An investigation of the damage mechanisms occurring during compressor blade rubbing

Experience with a wide range of industrial power generation gas turbines has shown that compressor blade or vane rubbing is relatively common and heavy contact between the blade and casing has, in the worst cases, led to failure of the blade through release of part or the whole of the aerofoil. This has meant carrying risk of high costs from repair and lost income over which operators have no ability to assess and manage other than by following manufacturer’s standard instructions. The research aim was to investigate the mechanism of compressor blade damage generated by tip rub contacts that would lead to a better understanding of the components involved in this complex mechanism. To achieve this aim, a complete test facility was designed, analysed, manufactured, assembled, developed and commissioned. The test facility delivered on the key requirements to measure strain and temperature close to the blade tip, measure casing tip loads and generate material samples during controlled rub depth experiments. It was highly reliable, safe and simple to operate with the only limitations being testing at 80% of design speed and a 2% speed drop during the test. This test facility contains the only high-speed test rig, designed specifically for the investigation of blade to casing rubbing in gas turbine compressors, in the UK. It is one of only three industrial scale, blade rub test rigs in the world.

From material examination, it was discovered that a very thin, modified microstructural layer was produced at the heaviest rub contact point and hardness data showed this layer could be up to double that of the base material. This was likely due to re-transformation of the microstructure, cold/hot work and the removal/deposition of material across the tip. Tip rubs also produced a finely grooved tip surface. Assessing the new test data and material examinations have shown that there are seven damage components: dynamic strain (impulse), dynamic strain (resonance), mean strain (thermal), plastic tip deformation, hardened (modified) surface layer, distributed areas of higher hardness and tip surface grooves (stress concentration). Four of the damage components: dynamic (impulse) strain, dynamic (resonance) strain, tip plastic deformation, and to a lesser extent mean strain had a linear correlation with rub depth. The fifth damage mechanism, amount of higher hardness areas in the blade tip, increased with rub depth. As for the remaining two damage components, there was no correlation of either the depth of the modified surface layer or the depth of the tip surface grooves with rub depth. There was however an increased distribution of higher hardness throughout the blade tip with an increased number of rubs. The finding of the relationships between the damage mechanisms and rub depth are considered novel as previously only tip load had been correlated (and plotted) with rub depth. Additionally, the measurements of plastic tip deformation, microstructural blade tip examination and the use of microhardness arrays have not been employed before in an investigation of blade-casing rub test experiment utilising gas turbine compressor blading. In a rub where the resonance did not become unstable, the dominant components would initially be the impulse (dynamic) and thermal (static) components. It was considered credible that lower range, high cycle fatigue crack initiation might occur based upon impulse and thermal stresses acting in combination with local plastic deformation and hardened, modified surface layers.

Testing has demonstrated that the visible tip deformation increased directly with rub depth that in turn corresponded to increased impulse peak strain, thermal strain, resonance strain and distribution of high hardness layers. Thus, tip deformation extent is directly related to damage potential and risk and this could be measured and assessed during regular borescope inspections with phase measurement. Additionally, damage remediation could be improved by removal of the tip surface height to the equivalent length of the tip deformation. Typically, removal of 0.6-0.7mm (in these tests), would eliminate all the hardened surface and sub-surface material and reduce future risk from tip rubbing.