TWI conducted a resonance fatigue test in their world-class facilities to assess the suitability of vibration analysis and acoustic emission techniques to detect crack initiation and growth on the geothermal well casing before it fails. A 4.4 meters well casing was tested to failure to its resonance frequency.
Fatigue resonance tests are used for crack investigation and involve the excitation of a test specimen close to its first mode of vibration, by applying a rotating radial force to one end. The resonance test method applies a fully alternating stress cycle and bending moment is generated in the specimen. Strain gauges are usually located along the structure to detect the presence of a crack, for this experiment acoustic emissions and vibration monitoring sensors were also used to monitor crack growth.
Figure 1 Schematic showing the principle of resonance fatigue testing.
The initial results are showing that a change in parameters recorded by both techniques, vibration and acoustic emission, led to early detection of crack initiation and propagation before the test failed.
The results from this test, along with previous testing and further research, will contribute to the assessment that TWI is carrying out for using the mentioned technologies for monitoring of geothermal well casing,