Mankind’s constant pursuit of knowledge founded on innovation and creativity has led to a more complex world. A world governed by structures stretching high into the skies, objects flying faster than the speed of sound and exploration of the stars. These advancements have manifested in structures with complex geometrical designs, new building and engineering materials and the need for constant structural health monitoring (SHM). Structures comprising of a handful of components to those enlisting thousands are not impervious to failure. The ‘perfect’ material, one that will never fail, does not exist and may never. The need to remove the possibility of failure from the ever growing list of materials, components and structures has never been greater. There is an abundance of well suited techniques for a myriad of engineering problems, all suited for individual problem areas but all suffering from different weaknesses. There is no complete solution, only partial, short-lived solutions which are eclipsed by the next.The objective of this PhD is to explore a new and interesting non-destructive testing and evaluation (NDT/E) technique; nonlinear ultrasound. The work looks at assessment of both metallic and composite structures. As of recently, the technique has become well studied and documented, which has highlighted the associated benefits. Theoretically it builds on the fundamental theories of ultrasound testing techniques, but provides solutions that have amplified sensitivity in early damage detection. The techniques follow the main principle that the excitation of damaged regions results in clapping/rubbing mechanisms that give rise to further harmonic production, these further harmonics can be correlated to damage.Nonlinear ultrasound techniques were used to assess various systems that are prone to failure, these systems included adhesively bonded structures, bolted structures, structures susceptible to fatigue, compression loaded structures and impact damage of a composite plate. A series of experiments were undertaken which evaluated the ability of novel nonlinear ultrasound techniques to detect damage within these systems. Initially the primary principles of nonlinear ultrasound techniques those that affect the accuracy and repeatability of these methods were explored. The effects of hysteresis, input and output voltages of piezoelectric transducers (PZTs), modal analysis, and various nonlinear parameters were evaluated. This allowed for a clear understanding of what factors affect the accurate generation of data from these techniques. After determining various influences on results, adhesive and bolted joints were evaluated. With the aim to determine accurate nonlinear techniques that would be able to assess kissing bonds in metallic and composite joints, cracks in loaded structures and the loosened state of an individual bolt. A novel frequency specific nonlinear acoustic moment method was used to evaluate the presence of kissing bonds. The acoustic moment experiments provided good kissing bond detection probability in both metallic and composite joints. Further experiments explored the effects of structurally loading on the production of nonlinear responses. The results provided valuable insight into the potential for these techniques to assess defects in loaded structures. Thus kissing bond detection as well as the loaded state of the structure was possible using the nonlinear acoustic moment method developed.Joints account for a large proportion of mechanical and engineering structures, robust evaluation techniques would provide great savings in terms of maintenance, complete failure and lifetime service costs. The main finding was the ability to measure the loosened state of an individual bolt. Individual bolt loosened state was possible by using only two PZTs while assessing a system which included four bolts. This piece of research provides significant advancement in SHM of these structures, which has implications for small and large scale industrial joints. Fatigue failure is the most prominent failure mechanism in metallic structures, an investigation into a baseline-free method using nonlinear ultrasound was undertaken. A fatigued component was examined over its useful life using a modulated nonlinear ultrasound technique, where the development of a nonlinear ultrasound theory was used to assess the residual fatigue life by comparing the nonlinear modulated response to a theoretical model. Findings showed good correlation between the theoretical and experimental results, paving the way for further studies of baseline-free methods. A baseline-free testing method would provide a great leap forward in current testing procedures; the work highlights the potential of such methods. Further studies explored the possibility of using a computational model to predict damage levels in a material from a measured experimental nonlinear parameter. The findings of this research found good correlation between the second harmonic measured experimentally and that of the computational model.A novel nonlinear ultrasound based thermosonic technique was developed using a dual frequency excitation method. The investigation looked at determining barely visible impact damage (BVID) of a composite plate. The speed and accuracy of the method provided many advantages over current testing methods, which can be slow and deliver inconsistent results. The research relied on the determination of damage-specific resonance frequencies (DSRF), which results in focused heating at the damaged zones. The methodology explored could be used in an autonomous setup which would provide rapid assessment of composites and higher probability of damage detection.Finally the nonlinear ultrasound research completed establishes the broad range of applications for these techniques. The ease of adaptability of the techniques from metallic to composite, loaded and unloaded structures and their ability to improve other NDT/E techniques shows the great potential of nonlinear ultrasound. Through methodical application of the various nonlinear ultrasound techniques to other SHM problems there is a great degree of certainty that these methods would provide further benefits.
|Date of Award||28 Oct 2014|
|Supervisor||Michele Meo (Supervisor)|