Structural integrity of bolted joints for pultruded GRP profiles

Presented in this thesis is a combined physical testing and computational modelling programme of research for the structural integrity of bolted joints for Pultruded Fibre Reinforced Polymer (PFRP) profiles. The work was carried out in a joint EPSRC funded Structural Integrity (SI) project between the Universities of Warwick and Lancaster. It will be shown in the thesis that it provides a significant new body of scientific understanding and relevant independent joint test data, which can be used in the future preparation of design guidance for the safe and rational design of joints. The contribution from the author to this SI project on plate-to-plate steel bolted joints can subdivided into four parts. The first part includes the evaluation of the 10 single-bolted and the 5 multi-bolted double lap-joint test series that appeared before 2001. This identifies that the 15 test series, from 10 centres, are often different because the researchers chose different parameters to study. Of the 800 concentric strength tests 640 were single-bolted and 160 were multi-bolted. It is noteworthy that few of these previous tests include environmental conditioning of the joints. A review of design methods is given in this part of the thesis. This shows that significant joint test data is required to appraise SI design methods for PFRP bolted joints, such as presented in the EUROCOMP Design Code and Handbook of 1996 and in 1981 by Hart-Smith. The EUROCOMP simplified and rigorous methods are expected to be generic and involve damage tolerance to increase the strength of the joint design. It is further found that application of these methods requires Finite Element (FE) stress calculations for the target analysis due to bolt bearing, and for the source analysis, for multi-bolted joints, to determine the bolt load distribution and by-pass loads. Parts two and three of the contribution from the author are combinations of experimental and FE analysis work that focuses on the target and the source problems, respectively. The strain field close to a notch is measured in part two using a pin-bearing test method and strain gauged specimens. To find out if FEA can predict the target strains/stresses an ABAQUS© FE model is developed. The PFRP material is assumed to be homogeneous and possess orthotropic properties. It is shown that a linear elastic analysis, with contact and friction modelled, gives strains that are similar to those measured. This observation is supported by a small photo-elastic strain analysis using the GFP 1000 system from Stress Photonics. By way of the evaluation of previous testing in part one it was decided in part three to conduct a comprehensive series of multi-bolted joint tests. Constant variables in this series of concentric loaded tests are the PFRP material, the plate thickness, the material orientation, the bolt diameter, the clearance hole size, and the bolt torque. This test series, design using a method by Taguchi, comprised four different joint configurations, twelve different joint geometries and four different environmental conditionings (including a temperature of 60oC and ‘wet’ ageing for 1000hrs). The author presents the testing procedure and 432 resistance results for 144 different joints, having a batch size of three. Mechanisms and modes of failure are identified, and several unrelated to the distinct modes of bearing, shear-out and net-tension used in current design approaches are presented. Using this fairly large number of consistent data points an evaluation is made on the influence of the various variables. It is shown that is not straightforward to establish simple strength trends. Furthermore, the findings of this test series suggest that a batch size of three is too small, as batch strength variability is relatively high. By way of a Round-Robin exercise between the two Universities, with joints having a batch size of 10, it is shown that the high strength variability can be associated with the non-homogeneity of the reinforcement arrangement in the PFRP material. Two multi-bolted joints having three rows of three bolts are instrumented with strain gauges to determine the strain distributions linked to bolt load distribution and by-pass loads. Measured strains are compared to numerical predictions from an ABAQUS FE analysis using the same modelling methodology as for the pin-bearing target analysis. The source analysis comparison is acceptable, but like many of the results presented in the thesis it appears likely that the experimental measurements are affected by the non-homogeneity in the macrostructure of the PFRP. Presented in part four is a preliminary appraisal of the EUROCOMP simplified design method and the Hart-Smith design method. The EUROCOMP method is shown to possess deficiencies, via a critique by Lancaster University, which indicates that it cannot be used in practice. By taking information from the multi-bolted joint test series in part three the author constructs semi-universal design charts using the Hart-Smith method, which can be used to design joints with changing geometry. An important finding from this preliminary appraisal is that the SI design methods are difficult to apply since they require lots of physical test data and relevant and reliable stress results from FEA. Finally, despite the programme of research providing a significant new body of scientific understanding and relevant independent joint test data it is concluded that further work is needed before an SI method for the universal design of plate-to-plate PFRP bolted joints can be considered for a code of practice