loss of, or increased contact between flanges and web/base plates.

  • Potential contact between the bolt shanks and holes.

Joint failure was to be regarded as yielding of the aluminium or, for the GRP model, exceeding the failure strength of the composite material. For this, predicted failure loads were obtained for positions at the interface between the flange and the base and web plates and for a point mid-way around the radii of the flange. Laboratory tests involving strain gauges were also carried out on T-joints as part of the investigation and the results from these were used to verify the accuracy of the LUSAS results.

Fatique crack in flange of bolted aluminium T-joint

Direct tension failure in flange of bolted aluminium T-joint

Results

Analysis with LUSAS showed that for the bolted aluminum joints under direct tension elongation of the bolt holes would initially occur in the base plate, followed by similar damage in the web holes. Ultimate failure occured midway around the web due to high stress and strain concentrations in this region. Mike Gower, Research Scientist at NPL said: "In the direct tension case for the aluminium joint we wanted to predict where yielding was occurring and to what extent in order to fatigue test a sample joint using a percentage of the yielding load. For the GRP T-joint, by looking at various stresses in the model we could determine at which load increment the material strength values were exceeded."

For the GRP T-joint, Mike Gower said: "We got very good agreement with LUSAS using the Hashin failure model. At first sight it looked as if the GRP flanges were delaminating mid-way around the web, but in fact LUSAS and physical measurements showed that the peak stresses predicted by the Hashin failure index actually occured under the bolt at the interface between the unidirectional and woven plies, before then propagating around the curve of the flange component"

For the direct tension load case LUSAS overpredicted the results slightly. This was thought to be due to modelling the base plate supports with infinite stiffness and with hindsight perhaps springs should have been used instead. The yield loads, however, agreed very well. Results obtained from laterally loading the joint were in similarly good agreement, indicating that the modelling of the restraints had less effect for this loading condition than for the direct tension loadcase.

"It was very useful to use LUSAS for this research work. I found it very intuitive."

Mike Gower, Research Scientist, National Physical Laboratory


 

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