Novel Robust Models for Damage Tolerant Helicopter Components

摘要

A new approach to the prediction of crack growth fatigue lives under variable amplitude loading, the K(PR) approach, is described. K(PR) is the minimum stress intensity for crack propagation during a loading cycle and is sensitive to the immediate loading history at the crack tip. Four parameters describe changes in K(PR) as a function of load history. Unlike other models the KPR model has no fitting parameter. The four KPR parameters for 7010-T73651 aluminium and SAE 4340 quenched and tempered steel, together with necessary crack growth rate data, were measured. Variable amplitude testing was performed on the same materials under two loading spectra, Rotarix, a standard spectrum for a helicopter rotorhead, and Falstaff, a fixed wing fighter aircraft spectrum. The variable amplitude crack growth data were used to validate the K(PR) model together with 4 other models for fatigue crack growth. These were FASTRAN, the 3 models within AFGROW, namely, Wheeler, Willenborg and the closure model. It was found that the K(PR) model provided the best agreement for Rotarix on 7010 aluminium, with errors of only 15-20%. FASTRAN was second best. All other models made non conservative predictions. On Falstaff, agreement was not as good on aluminium alloys, but K(PR) was still the most accurate model The KPR model performed not as well on the SAE 4340 steel. For Rotarix. K(PR)still was the closest, for Falstaff, other models achieved better accuracy. All predictions were made blind, in advance of knowledge of the validation test data.