Crack – fiber sensor interaction and characterization of the bridging tractions in mode I delamination

Fiber bridging is regularly encountered in mode I delamination tests of unidirectional fiber reinforced composites. However, characterization of the bridging tractions is rather difficult. One way to indirectly evaluate the bridging traction distribution is to embed a fiber Bragg grating (FBG) sensor close to the crack tip and to measure the distributed strain along this FBG. The strain measurements from the FBG sensor are used to characterize the fiber bridging tractions by an identification method. In this work, the sensor is embedded in a unidirectional carbon/epoxy composite. Firstly, it is treated as an inclusion near the crack plane and a numerical analysis is performed to study its effect on the measured strain field and energy release rate. The results demonstrate that the sensor, located at about two fiber diameters from the crack plane, has a negligible effect on the fracture process. Secondly, among the identified linear, bilinear, and exponential bridging traction distributions, the exponential one is found to be a suitable model. Characterization of the bridging tractions allows to calculate the energy release due to the bridging fibers GIb which is similar to the difference between the initiation energy release GIi and the propagation value GIP. The results also agree with the bridging tractions evaluated from the conventional energy release rate – crack opening displacement method.