Studies on Traction Separation Relations in Fracture of Layered Composites

Layered fiber reinforced composite materials are prone to fracture in planes parallel to the direction of fibers leading to extensive delamination or intralaminar fracture. This phenomenon of crack propagation is frequently accompanied by significant increase in fracture resistance due to different damage mechanisms active on the wake of the crack. The developed zone comprises intact pulled-out fibers bridging the crack faces forming the so-called, large scale fiber bridging (LSB). Several studies have dealt with the evaluation of the traction-separation relations mainly related to the bridging phenomena in delamination, employing different techniques. However, only a few recent studies dealt with the effect of specimen size on LSB and the traction-separation relations, challenging the applicability of the existing relations in structural design. This work initially focuses on the characterization of LSB phenomena in mode I intralaminar fracture of a unidirectional (UD) carbon fiber reinforced thermoplastic. Here an already developed semi-experimental technique based on quasi-continuous strain measurements by FBG sensors, adapted to the needs of the current study, is employed to identify the traction-separation relation. The outcome of the identification scheme compares very well with the results from a numerical micromechanical virtual test. The identified traction-separation relation is employed to calculate the energy release rate (ERR) and evaluate the resistance curves (R-curves) associated with LSB of this fracture response. An important toughening effect is demonstrated, which is about two times higher than the corresponding interlaminar values. The aforementioned semi-experimental technique is implemented in intralaminar fracture of a UD carbon/epoxy composite and is devoted in evaluating the effect of specimen thickness on the developed closing tractions due to LSB. Here, double cantilever beam (DCB) specimens of three different thicknesses, loaded with end opening forces are employed to conduct the characterization. A significant effect of specimen’s thickness is present on the three identified traction-separation relations. The results of the present study indicate a scaling relationship expressed as a function of the bridging traction profile exponent and the stiffness of the specimen. Nevertheless, a common maximum closing traction at the crack tip is evaluated. Similar to the thermoplastic composite, the measured ERRs are considerably higher than the corresponding interlaminar values. Furthermore, a testing apparatus able to apply pure moments on the bending arms of DCB specimens, by means of pairs of forces is designed, fabricated and used. This testing setup is employed in the fracture characterization of the mentioned carbon/epoxy system to investigate the effect of loading conditions. The acquired results show minor differences on the maximum ERRs, with some small variation on the shape of the R-curves. Finally, the traction-separation relation in delamination of a woven glass fiber reinforced epoxy with tufting through the thickness reinforcement (TTR) is investigated. Accordingly, a generalization of the relation between the closing traction profile and flexural rigidity of the DCB arms is attested and a concise modeling approach for the load history prediction is proposed comprising the effects of LSB and TTR.