No abstract

Bolted joints are ubiquitous structural elements, and form critical connections in mechanical and civil structures. As such, loosened bolted joints may lead to catastrophic failures of these structures, thus inspiring a growing interest in monitoring of bolted joints. A novel energy based wave method is proposed in this study to monitor the axial load of bolted joint connections. In this method, the time reversal technique was used to focus the energy of a piezoelectric (PZT)generated ultrasound wave from one side of the interface to be measured as a signal peak by another PZT transducer on the other side of the interface. A tightness index (TI) was defined and used to correlate the peak amplitude to the bolt axial load. The TI bypasses the need for more complex signal processing required in other energy-based methods. A coupled, electromechanical analysis with elasto-plastic finite element method was used to simulate and analyze the PZT based ultrasonic wave propagation through the interface of two steel plates connected by a single nut and bolt connection. Numerical results, backed by experimental results from testing on a bolted connection between two steel plates, revealed that the peak amplitude of the focused signal increases as the bolt preload (torque level) increases due to the enlarging true contact area of the steel plates. The amplitude of the focused peak saturates and the TI reaches unity as the bolt axial load reaches a threshold value. These conditions are associated with the maximum possible true contact area between the surfaces of the bolted connection.

X-ray micro-computed tomography (X-ray micro-CT) has been applied to visualise ice crystal structures formed during freezing of a number of foods. Materials were frozen unidirectionally at )5°C and then freeze-dried to remove the ice crystal structure and leave voids that can be measured by the X-ray. The system reconstructs the 3-D image based on a set of 2-D images, and is capable of micrometre-scale visualisation. This study demonstrates the capability of the technique to characterise the internal ice crystal microstructure of a range of frozen materials; meat, fish, chicken, potato, cheese and carrot. Results show the voids corresponding to the ice crystals formed within these materials at different directions to the heat flux and various axial positions. Electron microscopy of the same materials, both fresh and frozen at )5°C, indicates the same shape of voids seen by the tomographic technique. Ice crystal parameters such as size, area and width can be quantified by the technique. Ice crystals in carrot were larger than in the other materials, while cheese and potato had the lowest values. The ice crystal distribution of all the experimental materials varied with axial distance from cooling surface; the closer the measurement was to the cooling surface, the smaller the crystal size. The results demonstrate that X-ray micro-CT might be useful in the analysis of frozen foods.

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