Reusable Strategic Space Launcher Technologies and Operations (SALTO)
The SALTO project aimed to develop and validate an advanced structural monitoring methodology in vibration regime, applicable to metallic and composite structures used in the aerospace field.
Period: 14 July 2025 to 31 December 2025
Project code: PN-IV-P8-8.1-PRE-HE-ORG-2025-0277
The project included a single implementation stage, entitled “Experimental monitoring of structural response to vibrations. FEM analysis. Validation of the SHM methodology”. The activities carried out during the stage aimed at evaluating the vibration behavior of thin aluminum and composite structures subjected to controlled vibrations.
For these studies, flat and curved specimens instrumented with piezoelectric PZT (PWAS) sensors and optical FBG sensors were used. The experimental tests included measurements of the electromechanical impedance (EMI) signature and recording of the optical response under sinusoidal, random, and shock type vibrations. This approach allowed the characterization of the eigenmodes of the structures and the identification of local stiffness changes.
An important element of the stage was the assessment of the influence of external vibrations on the accuracy of EMI measurements. In particular, the differences were analyzed between the controlled vibrations applied during the experiment and the external vibrations to which structures are often exposed in real aerospace applications. The results showed that these external vibrations can significantly alter the EMI signature, which is a critical aspect in applying the technique for structural integrity monitoring in operational environments.
The positioning of the sensors on the surface of the plates represented another direction of investigation. The tests highlighted that central placement of the PWAS sensor for EMI measurements and the FBG sensor for optical measurements enables efficient capture of dominant modal phenomena, for both metallic and composite structures. This aspect contributes to increasing the sensitivity of the method and to obtaining stable and reproducible data.
In parallel with the experimental tests, the mechanical stability of sensor attachment was evaluated by comparing two types of adhesive materials, a permanent epoxy adhesive and a temporary one. The results confirmed that the epoxy adhesive ensures superior mechanical coupling, providing consistent EMI signatures, while the temporary adhesive showed detachment tendencies under repeated vibration, reducing measurement fidelity.
To validate the results, the experimental data were compared with numerical analysis performed using FEM methods. Simulations included modal frequency identification for metallic plates and Lamb wave propagation studies for composite plates. Comparison of simulated and measured responses showed good alignment of modal shapes and coherent behavior across experimental and numerical datasets, confirming the accuracy of the modeling and the correctness of the instrumentation.
The convergence of results obtained through EMI signatures, PWAS and FBG sensors, respectively through FEM analysis, demonstrates the reliability of the SHM methodology evaluated in the project. This methodology enables detection and characterization of early structural changes under vibration loading and provides a solid experimental and numerical framework for structural monitoring in the context of reusable space launchers and other aerospace structures exposed to dynamic loads.
Additional information is available at the I.N.C.A.S. Bucharest headquarters.