Bridging by intact fibers is an important toughening mechanism in composite materials. However, a direct experimental evaluation of its contribution is difficult to achieve and in the case of large scale bridging the thickness, h, may have an important influence on the energy release rate (ERR). In this work a semi-experimental method is adopted to quantify the effects of thickness to fracture of unidirectional glass fiber reinforced polymer (GFRP) double cantilever beam specimens in mode I fracture under monotonic loads. Several specimens with thickness in the range of h = 3.5-19 mm are tested. In two selected specimens, embedded optical fibers with an array of eight or ten wavelength division multiplexed fiber Bragg gratings were are to measure local strains close to the crack plane and employed in an inverse identification procedure to determine the bridging tractions. The results suggest that the stress at the start of the bridging zone and the crack opening displacement at its end are independent of the specimen thickness. However, the rate of change of the tractions with crack length depends on the specimen thickness. While the initiation value of the ERR is independent of h, at the steady state it varies from 900 (h = 3.5 mm) to 21 00 J/m(2) (h = 19 mm). Using the identified tractions, the total stress intensity factor (SIF) and the corresponding ERR are calculated. The results show that the SIP approach gives similar results. Thus thickness, via the bridging fibers, is responsible for the effects on steady state ERR observed in the specimens used herein. (C) 2013 Elsevier Ltd. All rights reserved.