Extended Non-destructive Testing for the Bondline Quality Assessment of Aircraft Composite Structures

We present the results of extended non-destructive testing (ENDT) methods for bond line quality assessment in adhesive joints. The results presented were derived for important application scenarios with regards to aircraft manufacturing and the in-service repair of composite structures. The electromechanical impedance (EMI), laser shock adhesion testing (LASAT), and nonlinear ultrasound scanning (NUS) were used on flat coupon samples, scarfed samples, and curved samples. The EMI method applied to the flat coupons showed some relation of the frequency shift to the level of contamination. For the curved samples, there was insufficient sensitivity to differentiate distinct levels of contamination, while for scarfed samples in most cases both detection and distinction were possible. The LASAT method gave good results for the coupon samples, which were also in accordance with the results of the $${\text{G}}_{\text{IC}}$$ G IC and $${\text{G}}_{\text{IIC}}$$ G IIC tests. For coupon samples with multiple contaminations, we obtained results with varying significance. In the case of NUS, the measurements revealed an increase in nonlinearity affected by contamination at the interphase between the CFRP adherend and the adhesive layer for the majority of scenarios comprising single contamination of flat coupons and scarfed samples. The effect of multiple contaminations was a decrease in nonlinearity for the curved samples.

Composite spirals and rings under flexural loading: Experimental and numerical analysis

Recent improvements in pultrusion and filament winding have allowed the manufacturing of spirals and rings composite profiles for applications such as fuselage reinforcements of small aircrafts, automotive bumper beams, automotive springs and structural reinforcement for pipes. However, the behavior of curved carbon fiber components is complex and hard to predict, and still demands deeper understanding. In this work, progressive damage and cohesive zone numerical models were used to simulate the behavior of unidirectional curved composite structures under flexural loading. Four-point bending tests were carried out on curved samples monitored by strain gages for model validation. The results have demonstrated a strong influence of delamination on samples with well-defined resin-rich areas. In contrast, curved structures with more homogeneous fiber distribution, i.e. those manufactured by curved pultrusion, showed increased flexural strength. Maximum stresses from numerical and experimental analyses were compared and the maximum difference found was below 3.5%.

Checking Plane Wave Approximation for Double Crystal Topography with Curved Samples

The double crystal diffractometer in n,-n position realizes plane wave conditions only approximately. Nevertheless topographs taken with such arrangements are usually interpreted on the basis of contrast simulations starting with plane waves. Observations of diffraction patterns obtained with curved samples in an n,-n setting have lead us to study the validity of such a treatment. Simulations made for different types of the starting wave actually show that plane waves are adequate even in that critical case. However for a full coincidence between simulation and experiment it proved necessary to account for the residual angular spread within the collimated beam by averaging several plane wave simulations.