Why did it break? - TWI explores root of blade failure

Unaccountable and sudden vibration detected by a TWI Industrial Member in one of its industrial gas turbines recently prompted it to shut down and decommission a major part of its plant. It turned to TWI to find the root of the problem.

Visual inspection revealed that a single turbine blade had fractured and damaged the power turbine. With only two pieces of broken blade to study in evidence, as well as several intact blades, TWI was asked to investigate the mechanism of failure.

Mobilising its multi-disciplinary approach to such problems TWI examined the blade specimens visually, by fluorescent penetrant, using scanning electron microscopy and metallographic sectioning.

It was concluded that aspects of the design of the blade were not ideal and that the failed blade exhibited a series of flaws. As a consequence of adopting this inferior design fatigue failure was initiated and led to the loss of the blade and subsequent damage in the rest of the turbine stage.

Lastly TWI confirmed that the chemical composition and microstructure of the failed and unfailed blades was the same. However it did discover two distinct zones to the fatigue crack with differing levels of oxidation. This suggested that propagation may have occurred at two separate stages.