Tuesday  |  Salon 9  |  10:00 AM–10:20 AM

#15847, Combining Thermoelastic Stress Analysis and Acoustic Emission to Detect and Monitor Cracks in Cyclically Loaded Aluminium

With metal additive manufacturing (AM) increasing in popularity, a reliable way to detect cracks and monitor their propagation is needed. AM is used in structural components such as on satellites, cars and in aerospace structures. It is therefore critical to be able to detect and monitor cracks and defects. One of the principle issues with AM is lack of fusion defects where a layer was not properly melted and did not fully adhere to the previous layer, this can weaken the material and cause cracks to initiate and grow. To this end, thermoelastic stress analysis (TSA) and acoustic emission (AE) were coupled to detect and monitor cracks using monolithic aluminium samples undergoing fatigue testing before implementation on AM samples.
TSA requires the use of an infrared camera which creates a temperature map of the surface of the sample while under a cyclic load. This temperature can be related to stress using the thermoelastic coefficient which then provides a surface stress map on the component. This can be used to determine the position of the peak stress and plot it through time, providing a crack position and growth rate.
As the crack grows it releases energy as an acoustic wave which can be detected using a piezoelectric sensors attached to the sample. AE captures the acoustic waveforms created during ‘hits’ or AE events, such as crack growth. The energy obtained from the waveforms and the number of hits provides a measure of activity or energy release through time. This can be used to determine when a crack is growing, due to a sharp rise in AE activity.
Location of cracks using only AE can be challenging, therefore the feasibility of using one AE sensor to signal crack initiation and growth and TSA for location was tested. AE can provide a measure of energy released which gives an indication of damage severity and progression, and TSA provides position and crack growth rate, assuming the surface stress distribution is affected. Combining both methods has enabled AE energy to be linked to TSA spatial information providing the severity, progression and location of cracks to be assessed.

Denis Jouan   University of Waikato
Rachael Tighe   University of Waikato
Rhys Pullin   Cardiff University