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The complexity of the microstructure in composites has led to major generalizations when predicting the strength of composites. In recent years composite strength predictions have progressed by improving the fidelity of randomly packed unidirectional models. Taking it one step, further we created digital twin models of real microstructures for discontinuous aligned composites. In these models we evaluated the impact of the mean fiber direction on the local stress concentration factors. We found that the local stress concentration factors can vary 71% within a 20 ° range of rotation. Nonetheless, this variation is load dependent and will be reduced with load progression.

Fatigue delamination growth in composites is accompanied by large scale bridging (LSB) that yields important toughening effects. However the extent of this mechanism depends on the laminate geometry rendering its modeling a challenging task. This work presents a combined experimental/numerical study on characterization of specimen thickness dependence of LSB in fatigue delamination. Double cantilever beam specimens of different thicknesses (h = 2 4 and 8 mm) equipped with arrays of multiplexed fiber Bragg grating sensors are subjected to mode I fatigue loads. Measured strain data with the sensors are employed to identify the bridging tractions and subsequently compute the energy release rate (ERR) due to the bridging as well as the ERR at the crack tip. The obtained results confirm that fatigue delamination growth strongly depends on the specimen geometry when LSB prevails. It is shown that both the extent of bridging and critical ERR at failure increase by increasing the specimen thickness while the maximum bridging traction at the crack tip is found independent of the specimen geometry. The identified traction separation relations serve to establish a power correlation between the crack growth rate and ERR at the crack tip which is independent of the specimen thickness.

It is well known that large scale bridging accompanying delamination and fracture in layered composites is among the most important toughening mechanisms. The resulting resistance to fracture however is depended on the specimen geometry rendering its modeling difficult. As a consequence characterization of the tractions on the wake of the crack the so called bridging zone is very important in the efforts to predict the loading response of composite structures. In this chapter experimental results and modeling of delamination and fracture in layered composite specimens are discussed. The experimental part consists of displacement controlled monotonic tests of interlaminar and intralaminar fracture. Selected specimens are equipped with wavelength multiplexed fiber Bragg grating (FBG) sensors to monitor crack propagation and strains over several mm in the wake of the crack. The modeling part involves an iterative scheme to calculate the traction separation relation due to bridging using the strains from the FBG sensors parametric finite elements and optimization. The results demonstrate an important effect of specimen thickness in interlaminar and intralaminar fracture: the bridging length at steady state linearly increases with specimen thickness while the maximum bridging stress is independent for thickness in each case. Results of a similar study in cross ply specimens limited to a selected specimen thickness show important effects of specimen width on the extent of large scale bridging. The obtained traction separation relations for each investigated case are employed in a cohesive zone simulation to predict the corresponding load displacement curves. On the basis of the experimental results a micromechanics model is used based on an embedded cell model to predict the observed specimen thickness effects on large scale bridging. The results of the reported studies demonstrate an important effect of specimen geometry on large scale bridging. Thus the so called bridging law is not a material parameter. Using the proposed methodologies the thickness effect on large scale bridging is quantified and used to predict the load displacement response.

One of the key issues in composite structures for aircraft applications is the early detection and localisation of damage. Often service induced damage does not involve visible plastic deformation, but internal matrix related damage, like transverse cracks and delaminations. Their detection imposes costly maintenance techniques. Vibration based damage identification methods are promising as an alternative for the time consuming and costly Non-Destructive Testing methods currently available. These methods also offer the potential to be used in a real-time health monitoring system. The measured change of the dynamic properties is employed to identify damage such as delaminations. Earlier performed research [1] showed that the Modal Strain Energy Damage Index algorithm [2] is a suitable method to identify impact induced damage in a fibre reinforced composite plate structure with stiffeners using laser vibrometer measurements. The damage identification algorithm requires the computation of the second derivative of the displacement mode shapes. The goal is to extent this research by applying fibre Bragg gratings since they can be valuable. Firstly, optical fibre sensors are suitable for integration, which is required in a Structural Health Monitoring environment. Secondly, measured strain mode shapes could be advantageous with respect to the numerical errors induced by the computation of second derivatives of the displacement mode shapes. Before applying the damage identification algorithm, it is a challenge to accurately extract the dynamic properties. The dynamic properties of a damaged composite T-shaped stiffener section, shown in figure 1, are investigated in this work using fibre Bragg gratings.