PSI - Issue 54

Vasiliki Panagiotopoulou et al. / Procedia Structural Integrity 54 (2024) 482–489 Vasiliki Panagiotopoulou/ Structural Integrity Procedia 00 (2023) 000 – 000

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A.W. Lees, 2005) has shown that vibrations can effectively indicate impact damage for structural monitoring. Specifically, diagnosis involves identifying frequency components and their alignment relative to the rotational axis. In undamaged conditions, spectrum analysis of rotating machinery typically reveals frequency multiples of the shaft rotational speed (1Ω). According to (AL-Shudeifat, M. A., 2010), certain mechanics-based rules can be followed. Vibrations at the 1Ω frequency may indicate unbalance or angular misalignment of the inclined shaft, with well maintained machines exhibiting relatively low amplitude at this frequency. Conversely, angular misalignment between shafts often leads to axial vibrations at the running speed, while parallel misalignment can result in radial vibrations occurring at the running speed or its multiples, depending on the coupling used. Initially, prior to any ballistic impact occurs, vibrations in the system primarily result from imperfections like machining errors, misalignments, and long-term operational wear. These imperfections lead to low-intensity vibrations due to eccentricity. However, following a ballistic impact, the rotor experiences eccentric loads due to changes in its properties, such as mass loss and stiffness asymmetry. Damaged conditions typically introduce more frequency components in the power spectrum, as observed by (C. Zhang, D. Wang, R. Zhu, J. Li, P. Cao, 2023), indicating the presence of damage. Stiffness asymmetry causes subharmonic resonance, while mass loss increases system resonance. In summary, an overall increase in resonance or the presence of a significant 2Ω frequency component in the system's response signifies that the shaft's structural integrity has been compromised due to a ballistic impact.

Fig. 2 The BERC test rig and the numerical model of the TRDL. The highlighted nodes are the retained nodes after performing the sub structuring method.

3.2. Experimental Activities Ground tests are being conducted on the Tail Rotor Drive Line (TRDL) slant shaft in both pristine and damaged conditions using a full-scale test rig known as the "BERC-Tail Rotor Electrical Test Bench." This BERC test rig includes a section of the Tail Boom, allowing for precise simulation of the helicopter's operational conditions. The Tail Rotor is powered by an electrical generator at its standard rotational speed. The transmission system's initial segment features a supercritical horizontal shaft linking the electrical engine to the Intermediate Gear Box (IGB). Beyond the IGB lies the slant shaft, which is the project focal point for vibration monitoring. It is particularly critical for assessing susceptibility to ballistic impacts, and dynamic tests will be conducted to evaluate its response both before and after such impacts occur. Finally, the slant shaft transmits power to the Tail Gear Box (TGB). All these components correspond to part numbers used in the AW139/189 LHD helicopter. The vibration levels of the structure are measured using accelerometers positioned at various locations along the BERC. Specifically, accelerometers were installed on the cases of both IGB and TGB. Additionally, a tri-axial accelerometer has been placed on the tail frame to monitor potential vibrational imbalances in the tail structure. The placement of these sensors was primarily determined based on engineering judgement. The primary factor affecting the level of vibration in the structure is the absorbed power. This power is contingent upon both the rotational speed of the TDRL and the pitch of the Tail Rotor, which in turn regulates the absorbed torque. Under normal conditions, the helicopter transmission operates at a constant rotational speed of 102%, equivalent to 1406 rpm in the case of the AW189. A Simplified Test Rig (STR) has been created as conducting impact tests on the BERC test rig is impractical. The STR is designed to perform impact tests on the slant shaft, mimicking various damage scenarios. Afterward, the