Burst Strength of Rotating Discs

Abstract

The rotating discs in the aircraft gas turbine engine are components in which the requirement for high strength/weight ratio and the complex stress produced by rotation have a significant influence on material strength and ductility and the study of material behaviour under these conditions has been the subject of numerous investigations. The opportunity to examine the most promising theories for predicting the plastic deformation and fracture of rotating discs arose when a large number of model discs in vacuum melted steel, having near uniform properties, became available for evaluation by burst testing. Fully ductile behaviour of a rotating disc is manifested in the instability phenomenon and correlation between theory and experiment shows that the rotational speed at which instability occurs may be predicted from a knowledge of the true stress-strain curve for the material in simple tension using the Hencky deformation theory. However, due to complex stress resulting from centrifugal loading, fracture in a rotating disc may precede the instability condition and it is not possible to assume fully ductile behaviour in a real material. The plastic stress analysis of model discs in which burst has resulted from fracture prior to instability shows that the materials tested favour a maximum principal stress criterion of fracture and good correlation has been achieved with the maximum true stress at fracture in the tensile test piece. The important influence of strain-hardening on the strength of a rotating disc is demonstrated by the theoretical analysis and by the burst results. For materials exhibiting low strain-hardening the average tangential burst stress in a disc agrees reasonably well with the tensile strength of the material and confirms the simple burst criterion. High strain-hardening materials, however, may be required to exhibit exceptional ductility to allow a disc to achieve a burst strength consistent with the tensile strength and in practical examples relatively low burst strengths have resulted.