Experimental and analytical study on net-tension strength formulae and stress concentration factors of multi-row bolted joints in pultruded fibre-polymer composites

This paper presents the test results from 13 batches of multi-row bolted double-lap shear joints and two batches of tensile coupon tests, each comprising five specimens. The study focuses on the net-tension mode of failure of steel bolted joints of a pultruded fibre-polymer composite with specimens fabricated from a 6.4 mm thick flat sheet laminate. It extends the characterisation work to non-staggered and staggered bolt configurations 3 × 3, 3 × 4, and 4 × 4 with composite material oriented either in the Longitudinal or Transverse direction of pultrusion. Test results show that a net-tension mode of failure primarily occurs at the first row of bolts. Evaluation of the test results shows when a fourth row of bolts is beneficial and that for staggered bolt configurations with the minimum geometrical requirements a lower strength efficiency per unit width is obtained compared to the equivalent non-staggered joints. A semi-empirical approach, aligned with the design procedure in CEN/TS 19101:2022, is used to establish that stress concentration factors for the 13 different joints are in the range 1.3–2.9. These new results can be used to extend the scope of non-staggered bolt configurations in CEN/TS 19101:2022.