Fatigue Substantiation and Damage Tolerance Evaluation of Fiber Composite Helicopter Main Rotor Blade Root End
Helicopter rotor systems are dynamically loaded structures with many composite components such as main and tail rotor blades and rotor hubs. The wings or blades of a helicopter are part of a larger dynamic rotor system, which is designed to provide the lift, thrust and control. It is therefore apparent that the safety of the helicopter is highly dependent on the reliability of the rotor and its blades. The loads experienced by the root end section of a main rotor blade during actual flight are flap load, lead-lag load, centrifugal load and the torsion. The dynamic components of a civil helicopter need to be certified according to the airworthiness guidelines containing increased safety demands. The guidelines addresses subjects like an investigation of growth rate of damages that may occur from fatigue, corrosion, intrinsic and manufacturing defects or damages from discrete sources under repeated loads expected in service, residual strength requirements, consideration of the effects of material variability and environmental conditions like hot/wet strength degradation etc. In general the fatigue substantiation of dynamically loaded structures is based on the Safe Life procedure by establishing of a safe fatigue strength working curve, derivation of load spectrum from in-flight measurements and the calculation service life by means of Modified Miner’s non-linear damage accumulation hypothesis. As the certification authorities demand for an improved damage tolerant behaviour, especially for dynamically loaded structures, basic damage tolerance material data were studied. This paper describes Multi-Scale modeling computational approach to assess the growth rate of damages from fatigue under spectrum loading and consideration of the effects of material variability to assess structural advantages of main rotor blade root end. Main emphasis was laid on the design of the dynamic components in order to achieve unlimited life with high flaw tolerance. A micromechanics based Multi-Scale Progressive Failure Analysis (MS-PFA approach that detects damage and fracture evolution is carried out to assess the durability and damage tolerance (D&DT;) of main rotor blade root end section with effect of defects: 1) ply-drop-off, and features at the reduced skin thicknesses along the root end span; 2) fiber waviness exhibited in thick sections; and 3) void shape, size, distribution. . Fatigue life is estimated under service spectrum block loading conditions by determining the material stiffness and strength degradation, failure loads and cycle. In all the simulation studies establishment of basic material fatigue and damage tolerance data of the fiber composite materials were measured with the help of coupon tests. Micromechanics based optimization subroutine is used to estimate the effective fiber and matrix properties by reverse engineer complete transversely anisotropic fiber and isotropic matrix modulus, Poisson’s ratio, and strength properties.
Author: Frank Abdi
Conference: SAMPE Seattle 2017