On the influence of resin pocket area on the failure of tapered sandwich composites

Structural defects such as resin pocket area are inevitably created between surface and core of composite structures during the production of wind turbine blades using vacuum infusion process. In this article, four-point bending tests were performed on tapered sandwich composites to investigate the effect of resin pocket area on the mechanical strength, crack growth path, and failure mode. Specimens were in similar shape to wind turbine blade profiles, and a shear-dominant load was applied to the resin pocket area during the experiments. The extended finite-element method was applied in order to predict crack growth path and failure mode. The average static strength of the specimens including the small size of resin pocket area had almost no change in compare with the specimen with no resin pocket area. Moreover, the medium size of resin pocket area decreased the strength for 3.5% while the large size one enhanced it for 1.75%. Thus, it can be deduced that the defect area does not have a significant effect on the flexural strength of the sandwich composite tapered specimens, but it can arrest the crack. Therefore, the crack propagates in the opposite direction at the interface of the face and core. Although the resin pocket area arrests the crack, it was observed that the size of resin pocket area directly affects the crack growth and its path. The smaller resin pocket area leads to slower crack growth, and early collapse occurs for the larger size of defect area. So, the size of resin pocket area has considerable importance during manufacturing of such structures. Finally, numerical results have shown good agreement with experimental ones.

Prediction of resin pocket geometry around rigid fiber inclusion in composite laminate by hot-pressing of prepregs

In this paper, two analytic methods are presented to predict the geometry of resin pockets formed around rigid fiber inclusions at the interlayer of unidirectional prepregs. The bending strain energy is calculated on fiber scale in one method, while it is calculated on layer scale in the other method. For the fiber scale method, several fibers in thickness direction are tied together to account for the bending stiffness increase caused by the interaction between fibers. And for the layer scale method, a single ply is divided into several sublayers to account for the bending stiffness decrease caused by the sliding between adjacent fibers. Both analytic methods can provide the closed-form solution for the resin pocket width, and the analytic results agree well with experimental results. The physical consistency of two methods is proved. It is found that the resin pocket size depends mainly on ply angles of the plies close to the inclusion, and the stacking sequence has some effect.

Numerical Modelling of Layered Composite Materials with Embedded Optical Fiber Sensors

One of the crucial points in making materials with embedded optical fiber sensors is obtaining information about stress-strain state which helps to asses the influence of embedded optical fibers on integral mechanical properties of the structure and redistribution of stresses and strains in the area around optical fibers. In this paper models of woven composite material with embedded optical fibers are studied with numerical results achieved with the help of finite element method in case of GFRP VPS-33 material with fabric filler T-10-14 and epoxy resin ENFB-2M. Numerical results about the change of stiffness parameters of plate specimens in cases of tension and bending are obtained. Evaluation of stress state change near embedded optical fiber in different cases of optical fiber orientation with respect to loading and in the presence and absence of resin pocket defect with polyamide optical fiber coating taken into account is given in this study.