Fiber metal laminates (FMLs) are of high interest for lightweight structures as they combine the advantageous material properties of metals and fiber-reinforced polymers (FRPs). However, low-velocity impacts can lead to complex internal damage. Therefore, structural health monitoring with guided ultrasonic waves (GUWs) is a methodology to identify such damage. Numerical simulations form the basis for corresponding investigations, but experimental validation of dispersion diagrams over a wide frequency range is hardly found in the literature. In this work the dispersive relation of GUWs is experimentally determined for an FML made of carbon FRP and steel. For this purpose, multi-frequency excitation signals are used to generate GUWs and the resulting wave field is measured via laser scanning vibrometry. The data are processed by means of a non-uniform discrete 2d Fourier transform and analyzed in the frequency-wavenumber domain. The experimental data are in excellent agreement with data from a numerical solution of the analytical framework. In conclusion, this work presents a highly automatable method to experimentally determine dispersion diagrams of GUWs in FML over large frequency ranges with high accuracy.

Residual stresses in fiber metal laminates (FML) inevitably develop during the manufacturing process. The main contributor to these stresses is the difference in the coefficients of thermal expansion (CTE) between fibers and metal in combination with high process temperatures. To quantify these stresses, the use of specimens with an asymmetric layup is an easily adaptable method. The curvature that develops after the manufacturing of flat laminates with an asymmetrical layer stack is a measure of the level of residual stresses evolving during cure. However, the accuracy of the curvature evaluation is highly dependent on specimen design and other influencing parameters. This leads to deviations when compared to other methods for residual stress quantification as can be seen from the literature. Therefore, in this work a large set of FML specimens is comprehensively investigated to identify relevant influencing parameters and derive conclusions about specimen design and evaluation techniques. For certain layups and process parameters, there is a good correlation between the curvature and the stress-free temperature, which is further covered by analytical solutions for bimetals. This correlation is the basis to transfer curvature into a stress-free temperature that can consequently be used for the quantification of residual stress levels in more complex FMLs. The transfer is validated by in situ strain measurements during cure using a strain gauge technique. Based on the results, the application of asymmetric specimens for residual stress characterization in more complex laminates is presented in the form of a workflow. The work shows the basic considerations and procedures necessary to use asymmetric specimens for residual stress quantification in FML. Furthermore, the results obtained can also be transferred to other composite materials.

Fiber metal laminates (FML) are a promising material as they combine the high specific properties of fiber-reinforced polymers with the ductility and damage tolerance of metals. However, during manufacturing in-plane residual stresses arise in the single layers of the laminate. The stresses mainly originate from the different coefficients of thermal expansion of the two materials and lead to an inhomogeneous stress state in the thickness direction of the FML. From homogeneous materials, it is known that stress due to external loads affects the propagation behavior of guided ultrasonic waves (GUW) in thin structures. Nevertheless, the magnitude of this effect is mode and frequency dependent. In the case of FML, the question is whether the inherent residual stresses also influence the dispersion characteristics of propagating GUW. With modified cure cycles, different residual stress states are realized in FML specimens made of carbon fiber-reinforced polymer and steel. An established measurement procedure using laser scanning vibrometry and multi-frequency excitation is used to determine the dispersion diagrams for the manufactured FML specimens. After processing the data with a nonuniform 2d discrete Fourier transform, the phase velocities over the measured frequency for the fundamental modes are compared for the different residual stress states. The results indicate that the variation of the residual stresses in the investigated FML does not show any significant effect on the GUW propagation for the measured frequencies.