Influence of fatigue crack growth on the dynamics of engineering components and structures
Despite of the fact that engineering components and structures are carefully designed against fatigue failures, 50 - 90 percent of all mechanical failures are due to fatigue. Cracks can exist as a consequence of deep machining marks and voids in welds during the manufacturing processes, and metallurgical discontinuities due to the presence of foreign particles and inclusions. A comprehensive study of the expenditure relating to component fracture in the United States indicated a cost of 119 billion in 1978, or about 4 percent of the gross national product. Beside cost, the consequence of fatigue failure may result in the loss of human life, in particular in the transportation, and the oil and gas industries. Therefore, a robust fatigue life assessment is crucial to provide significant economic and safety advantages. A good fatigue design involves, analysis and testing. The more closely analysis and testing simulate the real situation, the more confidence one can have in the results. To limit the extend and the consequences of such failures more fundamental understanding how fatigue crack develop, in particular under dynamic loading is required. Over the years, a number of fatigue life prediction models have been established. No doubt, these models show some promising results for predicting fatigue of engineering components and structures subjected to constant amplitude, however, they are not reliable for life prediction of components and structures subjected to variable amplitude loading. Therefore, a robust life prediction mathematical model has yet to be developed. In reality, dynamic structural interactions, which create conditions for fatigue crack growth occurs. To understand such behaviour, comprehensive studies have to be performed. Many experimental and theoretical work have been published which relates to the investigations of cracked structure subjected to various dynamic conditions. However, these works were mainly carried out assuming that the cracks remain stationary. Dynamic interactions due to a growing fatigue crack have not been studied extensively. To understand the dynamic interactions due to a growing crack which lacks robust analytical models, extensive experimental work has to be performed. Due to the restrictions of conducting realistic fatigue tests on conventional fatigue- testing machines, a new fatigue-testing rig has been developed and is presented in this thesis. This device comprised of two based excited oscillators situated at the top and bottom of a bending specimen and being excited by an electro-dynamic shaker. The main operating principle of the rig is that inertial forces generated by the oscillators act on the specimen. By changing the natural frequency of the oscillators, the extent of the pre-load, and the pattern of the excitation, the rig provides a new and robust means of fatigue testing. A mathematical model that allows to investigate the dynamics of the system under a propagating crack has been developed for the new fatigue-testing rig. The system responses have been examined using standard nonlinear dynamics tools such as Poincare maps and bifurcation diagrams. Finally, an extensive experimental work has been conducted on the prototype fatigue rig to confirm it's capability of inducing fatigue crack in the specimen as well as to examine the system behaviour during harmonic and chaotic excitations.