8.1 INTRODUCTION

Extensive analytical predictions and rig testing notwithstanding, blade failures in engines due to excessive vibrations still occur during final test phases. Even worse is the occurrence of failures after an engine has successfully passed a number of rigorous qualification and certification tests and goes into regular service. The failures may point to a lack of adequate design tools, but may indicate another issue arising from the business perspective. In the ongoing search for efficiency improvement and better thrust-to-weight ratio, engine designers and researchers are pursuing higher speeds and temperatures, radical new blade profiles, higher stage loads, and lower aspect ratios. A more exacting consideration of blade flutter, resonance, cooling, material characteristics, and manufacturing methods is required than hitherto.

The calculation of blade resonant frequencies and mode shapes of a bladed disk system calls for the definition of boundary conditions at shroud and dovetail interfaces. The maximum operating speed of the engine cannot be established unless vulnerable frequencies and response amplitudes at resonance are known to a satisfactory degree of confidence. Research efforts have focused on the need to develop a fundamental understanding of topics such as Coulomb and viscous damping, coolant passage turbulators, and unsteady viscous flow in turbomachines. Without adequate knowledge of damping, for example, there is no alternative but to guess values for the coefficient in determining the response from a particular excitation. Note that damping may arise from material characteristics, friction, aerodynamic flow, and possibly from impact.

Shrouds in the form of a protrusion are...