| Structural diagnosis and prognosis is still a challenging problem despite extensive progresses during the last few decades. As one of the most common failure modes of engineering materials and structures, fatigue has great impact on the long term durability of structures. The objective of this study is to develop a systematic framework for the concurrent fatigue damage diagnosis and prognosis at the structure level. First, a fatigue damage prognosis methodology is formulated based on a hierarchical state-space model and a novel time-derivative fatigue crack growth model. The method naturally couples the structural dynamics and the crack growth analysis, which provides the capability to perform concurrent fatigue crack growth analysis. The dynamic response (structural-level) and the fatigue crack growth (material-level) can be solved simultaneously. The time-derivative fatigue crack growth model can also be conveniently used for online and offline fatigue crack growth analysis given the loading information. In addition, the environmental effect for the material fatigue crack growth behavior is included to enhance the applicability of the model. Following this, an experimental investigation and modeling study for local damage identification based on activated ultrasonic wave is presented. The experimental work for damage detection and quantification in riveted lap joints is performed. Embedded lead zirconate titanate piezoelectric (PZT) ceramic wafer-type sensors are employed to perform in-situ non-destructive evaluation (NDE) during cyclical loading. PZT wafers are used as actuators to generate Lamb wave, as well as sensors, which collect the signals for damage detection (i.e., cracks at joints resulting from fatigue loading). The pitch-catch method is used to detect the signal which may carry damage information. The damage index relates fatigue crack length to three Lamb wave characteristics (correlation coefficient, normalized amplitude and phase change). Finally, a method for fatigue prognosis integrating damage diagnosis results, usage monitoring system, and mechanism observations is developed. The key concept is to reconstruct the dynamic responses for the critical spots which are vulnerable to fatigue damage and also are inaccessible for direct sensor measurements. The sensor data at limited locations measured from usage monitoring system are used as the basis of the reconstruction. Fatigue prognosis for the critical spot is performed using the reconstructed dynamic responses. |
