A Qualitative Study Of The Effect Of Fatigue Damage Growth On Helicopter Rotor Blade Modal Parameters

Structural damage in materials evolves over time due to growth of fatigue cracks in homogenous materials and a complicated process of matrix cracking, delamination, fiber breakage and fiber matrix debonding in composite materials. In this study, a finite element model of the helicopter rotor blade is used to analyze the effect of damage growth on the modal frequencies. Phenomenological models of material degradation for homogenous and composite materials are used. Results show that damage can be detected by monitoring changes in rotating frequencies, especially for composite materials where the onset of last stage of damage of fiber breakage is most critical. Curve fits are also proposed for mathematical modeling of the relationship between rotating frequencies and time to facilitate the use of modal frequencies for online health monitoring and life estimation. INTRODUCTION The damage detection in helicopter blades is based on some health measure of the system. Typical health measures are changes in blade flap (out of plane bending), lag (in plane bending) and torsion deflection between damaged and undamaged blades, vibratory hub loads, fuselage vibration, blade frequencies and mode shapes. Since frequencies can be estimated online [1-2], their use in rotor health monitoring has also been addressed. The health measure has a bounded value for a healthy system. However, when the system becomes unhealthy, the fault measure exceeds some threshold and maintenance action