Detection of a Blade Crack in Bladed Disks: Methodology and Validation

In turbomachinery, blade crack detection methods are beneficial for condition-based maintenance and for reducing the risk of catastrophic failures due to high cycle fatigue of bladed disks. A crack can be more easily detected in a perfectly tuned bladed disk (i.e., with identical blades and sectors). That is because the crack causes mode distortion and possibly response localization. Unfortunately, perfectly tuned bladed disks are an idealization since small blade mistuning is unavoidable and it can also cause mode distortion and response localization in bladed disks. A detection method has been recently developed to detect the presence of a crack in a mistuned bladed disk and to identify which blade is cracked. The method requires multi-point measurements of all blades (e.g., obtained by using tip-timing techniques) and is based on the observation/assumption that the dynamic response of a cracked bladed disk cannot be represented as a linear combination of modes of the tuned system. This paper introduces a method which uses a similar physical basis for detection, but requires fewer measurements, namely one measurement location per blade (e.g., obtained using tip-timing methods). The key idea is to focus on isolated modes (or isolated mode pairs) which are sensitive to the presence of a crack rather than on an entire mode family (with high modal density). The novel method also extracts spatially correlated vectors from the nonlinear (forced response) measurement data of the cracked bladed disk to be used for crack identification.